Phenylsilane

Abstract: [Tris(2-pyridylthio)methyl]zinc hydride, [κ3-Tptm]ZnH, is a multifunctional catalyst that is capable of achieving (i) rapid release of hydrogen by protolytic cleavage of silanes with either water or methanol and (ii) hydrosilylation of aldehydes, ketones, and carbon dioxide. For example, [κ3-Tptm]ZnH catalyzes the release of 3 equivalents of H2 by methanolysis of phenylsilane, with a turnover number of 105 and a turnover frequency surpassing 106 h–1 for the first 2 equivalents. Furthermore, [κ3-Tptm]ZnH also catalyzes the formation of triethoxysilyl formate by hydrosilylation of carbon dioxide with triethoxysilane. Triethoxysilyl formate may be converted into ethyl formate and N,N-dimethylformamide, thereby providing a means for utilizing carbon dioxide as a C1 feedstock for the synthesis of useful chemicals.

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. For example, stable singlet carbenes can activate CO, H2, [3b] and P4. [3c–e] Such reactions have long been known for transition metals. However, stable singlet carbenes can also activate NH3; [3b] a much more difficult task for transition metals. 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. Herein, we report the first examples of the activation of E H bonds (E=Si, B, P) at a single nonmetal center. On the basis of our successful results with H2, [3b] we began our study with the activation of Si H bonds. Indeed, silanes are similar to H2 in that they lack both nonbonding electron pairs and p electrons. They can bind to various metal centers to form stable Si H s complexes, which undergo subsequent oxidative addition. To test the possible activation of Si H bonds with carbenes, we treated the cyclic (alkyl)(amino)carbenes (CAACs) 1a and 1b with primary, secondary, and tertiary silanes. The addition of phenylsilane to 1a and 1b occurred readily at room temperature, and the corresponding adducts 2a,b were isolated in 91 and 83% yield, respectively (Scheme 1). As expected, in the case of the enantiomerically pure CAAC 1a, two diastereomers 2a,a’ were formed (in a 2:1 ratio), as shown by two singlets at d= 36.4 and 29.3 ppm in the Si NMR spectrum. The C NMR spectrum revealed the loss of the carbene signal and a new C H peak at d= 63.2 (2a) and 65.5 ppm (2b). The H NMR spectrum of the major isomer 2a 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 SiH2 fragment. The structure of 2a was confirmed by X-ray crystallography (Figure 1, top), whereas the presence of a triplet at d= 4.53 ppm and a doublet at d= 4.08 ppm in the H NMR spectrum confirmed the identity of adduct 2b. CAACs 1a,b also reacted with (EtO)3SiH to afford 3a (d.r. 3:1) and 3b in 64 and 73% yield, respectively. However, when Ph2SiH2 was used, only the less bulky carbene 1b underwent insertion into the Si H bond (to give 4b 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 H2, [11] we found that imidazolidin-2-ylidene 5 also reacted at room temperature with phenylsilane to afford the Si H insertion product 6 in 88% yield (Figure 1, bottom). The formation of 6 raises the question of the mechanism of the activation of Si H bonds with carbenes. Why should NHCs react with silanes although they are inert towards hydrogen? The evident difference is the presence of low-lying vacant orbitals in silanes. In other words, the observed reactivity might be due to the Lewis acid character of silanes; indeed, several NHC–SiX4 adducts are known. [13]

Abstract: A main group-catalyzed method for the synthesis of aryl- and heteroarylamines by intermolecular C–N coupling is reported. The method employs a small-ring organophosphorus-based catalyst (1,2,2,3,4,4-hexamethylphosphetane) and a terminal hydrosilane reductant (phenylsilane) to drive reductive intermolecular coupling of nitro(hetero)arenes with boronic acids. Applications to the construction of both Csp2–N (from arylboronic acids) and Csp3–N bonds (from alkylboronic acids) are demonstrated; the reaction is stereospecific with respect to Csp3–N bond formation. The method constitutes a new route from readily available building blocks to valuable nitrogen-containing products with complementarity in both scope and chemoselectivity to existing catalytic C–N coupling methods.