Photodriven Dehydrogenative Homocoupling of Benzylic C–H Bonds Forming Strained C–C Bonds

Abstract

A photoinduced dehydrogenative homocoupling reaction of alkylarenes is reported. Gaseous hydrogen is evolved as the sole byproduct and neither oxidants nor hydrogen acceptors are required. The present reaction offers an environmentally benign and atom-economical means for forming sterically strained C–C single bonds. It also gives a remarkable example of photodriven reactions overcoming a considerable rise in energy.

Key words

hydrocarbons - dehydrogenation - dimerization - green chemistry - radical reaction

• References and Notes

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• 14 Dehydrogenative Coupling of 1: a Typical Procedure To an oven-dried 5 mL Schlenk tube were added cumene 1 (24.3 mg, 0.20 mmol), (Ir[dF(CF₃)ppy]₂dtbbpy)PF₆ (4.5 mg, 0.004 mmol), NiBr₂(dtbbpy) (2.0 mg, 0.004 mmol), and ethyl acetate (5.0 mL) under a nitrogen atmosphere. The tube was capped with rubber septa, and the solution was stirred and irradiated with blue LEDs at room temperature for 24 h. Then, the resulting mixture was filtrated through a short column of silica gel using ethyl acetate as the eluent. The filtrate was concentrated under reduced pressure. The residue was purified by PTLC (hexane/dichloromethane = 5:1, R_f = 0.5) to give 2,3-dimethyl-2,3-diphenylbutane (2, 20.2 mg, 0.085 mmol, 84%) as white solids. ¹H NMR (400 MHz, CDCl₃): δ = 7.15–7.24 (m, 6 H), 7.03–7.12 (m, 4 H), 1.32 (s, 12 H). ¹³C NMR (100 MHz, CDCl₃): δ = 146.8, 128.6, 126.6, 125.5, 43.6, 25.2. The NMR spectra were in agreement with those reported.⁴