Silylated cyclohexadienes as new radical chain reducing reagents: preparative and mechanistic aspects

Approach to 3-Aminoindolin-2-ones via Oxime Ether Functionalized Carbamoylcyclohexadienes

O-Benzyloxime ether substituted amidocyclohexadienes were prepared in three steps in good yields from 2-aminoacetophenone. EPR spectroscopic observations and product analyses showed that peroxide-induced decompositions of model compounds led to indolin-2-ones with benzyloxyaminyl substitution at their 3-positions. The cyclization steps were very rapid and took place regioselectively at the C-atoms of the C=N bonds, by 5-exo ring closures. An O-trityloxime ether analogue was also prepared. The cyclohexadienyl intermediate smoothly yielded an alkoxylaminyl radical again by rapid 5-exo-cyclization. However, ring closure was quickly followed by another beta-scission step that released the persistent trityl radical and a 3-nitrosoindolin-2-one derivative. EPR spectroscopic evidence showed that the nitroso compound trapped other transient intermediates to afford a series of nitroxides. GC-MS analyses of products formed in reactions including methyl thioglycolate indicated that 1-benzyl-3-methyl-1,3-dihydro-2H-indol-2-one was derived from the indolinone moiety.

Fast Tin-Free Hydrodehalogenation and Reductive Radical Cyclization Reactions: A New Reduction Process

The photostimulated reactions of several aryl and alkyl chlorides and bromides with the monoanion of reduced ethyl benzoate 5H furnish the reduced products in high yields. If the aryl moieties have suitable double bonds, the cyclized reduced products are obtained in high yields. The photostimulated reaction of 1-allyloxy-2-bromobenzene (1a) with 5H affords 3-methyl-2,3-dihydro-benzofuran (2a) in 97% yield. When 1-allyloxy-2-chlorobenzene (1b) is used, the yield of 2a is only 55%, which increases up to 91% when acetone enolate ion is added to the reaction mixture as entrainment reagent. With diallyl-(2-bromophenyl)amine (3a), and 2-allyloxy-1-halonaphthalenes (chloro, 4b, and bromo, 4a) the cyclized reduced products are obtained in yields above 96%. By competition experiments, 5H reacts ca. 5 times faster with 1-naphthyl radicals than benzenethiolate ions do, which is near the diffusion limit rate.

Preparation of β-and γ-lactams via ring closures of unsaturated carbamoyl radicals derived from 1-carbamoyl-1-methylcyclohexa-2,5-dienes

1-Carbamoyl-1-methylcyclohexa-2,5-dienes produced the corresponding delocalised 1-carbamoyl-1-methylcyclohexa-2,5-dienyl radicals on treatment with radical initiators. At temperatures above ca. 300 K dissociation to produce toluene and aminoacyl (carbamoyl) radicals took place. The alternative dissociation of the 1-carbamoyl-1-methylcyclohexa-2,5-dienyl radicals to release methyl radicals and an aromatic amide did not compete. Aminoacyl radicals with allyl, butenyl or similar side chains underwent cyclisations. Moderate yields of N-benzyl-azetidin-2-ones and N-benzyl-pyrrolidin-2-ones were isolated for a range of substituents. The main by-products were N-benzyl-N-alkenylformamides. Ring closure did not take place to a significant extent for precursors with alk-2-ynyl or 2-cyanoalkyl side chains. An improved yield of 1,3-dibenzylazetidin-2-one was obtained by use of lauroyl peroxide as initiator and by inclusion of methyl thioglycolate as polarity reversal catalyst.