A series of cyclobutanes substituted 1,2- by polyenes of increasing radical-stabilizing power hasbeen investigated to test the proposition that stabilization energies obtained independently from apposite, cis,trans geometric isomerizations can be successfully transferred to another system, in this paper, cyclobutanes.The first member of the series, 3-methylenecyclohexene (
1), is photodimerized to
anti- and
syn-dispiro[5.0.5.2]tetradeca-1,8-dienes (
anti-
2 and
syn-
2), which undergo stereomutation (stereochemical interconversion) andcycloreversion (fragmentation) to
1 when heated in the range 72.1-118.2
![](/images/entities/deg.gif)
C:
anti-
2
syn-
2,
H![](/images/entities/thermod.gif)
= 30.3
kcal mol
-1,
S![](/images/entities/thermod.gif)
= 0.2 cal mol
-1 K
-1;
anti-
2
1,
H![](/images/entities/thermod.gif)
= 32.8
kcal mol
-1,
S![](/images/entities/thermod.gif)
= +8.0 cal mol
-1 K
-1.Agreement with an enthalpy of activation predicted by assuming full allylic stabilization in a hypotheticaldiradical intermediate is good. An example of further activation by a radical-stabilizing group is manifestedby the ~20 000-fold acceleration in rate shown by the system 1-phenyl-3-methylenecyclohexene (
3) and
anti-and
syn-2,9-diphenyldispiro[5.0.5.2]tetradeca-1,8-dienes (
anti-
4 and
syn-
4), measured, however, only at 43.6
![](/images/entities/deg.gif)
C. In both systems
2 and
4, volumes of activation for stereochemical interconversion and cycloreversionhave been determined and found to be essentially identical within experimental uncertainties,
V![](/images/entities/thermod.gif)
= +10.2± 1.0 and +12.6 ± 1.4 cm
3 mol
-1, respectively (weighted means). These strongly positive values are consistentwith the rate-determining step being the first bond-brea
king, while the near identity of the volumes of activationargues against the indispensable second bond-brea
king being a determining factor in fragmentation. Theseresults are consistent with the theoretically based construct of Charles Doubleday for the paradigm, cyclobutane,in which the ratio between two channels of exit from a "generalized common biradical" is not controlled byenthalpy
and entropy, as in the transition state model, but by
entropy alone.