Boranes, acyloxy, from

Fig. 13. Possible structures for (acyloxy)boranes formed from the reaction of 9-BBN with oxalic acid and solid state structure of the complex with 2,2-dimethylmalonic acid 30...

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-... 

The (acyloxy)borane complex 9, readily available from tartaric acid derivative 8, also catalyzes aldol additions of silyl enol ethers34 and silylketene acetals3 5 in an enantioselective manner. Thus,. u -ketones 10 and /Thydroxy esters 12 are available34, as well as a-unsubstituted ketones 1135. 

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... 

S)-(-)-CITRONELLOL from geraniol. An asymmetrically catalyzed Diels-Alder reaction is used to prepare (1 R)-1,3,4-TRIMETHYL-3-C YCLOHEXENE-1 -CARBOXALDEHYDE with an (acyloxy)borane complex derived from L-(+)-tartaric acid as the catalyst. A high-yield procedure for the rearrangement of epoxides to carbonyl compounds catalyzed by METHYLALUMINUM BIS(4-BROMO-2,6-DI-tert-BUTYLPHENOXIDE) is demonstrated with a preparation of DIPHENYL-ACETALDEHYDE from stilbene oxide. A palladium/copper catalyst system is used to prepare (Z)-2-BROMO-5-(TRIMETHYLSILYL)-2-PENTEN-4-YNOIC ACID ETHYL ESTER. The coupling of vinyl and aryl halides with acetylenes is a powerful carbon-carbon bond-forming reaction, particularly valuable for the construction of such enyne systems. 

A better accessible chiral mediator is the (acyloxy)borane (CAB) 2-64 prepared in situ from a tartaric acid derivative and arylboronic acid at room temperature. Hetero Diels-Alder reaction of benzaldehyde and Danishefsky s diene 2-10 in the presence of 2-64 gave the corresponding pyrone after acidic work up with 52 - 95 % ee depending on R. The best results were obtained with R = 2,4,6-Me3Ph and 2,4,6-iPr3Ph. Similarly, with 2-60 the pyrone 2-61 with up to 97% ee was found [107]. 

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]. 

Inl989, Yamamoto introduced the chiral (acyloxy)borane (CAB) complex for catalytic asymmetric Diels-Alder reactions [18], which has been utilized as a magic hand catalysis for the aldol synthesis and for the Sakurai-Hosomi reaction so far [19,20]. In contrast to R=H of 17, which is both air and moisture sensitive, the B-alkylated catalyst, R=Ph or alkyl, is stable and can be stored in closed containers at room temperature. This catalyst is easily prepared from phenyl- or alkylboric acid and 16 simple mixing of a 1 1 molar ratio of the ester 16 and phe-... 

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). 

Acyloxy)dialkylboranes are best prepared from carboxylic acids with tri-alkylboranes.1,12-16 Other methods involve reactions of halodiorganobo-ranes with carboxylic acids17 or with acetic anhydride.18 Alternative syntheses from acetic anhydride using dialkyl(alkylthio)boranes or dialkylamino compounds have also been described.19... 

In 1988, Yamamoto and coworkers reported the use of enantiomerically pure (acyloxy)borane (CAB) catalysts (8.16) for the enantioselective Diels-Alder reaction. These catalysts are derived from tartaric acid, and again, their reactions have been particularly selective with a-substituted enals as substrate. The dienophile... 

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... 

Reetz and coworkers introduced the cyclic chlorodialkylboron Lewis acid (75) (Equation 48) [46], and Kiyooka and coworkers made use of acyloxyborane (76) (Equation 49) [47] in enantioselective Mukaiyama-aldol reactions that employ stoichiometric amounts of the respective boron Lewis acids. Both species give high enantioselectivity in the formation of the desired aldol adducts. After Kiyooka s report of (76), various boron catalysts derived from chiral amino acids appeared in the literature. As such, Masamune and coworkers introduced (77) and (78) [48], Kiyooka and co workers introduced (79) [49], and Corey and co workers introduced (80) [50] as chiral acyloxy borane catalysts for enantioselective aldol reactions (Figure 5.7). 

In 1991, Yamamoto reported a simple catalyst for aldol reactions consisting of a chiral (acyloxy)borane (CAB) complex of tartrate-derived ligands (241, Scheme 4.28) [120-122]. This work, along with that of Mukaiyama and Kobayashi, stunningly showed that remarkably simple complexes for catalytic enantioselective aldol addition reactions could be identified. Both enantiomers of the catalyst are readily accessible from the enantiomers of tartaric acid. The syn aldol adducts were observed to be preferentially formed regardless of the enolate geometry (242 vs. 244) with high levels of enantios-... 

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]. 

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