Dibromoborane-Dimethyl sulfide

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... 

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... 

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). 

A )-and (Z)-alkenylboranes, or the corresponding boronic esters or potassium tri-fluoroborates, are conveniently available via hydroboration of terminal alkynes or 1-halo-1-alkynes using either disiamylborane, dicyclohexylborane, dibromoborane dimethyl sulfide, or catecholborane as hydroborating agents. ... 

Z)-1-Alkenylboronates are prepared by hydroboration of 1-bromo-l-alkynes with dibromoborane/dimethyl sulfide complex, followed by treatment of the intermediate (Z)-(l-bromo-l-alkenyl)boronates with potassium triisopropoxyborohydride. H. C. Brown and T. Imai, Organometallics, 1984, 3, 1392. 

Dibromoborane-dimethyl sulfide exhibits high reactivity toward internal alkynes . The relative reactivity of 1-octene, 1-hexyne, (Z)-3-hexene and 3-hexyne is 100, 290, 20 and 5900, respectively. This makes possible selective hydroboration of internal alkynes in the presence of terminal unsaturation. With 1-bromoalkynes Br2HB SMe2 provides [Z]-l-dibromoalkenylboranes which can be transformed into [E]- or [Z]-l-alkenylboronic esters . 

Dibromoborane-Dimethyl sulfide, HBBr2-S(CH3)2. Mol. wt. 233.79, viscous liquid. 

Hydroboration of Alkenes. Dibromoborane-dimethyl sulfide hydroborates alkenes directly without need for a decomplex-ing agent (eq 1). ... 

Hydroboration of Fluoroalkenes. Dibromoborane-dimethyl sulfide (DBBS) has demonstrated to be an excellent hydrob-orating agent. Based upon kinetic and mechanistic studies, it is clear that the hydroboration reaction exhibits a second-order kinetics (detaching of Me2S from the boron center follows a dissociative pathway, while the hydroboration process follows an associative mechanism).DBBS hydroborates a wide variety of alkenes and alkynes. Perfluoroalkylethylenes and 2, 3, 4, 5, 6 -pentafluorostyrene undergo Markovnikov hydroboration (>92% regioselectivity) (eq 12). ... 

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