Boranes, acyloxy, from aldehydes

Asymmetric Diels-AUer reactions The observation that simple acyloxy-boranes such as H2BOCOCH=CH2, prepared by reaction of BH3 with acrylic acid, can serve as Lewis acid catalysts for reactions of the a,P-unsaturated acids with cyclopentadiene (15, 2) has been extended to the preparation of chiral acyloxy-boranes derived from tartaric acid. The complex formulated as 3, prepared by reaction of BH3 with the monoacylated tartaric acid 2, catalyzes asymmetric Diels-Alder reactions of a,P-enals with cyclopentadiene with high enantioselectivity. The process is applicable to various dienes and aldehydes with enantioselectivities generally of 80-97 % ee. 

In 1989 Yamamoto et al. reported that the chiral (acyloxy)borane (CAB) complex 3 is effective in catalyzing the Diels-AIder reaction of a number of a,/ -unsaturated aldehydes [5]. The catalyst was prepared from monoacylated tartaric acid and bo-... 

Stable aryl boronates derived from tartaric acid catalyze the reaction of cyclo-pentadiene with vinyl aldehyde with high selectivity. Chiral acyloxy borane (CAB), derived from tartaric acid, has proved to be a very powerful catalyst for the enantioselective Diels-Alder reaction and hetero Diels-Alder reaction. Scheme 5 23 presents an example of a CAB 73 (R = H) catalyzed Diels-Alder reaction of a-bromo-a,/i-cnal 74 with cyclopentadiene. The reaction product is another important intermediate for prostaglandin synthesis. In the presence of... 

Yamamoto et al. [16] envisaged that acyloxy-boranes might behave as mixed anhydrides because of the electronegative trivalent boron atom and could serve as effective asymmetric catalysts in selected reactions. In the presence of 20 mole% of chiral acyloxy borane (CAB) complex 7 prepared from 2R,3R)-2-0-(2,6-diisopropoxybenzoyl)tartaric acid and BHs THF, various allyltrimethylsilanes react with achiral aldehydes to afford the corresponding homoallylic alcohols in good yield and high enantio- and diastereoselectivity (Scheme 3). 

Stereoselective Allylation Reaction. The enantioselective allylation of aldehydes with allyltrimethylsilane is now possible with good control. One example is the use of a chiral acyloxy borane (CAB) catalyst. Besides that, chiral Ti(OiPr)2X2-BINOL and TiFa-BINOL catalysts are effective for the enantioselective allylation of glyoxylates and aldehydes, respectively (eq 35). Chiral homoallylamines can be prepared from the reaction of imines and allyltrimethylsilane using chiral tt-allylpalladium complexes with TBAF (eq 36) or an (5)-Tol-BINAP-CuPFe system. Free radical allyl transfers from allyltrimethylsilane provides another method for this enantioselective C-C bond formation. Promoted by chiral Lewis acids, both... 

Yamamoto developed a remarkable boron-derived catalyst for enantioselec-tive Diels-Alder reactions which is easily assembled from monoacylated tartaric acid and borane. Spectroscopic data provided evidence that supports the proposed catalyst structure 144 depicted in Equation 16 [79, 80]. Such chiral (acyloxy)borane (CAB) catalysts have been employed in numerous cyclo-additions with unsaturated aldehydes to afford the corresponding products, such as 145, with high selectivity (98% ee, endo exo > 99 1) [80]. 

---