Pd–PEPPSI-type expanded ring N-heterocyclic carbene complexes: synthesis, characterization, and catalytic activity in Suzuki–Miyaura cross coupling

Figures & data

Scheme 1. Synthesis of NHC precursors 2a-f.

Table 1. ¹JCH coupling values of NHC Ligands 2a-f in Hz.

S. No	Ligands	^1JCH (Hz)
1.	2a	165.44
2.	2b	162.29
3.	2c	164.35
4.	2d	225.19
5.	2e	220.16
6.	2f	226.18

Scheme 2. Synthesis of Pd-PEPPSI complexes 3.

Table 2. The optimization for the Suzuki-Miyaura reaction of 4-bromoanisole^a.

Entry^a	Solvent	Base	Yield(%)^b
1	i-PrOH	K2CO3	75
2	i-PrOH	Na2CO3	58
3	i-PrOH	Cs2CO3	48
4	i-PrOH	K3PO4	59
5	i-PrOH	KOH	49
6	H2O	K2CO3	45
7	DMF	K2CO3	53
8	EtOH	K2CO3	69
9	MeOH	K2CO3	68
10	Acetone	K2CO3	45
11	i-PrOH/H2O(1:3)	K2CO3	97
12	i-PrOH/H2O(1:1)	K2CO3	94
13	DMF/H2O(1:3)	K2CO3	79
14	EtOH/H2O(1:3)	K2CO3	87
15^c	i-PrOH/H2O(1:3)	K2CO3	0.3

^a Reaction Condition:1 mmol of 4-bromoanisole, 1.1 mmol of phenylboronic acid, 1 mmol of base, 0,1 mol% complex 3a, 4 mL solvent, room temperature, 1 h.

^b Isolated yield.

^c The reaction was conducted under ligandless conditions and with a trace amount of palladium chloride.

Table 3. Catalytic activities of 3a-f complexes in the Suzuki coupling reactions under optimized conditions.^a

Entry	Product	[Pd-NHC]/yield (%)^b
		3a	3b	3c	3d	3e	3f
1		80	82	75	54	72	93
2^c		96	88	91	87	89	95
3		80	67	52	51	57	59
4		66	71	74	54	65	76
5^c		94	96	85	88	91	96
6^c		90	81	83	74	90	92

^a Reaction condition: 0.1 mol % [Pd], 1 mmol p-R-C₆H₄Br, 1.1 mmol phenylboronic acid, 1.0 mmol K₂CO₃, rt, 1h.

^b Isolated product.

^c 10 min.

Table 4. Catalytic activities of 3a complex in the Suzuki coupling reactions under optimized conditions.^a

Entry	R1	Time (h)	Yield^b
1	4-CH3	12 h	4
2	2-CH3	12 h	3
3	4-NO2	12 h	71
4	4-OCH3	12 h	1

^a Reaction condition: 0.1 mol % [Pd], 1 mmol p-R₁-C₆H₄Cl, 1.1 mmol phenylboronic acid, 1.0 mmol K₂CO₃, rt, 1h.

^b Isolated product.