Trialkylsilyl substituents

The unusually low-field 29Si NMR resonances (S 142-155) observed for tetrakis(trialkylsilyl)disilenes 22-24 are interesting,21 but whether these effects result from the trialkylsilyl substituents is open to question in view of their similarity to the chemical shifts between diarylbis(trimethylsilyl)-disilene 9 and dialkyldiaryldisilene 10 ((Z)-9 (8 97.68) vs. (Z)-10 (8 96.93)). 

Trialkylsilyl substituents have proved very useful for directing the course of cy-clizations involving vinyl ethers. The trialkylsilyl group stabilises a radical centre but destabilises a carbonium ion next to the silicon atom. An example of the di-... 

Kinetic analysis of the palladium catalyzed acylation reaction of 1 (R = i-Pr) and 23 indicates that the rate does not depend on the bulk of the trialkylsilyl substituent. Since the rate limiting step of this reaction is the interaction of a coordinatively unsaturated acylpalladium chloride with the cyclopropane (Cf. Eq. 59), the observed independence can reasonably be taken as an evidence that the Si—O bond remains intact in the transition state [56], Semiquantitative data on the cleavage of I (R = i-Pr) and 23 with ZnCl2 in ether, Eq. (13), led to the same conclusion [27]. 

Whereas much effort has been devoted to the search for triplet silylenes in the ground and excited states both experimentally189-194 and theoretically195 196, no successful result has been reported so far. Introduction of two bulky trialkylsilyl substituents to a silylene is predicted to lead to a ground state triplet196. 

The same technique was used950 to generate the l -(ferf-butyldimethylsilyl)bicy-clobutonium ion 529, which undergoes fast 1,3-hydride shift upon increasing temperature to yield 3-e ferf-butyldimethylsilyl)bicyclobutonium ion 530 [Eq. (3.135)]. Ion 530 has a static structure, which is due to the efficient stabilization by the y-ewiio-trialkylsilyl substituent. 

Trialkylsilyl substitution on the Si=Si bond, as in disilene 35, results in a substantial decrease in reactivity toward addition of alcohols relative to 103-10468. Product studies have been carried out for the addition of water and methanol to the tetrakisdert-butyldimethylsilyl) homologue 109, and show that the reaction proceeds to yield the expected 1,2-addition product149. The stabilizing effect of trialkylsilyl substituents on simple disilenes was predicted earlier by Kami and Apeloig on the basis of ab initio theoretical calculations150. 

The transition-metal-catalyzed decomposition of methyl trialkylsilyldiazoacetates 5 in the presence of styrene, hex-l-ene and cyclohexene has been studied.In all cases, where cyclopropanation occurs, the formation of the thermodynamically favored cyclopropane dia-stereomers E-6 and anti-1 are formed predominantly. The steric demand of the trialkylsilyl substituent has a relatively small influence on the diastereoselectivity on the other hand, catalytic cyclopropanation with the triisopropylsilyldiazoacetate is more difficult than with 5 (R = Me, Et) or fails completely. Copper(I) triflate is a more efficient catalyst for these cyclo-propanations than rhodium(II) acetate dimer and rhodium(II) bis(perfluorobutanoate) dimer the catalyst [Ru2(CO)4(/i-OAc)2] gives somewhat better results than copper(I) triflate for hex-l-ene, but not for styrene. The catalytic cyclopropanation of cyclohexene with 5 (R = Me, Et) to give bicyclo[4.1.0]heptanes 7 succeeds only with [Ru2(CO)4(/j-OAc)2] as catalyst. ... 

Keywords alkaline earth metals, cages, phosphanediides, tin, trialkylsilyl substituents... 

Mutual Effects between the Trialkylsilyl Substituents and the Phosphanediides 305... 

When Q ,/3-unsaturated acyl silanes are employed, the type of product formed varies depending on the trialkylsilyl substituent of the acyl silane five-membered carbocycles are produced from reaction with f-butyldimethylsilyl derivatives, whereas six-membered carbocycles are obtained from trimethylsilyl compounds (eq... 

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