Catalytic enantioselective synthesis of quaternary 3,3′-indolyloxindoles by combination of Rh(II) complexes and chiral phosphines

Abstract

Rh(II)/chiral phosphine combined catalysis has been developed for a highly efficient sequential C–H functionalization/asymmetric allylation or Michael addition reaction of indoles and 3-diazooxindoles with allenoates or vinyl ketones to afford a diverse range of quaternary 3,3′-indolyloxindole derivatives. Moreover, a significantly positive effect of LiCl in the sequential C–H functionalization/allylation reaction was observed, which led to an unusual ternary activation mechanism in the allylation step.

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Introduction

The optically active 3,3′-bisindole represents a unique structural element in the fascinating family of indole alkaloids exemplified in Fig. 1, which have been characterized by a wide spectrum of antimicrobial, antiviral and antitumor bioactivities.¹ All-carbon quaternary stereogenic center-linking patterns (QCPs) of the bisindole unit are potential sources of structural diversity that attracts great interest for its chemical synthesis, especially in a catalytic enantioselective fashion. Recent reports have described a number of intra-² or intermolecular^3–5 methodologies, and the latter chiefly centered on asymmetric substitution reactions of racemic 3,3′-disubstituted oxindoles³ and asymmetric addition reactions of 3-indolyloxindoles to electrophiles.⁴ Despite the great contributions that these existing methods have made in building up bisindole derivatives and total synthesis of natural alkaloids, the methods to access QCPs are still rather limited and hence novel strategies are extremely desired to circumvent daunting challenges in the highly straightforward and stereoselective construction of all-carbon quaternary stereogenic centers of this type.⁶

Fig. 1 Natural bisindole alkaloids.

Nucleophilicity-featured chiral phosphines⁷ have recently been at the center of attention in popular asymmetric organocatalysis arising from pioneering contributions from Lu⁸ and driven by important findings coming from Vedejs,⁹ Zhang,¹⁰ Fu,¹¹ Shi¹² and others.^13–17 Simple α-amino acid- or dipeptide-based bifunctional chiral phosphines, first discovered by Miller¹⁴ and Jacobsen,¹⁵ advanced by Zhao¹⁶ and Lu,¹⁷ have recently proven to be extremely effective in the promotion of a number of enantioselective transformations. For example, Lu's^17g and Zhao's^16c groups respectively reported asymmetric γ-additions of oxindoles or benzofuran to allenoates, providing direct access to highly enantioenriched quaternary 3,3′-indolyloxindoles or benzofuran (Scheme 1). In the last decade, asymmetric metal/organo-combined catalysis has been a valuable and versatile concept to enable the abundant creation of new transformations.^4d,18–20 However, to the best of our knowledge, chiral phosphines have rarely been in this category with the exception of a few non-enantioselective examples described by Krische and Wu.²¹ We believe that the combination of transition metals and chiral phosphines would be able to provide more opportunities to enrich the synthetic value than either type of individual catalysts. Herein, we will demonstrate our efforts directed towards developing a novel Rh(II)/chiral phosphine sequential catalysis for the efficient synthesis of enantioenriched 3,3′-indolyloxindoles with unprecedented QCPs (Scheme 2).

Scheme 1 Phosphine mediated asymmetric γ-additions.

Scheme 2 Rh(II)/chiral phosphine combined catalysis.

Results and discussion

We first investigated different conditions for a three-component reaction of 3-diazooxindole 1a, N-methyl indole 2a and allenoate 3 under the catalysis of 2 mol% of transition metal complexes and 10 mol% of organocatalysts, in CH₂Cl₂, at 25 °C (Table 1, entries 1–8). Since the Ru(II)/chiral squaramidebinary catalyst system previously developed^4d failed to afford the desired three-component transformation, instead only a C–H functionalization product was observed (see Fig. S1 in the ESI† for details), we turned to use more nucleophilic chiral phosphines, in combination with transition metal complexes to catalyze the reaction. In the presence of 2 mol% [RuCl₂(p-cymene)]₂, the L-Val derived bifunctional chiral phosphine 4a successfully enabled a sequential C–H functionalization/allylation reaction to give 5a, but in a low yield with a moderate 32% ee (entry 1). The use of rhodium complexes as metal catalysts did not exert an impact on the enantioselectivity while leading to obviously higher yields (entries 2 and 3). More interestingly, the Rh₂(esp)₂ showed good compatibility with the chiral phosphine organocatalyst to allow the sequential reaction to give a good yield (entry 3). Then, a series of bifunctional chiral phosphines 4b–4f were evaluated, and it was found that the original α-amino acid sources and hydrogen-bond donor moieties were both important for stereocontrol (entries 4–8). In particular, the combination of Rh₂(esp)₂ and L-Phe derived thiourea phosphine 4d enabled the reaction to give the best results of 50% yield and 77% ee (entry 6). Notably, the reaction in CHCl₃ displayed 87% ee (entry 9 and see Table S1 in the ESI† for more details). Surprisingly, even better results were obtained by lowering the loading of Rh₂(esp)₂ to 0.02 mol% probably due to more amounts of chiral phosphine catalyst 4d liberated from the coordination to rhodium (entry 10). The examination of additives (also see Table S1 in the ESI† for details) identified that 1.0 equivalent of LiCl continued to improve both the yield and enantioselectivity (entry 11, 81% yield and 91% ee). However, lowering the reaction temperature (0 °C) led to significantly eroded yield, albeit with a maintained enantiomeric excess (entry 12).

