最先端有機元素化学①：最新論文からのトピックス

(1)

最先端有機元素化学①：最新論文からのトピックス

Prof. Radosevich Pennsylvania State

University

タイトルと

TOC

グラフィックから読み取れること

・水素移動反応の話

・典型元素の酸化還元を利用した触媒

・リン原子が可逆的に

3

価と

5

価になる

Abstract: A planar, trivalent phosphorus compound is shown to undergo reversible two-electron redox cycling (P

^III

/P

^V

) enabling its use as catalyst for a transfer hydrogenation reaction. The trivalent phosphorus compound activates ammonia-borane to furnish a 10-P-5 dihydridophosphorane, which in turn is shown to transfer hydrogen cleanly to azobenzene, yielding diphenylhydrazine and regenerating the initial trivalent phosphorus species. This result constitutes a rare example of two-

electron redox catalysis at a main group compound and suggests broader potential for this nonmetal platform to support bond-modifying redox catalysis of the type dominated by transition metal catalysts.

Abstract

から読み取れること

有機元素化学特論第 12 回 2014.12.20

Main Group Redox Catalysis: Reversible PIII/PV Redox Cycling at a Phosphorus Platform Dunn, N. L.; Ha, M.; Radosevich, A. T.

J. Am. Chem. Soc. 2012, 134, 11330-11333.

・アンモニアボランが活性化されて

10-P-5

という化学種ができる

・アゾベンゼンが水素化されてヒドラジンになる

・非金属系で

2

電子の酸化還元は珍しく、応用の可能性は広い

(2)

Reviews

Frustrated Lewis Pairs

D. W. Stephan,* G. Erker* 46 – 76

Frustrated Lewis Pairs: Metal-free Hydrogen Activation and More

Communications

Surface Plasmon Polaritons

M. L. Pedano, S. Li, G. C. Schatz,*

C. A. Mirkin* 78 – 82

Periodic Electric Field Enhancement Along Gold Rods with Nanogaps

Self-Assembly

T. Moriuchi,* M. Nishina,

T. Hirao* 83 – 86

Arylimidovanadium(V) Complexes for a Tridendritic Centrosymmetric Structural Motif or Axial Chirality

Flow Lithography

K. W. Bong, K. T. Bong, D. C. Pregibon,

P. S. Doyle* 87 – 90

Hydrodynamic Focusing Lithography

For V’s a jolly good fellow: A one-pot reaction of aniline derivatives with VO- (OiPr)3in the presence of NaH affords either trinuclear arylimidovanadium(V) triisopropoxide with a tridendritic centrosymmetric structure (see picture, left) or axially chiral binuclear arylimidovanadium(V) triisopropoxide (right), wherein self-assembly creates a highly ordered molecular arrangement in the solid state.

Micro sandwiches: The new technique of hydrodynamic focusing lithography (HFL) utilizes stacked microfluidic flows for polymer microparticle synthesis. The method can improve the throughput of flow lithography for multifunctional particles and produce more complex chemi- cally patterned particles (see examples).

Introduction: 遷移金属と典型元素の反応性比較

Ref 9

Ref 12

Chem. Sci. 2011, 2, 389.

イントロを読む際の注意点

(1)

参考文献は精読しなくても良いので、その

abstract

と絵だけでも見て流れを掴め

(2)

まとまった本や総説

(review)

は何のトピックに関する内容なのかだけわかれば良い

Refs 1-5

は均一系触媒による有機合成化学反応の総説類

Refs 6-8

は均一系触媒反応にかかる遷移金属化合物の第一配位圏内での素反応についての総説類

Ref 11

Angew. Chem., Int. Ed.

2010, 49, 46.

Ref 10

trans-bent structure (Ar*, C6H3-2,6(C6H2-2,4,6-i-Pr3)2; i-Pr, CH(CH₃)₂). Also, there are formally double-bonded species such as the green Ar9GaGaAr9, which dissociates in solution to :GaAr9 monomers, which have donor and acceptor sites³⁴ (Ar9, C6H3- 2,6(C₆H₃-2,6-i-Pr₂)₂). In contrast, the heavier group 15 element multiple bonded compounds, exemplified by Yoshifuji’s landmark diphosphene Mes!€PP~PPMes€ ! (Mes*5C6H2-2,4,6-t-Bu3) (ref. 35;

t-Bu, C(CH3)3), have a conventional double bond withs- andp- components due top-orbital overlaps and lone pairs that are mainlys in character. These compounds have no non-Kekule´ character, and their chemistry more resembles that of unsaturated organic compounds. The heaviest element derivatives RMM~€ MMR (M€ 5Sb or Bi) have bond shortenings of 8–9%, which show thatp-bonding is main- tained in the higher rows of thep-block elements³⁶.

Reactivity of heavier main-group compounds

Reactions with saturated small molecules. In the previous section, it was argued that the multiple bonded heavier main-group compounds resemble transition-metal complexes owing to the presence of occu- pied and unoccupied frontier orbitals that are energetically accessible.

How is this reflected in the chemistry of the main-group species? In organotransition-metal chemistry, the most important or basic reaction types are oxidative additions, reductive eliminations and inser- tions. Most steps in homogenous catalytic cycles involve one or more of these reactions—usually with small molecules, such as hydrogen, olefins, carbon monoxide, ammonia and related molecules—under mild conditions. Until recently, no comparable reactions with main- group molecules were known. However, many such reactions have been recently discovered for the multiple bonded/unsaturated main- group compounds.

The simplest of these reactions involve the addition of an H₂molecule, which is analogous to a key step in many homogeneous transition- metal catalytic cycles such as olefin, alkyne or arene hydrogenation and hydroformylation. In several instances, the addition is reversible and, in a defining event in the development of coordination chemistry, Kubas showed that it was even possible to isolate transition-metal complexes of H₂itself, which represent the initial stage of an oxidative addition reaction³⁷. Until recently, no H2additions were known for main-group compounds under mild conditions. However, in 2005 it was shown that the Ge alkyne analogue Ar9GeGeAr9reacted directly with H₂under ambient conditions to give the hydrogenated products Ar9(H)GeGe(H)Ar9, Ar9(H)2GeGe(H)2Ar9and Ge(H)3Ar9; these were all structurally characterized by X-ray crystallography³⁸. Calculations indicate that in this reaction and that of its Sn analogue Ar9SnSnAr9 (which reacts to give Ar9Sn(m-H)2SnAr9exclusively) the initial step involves a synergic interaction of frontier orbitals with H₂(Fig. 2)³⁹.

The interaction involves donation from thes-orbital of H2into the LUMO of Ar9GeGeAr9, which is a symmetric unoccupied n1

non-bonding combination (compare Fig. 1e). There is a synergic electron donation from thep-HOMO orbital of the germanium species into thes*-orbital of H2. This weakens the H–H bond suffi- ciently to enable oxidative addition to occur. Clearly, the mechanism whereby H2 is activated by the main-group and transition-metal compounds is analogous in the symmetry of the frontier orbitals

and their energy separations. The main difference lies in the orbital labels. This type of activation by main-group species is general, as shown by the fact that several other unsaturated heavier main-group molecules, including the carbene-like :GeAr₂and :SnAr₂as well as the monovalent :GaAr species, have been recently shown to react directly with H2(Fig. 3a and b)^39,40.

