Identification and characterization of intermediates in the formation of the cyclobutadiene linked dimeric cobalt sandwich compound [(η5-RCp)Co(η4-C4Ph3)]2 [R = H, CH3C(O)O]

The synthesis of novel cyclobutadiene linked dimers of [eta(5)-RCp] Co(eta(4)-C4Ph4) was reported recently from a three-component reaction of 1,4-diphenylbutadiyne, diphenylacetylene and [eta(5)-RCp]Co(PPh3)(2) [R = H or MeOC(O)]. The chiral bis oxazoline derivative of this dimer was found to form cobalt oxazolinyl palladacycles which were excellent chiral catalysts for the Overman-Claisen rearrangement of trichloroacetimidates, similar to the well-known COP catalysts. In this paper, we report reactions carried out for identifying the intermediates in the formation of the dimeric sandwich compound {[eta(5)-MeOC(O)Cp]Co(eta(4)-C4Ph3)}(2) (1). On varying the stoichiometric ratio of the reactants of the three component reaction, two new cobalt sandwich compounds [eta(5)-MeOC(O)Cp]Co[eta(4)-C4Ph3(C CPh)] (2) and [eta(5)-MeOC(O)Cp]Co[eta(4)-C4Ph2(C CPh)(2)] (3) were found to form as viscous oily liquids. The identity of 2 was ascertained by oxidizing its alkyne moiety to yield crystalline [eta(5)-MeOC(O)Cp]Co{eta(4)-C4Ph3[C(O)-C(O)Ph]} (4) which was structurally characterized. Further, the role of phenylethynyl derived monomeric sandwich compound 2 as a precursor to the dimeric compound 1 was established by reacting 2 with diphenylacetylene and CpCo(COD) which yielded the first example of an unsymmetrical cyclobutadiene linked dimeric cobalt sandwich compound {eta(5)-[MeOC(O)Cp]Co(eta(4)-C4Ph3)}[(eta(5)-Cp)Co(eta(4)-C4Ph3)] (5) which was also structurally characterized.