Identification and characterization of intermediates in the formation of the cyclobutadiene linked dimeric cobalt sandwich compound [( ${\eta ^{5}}$ -RCp)Co( ${\eta^{4}}$ -C 4 Ph 3 )] 2 [R ${=}$ H, CH 3 C(O)O

Abstract

The synthesis of novel cyclobutadiene linked dimers of [η 5-RCp]Co(η 4-C4Ph4) was reported recently from a three-component reaction of 1,4-diphenylbutadiyne, diphenylacetylene and [η 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 {[η 5-MeOC(O)Cp]Co(η 4-C4Ph3)}2 (1). On varying the stoichiometric ratio of the reactants of the three component reaction, two new cobalt sandwich compounds [η 5-MeOC(O)Cp]Co[η 4-C4Ph3(C≡CPh)] (2) and [η 5-MeOC(O)Cp]Co[η 4-C4-Ph2(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 [η 5-MeOC(O)Cp]Co{η 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 {η 5-[MeOC(O)Cp]Co(η 4-C4Ph3)}[(η 5-Cp)Co(η 4-C4Ph3)] (5) which was also structurally characterized. The mechanism of the formation of cyclobutadiene linked dimers of the cobalt sandwich compound (η5-Cp)Co(η4-C4Ph4) has been studied. Two intermediates [η5-MeOC(O)Cp]Co[η4-C4Ph3(C≡CPh)] and [η5-MeOC(O)Cp]Co[η4-C4Ph2(C≡CPh)2] were isolated. The identity of the former was proved by oxidizing its alkyne moiety. Further, this compound was converted to a novel unsymmetrical cyclobutadiene linked dimer {η5-[MeOC(O)Cp]Co(η4-C4Ph3)} [(η5-Cp)Co(η4-C4Ph3)].