Bertrand and his group showed in 2007 that the reaction of the related stable carbenes such as :C(t-Bu)Ni-Pr₂ with H₂ (Fig. 3c) affords the addition product H2C(t-Bu)Ni-Pr2(ref. 41), in contrast to the reaction with the heavier tin carbene analogue (Fig. 3a), which leads to arene elimination (i-Pr, CH(CH₃)₂). All of the products were characterized spectroscopically and by X-ray crystallography.

In another major advance, Stephan and co-workers showed that H₂can be bound in a reversible manner by the use of a Lewis-acid–

base strategy, termed ‘frustrated Lewis pairs’, and incorporated in phosphine boranes (Fig. 3d)⁴². The important feature of the reaction is the steric prevention of the formation of a stable Lewis acid–base phosphine–borane adduct by the large phosphine substituents (hence the term frustrated Lewis pair) while permitting the synergic interaction of the empty borane 2porbital and the phosphine lone pair orbital with the H₂ molecule. In effect, there is an available unfilled acceptor orbital (on boron) and a donor orbital (phosphorus electron pair) that can fulfil a similar function to the frontier orbitals on transition metals and unsaturated, multiple bonded main-group compounds. The initial step is very likely to be the side-on reaction of thes-bond of H2with the borane, with population of the H₂s*- orbital by electron density from the phosphine. In other words, the activation of H2proceeds by a similar initial orbital interaction to those described above, although the ultimate disposition of H₂differs in that it is polar-H¹at phosphorus and H²at boron. Mild heating of the resulting phosphonium borate releases H₂to regenerate the phosphine and borane. Recent work has extended the list of H2frustrated Lewis pair activation systems to include borane complexes of carbenes^43,44, amines⁴⁵and phosphines^46,47. It seems very likely that other combinations of donor/acceptor species derived from various main- group elements will also be developed.

In addition to the activation of H₂, Bertrand and co-workers also showed that carbenes react with NH₃at278uC to afford N–H activation products (Fig. 3e)⁴¹. This was the first instance of the insertion of a main-group element into the N–H bond of ammonia under mild, uncatalysed conditions. The heavier germanium and tin carbene analogues :GeAr₂and :SnAr₂as well as the gallium species :GaAr were also shown insert into N–H bonds below room temperature^40,48. These reactions parallel those in transition metals where, for

M M

Ar

n₊(a_g) π(a_u)

σ*

H H σ

σ* M

H

H σ* σ σ*

a b

+

+ + +

+ +

+ – +

–

– +

Figure 2|Similarity of H2interactions with main-group and transition- metal compounds. a, The initial interaction of H2with thep(HOMO) and n1(LUMO) frontier orbitals of ArMMAr (M5Ge or Sn; Ar9 5C6H3- 2,6(C6H3-2,6-i-Pr2)2), which lie only,2 eV apart. This is comparable to b, the interaction of H2with the frontierdorbitals in a transition-metal complex (M is a transition metal).

2 :SnAr₂ + 2H₂ ArSn(μ-H)₂SnAr + 2ArH a

2 :GaAr + 2H₂ Ar(H)Ga(μ-H)₂Ga(H)Ar b

:C(t-Bu)Ni-Pr₂ + H₂ H₂C(t-Bu)Ni-Pr₂ c

B(C₆F₃)₃ + PR₃ H₂ [R₃PH][HB(C₆F₃)₃] 25 °C

:C(t-Bu)Ni-Pr₂ + NH₃

H

H₂N Ni-Pr₂ C

t-Bu e

Ir Pt-Bu₂

Pt-Bu₂

Me NH₃, 25 °C

<5 min

Pt-Bu₂

Me H

NH₂ + f

Ir d

Figure 3|The reactions of various main-group and transition-metal species with H2or NH3. Ar5C6H3-2,6(C6H2-2,4,6-Me3)2or C6H3-2,6- (C6H3-2,6-i-Pr2)2.

NATUREjVol 463j14 January 2010 REVIEWS

Nature 2010, 463, 171.

Acc. Chem. Res. 1991, 24, 202.

Ligand- Coupling Reactions of Hypervalent Species

Scheme IX

Acc. Chem. Res., Vol. 24, No. 7, 1991 207

R L

I

- \.

R-Cu(I) R

^I

- + -C-X / \. - L

R-Cu(I) + -C-X / - inversion

Cu (Ill) intermediate (L : solvent or other ligand) ligand coupling

* /

retention * R-C- \ + R-Cu(I)

shown by the three typical reactions shown in eqs 19-21.44 Since the perfluoroalkyl group is highly

I SAr CI

Me

RF :

Perllwroalkyl

^group

RF-I-OSO~H + phcH&QX

-

^RF-CH,Ph ⁽²⁰⁾

RF-I-OTS

+

PhOH

-

⁽²¹⁾

electronegative [i.e., the electronegativity of the RF

group is 3.45,& between that of C1 (3.0) and F (4.0)], a direct SN2-type nucleophilic attack on the RF group

may not give any substitution product. However, be- cause of the highly electron withdrawing effect of the

RF group, most nucleophiles can attack the central iodine atom to form transient iodine-centered hypervalent intermediates, which in a subsequent step readily undergo ligand coupling. Here again, there has been no stereochemical investigation to substantiate the mechanistic argument.

The reaction shown in eq 22 to form a-(trifluoro- methy1)sulfonoxy ketones (a-keto triflates) is another typical example of ligand coupling.46

Ph Ph

-

OTMS

I 1 1

^OTMS

I

-78%

H I

CeH5

1

L

The substitution reactions shown in eqs 23 and 24 were found to be stereospecific and proceeded with retention of the trans configuration of the double bond.

Ochiai proposed a reaction pathway involving the initial addition of the cuprate, ligand coupling on the iodine atom, and also ligand coupling on the copper.47

Among many other examples, the oxidation of sub- stituted thioanisoles with (diacetoxyiod0)benzene may

(44) Lyalim, V. V.; Orda, V. V.; Alekseeva, L. A.; Yagupolskii, L. M.

Zh. Org. Khim. 1971, 7, 1473. Yagupolskii, L. M.; Maletina, I. I.; Kon- dratenko, N. V.; Orda, V. V. Synthesis 1978, 835. Umemoto, T. Yuki Gosei Kagaku Kyokaishi 1983,41,251. Umemoto, T.; Kuriu, Y. Tetra- hedron Lett. 1981,22,5197. Umemoto, T.; Miyano, 0. Bull. Chem. SOC.

Jpn. 1984,57, 3361.

(45) Huheey, J. E. J. Phys. Chem. 1966,69, 3284.

(46) Moriarty, R. M.; EDa, W. R.; Penmasta, R.; Awasthi, A. K. Tet- rahedron Lett.-1989, 30, 663.

