RESUMEN
Cationic Lewis acids (LAs) are gaining interest as targets for frustrated Lewis pair (FLP)-mediated catalysis. Unlike neutral boranes, which are the most prevalent LAs for FLP hydrogenations, the Lewis acidity of cations can be tuned through modulation of the counteranion; however, detailed studies on such anion effects are currently lacking in the literature. Herein, we present experimental and computational studies which probe the mechanism of H2 activation using iPr3SnOTf (1-OTf) in conjunction with a coordinating (quinuclidine; qui) and noncoordinating (2,4,6-collidine; col) base and compare its reactivity with {iPr3Sn·base}{Al[OC(CF3)3]4} (base = qui/col) systems which lack a coordinating anion to investigate the active species responsible for H2 activation and hence resolve any mechanistic roles for OTf- in the iPr3SnOTf-mediated pathway.
RESUMEN
Enantioselective reduction of ketimines was demonstrated using chiral N-heterocyclic carbene (NHC)-stabilised borenium ions in frustrated Lewis pair catalysis. High levels of enantioselectivity were achieved for substrates featuring secondary N-alkyl substituents. Comparative reactivity and mechanistic studies identify key determinants required to achieve useful enantioselectivity and represent a step forward in the further development of enantioselective FLP methodologies.
RESUMEN
The catalytic conversion of N2 to N(SiMe3)3 by homogeneous transition metal compounds is a rapidly developing field, yet few mechanistic details have been experimentally elucidated for 3 d element catalysts. Herein we show that Fe(PP)2(N2) (PP = R2PCH2CH2PR2; R = Me, 1Me; R = Et, 1Et) are highly effective for the catalytic production of N(SiMe3)3 from N2 (using KC8/Me3SiCl), with the yields being the highest reported to date for Fe-based catalysts. We propose that N2 fixation proceeds via electrophilic Nß silylation and 1e- reduction to form unstable FeI(NN-SiMe3) intermediates, which disproportionate to 1Me/Et and hydrazido FeII[N-N(SiMe3)2] species (3Me/Et); the latter act as resting states on the catalytic cycle. Subsequent 2e- reduction of 3Me/Et leads to N-N scission and formation of [N(SiMe3)2]- and putative anionic Fe imido products. These mechanistic results are supported by both experiment and DFT calculations.
RESUMEN
The first cationic Fe silyldiazenido complexes, [Fe(PP)2(NN-SiMe3)]+[BArF4]- (PP = dmpe/depe), have been synthesised and thoroughly characterised. Computational studies show the compounds to be useful structural and electronic surrogates for the more elusive [Fe(PP)2(NN-H)]+, which are postulated intermediates in the H+/e- mediated fixation of N2 by Fe(PP)2(N2) species.