Table 1 Optimization of reaction conditions^a

Entry	[M]	4	Solvent	Yield^b (%)	ee^c (%)
a Unless indicated otherwise, reactions of 1a (0.10 mmol), 2a (0.10 mmol), 3 (0.11 mmol), metal (0.002 mmol), and 4 (0.010 mmol) were carried out in a solvent (1.0 mL), at 25 °C, for 12 h. b Isolated yield. c Determined by HPLC. d 0.02 mol% of Rh2(esp)2 was used. e 1.0 equivalent of LiCl was used as an additive. f The reaction was conducted at 0 °C. [Ru] = [RuCl2(p-cymene)]2.
1	[Ru]	4a	CH2Cl2	10	32
2	Rh2(OAc)4	4a	CH2Cl2	21	32
3	Rh2(esp)2	4a	CH2Cl2	53	32
4	Rh2(esp)2	4b	CH2Cl2	81	16
5	Rh2(esp)2	4c	CH2Cl2	60	58
6	Rh2(esp)2	4d	CH2Cl2	50	77
7	Rh2(esp)2	4e	CH2Cl2	55	58
8	Rh2(esp)2	4f	CH2Cl2	39	46
9	Rh2(esp)2	4d	CHCl3	52	87
10^d	Rh2(esp)2	4d	CHCl3	60	87
11^d^,^e	Rh 2 (esp) 2	4d	CHCl 3	81	91
12^d^,^e^,^f	Rh2(esp)2	4d	CHCl3	26	91

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With the optimized reaction conditions in hand, we then examined the substrate scope. In general, this Rh(II)/4d combined catalytic system tolerated a wide range of substituents on either the indole or 3-diazooxindole moiety, affording excellent yields and enantioselectivities in most cases (Table 2). The electronic nature of substituents was crucial for the formation of final products by accelerating or slowing down the first C–H functionalization step. For example, the substrates with electron–neutral or electron-donating substituents smoothly underwent a one-pot reaction in the presence of 0.01 mol% of Rh₂(esp)₂ (e.g., entries 2–4, 6, and 8–10), while the reaction of those with electron-withdrawing substituents required 0.05 mol% of Rh₂(esp)₂ to ensure the generation of the desired products in good yields (entries 7 and 12). The substituent on either the C6- or C7-position of 3-diazooxindoles 1 had a negative effect on the stereocontrol regardless of the electronic feature (entries 10–12). By converting 5a and a known compound into the same derivative, the absolute configuration of 5 was determined to be S (see Scheme S1 in the ESI† for details).

Table 2 Substrates scope of sequential C–H functionalization/allylation reaction^a

Entry	R^1/R^2	x	5	Yield^b (%)	ee^c (%)
a Unless indicated otherwise, reactions of 3-diazooxindoles 1 (0.10 mmol), 2 (0.10 mmol), and 3 (0.11 mmol) with 4d (0.010 mmol) and x mmol% Rh2(esp)2 were carried out in CHCl3 (1.0 mL), at 25 °C, for 12 h. b Isolated yield. c Determined by HPLC. d 20 mol% of 4d was used.
1	H/5-F	0.02	5b	69	87
2	H/5-Me	0.01	5c	80	90
3	H/5-OMe	0.01	5d	83	92
4	H/6-OMe	0.01	5e	81	90
5	H/6-Cl	0.02	5f	72	90
6	H/7-Me	0.01	5g	64	87
7^d	5-Br/H	0.05	5h	64	92
8	5-Me/H	0.01	5i	80	86
9	5-OMe/H	0.01	5j	66	88
10	6-OMe/H	0.01	5k	77	86
11	6-Br/H	0.02	5l	66	85
12	7-Br/H	0.05	5m	61	72

