Gold(I)-catalyzed intramolecular hydroarylation and the subsequent ring enlargement of methylenecyclopropanes to cyclobutenes

Abstract

The gold(I)-catalyzed intramolecular hydroarylation of methylenecyclopropanes containing aryl propargyl ethers proceeded smoothly to give 2H-chromene derivatives in good to excellent yields under mild conditions. Furthermore, in the presence of IPrAuSbF₆, these 2H-chromene derivatives containing a methylenecyclopropane could undergo ring enlargement to give the corresponding cyclobutenes.

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The metal-catalyzed hydroarylation of alkynes has long been recognized as an effective approach to synthesize 2H-chromene derivatives by 6-endo dig cyclization as chromenes, chromanes, coumarins and related heterocycles are key sections of structures in many natural products.¹ Since the intramolecular hydroarylation of substituted aryl propargyl ethers was first reported by Iwai et al. in 1963,² the transformation methods have been extensively investigated, with most of them were carried out under extremely harsh conditions, such as at high temperatures, even up to 200 °C. Afterwards, some metal-catalysts, such as Pd, Pt, Hg, Ag, Ru, Rh, In, Re, etc.³ including Au,⁴ were applied to facilitate this reaction (Scheme 1). The use of metal-catalysts allowed this reaction to proceed smoothly under milder conditions.

Scheme 1 Previous work and this work.

Methylenecyclopropanes (MCPs), as an easily available class of substrates with an exceedingly high ring strain, have served as useful building blocks in organic synthesis owing to their unique structure and electronic properties.⁵ In the past decade, the ring-opening functionalizations of methylenecyclopropanes have been extensively investigated under transition metal catalysis⁶ and Lewis acid catalysis.⁷ Ring enlargement of the methylenecyclopropanes to cyclobutenes is readily driven by release of the ring tension. This kind of rearrangement was reported by Fürstner⁸ and Shi⁹ (Scheme 1). Cyclobutenes are useful intermediates for organic synthesis, and are widely found as important structural motifs in an abundant number of biologically active molecules.¹⁰ However, few strategies have been disclosed to synthesize cyclobutenes, suggesting that the further development of new synthetic methods is highly desirable.

Concerning the previous studies, we successfully prepared a class of substituted ortho-(propargyloxy)aryl methylenecyclopropanes 1 and investigated their cyclization in the presence of a suitable gold complex. In this paper, we wish to report the intramolecular hydroarylation of 1 to give 2H-chromene and the simultaneous ring enlargement of methylenecyclopropane to cyclobutenes in the presence of gold(I) catalysts in 1,2-dichloroethane (DCE) (Scheme 1).

We initially attempted to optimize the reaction conditions for these transformations using 1a as a model substrate and the results are summarized in Table 1. As can be seen from Table 1, compound 2a was given in a 65% yield at room temperature within 5 hours in the presence of JohnPhosAuSbF₆ (2.5 mol%) in 1,2-dichloroethane (DCE) (Table 1, entry 1). Pt(cod)Cl₂ and Ph₃PAuCl/AgSbF₆ did not catalyze this reaction at room temperature (Table 1, entries 2 and 3). When (p-CF₃C₆H₄)₃PAuSbF₆ (2.5 mol%) was used as the catalyst, compound 2a was formed in an 86% yield at room temperature within 3 hours in DCE (Table 1, entry 4). With PtCl₂/CO or Rh(PPh₃)₃Cl as the catalyst, 2a was produced in moderate yields at 60 °C and 80 °C, respectively (Table 1, entries 5 and 6). By changing the reaction temperature in the presence of JohnPhosAuSbF₆, 2a and cyclobutene 3a were produced in 30% and 35% yields, respectively, at 80 °C, and 3a was afforded in an 83% yield as the sole product at 90 °C within 12 hours in DCE (Table 1, entries 7 and 8). When the counter ion of the gold complex was changed to trifluoromethanesulfonate, 2a was only given in a lower yield at 90 °C in DCE (Table 1, entries 9–11). None of the desired product was given in the presence of t-BuXPhosAuCl/AgSbF₆ at 90 °C in DCE (Table 1, entry 12). Using PPh₃AuSbF₆ and Cy–JohnPhosAuSbF₆ as the catalysts gave 2a in 48% and 80% yields, respectively, at 90 °C in DCE (Table 1, entries 13 and 14). In the presence of sterically bulky phosphine coordinated gold complexes, such as t-BuXPhosAuSbF₆ or Me₄-t-BuXPhosAuSbF₆, compound 3a was obtained as the sole product, but in a low yield at 90 °C in DCE (Table 1, entries 15 and 16). Using the electron-deficient phosphine coordinated gold complex (C₆F₅)₃PAuSbF₆ as the catalyst gave no desired product at 90 °C in DCE (Table 1, entry 17). However, compound 3a was exclusively produced in a 94% yield in the presence of the electron-rich gold complex IPrAuSbF₆ (IPr: 1,3-bis(2,6-diisopropylphenyl)-2,3-dihydro-1H-imidazole, Scheme 1) at 90 °C within 4 hours in DCE (Table 1, entry 18). Then, we screened the solvent for this reaction, and 69, 76, and 60% yields for 3a were given at 90 °C within 4 hours in toluene, 1,4-dioxane and nitroethane, respectively (Table 1, entries 19–21).

