Regiocontrolled 1,2-migration in cyclization of 1-(indol-2-yl)-3-alkyn-1-ols: (Ph₃P)Au⁺vs. PtCl₄

Abstract

An AuCl(PPh₃)/AgBF₄- and PtCl₄-catalyzed reaction of 1-(indol-2-yl)-3-alkyn-1-ols occurred smoothly in toluene to form a series of differently polysubstituted carbazole derivatives efficiently. The regioselectivity of the 1,2-migration may be tuned by using different metal catalysts: carbazoles 3 could be obtained exclusively in the presence of AuCl(PPh₃)/AgBF₄via a Wagner–Meerwein type 1,2-alkyl shift, whereas in some cases the use of PtCl₄ afforded differently substituted carbazoles 4 involving a platinum–carbene intermediate.

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Introduction

Control of selectivity remains one of the most important challenges in organic chemistry.¹ Among the many strategies employed, catalyst-based control of selectivity² has been proven to be very beneficial for the simple reason that the same substrates are applied for different products. Recently, we have described an approach to the carbazole skeleton through a AuCl₃-catalyzed cyclization of 1-(indol-2-yl)-3-alkyn-1-ols.³ With such simple substrates, the mechanism of such a transformation remains a puzzle. With the purpose of mechanistic study and extending the scope of this reaction, we prepared 2,2-dimethyl-substituted substrate 2a.⁴Quite unexpectedly, carbazoles 3a and 4a with one of the two methyl groups in the 1- or 3-position of the carbazole skeleton were formed in a ratio of 3a/4a = 34/18 (entry 1, Table 1). Based on these stimulating results, we were interested in the pathway(s) for the formation of both products and envisioned that if such a regioselectivity may be controlled, it would provide a highly selective entry to different carbazoles⁵ from the same substrates. In this paper, we wish to report the realization of such a concept (Scheme 1).

Scheme 1 Cyclization of 1-indolyl-3-alkynols.

Results and discussion

Studies were conducted to optimize the conditions to improve the selectivity of 3a and 4a with some typical results listed in Table 1. We attempted to apply the cationic gold catalyst⁶ first by the addition of different Ag⁺ salts to AuCl(PPh₃): interestingly, the yield of 3a was improved to 77% with AgSbF₆ as the co-catalyst (entry 2, Table 1);⁷ screening of other silver salts led to the observation that AgBF₄ is the best (entry 5, Table 1); no better results were obtained at a lower temperature (entries 6 and 7, Table 1); (IPr)AuCl (IPr = N,N′-bis(2,6-diisopropylphenyl)imidazol-2-ylidene)/AgBF₄ may catalyze this transformation affording 3a in 55% yield together with 4a in 9% yield (entry 8, Table 1); interestingly, InBr₃ also afforded a moderate yield of 3a with 94 : 6 selectivity (entry 9, Table 1).

Table 1 Optimization of the reaction conditions for the cyclization reaction of 1-(1-ethyl-5-methyl-1H-indol-2-yl)-2,2-dimethyloct-3-yn-1-ol 2a for selective formation of 3a^a

Entry	Catalyst (5 mol%)	Temp. (°C)	Time (h)	Yield^b (%)		3a : 4a^b
				3a	4a
a The reaction was conducted with 0.2 mmol of 2a and 5 mol% catalyst in 2 mL of toluene. b Determined by NMR using dibromomethane as the internal standard.
1	AuCl3	rt	42	34	18	65 : 35
2	AuCl(PPh3)/AgSbF6	rt	12	77	8	91 : 9
3	AuCl(PPh3)/AgOTf	rt	12	81	3	96 : 4
4	AuCl(PPh3)/AgPF6	rt	12	77	4	95 : 5
5	AuCl(PPh3)/AgBF4	rt	12	82	2	98 : 2
6	AuCl(PPh3)/AgBF4	10	5	74	2	97 : 3
7	AuCl(PPh3)/AgBF4	5	5	54	2	96 : 4
8	(IPr)AuCl/AgBF4	rt	12	55	9	86 : 14
9	InBr3	80	12	67	4	94 : 6

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The structures of 3a and 4a have been unambiguously determined by their X-ray single crystal diffraction study (Fig. 1).⁸

Fig. 1 ORTEP representation of products 3a and 4a.

