The reaction efficiency of 1b was then comparable to that achieved with its 4-fluorobenzoyl derivative (1a→2a (67%)), showing how the benefit of exogenous promotor H-TEDA(BF 4 ) 2 is at least comparable to the endogenous promoting photosensitization auxiliary. Clearly, the exogenous promotor is a more generally applicable and useful strategy. Regarding other Brønsted acidic additives, interestingly, those with a pK a ~9 were most effective promotors (entries 3,4), while AcOH with a higher pK a (entry 2) and those with lower pK a s (entries 5-7)