Allylboranes pinene-derived

Chiral addition of allyl metals to imines is one of the useful approaches toward the synthesis of homoallylic amines. These amines can be readily converted to a variety of biologically important molecules such as a-, / -, and y-amino acids. Itsuno and co-workers utilized the allylborane 174 derived from diisopropyl tartrate and cr-pinene for the enantioselective allylboration of imines. The corresponding iV-aluminoimines 173 are readily available from the nitriles via partial reduction using diisobutylaluminium hydride (DIBAL-H) <1999JOM103>. Recently, iV-benzyl-imines 176 have also been utilized for the asymmetric allylboration with allylpinacol boronate 177 in the presence of chiral phosphines as the chiral auxiliaries to obtain homoallylic A -benzylamines 178 in high yield and selectivity (Scheme 29) <2006JA7687>. 

Unsaturated aldehydes are poor substrates for allylation because of their inherent instability toward alkene isomerization accordingly, very few reports describe the allylation of these substrates. However, Lautens et al. were able to overcome this problem via an indirect approach by the use of 2-vinyloxiranes 219 as surrogates for the above-mentioned aldehydes. The in situ formation of these otherwise unstable aldehydes is achieved via the Lewis acid treatment of vinyloxiranes to furnish aldehydes 221 which further undergo allylation with chiral allyl boronates 174 in moderate ee (Scheme 37) <20020L83>. The chiral induction could be further enhanced by the use of allylboranes derived from a-pinene. 

Addition of certain homochiral allylboranes to simple prochiral aldehydes produces homochiral homoallylic alcohols. For example, the allyldialkylborane derived from (+)-a-pinene and allylmagnesium bromide adds to benzaldehyde to give the homoallylic alcohol product in 96% e.e. (Figure B5.3). 

Two novel routes to triethylborane have been reported. Irradiation of bromoethane and aluminium powder with ultrasound gives ethyl aluminium sesquibromide which on treatment with triethoxyborane gives triethylborane in good yields and a laser initiated gas phase reaction between diborane and ethene gives yields of upto 91%. Allylic boranes have been prepared from allylpotassiiim derivatives and chloroboranes. Hydrolysis leads to the Isomerised olefin and the technique has been used to transform (+)-a-pinene into (+)-3-pinene. Condensation reactions between allylboranes and acetylenes have been developed into a convenient method for the synthesis of bicyclo[3.3.l]nonane derivatives. Mainly linear alkyl derivatives of 9-BBN have been synthesised from the, reaction of iron carbonyls and the organoborane in a Fischer-Tropsch type reaction. ... 

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