(47) Ochiai, M. Reuiews on Heteroatom Chemistry; MYU: Tokyo, 1989; Vol. 2, pp 92-112.

(24) PhZCuLi

-

- +

-

^Ph

-I-Ph M F . . 30°C

B F ~ -

be cited (eq 25). In this reaction, the rates were found to be correlated with the Hammett

B

values, giving a

p value of -0.8>48 the rate being slower with bulky alkyl-substituted phenyl sulfides.

Ar-&-Me

I +

^PhlOAc

J

- 4

0 ⁽²⁵⁾

Recently, the reaction shown in eq 26 was found to give methyl p-tolyl sulfoxide in nearly optically pure form, due to the optically active nature of the starting This also involves a typical ligand coupling, with initial nucleophilic attack of the sulfide on the hypervalent iodine atom.

Ar-S-Me

+

PhI

+

PC9

I

Central Copper Atom

As was suggested earlier>l' many reactions involving organic copper reagents, such as the Ullman type re- actionsw and those involving the Gilman reagent: may proceed through ligand coupling. These reactions are highly stereoselective. A typical example of the Ullman type reaction was carried out by Cohen and Poeth, who showed the reaction to proceed with over 96% retention of the geometric configuration, as shown in eq 27.50

cu, 100°C

- * y (27)

H Y

Y

=

C02Et 96%

One example of the use of a Gilman reagent is the formation of optically active 2-phenyloctane in the treatment of optically active 2-octyl p-tosylate with lithium diphenyl~uprate,~' though octyl bromide be- haved somewhat differentl~.~~ Many similar reactions have been carried out with Gilman and all have been found to be stereoselective. The reaction obviously involves an initial SN2 and the

(48) Srinivasan, C.; Chellamani, A.; Kuthalingam, P. J. Org. Chem.

(49) Ray, D. G., 111; Koser, G. F. J. Am. Chem. SOC. 1990,112,5672.

(50) Cohen, T.; Poeth, T. J . Am. Chem. SOC. 1972, 94,4363.

(51) Dagurige, Y. MSc. Thesis, Okayama University of Science, 1988.

(52) Lipshutz, B. H.; Wilhelm, R. S. J . Am. Chem. SOC. 1982,

104,

4696. Lipshutz, B. H.; Wilhelm, R. S.; Kozlowski, J. A. The Chemistry of Higher Order Organocuprates, Tetrahedron Report, No. 176. Tetra- hedron 1984,40, 5005.

(53) Poaner, G. H. Substitution Reaction wing Organocopper Reag- ents; Organic Reactions, 22; John Wiley & Sons: New York, London, Sydney, and Toronto, 1975; pp 252-400. Posner, G. H. Conjugatiue Addition Reactions

of

Organocopper Reagents; Organic Reactions, 19;

John Wiley & Sons: New York, London, Sydney, and Toronto, 1972; pp 1-114. Johnson, C. R.; Dutra, G. A. J. Am. Chem. SOC. 1973,95,7783.

Johnson, C. R.; Dutra, G. A. J. Am. Chem. SOC. 1973,95,7777. Hebert, E. Tetrahedron Lett. 1982,23,415.

1982, 47, 428.

Ref 13

Chemistry of Hypervalent Compounds;

Wiley VCH: New York, 1999.

Ref 14

Ligand Coupling Reactions with Heteroatomic Compounds;

Elsevier: Oxford; 1998.

(3)

Introduction 2: 典型元素の酸化還元や触媒利用

Ref 16. Oxygenation of Al(III) species

Ref 17a. aza-Wittig with phosphine catalyst

J. Am. Chem. Soc. 2011, 133, 11865.

Ref 15. Reaction of carbene with H

₂

and NH

₃

Science 2007, 316, 439.

tuned by modifying the nature of the substituents, as shown by the differing reactivities of mono(amino)carbenes and diaminocarbenes with H

₂

and NH

₃

. Moreover, the nucleophilic activation of NH

3

, under very mild conditions, offers an alternative paradigm in the continuing search for catalytic systems capable of trans- forming ammonia efficiently into useful amino compounds.

References and Notes

1. P. Sabatier,Ind. Eng. Chem.18, 1005 (1926).

2. E. J. Lyonet al.,Eur. J. Biochem.271, 195 (2004).

3. O. Pilaket al.,J. Mol. Biol.358, 798 (2006).

4. A few species, which are only stable in matrices at 10 to 80 K, are also known to split H2: subvalent group 13 species, (5) triplet carbenes (6), and the highly electrophilic singlet difluorovinylidene (7).

5. H.-J. Himmel,J. Chem. Soc. Dalton Trans.2003, 3639 (2003).

6. H. Tomioka, inReactive Intermediate Chemistry, R. A. Moss, M. S. Platz, M. Jones Jr., Eds. (Wiley, New York, 2004), pp. 375–461.

7. C. Kötting, W. Sander,J. Am. Chem. Soc.121, 8891 (1999).

8. G. C. Welch, R. R. San Juan, J. D. Masuda, D. W. Stephan, Science314, 1124 (2006).

9. G. C. Welch, D. W. Stephan,J. Am. Chem. Soc.129, 1880 (2007).

10. G. H. Spikes, J. C. Fettinger, P. P. Power,J. Am. Chem.

Soc.127, 12232 (2005).

11. A. Werner,Z. Anorg. Chem.3, 267 (1893).

12. Y. Nakajima, H. Kameo, H. Suzuki,Angew. Chem. Int. Ed.

45, 950 (2006).

13. J. Zhao, A. S. Goldman, J. F. Hartwig,Science307, 1080 (2005).

14. E. G. Bryan, B. F. G. Johnson, K. Lewis,J. Chem. Soc.

Dalton Trans.1977, 1328 (1977).

15. G. L. Hillhouse, J. E. Bercaw,J. Am. Chem. Soc.106, 5472 (1984).

16. A. L. Casalnuovo, J. C. Calabrese, D. Milstein,Inorg.

Chem.26, 941 (1987).

17. M. M. B. Holl, P. T. Wolczanski, G. V. Duyne,J. Am. Chem.

Soc.112, 7989 (1990).

18. G. J. Kubas, R. R. Ryan, B. I. Swanson, P. J. Vergamini, H. J. Wasserman,J. Am. Chem. Soc. 106, 451 (1984).

19. G. J. Kubas,Adv. Inorg. Chem.56, 127 (2004).

20. G. J. Kubas,J. Organomet. Chem.635, 37 (2001).

21. G. S. McGrady, G. Guilera,Chem. Soc. Rev.32, 383 (2003).

22. R. H. Crabtree,Acc. Chem. Res.23, 95 (1990).

23. G. J. Kubas,Science314, 1096 (2006).

24. R. A. Moss, M. S. Platz, M. Jones Jr., Eds.,Reactive Intermediate Chemistry(Wiley, New York, 2004).

25. D. Bourissou, O. Guerret, F. P. Gabbaï, G. Bertrand, Chem. Rev.100, 39 (2000).

26. V. Lavalloet al.,J. Am. Chem. Soc.126, 8670 (2004).

27. V. Lavallo, Y. Canac, C. Praesang, B. Donnadieu, G. Bertrand,Angew. Chem. Int. Ed.44, 5705 (2005).

28. A. J. Arduengo III, R. L. Harlow, M. Kline,J. Am. Chem.

Soc.113, 361 (1991).

