Monosilane (SiH4) is far less well behaved than its carbon analogue methane (CH4). It is a colourless gas that is industrially relevant as a source of elemental silicon, but its pyrophoric and ...explosive nature makes its handling and use challenging. Consequently, synthetic applications of SiH4 in academic laboratories are extremely rare and methodologies based on SiH4 are underdeveloped. Safe and controlled alternatives to the substituent redistribution approaches of hydrosilanes are desirable and cyclohexa-2,5-dien-1-ylsilanes where the cyclohexa-1,4-diene units serve as placeholders for the hydrogen atoms have been identified as potent surrogates of SiH4. We disclose here that the commercially available Lewis acid tris(pentafluorophenyl)borane, B(C6F5)3, is able to promote the release of the Si-H bond catalytically while subsequently enabling the hydrosilylation of C-C multiple bonds in the same pot. The net reactions are transition-metal-free transfer hydrosilylations with SiH4 as a building block for the preparation of various hydrosilanes.
The reaction of trans‐M(N2)2(dppe)2 (M=Mo, 1Mo, M=W, 1W) with B(C6F5)3 (2) provides the adducts (dppe)2M=N=N‐B(C6F5)3 (3) which can be regarded as M/B transition‐metal frustrated Lewis pair (TMFLP) ...templates activating dinitrogen. Easy borylation and silylation of the activated dinitrogen ligands in complexes 3 with a hydroborane and hydrosilane occur by splitting of the B−H and Si−H bonds between the N2 moiety and the perfluoroaryl borane. This reactivity of 3 is reminiscent of conventional frustrated Lewis pair chemistry and constitutes an unprecedented approach for the functionalization of dinitrogen.
Dinitrogen is one of the missing small‐molecule targets of the frustrated Lewis pair (FLP) chemistry. The use of a Group 6 metal (Mo or W)/boron comound allows activation of N2 that is reminiscent of conventional FLPs. This mode of activation allows its mild and easy borylation and silylation, under a mechanism in which coordinated N2 acts as the Lewis base component of an FLP.
The present survey serves several purposes. Selected electron-deficient boron Lewis acids catalyze the release of hydrosilanes from cyclohexa-2,5-dien-1-yl-substituted silanes. The two-step process ...consists of a hydride abstraction to generate a silicon-stabilized Wheland complex and capture of the arene-stabilized silicon cation by the borohydride formed in the previous step. The same boron catalyst will then activate the Si–H bond for the reaction with representative π- and σ-donating substrates, alkenes/alkynes and ketones/ketimines, respectively. The net transformation is a transfer hydrosilylation, and the effect that the substitution pattern of the cyclohexa-1,4-diene core and the subsituents at the silicon atom exert on these hydrosilane surrogates is systematically investigated. The results are compared with those obtained employing the hydrosilane directly. Another part of this comprehensive analysis is dedicated to the comparison of literature-known fully or partially fluorinated triarylboranes in both the direct and the transfer hydrosilylation of the aforementioned substrates. The data are tabulated and color-coded, finally providing an overview of promising substrate/reductant/borane combinations. The often different reactivities of π- and σ-basic substrates are explained, and it is shown that the Lewis acidity of the boron atom, estimated by the Gutmann–Beckett method, is not the only decisive feature of these boron Lewis acids. Practical mechanistic models are presented to rationalize the interplay between the Lewis acidity and steric situation at the boron and, likewise, the silicon atom as well as the need for fluorination ortho to the boron atom in certain cases.
The first example of a formal 1,3‐B−H bond addition across the M−N≡N unit of an end‐on dinitrogen complex has been achieved. The use of Piers’ borane HB(C6F5)2 was essential to observe this ...reactivity and it plays a triple role in this transformation: 1) electrophilic N2‐borylation agent, 2) Lewis acid in a frustrated Lewis pair‐type B−H bond activation, and 3) hydride shuttle to the metal center. This chemistry is supported by NMR spectroscopy and solid‐state characterization of products and intermediates. The combination of chelate effect and strong σ donation in the diphosphine ligand 1,2‐bis(diethylphosphino)ethane was mandatory to avoid phosphine dissociation that otherwise led to complexes where borylation of N2 occurred without hydride transfer.
