THE TWOFOLD HECK REACTION ON 1,2-DIHALOCYCLOALKENES AND SUBSEQUENT 6-PI-ELECTROCYCLIZATION OF THE RESULTING (E,Z,E)-1,3,5-HEXATRIENES - A NEW FORMAL (2-ASSEMBLY OF 6-MEMBERED RINGS(2+2))

1,6-Disubstituted (E,Z,E)-1,3,5-hexatrienes (4 and 5) were prepared by vicinal twofold Heck coupling reactions from 1,2-dibromocyclopentene (1), 1,2-dibromocyclohexene (2), 1,2-diiodocyclopentene (8), 1,2-diiod ocyclohexene (9), -bromo-2-trifluoromethanesulfonyloxycyclohex-1-ene ( 11), or chloro-2-nonafluorobutanesulfonyloxycyclohex-1-ene (19) with a lkenes 3, e. g. methyl, tert-butyl, menthyl, 8-phenylmenthyl acrylate, styrene, and alkenylsilanes, respectively, in moderate to mostly good and very good yields (20-92 %). The coupling of alkenylsilanes 3f-k c ould only be achieved with the 1,2-diiodocycloalkenes 8 and 9, respect ively. The corresponding hexatrienes 5 with two different substituents in the 1- and 6-positions were prepared by a sequence of two coupling reactions with different alkenes from chloro-2-nonafluorobutanesulfon yloxycyclohex-1-ene (19) or by Wittig-Horner-Emmons olefination of 2-b romocyclohexene-1-carbaldehyde (24) and subsequent Heck reaction of th e resulting (E,Z)-bromodienes 25. Several of the hexatrienes (4aa, ee, 5aa, ee, al, am, el) readily underwent a 6 pi-electrocyclization upon heating in an inert atmosphere to give the 5- or 6-ring-annelated cis -5,6-disubstituted cyclohexadienes 26, 27 (50-95 %). Starting from 5am 2,3-disubstituted tetrahydronaphthalene 28am was formed under oxidati ve conditions (air) in the reaction and in the work-up procedure. The bissilyl-substituted derivatives 4ff, jj, 5ff did not cyclize under th ermal conditions, apparently due to the steric demand of the two silyl substituents which would have to end up cis with respect to each othe r in the cyclohexadiene products.