New Chromium Enolates

Abstract

Abstract Three new ( η 2 ‐acrylate)( η 6 ‐arene)dicarbonylchromium complexes are reported. They were obtained either by CO/acrylate exchange in [Cr( η 6 ‐benzene)(CO) 3 ] ( 1 ) via the photolytically generated η 2 ‐cyclooctene intermediate or by arene exchange in [Cr( η 2 ‐methyl acrylate)( η 6 ‐benzene)(CO) 2 ] ( 3 ) ( Scheme 1 ). On crystallization, [Cr( η 2 ‐methyl acrylate)( η 6 ‐ o ‐xylene)(CO) 2 ] ( 5 ) underwent partial resolution. The degree of this resolution was analyzed via X‐ray crystal‐structure determination ( Fig. 1 ) and was correlated to the CD spectra in solution ( Fig. 6 ), thus allowing the assignment of the absolute configuration. The reaction of [Cr( η 2 ‐acrylate)( η 6 ‐benzene)(CO) 2 ] complexes with cyclopentad‐1,3‐diene or 1 H ‐indene afforded new ( η 6 ‐cyclopenta‐2,4‐dien‐1‐yl)‐ or ( η 6 ‐1 H ‐inden‐1‐yl)( η 3 ‐oxaallyl)chromium complexes ( Scheme 2 ). A mechanism is proposed that involves arene‐ligand substitution by the diene ligand, initiated by haptotropic rearrangement of the acrylate from a η 2 ‐ to a η 4 ‐coordination mode, followed by hydride migration from the diene to the acrylate ( Scheme 3 ). An X‐ray crystal‐structure determination of [Cr(CO) 2 ( η 5 ‐1 H ‐inden‐1‐yl){ η 3 ‐CH(CH 2 CF 3 )C(O)OEt}] ( 8 ) reveals a metal enolate that is best described as η 3 ‐oxaallyl species ( Fig. 2 ). A shorter, more‐efficient route to the [Cr( η 5 ‐1 H ‐inden‐1‐yl)( η 3 ‐oxaallyl)] complexes was devised via the reaction of [Cr(CO) 2 ( η 2 ‐cyclooctene)( η 6 ‐1 H ‐indene)] with methyl acrylate ( Scheme 4 ).