And the B goes on. Application of frustrated Lewis pair (FLP)‐type reactions in dinitrogen coordination chemistry has led to the achievement of 1,3‐B−H bond addition across the M−N≡N unit of a N2 complex. A chelating, strongly σ‐donating phosphine ligand is necessary to observe the title reaction. The use of HB(C6F5)2 is essential as it plays a triple role: N2‐borylation agent, Lewis acid in a FLP‐type B−H bond activation, and hydride shuttle.
Working three shifts: Polyconjugated δ‐diketones are formed stereoselectively in high yields by the gold‐catalyzed rearrangement of 1,6‐diyn‐3‐yl esters. This cascade involves a 1,3‐sigmatropic ...acyloxy shift, a 5‐exo‐dig cyclization of the resulting allenyne, and an unprecedented 1,5‐sigmatropic shift of an acyl fragment. The usefulness of the products was shown by an efficient acid‐catalyzed transformation into a complex polycyclic framework.
1,6-Allenynes are useful mechanistic probes in noble-metal catalysis, since they can give rise to very distinct products in a highly selective fashion. Various cycloisomerization reactions have been ...described, and discrete mechanisms have been postulated. Of particular interest, whereas Alder-ene types of products can be obtained in a variety of ways using noble-metal catalysts (Au, Pt, Rh, ...), hydrindienes have been reported solely with gold and platinum under specific conditions. It was shown in a previous study that this intriguing transformation required the presence of chloride ligands at the active catalytic species. Herein, the factors governing the fate of 1,6-allenynes under cycloisomerization conditions have been studied more thoroughly, revealing a much more complex scenario. The nature of ligands, counterions, and metals was examined, showing that hydrindienes can be isolated in the absence of halides, using electron-rich, bulky triorganophosphines or carbene ligands. This crucial finding could also be used to access hydrindienes in high yields, not only with gold or platinum but also with silver. On the basis of mass spectrometry, NMR spectroscopy, and computations, refined mechanistic scenarios have been put forward, also rationalizing counterion effects. Notably, a metal vinylidene intermediate has been proposed for the formation of the hydrindiene derivatives. Finally, in the presence of tris((triphenylphosphine)gold)oxonium tetrafluoroborate as catalyst, a new pathway has been unveiled, involving gold alkyne σ,π complexes and leading to previously unobserved 2 + 2 cycloaddition compounds.
Set Me3SiH free! The strong Lewis acid B(C6F5)3 catalyzes the release of hydrosilanes from 3‐silylated cyclohexa‐1,4‐dienes with concomitant formation of benzene. Subsequent B(C6F5)3‐catalyzed SiH ...bond activation allows for alkene hydrosilylation (see scheme). The net reaction is an ionic transfer hydrosilylation. The new technique is particularly attractive in the case of otherwise gaseous, highly flammable hydrosilanes.
This Concept article highlights recent research on Lewis acid adducts of dinitrogen complexes, including our contributions. After a reminder of the early works, it is demonstrated that such kind of ...species offers a new platform for dinitrogen functionalization as well as valuable models for the understanding of elementary steps of (bio)catalytic cycles. When possible, parallels regarding this mode of activation from the orbital point of view are drawn between the different systems discussed herein.
Pairing off: After briefly giving an overview of the seminal examples of Lewis pairs formed of a dinitrogen complex with a Lewis acid of the p‐block, this Concept article highlights recent examples of such species and how they can help understand the functioning of nitrogenases or give snapshots of elusive catalytic intermediates in N2 reduction cycles. A parallel can be drawn with frustrated Lewis pair chemistry that may inspire new methods for N2 functionalization.
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