Table 3. Selected optimization for ring expansion of cyclobutenone 3a.

Entry	Ligand	Precatalyst	T (°C)	Solvent	Time	Yield a
1	dppp	[Rh(COD)Cl]2	130	Dioxane	24 h	0%
2	dppb	[Rh(COD)Cl]2	130	Dioxane	24 h	0%
3	PPh3 b	[Rh(COD)Cl]2	150	Dioxane	24 h	0%
4	dppb	[Rh(COD)Cl]2	150	Dioxane	24 h	0%
5	dppp	[Rh(COD)OH]2	150	Dioxane	24 h	70% c
6	dppp	[Rh(COD)OMe]2	150	Dioxane	24 h	56% c
7	dppp	[lr(COD)OMe]2	150	Dioxane	24 h	0%
8	dppe	[Rh(COD)OH]2	150	Dioxane	24 h	50%
9	dppb	[Rh(COD)OH]2	150	Dioxane	24 h	86% c
10	dpppent	[Rh(COD)OH]2	150	Dioxane	24 h	76%
11	dppb	[Rh(COD)OH]2	130	Dioxane	24 h	81% c
12	None	None	130	Dioxane	24 h	0%
13	None	[Rh(COD)OH]2	130	Dioxane	24 h	37% (67%)
14	dppp	None	130	Dioxane	24 h	0%
15	None	none d	130	Dioxane	24 h	0%
18	dppb	[Rh(COD)OH]2	110	Dioxane	48 h	92%
19	dppb	[Rh(COD)OH]2	100	Dioxane	48 h	74% (80%)
20	dppb	[Rh(COD)OH]2	90	Dioxane	48 h	65% (82%)
21	dppb	[Rh(COD)OH]2	80	Dioxane	48 h	42% (77%)
22	dppb	[Rh(COD)OH]2	110	THF	48 h	80% (93%)
23	dppb	[Rh(COD)OH]2	110	Toluene	48 h	90%
24	dppb	[Rh(COD)OH]2	110	CH3CN	48 h	58% (73%)
25	dppb	[Rh(COD)OH]2	110	nBu2O	48 h	30% (79%)
26	dppb	[Rh(COD)OH] 2	110	PhEt	48 h	95% c

^aUnless otherwise noted, all yields were determined by ¹H NMR using 1,1,2,2-tetrachloroethane as the internal standard. Values in the parentheses are yields based on recovered starting material.

^b30 mol% of PPh₃ was used as the ligand.

^cIsolated yields.

^d20 mol% of NaOH was used.