Intramolecular and intermolecular reactions of alkenylsilyl radicals

Abstract

The radicals formed during photolysis of di-t-butyl peroxide and a number of alkenyldimethylsilanes have been examined by e.s.r. spectroscopy. Only carbon-centred radicals were observed. These were either secondary alkyl radicals formed by the addition of the initially formed silyl radical to a double bond and/or ally1 radicals formed by hydrogen-atom abstraction from the alkenyl group. In most cases addition to the double bond was an intermolecular process. However, pent-4-enylsilyl radicals undergo intramolecular addition with endo-cyclization to form the six-membered ring being strongly favoured over exocycliration to form the five-membered ring. This preference was confirmed by product studies on 3,3dimethylpent-4-enyldimethylsilane. The factors that control homolytic ex0 : endo cyclization rates for species in which the radical centre is located on a second row element are adumbrated.Free-radical rearrangements are of growing importance since they can provide new approaches to the synthesis of organic compounds and can also be used to determine the absolute rate constants for intermolecular radical-molecule reactions.'S2 The most useful and most carefully studied rearrangement is undoubtedly the ring-closure of the hex-5-enyl radical (1; X = CH2) and related hydrocarbon and heteroatom-containing species including those in which the initial radical centre, X, is on a nitrogen atom or an oxygen a t ~m . ' * ~* ~

Tethered 1,2-Si-Group Migrations in Radical-Mediated Ring Enlargements of Cyclic Alkoxysilanes: An EPR Spectroscopic and Computational Investigation

“…A further point to notice is the small computed Δ H ‡ for 6- endo -cyclization of radical 18b (4.2 kcal mol –1 ). This implies a very rapid process; in agreement with literature kinetic data noted earlier, , …”

“…Kinetic data for cyclizations of Si-centered radicals is sparse. Available rate constants , of 6- endo ring closures are orders of magnitude greater (10 7 to 10 9 s –1 ) than that obtained for the rearrangement of 7a (see above and Table ). Kinetic evidence therefore also militates against the route (i) mechanism involving dissociation followed by 6- endo -cyclization of Si-centered radical 9 .…”

“…A further point to notice is the small computed ΔH ‡ for 6-endo-cyclization of radical 18b (4.2 kcal mol −1 ). This implies a very rapid process; in agreement with literature kinetic data noted earlier, 10,13 The calculated [UB3LYP/6-31+G(d,p)] 42 geometry and SOMO of the transition state of the 1,2-SiMe 2 migration (TS12) in radical 17b are displayed in Figure 6. The TSs for 5exo-and 6-endo-ring closures are in the Supporting Information (Figure S3).…”

“…Literature precedents favored the elimination route (i). First, because 6- endo -ring closure of Si-centered radicals was known to be preferred over the 5- exo -mode. − Then second, route (ii) is a particular example of a 1,2-silicon shift from carbon to carbon. 1,2-Silicon shifts from carbon to carbon were unknown in solution, although 1,2-silicon shifts from carbon to nitrogen , and from carbon to oxygen , were well established.…”

Sonochemical Synthesis of Polysilylenes by Reductive Coupling of Disubstituted Dichlorosilanes with Alkali Metals

“…Silyl radicals react very rapidly with 1-hexene34 (k = 5.0 x 106 mol-1 s™1 for the triethylsilyl radical at 25 °C) in the intermolecular reaction and even faster in the intramolecular process (107 s™1 < k < 109 s™1). 43 The second electron transfer must be 10-20 times faster than the intramolecular process in order to get the 10-20 Si-Si bond sequence. It should be noted that silyl radicals also react very rapidly with toluene (k = 1.2 x 106 mol™1 s™1 for the triethylsilyl radical at 25 °C).…”