Stereoselective chromium-and molybdenum- mediated transformations of arenes*

Abstract: Tricarbonylchromium-mediated dearomatization provides a rapid access to substituted cyclohexadienes. Efficient asymmetric routes to planar chiral arene complexes and to substituted cyclohexadienes have been developed. The article sums up the main features of this chemistry. Highly enantiomerically enriched ortho-substituted benzaldehyde complexes are accessible via asymmetric lithiation followed by trapping with electrophiles. In different solvents, the trimethylsilyl complex exhibits [α] values ranging from -174 to +108 for the same enantiomer. Details of two asymmetric syntheses of natural products are given: the alkaloid lasubine I starting from a highly enantiomerically enriched planar chiral arene complex and the marine furanosesquiterpene acetoxytubipofuran. The latter is assembled via asymmetric dearomatization of a benzaldehyde imine complex. Other key steps include an Eschenmoser-Claisen rearrangement and a regio-and diastereoselective Pd-catalyzed allylic substitution. The final section deals with labile arene metal complexes. For the first time, dearomatization reactions mediated by the Mo(CO) 3 group have been realized. The reactions show strong analogies to the Cr(CO) 3 -mediated reactions, but exhibit also marked differences: the arene-Mo bond is stronger, but more labile, and the sequential double additions show different selectivities compared to the chromium analogs. The search for ever more efficient chemo-, regio-, and stereoselective routes to complex molecules from simple starting materials is an important task in organic synthesis. Progress in this area continues to advance at a fast pace. Arenes, the subject of this article, are widely available, highly stable, and readily derivatized through reactions such as electrophilic and nucleophilic aromatic substitution [1], ortholithiation followed by reaction with electrophiles [2], or metal-catalyzed substitution and coupling reactions This notwithstanding, the synthesis of complex organic molecules via elegant dearomatization chemistry has undergone intensive investigation. Examples involve the Birch reduction, which achieves *Plenary and invited lectures presented at th

Highlights from the 1st Firmenich Innovation Through Synthesis (FITS) Symposium, Geneva, Switzerland, 26–27th June 2008: Conference Report

This conference report highlights the different lectures that took place for Geneva for the first organic synthesis symposium organized by Firmenich

New Chromium Carbonyl Catalysts for [6+2] Cycloaddition Reactions

The complexes, (benzene)chromiumdicarbonyl(methyl acrylate) and chromiumdicarbonylbis(cyclohexadiene), are precursors for the highly coordinatively unsaturated chromium dicarbonyl fragment 3, a catalyst for the cycloaddition of activated olefins to cycloheptatriene

New Chromium Enolates

Three new (?2-acrylate)(?6-arene)dicarbonylchromium complexes are reported. They were obtained either by CO/acrylate exchange in [Cr(?6-benzene)(CO)3] (1) via the photolytically generated ?2-cyclooctene intermediate or by arene exchange in [Cr(?2-methyl acrylate)(?6-benzene)(CO)2] (3) (Scheme 1). On crystallization, [Cr(?2-methyl acrylate)(?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(?2-acrylate)(?6-benzene)(CO)2] complexes with cyclopentad-1,3-diene or 1H-indene afforded new (?6-cyclopenta-2,4-dien-1-yl)- or (?6-1H-inden-1-yl)(?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 ?2- to a ?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(?5-1H-inden-1-yl){?3-CH(CH2CF3)C(O)OEt}] (8) reveals a metal enolate that is best described as ?3-oxaallyl species (Fig. 2). A shorter, more-efficient route to the [Cr(?5-1H-inden-1-yl)(?3-oxaallyl)] complexes was devised via the reaction of [Cr(CO)2(?2-cyclooctene)(?6-1H-indene)] with methyl acrylate (Scheme 4)

Stereoselective chromium- and molybdenum-mediated transformations of arenes

Tricarbonylchromium-mediated dearomatization provides a rapid access to substituted cyclohexadienes. Efficient asymmetric routes to planar chiral arene complexes and to substituted cyclohexadienes have been developed. The article sums up the main features of this chemistry. Highly enantiomerically enriched ortho-substituted benzaldehyde complexes are accessible via asymmetric lithiation followed by trapping with electrophiles. In different solvents, the trimethylsilyl complex exhibits [alpha] values ranging from −174 to +108 for the same enantiomer. Details of two asymmetric syntheses of natural products are given: the alkaloid lasubine I starting from a highly enantiomerically enriched planar chiral arene complex and the marine furanosesquiterpene acetoxytubipofuran. The latter is assembled via asymmetric dearomatization of a benzaldehyde imine complex. Other key steps include an Eschenmoser–Claisen rearrangement and a regio- and diastereoselective Pd-catalyzed allylic substitution. The final section deals with labile arene metal complexes. For the first time, dearomatization reactions mediated by the Mo(CO)3 group have been realized. The reactions show strong analogies to the Cr(CO)3-mediated reactions, but exhibit also marked differences: the arene–Mo bond is stronger, but more labile, and the sequential double additions show different selectivities compared to the chromium analogs