372490-64-7Relevant articles and documents
Beyond butadiene II: Thermal isomerization of the [2 + 2] photodimer of an all-trans-tetraene, (R)-4,4aβ,5,6,10,10a-hexahydro-10aβ-methyl-2(3H)-methyleanthracene, to a 16-membered [8 + 8] cycle
Doering,He,Shao
, p. 9153 - 9161 (2001)
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 °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 °C. Cycloreversion of the dimer 16 to monomer 1 occurs in the range 60.4-86.6 °C (ΔH? = 31.7 kcal mol-1, ΔS? = +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.
