Highly efficient pincer nickel catalyzed cross-coupling of aryltrimethylammonium triflates with arylzinc reagents

Abstract

N,N,P-Pincer-nickel-complex-catalyzed cross-coupling of aryltrimethylammonium triflates with aryl- or heteroaryl-zinc reagents was investigated. The reaction is suitable for a broad scope of substrates, exhibits good functional group compatibility and can be performed under mild conditions with extremely low catalyst loadings.

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Introduction

Since the 1970s a series of methodologies have been developed to construct C–C bonds through transition-metal-catalyzed cross-coupling, including Kumada coupling, Negishi coupling, Suzuki coupling, Stille coupling, and so on.¹ The electrophiles employed in these coupling reactions are principally organic halides and oxygen-containing compounds such as triflates, tosylates, mesylates, and carboxylates.^1–6 It is relatively rare to employ nitrogen-based electrophiles such as aromatic amines in these reactions due to the inertness of the C–N bonds. However, some trials have been performed over the years. An effective tactic is to transform aromatic amines to ammonium salts or diazonium salts to weaken the C–N bonds before catalytic cleavage.^7,8 For example, Wenkert and Reeves respectively carried out nickel- or palladium-catalyzed reactions of aryltrimethylammonium salts with Grignard reagents.^7a,d Blakey and MacMillan reported cross-coupling of aryltrimethylammonium triflates with arylboronic acids using the Ni(cod)₂/IMes catalyst.^7b Our group carried out the reaction of aryltrimethylammonium salts with organozinc reagents using Ni(PCy₃)₂Cl₂ or pincer nickel complexes (1–4, Chart 1) as the catalysts.^7e–h On the basis of the results achieved, we intended to further develop highly effective catalyst systems for the activation and transformation of C–N bonds of aromatic ammonium salts through tuning electronic and steric effects of the pincer ligands. Herein, we report P,N,N-pincer nickel (5a–5c,^3dChart 1) catalyzed cross-coupling of aryltrimethylammonium salts with arylzinc reagents.

Chart 1 Pincer nickel complexes.

Results and discussion

We performed a preliminary screen using the 5a-catalyzed cross-coupling of phenyltrimethylammonium triflate with p-MeOC₆H₄ZnCl as the model reaction (Table 1). We first examined the reaction using similar reaction conditions to those for 5a-catalyzed reaction of aryl chlorides with arylzinc reagents as we reported earlier.^3d Thus, the reaction was run in a 2 : 1 mixture of THF and NMP with 0.01 mol% catalyst loading. It was found that the reaction could proceed at room temperature and gave the corresponding cross-coupling product in 63% yield in 12 h. The product yield was improved when the reaction temperature was raised, being 92% at 45 °C and 94% at 65 °C (Table 1, entries 1–3). Further enhancing the reaction temperature, prolonging the reaction time or increasing the amount of the zinc reagent did not further improve the yields (Table 1, entries 4–6). Different solvents including a 1 : 1 mixture of THF and NMP, THF, a 2 : 1 mixture of THF and toluene, and a 2 : 1 mixture of THF and DMF were examined. The results showed that these solvents were less effective than the 2 : 1 mixture of THF and NMP (Table 1, entries 7–10). We also noticed that the zinc reagent prepared from ZnCl₂ and the corresponding Grignard reagent in the presence of 2 equiv. of LiCl gave better results than those prepared from ZnCl₂ and an equiv. of p-MeOC₆H₄Li or p-MeOC₆H₄MgBr in the absence of a LiCl additive (Table 1, entries 11 and 12). This implies that both magnesium ions and lithium ions play important roles in the reaction. The counterion effect of the ammonium salts was also examined with I⁻, Br⁻, Cl⁻, and BF₄⁻, respectively, as counterions in a 2 : 1 mixture of THF and NMP at 65 °C (Table 1, entries 13–16). The iodide salt exhibited the highest reactivity and the tetrafluoroborate salt showed the lowest reactivity. However, each of them was less reactive than the triflate salt. Complexes 5b and 5c exhibited a similar catalytic activity to complex 5a under comparable conditions. Complex 5b led to 93% product yield and complex 5c resulted in 91% product yield under the conditions as shown in Table 1 (entries 17 and 18). It seems that the substituents on the phosphorus atoms of the ligands affect the catalytic properties to a small extent.

Table 1 Evaluation of the catalysts and screening of reaction conditions^a

Entry	X	Cat. (mol%)	Solvent	T (°C)	Yield^b (%)
a Unless otherwise specified, the reactions were carried out on a 0.5 mmol scale according to the conditions indicated by the above equation; 0.75 mmol p-MeOC6H4ZnCl was employed. p-MeOC6H4ZnCl was prepared from ZnCl2 and an equiv. of p-MeOC6H4MgBr in the presence of two equiv. of LiCl. b Isolated yield. c The reaction was run for 20 h. d 1.0 mmol p-MeOC6H4ZnCl was employed. e p-MeOC6H4ZnCl was prepared from ZnCl2 and an equiv. of p-MeOC6H4Li. f p-MeOC6H4ZnCl was prepared from ZnCl2 and an equiv. of p-MeOC6H4MgBr.
1	OTf	5a	THF–NMP (2 : 1)	25	63
2	OTf	5a	THF–NMP (2 : 1)	45	92
3	OTf	5a	THF–NMP (2 : 1)	65	94
4	OTf	5a	THF–NMP (2 : 1)	90	94
5	OTf	5a	THF–NMP (2 : 1)	65	93^c
6	OTf	5a	THF–NMP (2 : 1)	65	92^d
7	OTf	5a	THF–NMP (1 : 1)	65	86
8	OTf	5a	THF	65	Trace
9	OTf	5a	THF–Toluene (2 : 1)	65	Trace
10	OTf	5a	THF–DMF (2 : 1)	65	45
11	OTf	5a	THF–NMP (2 : 1)	65	33^e
12	OTf	5a	THF–NMP (2 : 1)	65	41^f
13	I	5a	THF–DMF (2 : 1)	65	87
14	Br	5a	THF–NMP (2 : 1)	65	74
15	Cl	5a	THF–NMP (2 : 1)	65	76
16	BF4	5a	THF–NMP (2 : 1)	65	59
17	OTf	5b	THF–NMP (2 : 1)	65	93
18	OTf	5c	THF–NMP (2 : 1)	65	91

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Complexes 5a–5c are much more active than the reported catalysts for the cross-coupling of aryltrimethylammonium salts with arylzinc reagents. For example, the reaction of phenyltrimethylammonium salts with p-MeOC₆H₄ZnCl catalyzed by complexes 1–4 or Ni(PCy₃)₂Cl₂ requires 1–2 mol% catalyst loadings and 65–90 °C reaction temperature, giving 72–99% product yields.^7e–h By contrast, the same reaction catalyzed by 5a requires only 0.01 mol% catalyst loading and 45–65 °C reaction temperature, giving 92–94% product yields (Table 1).

