These complexes represent two of the first examples of nonplanar amide ligands in transition metal chemistry. One of the metal species was found to be a catalyst for the electrochemical oxidation of alcohols.17 A scheme showing the remarkable degradation of this ligand is illustrated in Scheme 1.1. An interesting feature of the oxo and imido ligands is the ability to form a triple bond with the metal for which the.
It was hoped that the electronic properties of the compound could be tuned by varying the nature of the PAC ligand.
The chemistry of osmium complexes is more developed, mainly because higher oxidation state osmium complexes still exhibit useful 1H NMR spectra despite their paramagnetism, which greatly aids in characterization. Most coordination chemistry with unsaturated nitrogen ligands was explored using a single PAC ligand, (TJ4-HBA-B), ,1,. One of the phosphine ligands is quite flexible and can be easily displaced, allowing coordination of a variety of ligands.
Much of this chemistry has been reported elsewhere.8 Developments in the chemistry of §_ with unsaturated nitrogen ligands are reported here.
PPh3 ff
The reaction was allowed to proceed and over several days it became a brilliant deep purple, simultaneously with the disappearance of the brown complex. The X-ray structure of the diimine complex is shown in Figure 2.4 and the bond length data are given in Table 2.3. The ligand is in a cis-/3 arrangement and is rotated out of the planar configuration in response to the structural constraints of the diimine ligand.
There are very few non-planar organic amides (examples include formamide, 16 lactams such as penicillin 17 and anti-Bredt bridge nitrogen compounds 18 ), and PAC ligand complexes represent the first examples of non-planar amide ligands.
This reaction is the well-known Staudinger reaction.34 Phosphines and azides will react quite easily and under various conditions to form phosphinimines; however, this reaction does not take place at room temperature. The phosphinimine reagent, 1§., was prepared independently and does not react with the starting material, §_, to form the final product, 14. 15 does not decompose to the original phosphinimine under the reaction conditions, but requires acid and a polar solution. solvent.34. This reaction is also a Staudinger reaction that will occur under the reaction conditions.34 Note:. one reaction to be performed is the treatment of §_ with HN3 to determine if 14 is produced.
The literature value is +0.1 ppm.) (Me3Si)2O (prepared independently) was added to the mother liquor and the peaks were found to coincide. In addition to the NMR evidence, the IR spectrum of the neat mother liquor shows bands corresponding to (Me3Si) 20. The integrations were consistent with a formulation of 1 :1 :1 in the PAC ligand, adamantyl group and triphenylphosphine.
This reagent was shown to be active in the ammoxidation of olefins.35 Because the complex 11 was somewhat persistent and could only be obtained in low yield, no further reactivity studies were performed. However, Gross and Trogler reported a notable C-F insertion in a similar system as discussed above (see equation 2.10).36. The black precipitate did not readily dissolve in solvents, with the exception of hot DMSO, in which it formed a black-purple solution.
However, the purple-black DMSO solution was pumped to dryness in vacuo, after which the compound was also soluble in benzene. The IR of this compound showed absorption peaks in the region of O-bonded DMSO complexes (925,940 cm-1)38 as well as the characteristic peaks of water.
22 S= DMSO
1H NMR spectra and microanalytical data after the black compound is dissolved in hot DMSO lead to the formulation of the compound as [Os(174-HBA-B)(DMSO)·H2O]x, 19a or 19b (where x = 1 or 2). There was no evidence at all from TLC, or from examination of the entire reaction mixture by 1H NMR, that any other byproduct was present. The connection appeared to be paramagnetic; the ligand and t-butylpyridine peaks were quite dispersed, and the incorporation of the t-butylpyridine peaks was only half of what was required for the dimeric osmium complex, £1.
It may be that after treatment one of the t-butylpyridine ligands was replaced by another solvent molecule (such as DMSO), or that only one t-butylpyridine ever coordinated and the other site was occupied by the solvent in the further treatment process, i.e. 22. It may happen that in the case of § 1. required a stronger PR3 ligand a-donor to cleave the Os-Os quadruple bond and that the t-butylpyridine ligand is simply not a strong enough donor to cleave the Os-Os bond. There are many examples of Mo-Mo and Re-Re dimeric compounds with four bonds.40 Despite the fact that the Os(IV) (d4-d4) metal centers are isoelectronic with Cr(II), Mo(II), W(II) and Re(III), all of which form quadruple bonded species, there are no examples of Os-Os quadruple bonds in the literature.
The electronic absorption spectrum of 1§_ is shown in Figure 2.11 and has an absorption band at 768 nm (13,200 cm-1) in methylene chloride; the absorption spectrum is identical when taken in toluene. The solutions were prepared and used quickly to prevent reaction with the solvent, but when they were examined a few hours later, there was no change.) Figure 2.12 shows the spectrum of the DMSO compound, .1Q, which also ' A band at 775 nm is displayed. (12,903 cm-1) in methylene chloride. The electronic spectrum of the starting monomeric complex, fl, shows no absorption band in this region. Metal-metal quadruple-bonded complexes have a characteristic vibronic structure in the o-5* band, due to metal-metal stretching, which can be seen at low temperature.
