Activation of SiH, BH, and PH Bonds at a Single Nonmetal Center

Abstract

For many years, it was believed that only transition-metal centers could activate small molecules and enthalpically strong bonds. However, it has recently been shown that several nonmetallic systems are capable of some of these tasks. [1,2] For example, stable singlet carbenes can activate CO, [3a] H 2 , [3b] and P 4 .[3c-e] Such reactions have long been known for transition metals. [4,5] However, stable singlet carbenes can also activate NH 3 ;[3b] a much more difficult task for transition metals. [6,7] The oxidative addition of hydrosilanes, hydroboranes, and hydrophosphines at vacant coordination sites of transition metals are well-exemplified and are considered as key steps in the transition-metal-catalyzed hydrosilylation, hydroboration, and hydrophosphination of multiple bonds.[8]Herein, we report the first examples of the activation of E À H bonds (E = Si, B, P) at a single nonmetal center.On the basis of our successful results with H 2 , [3b] we began our study with the activation of SiÀH bonds. Indeed, silanes are similar to H 2 in that they lack both nonbonding electron pairs and p electrons. They can bind to various metal centers to form stable Si À H s complexes, which undergo subsequent oxidative addition.[4] To test the possible activation of Si À H bonds with carbenes, we treated the cyclic (alkyl)-(amino)carbenes (CAACs) 1 a and 1 b [9] with primary, secondary, and tertiary silanes.The addition of phenylsilane to 1 a and 1 b occurred readily at room temperature, and the corresponding adducts 2 a,b were isolated in 91 and 83 % yield, respectively (Scheme 1). As expected, in the case of the enantiomerically pure CAAC 1 a, two diastereomers 2 a,a' were formed (in a 2:1 ratio), as shown by two singlets at d = À36.4 and À29.3 ppm in the 29 Si NMR spectrum. The 13 C NMR spectrum revealed the loss of the carbene signal and a new C À H peak at d = 63.2 (2 a) and 65.5 ppm (2 b). The 1 H NMR spectrum of the major isomer 2 a revealed a pseudotriplet at d = 4.78 ppm (SiCH) and two doublets at d = 4.29 and 4.21 ppm corresponding to the diastereotopic hydrogen atoms of the SiH 2 fragment. The structure of 2 a was confirmed by X-ray crystallography [10] (Figure 1, top), whereas the presence of a triplet at d = 4.53 ppm and a doublet at d = 4.08 ppm in the 1 H NMR spectrum confirmed the identity of adduct 2 b.CAACs 1 a,b also reacted with (EtO) 3 SiH to afford 3 a (d.r. 3:1) and 3 b in 64 and 73 % yield, respectively. However, when Ph 2 SiH 2 was used, only the less bulky carbene 1 b underwent insertion into the Si À H bond (to give 4 b in 65 % yield), and a reaction time of 16 hours at 80 8C was necessary for the reaction to reach completion. Surprisingly, although it has been shown that, in contrast to CAACs, N-heterocyclic carbenes (NHCs) do not react with H 2 , [11] we found that imidazolidin-2-ylidene 5[12] also reacted at room temperature with phenylsilane to afford the Si À H insertion product 6 in 88 % yield (Figure 1, bottom).

Nucleophilic addition and substitution at coordinatively saturated boron by facile 1,2-hydrogen shuttling onto a carbene donor

“…The π-electron deficiency at the carbene carbon atom of cAACs, and their consequent ability to accept electrons or other groups at this position, is now well-documented. 7 Despite this tendency, cAACs have in some cases formed well-behaved adducts (Fig. 1D) with trivalent hydroboranes ( e.g.…”

Germylene Cyanide Complex: A Reagent for the Activation of Aldehydes with Catalytic Significance

“…It is characterized by multinuclear NMR and IR spectroscopy and single-crystal X-ray diffraction studies. In its 1 H NMR spectrum, 2 shows a singlet resonance at d = 1.68 ppm due to the methyl protons of tert-butyl groups. The five protons attached to the seven-membered ring carbon atoms of the ATI ligand appear as a triplet, doublet, and pseudotriplet (in a 1:2:2 intensity ratio) at d = 6.51, 6.91, and 7.08 ppm, respectively.…”

“…Nevertheless, low-valent germanium compounds that show such a trait of activating small molecules with catalytic significances are hardly known. [1][2][3][4][5][6][7][8][9] Recently, Frenking, Jones, and co-workers have demonstrated the hydroboration of carbonyl compounds using a very reactive and in situ generated two-coordinate germylene hydride [( L 1 )GeH]. [10] Nevertheless, there is no example of a isolable and structurally characterized germylene that shows catalytic activity till now.…”

“…[1] Since a number of low-valent germanium compounds are inherently very reactive, [2] their usefulness for small-molecule activation has been gathering great interest in recent years. [1][2][3][4][5] The activation of dihydrogen using digermynes has been demonstrated by the groups of Power, [4] Jones, and Frenking. [5] Thus, a reaction of digermyne [ [5b] The activation of carbon dioxide at room temperature was reported by Roesky and co-workers using a germanium(II) hydride complex…”

“…[10] Nevertheless, there is no example of a isolable and structurally characterized germylene that shows catalytic activity till now. [1][2][3][4][5][6][7][8][9][10] In this regard, it was anticipated that a germanium(II) cyanide [GeCN(L')] (L' = a monoanionic bidentate ligand or bulky alkyl/ aryl group) may be a potential candidate due the presence of a reactive Ge II ÀCN bond. Accordingly, we searched the literature for the preparation of a germylene monocyanide and found to our surprise that there is no stable and structurally characterized example.…”