Although not officially a member of the Bercaw group (he possesses no synthetic skills), Bernie is still considered one of "the boys". Apart from proofreading the initial drafts of this thesis, Bernie performed much of the crystal-. Depending on the size of the alkyl ketene substituent, the hydrogenation of these compounds gives enolate hydride products with varying degrees of stereoselectivity.
These studies suggest a mechanism for CO insertion into metal-carbon bonds of the early transition metals. Permethylhafnocene hydride and permethylzirconocene hydride complexes react with diazoalkanes to give rf-N,N'-hydrazonido species in which the terminal nitrogen atom of the diazoalkane molecule is inserted into a metal hydride or metal-carbon bond. The proposed mechanism for the formation of the cis-enediolate dimer (Scheme 1. 2)5 involves initial attack of 1 on a carbonyl ligand.
The -OZrCp*2 group would necessarily have to come out of the "wedge" to minimize steric interactions between the bulky pentamethylcyclopentadienyl ligands. It was our hope to model some of the intermediates proposed in the cis-enediolate mechanism (Scheme 1.2) by treating a form of .
A thermodynamic rearrangement of the cis isomer can be invoked to explain the formation of the trans complex. Cp*2Zr(X)OCH=CHSiMe3 • The 1 H NMR spectrum of this mixture is identical to that observed for the iodine derivatives of 17c and 17t. 5 is therefore not the only possible mechanism for the formation of the trans isomer in this system.
Upon warming to room temperature, the NMR spectrum of the solution indicated that it was more than 95% formed. The volume of the solution was reduced to 15 mL in vacuo and the solution cooled to -78•C. CH4/mol !.· The volume of the solution was reduced to 15 mL in vacuum and the solution cooled to.
The color of the solution changed from colorless to bright orange upon addition to the allene. The proposed stereoselective hydrogenation of the permethyl-zirconocene-ketene intermediate 2 is the final step in the formation of. The solvent was removed in vacuo, the residue triturated with petroleum ether (10 mL) and the suspension was then filtered to remove MgBr 2 . The volume of the solution was reduced to 2 ml in vacuo.
After the addition of CO, the color of the solution changed from yellow to green.
CHAPTER 3
1 Only recently have these phosphoranes found use as reagents in the synthesis of organometallic compounds. Early work in this area focused on the use of phosphorus ylides in chelating or nucleophilic reactions. 2 Phosphorane has seen little use as a carbene transfer reagent (Wittig's reagent) or as a strong base; properties that have been exploited in synthetic organic chemistry.
The basicity of phosphorus ylides was used by Schrock in the preparation of several zirconium and tantalum complexes. The preparation of Cp2TaMeCH2 from Cp2TaMe2 and methylenetrimethylphosphorane (eq. 1) does not proceed via methylene transfer from the ylide, but rather by proton abstraction from one of the two methyl ligands. 3 Other compounds were prepared using an ylide first as a nucleophile and second as a dehydrohalogenation agent (equiv. 2).
Similar to their use as carbene transfer reagents in organic synthesis, phosphorus ylides were used to prepare several early types of transition metal carbenes by direct carbene transfer. These hydride complexes are known to act as powerful hydride transfer reagents8 and it was thought that hydride migration to the ylide nucleophilic carbon could be achieved to give a series of alkyl hydride compounds. Insertion of methylene into a metal-hydride bond to give a methyl-hydride derivative is known for some Group VI and VII transition metals using diazomethane as the carbon source, although the yields are usually low.
This chapter describes the use of methylenetrimethylphosphorane as a reagent in the synthesis of permethylzirconocene and permethylhafnocene methylhydride, as well as a permethylzirconocene. 1 Coordination of the ylide to the highly electrophilic metal center in Cp*2MH2 (M = Hf, Zr) should proceed quite readily. Because the pentamethylcyclopentadiene ligands are themselves quite large, the resulting ylide must be small to successfully enter the coordination sphere of the Cp*2MH2 complex.
The formation of Cp*Jif(H)Me from Cp*2Hfil2 and CH2PMe3 is unique because it is the first example of methylene insertion into a metal hydride bond using a phosphorylide as a carbene source. Hydrides of the later transition metals are apparently not hydride enough to effect this transformation; the protonic nature of later transition metal hydrides likely results in their deprotonation when treated with phosphorylides. 13 Although Cp2Zr(H)Me has been reported, it is undoubtedly polymeric and not yet well characterized.
