Chiral bromoboranes

Corey used a chiral bromoborane 75 (1.1 equiv.) to promote the addition of tert-butyl bromoacetate (76) to aromatic, aliphatic, and a,P-unsaturated aldehydes to give the halo alcohols 77 with high enantio- and diastereoselectivities (Table 1.10) [35]. 

Either antipode of bromoborane 332 can be prepared in a six-step sequence from benzil (330, Scheme 11-19) [127, 254, 258J. Reaction of benzil with cyclohexanone in the presence of ammonium acetate and acetic acid generates a cyclic bis-imine which is subsequently reduced with lithium in ammonia. The resulting racemic fra .s-imidazolidine is subsequently hydrolyzed to the diamine 331. Resolution of 331 is accomplished by crystallization with either antipode of tartaric acid. The enantiomerically enriched stein ligand 331 is then sulfonylated and condensed with boron tribromide, giving the chiral bromoborane 332. Transmetalla-tion of allyltri-n-butylstannane with bromoborane (R,/ )-332 then affords the allyl-boron reagent (R,/ )-198. 

Chiral bromoboranes. The chiral reagents, such as (1), can be prepared from BBrj, Me,SiH, and chiral terpenes (e.g., a-pinene). 

Chiral Bromoborane Reagents. Complexes made from chiral l-alkyl-2-(diphenylhydroxymethyl)pyrrolidines and BBra are effective catalysts for asymmetric Diels-Alder reactions. Bromoboranes prepared from chiral 1,2-diphenyl-l,2-bis(arenesulfonamido)ethanes are used to prepare chiral allylic boranes, allenylic borane, propargylic boranes, and enolates. The R-bromodiazaborolidinene (1), prepared... 

Cross-linked polymers bearing IV-sulfonyl amino acids as chiral ligands were converted to polymer bound oxazaborolidine catalysts by treatment with borane or bromoborane. In the cycloaddition of cyclopentadiene with methacrolein, these catalysts afforded the same enantioselectivities as their non-polymeric counterparts238. 

Corey and co-workers developed the highly enantioselective allylboron reagent 198 [127], whose chiral 1,2-diamino-1,2-diphenylethane (stein) auxiliary [254] serves as the source of asymmetry. In an extension of this methodology, Williams et al. have demonstrated the utility of the bromoborane 332 for the preparation of synthetically complex allylborane reagents [255] and have applied this methodology in two natural product syntheses [256, 257] (see below). 

The efficient transmetalation of allylic stannanes to allylboron reagents has generated an attractive methodology for asymmetric allylation. Corey and coworkers first described the use of enantiomers of bromoborane 228 (Scheme 5.2.51) for mild and quantitative transmetalation of allylstannane to yield the allylboron reagent 229. i The asymmetry in the bis-toluenesulfonamide of 228 is derived from l,2-diamino-l,2-diphenylethane, and both antipodes are readily available in high optical purity, by resolution of the starting diamines producing (R,R)- and (5, 5 )- Stein chiral auxiliaries in transmetalation product 229. 

Aldol Reactions. sjw-Aldol adducts can be formed enantio-selectively from the reaction of diethyl ketone and various aldehydes using bromoborane R,R)- as a chiral controller (eq 7). Reactions typically proceed in 85-91% yield with >95% ee. This process led to the highly efficient synthesis of the rice and com weevil aggregation pheromone sitophilure 19 (R = C2H5). Here the bis(tosyl)amide was easily recovered in high yield, since the aldol products were soluble in hexanes, but the chiral backbone... 

Formation of Chiral Propa-l,2-dienyl and Propargyl Alcohols. Reaction of bromoborane R,R)- with propadienyltri-... 

Some advancement has been described for the development of a catalytic enantioselective ester enolate Claisen rearrangement. The strong Lewis basic carboxylate functionality present in the Claisen product effectively coordinates with Lewis or Bronsted acids prohibiting the catalytic turnover. Corey has reported the use of stoichiometric bromoborane 175 to generate chiral boron enolate which undergoes [3,3]-sigmatropic rearrangement to yield 176. ° Kazmaier relied on excess quinidine 178 to provide for asymmetric induction in the conversion of 177 to amino acid 179. ... 

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