Dihalo boranes

With two molar amounts of the hydrogen chloride or bromide, the dihalo-borane adducts of phosphine were formed. 

Because they serve as versatile starting materials for a variety of other boranes, the dihalo boranes, particularly the fluoro and chloro derivatives, are the most important members of this class of compounds. Their chemistry, along with other pentafluor-ophenyl boron halides, has been reviewed recently from a personal perspective by Chi vers,15 whose PhD thesis described early explorations into the chemistry of these compounds. Most of the known literature on these compounds deals with pentafluor-ophenyl compounds, but the chemistry is likely extendable to other compounds with different AiF groups. 

The only practical synthesis of the highly reactive borole ring is based upon ( , )-(l,2,3,4-tetraaryl-l,3-butadien-l,4-ylidene)lithium (86) which is treated with an aryl (dihalo)borane in ether to form the relatively stable pentaarylboroles (87) (Equation (17)) <86JA379>. 

The reaction can be extended to ot,ot-dihalo esters1557 and a,a-dihalo nitriles.1558 It is possible to replace just one halogen or both. In the latter case the two alkyl groups can be the same or different. When dialkylation is applied to dihalo nitriles, the two alkyl groups can be primary or secondary, but with dihalo esters, dialkylation is limited to primary R. Another extension is the reaction of boranes with -y-halo-a.p-unsaturated esters.1559 Alkylation takes place in the -y position, but the double bond migrates, e.g.,... 

Selective reduction of ot,a-dihalo ketones." Reduction of a,a-dihalo ketones can he effected without hydrogenolysis of the halo groups with either DIBAH or borane dimethyl sulfide. Reactions with the former reagent are generally faster but work-up can be complicated by gelatinous aluminum salts. In general, the yields are roughly comparable. 

Tris(organoamino)boranes have been utilized to prepare, in reasonable yields,4,5 mono- and dihalo(organoamino)boranes which are often difficult to obtain by direct amination of the boron trihalides. Carboxylic acids, 1,3-diketones, ketones, and /3-ketoesters have been converted into carboxamides, enamino-ketones, enamines, and j -enamino-amides, respectively, by reaction with an appropriate tris(organoamino)borane under very mild conditions.6 Sulfenamides (R2NSC6H5) have also been prepared in high yield from selected tris(organoamino)boranes and sulfenic esters under relatively mild conditions.7... 

A general synthesis of mixed trialkylboranes and alkylalkenylalkynyl-boranes is based on a hydroboration-reduction sequence starting with dihalo-genoborane ... 

From dihalo(amino)boranes two products are possible, depending on stoichiometry. For a 1 1 ratio, only one chloride is displaced by RjSn ... 

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