The molecular structure of Cp*2Zr(H)CH2PMe2CH2 (ll) is represented in Figure 3.1 and a skeletal view of the immediate bond around zirconium with relevant bond distances and bond angles is given. The atoms of the metallocycle ring are nearly coplanar, with the phosphorus atom displaced from this plane by 0. The P-C(l) and P-C(2) bond distances are somewhat shorter than would be expected for a single P-C bond (1. 84A), 29 implying additional electron delocalization in the P-C bonds of the CH2-P-CH2 unit.
Formation of _!1 is most likely by methylene insertion into the Zr-H bond followed by elimination of the neutral phosphino ligand. 9 Mechanism of formation of Cp*2Zr(H)CH2PMe2CH2 (!.!) A possible mechanism of formation of ll from Cp*2Zr(H)Cl (_ID and CH2PMe3 is shown in scheme 3. Mechanism of formation of ll from Cp *2ZrH2 (_ID and CH2PMe3 is more complicated, however.
Typical values range from 1470-1625 em -1• 38 Because of this unusually low CO stretching frequency and because of the ambiguity in coordination around zirconium, the crystal structure was 15. 3, and a skeletal view of the immediate ligation around zirconium with relevant bond distances and bond angles are given in Figure 3. Although the hydride ligand was not located, the lateral displacement of the Zr-C(1)-C(2)- P moiety from the R1-Zr-~ plane.
However, the structural data is sufficient to confirm that the oxygen atom of the 772-acyl ligand occupies the central equatorial coordination site of the zirconium between the hydride. 5b, 27 The crystal structure of the thorium-772-acyl complex Cp*2 Th(COCHCMe3)Cl is the only example showing the same type of atomic arrangement around the metal center. 5 and a skeletal view of the immediate ligation around ziconium with relevant bond distances and bond angles is provided in Figure 3.
The coordination group's spatial layout on zirconium i!§. vide supra) contrasts with that of!.§. in that the 772-acyl oxygen atom in 16 occupies the central equatorial coordination site. Erker proposed that CO insertion into metal-carbon bonds of the early transition metals gives an intermediate if-acyl. 5b, 27 As previously mentioned, the only exception is the crystal structure of the oxygen-lateral thorium.
The formation of the metalated ylide complex, Cp*2Zr(H)CH2PMe2CH2 (_g), is interesting as the mechanism responsible for its formation differs from that of the common Cp ring analogue, Cp2Zr(H) CH2PMe2CH2• reaction. The kinetics of the approach to the acyl equilibrium provides an upper limit for the Zr-0 bond strength of ca. 4 mL C6D6• The 1H NMR spectrum (500 MHz) for this solution indicates CH4, CH3D, and CH2D2 in approximately equal concentrations.
The color of the solution changed from blue to yellow upon addition of the ylide. The color of the solution changed from yellow to deep amber upon addition of the ylide and a fine white solid (PMe4Cl) precipitated. The least squares planes of the pentamethylcyclopentadienyl rings and the ZrCOCP group are given in Tables 3.15 and 3.
40e-A -3• The C=C(H)PMe3 cluster appears to behave as a rigid body, and correction of bond lengths and angles for vibrational motion was performed using the Trueblood program. The least-squares plans of the pentamethylcyclopentadienyl rings and the Zr(H)COCHP group are given in Tables 3. Electrochemical reduction of tetraalkylphosphonium salts provides trialkylphosphines and alkyl radicals, C. The relative concentrations of CH4, CH20~ and were .CH20z and .CH20z and . measured by 500 MHz 1H NMR spectroscopy.
At the time this project began, no report of the reaction of an early transition metal complex (Groups TII-V) with a diazoalkane had appeared in the literature. Treatment of the monoalkyl complex Cp*2Zr(Me)OH (_ID with one equivalent of bis(p-tolyl)diazomethane (1_) at 6o•c gives r,Z-N, N'. The presence of the hydroxyl ligand is confirmed by an OH proton resonance in 0 2.
The molecular structure of Cp*2Zr(NMeNCTol2 )OH (1) is shown in Figure 4.1 and a skeletal view of the immediate ligation around it. Migration of the methyl ligand to N(2) via a 1,2-methyl shift yields the r( -N-bonded intermediate. Datative coordination of the N(l) nitrogen with the zirconium yields the 772-N,N on '-hydrazonido product 7. Coordination of the diazoalkane molecule with the lateral binding position should be preferred over coordination with the central binding position because of in.
The reaction rate of the phenyl complex with To~CN2 is extremely slow; complete reaction is. This may be due to the rate (or position of pre-equilibrium) of coordination of the diazoalkane molecule to the metal center.
