DIRECT OBSERVATION OF ALPHA-HYDROGEN TRANSFER FROM ALKYL TO ALKYLIDYNE LIGANDS IN (ME3CCH2)3W=CSIME3 - KINETIC AND MECHANISTIC STUDIES OF ALKYL ALKYLIDYNE EXCHANGE

Alpha-Hydrogen atoms in (Me3CCH2)3W = CSiMe3 are found to migrate to the alpha-carbon atom of the alkylidyne ligand leading to the equilibrium (Me3CCH2)3W = CSiMe3 (2a) reversible (Me3CCH2)2(Me3SiCH2)W = CCMe3 (2b). The equilibrium has been observed to follow reversible first-order kinetics with DELTA-H-degrees = 2.3 (0.5) kcal mol-1 and DELTA-S-degrees = 8.1 (1.3) eu. The activation parameters for the conversion 2a --> 2b are DELTA-H = 27.5 (0.6) kcal mol-1 and DELTA-S = -2.0 (1.7) eu and for the back-reaction 2b --> 2a are DELTA-H' = 25.4 (0.8) kcal mol-1 and DELTA-S' = -9.5 (1.9) eu. Isomerizations involving (Me3CCD2)3W = CSiMe3 (2a-d6) and (Me3CCHD)3W = CSiMe3 (2a-d3) revealed that the isomerization between 2a and 2b occurred by consecutive alpha-hydrogen/alpha-deuterium transfers with a large primary kinetic isotope effect k(HH)/k(DD) = 5.1 (0.3) and k(HH)/k(HD) = 3.0 (0.2) at 100-degrees-C. Crossover experiments indicate that the isomerization is a unimolecular process. These results are discussed in terms of a concerted four-center transition state leading to a bis(alkylidene) activated complex or reactive intermediate. An alternate reaction pathway involving the simultaneous transfer of two alpha-hydrogen atoms is ruled out by the rule of the geometric mean. The molecular structure of 2a-d6 was determined by X-ray crystallography. The molecule has rigorous C3 symmetry with W = C = 1.739 (8) angstrom and W-C(sp)3 = 2.096 (5) angstrom. At -169-degrees-C, crystal data are a = 15.232 (3) angstrom, c = 17.140(4) angstrom, Z = 6, d(calcd) = 1.36 g cm-3, and space group R3BAR.