Carbon-silicon bond, nucleophilic activation

The second major class of non-umpolung nucleophilic carbene catalysis comprises reactions by initial NHC-activation of various silicon compounds. Their proposed common pathway is thought to lead to a hypervalent silicon complex4 and thus provide carbene-catalyzed activation of the corresponding nucleophiles such as TMSCN, TMSCF3 etc. (Kano et al. 2006 Song et al. 2005 2006). It is not only certain carbon-silicon bonds that can be effectively activated, but a comparable activation of Si-O bonds, e.g. of trimethylsily enol ethers etc., allows for mild, NHC-promoted Mukaiyama aldol reactions (Scheme 6 Song et al. 2007). 

The first observation of penta- and hexaeoordinate silicon compounds was reported at the beginning of the 19th century by Gay-Lussac [87] and Davy [88], Subsequent investigation of hypercoordination in silicon compounds stimulated widespread use of nucleophilic activation and catalysis in the application of organosilicon compounds as reactive species in organic synthesis. Synthetic application for silicon-fluorine bond formation can be found in several reviews over the last two decades, and this section focuses on recent advances in the use of hypervalent organosilicon compounds in selective organic synthesis, in particular, selective carbon-carbon bond formation [89]. 

Introducing the Triazole Rina. The final step of the synthesis was displacement of the carbon-bound chlorine with triazole salts. Once again, silicon made life easy for us, since it activates such hindered systems toward displacement. The corresponding all-carbon compounds react very sluggishly with triazole salts. Luckily, silicon-carbon bond cleavage is not observed, provided water or other oxygen nucleophiles are excluded. The displacement reaction is illustrated in Equation 5 for DPX-H6573. 

Organosilanes bearing an electronegative heteroatom(s) at the a-carbon are susceptible to nucleophihc activation leading to silicon-carbon bond-cleavage, because of the electronic effect of the heteroatom. a-Heteroatom-substituted organosilanes are therefore quite valuable as protected carbon nucleophiles. The silicon-carbon bond is also readily activated by a transition metal complex. The reactivity is successfully utilized for catalytic carbon-carbon bond formation. 

Silicon Lewis acids have widely been utilized for carbonyl activation, which enables C-C, C-heteroatom, and C-H bond-forming reactions at the carbonyl carbon or the carbonyl-conjugated carbons. Combined use of silicon Lewis acids and silylated nucleophiles provides efficient catalytic systems for these reactions as shown in Scheme 9.1. The initial studies on catalysis by silicon Lewis acids demonstrated that acetalization and reduction of carbonyls were successfully carried out with alkoxysilanes and hydrosilanes, respectively [7e, 93]. Then, strong silicon Lewis acids and new reaction systems were developed for catalytic C-C bond-forming reactions of carbonyls. 

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