Substituent effects silyl

The non-planar nitrogen atoms in silylhydroxylamines are the result of substituent effects. Silyl... 

Aryl substitution on germanium, whether single or multiple, has only a small effect on the rate constants for hydrogen atom transfer, whereas the rate constant increases substantially with substitution of an alkyl group on Ge by a silyl group, much as observed with the silanes. A strong substituent effect also was observed for germane 19. 

Undoubtedly, further work will be necessary in order to get a solid background for silyl radical cyclizations. In particular, further investigations on the nature of the pendant alkenyl substituent as well as on the ring size and substituent effects on the silicon in the ring expansion can be expected in the future. 

Substituent effects opposite to those found for silane a-complexes were observed for the complexes with IHI, where the presence of one electron-withdrawing group trans to the hydride switches the IHI on in comparison with a silyl SiRs with the donating R s only. However, the IHI weakens with the further increase of the number of electron-withdrawing substituents at silicon. It also appears that electron-donating groups on silicon decrease the extent of H Si interactions in the recently discovered class of polyhydride complexes with multicenter H Si interactions. 

The low reactivity of 170 toward alcohols would be explained by the reduced polarity of the Si=C double bond due to the contribution of the resonance structure 168B, in addition to the silyl substituent effects. In agreement with experiment a theoretical study for the parent species 168 predicts that water addition to 168 is thwarted by an unusual high barrier (20.0 kcalmol-1 at MP2/6-311+- g )119,177. Furthermore, the calculations suggest that the transformation 170 - 341 proceeds in two steps via the cyclopropenylsilylene 342 (equation 85). The highest barrier for the isomerization of the parent 168 —> 343 is calculated to be 17 kcalmol-1. Thus, the isomerization can effectively compete with the water addition177. 

OCHEM), 313, 73 (1994). Substituent Effects of Second Row Elements on Silyl Centers. 

Substituent effects and the role of negative hyperconjugation in [l,2]-silyl migration and decarbonylation of methoxy(substituted-siloxy)carbenes have been investigated using quantum chemical calculations and natural bond orbital analysis (Scheme 38).62... 

FIGURE 18. First vertical ionization energies IE] (nS) (eV) (a) Comparison between thiomethyl and methoxy derivatives and (b) derivatives of H2S and its alkyl, silyl, trimethylsilyl and (trimethylsilyl)methyl derivatives (— — identical mono- and di-substitution, alkyl/silyl comparison,------- /3-silyl substituent effects)... 

The cross-coupling reaction of alkenyl(fluoro)silanes with aryl halides sometimes produces, in addition to the desired ipso-cowpled products, small amounts of cmc-coupled products [14]. The czne-coupling is often striking in the reaction with organotin compounds. The isomer ratio of products produced by the reaction of l-fluoro(dimethyl)silyl-l-phenylethene with aryl iodides is found to depend on the electronic nature of a substituent on aryl iodides (Eq. 11) an electron-withdrawing group like trifluoromethyl and acetyl favors the formation of the ipso-coupled product. To explain the substituent effect, the mechanism depicted in Scheme 3 is proposed for the transmetalation of alkenylsilanes with palladium(ll) complexes. It is considered that an electron-donating substituent on Ar enhances... 

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