Silyl carbonates

For the conversion of silyl-carbon bond in the cyclization products into hydroxy-carbon bond, several functionalized hydrosilanes were examined (Scheme 27).84,84a,84b Of the hydrosilanes examined, benzhydryldimethylsilane (HSiMe2CHPh2) was found to be most enantioselective in the reaction of diene 86a to give the cyclization product 87a with 93% ee. The second highest enantioselectivity (91% ee) was observed with hydrosiloxane HSiMe2OSiPh2Bu-/. The cyclization-hydrosilylation with the HSiMe2CHPh2 and catalyst 88 was very successful... 

Bottini M, Magrini A, Dawson MI, Rosato N, Bergamaschi A, Mustelin T (2007) Noncovalently silylated carbon nanotubes decorated with quantum dots. Carbon 45 673-676. 

Next to iodine there is also another class of neutral Lewis acids known. Tetracyanoethylene, dicyanoketene acetals and derivatives can catalyse reaction due to their tt-Lewis acid properties. They promoted the alcoholysis of epoxides [238], tetrahydropyranylation of alcohols [239], monothioacetahzation of acetals [240], and carbon-carbon bond formation of acetals [241,242] and imines [243] with silylated carbon nucleophiles. 

Similarly, C-glycosylation reaction of I-0-acylsugars with silylated carbon nucleophiles, such as silyl enol ethers, proceeded to give the corresponding C-glycosides in good yields in the presence of a catalytic amount of trityl perchlorate ( ). This reaction was also catalyzed by ploymer-bound trityl perchlorate (Mukaiyama, T. Kobayashi, S. Carbohydrate Research, in press.)... 

Silylation of lithiated 2-methyl-l-methylenecyclopropane is reported to give selectively tra s-2-methyl-3-trimethylsilyl-1 -methylenecyclopropane361. However, when 2-trimethyl-silyl-l-methylenecyclopropane is again lithiated in situ, and subsequently reacted with Me3SiCl, silylation occurs mainly at the already silylated carbon, giving 2,2-bis-(trimethylsilyl)-l -methylenecyclopropane, along with minor amounts of the exo-silylated isomeric cyclopropene and syn and anti trisilyl derivatives (equation 296)368. 

The dimethylisopropylsilyl-substituted 1 -p-lolylvinyl cation 389b can be prepared from the corresponding alkyne 388b at temperatures below —125 °C. At somewhat higher temperature (—105 °C) the cleavage of the /3-silyl carbon bond occurs rapidly and the a-(p-tolyl)vinyl cation 390 is formed as the only detectable product (equation 66). The -fragmentation is faster and occurs at lower temperatures compared with that of the... 

Deprotonation-silylation of 1-methyl-lH-tetrazole exhibits predominant C-silylation at the 5-position to form l-methyl-5-trimethylsilyl-lH-tetrazole (it is not a SMA, silylated carbon atom being sp2). When this position is blocked by a silyl or a phenyl group, silylation occurs on the /V-methyl to yield SMA derivatives.158... 

This behavior reflects the high basicity of the silylated carbon and, in fact, protonation of phenylsilane and aryltrimethylsilanes yields the //mo-protonatcd species as the... 

The reactivity of a silyl-zirconium complex is interesting because an unsaturated bond would be inserted into the silyl-zirconium bond to provide an alternative zirconium complex. It has zirconium-carbon and silyl-carbon... 

The codimerization reaction with unsymmetrically substituted silylalkynes is highly regioselective, specifically giving the isomer with the silylated carbon attached to the metal. In an analogous manner, metallabicyclo[3.1.0]hexene derivatives of both titanium and... 

The BCR cycloaddition is diastereoselective with respect to the TMM unit, with the alkene approaching from the side opposite to the metal center. This accounts for an overall net retention of configuration of the starting material in the cycloadducts obtained from the silyl carbonates (117) and (118) (equations 122, 123). Thus, no prior coordination of the electron-deficient alkene to the Pd center is necessary for reaction. This is quite different from the methylenecyclopropane mechanism as depicted in Scheme 5. It would also explain the need for a higher ligand-to-metal ratio of the catalyst. 

Silyl carbonates (124) possessing an anion-stabilizing function have been developed by Tsuji and co-woikers as TMM synthons in [3 + 2] reactions. In this case the in situ generated alkoxide serves as a... 

The high preference for C-T attack of the a-silyl carbon radical derived from 24 could be due to the formation of an incipient tertiary C-2 radical A, although steric hindrance of the 2 -methyl group cannot be ruled out. 

Deoxygenation of silyl epoxides. Alkenylsilanes are generated from the epoxy silanes with retention of configuration, when they are exposed to PhSLi in THF at —78°. The reagent attacks the silylated carbon atom regioselectively unless the silyl group is very bulky (e.g., r-butyldiphenylsilyl group). 

Oxyacid Derivatives. Both Mironov and Tarbell have investigated the synthesis of silyl carbonates. The routes shown in Scheme 23 gave high yields of the desired products. 

The aluminium enolates generated after ECA do not react directly with electrophiles, probably due to their high stability. However, they can be trapped in situ by silylation, carbonation and 0-acylation in good yields (Scheme 6). These intermediates 7-9 can eventually be used in Tsuji reactions or ozonolysis, for example, to generate more elaborated adducts [36]. 

With trimethylsilylacetylene, the same ketene forms the [2+2] cycloadduct 104, resulting from initial attack on the non-silicon substituted carbon of the alkyne . In contrast, PhCMTSiMes reacts with dichloroketene by initial attack at the silylated carbon atom to give 105. ... 

While in this case the silyl substituent just operates as passive volume, the second example makes use of a silyl-carbon substituent as a directing element first, which later serves as the precursor of a hydroxy group [126]. 

Generation of x-Silyl Carbon ions and their Peterson Reactions I 47... 

When the reaction conditions employed in all the steps are the same as those for the Peterson reaction, multi-step processes involving the Peterson reaction can be integrated in a one-pot process. Successive treatment of aDyl sulfones 234 with BuLi, MejSiCl, BuLi, aldehydes, and lithium hexamethyldisilazide(LiHMDS) gives the enynes in good to moderate overall yields for the four-step reactions (silylation, carbon-carbon bond formation, Peterson elimination, and sulfone elimination) (Scheme 2.144). The Peterson and sulfone eliminations can be successfully combined since both processes are carried out under basic conditions [152,153]. 

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