Regioselective BF₃·Et₂O-catalyzed C–H functionalization of indoles and pyrrole with reaction of α-diazophosphonates

Abstract

Facile regiospecific intermolecular C–H insertion reactions of α-diazophosphonates with indole or pyrrole derivatives catalyzed by trifluoroborane have been developed. The reaction protocol was effective for regioselective C–H insertion depending on the substitution pattern on the indole moiety and carbene migratory model. This represents the first straightforward access to N-unsubstituted β-(3-indol)-β-aminophosphonates and β-(2-pyrrol)-β-aminophosphonates containing quaternary carbon centers in moderate to good yields.

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The construction of quaternary carbon centers bearing a nitro atom, which is present in many natural products and biologically active compounds and therefore, the development of new synthetic procedures of the catalytic synthesis of these frameworks continues to be an intensely investigated field.¹ Carbenoid insertion reactions are important tools for C–C bond formation.² Generation of carbenoids and their insertion reactions with heterocyclic compounds, either intra-molecularly or inter-molecularly, has been described in the literature.³ Indole derivatives and pyrrole belong to the most important nitrogen heterocycles in both biologically active alkaloids and pharmaceutical agents and it is often considered a privileged scaffold.⁴ To access these privileged building blocks, the direct functionalization of preformed indole and pyrrole derivatives appears to be a remarkably efficient and versatile route to complex natural products.⁵ Among the enormous efforts devoted to this area, the application of carbenoid direct C–H functionalization of indoles under the catalysis of transition metals or Lewis acids has been realized as a powerful approach for constructing indol-3-position derivatives C–C bonds formation.⁴ In 2015, Wang and co-workers demonstrated the Cp*Co(III)-catalyzed aromatic C–H coupling reaction with α-diazomalonates to form tertiary stereocentres alkylation products with satisfactory yields and excellent regioselectivity.⁶ Zhou also reported an iron-catalyzed C–H functionalization of indoles with α-aryl-α-diazoesters to produce the useful α-aryl-α-indolylacetate derivatives containing tertiary stereocentres.⁷

β-Aminophosphonates continue to receive wide-spread attention due to their ubiquity in biological systems.⁸ On the other hand, recent studies have indicated that a number of heterocycle analogues containing phosphorus showed excellent bioactivities.⁹ Particularly, the presence of a quaternary stereocenter bearing a β-aminophosphate unit will interact with certain proteases and resist proteolytic degradation.¹⁰ We have recently developed a kind of novel α-diazophosphonyl compounds prepared from natural amino acid, which could afford tertiary β-alkoxy substituted β-aminophosphonates through a combined C–H functionalization/O–H insertion process (Scheme 1, eqn (1)).¹¹ As a natural extension of the carbenoid insertion reaction of α-diazophosphonyl compounds, we developed trifluoroborane-catalyzed C–H functionalization/S–H insertion reaction of α-diazophosphonate with mercaptans to produce N,S-acetals containing quaternary centers in good yields with moderate to good chemoselectivities (Scheme 1, eqn (2)).¹² Continuing with our interest in the chemistry of aminophosphorus derivatives,¹³ in this communication, we report the first example for converting dialkyl α-diazophosphonates into β-3-indole (or β-2-pyrrole) substituted β-aminophosphonates with quaternary centers. In this transformation, we employed BF₃·Et₂O as the catalyst to decompose α-diazophosphonates (Scheme 1, eqn (3) and (4)). We found that α-diazophosphonate could form a carbene complex, and then the β-hydrogen on the phosphonate migrated to the carbene center to form a tertiary carbocation intermediate. The C–H insertion process of indole and pyrrole derivatives leads producing quaternary β-3-indole (or β-2-pyrrole) substituted β-aminophosphonates in a regiospecific manner.

Scheme 1 Previous and proposed work.

