Proximate,syn-Periplanar, Rigid Imine(Nitrone)/Ene-, and Diazene(Diazeneoxy)/Ene Systems: Syntheses, Homoconjugate Reactivity and Photochemistry

Abstract

The chances for intramolecular imine/ene (→ azetidines), diazene/ene (→ 1,2‐diazetidines), diazeneoxy/ene (→ 1,2‐diazetidine oxides) and diazenedioxiene (→ 1,2‐diazetidine dioxides) [2+2]photocycloadditions and for the isolation of the respective photoproducts, have been probed with specifically designed substrates. Upon direct or sensitized excitation, [2+2]cycloaddition was found to be the exclusive or at least dominant chemical process for the CN/CC, NN/CC and ONN(O)/CC systems featuring very small π,π‐distances of 2.8‐3.0 Å and large π,π‐interorbital angles of 160‐170° (7 → 51, 17 → 55, 33 → 58 (competing N2 elimination), 22 → 62). This is not the case, however, in ONNO/CC (23, where electron transfer is a possibility), or in the more flexible, less “proximate” CN/CN (57) and CNO/CN (63) systems (π,π‐distances of >3.8 Å). While the corseted 1,2‐diazetidine photoadducts (55, 58) proved to be thermally stable, their N‐oxides (62, 65) were thermally too labile to be directly observable above ‐65 °C. For the latter's only fleeting existence, electronic rather than strain effects are held responsible (B3LYP/6‐31G* calculations). Very facile CNO/CC (12 → 13) and NNO/CC (22 → 24) [3+2]cycloadditions, homoconjugate addition of H2 and of dienophiles ([2+2+2]) to the diazene/ene 17 (→ 39, 41, 45) are manifestations of “proximity” in these bichromophoric skeletons.

Dynamic Covalent Chemistry: A Facile Room‐Temperature, Reversible, Diels–Alder Reaction between Anthracene Derivatives and N‐Phenyltriazolinedione

“…Ene reactions of 7 with various olefins have been reported 23. With certain olefins, compound 7 leads to the formation of 1,2‐azetidines 24a–c. Recently, it has been used as a dinitrogen equivalent where it undergoes a [4+2] cycloaddition reaction with aryl ketenimines to yield 1,2,4‐triazolo[1,2‐a]cinnolines 24d–e…”

Synthesis of azetidines by aza Paternò–Büchi reactions

“…14 ). 36 Relying on either direct or acetone-sensitized excitation, the desired azetidine ( 44 ) was isolated in 80–85% yield. Notably, the imine 43 used in this report did not require any conjugating group for successful reactivity.…”

First Higher‐Order Photocycloaddition to a CN Bond: 1,3‐Diazepines from Maleimides

“…Excitation of the CN bond is known to lead to isomerization, rearrangement, hydrolysis, oxidation, photoreduction, and photoalkylation 1. Although olefins have been reported to undergo cycloaddition to photoexcited imines containing electron‐withdrawing groups on the nitrogen atom,1a–c, 2 generally useful preparative examples of [2+2] photocycloadditions to CN bonds are very rare. Furthermore, the reverse situation in which the imine is the ground‐state partner during the cycloaddition with a photoexcited alkene has, to our knowledge, yet to be reported.…”