Organosilicon Lewis acids

It must be mentioned that the most versatile and representative organosilicon Lewis acid is undoubtedly trimethylsilyl triflate (la), the structure of which reflects the inertness of tetraalkylsilanes for Lewis basic substrates and the requirement of a counter-anion to increase the electrophilicity of the silicon center. Noyori s pioneering studies on the reactions of trimethylsilyl triflate not only provided a general outline for stoichiometric transformations, but also inspired an extraordinary number of subsequent studies of the catalytic use of Lewis acids in selective organic synthesis. 

Simple organosilicon Lewis acids, for example trimethylsilyl triflate (la) [5], trimethylsilyl nonaflate (2) [6], iodotrimethylsilane [7], fert-butyldimethylsilyl triflate (Ic) [8], triisopropylsilyl triflate (Id) [8], etc., are commercially available, but because of their easy hydrolysis and the appearance of traces of a protic acid during storage. 

Protodesilylation of allyl- or aryl-substituted trialkylsilanes with an acid partner is a cleaner method, producing propene or an aromatic hydrocarbon as a volatile inert byproduct. Although some methods of isolation have been reported, they can usually be prepared in situ in aprotic media and employed without further purification. Methods of preparation of representative organosilicon Lewis acids, la, trimethylsilyl bis(trifluoro-methanesulfone)imide (7) [9], trimethylsilyl perchlorate (6a) [10], iodotrimethylsilane, and trimethylsilyl tetrakis(trifluoromethanesulfonyl)borate [11] are described below. In principle, these analogs are also accessible in a similar manner (Table 1) [2d]. 

Organosilicon Lewis acid Representative method of preparation Ref. Main purpose... 

For reviews of organosilicon Lewis acids, see (a) Noyori, R. Murata, S. Suzuki, M. Tetrahedron,... 

The ring-opening of the cyclopropane nitrosourea 233 with silver trifiate followed by stereospecific [4 + 2] cycloaddition yields 234 [129]. (Scheme 93) Oxovanadium(V) compounds, VO(OR)X2, are revealed to be Lewis acids with one-electron oxidation capability. These properties permit versatile oxidative transformations of carbonyl and organosilicon compounds as exemplified by ring-opening oxygenation of cyclic ketones [130], dehydrogenative aroma-tization of 2-eyclohexen-l-ones [131], allylic oxidation of oc,/ -unsaturated carbonyl compounds [132], decarboxylative oxidation of a-amino acids [133], oxidative desilylation of silyl enol ethers [134], allylic silanes, and benzylic silanes [135]. 

It is well known that strong electrophiles such as carbocations are reduced by organosilicon hydrides (Eq. 1).3,70,71 On the other hand, simple mixtures of organosilicon hydrides and compounds with weakly electrophilic carbon centers such as ketones and aldehydes are normally unreactive unless the electrophilicity of the carbon center is enhanced by complexation of the carbonyl oxygen with Brpnsted acids3,70 73 or certain Lewis acids (Eq. 2).1,70,71,74,75 Using these acids, hydride transfer from the silicon center to carbon may then occur to give either alcohol-related or hydrocarbon products. 

Secondary Alkyl Alcohols. Treatment of secondary alkyl alcohols with tri-fluoroacetic acid and organosilicon hydrides results only in the formation of the trifluoroacetate esters no reduction is reported to occur.1,2 Reduction of secondary alkyl alcohols does take place when very strong Lewis acids such as boron trifluoride126 129 or aluminum chloride136,146 are used. For example, treatment of a dichlo-romethane solution of 2-adamantanol and triethy lsilane (1.3 equivalents) with boron trifluoride gas at room temperature for 15 minutes gives upon workup a 98% yield of the hydrocarbon adamantane along with fluorotriethylsilane (Eq. 10).129... 

Various chemical species with Lewis acid properties are also effective in promoting the direct conversion of aldehydes into ethers by organosilicon hydrides. 

This has recently been described as a new method of polymerizing a. /8-unsaturated esters, amides and nitriles with organosilicon derivatives of ketene as initiators and Lewis acids or bases as catalysts. The products formed have the characteristics of living polymers [9]. 

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