The substrate scope was tested using 5a as the catalyst under the optimized conditions. 1-Naphthyltrimethylammonium triflate is as active as phenyltrimethylammonium triflate in the catalytic reaction. Its reaction with either p-MeOC₆H₄ZnCl or p-Me₂NC₆H₄ZnCl in the presence of 0.01 mol% 5a gave the corresponding cross-coupling products in excellent yields (Table 2, entries 1 and 2). The electron-rich aryltrimethylammonium triflates involving m-MeOC₆H₄NMe₃⁺OTf⁻, m-PivOC₆H₄NMe₃⁺OTf⁻, p-MeOC₆H₄NMe₃⁺OTf⁻, and o-MeC₆H₄NMe₃⁺OTf⁻ also reacted smoothly with arylzinc chlorides under the catalysis of 5a (Table 2, entries 3–6). However, the reaction of p-MeOC₆H₄NMe₃⁺OTf⁻ required a higher catalyst loading (0.05 mol%) to reach completion compared with other substrates. The steric hindrance of the methyl group in o-MeC₆H₄NMe₃⁺OTf⁻ did not affect the reaction result; the reaction of o-MeC₆H₄NMe₃⁺OTf⁻ with p-MeOC₆H₄ZnCl in the presence of 0.01 mol% 5a led to 91% yield of 4′-methoxy-2-methylbiphenyl (Table 2, entry 6). However, more sterically hindered 2,4,6-Me₃C₆H₂NMe₃⁺OTf⁻ showed much lower reactivity in comparison with o-MeC₆H₄NMe₃⁺OTf⁻. Its reaction with p-MeOC₆H₄ZnCl using 0.01 mol% 5a as the catalyst gave only 37% yield of the cross-coupling product, and with p-Me₂NC₆H₄ZnCl in the presence of 0.05 mol% 5a led to 60% yield of the desired product (Table 2, entries 7 and 8). The electron-deficient aryltrimethyl-ammonium triflates involving p-PhC(O)C₆H₄NMe₃⁺OTf⁻, p-EtOC(O)C₆H₄NMe₃⁺OTf⁻, p-Et₂NC(O)C₆H₄NMe₃⁺OTf⁻, p-CF₃C₆H₄NMe₃⁺OTf⁻ and p-Et(O)₂SC₆H₄NMe₃⁺OTf⁻ exhibited good reactivity in the 5a-catalyzed reactions with p-MeC₆H₄ZnCl, p-MeOC₆H₄ZnCl or p-Me₂NC₆H₄ZnCl (Table 2, entries 9–17), affording the corresponding products in 76–99% yields. Among the reactions, the reaction of p-EtOC(O)C₆H₄NMe₃⁺OTf⁻ with p-MeC₆H₄ZnCl can be carried out at room temperature with 0.01 mol% catalyst loading; whereas the reaction of p-CF₃C₆H₄NMe₃⁺OTf⁻ required some higher catalyst loadings (0.03 mol%). The reaction of o-MeC₆H₄ZnCl with electron-deficient electrophiles such as p-PhC(O)C₆H₄NMe₃⁺OTf⁻ and p-EtOC(O)C₆H₄NMe₃⁺OTf⁻ proceeded smoothly and both gave 99% yields of the cross-coupling products. It seems that the steric hindrance of the o-methyl group of o-MeC₆H₄ZnCl did not affect the reaction results. However, the reaction of electron-rich p-MeOC₆H₄NMe₃⁺OTf⁻ with o-MeC₆H₄ZnCl was more difficult to perform. The higher catalyst loading (0.1 mol%) was required and only 67% product yield was achieved. The electron-deficient zinc reagent, p-CF₃C₆H₄ZnCl, was less reactive in the catalytic reaction. Its reaction with the activated aryltrimethylammonium salts proceeded smoothly with 0.01–0.05 mol% catalyst loadings, but resulted in little lower yields compared with those using p-MeC₆H₄ZnCl (Table 2, entries 23–25). The reaction of p-CF₃C₆H₄ZnCl with m-PivOC₆H₄NMe₃⁺OTf⁻ also gave a good product yield when 0.05 mol% 5a was employed. However, the reaction with p-MeOC₆H₄NMe₃⁺OTf⁻ afforded poor results. It required the loading of 1 mol% 5a and gave only 39% product yield (Table 2, entry 27). The catalyst was also demonstrated to be applicable for the reaction of the heteroarylammonium substrate. The reaction of 2-pyridyltrimethylammonium triflate with either p-MeC₆H₄ZnCl or p-MeOC₆H₄ZnCl gave desired products in 79% and 81% yields, respectively, but the catalyst loadings were higher than those for electron-deficient substituted phenyl systems (Table 2, entries 28 and 29).

