Bromoborane

Boron subhaHdes are binary compounds of boron and the halogens, where the atomic ratio of halogen to boron is less than 3. The boron monohaUdes, BCl, [20583-55-5] bromoborane(l) [19961-29-6] BBr, and iodoborane(l) [13842-56-3] BI, are unstable species that have been observed spectroscopicaHy when the respective ttihaUdes were subjected to a discharge (5). Boron dihaUde radicals have been studied, and stmctural and thermochemical data for these species ( BX2) have been deduced (5). 

Boron tnfluonde also reacts with hydroxy acndones to form difluoro boron complexes [111] (Table 28) Another route to fluoroboraties involves transhalogen-ation by fluonnation of the correspondmg chloro- and bromoboranes with lithium or potassium fluonde under mild conditions [112] (Table 29)... 

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]. 

Additionally, the sulfonamide precursor to 75 could be recovered and recycled to regenerate the bromoborane 75 [36], The resulting aldols could then be cyclized to the epoxy esters by treatment with potassium tert-butoxide (Scheme 1.22). 

Finally, allylboron reagents have been synthesized via reactions of bromoboranes and allylstan-nanes n. 

The following computer-designed bromoborane provides high induced and simple dia-stereoselectivity in aldol additions of thiopropionates64c. 

Although dichloroborane has been successfully used to reduce dialkyl sulphoxides, it was less useful for the reduction of diaryl sulphoxides and recent work26 has found that fully substituted bromoboranes can fill this gap. Three reagents were introduced, viz. bromodimethylborane, 9-bromo-9-borabicyclo[3.3.1]nonane (9-BBN-Br) and tribromo-borane, the reactions being shown in equation (5) ... 

Z-Boron enolates can also be obtained from silyl enol ethers by reaction with the bromoborane derived from 9-BBN (9-borabicyclo[3.3.1]nonane). This method is necessary for ketones such as 2,2-dimethyl-3-pentanone, which give E-boron enolates by other methods. The Z-stereoisomer is formed from either the Z- or E-silyl enol ether.20... 

The corresponding haloboranes are also useful for enantioselective hydrobo-ration. Isopinocampheylchloroborane can achieve 45-80% e.e. with representative alkenes.206 The corresponding bromoborane achieves 65-85% enantioselectivity with simple alkenes when used at —78° C.207... 

A similar equilibrium between reaction products is observed when n-butyldi-bromoborane is reacted with bromocyanide, whereas di-n-butylbromoborane forms only an adduct with the latter 26>. 

TABLE 3 5. Reaction of Aldehydes with the Enolate from Diethyl Ketone and Bromoborane (R,R)-55b... 

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. 

Caution All the operations should be carried out in a well-ventilated hood, since bromoborane derivatives fume in air and are rapidly hydrolyzed with the evolution of considerable heat. 

Bromoboronates 247 undergo a wide variety of reactions because of the inherent reactivity. The reduction of bromoborane to boron hydrides 248 can be effectively carried out in the presence of hydride reducing agents such as... 

Dioxa-, oxathia-, and dithiaboroles undergo efficient transmetallation with alkynyl stannanes, furnishing the 5-alkynyl boroles. Thus the reaction of bis(trimethylstannyl) acetylene 273 with 2 equiv of bromoboranes 272 in toluene at room temperature furnishes air-sensitive boroles 274 as colorless solids in good yield (Equation 10) <2004EJI4223>. 

This bromoborane can even effect enantioselective aldol reactions with an acetate ester and an aldehyde (equation II). 

Aldol reactions of propionate esters with aldehydes proceed more readily when promoted by the more Lewis acidic bromoborane (R,R)-5. This reagent is prepared... 

Allylation of aldehydes. The (R,R)-allylborane 6, obtained by reaction of the (R,R)-bromoborane 4 with allyltributyltin, adds to aldehydes to provide homoallylic... 

Alcohols protected as methyl ethers can be retrieved by reaction with tribromo-borane. Activation of the methyl ether by co-ordination of the Lewis acidic tri-bromoborane followed by nucleophilic cleavage of the O—Me bond (with concomitant formation of bromomethane) is typical behaviour. However, the cleavage took a different course with 39.1 [Scheme L39] instead of the O—Me bond being cleaved, the alternative C—O bond cleaved owing to participation of the remote acetoxy group.72 The formation of bromide 39.4 with retention of configuration is circumstantial evidence implicating dioxonium ion intermediate 393. 

The preparation of monochloroborane in ether is capricious and the product is not stable. A more convenient source of monochloroborane is its dimethyl sulfide complex, which is readily prepared by reaction of borane-dimethyl sulfide with either tetrachloromethane (equation 22) or trichloroborane-dimethyl sulfide (equation 23). The latter reaction has also been modified for synthesis of the bromoborane and iodoborane complexes. 

Monochloborane-dimethyl sulfide coexists with small amounts of the borane and dichloroborane complexes, but the bromoborane-dimethyl sulfide complex appears to be almost pure. - These complexes react readily with alkenes at 25 °C and can be used for hydroborations in a variety of solvents. Dialkylha-loboranes are obtained in high yield as their dimethyl sulfide complexes (equation 24), but dimethyl sulfide is readily removed under reduced pressure if required. - The reagents are also useful for cyclic hydroborations of dienes such as cyclooctadiene (equation 25). An alternative approach to dialkylbro-moboranes involves the reaction of dialkyl(methylthio)boranes with bromine. ... 

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