Acyloxyborane, chiral

The Diels Alder reaction of aldehydes 74 with dienes 75 in the presence of chiral acyloxyborane (CAB) catalysts 76 provides enantioselectively chiral 4-dihydropyranones (Equation 3.22) after CF3CO2H treatment of the cycloadducts [74]. 

The coupling of the achiral stannane 20 and aldehyde 21 was achieved with fair to good enantioselectivity and fair yield using chiral catalysts. Ti-BINOL gave 52% e.e. and 31% yield, whereas an acyloxyborane catalyst (see p. 127) gave 90% e.e. and 24% yield.189... 

Allylation of aromatic aldehydes with allyl trimethylsilane catalyzed by chiral acyloxyborane gives good results. In contrast, the results are normally poor for aliphatic aldehydes.89 Costa et al.90 introduced another enantioselective allylation procedure aiming to overcome this problem. In the following example, the enantioselective allylation of aldehyde octanal was carried out using... 

The enantioselective addition of an allylsilane to an aldehyde catalyzed by chiral acyloxyborane (CAB) 13 is an excellent method for obtaining optically active homoallyl alcohols.Itsuno and Kumagai reported that the synthesis of a new optically active polymer with chirality on the mainchain is possible by applying this reaction to the asymmetric polymerization of bis(allylsilane) and dialdehyde (Scheme 12.11). ... 

The remarkable reactivity of borane toward carboxylic acids over esters is one of the conspicuous characteristics of this element, which is rarely seen in any other hydride reagent. An acyloxyborane is recognized as an initial intermediate, hence it became of interest to evaluate the asymmetric induction ability of appropriate chiral auxiliaries by introducing them into such... 

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)... 

Acyloxyboranes. Yamamoto et a/.1 have used the known reactivity of borane with carboxylic acids to activate acrylic acids for Diels-Alder reactions. Thus addition of BH3-THF to acrylic acids at 0° furnishes an acyloxyborane formulated as 1, which undergoes cycloaddition (equations I and II). The reaction proceeds satisfactorily even when borane is used in catalytic amounts. A chiral acyloxyborane, BL, prepared from a tartaric acid derivative, can serve as a catalyst for an asymmetric Diels-Alder reaction (equation III). 

This catalytic aldol reaction has been extended to a,f -enals.2 Thus these reactions catalyzed by the chiral acyloxyboranes 3a and 3b in addition to Sn(OTf)2 and Bu3SnF lead to optically active products in as high as 97% ee even when 3 is... 

Most chiral organoboron Lewis acids reported to date are based on an organoborane that is attached to a chiral organic moiety such as a diol, aminoalcohol, or other readily available chiral substrates.Organoboron derivatives recently used as catalysts in enantioselective Diels-Alder reactions include the family of chiral acyloxyboranes (CAB) with (196) and (197) as representative examples and various cyclic boronic esters such as (198) and (199). An interesting system that combines the favorable Lewis acid properties of fluorinated arylboranes with a chiral Bronsted acid has been developed by Ishihara and Yamamoto. The Bronsted acid-assisted chiral Lewis acids (BLA) (200) was found to be highly effective in enantioselective cycloadditions of Q ,jS-enals with various dienes. The presence of the Bronsted acid functionality leads to significant acceleration of the reaction. 

Preparative Methods to a solution of (R,R)- or (5,5)-mono-(2,6-diisopropoxybenzoyl)tartaric acid (74 mg, 0.2 mmol) in dry dichloromethane or propionitrile (1 mL) is added BH3 THF (0.189 mL of 1.06 M solution in THE, 0.2 mmol) at 0 °C under an argon atmosphere. The reaction mixture is stirred for 1 h at 0 °C to produce the chiral acyloxyborane. Only 2 equiv of hydrogen gas should evolve under these reaction conditions (0°C). See also Euruta. ... 

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. 

Chelation cf prochiral dienophiles by chiral boron, aluminum and titanium Lewis acids 4.13,33 Tartrate-derived chiral acyloxyborane catalysts... 