Table 1 Optimization of the reaction conditions^a

Entry	Catalyst	Solvent	Temp. (°C)	Time (h)	Conv. (%)	Yield (%)
						2a	3a
a Reaction conditions: 1a (0.2 mmol), catalyst (2.5 mol%), solvent (2.0 mL), isolated yields.b Yields were determined by ^1H NMR spectroscopy.
1	JohnPhosAuSbF6	DCE	rt	5	>99	65	—
2	Pt(cod)Cl2	DCE	rt	24	<5	—	—
3	PPh3AuCl/AgSbF6	DCE	rt	0.5	>99	—	—
4	(p-CF3C6H4)3PAuSbF6	DCE	rt	3	>99	86	—
5	PtCl2/CO (1 atm)	DCE	60	24	>99	50^b
6	Rh(PPh3)3Cl	DCE	80	12	94	43	—
7	JohnPhosAuSbF6	DCE	80	12	>99	30	35
8	JohnPhosAuSbF6	DCE	90	12	>99	—	83
9	JohnPhosAuCl/AgOTf	DCE	90	3	>99	13	—
10	t-BuXPhosAuOTf	DCE	90	5	>99	39
11	XPhosAuOTf	DCE	90	12	>99	41	—
12	t-BuXPhosAuCl/AgSbF6	DCE	90	0.5	>99	—	—
13	PPh3AuSbF6	DCE	90	4	>99	48^b	—
14	Cy–JohnPhosAuSbF6	DCE	90	10	>99	80^b	—
15	t-BuXPhosAuSbF6	DCE	90	4	84	—	16^b
16	Me4-t-BuXPhosAuSbF6	DCE	90	10	>99	—	21^b
17	(C6F5)3PAuSbF6	DCE	90	4	>99	—	—
18	IPrAuSbF6	DCE	90	4	>99	—	94
19	IPrAuSbF6	Toluene	90	4	>99	—	69^b
20	IPrAuSbF6	Dioxane	90	4	>99	—	76^b
21	IPrAuSbF6	EtNO2	90	4	>99	—	60^b

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Having the optimal reaction conditions in hand, we next examined the scope of the reaction with respect to various substituted ortho-(propargyloxy)aryl methylenecyclopropanes 1. The results of these experiments are shown in Table 2. To vary the substituents at the benzene ring, no remarkable alteration in regard to the yield of 2 was observed when methyl or halogen atoms, such as F, Cl or Br, were introduced at the benzene ring. The position of the substituent at the benzene ring also had some impacts on the yield of 2. For instance, the yield of 2e was higher than that of 2h. The disparity in yield was because of the different electronic effect of the aromatic ring. Substituent Cl in 1h, as an electron-withdrawing group, reduced the electron density at C3, thereby disfavouring the intramolecular hydroarylation (Table 2, entries 5 and 8). When NO₂ was introduced at the benzene ring, the reaction could not take place, presumably due to its strongly electron-withdrawing effect (Table 2, entry 9). When the substituent was a MeO group at the benzene ring, complex products were afforded as a result of a strongly electron-donating effect (Table 2, entry 10). When R² was a phenyl group, the corresponding product 2g was formed in an 83% yield. However, when R² was CO₂Et, the reaction gave a complex product mixture (Table 2, entry 11).

