Reaction optimization^a.

Entry	λ [nm]	Time [min]	Yieldb	Z : E ratiob
1	400	15	76%	50 : 50
2	400	30	72%	68 : 32
3	400	45	74% (74%)	74 : 26 (73 : 27)
4	400	60	54%	73 : 27
5	400	90	49%	73 : 27
6	369	45	66%	27 : 73
7	374	45	61%	44 : 56
8	383	45	64%	53 : 47
9	414	45	67%	60 : 40
10	435	45	72%	26 : 74
11	520	45	67%	<5 : 95

^a(i) Cp₂ZrHCl (62 mg, 0.24 mmol, 1.2 eq.), CH₂Cl₂ (1.5 mL), alkyne A-1 (36 mg, 0.2 mmol, 1.0 eq.) in CH₂Cl₂ (0.5 mL); (ii) irradiation; (iii) NBS (39 mg, 0.22 mmol, 1.1 eq.).

^baverage yield and Z : E ratio of two reactions determined by ¹H-NMR with DMF as internal standard; isolated yield of the Z : E-mixture and Z : E-ratio in parentheses.