The scope was then explored with the optimized Au-catalyzed conditions: the 1-position of indoles may be substituted with various alkyl groups, such as methyl, ethyl, butyl, or even an allyl group. Substituents on the 5-position could be methyl (entries 1, 5, and 8, Table 2), methoxy (entry 3, Table 2) or bromo, which could easily be transformed to some useful functional groups in the synthesis of biologically active carbazole alkaloids⁹ (entry 4, Table 2). R³ could be an alkyl (entries 1–7 and 9, Table 2) or a phenyl group (entry 8, Table 2), although the selectivity for the substrate with the phenyl group 2h is slightly lower (93 : 7). The reaction can be easily conducted on a scale of 3.5 mmol of 2b (1.0401 g) in a slightly higher yield (entry 9, Table 2). We observed that ethyl may also migrate exclusively in good selectivity (entry 10, Table 2). Moreover, from the practical synthetic viewpoint, pure carbazoles 3 could be afforded by simple recrystallization (entries 1–5 and 7–9, Table 2).

Table 2 Gold-catalyzed cyclization reaction of 1-(indol-2-yl)-3-alkyne-1-ols 2^a

Entry	2	Yield (3/4)^b^,^c (%)	Yield of 3^d (%)
	R^1/R^2/R^3/R^4
a The reaction was conducted with 1.0 mmol of 2a and 5 mol% AuCl(PPh3)/AgBF4 in 10 mL of toluene at room temperature. b Isolated yield of 3 and 4. c The ratio of 3 : 4 is presented in parentheses. d Yield of 3 after recrystallization. e The reaction was completed in 18 h. f 10 mol% AuCl(PPh3)/AgBF4 was used. g The reaction was conducted with a 3.5 mmol (1.0401 g) scale of 2b.
1	Et/Me/Bu/Me (2a)	64 (97 : 3) (3a:4a)	52
2	Et/H/Bu/Me (2b)	64 (97 : 3) (3b:4b)	45
3	Et/OMe/Bu/Me (2c)	68 (97 : 3) (3c:4c)	58
4	Me/Br/Bu/Me (2d)	55 (97 : 3) (3d:4d)	47
5	Me/Me/Bu/Me (2e)	72 (97 : 3) (3e:4e)	59
6^e	Bu/H/Bu/Me (2f)	67 (97 : 3) (3f:4f)	—
7	Allyl/H/Bu/Me (2g)	69 (96 : 4) (3g:4g)	54
8^e^,^f	Et/Me/Ph/Me (2h)	67 (93 : 7) (3h:4h)	57
9^g	Et/H/Bu/Me (2b)	66 (97 : 3) (3b:4b)	51
10	Et/Me/Bu/Et (2i)	57 (98 : 2) (3i:4i)	—

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In addition, we have studied the reaction of the substrate with a 5-membered cycle 2j, which underwent a smooth cyclization with ring expansion to afford fused tetracyclic carbazole 3j in 65% yield (eqn (1)).

In order to invert the selectivity with the purpose of highly selective formation of carbazole 4a, different metal catalysts were then screened: AuCl, AgBF₄ or Cu(OTf)₂ failed to promote the reaction smoothly (entries 1–3, Table 3); it is exciting to observe an inverted regioselectivity when PtCl₂ was employed as the catalyst (entry 4, Table 3). In contrast to several Pt(II) catalysts (entries 4–8, Table 3), the higher oxidation state of the platinum catalyst, i.e., PtCl₄, shows a better selectivity (entry 9, Table 3). Several solvents were then tested for the PtCl₄-catalyzed reaction of 2a at room temperature with toluene still being the best (entries 9–13, Table 3). Furthermore, the effect of temperature was considered (entries 9 and 14–16, Table 3): the best yield (82%) and selectivity (4a : 3a = 93 : 7) were realized at −10 °C (entry 15, Table 3). Interestingly, the ratio of 4a : 3a dropped again at −20 °C (entry 16, Table 3).