Table 2 Substrate scope of substituted ortho-(propargyloxy)aryl methylenecyclopropanes 1 to products 2^a

Entry	R^1, R^2	Yield (%)
a Reactions were carried out using MCP 1 (0.2 mmol) in 1,2-dichloroethane (DCE) (2.0 mL) with (p-CF3C6H4)3PAuSbF6 (4.5 mg, 2.5 mol%), isolated yields.b Reaction time was 6 hours. N.R. means no reaction.
1	1a, R^1 = H, R^2 = H	2a, 86
2	1b, R^1 = 5-Me, R^2 = H	2b, 78
3	1c, R^1 = 4-Me, R^2 = H	2c, 73^b
4	1d, R^1 = 5-F, R^2 = H	2d, 80
5	1e, R^1 = 5-Cl, R^2 = H	2e, 95
6	1f, R^1 = 5-Br, R^2 = H	2f, 64
7	1g, R^1 = H, R^2 = Ph	2g, 83^b
8	1h, R^1 = 4-Cl, R^2 = H	2h, 61
9	1i, R^1 = 5-NO2, R^2 = H	N.R.
10	1j, R^1 = 4-MeO, R^2 = H	Complex
11	1k, R^1 = H, R^2 = CO2Et	Complex

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Next, we explored the generality for the production of cyclobutenes 3 under the optimal conditions, and the results are shown in Table 3. Substrates 1b–f and 1h bearing methyl and different halogen atoms, such as F, Cl or Br, at the benzene ring resulted in the formation of 3b–f and 3h in 41–64% yields. The influence of the substituent and its position was also observed. For example, introducing a Me at the ortho-position of the benzene ring as an electron-donating group increased the electron density of C3, favouring the intramolecular hydroarylation, to give 3c in a higher yield than that of 3b (Table 3, entries 2 and 3). As for substrate 1h bearing a Cl atom at the ortho-position, the electron density of C3 was reduced, giving 3h in a lower yield than that of 3e (Table 3, entries 5 and 8). As for substrate 1g, in which a phenyl group was introduced at the terminal alkyne, the expected product 3g could be also given, resulting in a 76% yield. Using 1i, 1j and 1k as the substrates, all afforded complex product mixtures, perhaps due to the electronic effect (Table 3, entries 9–11) (Scheme 2).

Table 3 Substrate scope of substituted ortho-(propargyloxy)aryl methylenecyclopropanes 1 to products 3^a

Entry	R^1, R^2	Yield (%)
a Reactions were carried out using MCP 1 (0.2 mmol) in 1,2-dichloroethane (DCE) (2.0 mL) with IPrAuSbF6 (4.0 mg, 2.5 mol%), isolated yields.b Reaction time was 10 hours.c Reaction time was 0.5 hour.
1	1a, R^1 = H, R^2 = H	3a, 94
2	1b, R^1 = 5-Me, R^2 = H	3b, 45
3	1c, R^1 = 4-Me, R^2 = H	3c, 63^b
4	1d, R^1 = 5-F, R^2 = H	3d, 41
5	1e, R^1 = 5-Cl, R^2 = H	3e, 64
6	1f, R^1 = 5-Br, R^2 = H	3f, 64
7	1g, R^1 = H, R^2 = Ph	3g, 76
8	1h, R^1 = 4-Cl, R^2 = H	3h, 46
9	1i, R^1 = 5-NO2, R^2 = H	Complex
10	1j, R^1 = 4-MeO, R^2 = H	Complex^c
11	1k, R^1 = H, R^2 = CO2Et	Complex

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Scheme 2 Reaction of methylenecyclobutanes 1l–1m.