Table 2 Cross-coupling of arylzinc chlorides with aryltrimethyl ammonium triflates catalyzed by 5a^a

Entry	Ar	Ar^1ZnCl	5a (mol%)	Yield^b (%)
a Unless otherwise specified, the reactions were carried out on a 0.5 mmol scale according to the conditions indicated by the above equation, 1.5 equiv. of ArZnCl were employed. The zinc reagents were prepared from ZnCl2 and an equiv. of Grignard reagents in the presence of two equiv. of LiCl. b Isolated yield. c The reaction was run at 100 °C. d The reaction was run at 25 °C.
1		p-MeOC6H4ZnCl	0.01	93
2		p-Me2NC6H4ZnCl	0.01	99
3		p-MeC6H4ZnCl	0.01	95
4		p-MeC6H4ZnCl	0.01	84
5		p-MeC6H4ZnCl	0.05	87
6		p-MeOC6H4ZnCl	0.01	91
7		p-MeOC6H4ZnCl	0.01	37
8		p-Me2NC6H4ZnCl	0.05	60^c
9		p-MeC6H4ZnCl	0.01	96
10		p-MeOC6H4ZnCl	0.01	93
11		p-MeC6H4ZnCl	0.01	99^d
12		p-MeOC6H4ZnCl	0.01	93
13		p-Me2NC6H4ZnCl	0.01	99
14		p-MeC6H4ZnCl	0.01	81
15		p-MeOC6H4ZnCl	0.01	88
16		p-MeC6H4ZnCl	0.03	92
17		p-MeOC6H4ZnCl	0.03	87
18		p-MeC6H4ZnCl	0.01	82
19		p-MeOC6H4ZnCl	0.01	76
20		o-MeC6H4ZnCl	0.01	99
21		o-MeC6H4ZnCl	0.01	99
22		o-MeC6H4ZnCl	0.1	67
23		p-CF3C6H4ZnCl	0.01	81
24		p-CF3C6H4ZnCl	0.05	82
25		p-CF3C6H4ZnCl	0.05	79
26		p-CF3C6H4ZnCl	0.05	80
27		p-CF3C6H4ZnCl	1	39
28		p-MeC6H4ZnCl	0.5	79
29		p-MeOC6H4ZnCl	0.5	81

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Two electron-rich heteroarylzinc reagents, 2-furylzinc chloride and 2-thienylzinc chloride, were also used in the coupling (Table 3). 2-Furyllithium and 2-thienyllithium are more convenient to prepare than the corresponding magnesium reagents. Hence the zinc reagents were prepared from the heteroaryllithiums and ZnCl₂. The reactions using the zinc reagents showed relatively low reaction efficiency (Table 3, entries 1–7). For example, their reactions required higher catalyst loadings and often higher reaction temperatures in comparison with those listed in Table 2. However, one equiv. of MgBr₂ and one equiv. of LiCl additives markedly improved the reactions (Table 3, entries 8, 9, 11 and 12). MgBr₂ alone as an additive was less effective than the combination of MgBr₂ and LiCl (Table 3, entry 10). 2-Thienylzinc chloride exhibited lower reactivity than 2-furylzinc chloride even in the presence of additives, the former requiring higher catalyst loading, higher reaction temperature and gave lower product yields.

Table 3 Cross-coupling of 2-furyl- or 2-thienylzinc chloride with aryltrimethylammonium triflates catalyzed by 5a^a

Entry	Ar	HetArZnCl	5a (mol%)	Yield^b (%)
a Unless otherwise specified, the reactions were carried out on a 0.5 mmol scale according to the conditions indicated by the above equation, 1.5 equiv. of ArZnCl were employed. The heteroarylzinc chlorides were prepared from ZnCl2 and an equiv. of heteroaryllithium. b Isolated yield. c The reaction was run at 90 °C. d One equiv. of MgBr2 and one equiv. of LiCl were added. e One equiv. of MgBr2 was added.
1			1	86
2			1	80
3			1	77^c
4			1	41
5			2	45^c
6			2	72^c
7			2	86
8			0.05	90^d
9			0.05	88^d
10			0.05	59^e
11			0.5	72^c^,^d
12			0.5	56^c^,^d

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This experimental fact on the role of additives further shows that both magnesium and lithium ions are important for the success of reactions. The effect of inorganic salt additives for the Negishi reaction has been reported by several groups.⁹ Recently, studies on the reactivity of RZnCl in the presence of LiCl and MgX₂ or of RMgX in the presence of LiCl and ZnCl₂ were also reported.¹⁰ The enhancement of the reactivity of the zinc reagents by using inorganic salt additives may result from the breaking-down of the ArZnX aggregation through formation of the adduct.^9g Whereas in the three-component system Hevia et al. attributed the dramatic increase in the chemoselectivity of the reactions to the existence of a trilateral Li/Mg/Zn synergistic effect.^10a In our reaction, the low reactivity of the zinc reagent prepared from a Grignard reagent and ZnCl₂ should be due to the aggregation of the arylzinc reagent with the coproduct MgCl₂.¹¹ One of the roles of LiCl might be to break the aggregation by forming trimetallic species.^10a,b On the other hand, LiCl and MgX₂ are likely to enhance the reactivity of ArZnCl by forming the more nucleophilic zincates.^9,10a

The exact catalytic cycle is still unknown at present. However, some experimental facts have been derived. The yield of the reaction between PhNMe₃⁺OTf⁻ and p-MeOC₆H₄ZnCl did not change in the presence of a drop of Hg. This result ruled out the possibility of nickel particles or colloids formed from the decomposition of the nickel complex, as the catalyst.¹² The catalytic reaction is inhibited by 1,1-diphenylethene. When 10 mol% of 1,1-diphenylethene was added into the reaction system composed of PhNMe₃⁺OTf⁻, p-MeOC₆H₄ZnCl and 0.01 mol% or 1 mol% 5a, no desired cross-coupling product was observed. We thought that the reaction may involve a radical intermediate based on the result. However, we could not confirm the process because the reaction of 1,1-diphenylethene with active nickel species formed in the process of the reaction may also give rise to inactivation of the catalyst. On the other hand, a combination of Ni(COD)₂ and [Li{N(2-Ph₂PC₆H₄)(2′-Me₂NC₆H₄)}] exhibited a similar catalytic activity to 5a; 0.01 mol% catalyst loading resulted in 90% product yield for the cross-coupling of PhNMe₃⁺OTf⁻ with p-MeOC₆H₄ZnCl. This catalytic reaction was also inhibited by the 1,1-diphenylethene additive (10 mol%). Hence the active catalyst may be a Ni(0) species. But we cannot rule out the possibility of a Ni(I) active intermediate¹³ formed through an electron transfer from Ni(0) to ArNMe₃⁺OTf⁻.¹⁴ Additional studies on the reaction mechanism are underway.

