Tk. Schoch et al., FORMATION OF OLEFINS UPON OXIDATION OF MOLYBDENUM ALKYL CARBYNES - ORGANIC RADICAL REACTIVITY IN AN ORGANOMETALLIC RADICAL-CATION, Journal of the American Chemical Society, 117(24), 1995, pp. 6475-6482
Decomposition of the complexes (eta(5)-C5H5)(CO){P(OMe)(3)}Mo=CR [R =
c-C4H9, (CH2)(3)CH3, and CH(CH2-CH2CH3)(2)] in CHCl3 results in conver
sion of the carbyne ligand to a terminal olefin. The reaction is initi
ated by oxidation and occurs during photolysis in CHCl3 or upon slow d
iffusion of O-2 into the reaction mixtures. Corroborating evidence for
initiation of the reaction by electron transfer has been obtained by
conversion of the butyl carbyne ligand of (eta(5)-C5H5)(CO){P(OMe)(3)}
Mo=C(CH2)(3)CH3 to 1-pentene upon electrochemical oxidation. Mechanist
ic studies were consistent with H-abstraction by the carbyne radical c
ation to yield a cationic carbene complex which forms the olefin in a
H-shift process. INDO calculations on the carbyne radical cation [(eta
(5)-C5H5)(CO){P(OMe)(3)}Mo=CCH2CH3](.+) indicate stabilization upon be
nding the Mo=C-C angle from 180 degrees to 120 degrees. This change in
geometry places spin density on the carbyne carbon in the radical cat
ion although the initial oxidation occurs from an orbital that is prim
arily nonbonding metal d in character. The oxidized carbyne is thus ab
le to function as a carbon-centered radical and abstract a hydrogen at
om at the carbyne carbon. Although the olefin-forming reaction is gene
ral far alkyl carbynes with a hydrogen on C2, the tert-butyl carbyne (
eta(5)-C5H5)(CO){P(OMe)(3)}Mo=CC(CH3)(3) (2e) did not form an olefin u
pon oxidation. Instead, photolysis of 2e in CHCl3 yielded the dichloro
molybdenum carbyne (eta(5)-C5H5)Cl-2{P(OMe)(3)}Mo=CC(CH3)(3) (5e) via
a Cl-abstraction pathway.