Photoisomerizations of 2-methylphenylcyclopropanes. Isotope effects and stereochemistry

Abstract

from an authentic sample.12 The surprisingly efficient lactonization-migration step13 is an intriguing aspect of this synthesis and is the subject of current investigation. The utility of this approach in preparing further analogs of rosenonolactone, e.g., the 11/3-hydroxy derivative, Rosein III,9 is also under examination. Supplementary Material Available.-Complete experimental details on all compounds described in this communication will appear following these pages in the microfilm edition of this volume of the journal. Photocopies of the supplementary material from this paper only or microfiche (105 X 148 mm, 20 X reduction, negatives) containing all of the supplementary material for the papers in this issue may be obtained from the Journals Department, American Chemical Society, 1155 16th St., N.W., Washington, D. C. 20036. Remit check or money order for 83.00 for photocopy or 82.00 for microfiche, referring to code number JOC-73-4090.

Photochemical studies on deuterium-labeled 2,2-dimethylphenylcyclopropanes

“…This system has the advantage that energy absorption, strain, and conformational considerations are common to all the compounds investigated. 20 We had guessed that two methyl groups at C2 would also decrease the rate of photochemical cis-trans isomerization in 12 vis-a-vis 1; i.e., either clockwise or counterclockwise rotation^a round the C2-C3 bond in diradical 15 will result in an increase in the steric interaction between the methyl group and the benzylic carbon. Since 180°rotation of Ci or C2 is necessary to interconvert geometrical isomers, to the extent that this steric interaction is important, isomerization will be disfavored.…”

“…A series of equations was written for compounds 16-19 and for a 1:1 mixture of 7 and 12 (20), relating the experimentally measurable (vide infra) ratio of 1,3-hydrogen to 1,3-deuterium migration (H/D) to the theoretical value for this quantity in terms of the primary = 7r, secondary type I = a, secondary type II = ß isotope effects and the fraction of migration originating from the m-methyl group = X.…”

“…Finally a mixture of dis-and conrotatory processes leading to a x cyclopropane type intermediate may be ruled out when the present results are considered in conjunction with stereochemical results on the benzylic carbon; i.e., the symmetry properties associated with the -cyclopropane intermediate, Figure 1, would require that hydrogen migration result in racemization at that position, whereas retention of stereochemistry is observed. 20 Possible mechanisms may be limited to a [ff2s + ct2s] reaction or one in which hydrogen migration takes place on cleavage of the cyclopropyl bond before rotation of the benzylic carbon has proceeded to any great extent. In fact differential rates of rotation around sp2 hybridized carbons with different substituents in thermally generated trimethylene diradicals are now well documented.3…”