Conclusions

We have demonstrated that N,N,P-pincer nickel complexes, [Ni(Cl){N(2-R₂PC₆H₄)(2′-Me₂NC₆H₄)}] (R = Ph, Prⁱ or Cy) are highly efficient catalysts for the cross-coupling of aryltrimethylammonium triflates and (hetero)arylzinc reagents. In most cases a very small amount of the catalyst and mild reaction conditions are required. The reaction is suitable for a wide scope of substrates, covering electron-rich and electron-deficient electrophiles and nucleophiles. A range of functional groups involving PhC(O), EtOC(O), Et₂NC(O), CF₃, EtSO₂, PivO, OMe, and NMe₂ can be tolerated. In comparison with the N,N,N- and P,N,P-pincer nickel complexes [Ni(Cl){N(2-Me₂NC₆H₄)₂}] and [Ni(Cl){N(2-R₂PC₆H₄)₂}] (R = Ph, Prⁱ or Cy) reported previously,^15,16 the N,N,P-pincer nickel complexes 5a–5c feature the aryl C–Cl, C–F or C–N bond activation.^3d The N,N,N-pincer nickel complex [Ni(Cl){N(2-Me₂NC₆H₄)₂}] is an excellent catalyst for the sp³ C–halide bond activation of alkyl halides; and the P,N,P-pincer nickel complexes [Ni(Cl){N(2-R₂PC₆H₄)₂}] can activate only the C–Br or C–I bonds of aryl halides rather than the unreactive C–Cl or C–F bonds. The difference in the catalytic properties of these complexes may result from the electronic effects of the ligands because the N,N,N-pincer ligand is “harder” compared with the “softer” P,N,P-pincer ligands.^15c This difference in coordination properties of the ligands may also lead to a different reaction mechanism in the nickel-complex-catalyzed cross-coupling reactions.

Experimental section

All reactions were performed under a nitrogen atmosphere using standard Schlenk and vacuum line techniques. THF was distilled under nitrogen over sodium/benzophenone and degassed prior to use. Toluene was distilled under nitrogen over sodium and degassed prior to use. NMP and DMF were dried over 4 Å molecular sieves, fractionally distilled under reduced pressure and stored under a nitrogen atmosphere. LiBuⁿ and methyl triflate were purchased from Acros Organics and used as received. CDCl₃ was purchased from Cambridge Isotope Laboratories and used as received. Aryldimethylamines were obtained from commercial vendors and purified by distillation under reduced pressure or recrystallization prior to use. Aryltrimethylammonium triflates,^7d Grignard reagents,¹⁷ 2-furyllithium,¹⁸ and 2-thienyllithium¹⁹ were prepared according to literature procedures. NMR spectra were recorded on a Bruker Avance III 400 NMR spectrometer at room temperature. The chemical shifts of the ¹H NMR spectra were referenced to a TMS or solvent residual signal; the chemical shifts of the ¹³C NMR spectra were referenced to the internal solvent resonances. High-resolution mass spectra (HRMS) were acquired using a Thermo Orbitrap XL ETD mass spectrometer (APCI).

General procedure for the cross-coupling of aryltrimethyl-ammonium triflates

A Schlenk tube was charged with aryltrimethylammonium triflate (0.5 mmol), NMP (0.80 cm³) and a solution of complex 5a (0.1 cm³, 0.0005 M solution in THF, 0.00005 mmol). To the stirred mixture ArZnCl solution (1.5 cm³, 0.5 M solution in THF, 0.75 mmol) was added by using a syringe. The reaction mixture was stirred at 65 °C for 12 h. Water (10 cm³) and several drops of hydrochloric acid were successively added. The mixture was extracted with Et₂O (3 × 10 cm³). The combined organic phases were dried over anhydrous Na₂SO₄, concentrated by rotary evaporation, and purified by column chromatography (silica gel).

Product characterization

4-Methoxybiphenyl²⁰. ¹H NMR (400 MHz, CDCl₃): δ 3.88 (s, 3H, OMe), 7.02 (d, J = 8.7 Hz, 2H, Ar), 7.35 (t, J = 7.4 Hz, 1H, Ar), 7.46 (t, J = 7.6 Hz, 2H, Ar), 7.59 (t, J = 8.6 Hz, 4H, Ar). ¹³C NMR (101 MHz, CDCl₃): δ 55.43, 114.31, 126.78, 126.85, 128.27, 128.85, 133.86, 140.93, 159.25.

1-(4-Methoxyphenyl)naphthalene²¹. ¹H NMR (400 MHz, CDCl₃): δ 3.92 (s, 3H, OMe), 7.07 (d, J = 8.7 Hz, 2H, C₆H₄), 7.43–7.56 (m, 6H, Ar), 7.87 (d, J = 8 Hz, 1H, Ar), 7.93 (d, J = 7.6 Hz, 1H, Ar), 7.97 (d, J = 8.4 Hz, 1H, Ar). ¹³C NMR (101 MHz, CDCl₃): δ 55.47, 113.82, 125.54, 125.83, 126.05, 126.19, 127.04, 127.46, 128.39, 131.24, 131.93, 133.22, 133.95, 140.01, 159.04.

N,N-Dimethyl-(4-naphthalen-1-yl)aniline²². ¹H NMR (400 MHz, CDCl₃): δ 3.09 (s, 6H, NMe), 6.94 (d, J = 8.8 Hz, 2H, C₆H₄), 7.47–7.60 (m, 6H, Ar), 7.88 (d, J = 8.1 Hz, 1H, Ar), 7.96 (dd, J = 1.2, 8 Hz, 1H, Ar), 8.11–8.14 (m, 1H, Ar). ¹³C NMR (101 MHz, CDCl₃): δ 40.72, 112.37, 125.61, 125.69, 125.81, 126.45, 126.86, 126.95, 128.33, 128.84, 130.92, 132.05, 134.03, 140.59, 149.88.

