Switching Shapes: Reversible Three Species Photoisomerization of Substituted 1,2-Dihydro-1,2-azaborinines

TL;DR

This paper describes a reversible photochemical process involving substituted 1,2-dihydro-1,2-azaborinines that can switch between three different isomers under light.

Contribution

The study introduces a new method for synthesizing tetrasubstituted dihydroazaborinines and reveals a reversible three-species photoisomerization mechanism.

Findings

Aryl substitution at positions 3 and 5 causes a bathochromic shift in absorption.

Photoisomerization proceeds via two distinct excited states leading to Dewar and benzvalene isomers.

Thermal cycloreversion can regenerate the original structure or a substituted isomer.

Abstract

Derivatives of 1,2-dihydro-1,2-azaborinines generally
undergo selective
photochemical electrocyclic ring-closure reactions to the corresponding
Dewar isomers (2-aza-3-borabicyclo[2.2.0]­hex-5-ene). Depending on
the substitution pattern, these photoreactions can also yield benzvalene
(3-aza-4-boratricyclo­[3.1.0.02.6]­hexane) analogues. Here,
we report the synthesis of 1,2,3,5-tetrasubstituted dihydroazaborinines
by transition-metal-catalyzed late-stage functionalization and the
investigation of their photophysical and photochemical properties
using transient absorption spectroscopy. The introduction of aryl
groups at the 3- and 5-positions induces a pronounced bathochromic
shift of the absorption maximum. Under broad-spectrum irradiation
(280–400 nm), quantitative conversion to the benzvalene isomer
can be achieved. The initial photoisomerization proceeds via excitation
to the short-lived singlet excited state (S1) yielding
the Dewar isomer, whereas the subsequent conversion of this intermediate
occurs through a long-lived excited state. Notably, the second isomerization
step is accompanied by an interchange of the carbons C3 and C4. Once
formed, the benzvalene isomers exhibit exceptional thermal stability.
Cycloreversion to the Dewar isomer and even to the dihydroazaborinine
structure can be triggered photochemically through targeted excitation
and during both processes the substituents return to the C3 and C5
positions. The thermal cycloreversion of the benzvalene isomer can
yield either the educt BN-benzene isomer (1,2,3,5-substitued) or its
1,2,4,5-substituted isomer. Computational studies revealed a stepwise
mechanism for the thermal back reaction reforming the educt, while
a concerted, energetically less-favorable pathway leads to the 1,2,4,5-substituted
analogue.

Figures