Substituent Directed Phototransformations of BN-Heterocycles: Elimination vs Isomerization via Selective B–C Bond Cleavage

Electron-rich and
-poor BN-heterocycles with benzyl-pyridyl backbones and two bulky
aryls on the boron (Ar = tipp, BN-1, Ar = MesF, BN-2) have been found to display distinct molecular transformations
upon irradiation by UV light. BN-1 undergoes an efficient
photoelimination reaction forming a BN-phenanthrene with ΦPE = 0.25, whereas BN-2 undergoes a thermally
reversible, stereoselective, and quantitative isomerization to a dark
colored BN-1,3,5-cyclooctatriene (BN-1,3,5-COT, BN-2a). This unusual photoisomerization persists for other BN-heterocycles
with electron-deficient aryls such as BN-3 with a benzyl-benzothiazolyl
backbone and MesF substituents or BN-4 with
a benzyl-pyridyl backbone and two C6F5 groups
on the boron. The photoisomerization of BN-4 goes beyond
BN-1,3,5-COT (BN-4a), forming a new species (BN-1,3,6-COT, BN-4b) via C–F bond cleavage and [1,3]-F atom sigmatropic
migration. Computational studies support that BN-4a is
an intermediate in the formation of BN-4b. This work
establishes that steric and electronic factors can effectively control
the transformations of BN-heterocycles, allowing access to important
and previously unknown BN-embedded species.