Dibromoborane

Retardation of the reaction rate by the addition of dimethyl sulfide is in accord with this mechanism. Borane—amine complexes and the dibromoborane—dimethyl sulfide complex react similarly (43). Dimeric diaLkylboranes initially dissociate (at rate to the monomers subsequentiy reacting with an olefin at rate (44). For highly reactive olefins > k - (recombination) and the reaction is first-order in the dimer. For slowly reacting olefins k - > and the reaction shows 0.5 order in the dimer. 

Dihalogenoboranes are conveniently prepared by the redistribution of borane—dimethyl sulfide with boron trihaUde—dimethyl sulfide complexes (82,83), eg, for dibromoborane—dimethyl sulfide [55671-55-1] (14). 

Dibromoborane—dimethyl sulfide is a more convenient reagent. It reacts directly with alkenes and alkynes to give the corresponding alkyl- and alkenyldibromoboranes (120—123). Dibromoborane differentiates between alkenes and alkynes hydroborating internal alkynes preferentially to terminal double and triple bonds (123). Unlike other substituted boranes it is more reactive toward 1,1-disubstituted than monosubstituted alkenes (124). 

Usually, organoboranes are sensitive to oxygen. Simple trialkylboranes are spontaneously flammable in contact with air. Nevertheless, under carefully controlled conditions the reaction of organoboranes with oxygen can be used for the preparation of alcohols or alkyl hydroperoxides (228,229). Aldehydes are produced by oxidation of primary alkylboranes with pyridinium chi orochrom ate (188). Chromic acid at pH < 3 transforms secondary alkyl and cycloalkylboranes into ketones pyridinium chi orochrom ate can also be used (230,231). A convenient procedure for the direct conversion of terminal alkenes into carboxyUc acids employs hydroboration with dibromoborane—dimethyl sulfide and oxidation of the intermediate alkyldibromoborane with chromium trioxide in 90% aqueous acetic acid (232,233). 

An alternative procedure for oxidation to ketones involves treatment of the alkylborane with a quaternary ammonium perruthenate salt and an amine oxide186 (see Entry 6 in Scheme 4.9). Use of dibromoborane-dimethyl sulfide for hydroboration of terminal alkenes, followed by hydrolysis and Cr(VI) oxidation gives carboxylic acids.187... 

The dimethyl sulfide complex of dibromoborane 215 and pinacolborane216 are also useful for synthesis of E-vinyl iodides from terminal alkynes. 

In some cases monomerization of dimeric iminoboranes (e.g., trichloro-methylchloromethyleneamino)dichloroborane and (trichloromethylbromo-methyleneamino)dibromoborane) can be observed on dissolving the materials in chlorinated saturated hydrocarbons. These compounds are monomeric in the gas phase and consequently have an unexpectedly low boiling point 24>. 

The corresponding haloboranes are also useful for enantioselective hydroboration. Isopinocampheylchloroborane can achieve 45-80% e.e. with representative alkenes.162 The corresponding dibromoborane achieves 65-85% enantioselectivity with simple... 

An efficient process for one-carbon homologation to aldehydes is based on cyclic boronate esters.17 These can be prepared by hydroboration of an alkene with dibromobor-ane, followed by conversion of the dibromoborane to the cyclic ester. The homologation step is carried out by addition of methoxy(phenylthio)methyllithium to the boronate ester. The migration step is induced by mercuric ion. Use of enantioenriched boranes and boronates leads to products containing the groups of retained configuration.18... 

Hydrometallation reactions of carbon-carbon MULTIPLE BONDS Hydroalumination Diisobutylaluminum hydride, 189 Hydroboration General methods Dibromoborane-Dimethyl sulfide,... 

Dibromoborane-Dimethyl sulfide, 92 Dichloro(dimethylamino)borane, 120 Diisobutylaluminum hydride-Boron trifluoride etherate, 116 Diisopinocampheylborane, 117, 182 Diisopinocampheylmethoxyborane, 86 Dimesitylmethylborane, 8 Dimethoxy[l-trimethylsilyl-l,2-buta-dienyl]borane, 218 (R,R)- and (S,S)-frans-2,5-Dimethyl-borolanes, 119... 

