Acyloxyborane complex

Recently, Yamamoto et al. have shown that the chiral acyloxyborane complex 31 is an excellent catalyst for the asymmetric Mukaiyama condensation of simple silyl enol ethers (Scheme 8B1.19 Table 8B1.11 entries 1-7) [43], The syn-aldol adducts are formed preferentially with high enantiomeric excess regardless of the stereochemistry (EI7) of the silyl enol ethers, suggesting an extended transition state (entries 4, 7). This methodology has been... 

TABLE 8B1.11. Asymmetric Aldol Reaction Catalyzed by Chiral Acyloxyborane Complex 31 (Scheme 8B1.19)... 

It is remarkable diat 0.1 mol equiv. of this chiral acyloxyborane complex induced fast (-78 C) and highly enantioselective Diels-Alder reactions of cyclic or acyclic 1,3- enes widi simple acrylic acid (entry 1), " or a,B-unsaturated aldehydes (entries 2-9)."" Table 32 reveals a striking steieo recting... 

Aldolization and related reactions. Tartaric acid-derived acyloxyborane complexes are shown to be useful catalysts for asymmetric aldol reactions. (5)-4-Isopropyl-3-tosyl-l,3,2-oxazaborolidin-5-one is an excellent cataly.st, not only for the aldolization " between a silyl enol ether and an aldehyde it also reduces the products to afford syn-l,3-diols. ... 

In the previous section, we described the acyloxyborane complexes which are highly... 

Form Supplied in the acyloxyborane-THF complex is available as a 0.1-0.2 M solution in dichloromethane or propionitrile. 

The rapid reaction between carboxylic acids and borane is related to the electrophilicity of the latter. The carbonyl group of the initially formed acyloxyborane intermediate, which is essentially a mixed anhydride, is activated by the Lewis acidity of the trivalent boron atom. Addition of 1/3 equiv of the Borane-Tetrahydrofuran complex to acrylic acid in dichloromethane followed by addition of a diene at low temperature results in the formation of Diels-Alder adducts in good yield (eq 1). Further, the reaction is successful even with a catalytic amount of borane. 

Asymmetric Diels-Alder Reaction of Unsaturated Carboxylic Acids. A chiral acyloxyborane (CAB) complex (1) prepared from mono(2,6-dimethoxybenzoyl)tartaric acid and 1 equiv of borane is an excellent catalyst for the Diels-Alder reaction of a,p-unsaturated carboxylic acids and dienes. In the CAB-catalyzed Diels-Alder reaction, adducts are formed in a highly diastereo- and enantioselective manner under mild reaction conditions (eq 2). The reaction is catalytic 10 mol % of catalyst is sufficient for efficient conversion, and the chiral auxiliary can be recovered and reused. 

Most acyloxyboranes [MMl] and aminoboranes [Al] do not react either with esters or with lactones. However, Ph2NH BH3 reduces aliphatic esters [GUI]. Substituted boranes are more efficient. 9-BBN reduces esters under reflux in THF [PSl], while ThexBHCl gives rise to alcohols with heating [BN5]. Finally, the ate complex Li 9-BBNH reduces esters to alcohols and lactones to diols. Acids, amides, nitriles, and halogenated derivatives remain intact under these conditions [BMl]. 

Uncomplexed acrolein, methacrolein, and crotonaldehyde all favor the s-trans conformer, and this preference is enhanced upon complexation to a Lewis acid. For example, Corey showed that the BFj-methacrolein complex adopts the s-trans conformation in the solid state as well as in solution by crystallographic and NMR spectroscopic methods (Fig. 8B) [27], while Denmark and Almstead found that methacrolein adopts the s-trans geometry upon complexation with SnCl4 (Fig. 8C) [28]. Yamamoto demonstrated that methacrolein is also observed in the s-trans conformation upon complexation to his chiral acyloxybo-rane (CAB) catalyst (Fig. 14A and Sect. 3.1.2) [40]. Interestingly, with the same CAB system, crotonaldehyde exhibited varying preferences for the two possible conformers depending on the exact substituents on the boron. On the basis of NOE enhancements, the s-trans conformer was observed exclusively with a hydrogen substituent on boron (Fig. 14B) the s-cis conformer was the only one detected in the case of the aryl-substituted acyloxyborane (Fig. 14C). 

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