Mechanistic Aspects of Bond Activation with Perfluoroarylboranes

Abstract

In the mid-1990s, it was discovered that tris(pentafluorophenyl)borane, B(C(6)F(5))(3), was an effective catalyst for hydrosilylation of a variety of carbonyl and imine functions. Mechanistic studies revealed a counterintuitive path in which the function of the borane was to activate the silane rather than the organic substrate. This was the first example of what has come to be known as "frustrated Lewis pair" chemistry utilizing this remarkable class of electrophilic boranes. Subsequent discoveries by the groups of Stephan and Erker showed that this could be extended to the activation of dihydrogen, initiating an intense period of activity in this area in the past 5 years. This article describes the early hydrosilylation chemistry and its subsequent applications to a variety of transformations of importance to organic and inorganic chemists, drawing parallels with the more recent hydrogen activation chemistry. Here, we emphasize the current understanding of the mechanism of this process rather than focusing on the many and emerging applications of hydrogen activation by fluoroarylborane-based frustrated Lewis pair systems.

Direct observation of a borane–silane complex involved in frustrated Lewis-pair-mediated hydrosilylations

“…This type of bond cleavage has come to be termed "frustrated Lewis pair" (FLP) bond activation. 20,21 In the hydrogenation chemistry, the role of an H2•B(C6F5)3 adduct (I, E = H) is still in question, 22 and proposals that involve the formation of a weak "encounter complex" II, stabilized by secondary C-H•••F interactions, prior to FLP activation of H2 via III have strong computational support. 23,24 Whether the path to III involves an EH•B(C6F5)3 adduct I or an encounter complex II, the bond activation transition state leads to an ion pair IV which proceeds to product upon transfer of hydride from the [HB(C6F5)3] -to the substrate carbon, regenerating the B(C6F5)3 catalyst.…”

“…8,28 This cumulative body of evidence has resulted in a high degree of acceptance for this mechanism in the literature, but the specific nature of the adducts I, or encounter complexes of type II, remain experimentally opaque. 22 29 Si NMR spectroscopy at 233 K. Noting, however, that their procedures did not appear to involve careful drying of the borane before use, it seemed likely that the peak assigned to this adduct is due in fact to Et3SiOSiEt3, formed rapidly when wet borane 35,36 is treated with silane via rapid dehydrogenative silation of water. 37,38 We confirmed this postulate by recording the 29 Si NMR spectrum of an authentic sample of Et3SiOSiEt3.…”

3‐Silylierte Cyclohexa‐1,4‐diene als Vorstufen für gasförmige Hydrosilane: die B(C6F5)3‐katalysierte Transferhydrosilylierung von Alkenen

“…[1] Meist durch Edelmetallkomplexe katalysiert, macht das Design von Katalysatoren auf der Grundlage häufiger vorkommender Übergangsmetalle derzeit erhebliche Fortschritte. [2] Eine Reihe von Triorganosilanen (R 3 SiH) und gesundheitsschäd-lichen Trialkoxysilanen [(RO) 3 SiH mit R = Me oder Et], ebenso wie entflammbares Trichlorsilan (Cl 3 SiH), werden üblicherweise in diesen Katalysen verwendet. Me 3 SiH und Me 2 SiH 2 kommen hingegen selten zum Einsatz, da die Handhabung dieser hochentzündlichen und potenziell explosiven Gase gefährlich sowie im Labormaßstab besonders umständlich ist.…”

“…[2] Eine Reihe von Triorganosilanen (R 3 SiH) und gesundheitsschäd-lichen Trialkoxysilanen [(RO) 3 SiH mit R = Me oder Et], ebenso wie entflammbares Trichlorsilan (Cl 3 SiH), werden üblicherweise in diesen Katalysen verwendet. Me 3 SiH und Me 2 SiH 2 kommen hingegen selten zum Einsatz, da die Handhabung dieser hochentzündlichen und potenziell explosiven Gase gefährlich sowie im Labormaßstab besonders umständlich ist. Praktikable Methoden zur Vermeidung dieser Probleme wären daher für etliche Bereiche der Siliciumchemie von Bedeutung.…”

“…Unsere Arbeitsgruppe klärte den Mechanismus der Si-HBindungsaktivierung durch die starke Lewis-Säure B(C 6 F 5 ) 3 auf, [3,4] bei dem ein durch Hydridabstraktion freigesetztes Siliciumkation als Zwischenstufe ausgeschlossen werden konnte (A!B, aber nicht C + ; Schema 1, oben).…”