indicates that this transformation is possibly triggered by SET process, which is confirmed by the observed HR-MS of TEMPO-trapped product 14 (Fig. ). Interestingly, when excessive H 2 O (1.0 mL) was subjected to this photocatalysis system, benzyl ether 15 (27%) and hydroxy-diazomethylation product 3-1 (22%) could be simultaneously obtained, indicating that cationic-polar crossover between alcohols and benzylic cation intermediates possibly occurs (Fig. ). To further probe whether the hydroxyl oxygen atom of 3-1 is from molecular O 2 or H 2 O, we conducted the hydroxy-diazomethylation of 1a with 2-2 under pure O 2 , and found that alkene 1a and hypervalent iodine diazoester 2-2 basically remained intact. This result implies that O 2 possibly inhibits the initiating SET process, resulting in very poor reaction conversions (Fig. ). By contrast, when H 2 O 18 (10.0 eq.) was subjected to the standard reaction conditions with alkene 1a and α-diazoester 2-2