In the initial studies, the reaction of diethyl α-diazophosphonate 1a (ref. 11) with indole 2a was performed in dichloromethane at room temperature in the presence of 5 mol% catalyst. The diazo compound was added in 1.5 hours to minimize the formation of dimerization products. The results revealed that transition metal catalysts AgOTf and Rh₂(OAc)₄ could not decompose 1a in the presence of 2a (Table 1, entries 1 and 2). With Cu(MeCN)₄PF₆ as catalyst, the corresponding reaction of α-diazophosphonate 1a and indole 2a did lead to the formation of diethyl((2-indole)-2-(1,3-dioxoisoindolin-2-yl)propyl)phosphonate 3a in 20% yield. In the meantime, one 1,2-hydrogen migration byproduct (Z)-diethyl-(2-(1,3-dioxoisoindolin-2-yl)prop-1-en-1-yl)phosphonate 4a was obtained in 8% yield (Table 1, entry 3). It is worthwhile to note that we have not obtained any product of 3a′ resulting from the potential competitive intermolecular N–H insertion reaction of the free carbenoid.¹⁴ Encouraged by this results, we set out to further optimize the reaction conditions. We turn our attention to Bronsted acid which could also decompose diazo compounds.^12,15

Table 1 Optimization of the reaction conditions^a

Entry	Catalysts	Solvent	(3a : 4a) ratio^b	Overall yield^c (%)
a Unless otherwise specified, all reactions were carried out using α-diazophosphonate 1a (0.28 mmol, 1 equiv.) and indole 2a (0.42 mmol, 1.5 equiv.) in 4 mL solvent with 10 mol% of catalyst at 25 °C for 3.5 h (of addition 1.5 h, after addition 2 h).b The product ratio was determined by ^31P NMR of the crude product.c Overall yield of the mixture of 3a and 4a after silica gel chromatograph.d 20 mol% catalyst was used.e 3 equiv. of indole 2a was used.f The reaction temperature is −20 °C.g The reaction temperature is 0 °C.h The reaction temperature is 40 °C.
1	AgOTf	CH2Cl2	—	N.R.
2	Rh2(OAc)4	CH2Cl2	—	N.R.
3	Cu(MeCN)4PF6	CH2Cl2	71 : 29	28
4	BF3·Et2O	CH2Cl2	88 : 12	65
5	H3PO4	CH2Cl2	22 : 78	33
6	AcOH	CH2Cl2	70 : 30	47
7	BF3·Et2O	Toluene	78 : 22	63
8	BF3·Et2O	DME	60 : 40	45
9	BF3·Et2O	ClCH2CH2Cl	80 : 20	58
10	BF3·Et2O	THF	—	N.R.
11	BF3·Et2O	CH3CN	—	N.R.
12^d	BF3·Et2O	CH2Cl2	88 : 12	71
13^e	BF3·Et2O	CH2Cl2	85 : 15	67
14^f	BF3·Et2O	CH2Cl2	—	N.R.
15^g	BF3·Et2O	CH2Cl2	—	N.R.
16^h	BF3·Et2O	CH2Cl2	72 : 28	74

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It was found that the diazo decomposition could be dramatically effected under various catalysts and that the ratio of the two products 3a and 4a depends on the catalysts employed. The other catalysts tested (e.g., BF₃·Et₂O, phosphoric acid and AcOH) were able to promote the reaction (Table 1, entries 4–6). Since BF₃·Et₂O gave a higher yield (65%) compared with other catalysts, it was used in further investigations. A screening of solvents showed that toluene, DME, and 1,2-dichloroethane were providing 3a in lower yields (Table 1, entries 7–9). On the contrary, the coordinating solvents THF and CH₃CN dramatically slowed down the reaction and gave very low yields (Table 1, entries 10 and 11). The most suitable solvent was found to be dichloromethane. Higher catalyst loading (20 mol% BF₃·Et₂O) gave slightly higher yield of product 3a and 4a (Table 1, entry 12). However, increasing the amount of indole 2a to 3 equiv., the yield of the reaction slightly decreased to 67% (Table 1, entry 13). When the reaction was proceeded at −20 °C and 0 °C, the diazophosphonate 1a could not be decomposed in the presence of 2a with BF₃·Et₂O as catalyst (Table 1, entries 14 and 15). Higher temperature favored this reaction. When the reaction was carried out at 40 °C, the overall yield increased to 74%. However the ratio of 3a : 4a changed to 72 : 28 (Table 1, entry 16). Therefore, the optimal reaction conditions for this transformation were determined to be 0.28 mmol α-diazophosphonate 1a, 1.5 equivalents of indole 2a, 20 mol% of BF₃·Et₂O as a catalyst in 3 mL CH₂Cl₂ as a solvent at 25 °C for 3.5 hours.

