Beyond butadiene II: Thermal isomerization of the [2+2] photodimer of an all-trans-tetraene, (R)-4,4a beta,5,6,10,10a-hexahydro-10a beta-methyl-2(3H)-methyleneanthracene, to a 16-membered [8+8] cycle

Enthalpies of stabilization of polyenyl radicals of increasing order previo usly obtained by thermal geometrical isomerization are applied to the ethyl ene-cyclobutane paradigm. Progressively lower enthalpies of activation for thermal cyclodimerization. and its reverse, cycloreversion, are predicted a nd realized. Photochemical dimerization at -75 degreesC of the optically pu re 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 t entative configuration anti-exo,exo is assigned. Equilibration among the th ree cyclobutanes (4), a slower rearrangement to a thermodynamically conside rably more stable, [8 + 8] cyclohexadecahexaene (16), and a surprisingly sl ow fragmentation to 1 are studied kinetically between -42.3 and -8.2 degree sC. Cycloreversion of the dimer 16 to monomer 1 occurs in the range 60.4-86 .6 degreesC (DeltaH double dagger = 31.7 kcal mol(-1), DeltaS double dagger = +10.8 cal mol(-1) K-1). The ratio of the rates of stereomutation and cyc loreversion is significantly larger in these 1,2-dihexatrienylcyclobutanes than in two less strongly stabilized, previously published examples. Them p ossible extension of Doubleday's calculational finding of entropic control of products from cyclobutane is considered.