29. F. E. Hahn,Angew. Chem. Int. Ed.45, 1348 (2006).

30. M. K. Denk, J. M. Rodezno, S. Gupta, A. J. Lough, J. Organomet. Chem.617–618, 242 (2001).

31. D. A. Dixon, A. J. Arduengo III, K. D. Dobbs, D. V.

Khasnis,Tetrahedron Lett.36, 645 (1995).

32. V. Lavallo, Y. Canac, B. Donnadieu, W. W. Schoeller, G. Bertrand,Angew. Chem. Int. Ed.45, 3488 (2006).

33. M. J. Frischet al.,Gaussian 03(Gaussian, Wallingford, CT, revision C.02, 2004).

34. In a first approximation, the singlet-triplet energy gap parallels the HOMO-LUMO (lowest unoccupied molecular orbital) splitting.

35. Carbenes1and2are highly moisture sensitive, and the low yields observed are due to the presence of water in the H2gas, which gives rise to the H2O-carbene adducts.

36. Preparation methods and spectroscopic data for compounds5,6a,6b,7,8a, and8bare available on ScienceOnline.

37. A. J. Arduengo IIIet al.,J. Am. Chem. Soc.116, 6641 (1994).

38. R. W. Alder, P. R. Allen, M. Murray, A. G. Orpen, Angew.

Chem. Int. Ed. Engl.35, 1121 (1996).

39. W. A. Herrmann, M. Elison, J. Fischer, C. Köcher, G. R. J. Artus,Chem. Eur. J.2, 772 (1996).

40. We are grateful to NSF (grant CHE 0518675) and RHODIA for financial support of this work, the University of Bielefeld for computer facilities, the Alexander von Humboldt Foundation for a fellowship (G.D.F.), and A. J. Arduengo for helpful discussions. Metrical data for the solid state structures of6aand8bare available free of charge from the Cambridge Crystallographic Data Centre under reference numbers CCDC-637184 and CCDC-637185, respectively.

Supporting Online Material

www.sciencemag.org/cgi/content/full/316/5823/439/DC1 Materials and Methods

Figs. S1 and S2 Tables S1 to S4 References and Notes

20 February 2007; accepted 13 March 2007 10.1126/science.1141474

Fig. 3. (A) (Alkyl)(amino)carbenes 1 and 2 split ammonia at subzero temperatures. (B) Bond distances (Å) and charges (in parentheses) for the transition state of the reaction of NH

₃

with carbene 2 ′ , calculated at the B3LYP/6-311 g** level of theory.

Fig. 4. Model carbenes used for the calculated values shown in Table 1.

Fig. 2. (A) Under mild conditions, (alkyl)(amino)carbenes 1 and 2 activate H

₂

, whereas (diamino)carbenes 3 and 4 are inert. (B) Some structures along the reaction pathway of the insertion of carbene 2 ′ into

H

₂

, calculated at the B3LYP/6-311 g** level of theory. Bond distances (Å) and charges (in parentheses) are given in the structure drawings, and relative energies are shown in the reaction coordinate diagram.

www.sciencemag.org SCIENCE VOL 316 20 APRIL 2007 441

REPORTS

Org. Lett. 2008, 10, 2589.

Angew. Chem., Int. Ed. 2009, 48, 6836.

Ref 17b. Wittig reaction with catalytic phosphine

(4)

Introduction 3: 典型元素の酸化還元を伴う触媒反応

この論文を読む上での鍵

→

Angew. Chem., Int. Ed. 2010, 49, 4976.

J. Org. Chem. 2010, 75, 5083.

Chem. Commun. 2010, 46, 3025.

Ref 17c.

Ref 17d.

Ref 17e.

Ref 18. review for iodine catalyst for oxidation Ref 19. Catalyst without precious metals

Refs 20. review for organocatalyst

(5)

pentane trituration of the crude reaction residue. Alternatively, the action of sodium cyanoborohydride on dichlorophosphor- ane 2 gives the same species in 59% isolated yield, which we assign as dihydridophosphorane 3 (Scheme 1). 27,28 The

magnitude and multiplicity of the 31 P NMR resonance are indicative of 1 J PH coupling of the phosphorus center with bound hydrogen nuclei of an apparently equivalent pair (i.e., a −P(H) 2 unit, vide infra) and longer range 3 J PH coupling to the remote vinylic hydrogen nuclei. The complementary coupling securing these assignments is evident in the 1 H NMR spectrum (Figure 2, top); phosphorus-coupled doublets are observed at δ 7.95 ppm ( 1 J PH = 670 Hz) and δ 5.83 ppm ( 3 J PH = 34 Hz).

X-ray di ff raction of a colorless single crystal obtained by sublimation of 3 in vacuo at 35 ° C a ff ords additional structural detail. The structure of 3 is found to deviate, albeit modestly, from an idealized 10-P-5 trigonal bipyramid toward a distorted square pyramid about phosphorus (Figure 3). The phosphorus center resides in a plane de fi ned by the nitrogen atom and the two P−H hydrogen atoms. The positions of the P−H hydrogens, both of which were located on the di ff erence Fourier map, are splayed obliquely with respect to the O,N,O- supporting structure and are crystallographically inequivalent (∠N−P−H 1 = 134.8(1) ° ; ∠N−P−H 2 = 116.1(1) ° ). 29 Tridentate binding of the O,N,O-motif is evident with oxygen atoms spanning the phosphorus center at an angle of 170.5(1) ° . The ring systems experience only a minor puckering with nitrogen displaying slight pyramidalization ( φ = 6.7 ° ). 30

Reaction of dihydridophosphorane 3 with an unsaturated organic substrate results in transfer of hydrogen from phosphorus, regenerating reduced three-coordinate 1. 31 In a stoichiometric experiment, exposure of 3 to 1 equiv of

azobenzene in acetonitrile-d 3 solution at 40 ° C for 19 h results in 72 ± 5% conversion of 3 to 1 with concomitant formation of 1,2-diphenylhydrazine in 63 ± 5% yield (eq 1). In the presence

of excess azobenzene (which mimics catalytic conditions, vide infra), the conversion of 3 to 1 can be driven cleanly to completion. When monitored by NMR, no other intermediates or products are evident in this hydrogen transfer. The consumption of 3 in acetonitrile-d 3 solution in the presence of excess azobenzene follows pseudo- ﬁ rst order kinetics, and an Eyring analysis over the temperature range 30−60 ° C furnishes Δ H ⧧ = 12.4 ± 0.7 kcal/mol and Δ S ⧧ = −36 ± 7 eu (see Supporting Information (SI)). Although a more detailed mechanistic description awaits further study, the sign and magnitude of the entropy of activation are consistent with a rate-limiting step involving a transition structure from bimolecular reaction.

