Boranes alcohols, tert

A more recent example, which involves an enantiomerically pure compound, reverts to the original lead by incorporating a hydroxyl group on the benzylic carbon. Preparation of this close relative of ibutilide (5-3) uses the same starting material. Acylation of w-dibutylamine with the acid chloride from the treatment of (6-1) with tert-butylcarbonyloxy chloride leads to the amide (6-2). Reduction of the carbonyl group in this compound with chloro-(+)-diisopropylcamphemyl borane (DIPCl) proceeds to afford the R alcohol (6-3) in high enantiomeric exess. 

With this strategy, tert-butylmethylphenylphosphine borane and PAMP-BH3 were obtained, although the latter presented low optical purity (41% ee) due to racemisation in the decarboxylation step, which had been performed in xylene at 130 °C for 1 h. The menthyl ester intermediate 60, for R = o-An, was also reduced to the alcohol, which was mesylated and iodinated affording the (I-iodophosphine borane 62 in 62% combined yield. This compound was reductively dimerised by activated copper to diphosphine borane 63 (64% yield), which is similar to DiPAMP but with a butyl bridge. 

New syntheses of tert. alcohols and oxo compds. are the result of investigations of the reaction of boranes with mercaptals C-Sulfenyla-tion of ketones and esters has been developed into a method for 1,2-carbonyl transposition, which furnishes isomeric ketones or converts esters into methyl ketones a-Keto esters can be obtained via a,a-di-sulfenylation of esters . 

In parallel with the search for catalytic systems, has emerged an impressive amount of results in the field of enantioselective allylation. The pioneering work of Marshall using a chiral (acyloxy)borane (CAB) system [216] was readily followed by titanium/BINOL catalysts [217], leading to homoallylic alcohols with enantiomeric excess up to 98%. An extension of this work in fluorous phase was also developed with 6,6 -perfluoroalkylated BINOLs [218]. Replacing the titanium by zirconium (IV) salts, led to more reactive catalyst for the allylation of aromatic and aliphatic aldehydes [219]. One of the more active catalyst is the zirconium-BINOL system associated with 4-tert-butylcalix [4]arene, which remains active with only 2% of the chiral inductor [220]. The use of activators, such as iPrSSiMe3, iPrSBEt2,... 

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