Table 1.

Optimisation of photocatalytic [2+2] cycloaddition.^[a]

	Deviation from standard conditions	dr (trans:cis)[b]	Yield [%][c]
1	none	3.6 : 1	83
2	[Ir{dF(CF3)ppy}2(dtbbpy)]PF6 (1 mol %)	3.3 : 1	41
3	DMF	1.6 : 1	6
4	HPLC grade THF	3.3 : 1	71
5	under air	3.2 : 1	23
6	argon sparged THF	3.5 : 1	63
7	no 4CzIPN	n.d.	<5
8	Reaction in the dark	n.d.	<5
9	0.5 mol % 4CzIPN, 1.00 mmol 1 b, 4 M, 48 h	3.4 : 1	56[d]

[a] The reaction mixture was degassed by three freeze–pump–thaw cycles prior to irradiation. For further details see the Supporting Information. [b] Diastereoselectivity was determined by ¹H NMR spectroscopy of the crude reaction mixture. [c] Yield estimated from the ¹H NMR of the reaction mixture relative to 1,3,5‐trimethoxybenzene as internal standard. [d] Isolated yield.