Silylation of C-H bonds

Catalytic C-H bond transformation is a very attractive research subject in organic and organometallic chemistry [1]. There are several procedures for C-H bond transformation. One is conversion of C-H bonds to C-Si bonds, i.e. the direct silylation of C-H bonds. Three procedures have been developed for silylation of C-H bonds - use of hydrosilanes (reaction 1 in Scheme 1), disilanes (reaction 2 in Scheme 1), and vinylsilanes (reaction 3 in Scheme 1) as silylating reagents. 

Scheme 2. One of the possible reaction pathways for the silylation of C-H bonds with hydrosilanes.

As mentioned above, three types of reaction are used for silylation of C-H bonds. In this section, we describe the reaction pathway of chelation-assisted silylation of C-H bonds with hydrosilanes (reaction 1). 

Several reaction pathways for reaction 1 are possible. A clear reaction mechanism has not been elucidated. Although it is premature to discuss the details of the reaction pathway for this silylation reaction, one possible pathway for the chelation-assisted silylation of C-H bonds is shown in Scheme 2. The catalytic reaction is initiated by oxidative addition of hydrosilane to A. Intermediate B reacts with an olefin to give C. Then, addition of a C-H bond to C leads to intermediate D. Dissociation of alkane from D provides Ru(silyl)(aryl) intermediate E. Reductive elimination making a C-Si bond gives the silylation product and the active catalyst species A is regenerated. Another pathway, addition of a C-H bond to A before addition of hydrosilane to A is also possible. At present, these two pathways cannot be distinguished. 

The first example of silylation of C-H bonds in arenes with hydrosilanes was reported by Curtis [2]. Later, silylation of C-H bonds with triethylsilane using a rhodium catalyst was reported (Scheme 3) [3, 4], The reaction of arenes with bis(hydrosilane) using a platinum catalyst involves a bis(silyl)platinum species in the coupling reaction (Scheme 3) [5]. In these non-chelation-assisted reactions possible regioisomers should be formed. 

Table 1. Silylations of C-H bonds in aromatic and heteroaromatic compounds and of benzyl C-H bonds 6mol%...

Scheme 4. Silylation of C—H bonds with disilanes (a) 5 mol% Ni(PPh3)4, disilane 1, toluene, reflux 20h (b) 1 mol% Pt2(dba)3-P(OCH2)3CEt, toluene, 160°C, 20 h.

Scheme 5. Silylation of C-H bonds with vinylsilanes (a) 6 mol% Ru3(CO)12, toluene, 115 °C, 20 h.

The direct silylation of C-H bonds with hydrosilanes or disilanes is one of the simplest procedures for obtaining arylsilanes. Curtis et al. [57] reported, to the best of our knowledge, the first example of the dehydrogenative silylation of benzene with pentamethyldisiloxane using an IrCl(CO)(PPh3)2 catalyst under thermal reaction conditions in the absence of a hydrogen acceptor. Unfortunately, however, the efficiency and the selectivity of this reaction were low. After this discovery, several attempts to achieve a high efficiency and selectivity were made. 

The use of C-H bonds is obviously one of the simplest and most straightforward methods in organic synthesis. From the synthetic point of view, the alkylation, alkenylation, arylation, and silylation of C-H bonds are regarded as practical tools since these reactions exhibit high selectivity, high efficiency, and are widely applicable, all of which are essential for practical organic synthesis. The hydroacylation of olefins provides unsymmetrical ketones, which are highly versatile synthetic intermediates. Transition-metal-catalyzed aldol and Michael addition reactions of active methylene compounds are now widely used for enantioselective and di-astereoselective C-C bond formation reactions under neutral conditions. 

The catalytic conversion of C-H bonds to C-C bonds is one of the most attractive and potentially useful reactions in organic synthesis. The silylation of C-H bonds... 

Hartwig, J. F. Borylation and silylation of C-H bonds a platform for diverse C-H bond functionalizations. Acc. Chem. Res. 2012,45, 864-873. 

In contrast to the asymmetric activation of C—H bonds in benzyl silyl ethers, the dirhodium tetraprolinate, Rh2(5-DOSP)2 (Figure 5.7), was found to be an efficient catalyst in an enantioselective C—H activation of acetals (Scheme 5.16). Interestingly, when the acetals had a methoxy substituent on the aromatic ring, the Stevens rearrangement was a main competing side reaction of the C—H activation of acetals. 

Scheme 5.15. Asymmetric activation of C—H bonds in benzyl silyl ethers.

In fact, unique cleavage of the C-Si bond from a tertiary SMA is not a general feature as cleavage of C-H bond can also occur. This is illustrated with the reaction of methylvinylketone with tertiary SMAs, leading either to silylated or non-silylated products or both.285... 

Details on oxidation and reduction reactions mediated by [(NHC)Pd] complexes can be found in Chapters 12 and 13, respectively. Further reports of interest disclosed include diboration of alkenes catalysed by a pincer complex, deuteration of C-H bonds with an N,0-functionalised NHC complex, and an intriguing Suzuki-type reaction of [FeI(Cp)(CO)2] with arylboronic acids. A number of useful C-heteroatom coupling reactions have also been reported recently, including alkene hydrophosphination and alkyne silylation. ... 

Therefore, attempts have been made to replace methane in the feedstock mixture of gases by carbosilanes with a smaller number of C—H bonds. Relevant candidates are methylsilane, di(silyl) methane, tri(silyl)methane, and tetra(silyl)methane C(S1H3) H4 with n = 1 — 4. The latter, n = 4 for C(SiH3)4, is the most attractive species since it is devoid of C—H bonds and the carbon atom is already surrounded only by silicon atoms (4—9). 

Cleavage of all the linkers described above provide a functional group (carboxylic acid, amide, amine, etc) at the anchoring position. Silyl-based handles 71,72, and 73 as well as germanium-based handle 74 insert a C-H bond at the anchoring position and are referred to as traceless (Fig. 15) [82-... 

---