Rate measurements for thermolysis of 4-azidothiazoles in p-xylene solu
tion have identified neighbouring group effects from nitro (19-fold ra
te increase), phenyliminomethyl (16), formyl (4.5) and acetyl (2.2) su
bstituents in the 5-position. These effects are very similar in magnit
ude to those measured for 3-azidothiophenes, whereas in azidobenzenes
the rate increases are much larger, 5-Substituents in 4-azidothiazole
which are capable of conjugative donation (phenyl, ethyl propenoate) a
lso increased the reaction rate. The reactions involving neighbouring
group participation led to cyclized products from the 4-azidothiazoles
though not all of these products were stable under the thermolysis co
nditions. (C) 1997 Elsevier Science Ltd.