Res. Time (minutes) a Reactions performed on a 0.3 mmol scale, 144 W of 365-nm LEDs. b Yields determined by 1 H NMR using trichloroethylene as an external standard. c A fresh solution of TBADT 2.5 mol% in 1.5 mL of the reaction mixture was added before the second run. d 2a/2b ratio determined to be 2:1 . See Supplementary Information for experimental details.