Furthermore, we prepared ortho-(propargyloxy)aryl methylenecyclobutanes 1l–m, and carried out the reactions under the same conditions. We found that using methylenecyclobutane to replace methylenecyclopropane produced the corresponding product 2l in only a 33% yield, along with some unidentified product complexes, presumably due to the fact that methylenecyclobutane does not have a higher strain energy as that of methylenecyclopropane.¹¹ Thus, its double bond can act the same as one in a normal olefinic compound, causing the production of some other byproducts. However, when a phenyl group was introduced at the terminal alkyne, the desired product 2m could also be obtained in an 80% yield, perhaps due to the phenyl being able to stabilize the in situ generated vinyl carbocation after gold(I) catalyst binding to the alkyne, as shown in Scheme 4, thus facilitating the intramolecular hydroarylation. The corresponding compounds 3l and 3m could not be obtained from the reactions of 1l and 1m under the standard conditions, probably due to the lower strain energy of methylenecyclobutane, compared with that of methylenecyclopropane.

A control experiment was conducted using 2a as the starting material, giving 3a in an 83% yield in the presence of IPrAuSbF₆ (2.5 mol%) in DCE at 90 °C, suggesting that 3a is derived from 2a under gold catalysis. In addition, no reaction occurred in the absence of gold catalyst (Scheme 3).

Scheme 3 Compound 2a was converted into product 3a upon addition of a gold catalyst in contrast to the reaction with no catalyst.

Based on Fürstner's^7,8 previous report and other groups' work,¹² a plausible mechanism for these reactions is depicted in Scheme 4. The initial species is the formation of a cyclized Au complex IM-I, generated likely through two paths: path (a) and path (b). Path (a) involves the gold(I) catalyst directly coordinating with the triple bond of the propargyl moiety in 1a, while path (b) involves the triple bond first binding covalently with the gold(I) catalyst, followed by the gold(I) catalyst coordinating with the double bond in the benzene ring. Then, through protonation, product 2a is produced and the gold(I) catalyst is regenerated at the same time if the employed catalyst is (p-CF₃C₆H₄)₃PAuSbF₆. Furthermore, the gold(I) catalyst can next coordinate to the double bond of MCP to produce a stabilized cyclopropylmethyl cation IM-II ¹³ which is a nonclassical carbocationic intermediate in the presence of IPrAuSbF₆. This carbocationic species is apt to rearrange to the corresponding cyclobutenyl cation IM-III, likely bearing some carbene character, and then gives the final product after a 1,2-hydrogen shift.⁷ The ligand in the gold catalyst plays an important role in controlling the reaction process that furnished the different products. Electron-poor phosphine ligands, such as (p-CF₃C₆H₄)₃P, produced the vinyl gold specie IM-I, which has a higher stability towards protodeauration; also after the protodeauration of the catalyst, it was more difficult to activate the double bond of methylenecyclopropanes than the triple bond of alkynes. Thus, the reaction was terminated by compound 2a.¹⁴ Electron-rich NHC ligands, such as IPr (1,3-bis(2,6-diisopropylphenyl)-2,3-dihydro-1H-imidazole), favoured protodeauration, and the regenerated cationic gold catalyst swiftly activated the double bond of methylenecyclopropane to afford the final product 3a.¹⁴

Scheme 4 Proposed mechanisms.

In conclusion, we developed gold(I)-catalyzed substituted aryl propargyl ethers to effectively produce 2H-chromene derivatives through intramolecular hydroarylation in good to excellent yields. Simultaneously, we also demonstrated that the further transformation of 2H-chromene derivatives containing a methylenecyclopropane moiety was possible to attain cyclobutenes through ring enlargement of the methylenecyclopropanes under gold(I) catalysis using IPrAuSbF₆ as the catalyst, which is another effective method to synthesize cyclobutene. Further investigations on this and related transition metal-catalyzed hydroarylations and further transformations are ongoing.

