Acyloxy borane catalyst

Kumagai, T., Itsuno, S. Asymmetric Aiiyiation Polymerization Novel Polyaddition of Bis(allylsilane) and Dialdehyde Using Chiral (Acyloxy)borane Catalyst. Macromolecules 2000, 33,4995-4996. 

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 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... 

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. 

Enantioselective condensation of aldehydes and enol silyl ethers is promoted by addition of chiral Lewis acids. Through coordination of aldehyde oxygen to the Lewis acids containing an Al, Eu, or Rh atom (286), the prochiral substrates are endowed with high electrophilicity and chiral environments. Although the optical yields in the early works remained poor to moderate, the use of a chiral (acyloxy)borane complex as catalyst allowed the erythro-selective condensation with high enan-tioselectivity (Scheme 119) (287). This aldol-type reaction may proceed via an extended acyclic transition state rather than a six-membered pericyclic structure (288). Not only ketone enolates but ester enolates... 

The characteristic feature of the aforementioned oxazaborolidine catalyst system consists of a-sulfonamide carboxylic acid ligand for boron reagent, where the five-membered ring system seems to be the major structural feature for the active catalyst. Accordingly, tartaric acid-derived chiral (acyloxy)borane (CAB) complexes can also catalyze the asymmetric Diels-Alder reaction of a,P-unsaturated aldehydes with a high level of asymmetric induction [10] (Eq. 8A.4). Similarly, a chiral tartrate-derived dioxaborolidine has been introduced as a catalyst for enantioselective Diels-Alder reaction of 2-bromoacrolein [11] (Eq. 8A.5). 

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 parallel with the search for catalytic systems, an impressive amount of results in the field of enantioselective allylation has emerged (equation 8). The pioneering work of Marshall and Tang, using a chiral (acyloxy)borane 6 (CAB) system57, was followed by titanium-based catalysts 758 and 859-62 leading to various homoallylic alcohols with enantiomeric excess up to 98%. 

Chiral (acyloxy)borane (CAB) is known as an effective chiral Lewis acid catalyst for enantioselective allylation of aldehydes. Marshall applied the CAB complex 1 to the addition of crotylstannane to achiral aldehydes and found that the CAB catalyst gives higher syn/anti selectivity than BINOL/Ti catalysts in the reaction [4]. CAB complex 2 was utilized in asymmetric synthesis of chiral polymers using a combination of dialdehyde and bis(allylsilane) [5] or monomers possessing both formyl and allyltrimethylsilyl groups [6]. 

Chiral(acyloxy)boranes 523-525 were reported to be excellent catalysts for the enantioselective Mukaiyama condensation (Equation (248)) and allylation reaction of aldehydes leading to homoallylic alcohols (Equation (249)) 1042-1045... 

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]. 

Ishihara, K., Mouri, M., Gao, Q., Maruyama, T., Furuta, K., Yamamoto, H. Catalytic asymmetric aiiyiation using a chiral (acyloxy)borane complex as a versatile Lewis acid catalyst. J. Am. Chem. Soc. 1993, 115,11490-11495. 

The first example of the Lewis acid-catalyzed asymmetric addition of achiral allylstannanes to achiral aldehydes was reported by Marshall in 1992 using Yamamoto s chiral (acyloxy)borane (CAB) catalyst [87]. In initial studies with this catalyst, both aliphatic and aromatic aldehydes could be employed with substituted... 

The allylation reactions of carbonyl compounds catalyzed by chiral Lewis acids represent a powerful new direction in allylmetal chemistry. Yamamoto and coworkers reported the first example of the catalytic enantioselective allylation reaction in 1991, using the chiral (acyloxy)borane (CAB) catalyst system (see below) [288]. Since then, several additional reports of the catalytic allylation reaction have appeared. To date, the most effective catalyst systems reported for the enantioselective reaction of aldehydes and Type II allyl- and crotylstannane and silane reagents include the Yamamoto CAB catalyst and catalysts complexes composed of various Lewis acidic metals and either the BINOL or BINAP chiral ligands [289-293]. Marshall and Cozzi have recently reviewed progress in the enantioselective catalytic allylation reaction [294, 295]. 

To accelerate aUylation with aUylstaimanes, addition of a Lewis acid is often required, because coordination of the Lewis acid to the carbonyl can enhance the electrophilicity of the substrate and facilitate the couphng reaction. Since Yamamoto showed that a nonracemic Lewis acid, chiral (acyloxy)borane (CAB), catalyzed enantioselective allylation [73], chiral Lewis acid catalysts have been extensively developed. Above all, easily available chiral compounds such as BINOL and BINAP have been most frequently used as chiral auxiliaries [74], and enantioselective allylations of C-N double bonds and of carbonyl groups have been achieved [48a, 75]. 

Yamamoto and coworkers have developed a practical Diels-Alder catalyst for aldehyde dienophiles. Treatment of a monoacylated tartaric acid with borane released ca. 2.2 equiv of H2 gas, affording a complex that has been assigned structure 7. Circrunstantial evidence for structure 7 was found in the comparable enan-tioselectivity of a catalyst in which the free carboxyl group was esterified (see below). The chiral (acyloxy)borane (CAB) complex is effective in catalyzing a number of aldehyde-based Diels-Alder reactions (Scheme 9) [56]. Reactions with... 

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-... 

The dibenzyl ester 27 can be obtained by slowly distilling a mixture of tartaric acid and benzyl alcohol, without adding an acidic catalyst34. It is a key intermediate in the synthesis of mono-O-acylated tartaric acids 28-31 33,35 the benzyl ester groups can be selectively cleaved to the free carboxylic acid groups by catalytic hydrogenation. Such derivatives form acyloxy-boranes with diborane which are used as chiral catalysts in enantioselective Diels-Alder reactions (Section D. 1.6.1.1.1.). 

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). 

A stable chiral acyloxy borane (CAB) complex 7 is also an effective catalyst for hetero Diels-Alder reaction to produce dihydropy-rans in high optical purity. [18] (Scheme 5). 

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... 

Catalytic enantioselective allylation with allylsilanes has also been achieved with chiral Lewis acid catalysts such as Ti/binol " and chiral (acyloxy)borane complexes (Scheme 3-89). 

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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