3-Methoxy-4′-methylbiphenyl^7e. ¹H NMR (400 MHz, CDCl₃): δ 2.37 (s, 3H, Me), 3.81 (s, 3H, OMe), 6.84–6.87 (m, 1H, C₆H₄), 7.09–7.11 (m, 1H, C₆H₄), 7.15 (dt, J = 1.2, 7.6 Hz, 1H, C₆H₄), 7.22 (d, J = 7.9 Hz, 2H, C₆H₄), 7.31 (t, J = 7.9 Hz, 1H, C₆H₄), 7.47 (d, J = 8.1 Hz, 2H, C₆H₄). ¹³C NMR (101 MHz, CDCl₃): δ 21.20, 55.33, 112.45, 112.80, 119.59, 127.12, 129.56, 129.81, 137.29, 138.30, 142.78, 160.02.

4′-Methyl-[1,1′-biphenyl]-3-yl pivalate. ¹H NMR (400 MHz, CDCl₃): δ 1.38 (s, 9H, tBu), 2.39 (s, 3H, Me), 6.99–7.04 (m, 1H, C₆H₄), 7.22–7.26 (m, 3H, C₆H₄), 7.41–7.44 (m, 2H, C₆H₄), 7.48 (d, J = 8.1 Hz, 2H, C₆H₄). ¹³C NMR (101 MHz, CDCl₃): δ 21.26, 27.31, 39.24, 120.11, 120.15, 124.32, 127.15, 129.62, 129.70, 137.51, 137.58, 142.86, 151.59, 177.26. HR-MS: m/z = 269.15332 [M + H]⁺, calcd for C₁₈H₂₁O₂: 269.15361.

4-Methoxy-4′-methylbiphenyl²³. ¹H NMR (400 MHz, CDCl₃): δ 2.37 (s, 3H, Me), 3.83 (s, 3H, OMe), 6.95 (d, J = 8.8 Hz, 2H, C₆H₄), 7.21 (d, J = 8 Hz, 2H, C₆H₄), 7.44 (d, J = 8.1 Hz, 2H, C₆H₄), 7.50 (d, J = 8.8 Hz, 2H, C₆H₄). ¹³C NMR (101 MHz, CDCl₃): δ 21.18, 55.45, 114.29, 126.71, 128.08, 129.57, 133.87, 136.47, 138.10, 159.06.

4′-Methoxy-2-methylbiphenyl²⁴. ¹H NMR (400 MHz, CDCl₃): δ 2.35 (s, 3H, Me), 3.91 (s, 3H, OMe), 7.02 (d, J = 8.7 Hz, 2H, C₆H₄), 7.27–7.35 (m, 6H, C₆H₄). ¹³C NMR (101 MHz, CDCl₃): δ 20.67, 55.35, 113.57, 125.87, 127.08, 130.01, 130.35, 130.41, 134.44, 135.57, 141.64, 158.60.

4′-Methoxy-2,4,6-trimethyl-biphenyl²⁵. ¹H NMR (400 MHz, CDCl₃): δ 2.06 (s, 6H, Me), 2.37 (s, 3H, Me), 3.89 (s, 3H, OMe), 6.97–7.01 (m, 4H, Ar), 7.10 (d, J = 8.7 Hz, 2H, C₆H₄). ¹³C NMR (101 MHz, CDCl₃): δ 20.93, 21.13, 55.28, 113.86, 128.14, 130.42, 133.39, 136.53, 138.78, 158.29.

N,N,3′,4′,5′-Pentamethylbiphenyl-4-amine²⁶. ¹H NMR (400 MHz, CDCl₃): δ 2.05 (s, 6H, Me), 2.33 (s, 3H, Me), 3.00 (s, 6H, NMe), 6.80 (d, J = 8.8 Hz, 2H, C₆H₄), 6.94 (s, 2H, C₆H₂), 7.02 (d, J = 8.8 Hz, 2H, C₆H₄). ¹³C NMR (101 MHz, CDCl₃): δ 21.04, 21.15, 40.77, 112.50, 128.09, 129.20, 130.09, 136.21, 136.87, 139.31, 149.20.

(4′-Methylbiphenyl-4-yl)(phenyl)methanone²⁷. ¹H NMR (400 MHz, CDCl₃): δ 2.42 (s, 3H, Me). 7.30 (d, J = 7.9 Hz, 2H, C₆H₄), 7.51 (t, J = 7.5 Hz, 2H, C₆H₅), 7.56 (d, J = 8.1 Hz, 2H, C₆H₄), 7.61 (t, J = 7.4 Hz, 1H, C₆H₅), 7.70 (d, J = 8.4 Hz, 2H, C₆H₄), 7.82–7.86 (m, 2H, C₆H₅), 7.89 (d, J = 8.4 Hz, 2H, C₆H₄). ¹³C NMR (101 MHz, CDCl₃): δ 21.33, 126.84, 127.27, 128.43, 129.84, 130.13, 130.89, 132.47, 136.05, 137.18, 137.94, 138.32, 145.33, 196.54.

(4′-Methoxybiphenyl-4-yl)(phenyl)methanone²⁸. ¹H NMR (400 MHz, CDCl₃): δ 3.87 (s, 3H, OMe), 7.02 (d, J = 8.8 Hz, 2H, C₆H₄), 7.50 (t, J = 7.5 Hz, 2H, Ph), 7.58–7.63 (m, 3H, Ar), 7.67 (d, J = 8.5 Hz, 2H), 7.81–7.85 (m, 2H, Ph), 7.88 (d, J = 8.5 Hz, 2H, C₆H₄). ¹³C NMR (101 MHz, CDCl₃): δ 55.54, 114.54, 126.52, 128.43, 128.54, 130.12, 130.96, 132.44, 132.48, 135.70, 137.98, 144.99, 160.00, 196.52.

Ethyl 4′-methylbiphenyl-4-carboxylate²⁹. ¹H NMR (400 MHz, CDCl₃): δ 1.42 (t, J = 7.1 Hz, 3H, Et), 2.41 (s, 3H, Me), 4.40 (q, J = 7.1 Hz, 2H, Et), 7.28 (d, J = 8 Hz, 2H, C₆H₄), 7.53 (d, J = 8 Hz, 2H, C₆H₄), 7.65 (d, J = 8.5 Hz, 2H, C₆H₄), 8.10 (d, J = 8.5 Hz, 2H, C₆H₄). ¹³C NMR (101 MHz, CDCl₃): δ 14.50, 21.29, 61.06, 126.87, 127.23, 129.04, 129.77, 130.16, 137.25, 138.20, 145.58, 166.71.

