Aldehydes reactions with organoboranes

Usually, organoboranes are sensitive to oxygen. Simple trialkylboranes are spontaneously flammable in contact with air. Nevertheless, under carefully controlled conditions the reaction of organoboranes with oxygen can be used for the preparation of alcohols or alkyl hydroperoxides (228,229). Aldehydes are produced by oxidation of primary alkylboranes with pyridinium chi orochrom ate (188). Chromic acid at pH < 3 transforms secondary alkyl and cycloalkylboranes into ketones pyridinium chi orochrom ate can also be used (230,231). A convenient procedure for the direct conversion of terminal alkenes into carboxyUc acids employs hydroboration with dibromoborane—dimethyl sulfide and oxidation of the intermediate alkyldibromoborane with chromium trioxide in 90% aqueous acetic acid (232,233). 

Organoboranes do not normally react with carbonyl compounds in Grignard-like fashion with the exception of allylboranes. Aromatic aldehydes reacted with dialkylboron chloride derivatives in the presence of base to generate arylalkylmethanols in good yields (Equation (132)). On the other hand, reactions of aromatic aldehydes with dialkylboron chlorides in the presence of oxygen resulted in chlorination (Equation (133)).595... 

The reaction of organoboranes with carbon monoxide is one of the most versatUe reactions among synthetic applications of organoboranes. Triorganoborane-CO adducts, although not stable enough to be isolated, are isoel Aronic with cyano-trialkylborates. A wide variety of primary, secondary and tertiary alcohols, aldehydes, and ketones have been synthesized by this reaction. Since such reactions have been extensively reviewed only recent progress is presented here. 

Homopropargylic alcohols ot-allenic alcohols. The synthesis of alkylallenes (5, 397) by the reaction of trialkylboranes and lithium chioropropargylide, ClCH2C=CLi (1), has been extended to these two systems. Thus addition of acrolein to the organoborane a can lead to either 2 or 3, depending on the temperature at which a is kept before reaction with the aldehyde (equation I). If the aldehyde is... 

Enol borinates are also intermediates in the reaction of organoboranes with a, -unsaturated ketones and aldehydes. Thus the reaction of triethylborane and 3-mcthyl-3-butene-2-one (5) followed by addition of (2, in DMSO) affords the Mannich base (6) in 87% yield. 

B-l-Alkenyl-9-borabicyclo[3.3.1]nonanes (1). These substances are prepared by hydroboration of alkynes with 9-BBN (2, 31 3, 24-29 4, 41 5, 46-47 6, 62-64). -Allylic alcohols. Usually reaction of organoboranes with carbonyl groups results in reduction. However, B-l-alkenyl-9-borabicyclo[3.3.1.]nonanes (1) add to simple aldehydes to give, after oxidation, allylic alcohols, with retention of configuration (equation I). The reaction is a Grignard-like synthesis and should be a useful route to allylic alcohols, even to those containing functional groups. 

The reduction is bimolecular and thus the rate is dependent on concentration. Running the reaction neat provides the fastest rates. Usually an excess of Alpine-Borane is used to insure that the reaction does not become excessively slow at the end of the reduction. The excess organoborane may be destroyed by addition of an aldehyde such as Acetaldehyde. The resulting alkoxy-9-BBN may be treated with Ethanolamine to liberate the alcohol and precipitate the majority of the 9-BBN. Any remaining borane impurities may be removed by oxidation with basic Hydrogen Peroxide. 

The reaction of alkenes with borane, monoalkyl and dialkylboranes leads to a new organoborane (see 15-16). Treatment of organoboranes with alkaline H2O2 oxidizes trialkylboranes to esters of boric acid." This reaction does not affect double or triple bonds, aldehydes, ketones, halides, or nitriles that may be present elsewhere in the molecule. There is no rearrangement of the R group itself, and this reaction is a step in the hydroboration method of converting alkenes to alcohols (15-16). The mechanism has been formulated as involving initial formation of an ate complex when the hydroperoxide anion attacks the electrophilic boron... 

Condensation reactions. Nitriles activated through coordination to Rh become nucleophilic toward aldehydes in DMSO such that p-hydroxy alkanitriles are formed at room temperature. The Rh complex also promotes transfer reaction of an organoborane to conjugated esters, and those with additional bonding opportunities cyclic structures may be erected. ... 

The reactions may be run on a large scale but is advisable to cool the reaction to 0 "C or 25 "C since the reduction may become exothermic. Usually an excess of organoborane is used since the bimolecular process becomes very slow towards the end of the reduction. Excess organoborane is destroyed by the addition of a low-boiling aldehyde such as acetaldehyde. This helps prevent contamination of the desired product with isopinocampheol. The borane component may be removed by precipitation as an ethanolamine complex or by an oxidative workup using hydrogen peroxide and base. Alternatively, these two steps may be combined, with the majority of the borane removed with ethanolamine and any remaining boron components removed by oxidation. 

Oxidation of organoboranes to alcohols is usually effected with alkaline hydrogen peroxide. The reaction is of wide applicability and many functional groups are unaffected by the reaction conditions, so that a variety of substituted alkenes can be converted into alcohols by this procedure. Several examples have been given above. A valuable feature of the reaction is that it results in the overall addition of water to the double (or triple) bond, with a regioselectivity opposite to that from acid-catalysed hydration. This follows from the fact that, in the hydroboration step, the boron atom adds to the less-substituted carbon atom of the multiple bond. Terminal alkynes, for example, give aldehydes in contrast to the methyl ketones obtained by mercury-assisted hydration. 

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