Sulfur-bridged molecular racks : O,S-sesquinorbornadienes, CNS-[3] and CNOS [4]polynorbornanes

Thiophene has been reacted under high-pressure (10 kbar) and temperature (1000C) with Nmethylmaleimide and N-phenylmaleimide to produce Diels-Alder exo- and endo-adducts in modest yields (25-36%). Reaction of 7-oxanorbornadiene-2,3-dicarboxylic anhydride 6, a highly reactive dienophile, with thiophene occurred under high-pressure (10 kbar) at 100oC to yield stereoisomeric 1:1-adducts by site selective attack at the maleic anhydride type of π-bond. This approach afforded the first examples of syn- and anti- heterobridged sesquinorbornadiene anhydrides 8 and 9 containing a sulfur bridge. Similar reaction of isobenzothiophene with 6 was even more facile as reaction occurred at room temperature and atmospheric pressure to yield benzo-analogues 12 and 13. Thermal fragmentation involving loss of furan and sulfur occurred from both classes of adducts under FVP (370 0C, 0.005 mbar) to produce phthalic anhydride or naphthalene-2,3-dicarboxylic anhydride respectively. The putative 7-thianorbornadiene intermediates 20 and 22, generated by loss of furan, were not detected. Reaction of exo-N-methyl 7-thianorbornene-5,6-dicarboxylic imide 4a with the ester-activated cyclobutenoaziridine 16 provided access to CNS-[3]polynorbornane 18, while similar addition of the exo,endo-isomer of O,S-benzosesquinorbornadiene 13 to 16 afforded the CNOS-[4] olynorbornane 19. These are the first S-bridged [n]polynorbornanes to be reported. Molecular modelling (AM1) has shown that Sn-[n]polynorbornadienes have a curved topology greater than On- n]polynorbornadienes but less than Nn-[n]polynorbornadienes.