Beyond butadiene II: thermal isomerization of the [2 + 2] photodimer of an all-trans tetraene, (R)-4,4abeta,5,6,10a-hexahydro-10abeta-methyl-2(3H)-methyleneanthracene, to a 16-membered [8 + 8] cycle.
Enthalpies of stabilization of polyenyl radicals of increasing order previously obtained by thermal geometrical isomerization are applied to the ethylene-cyclobutane paradigm. Progressively lower enthalpies of activation for thermal cyclodimerization and its reverse, cycloreversion, are predicted and realized. Photochemical dimerization at -75 degrees C of the optically pure tetraene of the title (1) at the semicyclic double bond produces in the main only one (4-axx) of the three allowed cyclobutanes (4), to which the tentative configuration anti-exo,exo is assigned. Equilibration among the three cyclobutanes (4), a slower rearrangement to a thermodynamically considerably more stable, [8 + 8] cyclohexadecahexaene (16), and a surprisingly slow fragmentation to 1 are studied kinetically between -42.3 and -8.2 degrees C. Cycloreversion of the dimer 16 to monomer 1 occurs in the range 60.4-86.6 degrees C (DeltaH = 31.7 kcal mol(-1), DeltaS = +10.8 cal mol(-1) K(-1)). The ratio of the rates of stereomutation and cycloreversion is significantly larger in these 1,2-dihexatrienylcyclobutanes than in two less strongly stabilized, previously published examples. The possible extension of Doubleday's calculational finding of entropic control of products from cyclobutane is considered.
Doering
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Department of Chemistry and Chemical Biology, Harvard University, Cambridge, MA 02138-2902, USA. doering@chemistry.harvard.edu
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