Dibromoborane, with alkynes

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). 

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). 

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)." ... 

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 . 

With hydride reagents such as dibal, Z-alkenes can be selectively obtained from alkynes in the phenolic lipid series (ref. 162), and related series of boron reagents greatly supplement the chemical methods of selective reduction and alkyiation. This selectivity has been achieved by the use of less reactive dialkylboranes such as bis(3-methyl-2-butyl)borane (di-isoamylborane), bis(2,3-dimethyl-2-butyl)borane (thexylborane), 9-boraUcyclo[3.kl]nonane (9-BBN) and dicyclohexylborane. Some applications in the polyethenoid field have been summarised (refs. 135,163) and the synthesis of alkenyl compounds generally reviewed (ref. 164). By the use of dibromoborane dimethylsulphide, an internal alkyne can be reduced selectively (ref. 165) as for example in the following way (R = n-alkyl). 

Alternatively, the boronates 25 can be prepared via hydroboration of alkenes/alkynes 20 with dibromoborane followed by alcoholysis of the resulting alkyldibromoborane 24 (Scheme 28.8). As... 

Bromoboration Reactions. BBrj does not add to isolated double bonds, but reacts with allene spontaneously even at low temperature to give (2-bromoallyl)dibromoborane, which provides stable (2-bromoallyl)diphenoxyborane by the addition of anisole. The diphenoxyborane derivative reacts with carbonyl compounds to give 2-bromohomoallylic alcohols in high yields (eq 12). Bromoboration of 1-alkynes provides (Z)-(2-bromo-l-alkenyl)dibromoboranes stereo- and re-gioselectively (eq 13), which are applied for the synthesis of trisubstituted alkenes, a,p-unsaturated esters, and 7,8-unsaturated ketones, bromodienes, 1,2-dihalo-l-alkenes, 2-bromoalkanals, and (3-bromo-oi,3-unsaturated amides. ... 

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