On the addition of silyl radicals to unsaturated carbonyl compounds: Regioselectivity of the attack and 1,3 carbon to oxygen silicon migration

The addition reactions of silyl radicals with 2,6-di-tert-butyl- (l), 2,6-dimethoxy- (2), and 2,6-dimethyl-p-benzoquinone (3), ,6-dimethylthieno[3,2-b] thiophene-2,Sdione (4), 9-methyleneanthrone (5), and 5-benzylidene-3,6-dimethylthieno[3,2-blthiophen-2-one (6) have been studied by EPR spectroscopy. With substrates 5 and 6 only the adducts resulting from attack at the exocyclic olefinic double bond were detected up to 400 K. With the quinones and dithiolactone 4 the nature of the adducts depended on the experimental temperature: thus at low temperature preferential addition of silyls to the less hindered ring carbon atom was evident, while at higher temperature only the adducts to a carbonyl oxygen were detected. Experimental evidence has been obtained that the kinetically favored carbon adduct converts to the thermodynamically more stable oxygen adduct via a 1,3 carbon to oxygen internal migration of the organometallic group involving a four-membered cyclic transition state. The kinetics of the rearrangement has been followed for the triphenylsilyl adduct of 1 and the Arrhenius equation log ( k l s - ' ) = [ ( I 3 3 & 0.3) - (18.2 & 0.3)]/0 was determined. The rearrangement of the corresponding adduct of 4 was much slower and its kinetic analysis was complicated by a fast equilibration with its dimer. A kinetic scheme is outlined according to which the observed rate constants should refer to the rearrangement of the dimer rather than of the rddical. An alike behavior is also reported for the triphenylgermyl adducts of compounds 1 and 4.