Ethyl 4′-methoxybiphenyl-4-carboxylate³⁰. ¹H NMR (400 MHz, CDCl₃): δ 1.41 (t, J = 7.1 Hz, 3H, Et), 3.86 (s, 3H, OMe), 4.40 (q, J = 7.1 Hz, 2H, Et), 7.00 (d, J = 8.8 Hz, 2H, C₆H₄), 7.57 (d, J = 8.8 Hz, 2H, C₆H₄), 7.62 (d, J = 8.4 Hz, 2H, C₆H₄), 8.09 (d, J = 8.4 Hz, 2H, C₆H₄). ¹³C NMR (101 MHz, CDCl₃): δ 14.48, 55.47, 61.01, 114.48, 126.53, 128.46, 128.71, 130.17, 132.56, 145.21, 159.93, 166.70.

Ethyl 4′-(N,N-dimethoxybiphenyl)-4-carboxylate²⁶. ¹H NMR (400 MHz, CDCl₃): δ 1.41 (t, J = 7.1 Hz, 3H, Et), 3.02 (s, 6H, NMe), 4.39 (q, J = 7.1 Hz, 2H, Et), 6.80 (d, J = 8.8 Hz, 2H, C₆H₄), 7.56 (d, J = 8.8 Hz, 2H, C₆H₄), 7.62 (d, J = 8.8 Hz, 2H, C₆H₄), 8.06 (d, J = 8.8 Hz, 2H, C₆H₄). ¹³C NMR (101 MHz, CDCl₃): δ 14.53, 40.56, 60.92, 112.71, 125.86, 127.64, 127.86, 128.02, 130.16, 145.64, 150.61, 166.89.

N,N-Diethyl-4′-methylbiphenyl-4-carboxamide³¹. ¹H NMR (400 MHz, CDCl₃): δ 1.15 (b, 3H, Et), 1.26 (b, 3H, Et), 2.40 (s, 3H, Me), 3.32 (b, 2H, Et), 3.57 (b, 2H, Et), 7.26 (d, J = 8 Hz, 2H, C₆H₄), 7.44 (d, J = 8.3 Hz, 2H, C₆H₄), 7.50 (d, J = 8.1 Hz, 2H, C₆H₄), 7.60 (d, J = 8.3 Hz, 2H, C₆H₄). ¹³C NMR (101 MHz, CDCl₃): δ 13.00, 14.34, 21.19, 39.30, 43.40, 126.88, 126.93, 126.99, 129.64, 135.81, 137.53, 137.56, 141.97, 171.24.

N,N-Diethyl-4′-methoxybiphenyl-4-carboxamide³². ¹H NMR (400 MHz, CDCl₃): δ 1.15 (b, 3H, Et), 1.26 (b, 3H, Et), 3.32 (b, 2H, Et), 3.57 (b, 2H, Et), 3.86 (s, 3H, OMe), 6.99 (d, J = 8.8 Hz, 2H, C₆H₄), 7.42 (d, J = 8.3 Hz, 2H, C₆H₄), 7.54 (d, J = 8.8 Hz, 2H, C₆H₄), 7.57 (d, J = 8.3 Hz, 2H, C₆H₄). ¹³C NMR (101 MHz, CDCl₃): δ 12.98, 14.37, 21.21, 39.34, 43.47, 126.93, 126.97, 127.04, 129.67, 135.88, 137.60, 142.03, 171.29.

4′-Methyl-4-(trifluoromethyl)biphenyl³³. ¹H NMR (400 MHz, CDCl₃): δ 2.45 (s, 3H, Me), 7.31 (d, J = 7.9 Hz, 2H, C₆H₄), 7.53 (d, J = 8.2 Hz, 2H, C₆H₄), 7.70 (s, 4H, C₆H₄). ¹³C NMR (101 MHz, CDCl₃): δ 21.25, 124.11 (q, J = 272.9 Hz), 125.80 (q, J = 3.8 Hz), 127.23, 127.29, 129.15 (q, J = 32.5 Hz), 129.85, 136.97, 138.30, 144.77.

4′-Methoxy-4-(trifluoromethyl)biphenyl³³. ¹H NMR (400 MHz, CDCl₃): δ 3.87 (s, 3H, Me), 7.01 (d, J = 8.8 Hz, 2H, C₆H₄), 7.55 (d, J = 8.8 Hz, 2H, C₆H₄), 7.66 (s, 4H, C₆H₄). ¹³C NMR (101 MHz, CDCl₃): δ 55.51, 114.54, 124.52 (q, J = 272.8 Hz), 125.81 (q, J = 3.8 Hz), 126.99, 128.48, 128.79 (q, J = 32.6 Hz), 132.29, 144.41, 159.96.

4-Ethanesulfonyl-4′-methylbiphenyl. ¹H NMR (400 MHz, CDCl₃): δ 1.31 (t, J = 7.4 Hz, 3H, Et), 2.42 (s, 3H, Me), 3.15 (q, J = 7.4 Hz, 2H, Et), 7.30 (d, J = 8 Hz, 2H, C₆H₄), 7.52 (d, J = 8 Hz, 2H, C₆H₄), 7.75 (d, J = 8.5 Hz, 2H, C₆H₄), 7.94 (d, J = 8.5 Hz, 2H, C₆H₄). ¹³C NMR (101 MHz, CDCl₃): δ 7.67, 21.33, 50.86, 127.36, 127.73, 128.87, 129.97, 136.36, 136.77, 138.91, 146.72. HR-MS: m/z = 261.09393 [M + H]⁺, calcd for C₁₅H₁₇O₂S: 261.09438.

