Carbonylation reactions boranes

Moderate yields of acids and ketones can be obtained by paHadium-cataly2ed carbonylation of boronic acids and by carbonylation cross-coupling reactions (272,320,321). In an alternative procedure for the carbonylation reaction, potassium trialkylborohydride ia the presence of a catalytic amount of the free borane is utilized (322). FiaaHy, various tertiary alcohols including hindered and polycycHc stmctures become readily available by oxidation of the organoborane iatermediate produced after migration of three alkyl groups (312,313,323). 

In Scheme 13.8, the side chain was added in one step by a borane carbonylation reaction. This synthesis is very short and the first four steps were used to transform the aldehyde group in the starting material to a methyl ester. The stereochemistry at C(4)-C(7) is established in the hydroboration in Step B, in which the C(7)-H bond is formed. A 1 1 mixture of diastereomers resulted, indicating that the configuration at C(4) has little influence on the direction of approach of the borane reagent. 

This sequence of steps is an important part of the mechanism of the hydro-formylation of alkenes (oxo reaction), to be discussed in Section 31-4B, and also is related to the carbonylation reactions of boranes discussed in Section 16-9G. 

Another major difference with hydrosilylation and organo-boranes, Alkyl-Zr do not react with functional groups such as aldehydes, ketones and alkylating agents [46]. Carbon—carbon bond formation is nevertheless possible through transfer or carbonylation reactions [45]. 

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. ... 

A selectivity determining hydride transfer identical to that for a related B(C6H5)3-catalysed carbonyl reaction has been proposed for hydrosilylation of imines by a silane reactant catalysed by an axially chiral borane (Scheme 8). ... 

If boranes (K. Utimoto, 1973 H.C. Brown, 1975, 1980 A. Pelter, 1979) are used as donor synthons for the alkylation of a, -unsatarated carbonyl compounds, no enolate anion is formed, and the, 8-position of the C=C bond is the only reaction site. 

The reaction between a trinuclear metal carbonyl cluster and trimetbyl amine borane has been investigated (41) and here the cluster anion functions as a Lewis base toward the boron atom, forming a B—O covalent bond (see Carbonyls). Molecular orbital calculations, supported by stmctural characterization, show that coordination of the amine borane causes small changes in the trinuclear framework. 

Several studies of reactions of configurationally unstable a-substituted allylboranes have also been reported19,29. The reactions of dialkyl[( )-l-alkyl-2-butenyl]boranes and aldehydes at — 78 °C provide a mixture of syn- and an/i -diastereomers. reflecting reactions by both the Z-and /f-isomers. When generated and used at — I00°C, however, the ff/m-diastereomer is obtained with >95% diastercoselectivity and >90% selectivity for the /T-olefin isomer by way of a transition state analogous to 429. This result suggests that the allylboron isomerization is slow at —100 JC relative to carbonyl addition. 

Reaction of halo ketones or diazo ketones with boranes 10-112 Carbonylation of alkyl halides... 

Comparable reactions between cyclic divinyl boranes and metal carbonyls lead to borinato (borabenzene) complexes... 

The Leuckart-Wallach reaction is the oldest method of reductive amination of carbonyl compounds. It makes use of formamide, formic acid or ammonium formate at high temperature. The final product is a formamide derivative, which can be converted to an amine by reduction or hydrolysis. The method has been applied to the preparation of 1,2-diamines with a norbornane framework, which are interesting rigid analogues of 1,2-diaminocyclohexanes. As a matter of fact, starting from N-acetyl-2-oxo-l-norbornylamine 222, the diamide 223 was obtained with excellent diastereoselectivity and then converted to the M-methyl-N -ethyl derivative 224 by reduction with borane [ 104] (Scheme 34). On the other hand, when the reac-... 

Diborane also has a useful pattern of selectivity. It reduces carboxylic acids to primary alcohols under mild conditions that leave esters unchanged.77 Nitro and cyano groups are relatively unreactive toward diborane. The rapid reaction between carboxylic acids and diborane is the result of formation of a triacyloxyborane intermediate by protonolysis of the B-H bonds. The resulting compound is essentially a mixed anhydride of the carboxylic acid and boric acid in which the carbonyl groups have enhanced reactivity toward borane or acetoxyborane. 

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