Dichotomy of 1,3‐dipolar cycloreversions in a tetrasubstituted 2,5‐dihydro‐1,3,4‐thiadiazole

The cycloaddition of diphenyldiazomethane ( 8 ) to 16 thioketones at 40°C which furnishes tetrasubstituted 2,5‐dihydro‐1,3,4‐thiadiazoles, is followed by rapid N₂ loss (see the preceding paper), with one exception: For the dihydrothiadiazole 10 , the N₂ extrusion is slower by a factor of 4900 than its formation from 8 and 2,2,6,6‐tetramethycyclohexanethione ( 7 ). This elimination of N₂ is a 1,3‐dipolar cycloreversion which affords a thiocarbonyl ylide + N₂ As a consequence of steric hindrance in the example of 10 , a concomitant second cycloreversion furnishes thiobenzophenone ( 12 ) and the diazocyclohexane derivative 13 in an equilibrium. The complex kinetic system of Scheme 2 is confirmed by the irreversible interception of 13 with thioketone 14 . The structural conditions for retarded N₂ extrusions from dihydrothiadiazoles are discussed. © 2006 Wiley Periodicals, Inc. Heteroatom Chem 17:443–448, 2006; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/hc.20263