Silylation of aromatic C-H bonds

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 

9 Ruthenium-Catalyzeci Reactions via sp C-H, sp C-H, sp C-H, and C-Halogen Bond Activations 

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Scheme 21 Scandiiim-catalyzed silylation of aromatic C-H bonds...

Catalytic Silylation of Aromatic C-H Bonds. Direct, catalytic functionalizations of ortho C-H bonds in aromatic ox-azolines have been extended to silylation. Treatment of pheny-loxazolines with triethylsilane using Ru3(CO)i2 as a catalyst and tert-butylethylene as a hydrogen scavenger gives 2-(2-triethyl-silyl)phenyloxazolines in good yields (eq Vajjous func-... 

Iridium-Catalyzed Silylation and Borylation of Aromatic C—H Bonds... 

The silylation of benzylic G-H bonds is achieved by using Ru3(GO)12 catalyst in the presence of norbornene as a hydrogen acceptor.145 The reaction of 2-(2,6-dimethylphenyl)pyridine with triethylsilane in the presence of Ru3(CO)i2 catalyst and norbornene affords mono- and disilylation products in 30% and 55% yields, respectively (Equation (106)). The reaction of 2-(2-tolyl)pyridine shows that the silylation of the aromatic C-H bond is more facile than that of the benzylic C-H bond. 

The direct silylation of arenes through C—H bond activation provides an attractive route for the synthesis of useful aromatic compounds [64]. Vaska s complex was the first of the iridium catalysts to be reported for activation of the C—H bond in benzene by Si—H of pentamethyldisiloxane to yield phenylsubstituted siloxane [65]. However, a very attractive method for the aromatic C—H silylation with disilanes has been recently reported by the groups of Ishiyama and Miyaura [66-68]. 

Insertion can also be carried out on the C-H bonds of heteroaromatics. Masahiro Murakami of Kyoto University has described (J. Am. Chem. Soc. 2003,125,4720) a Ru catalyst that will effect rearrangement of a silyl alkyne such as 10 into the vinylidene carbene. The intermediate Ru carbene complex is then electrophilic enough to insert into the aromatic C-H bond. The insertion is highly regioselective. The Au and the Ru alkylidene insertions are geometrically complementary, as Ru gives the E-alkcne. 

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

The alkynylation of sp -C-H bonds has in general been much less developed than that of sp -C-H bonds. Metal-mediated methods have been limited to the use of alkynyl bromides [143], whereas radical approaches have been dominated by alkynyl sulfones [21, 22]. Nevertheless, Yu and Chen and co-workers recently reported that aromatic EBX reagents were highly efficient for the interception of radicals generated in a-position to heteroatoms [144]. Silyl EBX could also be used. The inherent limitations of this radical-mediated approach are the requirement for a... 

Direct silylation of aromatic compounds is carried out with 1,2-di-tert-butyl-1,1,2,2-tetrafluorodisilane (165) that serves as a silylating reagent in the presence of an iridium catalyst [60]. For example, the Irotalyzed C-H activation reaction of o-xylene selectively proceeds at the aromatic C-H bond rather than the benzylic one to give 166 in a high yield (Scheme 5.43). The synthetic utility of the products... 

For /8-substituted 7t-systems, silyl substitution causes the destabilization of the 7r-orbital (HOMO) [3,4]. The increase of the HOMO level is attributed to the interaction between the C-Si a orbital and the n orbital of olefins or aromatic systems (a-n interaction) as shown in Fig. 3 [7]. The C-Si a orbital is higher in energy than the C-C and C-H a orbitals and the energy match of the C-Si orbital with the neighboring n orbital is better than that of the C-C or C-H bond. Therefore, considerable interaction between the C-Si orbital and the n orbital is attained to cause the increase of the HOMO level. Since the electrochemical oxidation proceeds by the initial electron-transfer from the HOMO of the molecule, the increase in the HOMO level facilitates the electron transfer. Thus, the introduction of a silyl substituents at the -position results in the decrease of the oxidation potentials of the 7r-system. On the basis of this j -efleet, anodic oxidation reactions of allylsilanes, benzylsilanes, and related compounds have been developed (Sect. 3.3). 

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. 

Fluoride ion-assisted desilylation has been extensively used to create an ylid from a /V-silyl methyl-quaternary ammonium salt. Its evolution to final produces) is variable and Sommelet-Hauser and Stevens rearrangement products were obtained (often as major products) in a ratio that can be shifted from one structure to another very close one, as in examples 1 and 2 dealing with //-benzyl salts.246,366 Differences in the solvents used are not significant because in the first example, HMPA does not reverse the ratio, yields and selectivity being just a bit lower, /so-toluene was proposed as an intermediate in example 1 it might also be the intermediate in example 2. Thus product partition reflects the relative ability of the C-H or the C-C bonds to be cleaved to produce aromatization with proton or a-amine carbocation migration. 

Ishikawa found the Ni(PEt3)4-catalyzed silylation of aromatic compounds with 3,4-benzo-1,1,2,2-tetraethyl-1,2-disilacyclobutene, providing the l-(di-ethylarylsilyl)-2-(diethylsilyl)benzene in high yields (Eq. 44) [89]. In the case of the reaction of mesitylene, interestingly, the sp3 C-H bond adds to the disilacy-clobutene, albeit in low yield (28% yield) [89]. Platinum(O) complexes are also applicable to this silylation reaction as the catalyst [90]. 

Metal Free Transition metal catalysts are highly effective for C—H bond activation. However, transition metal complexes are not only expensive, but also difficult to remove from the reaction products, resulting in toxicity concerns. DDQ is a well-known oxidant in organic chemistry [33]. For many years, it has been used for the oxidation of alcohols to ketones and aromatization. The first intermolecular C—C bond formation was realized by DDQ-mediated Mukaiyama-type aldol reactions [34], The reactions of electron-rich benzyl ethers and silyl enol ethers afforded 3-alkoxy-3-phenylpropionyl derivatives at ambient temperature with moderate to excellent yields (Equation 11.12). 

The next step involves a re-aromatization of the bicationic species with a cleavage of the C-Si bonds. C-Si bonds are much weaker than C-H bonds and can be cleaved more easily. Moreover, this cleavage can be assisted by nucleophilic attack of the silicium by nucleophilic species which are present in the electrolytic medium. Evidence for the formation of McaSiF [13] was obtained in the electropolymerization of silylated thiophene. It was proposed that it resulted from the attack of Bp4 on the C-Si bonds. 

Therefore, direct functionalization by iridium-catalyzed reactions involving C-H bond activation would provide an alternative protocol to the existing multi-step organic synthesis [116-130]. To date, iridium-catalyzed C-H activation of aromatic rings for reactions such as borylation [131 -136], alkylation/alkenylation [137-143] and silylation [144-146], and cross-coupling [147, 148] has been investigated. 

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