Table 3 Optimization of the reaction conditions for the formation of 4a^a

Entry	Catalyst	Temp. (°C)	Solvent	Time (h)	Yield^b (%)		4a : 3a^b
					4a	3a
a The reaction was conducted with 0.2 mmol of 2a and 5 mol% catalyst in 2 mL of solvent. b Determined by NMR using dibromomethane as the internal standard. c The recovery of 2a was 53%. d The recovery of 2a was 65%. e The recovery of 2a was 100%. f The recovery of 2a was 94%. g The recovery of 2a was 92%.
1^c	AuCl	rt	Toluene	18	5	13	28 : 72
2^d	AgBF4	rt	Toluene	13	—	4	—
3	Cu(OTf)2	80	Toluene	12	1	8	11 : 89
4	PtCl2	rt	Toluene	18	71	15	83 : 17
5^e	PtI2	rt	Toluene	12	—	—	—
6	PtI2	80	Toluene	12	72	19	79 : 21
7^f	Pt(CN)2	rt	Toluene	24	—	—	—
8^g	PtCl2(PhCN)2	rt	Toluene	18	—	—	—
9	PtCl4	rt	Toluene	18	74	11	87 : 13
10	PtCl4	rt	DCM	5	24	4	86 : 14
11	PtCl4	rt	Xylenes	8	73	12	86 : 14
12	PtCl4	rt	THF	8	26	26	50 : 50
13	PtCl4	rt	Dioxane	8	53	27	66 : 34
14	PtCl4	−5	Toluene	18	77	10	89 : 11
15	PtCl4	−10	Toluene	18	81	6	93 : 7
16	PtCl4	−20	Toluene	18	70	7	91 : 9

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Under the PtCl₄-catalyzed conditions (entry 15, Table 3), the reversed regioselective 1,2-methyl migration¹⁰ was then explored: R¹ could be a series of alkyl groups, such as methyl, ethyl, and butyl. Substituents on the 5-position of indoles could be methyl (entries 1, 4, and 6, Table 4) and methoxy (entry 3, Table 4). R³ could be an alkyl (entries 1–5, Table 4) or a phenyl group (entry 6, Table 4). Moreover, again from the practical synthetic viewpoint, the ratio of 4 and 3 could be improved by simple recrystallization (entries 1, 3, and 6, Table 4).

Table 4 Platinum-catalyzed cyclization reaction of 1-(indol-2-yl)-3-alkyne-1-ols^a

Entry	2	Yield (4/3)^b^,^c (%)	Yield and ratio^d (%)
	R^1/R^2/R^3
a The reaction was conducted with 1.0 mmol of 2a and 5 mol% PtCl4 in 10 mL of toluene at −10 °C. b Isolated yield of 4 and 3. c The ratio of 4 : 3 is presented in parentheses. d Yield and ratio after recrystallization. e The reaction was completed in 12 h. f The reaction was completed in 72 h.
1	Et/Me/Bu (2a)	80 (92 : 8) (4a : 3a)	61 (96 : 4)
2	Et/H/Bu (2b)	81 (90 : 10) (4b : 3b)	—
3	Et/OMe/Bu (2c)	80 (89 : 11) (4c : 3c)	65 (98 : 2)
4^e	Me/Me/Bu (2e)	77 (91 : 9) (4e : 3e)	—
5	Bu/H/Bu (2f)	78 (89 : 11) (4f : 3f)	—
6^f	Et/Me/Ph (2h)	78 (93 : 7) (4h : 3h)	61 (97 : 3)

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With this information, a rationale was proposed for this reaction (Scheme 2). The reaction of Au or Pt with 2 would form intermediate M1via the coordination of the alkyne with the gold or platinum atom followed by a nucleophilic attack of indolyl C3 to these metal-activated C–C triple bonds.¹¹ The intermediate M2_a may be afforded via protonation of the hydroxyl group in M1 followed by elimination of H₂O.¹² The R³ group subsequently migrates to the carbocationic center to form a new carbocationic intermediate M3.¹³ Subsequent elimination of H⁺ of M3 would afford intermediate M4. Finally, protonolysis would release the gold catalyst into the catalytic cycle to afford the target carbazole 3 (path A, Scheme 2). With PtCl₄ as the catalyst, the reaction proceeds via the resonance structure vinylic platinum carbene (M2_b), affording the final product 4via 1,2-alkyl migration¹⁴ (path B, Scheme 2). The real role of each catalyst for the different selectivity is still not clear.

