On the radical Brook rearrangement. Reactivity of α-silyl alcohols, α- silyl alcohol nitrite esters, and β-haloacylsilanes under radical-forming conditions

Article abstract of DOI:10.1021/jo9912855

Two alkoxyl radical generation methods, lead tetraacetate treatment of alcohols and photolysis of nitrites, were applied to α-silyl alcohols 21 and to the corresponding nitrites 25 with a view to forming α-silyl alkoxyl radicals 23 and studying their possible radical Brook rearrangement to α- silyloxy carbon radicals 24. LTA treatment of 21 led to their quick and efficient conversion into mixed acetyl-silyl acetals 33 under very mild conditions. Photolysis of α-alkylmonosubstituted α-silyl nitrites 25 to the corresponding aldehydes is considered to proceed through α-silyl alkoxyl radical intermediates 23. A concerted process is, however, proposed for the case of the benzyl nitrites analogues. On the basis of these results, it is postulated that resonance stabilization can have a major influence on the evolution of α-silyl alkoxyl radicals: should this stabilization be possible, they quickly evolve by α-silyl fragmentation; otherwise, they tend to undergo radical Brook rearrangement. It was also found that the radical Brook rearrangement of α-silyl cyclopropyloxyl radicals generated from β- bromoacylsilanes under standard tin-radical conditions is too slow to compete with β-fragmentation.