Computational Studies of the Reactions of B10H13− with Alkynes and Olefins: Pathways for Dehydrogenative Alkyne-Insertion and Olefin-Hydroboration Reactions

Abstract

Quantum mechanical computational studies of possible mechanistic pathways for B10H13(-) dehydrogenative alkyne-insertion and olefin-hydroboration reactions demonstrate that, depending on the reactant and reaction conditions, B10H13(-) can function as either an electrophile or nucleophile. For reactions with nucleophilic alkynes, such as propyne, the calculations indicate that at the temperatures (approximately 110-120 degrees C) required for these reactions, the ground-state B10H13(-) (1) structure can rearrange to an electrophilic-type cage structure 3 having a LUMO orbital strongly localized on the B6 cage-boron. Alkyne binding at this site followed by subsequent steps involving the formation of additional boron-carbon bonds, hydrogen elimination, protonation, and further hydrogen elimination then lead in a straightforward manner to the experimentally observed ortho-carborane products resulting from alkyne insertion into the decaborane framework. A similar mechanistic sequence was identified for the reaction of propyne with 6-R-B10H12(-) leading to the formation of 1-Me-3-R-1,2-C2B10H11 carboranes. On the other hand, both B10H13(-) and 4,6-C2B7H12(-) have previously been shown to react at much lower temperatures with strongly polarized alkynes, and the DFT and IRC calculations support an alternative mechanism involving initial nucleophilic attack by these polyborane anions at the positive terminal acetylenic carbon to produce terminally substituted olefinic anions. In the case of the B10H13(-) reaction, subsequent cyclization steps were identified that provide a pathway to the experimentally observed arachno-8-(NC)-7,8-C2B10H14(-) carborane. The computational study of B10H13(-) propylene hydroboration also supports a mechanistic pathway involving a cage rearrangement to the electrophilic 3 structure. Olefin-binding at the LUMO orbital localized on the B6 cage-boron, followed by addition of the B6-H group across the olefinic double bond and protonation, then leads to the experimentally observed 6-R-B10H13 products.