"Front Matter". In: Organosilanes in Radical Chemistry - Index of
"Front Matter". In: Organosilanes in Radical Chemistry - Index of "Front Matter". In: Organosilanes in Radical Chemistry - Index of
6 Unimolecular Reactions 6.1 CYCLIZATION REACTIONS OF SILYL RADICALS 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 siliconcentred 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. Early work was focused to establish the preference for exo- vsendo-mode of cyclization. However, the absence of an effective method for generation of alkyl and/or aryl substituted silyl radicals made this task difficult. The reaction of prototype alkanesilane 1 with thermally generated t-BuO: radicals at 145 8C after 4 h afforded a 48 % yield of unreacted starting material and 19 % yield of a six-membered cyclic product (Scheme 6.1) [1]. Moreover, EPR studies of the same reaction recorded the spectra at temperatures between 30 and 0 8C, which were identified as the superimposition of two species having allylic-type (2) and six-membered ring (3) structures, respectively [2]. At higher temperatures radical 2 predominates; therefore, the low yield detected in the product studies could derive from the extensive t-BuO: attack on the allylic hydrogens. On the other hand, product studies of volatile materials for the reaction of silane 4 (no allylic hydrogen available) with thermally generated t-BuO: radicals at 46 8C revealed the formation of cyclic silanes in a 46 % overall yield Organosilanes in Radical Chemistry C. Chatgilialoglu # 2004 John Wiley & Sons, Ltd ISBN: 0-471-49870-X
120 Unimolecular Reactions 1 Si(H)Me 2 Si(H)Me 2 t-BuO + t-BuOOBu-t 145 �C 2 3 SiMe 2 Scheme 6.1 Summary from product and EPR studies SiMe 2 (Scheme 6.2) [2]. Evidence for a five-membered ring product in 1 % yield has also been obtained. Furthermore, EPR studies of the same reaction allowed for the identification of the silacyclohexyl radical 6 with two unequivalent bhydrogen atoms in the temperature range between 73 and 25 8C, as the only observable intermediate. A kinetic investigation placed the rate constant for the cyclization of radical 5 between 10 7 < k < 10 9 s 1 [2]. From the above description, it is clear that the cyclization of pent-4-ene substituted silyl radicals is a fast reaction and that formation of the sixmembered ring is very strongly favoured. This is of course in antithesis with the analogous carbon-centered radicals, where the kinetically favored product has the five-membered ring structure produced by an exo-cyclization (cf. Section 7.3). The reversal in the regioselectivity of the silyl radicals was initially explained based on structural factors, such as the longer C w Si bond length and pyramidal geometry of the silicon centre. Semi-empirical calculations (MINDO/3) suggested that such behaviour of silyl radicals is due to an enthalpy factor caused by electronic effects [3]. The six-membered ring formation was also observed in the reaction of analogous heterosubstituted derivatives 7 (Reaction 6.1) [1]. 4 Si(H)Me 2 t-BuO• t-BuONNOBu-t C6D6 , 46 �C Si 6-endo-trig 35% Si Si 5 6 Scheme 6.2 Preference for 6-endo-trig cyclization of silyl radicals + H a He H 11% Si
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6 Unimolecular Reactions<br />
6.1 CYCLIZATION REACTIONS OF SILYL RADICALS<br />
The importance <strong>of</strong> carbon-centred radical cyclizations <strong>in</strong> organic chemistry has<br />
been documented <strong>in</strong> the large number <strong>of</strong> papers published each year and<br />
numerous reviews and books deal<strong>in</strong>g with this subject. <strong>In</strong> Chapter 7 the reader<br />
can f<strong>in</strong>d a collection <strong>of</strong> such processes mediated by organosilanes. The siliconcentred<br />
radical cyclizations have <strong>in</strong>stead received very little attention, although<br />
there has been a grow<strong>in</strong>g <strong>in</strong>terest <strong>in</strong> silicon-conta<strong>in</strong><strong>in</strong>g compounds from a<br />
synthetic po<strong>in</strong>t <strong>of</strong> view, due to their versatility and applicability to material<br />
science. As we shall see, this area <strong>of</strong> research is very active and some recent<br />
examples show the potentiality <strong>of</strong> silyl radical cyclization <strong>in</strong> the construction <strong>of</strong><br />
complex molecules.<br />
Early work was focused to establish the preference for exo- vsendo-mode <strong>of</strong><br />
cyclization. However, the absence <strong>of</strong> an effective method for generation <strong>of</strong> alkyl<br />
and/or aryl substituted silyl radicals made this task difficult. The reaction <strong>of</strong><br />
prototype alkanesilane 1 with thermally generated t-BuO: radicals at 145 8C<br />
after 4 h afforded a 48 % yield <strong>of</strong> unreacted start<strong>in</strong>g material and 19 % yield <strong>of</strong> a<br />
six-membered cyclic product (Scheme 6.1) [1]. Moreover, EPR studies <strong>of</strong> the<br />
same reaction recorded the spectra at temperatures between 30 and 0 8C,<br />
which were identified as the superimposition <strong>of</strong> two species hav<strong>in</strong>g allylic-type<br />
(2) and six-membered r<strong>in</strong>g (3) structures, respectively [2]. At higher temperatures<br />
radical 2 predom<strong>in</strong>ates; therefore, the low yield detected <strong>in</strong> the product<br />
studies could derive from the extensive t-BuO: attack on the allylic hydrogens.<br />
On the other hand, product studies <strong>of</strong> volatile materials for the reaction <strong>of</strong><br />
silane 4 (no allylic hydrogen available) with thermally generated t-BuO: radicals<br />
at 46 8C revealed the formation <strong>of</strong> cyclic silanes <strong>in</strong> a 46 % overall yield<br />
<strong>Organosilanes</strong> <strong>in</strong> <strong>Radical</strong> <strong>Chemistry</strong> C. Chatgilialoglu<br />
# 2004 John Wiley & Sons, Ltd ISBN: 0-471-49870-X
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