Syntheses and Structural Characterizations of Benzoxazolyl N-Heterocyclic Carbene-Palladium Complexes and Their Applications

doi:10.6023/cjoc202311013

Abstract

Six seven-membered benzoxazolyl N-heterocyclic carbene (Benzoxa^NHC) palladium complexes with flexible linear alkyl or benzyl linkers were synthesized. Their structures were characterized by NMR, HRMS, and FTIR spectroscopies. The structures of three complexes were further confirmed by X-ray single crystal diffraction. Palladium center adopted slight distorted square planer geometry, and the benzoxazole and NHC planes in its ligand formed a certain angle with each other, not coplanar. The first methylene group connected to a nitrogen-containing heterocyclic carbene fragment exhibited magnetic anisotropy in the complex. The shift difference in the methylene group (Δδ_g) changed with the increase of the length of the side chain. When the side chain was a linear chain alkyl group of four carbons, Δδ_g reched maximum. Furthermore, these complexes showed excellent catalytic performance in Suzuki-Miyaura coupling, and could be used to synthesize biphenyl gem-difluoroethenes in the manner of two steps in one pot.

Key words: N-heterocyclic carbene-palladium complex, Suzuki-Miyaura reaction, gem-difluoroalkene

Cite this article

Chao Sun, Quan Zhou, Chuanying Li, Lei Wang. Syntheses and Structural Characterizations of Benzoxazolyl N-Heterocyclic Carbene-Palladium Complexes and Their Applications[J]. Chinese Journal of Organic Chemistry, 2024, 44(6): 1957-1966.

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Fig. & Tab. 11

Figure 1. Previous study in bidentate nitrogen NHC pre-ligands

Scheme 1. Synthesis route of NHC-palladium complexes

Figure 2. gem-Methylene diastereotopic protons chemical shift difference (Δδg) of Pd1 (a), Pd2 (b), Pd3 (c), Pd4 (d), Pd5 (e), Pd6 (f) in CDCl3 (298 K)

Figure 3. Molecular structure (ORTEP, 30% probability level) of Pd2 (CCDC 2280218) H atoms have been omitted for clarity. Selected bond lengths in nm, angles in (°): Pd(1)—C(1) 1.946(3), Pd(1)—N(3) 2.016(2), Pd(1)—Cl(2) 2.292(1), Pd(1)—Cl(1) 2.352(1), N(1)—C(1) 1.340(5), N(2)—C(1) 1.360(4), C(1)—Pd(1)—N(3) 86.10(12), C(1)—Pd(1)—Cl(2) 89.12(10), Cl(2)—Pd(1)—Cl(1) 92.50(2), Cl(1)—Pd(1)—N(3) 92.30(7), N(2)— C(1)—Pd(1) 125.47(22), N(1)—C(1)—Pd(1) 129.27(23), N(1)—C(1)—N(2) 105.26(27); C(1)—N(2)—C(4)—C(9) 52.89(48), C(4)—C(9)—C(10)—N(3) 46.69(58)

Figure 4. Molecular structure (ORTEP, 30% probability level) of Pd4 (CCDC 2280219) H atoms have been omitted for clarity. Selected bond lengths in nm, angles in (°): Pd(1)—C(1) 1.936(1), Pd(1)—N(3) 2.0128(1), Pd(1)—Cl(1) 2.3511(2), Pd(1)—Cl(2) 2.2991(1), N(1)—C(1) 1.3430(1), N(2)—C(1) 1.3679(1); C(1)—Pd(1)—N(3) 86.529(3), C(1)—Pd(1)—Cl(2) 91.606(3), Cl(1)—Pd(1)—Cl(2) 92.894(3), N(3)—Pd(1)—Cl(1) 89.020(3), N(2)—C(1)—Pd(1) 123.971(5), N(1)—C(1)—Pd(1) 130.839(4), N(2)—C(1)—N(3) 105.185(6); C(1)—N(2)—C(4)—C(9) 53.807(13), C(4)—C(9)—C(10)—N(3) 48.155(13)

Figure 5. Molecular structure (30% probability level) of Pd6 (CCDC 2280260) H atoms have been omitted for clarity. Selected bond lengths in nm, angles in (°): Pd(1)—C(1) 1.9663(2), Pd(1)—N(1) 2.0304(1), Pd(1)—Cl(1) 2.3506(2), Pd(1)—Cl(2) 2.2868(2), N(1)—C(1) 1.3433(1), N(2)—C(1) 1.3519(1), C(1)—Pd(1)—N(3) 86.861(4), C(1)—Pd(1)—Cl(2) 89.407(3), Cl(2)—Pd(1)—Cl(1) 91.960(3), N(3)—Pd(1)—Cl(1) 91.770(3), N(2)—C(1)—Pd(1) 126.168(5), N(1)—C(1)—Pd(1) 128.215(5), N(2)—C(1)—N(1) 105.603(5); C(1)—N(2)—C(4)—C(9) 51.851(11), C(4)—C(9)—C(10)—N(3) 45.316 (12)

Pd complex	Pd—H^21N	Pd—H^21F	Cl—H^21N	Cl—H^21F
Pd2	0.30878(3)	0.43335(4)	0.28112(10)	0.41352(10)
Pd4	0.29863(3)	0.42461(4)	0.28426(13)	0.39054(13)
Pd6	0.29519(5)	0.42365(5)	0.27876(12)	0.38604(12)

Table 1. Methylene protons distance (nm) with Pd/Cl

Entry	Base	Solvent	Yield/%
1	K₃PO₄	DMF	87
2	KH₂PO₄	DMF	74
3	K₂CO₃	DMF	98
4	Na₂CO₃	DMF	93
5	NaHCO₃	DMF	88
6	N(C₂H₅)₃	DMF	51
7	K₂CO₃	DMA	95
8	K₂CO₃	DME	30
9	K₂CO₃	MeOH	61
10	K₂CO₃	DMF	28^b

Table 2. Conditions optimization for Suzuki-Miyaura reactiona

Pd complex	Yield/%
Pd complex	100 mmol%	10 mmol%	1 mmol%
Pd1	98	84	46
Pd2	98	85	50
Pd3	97	82	54
Pd4	98	87	56
Pd5	98	89	57
Pd6	94	71	34

Table 3. Cross coupling yields of 4a with Pd1~Pd6 as catalystsa

Table 4. Substrate scopea

Scheme 2. One-pot synthesis of gem-difluoroethene via coupling Suzuki-Miyaura and Wittig olefination Reagent and condition: (i) arylboronic acid (10 mmol), 4-bromo benzaldehyde (10 mmol), Pd5 (10 mmol%), K2CO3 (30 mmol), 110 ℃, 16 h; (ii) PPh3 (15 mmol), sodium 2-chloro-2,2-difluoroacetate (2.0 equiv.). a Pd(PPh3)4 (5 mol%) instead of Pd5 (10 mmol%), N2.

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