It has been determined from previous computational and experimental data that (F3C)2B=N(CH3)2, 1, undergoes a variety of pericyclic reactions with alkenes, including Diels-Alder cyclization, ene-type reactions, and hydrogen-transfer reactions. Ene-type reactions are of particular interest because they generate zwitterionic acyclic products containing boron, and so might find use in boron neutron capture therapy (BNCT) applications. In this project we systematically studied ene-type reactions between aminoborane 1 and propene derivatives (R1)(R2)C=C(R3)−C(H)(R4)(R5) where the R groups were either (H, CH3), (H, F), or (H, CF3). Alkenes containing electron-donating methyl groups at all sites were predicted to undergo the ene reaction with 1 exothermically, even with the extremely bulky "pentamethylpropene." Exothermicities ranged from ca. –20 to –70 kJ mol–1. Alkenes containing less bulky, but electron-withdrawing fluorine substituents also gave exothermic ene reactions with 1, with exothermicities greater than those for the methyl-substituted alkenes. In contrast, alkenes containing bulky, electron-withdrawing CF3 substituents were predicted to undergo significantly less exothermic ene reactions (&gt; –40 kJ mol–1), with the ene reaction between 1 and "penta-CF3-propene" predicted to be uniquely endothermic at ca. +60 kJ mol–1. The data clearly point to a relationship between steric bulk on the alkene and reaction exothermicity, one not affected much by the electron donating or withdrawing characteristics of the substituents. Transition state barriers behaved similarly, although steric effects appeared to dominate electronic effects more dramatically. When transition states involving methyl-substituted alkenes were examined, barriers ranged from &lt; 30 kJ mol–1 for mono-substituted alkenes to &gt; 100 kJ mol–1 for tetra- and penta-substituted alkenes. Moreover, substitution at the 1 and 2 positions raised barriers far more than substitution at the 3, 4 or 5 positions. Similar behavior was observed for fluoro- and CF3-substituted alkenes. The most extreme behavior was observed for the transition state between 1 and "penta-CF3-propene", with a barrier of &gt; 150 kJ mol–1 and an incipient B–C bond-forming distance of 3.65 Å. This reaction is almost certainly not concerted in the conventional sense, but closer to stepwise. Most other ene reactions observed in this work appeared concerted.

Systematic computational studies of ene reactions between (F3C)2B=N(CH3)2 and substituted alkenes

255th National Meeting (2018)

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