Trimethylamine-dibromoborane has been reported in the literature only once.7 It was prepared by the reaction of boron tribromide with trimethylamine-borane, in carefully controlled... 

Divalent cations of boron have been synthesized by partial displacement of bromide from boron tribromide adducts by using substituted pyridines.1 This reaction may lead to complete substitution of bromine with sufficiently basic amines or if the reaction conditions are not properly controlled. The present synthesis avoids this side reaction by blocking the fourth coordination position on boron with hydride which is not readily displaced by amines.2 The starting material in this case is an adduct of dibromoborane, (CH3)8NBHBr2, which is readily synthesized and is described in Sec. 20B. 

To prevent the loss of alkyl groups in the 1,2-migration step, easily available organodichloroboranes [43] proved to be the organoboranes of choice. However, when dibromoboranes were used instead, competitive migration of the alkyl group and bromide ocurred which led to the simultaneous formation of the expected secondary amine and tetraazaboroline [44] (Scheme 18). 

E)-(2-Bromoethenyl)dibromoborane, BrCH==CHBBr2, can be used in place of 1 to couple alkylzinc chlorides with an alkyl chloride to form a monoene, but this reagent is unsatisfactory for coupling of alkenylzinc chlorides.1... 

Among the haloboranes, dibromoborane-methylsulfide has proven to be an excellent hydroborating reagent. Its reaction with terminal and internal alkynes cleanly affords the corresponding vinylic dibromoboranes without the concomitant formation of 1,1-dibora derivatives. This reagent reacts only with internal triple bond in the presence of terminal double bond, and with disubstituted terminal double bond in the presence of monosubstituted one 36c). 

However, this method has an obvious disadvantage one of the R group is wasted. The final solution to this problem has now been provided with dibromoborane-methyl sulfide (Eq. 67) 122). 

A more convenient procedure for the synthesis of ( )-alkenes has been reported 75). This involves the hydridation step with lithium aluminium hydride at 0 °C. The loss of thexyl group is also undesirable and it can be avoided by use of dibromoborane-dimethylsulfide (Eq. 73) 25 l22>. This procedure appears to be general for essentially all R groups with no disadvantages known. There is no loss of R groups. 

E and Z boronic esters (13) and (14) were made easily accessible by bromoboration of acetylene followed by esterification of the resulting dibromoboranes 2. The Pd(O) cross-... 

The most convenient dihaloborane reagent is the dibromoborane-dimethyl sulfide complex. It is commercially available as a neat liquid or is readily prepared by a redistribution reaction, of a type which is also applicable to chloro and iodo analogs (equation 43). Unlike the chloro analog, however, it reacts readily with alkenes ° " ° and alkynes in dichloromethane without need for a decomplexing agent such as trichloroborane. 

The selectivities exhibited by dibromoborane-dimethyl sulfide are also interesting. In contrast to the situation with 9-BBN-H, internal alkynes are hydroborated faster than either terminal alkynes or terminal alkenes. Unlike reactions with disiamylborane, 1,1-disubstituted ethylenes are hydroborated in preference to simple 1-alkenes. These properties can lead to interesting possibilities for selective hydroboration of polyunsaturated molecules (e.g. Schemes 8 and 9)." ... 

Dibromoborane-dimethyl sulfide exhibits regioselectivity in hydroboration of alkenes which is comparable to that exhibited by dialkylboranes. Excess alkene should be avoided in reactions of trisubstituted ethylenes, otherwise hydrogen bromide liberated during work-up may add to the excess ethylene and cause problems. 

Alkyldibromoboranes isomerize only slowly and dibromoborane-dimethyl sulfide may have advantages over borane-THF for hydroboration of problematical alkenes such as 1-methylcyclooctene" (compare equation 30, Section 3.10.4.2). 

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