Chiral Lewis Acids as Catalytic Reagents

The first example of an enantioselective catalytic Diels-Alder reaction is the report in 1976 of cycloaddition between methyl acrylate and cyclopentadiene [3]. The catalyst was BF3 complexed to (-)-methyl menthyl ether. The enantiomeric excess is very small (3 %), and it seems unlikely that chiral catalysis occurred (e.g., the undissociated complex). 

About 10 years later the catalytic use of boron derivatives was again investigated by Kaufmann et al. Compounds of the type RBBr2 (R = pinanyl) catalyze the Diels-Alder reaction at -78 °C with low ee (Eq. 3) [4]. Modified isopinocamphenyl dibro-moborane, which has been synthesized since then, gives better results in the reaction of cyclopentadiene and methyl acrylate (Eq. 4) [5]. 

Kaufmann et al. greatly improved on their results by using a boron compound derived from 1,1-binaphthol (Eq. 5) [6]. This compound was unexpectedly obtained from monobromoborane dimethyl sulfide and l,l -binaphthol. A diborate structure with a propeller-like shape has been established by X-ray analysis. The reaetion between methacrolein and cyclopentadiene is catalyzed at -78 °C in dichloromethane by 3 mol % of the chiral borate, and the exo cyeloadduct is obtained with high exo selectivity (97.4 2.6) and 90 % ee. The mechanistic details and scope of the reaction are still unclear. 

Bao and Wulff compared catalysts prepared from vaulted biaryls and from bromo-borane dimethylsulfide with those generated from linear biaryls with regard to their capacity to provide enantioselective induction in the Diels-Alder reaction of cyclo-pentadiene and methacrolein (Eqs 6 and 7) [7]. Because the (5) enantiomers of vaulted biaryls result in induction opposite to that resulting from use of the (5) enantiomer of binaphthol, and because effective catalysts cannot be generated from binaphthol and phenylboron dichloride, suggest that the catalysts obtained from vaulted biaryls do not have the same structure as the Cs-symmetrical catalyst produced from binaphthol. 

Another promising approach has been devised by Yamamoto and co-workers [8]. They found that the action of a controlled amount of diborane on a carboxylic acid leads to an (acyloxy)borane RC02BR 2 which behaves as a Lewis acid. The chiral (acyloxy)borane (CAB) complex 1 formed in situ from monoacyl tartaric acid and diborane is an excellent asymmetric catalyst (Eq. 8) for the Diels-Alder reaction of cyclopentadiene and acrylic acid (78 % ee) (Eq. 9) [8] or of cyclopentadiene and methacrolein (96 % ee) (Eq. 10) [9]. 

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