Alkynes dibromoborane-dimethyl sulfide

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). 

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)." ... 

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 . 

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). ... 

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

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