Hydroborations dichloroborane-dimethyl sulfide

If alkyldichloroboranes are specifically required, dichloroborane-dimethyl sulfide is the reagent of choice. It is more stable and more convenient than the dichloroborane-diethyl ether complex, but its hydroborating properties are very similar." Dichloroborane complexes ethers even more strongly than monochloroborane, and its reactions with alkenes in this solvent are slow and lead to mixtures. Therefore, it is generally used in pentane and trichloroborane is added to liberate uncomplexed dichloroborane. Under these conditions it readily gives alkyldichloroboranes on reaction with alkenes or alkenyldichloro-boranes on reaction with alkynes. " " The latter reaction has been applied to alkynylsilanes (equation 44)." ... 

Monochioroborane-Dimethyl sulfide, H2BCI-S(CH3)2 Dichloroborane-Dimethyl sulfide, HBCI2 S(CH3)2. These stable hydroborating agents are prepared from borane-dimethyl sulfide (4, 124, 191 5, 47) and boron trichloride dimethyl sulfide (equations I and II). They possess the advantage over chloroborane etherates of indefinite stability. They show regiospecificity comparable to the... 

Hydroboration. This reagent, in contrast to dichloroborane-dimethyl sulfide, hydroborates alVenes directly without necessity for an added Lewis acid such as BCI3. Actually HBBr2 - S(CH3)2 would be expected to be less reactive than HBCl2 S(CH3)3, since it should be more acidic. Irrespective of the theoretical interpretation of the unexpected activity, hydroboration with this reagent leads to synthesis of the previously unknown alkyldibromoboranes, obtained as the S(CH3>2 addition compounds. These complexes can be freed from dimethyl sulfide by distillation in the presence of BBrs. The directive effects are similar to those observed with other borane reagents. 

Hydroboration of Alkenes and Alkynes. The hydroboration of alkenes with dichloroborane-dimethyl sulfide (DCBS) in refluxing dichloromethane produces considerable quantities of R2BCI and R3B as impurities, presumably as the result of disproportionation of the reagent. The addition of boron trichloride liberates dichloroborane and the hydroboration proceeds cleanly at25°C (eq 1). ... 

The products are Hquids, soluble in various solvents and stable over prolonged periods. Monochloroborane is an equiUbtium mixture containing small amounts of borane and dichloroborane complexes with dimethyl sulfide (81). Monobromoborane—dimethyl sulfide complex shows high purity (82,83). Solutions of monochloroborane in tetrahydrofuran and diethyl ether can also be prepared. Strong complexation renders hydroboration with monochloroborane in tetrahydrofuran sluggish and inconvenient. Monochloroborane solutions in less complexing diethyl ether, an equiUbtium with small amounts of borane and dichloroborane, show excellent reactivity (88,89). Monochloroborane—diethyl etherate [36594-41-9] (10) may be represented as H2BCI O... 

Although dichloroborane reacts direcdy with alkenes in the gas phase (118), its complexes with diethyl ether and dimethyl sulfide are so strong that direct hydroboration does not proceed (119,120). The addition of a decomplexing agent, eg, boron trichloride, is necessary for hydroboration. 

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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