Silyl groups, stabilizing effect

The effects of silyl groups on the chemical behavior of the anion radicals generated by cathodic reduction is also noteworthy. It is well known that silyl groups stabilize a negative charge at the a position. Therefore, it seems to be reasonable to consider that the anion radicals of re-systems are stabilized by a-silyl substitution. The interaction of the half-filled re orbital of the anion radical with the empty low-lying orbital of the silicon (such as dx-pK interaction) results in partial electron donation from the re-system to the silicon atom which eventually stabilizes the anion radical. 

A fi-silyl group stabilizes (/i-silyl effect) the bicyclo[2.2.1]heptyl cation. For example, endo-2-(4-bromophenylsulfonyloxy)-enc/o-6-trimethylsilylbicyclo[2.2.1]heptane on solvolysis in trifluo-roethanol, in the presence of 2,6-dimethylpyridine, gave predominantly bicyclo[3.1.1]hept-2-ene (norpinene, 29 86% by GC 83% overall yield of solvolysis products).75... 

Silyl groups are well-known for their abilities to stabilize carbocationic centers, most notably through the a- and -silyl effects. A report described the alkenylation of a,j6-unsaturated ketones and a carbocation intermediate is thought to be stabilized by a y-silyl effect (Scheme 4). Thus, the ketone (9) reacts with 4-methylstyrene with acid catalysis to provide a good yield of the alkenylation product (10). The proposed mechanism involves formation of the electrophile (11), followed by the silyl-group-stabilized carbocation (12). 

In fl-trimethylsilylcarboxylic acids the non-Kolbe electrolysis is favored as the carbocation is stabilized by the p-effect of the silyl group. Attack of methanol at the silyl group subsequently leads in a regioselective elimination to the double bond (Eq. 29) [307, 308]. This reaction has been used for the construction of 1,4-cyclohexa-dienes. At first Diels-Alder adducts are prepared from dienes and P-trimethylsilyl-acrylic acid as acetylene-equivalent, this is then followed by decarboxylation-desilyl-ation (Eq. 30) [308]. Some examples are summarized in Table 11, Nos. 12-13. 

Full accounts have appeared of the exchange of trimethylsilyl for chlorosilyl groups on treatment of trimethylsilylylides with chlorosilanes and of the stabilizing effects of the simple silyl (SiHa) group on ylides. The exchange process involves nucleophilic attack of the ylide on the silicon of the chlorosilane, e.g. 

The regioselective ring opening of the silyl epoxides is facilitated by the stabilizing effect that silicon has on a positive charge in the -position. This facile transformation permits vinylsilanes to serve as the equivalent of carbonyl groups in multistep synthesis.149... 

It should be recognized that the stability of cation radicals generated by anodic oxidation is also affected by jS-silyl substitution. Stabilization of car-bocations by a silyl group situated at the -position is well known as the / effect . The interaction of the C Si a orbital with the empty p orbital of the carbon stabilizes the carbocation. Therefore, we can expect similar effects of silicon for cation radical species. The interaction of the filled C-Si a orbital with the half-filled orbital of the carbon may stabilize the cation radical. 

Due to the benzylic p-Tc resonance stabilization the C+-C,pso bond has partial double bond character and the ortho, ortho and meta, meta methine groups syn and anti to the silyl group are non-equivalent. The effect pf the a-silyl group on the positive charge in benzyl cations can be estimated by comparison of the NMR spectroscopic data of the 1 -phenyl-1 -(trimethylsilyl)ethyl cation 1 with those for the 1-phenylethy 1 cation 5 (P) and the cumyl pation 3 (15, 16, 17) (Table 1). 

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