So far the most promising chiral Diels-Alder catalyst has been obtained in situ by treatment of monoacylated tartaric acid (470) with BHs-THF (1 mol equiv.)." The resulting non-isolated acyloxyborane was assumed to feature a five-membeied ring derived from the a-hydroxy acid moiety of (470) with die boron atom bound to the carboxylate and (Za-positioned oxygen atoms cf. formula 471) (Scheme 111, Table 32). 

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... 

Table 16. Additions of crolyl tributylstannanes to aldehydes catalyzed by a chiral acyloxyborane (CAB). ArC02...

Two related chiral Lewis acids are the acyloxyborane 68 [46] and the oxazabo-rolidine 69 [47] which are derived from (/ ,/ )-tartrate and L-valine, respectively. Each uses borane to generate the active catalyst, and they promote a variety of... 

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). 

Chiral boronales are generated m situ by reaction of binaphthols 3.7 (R = H, Ph) [231] with BH3 in the presence of acetic acid [778], with H BBr [781] or with B(OPh)3 [782, 783], Chiral borates are formed by reactions of substituted (S)-prolinol derivative 2.13 (R =- CPl OH) and BBr3 [784], These boronates and borates are valuable catalysts in asymmetric Diels-Alder reactions [73, 231, 601, 780], Tartaric acid derivatives, such as borate 3.8 and acyloxyboranes 3.9 recommended by Yamamoto and coworkers [73,601,778,780,785-791], are very efficient catalysts in asymmetric Diels-Alder reactions and in condensations of aldehydes with allylsilanes, enoxysilanes or ketene acetals. These catalysts are generated in situ from substituted monobenzoates of (RJl)- or (S -tartaric acid and BH3 (R = H) or an arylboric acid (R = Ar). The best asymmetric inductions are observed with catalysts 3.9, R = /-Pr. 1,3,2-OxazaboroMnes 3.10, prepared from a-aminoacids [44, 601, 780, 792, 793], are efficient catalysts in asymmetric Diels-Alder reactions. The catalyst generated from A -tosyltrytophan 3.11 is more efficient than borolidines 3.10 (R = Et, /-Pr). The catalysts 3.10 prepared from 3.11, 3.12 and 3.13 are also useful in asymmetric condensations of aldehydes with ketene acetals [794-797]. 

The additions of allylsilanes and -stannanes to carbonyl groups are usually catalyzed by a Lewis add. Therefore, chirality has been introduced either on the Lewis add [786, 816] or on the reagent [569, 698, 737, 739, 740, 1213-1217]. The use of a chiral Lewis add has been proposed by Yamamoto and coworkers [777a, 786, 791], Acyloxyboranes (R,R)- or (S,S)-3.9 (R = /-Pr, R = H or 3,5-(CF3)2C5H3) catalyze the asymmetric addition of allylsilanes and -stannanes to aldehydes at -78°C. Provided that R" H, the chemical yields and the selectivities of the reactions of allylsilanes are good only with PhCHO or unsaturated aldehydes [791], Better results are obtained from allylstannanes and any aldehyde [816] (Figure 6.53). 

CHIRAL ACYLOXYBORANE CATALYZED ASYMMETRIC DIELS-ALDER REACTION... 

Asymmetric addition of allyltins to RCHO Addition of y-substituted allylstan-nancs to aldehydes to form homoallylic alcohols can proceed in high diastcrco- and cnantioselectivity when carried out in the presence of 1 cquiv. of the Yamamoto chiral acyloxyborane 1 (16, 314). If trifluoroacctic anhydride is used as promotor, then only catalytic amounts of 1 are required. Virtually quantitative yields are obtained if 1 and (TFA)20 are used in the ratio 1 eq 1.2 eq. 

The chiral acyloxyborane 7 (CAB) has also been found to be an excellent catalyst for asymmetric Mukaiyama-Michael type aldol reaction between silyl enol ethers and aldehydes (Scheme 8). Yamamoto et al. [27] have used 20 mol % of CAB in propionitrile at -78 °C as a highly efficient catalyst for the condensation of several E and Z silyl enol ethers and ketene acetals with a variety of aldehydes (yields 49-97 %, 80-97 % ee). 

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