Pergamon Tetrahedron Letters, Vol. 37, No. 40, pp. 7209-7212, 1996 Copyright O 1996 Elsevier Science Ltd Printed in Great Britain. All rights reserved PII: S0040-4039(96)01569-9 0040-4039/96 $15.00 + 0.00 Conformational Effects on the Excited State 1,2-Hydrogen Migration in Alkyldiazomethanes Sasha C. Reed and David A. Modarelli* Departmentof Chemistry Colgate University Hamilton,NY 13346 Abstract: Excited state rearrangements of alkyldiazo compounds are well-known. In particular, 1,2-hydrogen migrations to give carbene products are common. In this communication we provide evidence showing the migration in the diazo excited state is dependent upon the dihedral angle formed between the migrating hydrogen and the diazo carbon. Copyright © 1996 Elsevier Science Ltd The involvement of the excited state in diazo compounds and diazirines has recently been implicated in many reactions previously thought to proceed by a carbene intermediate.1 In particular, the recent efforts of Platz and co-workers have extensively documented the effects of bond strength on the excited state migration of hydrogen concurrent with nitrogen loss in arylalkyldiazo 2 compounds (i.e., Scheme 1) and alkyldiazirines. 3 Scheme 1 R/~CH2 x h~ R~LOH2 x ~ H R,"~CHX These workers have found that the amount of excited state migration linearly tracks the ¢t C-H bond strength of model systems. Inspired by their work on this rearrangement, and work showing a relationship between the rate of migration in alkylcarbenes as a function of the dihedral angle between the empty p-orbital of a singlet earbene and the migrating hydrogen, 4 we decided to study conformational effects on the excited state 1,2-H rearrangement.5 Herein, we report evidence that the rate of hydrogen migration in the excited state is dependent on the dihedral angle between the ~ C-H bond and the diazo group in alkyldiazo precursors. Laser flash photolysis experiments utilizing the pyridine-ylideprobe technique6 at saturating concentrations of pyrldine (Ay sat) reveals diazo compound 1 to have the highest yield of yllde (Table 1), 1 2 3 with 2 having 10% less and :3 yielding nearly 50% less. At these concentrations of pyridine, every carbene generated within the laser pulse is captured by pyridine (Scheme 2). Thus, any differences in yield must result from formation of olef'm by way of the excited state migration, and we can conclude that twice as much carbene is generated from 1 (and 36% from 2) than from 3. Product studies confirm these results (Table 1).7 When 3 is photolyzed in a static reactor at 300 nm in methanol, more than twice as much olefin is formed than for diazo 7209