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Other electrophiles such as α,β-unsaturated carbonyls could also be adopted in the C–H functionalization/asymmetric Michael addition^17f sequentially catalyzed by Rh₂(esp)₂ and the chiral phosphine 4g (see Fig. S2 in the ESI† for the detailed evaluation of chiral phosphines). Under the optimal conditions (see Table S2 in the ESI† for details), high yields and excellent enantioselectivities were generally provided (Table 3). Non-protected indoles were also permitted to undergo a clean reaction, however with a much diminished enantioselectivity of 56% ee (entry 2), presumably due to the fact that the H-bonding interaction between the catalyst and the substrate was disrupted.¹⁷ⁱ Again, the presence of electron-donating substituents at the indole moiety gave higher enantioselectivity than those with an electron-deficient substituent (entries 3–8). Moreover, ethyl- and phenyl vinyl ketones underwent a clean reaction to generate 7i and 7j in high yields with excellent or good levels of stereocontrol. By converting 7b and a known compound into a similar derivative, the absolute configuration of 7 was determined to be S (see Scheme S2 in the ESI† for details).

Table 3 Rh(II)/chiral phosphine sequentially catalytic C–H functionalization/Michael addition reaction^a

Entry	R^1/R^2/R^3	x	7	Yield^b (%)	ee^c (%)
a Unless indicated otherwise, reactions of 3-diazooxindoles 1 (0.10 mmol), 2 (0.10 mmol, PG = Me), and 6 (0.20 mmol) with 4g (0.010 mmol) and x mmol% Rh2(esp)2 were carried out in CHCl3 (1.0 mL), at −60 °C, for 12 h. b Isolated yield. c Determined by HPLC. d PG = H. e 10 mol% of 4c was used instead of 4g.
1	H/H/Me	0.02	7a	92	91
2^d	H/H/Me	0.02	7b	82	56
3	H/5-OMe/Me	0.01	7c	75	92
4	H/6-OMe/Me	0.01	7d	88	92
5	H/6-Cl/Me	0.02	7e	88	87
6	H/7-Me/Me	0.01	7f	61	89
7	5-Br/H/Me	0.05	7g	87	92
8	6-OMe/H/Me	0.01	7h	77	90
9	H/H/Et	0.02	7i	86	90
10^e	H/H/Ph	0.02	7j	94	77

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Products obtained from these Rh(II)/chiral phosphine sequentially catalytic protocols could be transformed into a structurally more complicated spirocyclic 3,3′-indolyloxindole through classical procedures. For example, in the presence of excess amounts of BF₃·Et₂O, the Boc group of 7a was first removed to give A, which then underwent an intramolecular Friedel–Crafts/dehydration process to generate a spirocycle 8 (Scheme 3).

Scheme 3 Synthesis of a spirocycle from 7a.

According to previous findings,^4d the one-pot reactions might principally encompass two catalytic cycles (Fig. 2), including an insertion reaction of rhodium carbene species to the C–H bond²² to generate a nucleophilic intermediate 9. In the sequential C–H functionalization/allylation reaction, a lithium-stabilized phosphonium enolate 10 would be generated, as suggested by previous LiCl-promoted nucleophilic catalysis,²³ which worked as a stronger base to facilitate the enolization of 9, thereby leading to an improved yield (entry 10 vs. 11, Table 1). More importantly, LiCl, in combination with the bifunctional phosphine, constitutes a ternary synergistic activation system (that is, Lewis acid, hydrogen bonds and phosphine synergistic activation), as shown in the transition state TS-I, wherein the oxindole lithium enolate interacted with a chiral thiourea–chloride complex through Li–Cl coordination, as indicated by Jacobsen hypothesis,²⁴ to deliver relatively higher levels of stereocontrol than in the absence of LiCl (entry 10 vs. 11, Table 1). In contrast, the sequential process of C–H functionalization and Michael addition also involves the C–H insertion reaction to generate 9 while the formation of products 7 from 9 is considered to follow the general mechanism reported in the literature.^17f

Fig. 2 Proposed mechanism for the Rh(II)/chiral phosphine sequentially catalytic C–H functionalization/asymmetric allylation reaction.

Conclusions

We have developed a combined catalyst system comprised of Rh(II) complexes and amino acid-derived chiral phosphines, enabling highly enantioselective sequential C–H functionalization/asymmetric addition reactions of indoles and 3-diazooxindoles with allenoates or vinyl ketones. These reactions afforded enantioenriched 3,3′-indolyloxindoles with unprecedented quaternary stereogenic center-linking patterns, which allowed further transformations into more complex molecules. Moreover, the positive effect of LiCl led to an unusual possibility that the lithium enolate might be able to form a chiral thiourea–chloride complex through Li–Cl coordination to assist the stereochemical control.

Acknowledgements

We are grateful for financial support from NSFC (21232007) and the Ministry of Education.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5qo00151j

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