Organocobalt complexes reduction

Only a few other cobalt complexes of the type covered in this review (and therefore excluding, for example, the cobalt carbonyls) have been reported to act as catalysts for homogeneous hydrogenation. The complex Co(DMG)2 will catalyze the hydrogenation of benzil (PhCOCOPh) to benzoin (PhCHOHCOPh). When this reaction is carried out in the presence of quinine, the product shows optical activity. The degree of optical purity varies with the nature of the solvent and reaches a maximum of 61.5% in benzene. It was concluded that asymmetric synthesis occurred via the formation of an organocobalt complex in which quinine was coordinated in the trans position (133). Both Co(DMG)2 and cobalamin-cobalt(II) in methanol will catalyze the following reductive methylations ... 

These reactions are believed to proceed via a reductive process to give an organocobalt complex 37a, which undergoes an intramolecular cyclization of the radical 37b generated by homolytic cleavage of C-Co bond (Scheme 15). 

Brookhart s group has discovered a new hydrido organocobalt complex in which the cobalt atom is in a high oxidation state. They thus reported a rare example of an organometallic cobalt derivative with a +v formal oxidation state for the cobalt center 10. This species contained additional silyl ligands that stabilized such high oxidation state complexes. These species were remarkably resistant toward reductive elimination. The 2.28 A H-H and the... 

Reduction to Alcohols. The organosilane-mediated reduction of ketones to secondary alcohols has been shown to occur under a wide variety of conditions. Only those reactions that are of high yield and of a more practical nature are mentioned here. As with aldehydes, ketones do not normally react spontaneously with organosilicon hydrides to form alcohols. The exceptional behavior of some organocobalt cluster complex carbonyl compounds was noted previously. Introduction of acids or other electrophilic species that are capable of coordination with the carbonyl oxygen enables reduction to occur by transfer of silyl hydride to the polarized carbonyl carbon (Eq. 2). This permits facile, chemoselective reduction of many ketones to alcohols. 

In this section, we present material dealing with the direct oxidation and reduction of a variety of organocobalt species, including complexes with more than one cobalt center, electrodes functionalized with cobalt complexes, cobalt-containing SchifF-base complexes, cobalt porphyrins and corroles, and macrocyclic tetraamines. 

A tandem radical 5-exo cyclization/radical addition/allylic substitution reaction was subsequently described [292]. Allylic ot-bromo acetal 242b cyclized cobalt-catalyzed. Addition to diene 245 and subsequent coupling with coformed organocobalt(I) species generates an allylcobalt complex, which undergoes reductive elimination to cyclic product 246 in 93% yield (cf. Fig. 56). 

Since the butenylcobalt complex was too unstable, the PMR spectrum of the allylcobalt complex was studied 21, 22) to obtain information on the structure of the organocobalt intermediates in these stereoselective reductions (20). This investigation indicated that a-bonded allylic complexes are in equilibrium with 7r-allylic complexes and cyanide ion, thus providing a rationale for the stereoselectivities observed in the reduction of butadiene and butenyl chlorides. 

Cobalt(II) complexes (such as cob(II)alamin, formerly called B,2,) can be obtained by several methods including controlled potential reduction [14,15], anaerobic photolysis of some organocobalt species [16] (see Section 5.2(b)), partial oxidation of cobalt(I) complexes, in some cases acidification of solutions of cobalt(I) complexes (see below) and chemical reduction. Chemical reduction of cobalt(III) cobalamins to the +2 oxidation state has been achieved with hydrogen over platinum oxide [16], with neutral and acidic solutions of vanadium(III) [17], with chromous acetate at pH 3 [18] and with amalgamated zinc in 0.1 M aqueous perchloric acid [19]. All such cobalt(II) species are probably 5-coordinate, and are low spin d systems containing an unpaired electron and thus displaying an ESR spectrum [20]. 

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