Photochemistry of 3‐Substituted 1‐Iminopyridinium Ylides . Regiospecific versus non‐regiospecific photoisomerization patterns [1]

3‐Substituted 1‐iminopyridinium ylides 1 undergo photo‐induced ring enlargement to 1 H ‐1,2‐diazepines. With strongly electron‐withdrawing substituents the ring expansion process is regiospecific and leads exclusively to 4‐substituted 1 H ‐1, 2‐diazepines. Weak electron‐donating substituents, like a methyl group and halogen atoms, do not have any directing effect since both 4‐ and 6‐substituted 1 H ‐1,2‐diazepines are obtained. With strong electron‐donating substituents no diazepines are formed; instead one observes photo‐induced isomerization to the 2‐aminopyridine derivatives, the process being non‐regiospecific. Regiospecific photo‐induced ring expansion processes are explained in terms of a simple HMO model.