PYROLYSIS OF 9-METHYLENESPIRO[3.5]NONA-5,7-DIENE - A ROUTE TO BENZO-2-HEXENE-1,6-DIYL, A PUTATIVE INTERMEDIATE IN THE RETRO-DIELS-ALDER REACTION OF TETRALIN

A precursor to the 1,6-biradical potentially formed in the retro-Diels-Alder reaction of tetralin, namely, 5-methylenespiro[bicyclo[2.2.1]hept-2-ene-6,T-cyclobutan]-7-one (9) has been prepared and found to give tetralin and o-adlyltoluene upon pyrolysis in solution below 100 °C and upon flash vacuum pyrolysis around 250 °C. The product ratio changes from 1:1 to 6:1 at higher temperatures. Rate-determining loss of CO from 9 to give 9-methylenespiro[3.5] nona-5,7-diene, 2, has been demonstrated by trapping the triene with N-methyl- and N-phenyltriazolinedione in a reaction whose rate is independent of trapping agent concentration. The kinetics for loss of ketone, 9, gave log k (s-1) - 14.628 ± 0.038 - (30.554 ± 0.064)/2.3RT. Pyrolysis of cis-syn(to methylene)-and traNS-l,2-dimethyl-9-methylenespiro[3.5]nona-5,7-diene gives the vinylcyclobutane rearrangement product, 2,3-dimetyltetralin, with a 4:1 and 2.7:1, respectively, preference for retention over inversion at the migrating carbon. Hydrogen shift products are also formed and by highly ordered transition states. One of these hydrogen shifts involves an unprecedented shift from the carbon remote from the vinyl group. © 1990, American Chemical Society. All rights reserved.