STEREOCHEMICAL INFLUENCE OF THE PENTAMETHYLCYCLOPENTADIENYL LIGAND ONTHE ISOCYANIDE INSERTION REACTIONS OF THE 1-SILA-3-HAFNACYCLOBUTANE COMPLEX, (C(5)ME(5))(C5H5)HF(CH(2)SIME(2)CH(2))

The insertion reactions of CNMe and CN-t-Bu with CpCpHf(CH(2)SiMe(2)C H(2)), 1, have been investigated to evaluate the stereoelectronic infl uence of the Cp ligand on the competitive 1,2-silyl shift and reducti on coupling pathways previously observed for Cp(2) Hf(CH(2)SiMe(2)CH(2 )). Under ambient conditions the addition CNMe to 1 proceed with the s equential formation of CpCpHf(N(Me)C(=CH2) SiMe(2)CH(2)), 2, and Cp*C pHf(N(Me) C(=CH2)SiMe(2)(CH2=)CN(Me)), 3. Alternatively, the addition of excess CNMe below - 5 degrees C to 1 occurs with reductive coupling to produce CpCpHf(N(Me)C(CH(2)SiMe(2)CH(2)) =CN(Me)), 4. In contrast , the addition of two equivalents of CN-t-Bu to 1 does not afford the corresponding cyclic enamide species but proceeds with the sequential formation of CpCpHf(N(CMe(3))CCH(2)SiMe(2)CH(2)), 5, and Cp*CpHf(N(CM e(3))C-C(=NCMe(3))CH(2)SiMe(2)CH(2)), 6. The molecular structures of 3 and 6 have been determined by X-ray crystallography. The puckered 1-s ila-3,5-diaza-4-hafnacyclohexane ring of 3 adopts a chair conformation with the folding along the N...N' vector being 19.3 degrees out of th e N', Hf, N plane toward the smaller Cp ligand and the folding along t he C(1)...C(1)' vector being 43.3 degrees in the opposite direction to ward the Cp ligand. The molecular structure of 6 is consistent with n ucleophilic attack of CN-t-Bu at the eta(2)-iminoacyl carbon of 5 occu rring exclusively from the side opposite to the Cp ligand. The second equivalent of CN-t-Bu displaces the eta(2)-iminoacyl group away from the Hf atom, eventually placing the resultant imine group of 6 syn to the Cp ligand.