Hydroboration reductive coupling

Scheme 25 Hydroboration and hydrozirconation strategies for aldehyde/alkyne reductive coupling...

Reductive Coupling Reaction via Hydroboration of Allenylidene Intermediates... 

Dialkyl ketones/ The reductive coupling of alkenes and carbon monoxide at room temperature is promoted by CoCl2-Ph(Et)2N BH3 (7 examples, 50-70%). The generation of ClBHj as hydroborating agent for the alkenes is implicated, and the subsequent carbonylation is assisted by cobalt carbonyl species formed in situ. 

Of all the hydrocarbon derivatives of [60]fullerene, C60H2, first prepared by Henderson and Cahill using a hydroboration-hydrolysis procedure, is the simplest. Multiple other methods have been demonstrated for the formation of C60H2 including two (i.e., a NaBH4 reduction and a Zn(Cu) couple reduction) that produce C60H2 in good isolated yield. 

The epoxidation-epoxide opening sequence with this reagent provides a convenient access to the products of an //-Markovnikov addition of water to olefins. Interestingly, the Cp2TiCl/H20 couple combination shows anti stereoselectivity in the reduction step [73, 74], which is complementary to the hydroboration-oxidation method (Scheme 32). 

Diketo diester 512 has played a key role in one of Paquette s approaches to the pentagonal dodecahedrane. Direct hydroboration-oxidation of502 provided as the principal product the unwanted isomer 511 (49 %) rather than 512 (30 %). This complication was circumvented by the improvisation of the cross-corner oxygenation sequence outlined in Scheme 75.420,42 The iodolactonization of diacid 510 proceeded with high efficiency to give 513 which underwent cleavage to 514 in the presence of methanolic sodium methoxide at room temperature. Oxidation to a-iodo ketone 515 followed by reductive deiodination with zinc-copper couple and ammonium chloride in methanol solution furnished isomerically pure 512. 

Vinyl triflates can be coupled efficiently to various organotin compounds. Further details of this chemistry have now appeared including efficient preparations of trimethylsilyldienes (Scheme 33).71 Two other routes to silyldienes are outlined in Scheme 34. Reduction of allenic alcohols (21) gave moderate yields of the dienyl products, although the stereoselectivity of the process leaves much to be desired. The second method, involving hydroboration of the allene (22) is much more stereoselective and enables either isomer of the final product to be obtained depending on the elimination conditions used. A key application for such dienes is in Diels-Alder reactions, where the products have vinylsilane functionality for further modification. This type of chemistry has now been explored with 2,3-bis(trimethylsilyl)buta-1,... 

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