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Keywords:

  • Amines;
  • Boron;
  • Dehydrogenation;
  • Lewis acids;
  • Lewis bases

Abstract

The ability of trialkyl Group 14 triflates in combination with amine and pyridine bases to dehydrogenate amine– and phosphane–borane adducts has been investigated. By using multinuclear NMR spectroscopy, it has been shown that Me2NH·BH3 (11) is efficiently converted to [Me2N–BH2]2 (12) by the so-called “frustrated Lewis pair” (FLP) of nBu3SnOTf (4, OTf = OSO2CF3) and 2,2,6,6-tetramethylpiperidine (6). Within the scope of the study, exchange of the Lewis acid effects the rate of dehydrogenation in the order: 4 > Me3SiOTf (2) > Et3SiOTf (3). Exchange of the Lewis base for 2,6-di-tert-butylpyridine (5) has also been shown to reduce the rate of reaction, whereas 1,3-di-tert-butylimidazol-2-ylidene (7) reacted directly with 2 to afford 1,3-bis-tert-butyl-4-(trimethylsilyl)imidazolium triflate (8[OTf]). For FLP combinations for which dehydrogenation reaction times are longer, detectable quantities of [H2B(μ-H)(μ-NMe2)BH2] (14) are observed. Both the dehydrogenation reaction and competitive formation of this product are proposed to proceed by initial hydride abstraction by the Lewis acid, followed by deprotonation by the Lewis base, or combination with further dimethylamine–borane and elimination of [Me2NH2]OTf (18[OTf]), respectively. In contrast to 11, MeNH2·BH3 (22) was not found to cleanly dehydrogenate to either [MeNH–BH2]3 or [MeN–BH]3 under the same conditions. An alternative reaction pathway was observed with either 2 or 4 and 6 with Ph2PH·BH3 (23), resulting in P-silylation or P-stannylation of the phosphane–borane, respectively.