The combined reactivities of 1 as hydrogen acceptor from ammonia-borane and 3 as hydrogen donor to an organic substrate suggested the possibility of employing this phosphorus platform as a redox catalyst for transfer hydro- genation. The direct reaction of azobenzene with a 4-fold excess of ammonia-borane in acetonitrile solution at 80 ° C proceeds only to minimal conversion (ca. 5%) after 24 h (Table 1, entry 1). However, the addition of 10 mol % of 1 yields diphenylhydrazine in 80% yield in the same time frame (entry 2), even under less forcing conditions (40 ° C). Longer reactions times lead to modestly increased yields (entry 3).

Presumably, under these conditions, three-coordinate 1 accepts hydrogen from ammonia-borane forming dihydridophosphorane 3, then subsequent hydrogen transfer to azobenzene regenerates 1 and closes the catalytic cycle. Consistent with this scheme, the use of a catalytic amount of 3 also promotes Figure 2. (Top) Abridged

¹

H NMR spectrum for 3 in CDCl

₃

. An

additional

¹

H signal for −C(CH

3

)

₃

is found at δ 1.06 ppm. (Bottom) Annotated

³¹

P NMR spectrum for 3 in CDCl

₃

. Units are ppm relative to Me

₄

Si (

¹

H) and H

₃

PO

₄

(

³¹

P).

Scheme 1. Preparation of Dihydridophosphorane 3

Figure 3. (Left) Thermal ellipsoid plot (50%) of 3. Hydrogen atoms on carbon omitted for clarity. Selected bond distances (Å) and angles (deg): P(1)−N(1) = 1.676(2); P(1)−O(1) = 1.721(1); P(1)−O(2) = 1.699(1); P(1)−H(1) = 1.29(2); P(1)−H(2) = 1.31(2); O(1)−

P(1)−O(2) = 170.5(1); N(1)−P(1)−H(1) = 134.8(1); N(1)−P(1)−

H(2) = 116.1(1); H(1)−P(1)−H(2) = 109.1(2). (Right) Schematic illustrating bond angles.

Journal of the American Chemical Society Communication

dx.doi.org/10.1021/ja302963p|J. Am. Chem. Soc. 2012, 134, 11330−11333

11331

This Work 1: Synthesis of dihydrophosphorane

ジヒドロホスフィン

3

の合成と性質

Cf. Ref 22-23.

pentane trituration of the crude reaction residue. Alternatively, the action of sodium cyanoborohydride on dichlorophosphor- ane 2 gives the same species in 59% isolated yield, which we assign as dihydridophosphorane 3 (Scheme 1).

^27,28

The

magnitude and multiplicity of the

³¹

P NMR resonance are indicative of

¹

J

_PH

coupling of the phosphorus center with bound hydrogen nuclei of an apparently equivalent pair (i.e., a −P(H)

2

unit, vide infra) and longer range

³

J

_PH

coupling to the remote vinylic hydrogen nuclei. The complementary coupling securing these assignments is evident in the

¹

H NMR spectrum (Figure 2, top); phosphorus-coupled doublets are observed at δ 7.95 ppm (

¹

J

_PH

= 670 Hz) and δ 5.83 ppm (

³

J

_PH

= 34 Hz).

X-ray di ff raction of a colorless single crystal obtained by sublimation of 3 in vacuo at 35 ° C a ff ords additional structural detail. The structure of 3 is found to deviate, albeit modestly, from an idealized 10-P-5 trigonal bipyramid toward a distorted square pyramid about phosphorus (Figure 3). The phosphorus center resides in a plane de fi ned by the nitrogen atom and the two P−H hydrogen atoms. The positions of the P−H hydrogens, both of which were located on the di ff erence Fourier map, are splayed obliquely with respect to the O,N,O- supporting structure and are crystallographically inequivalent (∠N−P−H

1

= 134.8(1) ° ; ∠N−P−H

2

= 116.1(1) ° ).

²⁹

Tridentate binding of the O,N,O-motif is evident with oxygen atoms spanning the phosphorus center at an angle of 170.5(1) ° . The ring systems experience only a minor puckering with nitrogen displaying slight pyramidalization ( φ = 6.7 ° ).

³⁰

Reaction of dihydridophosphorane 3 with an unsaturated organic substrate results in transfer of hydrogen from phosphorus, regenerating reduced three-coordinate 1.

³¹

In a stoichiometric experiment, exposure of 3 to 1 equiv of

azobenzene in acetonitrile-d

3

solution at 40 ° C for 19 h results in 72 ± 5% conversion of 3 to 1 with concomitant formation of 1,2-diphenylhydrazine in 63 ± 5% yield (eq 1). In the presence

of excess azobenzene (which mimics catalytic conditions, vide infra), the conversion of 3 to 1 can be driven cleanly to completion. When monitored by NMR, no other intermediates or products are evident in this hydrogen transfer. The consumption of 3 in acetonitrile-d

3

solution in the presence of excess azobenzene follows pseudo- ﬁ rst order kinetics, and an Eyring analysis over the temperature range 30−60 ° C furnishes ΔH

^⧧

= 12.4 ± 0.7 kcal/mol and ΔS

^⧧

= −36 ± 7 eu (see Supporting Information (SI)). Although a more detailed mechanistic description awaits further study, the sign and magnitude of the entropy of activation are consistent with a rate-limiting step involving a transition structure from bimolecular reaction.

The combined reactivities of 1 as hydrogen acceptor from ammonia-borane and 3 as hydrogen donor to an organic substrate suggested the possibility of employing this phosphorus platform as a redox catalyst for transfer hydrogenation. The direct reaction of azobenzene with a 4-fold excess of ammonia-borane in acetonitrile solution at 80 ° C proceeds only to minimal conversion (ca. 5%) after 24 h (Table 1, entry 1). However, the addition of 10 mol % of 1 yields diphenylhydrazine in 80% yield in the same time frame (entry 2), even under less forcing conditions (40 ° C). Longer reactions times lead to modestly increased yields (entry 3).

Presumably, under these conditions, three-coordinate 1 accepts hydrogen from ammonia-borane forming dihydridophosphor- ane 3, then subsequent hydrogen transfer to azobenzene regenerates 1 and closes the catalytic cycle. Consistent with this scheme, the use of a catalytic amount of 3 also promotes Figure 2. (Top) Abridged

¹

3

. An

additional

¹

H signal for −C(CH

3

)

3

³¹

3

4

Si (

¹

H) and H

3

PO

4

(

³¹

P).

