The Dewar Isomer of 1,2‐Dihydro‐1,2‐azaborinines: Isolation, Fragmentation, and Energy Storage

The photochemistry of 1,2‐dihydro‐1,2‐azaborinine derivatives was studied under matrix isolation conditions and in solution. Photoisomerization occurs exclusively to the Dewar valence isomers upon irradiation with UV light (>280 nm) with high quantum yield (46 %). Further photolysis with UV light (254 nm) results in the formation of cyclobutadiene and an iminoborane derivative. The thermal electrocyclic ring‐opening reaction of the Dewar valence isomer back to the 1,2‐dihydro‐1‐tert‐butyldimethylsilyl‐2‐mesityl‐1,2‐azaborinine has an activation barrier of (27.0±1.2) kcal mol−1. In the presence of the Wilkinson catalyst, the ring opening occurs rapidly and exothermically (ΔH=(−48±1) kcal mol−1) at room temperature. NB: A boron–nitrogen derivative of Dewar benzene was generated by irradiation of a BN arene with UV light (see scheme) and found to be stable in solution. The thermal reverse reaction has a high activation barrier and can be catalyzed by the Wilkinson catalyst. The system comprises a new valence isomer pair that is in principle suitable for energy storage.