Optimization of regioselective dihalogenation^a.

Entry	Z	[Z+] (equiv.)	T, °C	Yieldb, %
1	I (5b)	I2 (1.2)	rt	< 5
2		NIS (3.0)	rt	< 5
3		I–Cl (3.0)	rt	< 5
4		CHI3 (3.0)	rt	nrc
5		nC6F13–I (3.0)	rt	nr
6		nC 4 F 9 –I (5.0)	rt	(86)
7	Br (5e)	CBr4 (5.0)	rt	< 5
8		NBS (5.0)	rt	< 5
9		nC6F13–Br (5.0)	rt	6
10		C6F5–Br (20)	rt	17
11		nC8F17–Br (15)	50	(16)
12		C 6 F 5 –Br (15)	50	(91)
13	Cl (5h)	CCl4 (5.0)	rt	< 5
14		CCl4 (1.0 mL)	rt	nr
15		NCS (5.0)	rt	nr
16		C6F5–Cl (5.0)	rt	< 5
17		C6F5–Cl (15)	50	(30)
18		CCl 4 (15)	50	(53)

^aStandard conditions: 1a (0.10 mmol), Cs₂CO₃ (2.0 equiv.), TBAB (2.0 equiv.), [Z⁺] electrophile, THF (2.0 mL), argon, Temperature, 16 h.

^bYields determined by ¹H NMR analysis using CH₂Br₂ as internal standard; isolated yields in parentheses.

^cNo reaction.