P(1)−O(2) = 170.5(1); N(1)−P(1)−H(1) = 134.8(1); N(1)−P(1)−

Journal of the American Chemical Society

Communication

dx.doi.org/10.1021/ja302963p|J. Am. Chem. Soc.2012, 134, 11330−11333 11331

δ

_H

7.95 ppm (d)

1

J

_PH

= 670 MHz H

H

δ

_H

5.83 ppm (d)

3

J

_PH

= 34 MHz

δ

_P

–43.7 ppm (tt)

1

J

_PH

= 670 MHz

3

J

_PH

= 34 MHz

1

H NMR (CDCl

₃

)

31

P NMR (CDCl

₃

)

pentane trituration of the crude reaction residue. Alternatively, the action of sodium cyanoborohydride on dichlorophosphor- ane 2 gives the same species in 59% isolated yield, which we assign as dihydridophosphorane 3 (Scheme 1).

^27,28

The

magnitude and multiplicity of the

³¹

P NMR resonance are indicative of

¹

J

_PH

coupling of the phosphorus center with bound hydrogen nuclei of an apparently equivalent pair (i.e., a −P(H)

2

unit, vide infra) and longer range

³

J

_PH

coupling to the remote vinylic hydrogen nuclei. The complementary coupling securing these assignments is evident in the

¹

H NMR spectrum (Figure 2, top); phosphorus-coupled doublets are observed at δ 7.95 ppm (

¹

J

_PH

= 670 Hz) and δ 5.83 ppm (

³

J

_PH

= 34 Hz).

X-ray di ff raction of a colorless single crystal obtained by sublimation of 3 in vacuo at 35 °C a ff ords additional structural detail. The structure of 3 is found to deviate, albeit modestly, from an idealized 10-P-5 trigonal bipyramid toward a distorted square pyramid about phosphorus (Figure 3). The phosphorus center resides in a plane de fi ned by the nitrogen atom and the two P−H hydrogen atoms. The positions of the P−H hydrogens, both of which were located on the di ff erence Fourier map, are splayed obliquely with respect to the O,N,O- supporting structure and are crystallographically inequivalent (∠N−P−H

1

= 134.8(1)°; ∠N−P−H

2

= 116.1(1)°).

²⁹

Tridentate binding of the O,N,O-motif is evident with oxygen atoms spanning the phosphorus center at an angle of 170.5(1)°.

The ring systems experience only a minor puckering with nitrogen displaying slight pyramidalization (φ = 6.7°).

³⁰

Reaction of dihydridophosphorane 3 with an unsaturated organic substrate results in transfer of hydrogen from phosphorus, regenerating reduced three-coordinate 1.

³¹

In a stoichiometric experiment, exposure of 3 to 1 equiv of

azobenzene in acetonitrile-d

₃

solution at 40 °C for 19 h results in 72 ± 5% conversion of 3 to 1 with concomitant formation of 1,2-diphenylhydrazine in 63 ± 5% yield (eq 1). In the presence

of excess azobenzene (which mimics catalytic conditions, vide infra), the conversion of 3 to 1 can be driven cleanly to completion. When monitored by NMR, no other intermediates or products are evident in this hydrogen transfer. The consumption of 3 in acetonitrile-d

₃

solution in the presence of excess azobenzene follows pseudo- ﬁ rst order kinetics, and an Eyring analysis over the temperature range 30−60 °C furnishes Δ H

^⧧

= 12.4 ± 0.7 kcal/mol and Δ S

^⧧

= −36 ± 7 eu (see Supporting Information (SI)). Although a more detailed mechanistic description awaits further study, the sign and magnitude of the entropy of activation are consistent with a rate-limiting step involving a transition structure from bimolecular reaction.

The combined reactivities of 1 as hydrogen acceptor from ammonia-borane and 3 as hydrogen donor to an organic substrate suggested the possibility of employing this phosphorus platform as a redox catalyst for transfer hydro- genation. The direct reaction of azobenzene with a 4-fold excess of ammonia-borane in acetonitrile solution at 80 °C proceeds only to minimal conversion (ca. 5%) after 24 h (Table 1, entry 1). However, the addition of 10 mol % of 1 yields diphenylhydrazine in 80% yield in the same time frame (entry 2), even under less forcing conditions (40 °C). Longer reactions times lead to modestly increased yields (entry 3).

Presumably, under these conditions, three-coordinate 1 accepts hydrogen from ammonia-borane forming dihydridophosphorane 3, then subsequent hydrogen transfer to azobenzene regenerates 1 and closes the catalytic cycle. Consistent with this scheme, the use of a catalytic amount of 3 also promotes Figure 2. (Top) Abridged

¹

₃

. An

additional

¹

H signal for −C(CH

₃

)

₃

³¹

3

₄

Si (

¹

H) and H

₃

PO

₄

(

³¹

P).

P(1)−O(2) = 170.5(1); N(1)−P(1)−H(1) = 134.8(1); N(1)−P(1)−

Communication

dx.doi.org/10.1021/ja302963p|J. Am. Chem. Soc.2012, 134, 11330−11333 11331

3の結晶構造

少し歪んだ三方両錐構造

[O-P-O = 170.5(1)°]

apical

equatorial equatorial equatorial

昇華による

再結晶

ジクロロホスホランからも合成可能

結合

3

本による角度の和が

360°

=

その原子周りは平面

X

線で得られる結合距離・角度は

末尾の桁の後ろに括弧書きで標準偏差が記されている

補足：二つの水素が対称に配置する異性体は少し不安定

(SI

に

DFT

あり

)

＝平方錐構造だと考えることも可能

(6)

This Work 2: Elementary Reaction of 3

3

とアゾベンゼンの反応

pentane trituration of the crude reaction residue. Alternatively,

the action of sodium cyanoborohydride on dichlorophosphor- ane 2 gives the same species in 59% isolated yield, which we assign as dihydridophosphorane 3 (Scheme 1). 27,28 The

magnitude and multiplicity of the 31 P NMR resonance are indicative of 1 J PH coupling of the phosphorus center with bound hydrogen nuclei of an apparently equivalent pair (i.e., a −P(H) 2 unit, vide infra) and longer range 3 J PH coupling to the remote vinylic hydrogen nuclei. The complementary coupling securing these assignments is evident in the 1 H NMR spectrum (Figure 2, top); phosphorus-coupled doublets are observed at δ 7.95 ppm ( 1 J PH = 670 Hz) and δ 5.83 ppm ( 3 J PH = 34 Hz).

