Triphenylsilyl radical

The site of reaction on an unsaturated organometallic molecule is not restricted to the most probable position of the metallic atom or cation or to a position corresponding to any one resonance structure of the anion. This has been discussed in a previous section with reference to the special case of reaction with a proton. Although the multiple reactivity is particularly noticeable in the case of derivatives of carbonyl compounds, it is not entirely lacking even in the case of the derivatives of unsaturated hydrocarbons. Triphenylmethyl sodium reacts with triphenylsilyl chloride to give not only the substance related to hexaphenylethane but also a substance related to Chichi-babin s hydrocarbon.401 It will be recalled that both the triphenyl-carbonium ion and triphenylmethyl radical did the same sort of thing. 

In this reaction, the diphenylmethylsilyl radical is produced. Although its Si HF coupling constant has not yet been measured, it may be similar to that of the triphenylsilyl radical (A/g B = 7.96 mX). Typical a values for the Si-enrichment observed for this reaction in the absence and presence of external magnetic fields are listed in Table 9-2. 

Triphenylsilyl ether aluminium acetylacetonate and triphenylarsonium-p-nitrophenacylide have been found to induce the photopolymerisation of acrylic monomers by a free radical mechanism while arylthiyl radicals induce the photopolymerisation of vinyl monomers (CH2=CHX) in the order X= -C(CH3)3 < -( 2)3 3 < -(CH2)Si(OCH3)3 <= -CH2Si(CH3)3 -Si(OCH3)3 -Si(CH3)3 ... 

Sommer, a pioneer in the use of photochemistry to generate silenes, showed that pentaphenylmethyldisilane on photolysis gave rise to 1,1-diphenylsilene38. The reaction almost certainly occurred via homolysis, followed by hydrogen abstraction by the triphenylsilyl radical since the silene was unstable it was trapped with methanol, confirming its formation (equation 23). 

The reactivity of free radicals is linked to their stability. Reactivity is a function of spin density of the atom and the type of orbital occupied by the unpaired electrons. Increasing the number of atoms in a radical generally decreases reactivity, making monoatomic radicals such as Li and F very reactive. Conjugation decreases the spin density and therefore the reactivity. The resultant delocalization does not necessarily, however, lead to a very stable radical. Steric effects play a significant role in radical reactivity. "If the bulkiness of the surrounding substituents exceeds 12-fold that of the central atom containing a radical site", the radical is unreactive and fails to react at all in many cases. Triphenylsilyl radical (45), for example, is very reactive but perchlorobenzyl radical (46) is not. 

Reduction of primary, secondary and tertiary acetates can also be conveniently performed by reaction with triphenylsilyl hydride or p-(his-diphenylhydrosilyl)benzene under radical conditions with the latter reagent being more effective (equations 22 and 23). It is to be noted that AIBN and benzoyl peroxide are not effective as radical initiators in this reduction. ... 

Laser photolysis of hexaphenylsilane (and hexaphenylgermane) was used to observe the TR EPR spectra of triphenylsilyl and triphenylgermyl radicals for the first time in fluid solutions. The spectra show TM and RPM CIDEP, which indicates that the homolysis of the M-M bond occurs via the triplet state. The triphenylstannyl radical was not observed. 

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