Acyloxyborane

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]. 

Acid derivatives that can be converted to amides include thiol acids (RCOSH), thiol esters (RCOSR), ° acyloxyboranes [RCOB(OR )2]. silicic esters [(RCOO)4Si], 1,1,1-trihalo ketones (RCOCXa), a-keto nitriles, acyl azides, and non-enolizable ketones (see the Haller-Bauer reaction 12-31). A polymer-bound acyl derivative was converted to an amide using tributylvinyl tin, trifluoroacetic acid, AsPh3, and a palladium catalyst. The source of amine in this reaction was the polymer itself, which was an amide resin. 

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... 

Although simple alkyl esters (ethyl propionate) fail to enolize with the boryl triflate reagents under normal conditions, the more acidic acyloxyboranes 66 readily form the diboryl enediolates 67 (eq. [52]) (6a,66). Several interesting trends are noted in the data included in Table 23. Since previous studies have demonstrated that enolate geometry strongly correlates with product stereochemistry, enediolate 67 has been employed to directly compare the reactivities... 

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). ... 

Acid derivatives that can be converted to amides include thiol acids RCOSH, thiol esters RCOSR,911 acyloxyboranes RCOB(OR )2,912 silicic esters (RCOO)4Si, 1,1,1-trihalo ketones RCOCX3,913 a-keto nitriles, acyl azides, and nonenolizable ketones (see the Haller-Bauer reaction 2-33). 

The best results are obtained when the acyloxyboranes are made from a carboxylic acid and catecholboranc (p. 615) Collum Chen Ganem J. Org. Chem. 1978, 43, 4393. 

The reason for the high reactivity lies in the fact that the acid first converts the borane to an acyloxyborane, which then undergoes an intramolecular rearrangement in which the carbonyl group is reduced. Hydrolysis gives the alcohol ... 

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. 

Amines lactams. Simple acyloxyboranes are known to react with amines to form amides, but in low yield. This reaction has been improved by use of catecholborane (equation I). It can be extended to synthesis of lactams. Thus addition of catecholborane to a suspension of an w-amino acid in pyridine (80°) results in a lactam. Yields are high (85-95%) for 3- and 5-membered lactams. Yields are low (—6 (.) in the case of medium-size lactams, but improve to 10-15% in the synthesis of 14- and 16-mcmbcrcd lactams. (Dimers are formed preferentially.) Yields are... 

Form Supplied in the acyloxyborane-THF complex is available as a 0.1-0.2 M solution in dichloromethane or propionitrile. 

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. ... 

Handling, Storage, and Precautions the acyloxyborane solution should be flushed with Ar and stored tightly sealed (to preclude contact with oxygen and moisture) below 0°C. Use in a fume hood. 

Acyloxyborane as an Activating Device for Carboxylic Acids . The reduction of carboxylic acids by borane is an important procedure in organic synthesis. The remarkable reactivity of borane towards carboxylic acids over esters is characteristic of this reagent. Such selectivity is rarely seen with other hydride reagents. 

The rapid reaction between carboxylic acids and borane is related to the electrophilicity of the latter. The carbonyl group of the initially formed acyloxyborane intermediate, which is essentially a mixed anhydride, is activated by the Lewis acidity of the trivalent boron atom. Addition of 1/3 equiv of the Borane-Tetrahydrofuran complex to acrylic acid in dichloromethane followed by addition of a diene at low temperature results in the formation of Diels-Alder adducts in good yield (eq 1). Further, the reaction is successful even with a catalytic amount of borane. 

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. 

Asymmetric Diels-Alder Reaction of Unsaturated Aldehydes . The boron atom of acyloxyborane is activated by the electron-withdrawing acyloxy groups, and consequently acyloxyborane derivatives are sufficiently Lewis acidic to catalyze certain reactions. Thus, asymmetric Diels-Alder reactions of a,p-enals with dienes using (1) as a Lewis acid catalyst have been developed. For example, the reaction of cyclopentadiene and methacrolein gives the adduct in 85% yield (endo exo= 11 89) and 96% ee (major exo isomer) (eq 3). Some additional examples are listed in Figure 1. The a-substituent on the dienophile increases the enantioselectivity, while p-substitution dramatically decreases the selectivity. In the case of a substrate having substituents in both a- and p-positions, high enantioselectivity is observed thus the a-substituent effect overcomes that of the p-substituent. 

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... 

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