Ketones reaction 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). 

The syntheses of carbon-13 labelled trans-tricyclic ketones 93 and 94 have been carried out79 via the cyanidation reaction of organoborane (equation 43). The isomerically pure tram -tricyclic ketones 93 and 94 were needed in connection with the undertaken synthesis... 

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. 

The reaction of organoboranes with catechol dichloromethylene ether in the presence of methyllithium provides a synthesis of symmetrical ketones (Eq. 34) From the result of a crossover experiment by using a mixture of two different trialkylboranes, the reaction was confirmed to proceed through intramolecular reaction pathways. 

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. 

Stereoselectivity in addition reactions of borane with bicyclic alkenes is similar to that observed for reduction of bicyclic ketones with hydride reducing agents. Boranes add to norbornene derivatives from the exo face (27 gave 85% exo attack upon reaction with diborane). In 27-30, all values are percentage yields of the organoborane products obtained by reaction from that face. Interestingly, only 80% exo attack was observed when 27 reacted with disiamylborane, and the size of the borane appears to have little effect on selectivity. 

As early as 1986, Kojima and colleagues reported on the carbonylative coupling of aryl iodides or benzyl halides with organoboranes in the presence of a catalytic amount of a palladium catalyst [9]. This was the first application of organoboranes in carbonylative coupling reactions mediated by 1.1 equivalent of Zn(acac)2 to favor the transmetallation. Various ketones have been produced in good yields starting from aryl iodides and benzyl chloride (Scheme 4.2). 

The base, however, acts to abstract the proton from the a-haloketones to produce the a-halocarbanion. The latter is immediately removed by the reaction with the free, uncomplexed organoborane, and the transfer of alkyl group from boron occurs and product methyl ketone is obtained by the hydrolysis of the intermediate (Scheme 7.4) [2c]. It is significant to mention that protonolysis of the reaction intermediate by 2,6-di-f-butylphenol produced in the reaction is relatively difficult. Consequently, ethyl alcohol is added to liberate ketone. 

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