Under the optimal reaction conditions, various α-diazophosphonates 1 were examined in the C–H functionalization of indoles 2. The tested (S)-diethyl(1-diazo-2-(1,3-dioxoisoindolin-2-yl)propyl) phosphonate 1a which derived from L-alanine gave the desired product 3a in good yield (Table 1, entry 1). The tested α-diazophosphonates 1b and 1c with different substituents on β-position, such as isobutyl and benzyl groups could not undergo this reaction to give the desired products. The starting materials 1b and 1c were decomposed under the reaction conditions (Table 2, entries 2 and 3). The substituent effect on the indole ring was then investigated. Substitution on the benzenoid position of the indole was well tolerated in most cases (Table 2, entries 4–11). Both electron-withdrawing and electron-donating moieties were well behaved. However, an electron-withdrawing group at the 5-position of indole had a negative effect in the reaction (Table 2, entry 12).

Table 2 Scope of the reaction^a

Entry	Product	R^1	R^2	R^3	(3 : 4) ratio^b	Overall yield^c (%)
a Reaction conditions: α-diazophosphonate 1 (0.28 mmol) and 2 (0.42 mmol, 1.5 equiv.) in 4 mL of CH2Cl2 at 25 °C in the presence of 20 mol% of BF3·Et2O for 3.5 h (of addition 1.5 h, after addition 2 h).b The product ratio was determined by ^31P NMR of the crude product.c Overall yield of the mixture of 3 and 4 after silica gel chromatograph.
1	3a	CH3 (1a)	CH3CH2	H (2a)	88 : 12	71
2	3b	CH2CH(CH3)2 (1b)	CH3CH2	H (2a)	—	N.R.
3	3c	C6H5CH2 (1c)	CH3CH2	H (2a)	—	N.R.
4	3d	CH3 (1a)	CH3CH2	4-CO2CH3 (2b)	78 : 22	64
5	3e	CH3 (1a)	CH3CH2	5-CH3 (2c)	83 : 17	67
6	3f	CH3 (1a)	CH3CH2	6-OCH3 (2d)	81 : 19	46
7	3g	CH3 (1a)	CH3CH2	6-Br (2e)	84 : 16	60
8	3h	CH3 (1a)	CH3CH2	6-CO2CH3 (2f)	77 : 23	46
9	3i	CH3 (1a)	CH3CH2	7-CH3 (2g)	82 : 18	73
10	3j	CH3 (1a)	CH3CH2	7-Br (2h)	79 : 21	63
11	3k	CH3 (1a)	CH3CH2	7-NO2 (2i)	36 : 64	77
12	3l	CH3 (1a)	CH3CH2	5-NO2 (2j)	—	N.R.
13	3m	CH3 (1d)	CH3	H (2a)	83 : 17	73
14	3n	CH3 (1e)	CH(CH3)2	H (2a)	85 : 15	72
15	3o	CH3 (1f)	(CH2)3CH3	H (2a)	82 : 18	87

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To access the effect of substrates on product selectivity, we set out to study reactions of a series of dialkyl α-diazophosphonates 1d–f under BF₃·Et₂O catalytic condition. The results show that the size of the R² group in the α-diazophosphonates 1 has almost no influence on the reactivity in the C–H insertion reaction (Table 2, entries 13–15). Interestingly, in case of 3-methyl-substituted indole 2k, the C–H functionalization at C-2 and N–H insertion of the indole ring took place to give the corresponding products 3p and 3p′ in a significant amount. The 1,2-hydride migration product 4a was not found. This was isolated and the yields were determined after column chromatographic purification (Scheme 2). The structure of 3a was confirmed by single crystal X-ray diffraction (Fig. 1).¹⁶

Scheme 2 Reaction of 1a with 3-methyl indole 2k.

Fig. 1 X-ray crystal structure of 3a.