Experimental section

Compound 2a

To a flame-dried flask were added methylenecyclopropane 1a (37 mg, 0.2 mmol, 1.0 equiv.) and (p-CF₃C₆H₄)₃PAuSbF₆ (0.005 mmol, 0.025 equiv.). The flask was evacuated and backfilled with Ar, with this process carried out a total of three times. DCE (2.0 mL) was then added to this flask via a syringe under Ar. The reaction mixture was stirred for 3 hours at room temperature. An appropriate amount of silica gel was added to the reaction mixture and the solvent was removed under vacuum pump at low temperature; finally, the crude product was purified by silica gel chromatography (PE) to obtain the desired product 2a (32 mg, 86%) as a colourless oil. ¹H NMR (CDCl₃, 400 MHz, TMS) δ 1.14–1.18 (m, 2H, CH₂), 1.36–1.40 (m, 2H, CH₂), 4.85 (dd, J₁ = 3.6 Hz, J₂ = 2.0 Hz, 2H, CH₂), 5.79 (dt, J₁ = 9.6 Hz, J₂ = 3.6 Hz, 1H, =CH), 6.42 (d, J = 9.6 Hz, 1H, =CH), 6.83–6.84 (m, 2H, Ar), 7.05 (s, 1H, =CH), 7.58 (dd, J₁ = 4.8 Hz, J₂ = 4.8 Hz, 1H, Ar). ¹³C NMR (CDCl₃, 100 MHz, TMS) δ 0.6, 3.9, 65.5, 111.6, 120.9, 121.8, 122.5, 124.5, 124.9, 125.0, 125.7, 126.3, 150.5. IR (neat) ν 3291, 3046, 2972, 2846, 2123, 1784, 1638, 1581, 1460, 1323, 1237, 1199, 1086, 975, 745 cm⁻¹. MS (%) m/z 184 (M⁺, 84.89), 183 (100.00), 169 (34.64), 155 (37.71), 141 (35.89), 128 (37.02), 115 (56.60), 91 (24.40), 51 (16.02). HRMS (EI) calcd for C₁₃H₁₂O: 184.0888, found: 184.0886.

Compound 3a

To a flame-dried flask were added methylenecyclopropane 1a (37 mg, 0.2 mmol, 1.0 equiv.) and IPrAuSbF₆ (0.005 mmol, 0.025 equiv.). The flask was evacuated and backfilled with Ar, with this process carried out a total of three times. DCE (2.0 mL) was added to this flask via a syringe under Ar. The reaction mixture was stirred for 4 hours at 90 °C. An appropriate amount of silica gel was added to the reaction mixture and the solvent was removed under vacuum pump at low temperature; finally, the crude product was purified by silica gel chromatography (PE) to obtain the desired product 3a. (35 mg, 94%) as a colourless oil. ¹H NMR (CDCl₃, 400 MHz, TMS) δ 2.55–2.57 (m, 2H, CH₂), 2.82–2.83 (m, 2H, CH₂), 4.88 (dd, J₁ = 3.6 Hz, J₂ = 2.0 Hz, 2H, CH₂), 5.76 (dt, J₁ = 10.0 Hz, J₂ = 3.6 Hz, 1H, =CH), 6.32 (s, 1H, =CH), 6.40 (d, J = 10.0 Hz, 1H, =CH), 6.79–6.84 (m, 2H, Ar), 7.01 (dd, J₁ = 7.6 Hz, J₂ = 2.4 Hz, 1H, Ar). ¹³C NMR (CDCl₃, 100 MHz, TMS) δ 27.5, 29.9, 65.4, 120.6, 121.6, 122.1, 122.5, 124.6, 125.7, 126.4, 132.0, 142.5, 152.4. IR (neat) ν 3048, 2919, 2849, 1639, 1594, 1460, 1392, 1235, 1215, 1079, 1017, 927, 805, 750, 698 cm⁻¹. MS (%) m/z 184 (M⁺, 100.00), 169 (42.04), 155 (56.12), 141 (30.97), 128 (47.38), 115 (50.82), 102 (15.29), 91 (23.72), 51 (22.85). HRMS (EI) calcd for C₁₃H₁₂O: 184.0888, found: 184.0892.

Acknowledgements

We are grateful for the financial support from the National Basic Research Program of China (973)-2015CB856603, and the National Natural Science Foundation of China (20472096, 21372241, 21361140350, 20672127, 21102166, 21121062, 21302203, 20732008 and 21572052).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental procedures, characterization data of new compounds. See DOI: 10.1039/c6ra02549h

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