Scheme 2 Proposed mechanisms.

In conclusion, we have developed a simple and efficient AuCl(PPh₃)/AgBF₄- or PtCl₄-catalyzed reaction of 1-(indol-2-yl)-2,2-dialkyl-substituted-3-alkyne-1-ols, providing differently substituted carbazoles in good isolated yields under very mild conditions. Different regioselective 1,2-alkyl migration pathways have been established: carbazoles 3 could be obtained exclusively in the presence of AuCl(PPh₃)/AgBF₄via a Wagner–Meerwein type 1,2-alkyl shift, whereas in some cases the use of PtCl₄ afforded inverted regioselectivity forming carbazoles 4 involving a platinum–carbene intermediate. Due to the potential of the products and unique pathways, this method may be useful in organic synthesis and medicinal chemistry. The observed selectivity with different metal catalysts is quite informative for further study. Further studies including new ways for the synthesis of the starting materials and synthetic applications of this reaction and the effect of the catalyst on the selectivity are being carried out in our laboratory.

Experimental

1. AuCl(PPh₃)/AgBF₄-catalyzed cyclization reaction of 1-(indol-2-yl)-2,2-dialkyl-3-alkyne-1-ols: synthesis of 4-butyl-9-ethyl-1,2,6-trimethyl-9H-carbazole (3a)

Typical procedure. To a dry Schlenk tube were added AgBF₄ (10.8 mg, 0.055 mmol, weighed in a glove box), AuCl(PPh₃) (24.6 mg, 0.05 mmol), 2a (310.2 mg, 1.0 mmol), and toluene (10 mL) under N₂. After continuous stirring for 12 h at rt, the reaction was complete as monitored by TLC. Filtration through a short pad of silica gel (eluent: Et₂O (20 mL × 3)), evaporation, 3a : 4a = 97 : 3 determined by ¹H NMR of crude product, column chromatography on silica gel (petroleum ether–dichloromethane = 30/l for the first round, petroleum ether–dichloromethane = 30/l for the second round (impure part)) afforded 3a (188.1 mg, 64%, 3a : 4a = 97 : 3), which was further purified by recrystallization to afford pure 3a (152.8 mg, 52%) as a solid: m.p. 80–82 °C (n-hexane–ethyl acetate); ¹H NMR (300 MHz, CDCl₃) δ 7.94 (s, 1H, ArH), 7.34 (d, J = 8.4 Hz, 1H, ArH), 7.31 (d, J = 8.4 Hz, 1H, ArH), 6.92 (s, 1H, ArH), 4.61 (q, J = 7.1 Hz, 2H, NCH₂), 3.22 (t, J = 7.8 Hz, 2H, ArCH₂), 2.73 (s, 3H, CH₃), 2.62 (s, 3H, CH₃), 2.53 (s, 3H, CH₃), 1.96–1.82 (m, 2H, CH₂), 1.71–1.58 (m, 2H, CH₂), 1.48 (t, J = 6.9 Hz, 3H, CH₃), 1.10 (t, J = 7.4 Hz, 3H, CH₃); ¹³C NMR (75 MHz, CDCl₃) δ 140.3, 139.7, 135.2, 134.2, 127.9, 125.7, 123.4, 122.7, 122.4, 120.0, 115.7, 108.2, 39.9, 33.9, 31.9, 23.0, 21.6, 20.9, 15.5, 14.8, 14.1; IR (KBr) ν (cm⁻¹) 3011, 2955, 2928, 2862, 1592, 1574, 1484, 1377, 1343, 1308, 1229, 1172, 1150, 1081; MS (70 ev, EI) m/z (%) 294 (M⁺ + 1, 21.92), 293 (M⁺, 100); elemental analysis calcd (%) for C₂₁H₂₇N: C, 85.95; H, 9.27; N, 4.77; found: C, 85.64, H, 9.31; N, 4.84.

