Additions to Carbonyls

Me- SiCl also affects the stereoselectivity of 1,2-additions to carbonyl compounds [ 133]. Witli the aid of suitable activators, these mildly reactive reagents show selec-tivities unattainable by the conventional reagents, as ilustrated below for Me- SiCl-dependent Chemoselectivity fEq. 10.13) [134]. 

A more versatile method to use organic polymers in enantioselective catalysis is to employ these as catalytic supports for chiral ligands. This approach has been primarily applied in reactions as asymmetric hydrogenation of prochiral alkenes, asymmetric reduction of ketone and 1,2-additions to carbonyl groups. Later work has included additional studies dealing with Lewis acid-catalyzed Diels-Alder reactions, asymmetric epoxidation, and asymmetric dihydroxylation reactions. Enantioselective catalysis using polymer-supported catalysts is covered rather recently in a review by Bergbreiter [257],... 

Conjugate additions to enals and enones.1 In the presence of zinc (or tin) allyl halides undergo 1,2-addition to carbonyl compounds. The reaction is facilitated by sonication and proceeds in highest yield in saturated aqueous ammonium chlo-ride/THF (5.1).1... 

Thus, if trimethylsilyl anion were used as the nucleophile, then requisite diastereomer 30 could become available. Of the various counterions, Li+, Na+, and K+ have been examined by others. None of these species were suitable for direct 1,2-addition to carbonyl compounds. For example, trimethylsilyl lithium (Me3SiLi) adds nicely 1,4- to enones by a one-electron-transfer process, but does not provide a-silyl alcohols from ketones or aldehydes. In contrast tris(trimethylsilyl)alumi-... 

RCeCl2. The reagents, prepared in situ from RLi + CeCK, are more reactive than RLi in 1,2-addition to carbonyl groups. An example is the reaction with 2-alkoxypropionoyl-l,3-dithianes (1), prepared from methyl (S)-lactate.2 The dias-tereoselectivity obtains even when the protecting group is incapable of chelation. 

Addition of a nucleophile to a carbonyl group can create a new chiral center. This type of asymmetric induction has been of active interest for more than 50 years. While countless examples of 1,2-addition to carbonyl compounds resulting in enantiomeric or diasteriomeric excess are known, the origin of this asymmetric induction remains in dispute. 

Although the Michael addition of metal ynolates to a,/ -unsaturated carbonyl compounds is expected to give six-membered cycloadducts, 1,2-addition to carbonyl groups usually precedes 1,4-addition. The cycloaddition of the lithium-aluminum ate complex of silyl-substimted ynolate 112 with ethyl benzylideneacetoacetate (113), which is doubly activated by the ester and keto functions, gives the y-lactone 114 via a [4 4- 2] type cycloaddition (equation 46). Diethyl benzylidenemalonate (115) affords the uncyclized ketene 116 by reaction with 112 (equation 47). This could be taken as evidence for a stepwise mechanism for equation 46. ... 

The hydrosiiyiation of a,jS-unsaturated ketones and aldehydes using monohydrosilanes catalyzed by Rh(PPh3)3Cl proceeds by 1,4-addition, while dihydrosilanes or trihydrosilanes undergo 1,2-addition to carbonyl functionalities - . 

It is most striking that the hydrosilylation of aj3-unsaturated carbonyl compounds using wono-hydrosilanes was found to proceed in a manner of 1,4-addition, while c i-hydrosilanes very specifically underwent 1,2-addition to carbonyl functionalities [35]. Since the resulting silyl enol ethers and allylic silyl ethers can readily be converted by hydrolysis to saturated carbonyl compounds and a,j3-unsaturated alcohols, respectively, these reactions may furnish a unique method for selective reduction of carbonyl compounds, equation (13). The results are summarized in Table 4. 

Abstract The use of organoaluminum-based Lewis acids (A1R X3 R = alkyl, alkynyl, X = halide or pseudohalide) in the period 2000 to mid-2011 is overviewed with a focus on (1) stoichiometric reactions in which one of the organoaluminum substituents is transferred to the substrate (e.g., the opening of epoxides, 1,2-additions to carbonyl compounds, coupling with C-X, and Reissert chemistry) and (2) asymmetric, often catalytic, reactions promoted by Lewis acid catalysts derived from organoaluminum species (e.g., use of auxiliaries with alanes, Diels-Alder, and related cycloaddition reactions, additions to aldehydes and ketones, and skeletal rearrangement reactions). 

Majority of 1,2-additions to carbonyl compounds using In(III) salts as promoters or catalysts are in the area of allylation reactions. For several such reactions, In(III) salts may participate mainly through transmetallation, and thus strictly not acting as Lewis acids. However, such reactions are presented here, both for the sake of completion and for illuminating pertinent coordinating characteristics of In(III) complexes in reactions. 

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