Optimization of reaction conditions^a.

Entry	Activator	Solvent	Temp.	Yieldb of 3a : 4a
1	TFAA	MeCN	−50 °C	57% : 15%
2	TFAA	DCM	−50 °C	0% : 0%
3	TFAA	MeCN/DCM (10/1)	−50 °C	50% : 20%
4	TFAA	MeCN/DCM (9/1)	−50 °C	65% : 5%
5	TFAA	MeCN/DCM (8/1)	−50 °C	27% : 29%
6	Ts2O	MeCN/DCM (9/1)	−50 °C	0% : trace
7	Tf2O	MeCN/DCM (9/1)	−50 °C	0% : 17%
8	(ClF2CO)2O	MeCN/DCM (9/1)	−50 °C	61% : 8%
9	Tf2O	DCM	−78 °C	0% : 48%b
10	Tf2O	MeCN	−50 °C	0% : 19%b
11	Tf2O	DCM	−50 °C	0% : 35%b
12	Tf2O	DCM	−100 °C	0% : 73%b

^aUnless otherwise noted, the reaction was performed with 1a (0.3 mmol), 2a (1.5 equiv.), activator (1.5 equiv.) and Et₃SiH (3.0 equiv.).

^bThe reaction of 1a and 2a (3.0 equiv.) was performed with Tf₂O (1.5 equiv.) at the indicated temperature for 24 h and Et₃SiH was not used herein. For the investigation of other reaction parameters, see the ESI.