Table 1.

Screen of Possible Trace Oxygen Sources, Added in Substoichiometric Quantities

entry	Ta source	additivea	solvent	yield of 2 (%)b
1	1		C6H6	trace
2	1	O2	C6H6	0
3	1	O2c	C6H6	0
4	1d		C6H6	5
5	1d	H2O/O2	C6H6	5
6	1	H2O	C6H6	20
7	1	H2Oc	C6H6	0
8	1	H2O	C6H6/hexane	57
9	1	Ta2O5	C6H6	36

^a0.2 equiv except Ta₂O₅.

^b0.2 mmol of 1, 5 equiv of aniline, 5 mL of solvent, 135 °C, 48 h, N₂ atmosphere.

^cAdded after heating.

^dPartially oxidized.