This reaction can be extended to pyrrole as shown in Table 3. Pyrroles are known to be important building blocks for many biologically active compounds.⁴ Initially, we treated pyrrole 5 (0.5 mmol) with 1a (0.25 mmol) and BF₃·Et₂O (20 mol%) in CH₂Cl₂ at room temperature for 3.5 h, and diethyl(2-(1,3-dioxoisoindolin-2-yl)-2-(1H-pyrrol-2-yl)propyl) phosphonate 6a and 4a were obtained in 55% combined yield in a 87 : 13 ratio (Table 3, entry 1). Notably, GC-MS analysis did not reveal any 3- and N-alkylated products. The reaction of diazo reactant 1 and pyrrole 5 exhibits good tolerance to substituents on β-position of α-diazophosphonates 1, and combined yields of 55% and 64% were obtained with 1a and 1b, respectively (Table 3, entries 1 and 2). Similarly, the reactions of substituted diazo phosphonates 1c and 1g gave the desired products in 61% and 57% yields (Table 3, entries 3 and 4). The low yield observed when (S)-diethyl-[1-diazo-2,6-bis(1,3-dioxoisoindolin-2-yl)hexyl]phosphonate 1h was employed in this reaction (Table 3, entry 5). It is astonishing to note that diethyl α-diazophosphonates 1i and 1j which are derived from valine and methionine could not undergo this reaction to give the desired products. The starting materials 1i and 1j were decomposed under the reaction conditions (Table 2, entries 6 and 7). The absolute configuration of 6a was unambiguously assigned by X-ray crystallography (Fig. 2).¹⁷

Table 3 Scope of the reaction^a

Entry	Product	R^1	(6 : 4) ratio^b	Overall yield^c (%)
a Reaction conditions: α-diazophosphonate 1 (0.28 mmol) and 5 (0.42 mmol, 1.5 equiv.) in 4 mL of CH2Cl2 at 25 °C in the presence of 20 mol% of BF3·Et2O for 3.5 h (of addition 1.5 h, after addition 2 h).b The product ratio was determined by ^31P NMR of the crude product.c Overall yield of the mixture of 6 and 4 after silica gel chromatograph.
1	6a	CH3 (1a)	87 : 13	55
2	6b	CH2CH(CH3)2 (1b)	67 : 33	64
3	6c	C6H5CH2 (1c)	71 : 29	61
4	6d	p-AcOPh (1g)	71 : 29	57
5	6e	(CH2)4NPht (1h)	67 : 33	38
6	6f	CH(CH3)2 (1i)	—	N.R.
7	6g	CH2CH2SCH3 (1j)	—	N.R.

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Fig. 2 X-ray crystal structure of 6a.

A plausible mechanism for the reactions of α-diazophosphonates 1 with indoles 2 in the presence of BF₃·Et₂O may be proposed as given in Scheme 3.¹⁸ Diazo compounds are ambiphilic reagents.¹⁹ From the resonance structure 1 and A, it is found that the α-position of carbon to which the diazo group attached has a partial negative charge and thus is nucleophilic.¹² The α-diazophosphonate was decomposed by BF₃·Et₂O to generate the intermediate B. Then the β-hydrogen on the phosphonate migrated to the carbine center through a resonance complex B to form a tertiary carbocation intermediate C. Formation of a Lewis acid associated zwitterionic intermediate D can be envisaged by the nucleophilic addition of indole 2 onto electrophilic resonance complex C. A proton migration from C-3 of the indole to the α-position of the phosphonate took place to give the product 3 and regenerate the BF₃·Et₂O catalyst. On the other hand, intermediate C may also be transformed to by-product 4 through loss of BF₃·Et₂O and β-hydrogen migration before the attack of indole. In fact this mechanism would also agree with the substitution pattern observed, C-3 for indole and C-2 for pyrrole.²⁰

Scheme 3 Proposed reaction mechanism.

Conclusions

In conclusion, we have demonstrated the facile synthesis of N-unsubstituted β-(3-indol)-β-aminophosphonates 3 and β-(2-pyrrol)-β-aminophosphonates 6 from α-diazophosphonates 1 with indole and pyrrole derivatives via intermolecular C–H insertion catalyzed by BF₃·Et₂O. This methodology forms the first regiospecific synthesis of structurally interesting substrates that containing quaternary carbon centers bearing a nitro atom. In the case to unprotected derivatives, C–H insertion products are obtained, with a preference for position 3 in indoles and position 2 in pyrrole. Further research studies for control of the absolute stereochemistry of the products are currently underway in our laboratory.

Acknowledgements

We thank the National Natural Science Foundation of China (21072102), the Committee of Science and Technology of Tianjin (15JCYBJC20700) and State Key Laboratory of Elemento-Organic Chemistry in Nankai University for financial support.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. CCDC 982309 and 992526. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra15329a

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