2. PtCl₄-catalyzed cyclization reaction of 1-(indol-2-yl)-2,2-dimethyl-3-alkyne-1-ols: synthesis of 4-butyl-9-ethyl-2,3,6-trimethyl-9H-carbazole (4a)

Typical procedure. To a dry Schlenk tube were added PtCl₄ (17.1 mg, 0.05 mmol, weighed in a glove box), 2a (311.6 mg, 1.0 mmol), and toluene (10 mL) under N₂. After continuous stirring for 18 h at −10 °C, the reaction was complete as monitored by TLC. Filtration through a short pad of silica gel (eluent: Et₂O (20 mL × 3)), evaporation, 4a : 3a = 92 : 8 determined by ¹H NMR of crude product, and column chromatography on silica gel (petroleum ether–ethyl acetate = 100/l) afforded 4a and 3a (235.0 mg, 80%, 4a : 3a = 92 : 8 determined by ¹H NMR), which was further purified by recrystallization to afford 4a (179.3 mg, 61%, 4a : 3a = 96 : 4) as a solid: m.p. 84–86 °C (n-hexane/ethyl acetate); ¹H NMR of 4a (300 MHz, CDCl₃) δ 7.99 (s, 1H, ArH), 7.37–7.27 (m, 2H, ArH), 7.14 (s, 1H, ArH), 4.34 (q, J = 7.1 Hz, 2H, NCH₂), 3.33 (t, J = 8.0 Hz, 2H, ArCH₂), 2.62 (s, 3H, ArCH₃), 2.56 (s, 3H, ArCH₃), 2.43 (s, 3H, ArCH₃), 1.92–1.62 (m, 4H, 2 × CH₂), 1.44 (t, J = 7.1 Hz, 3H, CH₃), 1.13 (t, J = 7.2 Hz, 3H, CH₃); the following signals are discernible for 3a: 7.93 (s, 1H, ArH), 6.91 (s, 1H, ArH), 4.60 (q, J = 7.1 Hz, 2H, NCH₂), 3.21 (t, J = 7.7 Hz, 2H, ArCH₂), 2.73 (s, 3H, ArCH₃); ¹³C NMR of 4a (75 MHz, CDCl₃) δ 138.8, 138.2, 136.4, 134.8, 127.4, 125.4, 124.6, 123.2, 122.4, 119.1, 107.7, 107.1, 37.1, 31.4, 30.3, 23.4, 22.3, 21.7, 14.5, 14.1, 13.6; IR (neat) ν (cm⁻¹) 2956, 2929, 2871, 1623, 1605, 1576, 1487, 1471, 1377, 1349, 1307, 1266, 1192, 1147, 1015; GC-MS (GC conditions: injector: 280 °C; column: DB5 column 30 m × 0.25 mm, temperature programming: 60 °C (2 min), 20 °C min⁻¹ to 280 °C, 280 °C (30 min); detector: 280 °C) (70 ev, EI) m/z (%) for 4a: T_R 5.290 min: 294 (M⁺ + 1, 22.77), 293 (M⁺, 100), for 3a: T_R 5.313 min: 294 (M⁺ + 1, 23.31), 293 (M⁺, 100); elemental analysis calcd (%) for C₂₁H₂₇N: C, 85.95; H, 9.27; N, 4.77; found: C, 85.91; H, 9.46; N, 4.91.

Acknowledgements

Financial support from the National Natural Science Foundation of China (21232006) and the National Basic Research Program (2011CB808700) is greatly appreciated. Shengming Ma is a Qiu Shi Adjunct Professor at Zhejiang University. We thank Mr X. Tang of this group for reproducing the preparation of 3e and 3g in Table 2 and 4e in Table 4.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental procedures and detailed characterization data for all new compounds. CCDC 944138 and 944139. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3qo00006k

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