Table 1. Screening of Phosphorus-Based Acid Catalysts^a.

entry	catalyst	(mol %)	1a (%)b	3aa (%)b	4aa (%)b
1	(HO)P(O)H2	10	<5	>95	<5
		10c	<5c	79c	13c
		5	<5	>95	<5
2	(HO)2P(O)H	10	<95	<5	<5
3	(HO)3P(O)	10	<95	<5	<5
4	(HO)P(O)(OPh)2B	10	<5	72	<5
		5	<5	72	<5
		1	14	83	<5
5	(HO)P(OPh)2	10	<5	90	5
		5	31	62	<5
		1	70	26	<5
6	(HO)P(OEt)2A	10	<5	94	<5
		5	9	84	<5
		1	47	43	<5
7	P(OPh)3	10	5	69	11
8	P(OEt)3	10	<5	67	11

^aReaction conditions: 1a (0.5 mmol) and 2a (2 equiv) in technical cumene (0.5 mL).

^b1H NMR yields/recoveries are the average of two independent experiments and were determined with respect to 1,3,5-trimethoxybenzene as internal standard.

^c1.5 equiv of 2a.