Alkynyl-Dicobalt Hexacarbonyl Complexes of Menthyl Cations: Isolobal Substitution of [Co(CO)3]+ by Fe(CO)3 as a Structural Model

The addition of phenylethynyl- or trimethylsilylethynyl-lithium to (−)-menthone yields primarily the alkynyl-menthols in which the alkynyl substituent occupies an equatorial site. The stereochemistry of both the axial and equatorial isomers was established unequivocally by an X-ray crystallographic study of their dicobalt hexacarbonyl derivatives. Treatment of these alkynol-Co2(CO)6 complexes with iron pentacarbonyl yields neutral iron−cobalt clusters whose structures provide excellent models for cobalt-stabilized propargyl cations found as intermediates in the Nicholas reaction. Although the replacement of a cationic Co(CO)3 vertex by a neutral Fe(CO)3 moiety was originally classified as a dehydroxylation, the isolation of bridged hydride species suggests that the mechanism more likely involves a decarboxylation process. When this reaction was applied to enantiomerically pure dicobalt hexacarbonyl complexes of axial or equatorial phenylethynyl-menthols, the mixed iron−cobalt products that resulted always positioned the planar phenyl substituent, rather than the much bulkier cobalt tricarbonyl moiety, adjacent to the isopropyl substituent of the menthyl ring. Mechanisms to account for these observations are proposed, and all new metal clusters were characterized by X-ray crystallography.