Fast tin-free hydrodehalogenation and reductive radical cyclization reactions: A new green reduction process

S. E. VAILLARD, J. I. BARDAGÍ, R. A. ROSSI,

Fortaleza - Brasil

Workshop; 1st Brazilian Workshop on Green Chemistry; 2007

Centro de Energias Alternativas e Meio Ambiente

Tin hydrides, mainly tri-n-butylstannane and trimethylstannane, play a central role in radical chemistry. However, their utility at preparative levels is usually limited due to their toxicity and difficulty of elimination from reaction mixtures. Special efforts have been attempted in order to replace the toxic tin hydrides, and potential substitute candidates were reported in the last years. I wish to report a new non-toxic and readily available reactive intermediate that can be used for both tin-free reductive cyclization of suitably-substituted aryl halides, and for hydrodehalogenation of aryl and alkyl halides         The photostimulated reactions of several aryl and alkyl chlorides and bromides with the monoanion of reduced ethyl benzoate 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 with the monoanion of reduced ethyl benzoate affords 3- methyl-2,3-dihydro-benzofuran in 97% yield.          When 1-allyloxy- 2-chlorobenzene is used,  the yield of  3-methyl-2,3- dihydro- benzofuran  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  and 2-allyloxy-1-halonaphthalenes (chloro, and bromo) the cyclized reduced products are obtained in yields above 96%. By competition experiments, the monoanion of reduced ethyl benzoate reacts ca. 5 times faster with 1-naphthyl radicals than benzenethiolate ions do, which is near the diffusion limit rate.          With 4-substituted dihydro ethyl benzoates the rate of hydrogen transfer decreases and excellent yields of products are obtained by 6- exo-trig ring closure reaction followed by reduction.          Considering the availability and/or simplicity of the starting materials, and the readiness and mild conditions of the procedure, we have demonstrated that this can be a general methodology reduction and free reductive cyclization.    J. Org. Chem. 2004, 69 (6), 2037-2041.Tetrahedron Lett. 2006, 47 (18), 3149-3152.