Sequential cycloaddition-cycloreversion-cycloaddition-cope rearrangement with an annelated norbornadiene and electrophilic dienes. unusual [π 2s + π 2a + σ 2a] transformation of a pentacyclo-[11.4.0.1 7,10.0 4,13.0 6,11]octadeca-3,8,14,17-tetraene.1

Abstract

Hexachloronorbornadiene efficiently forms a 1:1 adduct 3 with 5,6-bismethylenenorbornene. Adduct 3 on reaction with 1,2,3,4-tetrachlorothiophene dioxide (TCTD) under mild conditions gives an SO 2 bridged adduct, which loses SO 2, the product cycloreverting to give 1,2,3,4-tetrachlorobenzene and an annelated cyclopentadiene 8; 8 reacts further with TCTD to give, in 1:1 ratio, two compounds ( 14 and 15) shown by mass, 13C and 1H NMR spectroscopy and single crystal X-ray structure determination to be the stereoisomeric products of a rare type of specific diene capture by TCTD. Evidence is presented that this result (and analogous examples) is best understood in terms of “normal” [4+2]π cycloadditions to 8 as dienophile towards electron-deficient TCTD, followed by rapid Cope rearrangement of the SO 2-bridged intermediate adducts to give the observed products. At low temperatures adduct 3 behaves as expected towards electron-deficient diene tetracyclone ; the [4+2]π cycloadduct decarbonylates on mild thermolysis, concomitant cycloreversion also giving diene 8. At much higher temperatures in the presence of excess tetracyclone, diene 8 likewise behaves as dienophile; the carbonyl-bridged major cycloadduct decarbonylates to give an intermediate which undergoes an unusual thermally-allowed [π 2 s + π 2 a + σ 2 a] 1,3-shift/cycloaddition, delivering a symmetrical cage-like structure 24, as indicated by mass and 13C and 1H NMR spectroscopy.