Radicals silyl-centred

Homolytic substitution reactions including homolytic allylation, radical [2,3]-migrations and stereochemical reactions been reviewed. The review also highlights the possible applications of homolytic substitution reactions. ni reactions at silicon (by carbon-centred radicals in the a-position of stannylated silyl ethers) are efficient UMCT reactions producing cyclized alkoxysilanes. Bimolecular reactions can also be facilitated in good yield (Schemes 32 and 33). ... 

The reaction of atoms, radicals or excited triplet states of some molecules with silicon hydrides is the most important way for generating silyl radicals [1,2]. Indeed, Reaction (1.1) in solution has been used for different applications. Usually radicals X are centred at carbon, nitrogen, oxygen, or sulfur atoms... 

For a long time, this knowledge on carbon-centred radicals has driven the analysis of spectroscopic data obtained for silicon-centred (or silyl) radicals, often erroneously. The principal difference between carbon-centred and silyl radicals arises from the fact that the former can use only 2s and 2p atomic orbitals to accommodate the valence electrons, whereas silyl radicals can use 3s, 3p and 3d. The topic of this section deals mainly with the shape of silyl radicals, which are normally considered to be strongly bent out of the plane (a-type structure 2) [1]. In recent years, it has been shown that a-substituents have had a profound influence on the geometry of silyl radicals and the rationalization of the experimental data is not at all an extrapolation of the knowledge on alkyl radicals. Structural information may be deduced by using chemical, physical or theoretical methods. For better comprehension, this section is divided in subsections describing the results of these methods. 

Persistent and stable silyl radicals have attracted considerable attention [42]. Bulky aryl or alkyl groups that generally make carbon-centred radicals persistent [43,44] have a much weaker effect on the silyl radicals. The high reactivity of the Ph3Si radical contrary to the stable Ph3C radical is mentioned above. The decay of the trimesitylsilyl radical at 63°C follows a first-order kinetics with a half-life of 20 s [37]. Tri-tert-butylsilyl radical is also not markedly persistent showing the modest tendency of tert-h Ay groups to decrease pyramidalization... 

The kinetics data on the reactions of silyl radicals with carbon-centred radicals are also available. The rate constant for the cross-combination of CHs with MesSi was measured to be 6.6 x 10 M s in the gas phase [19]. Studies on the steady-state and the pulse radiolysis of EtsSiH in methanol showed that the cross-combination of Et3Si with CH30 andHOCH2 occurs with rate constants of 1.1 x 10 and 0.7 x 10 M s , respectively [20]. 

The importance of carbon-centred radical cyclizations in organic chemistry has been documented in the large number of papers published each year and numerous reviews and books dealing with this subject. In Chapter 7 the reader can find a collection of such processes mediated by organosilanes. The silicon-centred radical cyclizations have instead received very little attention, although there has been a growing interest in silicon-containing compounds from a synthetic point of view, due to their versatility and applicability to material science. As we shall see, this area of research is very active and some recent examples show the potentiality of silyl radical cyclization in the construction of complex molecules. 

Silyl substituted carbon-centred radicals, which are produced when adding RsSi to unsaturated bonds can participate in consecutive reactions other than cyclization. A simple example is given in Reaction (7.66) where the adduct of silyl radical to (3-pinene rearranged by opening the four-membered ring prior to H atom transfer [33,77],... 

Lycoramine. Lycoramine (300)[79] synthesised by Parker et al [80] utilised an intramolecular 5-exo radical addition to an appropriately located double bond to generate the quaternary carbon centre (Scheme 46). The starting material, the silyloxybromo ketone 301, was secured initially as a mixture of isomers from 4-methoxy-3-cyclohexen-l-ol (302) by silylation to 303 followed by bromination. On standing or in contact with silica, the mixture was converted essentially into 304. The overwhelming preference for the substance 301 to exist... 

Since 19951, to the best of our knowledge, there have appeared few papers detailing structural investigations of silyl radicals. Of those few, Matsumoto and coworkers investigated the isomerization of silyl radicals derived from 9,10-di-terf-butyl-9,10-dihydro-9, 10-disilaanthracenes (2)6. Irradiation of a di-terf-butyl peroxide (DTBP)/pentane solution of either cis-2 or trans-2 affords the same 81% cist 19% trans mixture of 2. In the absence of DTBP and irradiation, solution NMR studies indicate that each isomer of 2 is unchanged in the —85 to 20 °C temperature range. The authors propose that the radicals 3 derived from 2 isomerize to each other via inversion of the radical centre (equation 1) followed by hydrogen abstraction from the parent compound 2 (an identity reaction). 

Very recently, Matsumoto and coworkers have been interested in the generation of persistent silyl radicals where the substituents on the silicon centre enforce highly planar architectures11. A series of silylated silyl radicals (6-10) were generated by photolysis of the corresponding silane or disilane, or by oxidation of the silylsodium precursor, and their EPR spectra recorded (equations 2-4). 

Radical reactions have some stereochemical features that can be compared directly with their ionic counterparts, especially when the radical centre is adjacent to an existing stereogenic centre. The tris(trimethylsilyl)silyl radical adds to chiral ketones like 3-phenyl-2-butanone 7.59 to give a radical 7.60 flanked by a stereogenic centre. The hydrogen atom abstraction from a thiol, determines the relative stereochemistry, and the products 7.61 and 7.62 are analogous to those from the hydride reduction of the ketone. They are formed in the same sense, and the stereochemistry is explained by the Felkin-Anh picture 7.60. 

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