Bromides organoboranes

Organoboranes are reactive compounds for cross-coupling[277]. The synthesis of humulene (83) by the intramolecular cross-coupling of allylic bromide with alkenylborane is an example[278]. The reaction of vinyiborane with vinyl-oxirane (425) affords the homoallylic alcohol 426 by 1,2-addition as main products and the allylic alcohol 427 by 1,4-addition as a minor product[279]. Two phenyl groups in sodium tetraphenylborate (428) are used for the coupling with allylic acetate[280] or allyl chloride[33,28l]. 

Olefin metathesis of vinylboronates [102] and allylboronates [103, 104] has been investigated over the past few years because organoboranes are versatile intermediates for organic synthesis. Cross metathesis of vinylboronate 108 and 2-butene 109, for example, yields the boronate 110, which can be converted to the corresponding vinyl bromide 111 with high Z selectivity. Vinyl iodides can be obtained analogously. It should be noted that vinyl bromides and vinyl... 

The mechanism of these alkylations involves a tetracoordinate boron intermediate formed by addition of the enolate of the a-bromo ester to the organoborane. The migration then occurs with displacement of bromide ion. In agreement with this mechanism, retention of configuration of the migrating group is observed.23... 

Show how organoborane intermediates can be used to synthesize the gypsy moth pheromone E, Z-CH3C02(CH2)4CH=CH(CH2)2CH=CH(CH2)3CH3 from hept-6-ynyl acetate, allyl bromide, and 1-hexyne. 

To prevent the loss of alkyl groups in the 1,2-migration step, easily available organodichloroboranes [43] proved to be the organoboranes of choice. However, when dibromoboranes were used instead, competitive migration of the alkyl group and bromide ocurred which led to the simultaneous formation of the expected secondary amine and tetraazaboroline [44] (Scheme 18). 

Alkyl bromides can also be obtained from organoboranes by treatment with bromine or bromine chloride 170). 

The Overall process of reduction of an alkene to an alkane, or of reduction of an alkyne to either an al-kene or an alkane, via hydroboration requires that the intermediate organoborane be protonolyzed (Section 3.10.1.1). However, simple trialkylboranes are remarkably resistant to hydrolysis. For example, trimethylborane gives only 69% of hydroxydimethylborane after 7 h at 180 °C with 1 equiv. of water. Similar resistance is shown to alcohols, phenols and amines, and even mineral acids do not completely protonolyze trialkylboranes with any ease. Thus, aqueous or anhydrous hydrogen bromide removes only one alkyl group from tributylborane after reflux for 1 h. ... 

The reaction of trialkylboranes with bromine to give primary bromides is also induced with base, but in this case sodium methoxide in methanol is used as base in order to prevent an undesirable oxidative side reaction leading to alcohols.6 Two procedures have been employed. In one. bromine (33% excess) is added to the organoborane in THF followed by a solution of sodium methoxide in methanol, all at 0°. In the other, bromine and base (10% excess) are added simultaneously at 25°. The former procedure is preferable for monosubstituted olefins, the latter is preferable for hindered olefins. Apparently, in this case, all three alkyl groups are utilized since yields of primary bromides are 30-99%. 

A rather general method for homologating secondary alkyl hahdes (e.g., bromides) is by the Ni-catalyzed reaction with organoboranes. Asymmetric induction by the C2-symmetric diamine 19 is now realized ... 

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