X-ray di ff raction of a colorless single crystal obtained by sublimation of 3 in vacuo at 35 ° C a ff ords additional structural detail. The structure of 3 is found to deviate, albeit modestly, from an idealized 10-P-5 trigonal bipyramid toward a distorted square pyramid about phosphorus (Figure 3). The phosphorus center resides in a plane de fi ned by the nitrogen atom and the two P−H hydrogen atoms. The positions of the P−H hydrogens, both of which were located on the di ff erence Fourier map, are splayed obliquely with respect to the O,N,O- supporting structure and are crystallographically inequivalent (∠N−P−H 1 = 134.8(1) ° ; ∠N−P−H 2 = 116.1(1) ° ). 29 Tridentate binding of the O,N,O-motif is evident with oxygen atoms spanning the phosphorus center at an angle of 170.5(1) ° . The ring systems experience only a minor puckering with nitrogen displaying slight pyramidalization ( φ = 6.7 ° ). 30

Reaction of dihydridophosphorane 3 with an unsaturated organic substrate results in transfer of hydrogen from phosphorus, regenerating reduced three-coordinate 1. 31 In a stoichiometric experiment, exposure of 3 to 1 equiv of

azobenzene in acetonitrile-d 3 solution at 40 ° C for 19 h results in 72 ± 5% conversion of 3 to 1 with concomitant formation of 1,2-diphenylhydrazine in 63 ± 5% yield (eq 1). In the presence

of excess azobenzene (which mimics catalytic conditions, vide infra), the conversion of 3 to 1 can be driven cleanly to completion. When monitored by NMR, no other intermediates or products are evident in this hydrogen transfer. The consumption of 3 in acetonitrile-d 3 solution in the presence of excess azobenzene follows pseudo- ﬁ rst order kinetics, and an Eyring analysis over the temperature range 30−60 ° C furnishes ΔH ⧧ = 12.4 ± 0.7 kcal/mol and ΔS ⧧ = −36 ± 7 eu (see Supporting Information (SI)). Although a more detailed mechanistic description awaits further study, the sign and magnitude of the entropy of activation are consistent with a rate-limiting step involving a transition structure from bimolecular reaction.

The combined reactivities of 1 as hydrogen acceptor from ammonia-borane and 3 as hydrogen donor to an organic substrate suggested the possibility of employing this phosphorus platform as a redox catalyst for transfer hydrogenation. The direct reaction of azobenzene with a 4-fold excess of ammonia-borane in acetonitrile solution at 80 ° C proceeds only to minimal conversion (ca. 5%) after 24 h (Table 1, entry 1). However, the addition of 10 mol % of 1 yields diphenylhydrazine in 80% yield in the same time frame (entry 2), even under less forcing conditions (40 ° C). Longer reactions times lead to modestly increased yields (entry 3).

Presumably, under these conditions, three-coordinate 1 accepts hydrogen from ammonia-borane forming dihydridophosphor- ane 3, then subsequent hydrogen transfer to azobenzene regenerates 1 and closes the catalytic cycle. Consistent with this scheme, the use of a catalytic amount of 3 also promotes

Figure 2. (Top) Abridged

¹

₃

. An additional

¹

H signal for −C(CH

₃

)

₃

³¹

₃

₄

Si (

¹

H) and H

₃

PO

₄

(

³¹

P).

P(1)−O(2) = 170.5(1); N(1)−P(1)−H(1) = 134.8(1); N(1)−P(1)−

dx.doi.org/10.1021/ja302963p|J. Am. Chem. Soc.2012, 134, 11330−11333

11331

Figure S2. Plots ln (k/T) versus (1/T) from data in table at right.

Figure S1. Plots ln [3] versus time (s) for samples at T = 30, 40, 50, 60 °C.

40 °C, 19 hで72±5%の転化率

63±5%の収率でPhNH-NHPhが生成

副生成物および中間体は観測されずいずれもNMRスペクトルで確認

反応速度測定

1

H NMR

にて

3

の^t

Bu

シグナルの減衰を追跡

CD

₃

CN

溶媒中で擬一次反応条件を設定

＝大過剰のアゾベンゼン存在下で反応

3

の減少は以下の式で示される

d [3]

– dt = k₁[3][Ph₂N₂]

d [3]

– dt = k_obs[3]

アゾベンゼンが大過剰にある

＝反応が進行しても[Ph₂

N

₂

]が

ほとんど変わらない

＝[Ph₂

N

₂

]は定数とみなせるので k

_obs

= k

₁

[Ph

₂

N

₂

]

とおけば

となる

これは一次反応速度式なので時間0からtで微分方程式を解けば

となる

一次反応速度定数の算出

Eyring plot

ln [3] = k– _obst + ln [3]₀

温度可変測定後に

Eyring plot

を行い傾きと切片から

活性化エンタルピーおよび活性化エントロピーを算出

(7)

This Work 3: Catalytic Hydrogen Transfer by Using 3

transfer hydrogenation (entries 4 and 5). Interrogation of the phosphorus speciation by

³¹

P NMR under the catalytic conditions supports this mechanistic outline; the signal corresponding to 1 ( δ 186.7 ppm) is replaced by dihydridophosphorane 3 ( δ −43.7 ppm) at early time points, after which compound 3 persists in solution as the sole observable phosphorus species in the range +250 to −150 ppm until the catalytic reaction is terminated at 48 h (Figure 4).

These results implicate 3 as the resting state of the catalytic cycle. The redox platform 1/3 is unique among phosphorus species surveyed in enabling this catalytic transfer hydrogenation reactivity; none of the other trivalent phosphorus compounds examined promote the transfer hydrogenation at a comparable level, even at higher loadings and temperatures (entries 4−6).

These results demonstrate the capacity for a three-coordinate phosphorus compound to support two electron redox chemistry of the type commonly observed in the transition metals. This constitutes a rare example of two-electron bond modifying redox catalysis outside of the d-block. Moreover, whereas phosphines are well-known nucleophilic

³²

and frustrated Lewis pair (FLP) catalysts,

³³

the behavior of the phosphorus center described above is of a di ﬀ erent type. In both nucleophilic and FLP paradigms, the phosphorus behaves exclusively as an electron donor.

³⁴

By contrast, catalysis with 1 results from cycling between discrete tricoordinate P

^III

and pentacoordinate P

^V

states, capitalizing on the colocalization of electron-donating and electron-accepting function at a single atomic center. It is precisely this coordinative and oxidative reactivity that enables the valuable two-electron redox trans- formations observed in transition metal catalysis. The identi ﬁ cation of this reactivity in 1 suggests broader potential for this nonmetal platform to support additional bond- modifying redox catalysis of the type currently dominated by transition metal catalysts. Research along these lines is currently underway in our laboratory.

■ ASSOCIATED CONTENT

*

^S

Supporting Information

Synthetic procedures and spectra, kinetic data, crystallographic and computational details. This material is available free of charge via the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION Corresponding Author

[email protected] Notes

The authors declare no competing ﬁ nancial interest.

■ ACKNOWLEDGMENTS

We gratefully acknowledge the Pennsylvania State University for ﬁ nancial support of this work. We thank Dr. Hemant Yennawar for crystallographic structure determination of 3.

■ REFERENCES

(1) Parshall, G. W.; Ittel, S. D. Homogeneous Catalysis; Wiley: New York, 1992.

(2) Beller, M.; Bolm, C.; Eds. Transition Metals for Organic Synthesis, 2nd ed.; Wiley: Weinheim, 2004.

(3) Diederich, F.; Stang, P. J. Metal-Catalyzed Cross Coupling Reactions; Wiley-VCH: Weinheim, 1998.

(4) De Vries, J. G.; Elsevier, C. J. Handbook of Homogeneous Hydrogenation; Wiley-VCH, Weinheim, 2007.

(5) Ojima, I, Ed. Catalytic Asymmetric Synthesis, 3rd ed.; Wiley: New York, 2010.

