Boranes applications, synthetic

The section will cover various aspects of the synthetic route development and the scale up to make several kilograms of final compound 1. As will be seen, a novel sulfoxide-directed stereospecific borane reduction was discovered in this effort and the scope and application of this reaction will be discussed in Section 5.2 [5],... 

Another example of great synthetic interest, involves the hydroboration reaction of alkenes [62], In general, the addition of borane to alkenes proceeds stepwise, the final product being the trialkylborane. However, hindered alkenes react slowly, especially when the dialkylborane precipitates from the medium. It was found that trialkyl bor-anes could be obtained rapidly under sonication, even with highly hindered substrates (Eq. 3.5). Applications of this useful modification were published, among which were the reduction-hydroxylation of vinyl groups by 9-BBN [63,64]. 

A relatively new method for thermal radical initiation has gained popularity in synthetic applications. A small amount of Et3B is added to the reaction mixture that has not been rigorously deoxygenated." " The borane reacts with oxygen, apparently generating ethyl radicals. A major advantage for this mode of radical initiation is the low reaction temperatures (as low as —78 °C) that can be achieved when stereoselective reactions are desired." ... 

SCHEME 17. Synthetic application chiral secondary alcohols produced by the asymmetric borane reduction. 

The chemistry of unsaturated organoboranes often differs markedly from that of their saturated analogues. Both vinylic and allylic boranes react readily with many substrates toward which trialkylboranes are inert90 . Allylic boranes can be synthesized selectively via direct hydroboration of an appropriate allene or conjugated diene and are of immense synthetic importance 91 93). Mikhailov in his book and review 7,94) has documented the synthetic applicability of allylic boranes with caution of high thermal reactivity with respect to allylic rearrangement. For example, (l-methyl-2-propenyl)dialkylboranes rearrange spontaneously to the 2-butenyl isomer even at -78 °C (Eq. 40). 

Photochemical transformations are widely employed in both organic and organoinetallic chemistry and have been extensively used as synthetic strategies for the formation of a variety of new compounds.1 Part of our recent research has focused upon an exploration of the photochemical reactions of borane and metallaborane clusters. As a contextual setting for these photochemical studies, we have also explored several aspects of the thermal and redox chemistry of these clusters. In this paper, we present a summary of our recent work on the thermal, photochemical, and redox reactions of borane and metallaborane clusters. In addition, recent work directed toward the application of these clusters to several aspects of molecular electronics will be presented. 

The most important synthetic application of borane is for the preparation of alkyl boranes by addition to alkenes, aprocess known as hydroboration (5.1). Borane and its derivatives can also be used for reduction (see Section 7.3). The hydroboration reaction has been applied to a large number of alkenes of widely differing structures. In nearly all cases the addition proceeds rapidly at room temperature, and only the most hindered alkenes do not react. 

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