Silicon-containing substituent effects

In this work we present models for the quantitative description of the structural effects of substituents whose first or second atom is silicon, germanium, tin or lead. Silicon has been included in this work because its behavior is analogous to that of the remaining elements of the group and there is much more information available for silicon containing substituents than there is for all of the other elements. There are only two types of substituent we shall consider here. They are ... 

Norbornadienes with silicon-containing substituents at the 2-position were also effectively polymerized according to the ROMP way (Table 3). Substituted double bond is inactive in this reaction and therefore polymers obtained were soluble in contrast to polynorbomadiene itself. [153]. Norbornadienes did not polymerize in the presence of simple RuCl3-catalysts, but W-containing and Grubbs systems were very effective. 

The acid cleavage of the aryl— silicon bond (desilylation), which provides a measure of the reactivity of the aromatic carbon of the bond, has been applied to 2- and 3-thienyl trimethylsilane, It was found that the 2-isomer reacted only 43.5 times faster than the 3-isomer and 5000 times faster than the phenyl compound at 50,2°C in acetic acid containing aqueous sulfuric acid. The results so far are consistent with the relative reactivities of thiophene upon detritia-tion if a linear free-energy relationship between the substituent effect in detritiation and desilylation is assumed, as the p-methyl group activates about 240 (200-300) times in detritiation with aqueous sulfuric acid and about 18 times in desilylation. A direct experimental comparison of the difference between benzene and thiophene in detritiation has not been carried out, but it may be mentioned that even in 80.7% sulfuric acid, benzene is detritiated about 600 times slower than 2-tritiothiophene. The aforementioned consideration makes it probable that under similar conditions the ratio of the rates of detritiation of thiophene and benzene is larger than in the desilylation. A still larger difference in reactivity between the 2-position of thiophene and benzene has been found for acetoxymercuration which... 

This section will describe the Friedel-Crafts alkylation reactions of aromatic hydrocarbons with alkenylchlorosilanes containing short chain alkenyl groups such as allyl and vinyl. The reaction will be discussed in terms of the substituent effect on silicon and the arene rings. 

E. Magnusson, Tetrahedron, 41,2945 (1985). Substituent Effects in Second Row Molecules. Silicon-Containing Compounds. 

There exists a correlation between the NMR spectral parameters of the donor fragment (or silicon-containing group) and electronic effects of substituents at the silicon atom (or at the donor fragment) which cannot be explained in terms of transmission through covalent bonds. 

Due to their ready isomerization simple cyclopentenones present a particular challenge in the Nazarov cyclization. In all of the cases studied in a- and -monosubstituted and a, -disubstituted systems the cy-clopentenone product contained the double bond in the less substituted position, as required by loss of the silicon electrofuge (Scheme 17). The relative conBguration of substituents in disubstituted cases is controlled by kinetic protonation and weakly favors the cis isomers. Substituent effects in rate were particularly noted in these cases where substitution with a- and -alkyl groups greatly accelerated and decelerated the reactions, respectively. 

Summary The reactions between silicon atoms and a series of oxygen-containing donor molecules like water, methanol and dimethyl ether were studied by means of IR and UVA is spectroscopy. General trends in reactivity as well as substituent effects — especially the effect of isotopic substitution — can be derived ftom the observed photochemical reactions. As a tribute to the famous direct process the reaction of atomic silicon and methyl halides was also investigated. 

Most optically active polysilanes owe their optical activity to induced main-chain chirality, as outlined above. However, backbone silicon atoms with two different side-chain substituents are chiral. Long-chain catenates, however, are effectively internally racemized by the random stereochemistry at silicon, and inherent main-chain chirality is not observed. For oligosilanes, however, inherent main-chain chirality has been demonstrated. A series of 2,3-disubstituted tetrasilanes, H3Si[Si(H)X]2SiH3 (where X = Ph, Cl, or Br), were obtained from octaphenylcyclote-trasilane and contain two chiral main-chain silicon atoms, 6.16 These give rise to four diastereoisomers the optically active S,S and R,R forms, the activity of which is equal but opposite, resulting in a racemic (and consequently optically inactive) mixture and the two meso-forms, S,R and R,S, which are optically inactive by internal compensation. It is reported that the diastereoisomers could be distinguished in NMR and GC/MS experiments. For the case of 2-phenyltetrasilane, a racemic mixture of (R)- and (A)-enantiomers was obtained. 

An alternate pathway is possible for systems containing silylamino substituents at phosphorus. This most likely involves attack of the CCl3 anion at the electrophilic silicon resulting in elimination of Me3SiCCl3 as shown in pathway B. In the systems investigated thus far, the reaction pathway preference appears to be influenced by (1) solvent polarity, and (2) steric and electronic effects of the substituents at phosphorus ( ). 

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