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

Abstract

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.

Organotransition Metal Chemistry of Terpenes: Syntheses, Structures, Reactivity and Molecular Rearrangements

“…In contrast to the earlier cases, alkynyl anion attack on menthone yields a mixture of alkynyl-menthols, 210 and 211 , in which the predominant isomer is the latter, whereby the alkynyl substituent occupies the equatorial site. The original attribution of these isomers was based on NMR data of their derived allenyl-phosphine oxides [ 115 ], and the X-ray crystallographic structure determinations of their phenylethynyl-Co 2 (CO) 6 tetrahedral clusters, 212 and 213 , respectively, validated these assignments [ 116 ]. As anticipated, protonation of these alkynol clusters yielded cationic complexes but, since they were not amenable to X-ray structural analysis, they were also treated with iron pentacarbonyl in acetone, a reaction previously shown to yield neutral mixed-metal Fe-Co complexes [ 117 ] in which Fe(CO) 3 replaced the isoelectronic, and isolobal, [Co(CO) 3 ] + fragment.…”