From organoboranes with ketones

Diiron enneacarbonyl, 50, 2J Diketones, from diazoketones and organoboranes, 53/ 82 3-DIKETONES FROM METHYL ALKYL KETONES 3-n-BUTYL-2,4-PENTANEDIONE, 51, 90 2,6-Dimethoxybenzaldehyde, by reduction of 2,6-dime thoxy-benzonitrile with Raney nickel alloy in formic acid,... 

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. 

However, this methodology to homologate a-bromoacetone for the synthesis of methylketones fails. This ketone is extraordinarily sensitive to the action of potassium t-butoxide, an butoxide, an exceptionally strong base. The problem is circumvented by utilizing potassium 2,6-di-t-butylphenoxide. The presence of bulky substituents at ortho positions prevent the organoborane from coordinating with base. 

AUylic organoboranes react via cyclic transition states not only with aldehydes and ketones, but also with alkynes, aHenes, and electron-rich or strained alkenes. Bicyclic stmctures, which can be further transformed into boraadamantanes, are obtained from triaHyl- or tricrotylborane and alkynes (323,438,439). 

Organoboranes from hydroboration reactions can be oxidized directly to ketones with PCC, thus eliminating the need to isolate the intermediate alcohol. ... 

Propargylic organoboranes derived from the corresponding lithium reagents react with aldehydes and certain ketones with high regioselectivity to give trimethylsilyl-substituted a-allenic alcohols (Scheme... 

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. 

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. 

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). 

Additionally, copper was used as a powerful catalyst in the carbonylative coupling of hypervalent iodonium salts with organostannanes and organoboranes as well [75]. In the presence catalytic amount of Cul, ketones were produced in good yields from their parent molecules under mild conditions (Scheme 4.38). 

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