Breaking the bottleneck: stilbene as a model compound for optimizing 6π e - photocyclization efficiency.

TEMPO was more suitable at photocyclizing stilbene than iodine. As stilbene concentration increased, TEMPO produced a higher yield of phenanthrene at shorter times and significantly reduced the potential for undesired [2+2] cycloadditions. Iodine retarded phenanthrene formation because it promoted isomerization to ( E )-stilbene which encouraged [2+2] cycloaddition.