Disiamyl borane

The hydroboration of a trisubstituted olefin, exemplified by the reaction of 2-methyl-2-butene with diborane, is conveniently stopped at the dialkylborane stage to produce disiamyl-borane. As a result of its rather large steric requirements this... 

The required vinyl boranes and vinyl iodides can both be easily made by tlie hydroboration of alkynes witli disiamyl borane (Sia). Thus the Suzuki reaction is an important methodology for the synthesis of conjugated polyene natural products. 

The mildness of these reagents tolerates the presence of various functional groups such as ester, ether, halogen, and nitrile. The stereospecific cis nature of hydroboration gives exclusively the tram alkenylboranes, often also in high regioisomeric purity (Eq. 53). On the other hand, highly pure (Z)-l-alkenyl-dialkylboranes are prepared without any difficulty via the monohydroboration of 1-halo-1-alkynes with disiamyl-borane or dicyclohexylborane, followed by treatment with t-butyllithium (Eq. 55)106). 

Trialkylboranes including 9-alkyl-9-BBN and alkyl(disiamyl)borane readily undergo the cross-coupling with 1-alkenyl or aryl halides or triflates [8] (Scheme 20). The reaction is limitedly used for primary alkylboranes thus, hydroboration of terminal alkenes with 9-BBN or HB(Sia)2 is the most convenient to furnish the desired boron reagents. Trialkylboranes thus obtained are... 

Conjugated diynes can be reduced to cis enynes by the well-known hydroboration-protonolysis sequence . cw-5-Dodecen-7-yne (215) is obtained from the diyne 213 in 76% yield as shown in equation (23). The orientation of addition of the disiamyl-borane was established by using CH3CO2D in the second step. Addition of a second mole of disiamylborane to 214 is very slow, but reduction to the cis,cis diene can be accomplished by using dicyclohexylborane instead . 

Conjugated enynes are of importance for themselves, as well as for the synthesis of conjugated dienes. The cross-coupling reaction of 1-alkenyl(disiamyl)boranes (3c) with 1-bromo-l-alkynes (Scheme 2-34) provides conjugated enynes in high yields [45]. 

Bis-3-methyl-2-butylborane ( Disiamyl-borane ), 29, 31, 242 Bis-(p-nitrophenyl)hydrogen phosphate, 29-30... 

Two new syntheses of the juvenile hormone, juvabione (57a) have been eompleted. In the first synthesis, i -(-f )-limonene was treated with disiamyl-borane and the adduct oxidised to yield the two alcohols (58a) and (58b) which were separated by fractional crystallisation of their diastereoisomeric 3,5-dinitrobenzoates and subsequent hydrolysis. Eaeh alcohol was separately converted to the corresponding nitrile and these on treatment with isobutyl-lithium gave the optically pure ketones (59a) and (59b). Oxidative modification yielded... 

An a-methyl substituent increases the rate toward 9-BBN over that of the parent 1-alkene due to hyperconjugative affect. However, opposite is true for disiamyl-borane. 

The difficulties encountered are, however, overcome by the use of disiamyl borane and sodium triethylborohydride [3] as the base, in a nonaqueous system. However, 9-BBN is an ideal for this purpose, and a wide variety of cyclopropane derivatives (Chart 16.1) [4-6] are prepared in good yields. 

The great application of boranes in organic synthesis is the regio- and stereoselective hydroboration cis addition) of terminal alkenes (anti-Markovnikov) that is selective when the R groups are large enough. Boranes currently used are disiamyl-borane and 9-BBN. - ... 

In a total synthesis of the macrolactam herbimycin A, the known mannose-derived epoxide (53) (Vol. 22, p. 150) was converted to (54), and hence to the ansa-chain unit (55). The epoxide ring in (53) was opened regioselectively using disiamyl borane/NaBH4. In a route to glidobamine (56), the core structure of the antibiotic glidobactin, the chirality of the allylic... 

FIGURE 14.2 Carbonyl compounds synthesized by addition of water to alkynes (SiajBH is disiamyl-borane, lUPAC name bis (l,2-dimethylpropyl)borane. 9-BBN is 9-borabicyclo[3.3.1]nonane). 

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