Boranes carbinols

The aziridine carbinols are also effective ligands in the preparation of oxazaborolidine catalysts for the asymmetric ketone reduction with borane (Fig. 4) [551. 

Allylic boranes such as 9-allyl-9-BBN react with aldehydes and ketones to give allylic carbinols. The reaction begins by Lewis acid-base coordination at the carbonyl oxygen, which both increases the electrophilicity of the carbonyl group and weakens the C-B bond to the allyl group. The dipolar adduct then reacts through a cyclic TS. Bond formation takes place at the 7-carbon of the allyl group and the double bond shifts.36 After the reaction is complete, the carbinol product is liberated from the borinate ester by displacement with ethanolamine. Yields for a series of aldehydes and ketones were usually above 90% for 9-allyl-9-BBN. 

The allylation reaction has been extended to enantiomerically pure allylic boranes and borinates. For example, the 3-methyl-2-butenyl derivative of (Ipc)2BH reacts with aldehydes to give carbinols of greater than 90% e.e. in most cases.39... 

Scheme 9.3 illustrates some examples of syntheses of allylic carbinols via allylic boranes and boronate esters. Entries 1 and 2 are among the early examples that... 

A 1 1 mixture of sodium L-prolinate (68) and borane gives rise to carbinols with as much as 62% ee, and the complex prepared by mixing borane, m-nitroaniline and (5)-(69) in a 1 1 1 ratio reduces acetophenone in ee. ... 

Reaction with trialkylboranes. Carbon monoxide added to a solution of a trialkyl-borane formed in situ in diglyme solution reacts at 100° at atmospheric pressure the resulting organoboron intermediate is oxidized by hydrogen peroxide to a tri-alkylcarbinol in excellent yield.1 /-Carbinols with highly branched alkyl groups... 

TETRAHYDROFURFURYL CARBINOL (97-99-4) Combustible liquid (flash point 167°F/75°C). Violent reaction with strong oxidizers. Incompatible with strong acids, aliphatic amines, boranes, caustics, isocyanates. Unless inhibited, can form unstable and explosive peroxides. 

The use of elevated pressures (2,000-6,000 atm.) accelerates asym. reduction of prochiral ketones with B-3-pinanyl-9-borabicyclo[3.3.1]nonane while suppressing competing dehydroboration-reduction which erodes enantioselectivity at atm. pressure. E Neat Alpine-Borane at 0° treated with 3-acetylpyridine via a syringe under Nj, stirred for 20 min at room temp., the mixture [in a capped syringe] placed in a high-pressure cell, pressurized to 6,000 atm. for 1.5 days, and worked up by the standard oxidative procedure - (S)-methyl(3-pyridyl)carbinol. Y 67% (100% e.e. ... 

A new route to monoalkylboranes from borane and alkylidene triphenylphos-phoranes, when used in combination with hydroboration, forms the basis for a synthesis of tertiary carbinols (Scheme ) the free monoalkylboranes are obtained in solution from their triphenylphosphine complexes by quaternization (and hence removal) of the phosphine. 

Reactions of Organoborates. Scheme 9 reveals a fundamental synthetically useful difference between the 3-migration cyanoborate and borane carbonylation reactions.The thermodynamically more stable cyanide adduct is formed prior to electrophile-induced migrations, in contrast to the kinetically determined product resulting from carbonylation. Carbonylation of R3B in the presence of LiAlH(OMe)3 followed by treatment with acid and oxidizing agent is a particularly mild method for the preparation of secondary carbinols. ... 

Aldehydes of different types can be conveniently prepared from the next lower halides through carbinols which are split by dia-zotized sulfanilic acid Aliphatic aldehydes in particular can be easily obtained from carboxylic acids through N-acylethylenimi-es Aldehydes, in turn, can be converted directly into nitriles with hydroxylamine hydrochloride Ketones can be efficiently prepared from ethylene derivatives via boranes and, in high purity, from labile alcohols by oxidation in an aq.-ethereal two-phase medium Heating aliphatic acids with reduced iron powder proved to be an excellent method for the prepn. of sym. straight-chain ketones /5-Aminoketones unobtainable by Mannich reaction may be prepared via halogenomagnesium enolates... 

---