Acyloxy borane

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. 

Fig. 12. Possible monomeric (I and II) and dimeric (III) structures for (acyloxy)boranes in gas phase, solution and solid-state...

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...

Chiral (Acyloxy)borane Complex Catalyzed Asymmetric Diels-Alder Reaction (1 R)-1,3,4-Trimethyl-3-cyclohexene-1-carboxaldehyde. 

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... 

CHIRAL (ACYLOXY)BORANE COMPLEX-CATALYZED ASYMMETRIC DIELS-ALDER REACTION (1R)-1,3,4-TRIMETHYL-3-CYCLOHEXENE-1-CAR BOX ALDEHYDE (3-Cyclohexene-1-carboxaldehyda, 1,3,4-trlmethyl-, (-)-)... 

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). 

Furuta, K. Gao, Q.-Z. Yamamoto, H. Chiral (acyloxy)borane complex catalyzed asymmetric Diels-Alder reaction (lR)-l,3,4-trimethyl-3-cyclohexene-l-carboxaldehyde. Org. Synth. 1998, Coll. Vol. IX, 722-727. 

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... 

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]. 

The reaction with acrylic acid deserves special attention, because acrylic acid is not usually a good Diels-Alder reagent. That the reaction proceeds catalytically and with high ee indicates the facile exchange of the (acyloxy)borane of the cycloadduct with the carboxylic group of unreacted acrylic acid, whereas the monoacrylated tartaric acid remains bound to boron (Fig. 1). 

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. 

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

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