4-Ethanesulfonyl-4′-methoxybiphenyl. ¹H NMR (400 MHz, CDCl₃): δ 1.30 (t, J = 7.4 Hz, 3H, Et), 3.14 (q, J = 7.4 Hz, 2H, Et), 3.86 (s, 3H, OMe), 7.01 (d, J = 8.8 Hz, 2H, C₆H₄), 7.56 (d, J = 8.8 Hz, 2H, C₆H₄), 7.72 (d, J = 8.5 Hz, 2H, C₆H₄), 7.92 (d, J = 8.5 Hz, 2H, C₆H₄). ¹³C NMR (101 MHz, CDCl₃): δ 7.64, 50.82, 55.52, 114.65, 127.33, 128.63, 128.85, 131.54, 136.32, 146.30, 160.32. HR-MS: m/z = 277.08926 [M + H]⁺, calcd for C₁₅H₁₇O₃S: 277.08929.

(2′-Methylbiphenyl-4-yl)(phenyl)methanone³¹. ¹H NMR (400 MHz, CDCl₃): δ 2.31 (s, 3H, Me), 7.24–7.33 (m, 4H, Ar), 7.45 (d, J = 8.5 Hz, 2H, C₆H₄), 7.51 (t, J = 7.6 Hz, 2H, Ar), 7.61 (t, J = 7.4 Hz, 1H, Ar), 7.84–7.90 (m, 4H, Ar). ¹³C NMR (101 MHz, CDCl₃): δ 20.60, 126.08, 128.02, 128.45, 129.32, 129.72, 130.17, 130.18, 130.68, 132.54, 135.37, 136.07, 137.84, 140.95, 146.49, 196.66.

Ethyl 2′-methylbiphenyl-4-carboxylate²³. ¹H NMR (400 MHz, CDCl₃): δ 1.41 (t, J = 7.1 Hz, 3H, Et). 2.26 (s, 3H, Me), 4.41 (q, J = 7.1 Hz, 2H, Et), 7.20–7.29 (m, 4H, C₆H₄), 7.39 (d, J = 8.4 Hz, 2H, C₆H₄), 8.09 (d, J = 8.4 Hz, 2H, C₆H₄). ¹³C NMR (101 MHz, CDCl₃): δ 14.50, 20.51, 61.09, 126.02, 127.93, 129.05, 129.34, 129.50, 129.65, 130.60, 135.30, 141.02, 146.75, 166.70.

Phenyl(4′-(trifluoromethyl)biphenyl-4-yl)methanone^7g. ¹H NMR (400 MHz, CDCl₃): δ 7.52 (t, J = 7.6 Hz, 2H, Ph). 7.62 (tt, J = 1.2, 7.2 Hz, 1H, Ar), 7.72 (d, J = 8.4 Hz, 2H, C₆H₄), 7.75 (s, 4H, Ar), 7.82–7.87 (m, 2H, Ar), 7.92 (d, J = 8.4 Hz, 2H, C₆H₄). ¹³C NMR (101 MHz, CDCl₃): δ 124.27 (q, J = 273.1 Hz), 126.05 (q, J = 3.8 Hz), 127.31, 127.76, 128.51, 130.16, 130.29 (q, J = 32.7 Hz), 130.94, 132.72, 137.23, 137.61, 143.63, 143.71, 196.30.

Ethyl 4′-(trifluoromethyl)biphenyl-4-carboxylate³⁴. ¹H NMR (400 MHz, CDCl₃): δ 1.42 (t, J = 7.1 Hz, 3H, Et), 4.42 (q, J = 7.1 Hz, 2H, Et), 7.66 (d, J = 8.6 Hz, 2H, C₆H₄), 7.72 (s, 4H, C₆H₄), 8.14 (d, J = 8.6 Hz, 2H, C₆H₄). ¹³C NMR (101 MHz, CDCl₃): δ 14.49, 61.29, 124.28 (q, J = 273.1 Hz), 126.00 (q, J = 3.7 Hz), 127.35, 127.73, 130.25 (q, J = 32.6 Hz), 130.28, 130.37, 143.71, 144.09, 166.42.

N,N-Diethyl-4′-(trifluoromethyl)biphenyl-4-carboxamide^7h. ¹H NMR (400 MHz, CDCl₃): δ 1.15 (b, 3H, Et), 1.27 (b, 3H, Et), 3.32 (b, 2H, Et), 3.58 (b, 2H, Et), 7.48 (d, J = 8.3 Hz, 2H, C₆H₄), 7.62 (d, J = 8.3 Hz, 2H, C₆H₄), 7.70 (s, 4H, C₆H₄). ¹³C NMR (101 MHz, CDCl₃): δ 13.05, 14.45, 39.43, 43.46, 124.34 (q, J = 273.1 Hz), 125.94 (q, J = 3.8 Hz), 127.17, 127.47, 127.57, 129.84 (q, J = 32.7 Hz), 137.17, 140.62, 144.03, 170.93.

4′-(Trifluoromethyl)biphenyl-3-yl pivalate. ¹H NMR (400 MHz, CDCl₃): δ 1.39 (s, 9H, tBu), 7.11 (dt, J = 2, 7.2 Hz, 1H, C₆H₄), 7.29 (t, J = 2 Hz, 1H, C₆H₄), 7.44–7.50 (m, 2H, C₆H₄), 7.69 (s, 4H, C₆H₄). ¹³C NMR (101 MHz, CDCl₃): δ 27.28, 39.27, 120.62, 121.41, 124.35 (q, J = 273 Hz), 124.65, 125.87 (q, J = 3.8 Hz), 127.64, 129.81 (q, J = 32.7 Hz), 130.05, 141.39, 143.87, 151.74, 177.22. HR-MS: m/z = 323.12482 [M + H]⁺, calcd for C₁₈H₁₈O₂F₃: 323.12491.

(4-(Furan-2-yl)phenyl)(phenyl)methanone³¹. ¹H NMR (400 MHz, CDCl₃): δ 6.53 (dd, J = 2, 3.6 Hz, 1H, furyl), 6.82 (d, J = 3.2 Hz, 1H, furyl), 7.49 (t, J = 7.5 Hz, 2H, Ph), 7.54 (d, J = 1.6 Hz, 1H, furyl), 7.57–7.62 (m, 1H, Ph), 7.77 (d, J = 8.5 Hz, 2H, C₆H₄), 7.79–7.82 (m, 2H, Ph), 7.85 (d, J = 8.5 Hz, 2H, C₆H₄). ¹³C NMR (101 MHz, CDCl₃): δ 107.51, 112.22, 123.43, 128.42, 130.05, 130.96, 132.45, 134.54, 135.97, 137.88, 143.37, 152.99, 196.16.

