A Deceiving X‐ray Single‐Crystal Structure Determination: Amino‐Hydrogen Exchange in Amino‐alkynylboranes and ab initio Investigations of Alkynylboranes, Borirenes, and Boraallenes

Publisher Website
Google Scholar
Abstract
Dimeric(dimethylamino)(phenylethynyl)borane (2)2 is formed by the reaction of bis(dimethylamino)(phenylethynyl)‐borane (1) with 9‐BBN‐H. An X‐ray single‐crystal diffraction study revealed a central B2N2 four‐membered ring for (2)2 with both alkynyl groups pointing to the same side in a cisoid arrangement. However, solution and solid‐state NMR as well as ab initio calculations on model compounds show that the cis arrangement in the crystal chosen for X‐ray diffraction is not representative of the bulk material, which consists of both cis and trans isomers. Further investigations of the competition between hydrogen‐amino group exchange and hydroboration in the reaction of amino‐alkynylboranes with hydroborating agents (9‐BBN‐H and catB‐H) show the exchange to be much faster even in the presence of Wilkinson's catalyst and with cyclic amino‐alkynylboranes such as 1,3‐dimethyl‐2‐[(trimethylsily1)ethynyl]‐1,3,2‐diazaborolidine (3). Ab initio calculations on alkynylboranes I, borirenes II, and boraallenes III, which are all geometrical isomers, show alkynylboranes to be the most stable isomers only if strongly π‐donating groups X (X = NH2, F) are attached to the boron atom. In any case, boraallenes are highest in energy.