Radical rearrangements of Ph3Ge, Me3Si, 1,2-Aryl, and imide carbonyl groups

Abstract

New types of trimethylsilyl and triphenylgermyl rearrangements were discovered in the radical reactions of spiro epoxy ketones and spiro epoxy trimethylsilyl enol ethers. The 1,5-rearrangements of the germyl and silyl group from enoxy oxygen to alkoxy oxygen proceeded rapidly prior to the reduction of an intermediate alkoxy radical. The rate constants of the rearrangements were estimated to exceed over $10^9 sec^{-1}$ at 80℃. 1,2-Aryl rearrangement from carbon to nitrogen, neophyl-like rearrangement of aminyl radicals, was observed for the first time. The rearrangement would occur via an intermediate spiro cyclohexadienyl radical and thus, be largely affected by substituents to stabilize the intermediate. Tertiary azides having at least more than two aryl groups underwent the rearrangement with high efficiency. Migration of 5-membered aryl group such as 2-thiophenyl 2-furanyl group over pheny and substituted phenyl with electron donating or electron withdrawing groups was observed. The synthetic utility of the 1,2-aryl rearrangement was demonstrated by 9-azidofluorenes into phenanthridine derivatives. The radical rearrangement of imide carbonyl group was studied. An amide function was predicted to participate in radical reactions as a radical acceptor by the introduction of certain protection groups, where the protection would lower LUMO-energy of the amide carbonyl. Thus, 1,4- and 1,5-carbonyl rearrangements were achieved with protected acyclic amides. The reactions occurred via the addition-elimination mechanism as well-recognized ketonic carbonyl rearrangements. Trifluoroacetyl group among many protection groups induced the rearrangements most efficiently, whereas benzoyl and Cbz group gave relatively low yields of the rearranged products. 1,2-Rearrangements were also observed in radical reaction of protected lactams in which initial 1,5- or 1,6-hydrogen atom transfer and subsequent 1,2-carbonyl rearrangement afforded ring-contracted lactams.