The shape of the band is similar to the shape of the low-temperature (5 ° K) spectrum of Re2C18·2 (see Figure 2.13b) and is characteristic of s-s* excitations in an a antibonding orbital.43 The bond order of the metal -metal bonding is reduced, causing a "displacement shift" of the potential energy wells (along some coordinate axis). The vibronic structure observable at low temperature for 18 has an average progression of 305 cm·1, which is in the middle of the known range.
WAVELENGTH
UAUELEHGTH
2.14b) Ru(OEP)(CO)L
The IR for this compound has a CO stretch at 1985cm-1 .8 The biscarbonyl complex, Os(774-HBA-B)(CO)2, was highly desirable because it would have provided another possible access to bisimido complexes . Metal oxo complexes react with phosphinimines to yield imido complexes.3 The driving force behind this reaction is apparently the formation of the strong P=O bond (130 kcal/mol). The first example of a neutral octahedral Os(Vlll) compound, formed as an intermediate in the formation of the diimine complex, has been postulated.
The four basic ligands are α-antibonding to Md/. The deformation of the basal plane shifts the ligand orbitals of the d/torus to a node causing the 2a1 orbital to drop to a lower energy. Coordination of donor ligand in the transaxial position changes the electronic environment of the nitride complex. This is consistent with the observed protonation reaction in an analogous molybdenum compound (see equation 2.25) and the formation of the thionitrosyl complexes shown in the equations.
After rotary evaporation of the solvent, the solid was dissolved in THF from which the yellow mother liquor could be decanted from a white precipitate. An IR of the crude reaction mixture was quite similar to that of 26 (see Scheme 2.7). An IR spectrum showed the absence of the nitride peak at 1067cm-1 and a new peak at 1105cm-1 which was assigned to Os=N.
Systematic absences led to the assignment of the space group P21/n (around fork absent odd, hol absent for h+1 odd). A THF/MeCl2 solution of the resulting purple solid was chromatographed on TLC plates eluting with MeCl2. Further workup, including vacuum drying, resulted in the loss of IR bands at >1700 and the compound was not further characterized.
One of the other products was identified as N-methylpyridinium iodide by comparison with an independently prepared sample.
ASYMMETRIC HYDROGENATION USING A RESOLVED CHIRAL SCANDIUM HYDRIDE COMPLEX
1 TiC 1 1
LiAIH2( OCH2CH2 OCH3)2 LiAlH 2 (0tBut )2
The reported optical yields* were between 22 and 34%, the highest yet achieved for the asymmetric hydrogenation of simple unfunctionalized olefins (see Table 1.3).6.
It is the aim of the present work to show that it is possible to extend this reactivity to the asymmetric hydrogenation of a prochiral olefinic substrate by incorporating chirality into one of the Cp ligands. A more convenient starting material, §, can be prepared from I by metathesis of the chloride with the bulky lithium reagent, (Me3Si)2CHLi (see scheme 2.3). The presence of the chiral menthyl group allows two diastereomers (§ and §') to be formed, which can be distinguished by the nonequivalence in the 1H NMR spectrum of the two trimethylsilyl (Sc-CH(SiMe3)(SiMei)) groups.
These are easily distinguished by the disparity of the three downfield protons in the Cpm ring. Isomerization was slowed considerably and the only deuterium incorporation seen was in the isopropyl group of the menthyl moiety. Initially, the prochiral diolefin 3-methyl-1,5-hexadiene was chosen as the substrate of choice; ~ was treated with 20 equivalents of substrate according to equation 2.4.
The ring-closing step can occur from either side, creating one chiral site; however, the chiral center at the 3-methyl position may be too far from the reaction site to have any major influence, and subsequent decomplexation of the substrate produces an identical compound (see Scheme 2.7). As shown in Scheme 2.8, the product of the cyclization of divinylbenzene is methylene indane, and its hydrogenation is 1H NMR splitting of the enantiomeric signals for various alkene and arene hydrocarbons has been shown to form heptafluoro-2,2,-dimethyl-3,5-octane-dionato)silver(I) (Ag(fod}), _1_g (see Figure 2.4 ).complexes with arenes and alkenes that are in solution in equilibrium that is fast on the NMR time scale.8 Silver complexation shifts and broadens the proton resonances to lower field.
Yb(hfbc)s binds to the silver complex of the alkene or arene and confers chiral recognition regardless of the "dilution" of the optically active environment around the substrate.6. To determine the enantiomeric purity of the methylindane produced by the methylene indane reaction, it was first necessary to separate the organic product from the organometallic complex. Most of the solvent was removed in vacuo, the vessel was opened, and the remaining solution was loaded onto a thin layer of silica gel.
Clear solution was taken from the top of the centrifuge tube and used for the 1H NMR studies.
