Cobalt enolates aldol reaction

Although a cobalt-catalyzed intermolecular reductive aldol reaction (generation of cobalt enolates by hydrometal-lation of acrylic acid derivatives and subsequent reactions with carbonyl compounds) was first described in 1989, low diastereoselectivity has been problematic.3 6 However, the intramolecular version of this process was found to show high diastereoselectivity (Equation (37)).377,377a 378 A Co(i)-Co(m) catalytic cycle is suggested on the basis of deuterium-labeling studies and the chemistry of Co(ll) complexes (Scheme 81). Cobalt(m) hydride 182, which is... 

Other examples of the uses of enolate derivatives of thioesters for highly diastereoselecave aldol reactions were reported by Gennari [373] and Hanaoka et al. [374. 375]. The latter reported the reaction between a chromium-complexed benzaldehyde (1) and the titanium enolate of a thioester (2) as the first step of sequences directed towards the synthesis of (+)-goniofufurone [374] and the taxol C13 side-chain [375]. They also used cobalt-complexed acetylenic aldehydes for the selective formation of syn-aldols [376]. 

Cyclic cobalt-acyl complexes can be deprotonated, and subsequent reaction of these enolates with aldehydes gives predominantly the anti/threo product (Scheme 63). Rhenium-acyl complexes can be deprotonated in the same manner. These lithium enolates can be alkylated or can react with [M(CO)5(OTf)] (M = Re, Mn) to give the corresponding enolates (Scheme Many transition metal enolates of type (21) or (22) are known, - but only a few have shown normal enolate behavior , e.g. aldol reaction, reaction with alkyl halides, etc. Particularly useful examples have been developed by Molander. In a process analogous to the Reformatsky reaction, an a-bromo ester may be reduced with Smia to provide excellent yields of condensation products (Scheme 65) which are generated through intermediacy of a samarium(III) enolate. ... 

Lewis-acid-promoted alkylations of silylenol ethers and silyl ketene acetals [195] with Co-complexed acetylenic acetals [196] and acetylenic aldehydes [197,198] (Scheme 4-56) also proceed with fair to excellent syn diastereoselectivity, in contrast to the low selectivity reactions of the free acetylenic derivatives [199, 200]. Reactions of the complexed aldehydes with lithium enolates are stereospecific, with (Z)-enolates giving syn selectivity and ( )-enolates anti selectivity [201]. The complementary stereoselectivity of the crossed aldol reactions of free and cobalt-complexed propynals with silyl ketene 0,S-acetals has been elaborated by Hanoaka exclusive syn selectivity is exhibited by the complexes and high anti selectivity is found with pro-... 

The formal conjugate addition of a hydride to a,f(-unsaturated carbonyl compounds with a subsequent aldol reaction of the in situ formed enolate has been frequently employed in organic synthesis. A broad range of procedures have been developed using various metals (e.g., rhodium, cobalt, iridium, mthenium, copper) and different reductants (typically silanes, boranes, or elemental hydrogen) [37]. 

A second, less used, strategy encompasses the Lewis acid catalyzed intramolecular reaction of a silyl enol ether with a propargyl cation. The latter can be conveniently generated by a cobalt complexed propargyl ether. This complexation strongly helps the carbocation formation. By using cobalt complexation, intramolecular aldol type reactions (for R = OR ) have been accomplished. ... 

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