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The Reactivity of Borocations with Alkynes
[Thesis]. Manchester, UK: The University of Manchester; 2015.
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Abstract
It has been found that by combining various borocations or neutral electrophilic boranes with a multitude of alkynes, interesting and often highly selective borylation reactivity is observed. The reactions can be loosely categorised as de-(elemento)boration of the alkyne, haloboration and carboboration. Direct alkyne de-(elemento)boration was achieved as both dehydroboration and desilylboration, generating a selection of borylated alkynes. Other examples of more complicated alkyne borylation were also observed, such as the boroamination of TMS-ethylene.Upon discovering that 2-N,N-dimethylaminopyridine ligated dichloro-boronium salt underwent selective 1,2-haloboration with a range of terminal alkynes, and that the dibromo-analogue could haloborate a limited number of internal alkynes, the more Lewis acidic borenium salt [Cl2B(2,6-lutidine)][AlCl4] was synthesised and reacted with a series of internal alkynes to give haloborated products. These included dialkyl and diaryl internal alkynes containing a range of functional groups including thioether, methoxyphenyl, vinyl and halide. Each proceeded with excellent stereo- and regio-selectivity, with the boron and the halide added mutually cis, and the regioselectivity determined by the electronically most stable form of the carbocation intermediate. The initial dihalovinyl borane products were esterified in-situ to provide the more stable pinacol boronate esters.The elementoboration method was expanded via modification of the borocations to include different transfer groups that, upon reaction with trimethylsilyl-substituted alkynes, underwent 1,1-carboboration, with migration of the TMS-group. This method also provided access to a small selection of borylated dienes, resulting from reactions with excess alkyne. It was also shown that some neutral boranes reacted analogously with certain TMS-alkynes, albeit with limitations on scope. In addition to this, two types of 1,2-carboboration were discovered. The first involved intercepting products from alkyne haloboration with a TMS-alkyne, undergoing a vinylboration to produce 1-boradiene products. The second was an example of an intermolecular trans carboboration, where the initially formed vinyl carbocation is intercepted by a thiophene.