The Photochemistry of N-Iminopyridinium Ylides in Retrospect. From a Simple Concept to Some Applications

Abstract

Azomethine-imines are well known to undergo ground-state 1,3-dipolar cycloaddition reactions with olefins or acetylenes, leading thereby to five-membered heterocyclic systems. When excited by UV light, some of these 1,3-dipolar azomethine-imines lead to the isomeric three-membered diaziridines via disrotatory electrocyclisation. The following review article shows that many N-iminopyridinium ylides – which are aromatic analogues of azomethine-imines – also lead to electrocyclisation from their photo-excited electronic (singlet) state to give the expected non-aromatic and boat-shaped 1H-1,2-diazepine isomers. Diazanorcaradienes – i.e. bicyclic diaziridines – are the postulated intermediates for these photoinduced ring-expansion processes.Ylides Y (Scheme 3) being mesoionic compounds have a large dipole moment; as a consequence, they are hydrophilic/lipophobic species, whereas diazepines D are poorly polar and, therefore, lipophilic/hydrophobic molecules. This very pronounced change of polarity triggered off the synthesis of some novel negative photoresists which are based on the photoisomerisation of water-soluble polymeric pyridinium ylides to the corresponding hydrophobic polymeric diazepines.Thermal activation of diazepines D led to ring contraction back to the corresponding ylides Y – obviously via norcaradiene intermediates N – and permitted to establish the kinetic and thermodynamic parameters for this process. Diazepines D are higher-energy (by ca. 85 kJ/mol) isomers of ylides Y. They react smoothly and chemospecifically as dipolarophiles with diazoisopropane to give pyrazoline cycloadducts. From these latter ones, novel small-ring compounds could be prepared, e.g. homodiazepines (cyclopropane derivatives), their tricyclic staircase-like isomers (fused cyclopropane/cyclobutane derivatives) as well as the first example of a cyclopropapyridine (a cyclopropene derivative).