Lewis Base Activation of Di- and Tetrahalodiboranes for Organic and Inorganic Chemistry
Final Report Abstract
The proposal for this project was motivated by a series of discoveries made by our group involving the reactions of di- and tetrahalodiboranes(4), in addition to new, convenient syntheses of the very rare tetrahalodiboranes(4) and their labile-base adducts. These two related families of molecules, despite their long histories, were surprisingly underexplored at the start of this project, a situation thought to be mainly due to their relatively low stability and difficult syntheses. This was particularly true of the tetrahalodiboranes(4), some of which were, until our new synthetic routes, only accessible via gas-phase syntheses. Our development of new routes to these compounds prior to the start of the project opened up a wide range of new avenues to explore. The project was divided into two subprojects: one on the chemistry of dihalodiboranes(4) and one on the chemistry of tetrahalodiboranes(4). It should be noted that the many of the originally-proposed reactivity patterns did not lead to promising results, or provided entirely different results from those we had predicted. In part this is the nature of chemical research, but in this case the situation is amplified by the high reactivity of the diborane species and, more specifically, the large number of labile bonds in each (i.e. one B–B bond, two / four B–Hal bonds, sometimes also labile dative L®B bonds). While these particular properties of di- and tetrahalodiboranes(4) provided unpredictability of the reaction outcomes and led to numerous dead-ends, they also provided a number of surprising results. The more chemistry we did on these species, the more we learned about how to control their reactivity, as some of the later results attest, e.g. the controllable reaction at either the B-B or CºC bonds of alkynyl-appended diboranes(4). This level of control facilitated often-temperamental reactions such as a Pd-catalysed cross-coupling at diborane(4)-bound terminal alkynyl groups, and hydroboration-mediated synthesis of oligomers containing B(sp2)-B(sp2) bonds (vide infra).
