Browsing by Author "Culotta, Anne Marie"

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    Photolysis of azoalkanes. Reactions and kinetics of the 1-cyclopropyl-1,3-cyclopentanediyl biradical and 1-cyclopropylcyclopentyl radical
    (1991) Culotta, Anne Marie; Engel, Paul S.
    The 1-cyclopropylcyclopentyl radical (114a) and 1-cyclopropylcyclohexyl radical (114b) were generated in the presence of PhSH via 366 nm photolysis of azo-1-cyclopropylcyclopentane and azo-1-cyclopropylcyclohexane respectively. These radicals undergo cyclopropylcarbinyl (CPC) rearrangement to primary homoallylic radicals 115a and 115b, respectively, at rates of 1.45 $\times$ 10$\sp7$ s$\sp{-1}$ and 1.1 $\times$ 10$\sp7$ s$\sp{-1}$ based on competitive trapping with PhSH. These rates are 6-9 times slower than the parent CPC radical. E$\sb{\rm a}$ for 114a$\to$115a was found to be 7.0 kcal/mol and log A = 12.3. Deuterium labeling studies show that homoallylic radical 115a recloses to 114a with k$\sb{\rm rr}$ = 5.5 $\times$ 10$\sp4$ s$\sp{-1}$. Unsymmetrical triplet biradical 1-cyclopropyl-1,3-cyclopentanediyl (104T) is produced from the triplet sensitized photolysis of 1-cyclopropyl-2,3-diazabicyclo (2.2.1) hept-2-ene (123). Biradical 104T undergoes CPC rearrangement to a pair of 1,6-biradicals 105E and 105Z the latter of which undergoes rapid intramolecular disproportionation to Z-3-propylidenecyclopentene (108Z). On account of its geometry, 105E cannot lead directly to a stable product; hence, it recloses to 104T (k$\sb{\rm ra}$ = 1.2 $\times$ 10$\sp5$ s$\sp{-1}$) but interestingly, not to 104S. If the CPC rearrangement rate of 104T is taken as that of 114a, we calculate from the product distribution that the lifetime ($\tau$ = 1/k$\sb{\rm isc}$) of 104T is 59 ns. This figure is only one half the lifetime of the parent 1,3-cyclopentanediyl and is close to $\tau$ = 42 ns for 1,3-dimethyl-1,3-cyclopentanediyl, showing that the effect of the cyclopropyl group is similar to that of methyl. ISC of 104T displays linear Arrhenius behavior over 3.4-61.0$\sp\circ$C. Activation parameters for ISC of 104T were determined via competition with the CPC rearrangement and were found to be E$\sb{\rm a}$ (ISC) = 1.6 kcal/mol and log A = 8.4. Thus 104T is shown to be a ground state triplet and the low value of log A is characteristic of spin-forbidden ISC.