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

✍ Scribed by Elke Langhals; Rolf Huisgen

• Publisher: John Wiley and Sons
• Year: 2006
• Tongue: English
• Weight: 192 KB
• Volume: 17
• Category: Article
• ISSN: 1042-7163
• DOI: 10.1002/hc.20263

✦ Synopsis

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 N 2 loss (see the preceding paper), with one exception: For the dihydrothiadiazole 10, the N 2 extrusion is slower by a factor of 4900 than its formation from 8 and 2,2,6,6tetramethycyclohexanethione ( ). This elimination of N 2 is a 1,3-dipolar cycloreversion which affords a thiocarbonyl ylide + N 2 As a consequence of steric hindrance in the example of 10, a concomitant second cycloreversion furnishes thiobenzophenone ( ) 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 N 2 extrusions from dihydrothiadiazoles are discussed.