(6) (a) Collman, J. P. Acc. Chem. Res. 1968, 1, 136. (b) Halpern, J.

Acc. Chem. Res. 1970, 3, 386.

(7) Niu, S.; Hall, M. B. Chem. Rev. 2000, 100, 353.

(8) Hartwig, J. F. Organotransition Metal Chemistry; University Science Books: Sausalito, CA, 2010; p 261.

(9) Martin, D.; Soleilhavoup, M.; Bertrand, G. Chem. Sci. 2011, 2, 389.

(10) Power, P. P. Nature 2010, 463, 171.

(11) Stephan, D. W.; Erker, G. Angew. Chem., Int. Ed. 2010, 49, 46.

(12) Oae, S.; Uchida, Y. Acc. Chem. Res. 1991, 24, 202.

(13) Akiba, K. Y. Chemistry of Hypervalent Compounds; Wiley VCH:

New York, 1999.

(14) Finet, J.-P. Ligand Coupling Reactions with Heteroatomic Compounds; Elsevier: Oxford; 1998.

Table 1. Catalytic Transfer Hydrogenation with 1/3

a

Loading relative to azobenzene substrate.

^b

Determined by

₁

H NMR of crude reaction mixtures. Error estimate ±5%.

Figure 4. Evolution of phosphorus species during transfer hydrogenation catalysis with 1 (Table 1, entry 3). Time-stacked

³¹

P NMR spectra at t = 0, 3, 8, and 48 h. See SI for full, unabridged spectra.

Communication

11332

触媒無しだと反応はほとんど進行しない

1,3どちらも触媒になる

通常の3価ホスフィンは触媒活性がほとんど無い

transfer hydrogenation (entries 4 and 5). Interrogation of the phosphorus speciation by ³¹P NMR under the catalytic conditions supports this mechanistic outline; the signal corresponding to 1 (δ 186.7 ppm) is replaced by dihydridophosphorane3 (δ −43.7 ppm) at early time points, after which compound 3 persists in solution as the sole observable phosphorus species in the range +250 to−150 ppm until the catalytic reaction is terminated at 48 h (Figure 4).

These results implicate 3 as the resting state of the catalytic cycle. The redox platform 1/3 is unique among phosphorus species surveyed in enabling this catalytic transfer hydrogenation reactivity; none of the other trivalent phosphorus compounds examined promote the transfer hydrogenation at a comparable level, even at higher loadings and temperatures (entries 4−6).

These results demonstrate the capacity for a three-coordinate phosphorus compound to support two electron redox chemistry of the type commonly observed in the transition metals. This constitutes a rare example of two-electron bond modifying redox catalysis outside of the d-block. Moreover, whereas phosphines are well-known nucleophilic³² and frustrated Lewis pair (FLP) catalysts,³³ the behavior of the phosphorus center described above is of a different type. In both nucleophilic and FLP paradigms, the phosphorus behaves exclusively as an electron donor.³⁴By contrast, catalysis with1 results from cycling between discrete tricoordinate PÎII and pentacoordinate P^Vstates, capitalizing on the colocalization of electron-donating and electron-accepting function at a single atomic center. It is precisely this coordinative and oxidative reactivity that enables the valuable two-electron redox trans- formations observed in transition metal catalysis. The identification of this reactivity in 1 suggests broader potential for this nonmetal platform to support additional bond- modifying redox catalysis of the type currently dominated by transition metal catalysts. Research along these lines is currently underway in our laboratory.

■

ASSOCIATED CONTENT

*

^S Supporting Information

Synthetic procedures and spectra, kinetic data, crystallographic and computational details. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION Corresponding Author [email protected] Notes

The authors declare no competingﬁnancial interest.

■

ACKNOWLEDGMENTS

We gratefully acknowledge the Pennsylvania State University for ﬁnancial support of this work. We thank Dr. Hemant Yennawar for crystallographic structure determination of3.

■

^REFERENCES

(1) Parshall, G. W.; Ittel, S. D.Homogeneous Catalysis; Wiley: New York, 1992.

(2) Beller, M.; Bolm, C.; Eds.Transition Metals for Organic Synthesis, 2nd ed.; Wiley: Weinheim, 2004.

(3) Diederich, F.; Stang, P. J. Metal-Catalyzed Cross Coupling Reactions; Wiley-VCH: Weinheim, 1998.

(4) De Vries, J. G.; Elsevier, C. J. Handbook of Homogeneous Hydrogenation; Wiley-VCH, Weinheim, 2007.

(5) Ojima, I, Ed.Catalytic Asymmetric Synthesis, 3rd ed.; Wiley: New York, 2010.

(6) (a) Collman, J. P.Acc. Chem. Res.1968,1, 136. (b) Halpern, J.

Acc. Chem. Res.1970,3, 386.

(7) Niu, S.; Hall, M. B.Chem. Rev.2000,100, 353.

(8) Hartwig, J. F. Organotransition Metal Chemistry; University Science Books: Sausalito, CA, 2010; p 261.

(9) Martin, D.; Soleilhavoup, M.; Bertrand, G.Chem. Sci.2011,2, 389.

(10) Power, P. P.Nature2010,463, 171.

(11) Stephan, D. W.; Erker, G.Angew. Chem., Int. Ed.2010,49, 46.

(12) Oae, S.; Uchida, Y.Acc. Chem. Res.1991,24, 202.

(13) Akiba, K. Y.Chemistry of Hypervalent Compounds; Wiley VCH:

New York, 1999.

(14) Finet, J.-P. Ligand Coupling Reactions with Heteroatomic Compounds; Elsevier: Oxford; 1998.

Table 1. Catalytic Transfer Hydrogenation with 1/3

aLoading relative to azobenzene substrate.^bDetermined by1H NMR of crude reaction mixtures. Error estimate±5%.

Figure 4. Evolution of phosphorus species during transfer hydrogenation catalysis with1(Table 1, entry 3). Time-stacked³¹P NMR spectra att= 0, 3, 8, and 48 h. See SI for full, unabridged spectra.

11332

1 3

アンモニアボランによるアゾベンゼンの触媒的水素化

反応溶液の³¹

P NMR

追跡

(8)

Other Experiments and Next Approach

他の実験により何かわかるか？

次週の宿題

次のアプローチはどうすべきか？ → そのために何を調べてみる？

一つの解答：同じ筆者らの続編(N-H結合切断への展開)

McCarthy, S. M.; Lin, Y.-C.; Devarajan, D.; Chang, J. W.;

Yennawar, H. P.; Rioux, R. M.; Ess, D. H.; Radosevich, A. T.

J. Am. Chem. Soc. 2014, 136, 4640-4650.

He, G.; Torres Delgado, W.; Schatz, D. J.; Merten, C.; Mohammadpour, A.;

Mayr, L.; Ferguson, M. J.; McDonald, R.; Brown, A.; Shankar, K.; Rivard, E.

"Coaxing Solid-State Phosphorescence from Tellurophenes"

Angew. Chem. Int. Ed. 2014, 53, 4587.

最先端有機元素化学①：最新論文からのトピックス