1,x-elimination reactions: Extending the limits of a classical organic reaction

alpha,omega-Dibromo derivatives in which the two terminal carbon atom are separated by an unsaturated spacer unit (pi spacer) undergo I,x-elimination reactions (with x = 6 8, 10, and 14) using Mori's reagent (nBu(3)SnSiMe(3)/CsF). The resulting cumulenic intermediates cyclodimerize in a subsequent step yielding novel macrocyclic acetylenic and bridged aromatic compounds (cyclophanes). Thus 1,6-eliminations were carried out with dibromide 17 to yield 1,3,7,9-cyclodode-catetrayne (20) and with benzylbromide 24 to provide cyclophanes 26 and 27. By 1,8-eliminations the 16-membered macrocycle 33 could be prepared from enediyne 31, the benzannelated 1,5-cyclooctadiyne 41 from dibromide 38, and a mixture of cyclophanes 45 and 46 from the precursor 43. 1,10-Eliminations were carried out successfully with dibromides 47, 50, and 53 yielding the corresponding unsaturated cyclophanes (cyclophynes) 49, 52, and 55. The influence of the solvent on the cyclodimerization 47 49 was investigated, with acetonitrile providing the highest yields. The heterophanes 59a and b were obtained by 1,10-elimination of the precursor dibromides 57a and b, and in an elimination experiment involving a 1: 1 mixture of the dibromides 50 and 57b the mixed dimer 60 was isolated, besides the homodimers 52 and 59b. The method reached its limits with the 1,14-elimination of 68, 70, and 74 providing the cyclophanes 69, 71, and 75 in varying amounts. Two final debrominations with 76 and 77, which in principle could undergo 1,16- and 1,20-eliminations reactions, respectively, failed. The structures of the new cyclophanes 49, 50, 59a, and 59b were established by X-ray structural analysis; all other structure assignments rest on the usual spectroscopic and analytical data.