Insertion and Thermal Decomposition Reactions of Cyclopentadienylcobalt (III) Dialkyl Complexes

PMe3 replaces PPh3 in CpCo (PPh3) (Me) 2 (1, Cp = ƞ5- C5H5) in a dissociative substitution reaction to give CpCo (PMe3) (Me) 2; the first order rate constant for PPh3 loss from 1 is 4 x 10-4 sec-1 at 30°C in toluene-d8. Dialkyl complex 1 doubly alkylates CO in an intrarnolecular process to give acetone in high yield; this proceeds via CO substitution in 1 to give the spectroscopically observed intermediate CpCo (CO) (Me) 2 (6), followed by migratory insertion, and reductive elimination from an acyl-alkyl complex. Heating and then carbonylating mixtures of 1-d0 and 1-d6 (deuterated methyl groups) in benzene revealed the presence of an intermolecular methyl exchange process in 1. We have reinvestigated the reaction of. 2: _ with diphenylacetylene and, in addition to the organometallic products observed by previous workers, isolated two new organic products 2, 3- diphenyl-1-butene (11) and Z-2, 3-diphenyl-2-butene; these products account for >95% of the methyl groups in starting 1 and were formed in the ratio 14: 1 in the absence of added PPh3. Additional experiments designed to probe the mechanism of product formation led to the following conclusions: (1) high yield stereospecific double alkylation of diphenylacetylene is a feasible process; (2) reactions of 11 in this system involve two diastereomeric ƞ2-olefin complexes which are interconvertible only by dissociation of the olefinic ligand from the metal; (3) hydrogen shifts involved in the Cp (PPh3) Co-catalyzed isomerization of alkenes occur more rapidly than dissociation of the alkene from the metal center; and (4) interconversion of ƞ3-allyl complexes via rotation in ƞ1-allyl complexes is much slower than hydrogen transfer and product formation. Ethylene reacts with 1 to give methane, propene, and the new ethylene complex CpCo (PPh3) (C2H4) (14) ; this was independently generated by thermal PPh3 or photochemical CO substitution in CpCo (PPh3) 2 and CpCo (CO) (PPh3), respectively. The production of propene from ethylene and 1 is a model for the chain-growth step in the Ziegler-Natta polymerization of olefins. The products can be explained by a classical mechanism involving insertion of ethylene into a cobalt-carbon a-bond or an alternative one involving ethylene addition to an intermediate metal-carbene complex formed by a-elimination. Labeling experiments in this system have shown propene is formed by transfer of one intact methyl group to ethylene, in agreement with the classical view. Thermal decomposition of 1 is competitive with the alkylation of ethylene and gives mostly methane, abstracting the fourth methane hydrogen from the cyclopentadienyl ring. The decomposition has been monitored by 1H NMR spectroscopy in the presence of added PPh3 and ethylene (conditions where no propene is generated) and CpCo (PPh3) (C2H4) (Cp' = ƞ5CH3C5H4) and Cp'Co (PPh3) (Me) 2 have been observed spectroscopically and identified as the primary and secondary products of the decomposition, respectively. Together, these reactions show reductive elimination in alkyl-acyl and alkyl-vinyl complexes is more favorable than alkyl-alkyl reductive elimination and this is attributed to the ability of the former systems to donate an additional pair of electrons to the metal center in the reductive elimination transition state. ƞ5-cyclopentadieny (triphenylphosphine) cobaltacyclopentane (18) has been prepared by reaction of CpCo (PPh3) I2 with 1, 4- dilithiobutane at low temperature. This metallacycle is thermally more sensitive than its dimethyl analog 1 and decomposes to give 1-butene which is subsequently isomerized by the organometallic products to a mixture of 2-butenes. In this system, ethylene insertion cannot compete with thermal decomposition, but CO and diphenylacetylene insertion can. CO reacts with 18 to give cyclopentanone via the CO substituted analog of 18; this has been observed spectroscopically and builds up to a greater degree than the dimethyl analog observed during the reaction of 1 with CO. Diphenylacetylene reacts with 18 to give the expected products of double alkylation, 1, 2-diphenylcyclohe. xene and 2, 3-diphenylcyclohexene, and Z-1, 2-diphenyl-1, 5-hexadiene as well. The latter product is presumably derived from β-elimination in an intermediate metallacycloheptene which also generates the cyclohexenes via reductive elimination.