Ethyl 4-(furan-2-yl)benzoate³¹. ¹H NMR (400 MHz, CDCl₃): δ 1.40 (t, J = 7.1 Hz, 3H, Et), 4.38 (q, J = 7.1 Hz, 2H, Et), 6.50 (dd, J = 1.8, 3.4 Hz, 1H, furyl), 6.78 (d, J = 3.4 Hz, 1H, furyl), 7.52 (d, J = 1.2 Hz, 1H, furyl), 7.72 (d, J = 8.6 Hz, 2H, C₆H₄), 8.05 (d, J = 8.6 Hz, 2H, C₆H₄). ¹³C NMR (101 MHz, CDCl₃): δ 14.47, 61.08, 107.28, 112.13, 123.47, 128.99, 130.18, 134.78, 143.20, 153.07, 166.46.

2-Naphthalen-1-yl-furan³⁵. ¹H NMR (400 MHz, CDCl₃): δ 6.61 (dd, J = 2, 3.6 Hz, 1H, furyl), 6.75 (d, J = 3.2 Hz, 1H, furyl), 7.51–7.58 (m, 3H, Ar), 7.65 (d, J = 2 Hz, 1H, furyl), 7.75 (dd, J = 1.2, 7.2 Hz, 1H, Ar), 7.86 (d, J = 8.2 Hz, 1H, Ar), 7.89–7.93 (m, 1H, Ar), 8.41–8.46 (m, 1H, Ar). ¹³C NMR (101 MHz, CDCl₃): δ 109.35, 111.51, 125.45, 125.69, 126.05, 126.30, 126.68, 128.64, 128.69, 128.74, 130.52, 134.08, 142.56, 153.60.

Phenyl-(4-thiophen-2-yl-phenyl)methanone^7g. ¹H NMR (400 MHz, CDCl₃): δ 7.14 (dd, J = 3.7, 5.1 Hz, 1H, thienyl), 7.38 (dd, J = 0.9, 5.1 Hz, 1H, thienyl), 7.45 (dd, J = 0.9, 3.6 Hz, 1H, thienyl), 7.50 (t, J = 7.6 Hz, 2H, Ph), 7.60 (t, J = 7.4 Hz, 1H, Ph), 7.72 (d, J = 8.4 Hz, 2H, C₆H₄), 7.80–7.83 (m, 2H, Ph), 7.84 (d, J = 8.4 Hz, 2H, C₆H₄). ¹³C NMR (101 MHz, CDCl₃): δ 124.70, 125.58, 126.55, 128.46, 128.53, 130.09, 131.12, 132.52, 136.23, 137.84, 138.43, 143.17, 196.12.

Ethyl 4-(thiophen-2-yl)benzoate^7g. ¹H NMR (400 MHz, CDCl₃): δ 1.41 (t, J = 7.1 Hz, 3H, Et), 4.39 (q, J = 7.1 Hz, 2H, Et), 7.12 (dd, J = 3.7, 5.1 Hz, 1H, thienyl), 7.36 (dd, J = 1, 5.1 Hz, 1H, thienyl), 7.42 (dd, J = 1, 3.6 Hz, 1H, thienyl), 7.67 (d, J = 8.5 Hz, 2H, C₆H₄), 8.05 (d, J = 8.5 Hz, 2H, C₆H₄). ¹³C NMR (101 MHz, CDCl₃): δ 14.50, 61.14, 124.58, 125.62, 126.37, 128.45, 129.27, 130.38, 138.66, 143.27, 166.44.

2-Naphthalen-1-yl-thiophene³⁴. ¹H NMR (400 MHz, CDCl₃): δ 7.19 (dd, J = 3.5, 5.1 Hz, 1H, thienyl), 7.25–7.26 (m, 1H, thienyl), 7.44 (dd, J = 1.2, 5.1 Hz, 1H, thienyl), 7.47–7.53 (m, 3H, Ar), 7.58 (dd, J = 1.3, 7.1 Hz, 1H, Ar), 7.85–7.92 (m, 2H, Ar), 8.20–8.24 (m, 1H, Ar). ¹³C NMR (101 MHz, CDCl₃): δ 125.38, 125.77, 125.90, 126.14, 126.58, 127.41, 127.53, 128.34, 128.46, 128.53, 132.01, 132.58, 133.98, 141.91.

2-p-Tolylpyridine^31,36. ¹H NMR (400 MHz, CDCl₃): δ 2.40 (s, 3H, Me), 7.17–7.20 (m, 1H, Py), 7.28 (d, J = 8 Hz, 2H, C₆H₄), 7.67–7.74 (m, 2H, Py), 7.89 (d, J = 8.2 Hz, 2H, C₆H₄), 8.66–8.69 (m, 1H, Py). ¹³C NMR (101 MHz, CDCl₃): δ 21.39, 120.35, 121.90, 126.86, 129.58, 136.70, 136.77, 139.03, 149.69, 157.55.

2-(4-Methoxyphenyl)pyridine³⁷. ¹H NMR (400 MHz, CDCl₃): δ 3.86 (s, 3H, OMe), 7.00 (d, J = 8.9 Hz, 2H, C₆H₄), 7.14–7.18 (m, 1H, Py), 7.65–7.72 (m, 2H, Py), 7.95 (d, J = 8.9 Hz, 2H, C₆H₄), 8.64–8.66 (m, 1H, Py). ¹³C NMR (101 MHz, CDCl₃): δ 55.46, 114.21, 119.91, 121.52, 128.26, 132.13, 136.77, 149.65, 157.21, 160.54.

Acknowledgements

This research was supported by the National Natural Science Foundation of China (grant no. 21372212) and the National Basic Research Program of China (grant no. 2015CB856600).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: ¹H and ¹³C NMR copies of all products. See DOI: 10.1039/c4qo00321g

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