Dual C–H Functionalization of N-Aryl Amines: Synthesis of Polycyclic Amines via an Oxidative Povarov Approach

Abstract

Iminium ions generated in situ via copper(I) bromide catalyzed oxidation of N-aryl amines readily undergo [4 + 2] cycloadditions with a range of dienophiles. This method involves the functionalization of both a C(sp³)–H and a C(sp²)–H bond and enables the rapid construction of polycyclic amines under relatively mild conditions.

First reported in 1963,¹ the Povarov reaction comprises a formal [4 + 2] cycloaddition of an electron-rich dienophile and a 2-azadiene, typically an N-aryl imine or iminium ion.² This transformation generally requires a Lewis or Brønsted acid catalyst or promoter and provides an efficient route to tetrahydroquinolines (Scheme 1).³ Povarov reactions are commonly performed as two-component transformations in which a preformed imine 1 engages an electron-rich dienophile 2 to form tetrahydroquinoline product 3. Three-component variants are also popular and involve the condensation of an amine 4 with an aldehyde (or a ketone) and a dienophile 2.² While the two-component method is limited to imines derived from primary amines, the three-component approach is also applicable to secondary N-aryl amines, including cyclic amines.⁴ The latter enables the formation of polycyclic products 5 in which two rings are fused to the same aryl group. In contrast, the classic Povarov reaction is not readily applicable to the synthesis of polycyclic amines of type 6. A typical two-component Povarov approach to these compounds would require an aminoaldehyde 7, species that are not readily accessible. Other approaches to polycyclic frameworks related to 6 typically require ortho-functionalized N-aryl amines and/or additional steps. These methods include intramolecular redox transformations,⁵ oxidative couplings,⁶ C–N bond formation via Pd-catalysis⁷ or benzyne intermediates,⁸ and Bischler–Napieralski reactions followed by reduction.⁹ As part of our continuing efforts to develop practical methods for the C–H functionalization of amines,¹⁰ herein we report an alternate Povarov approach to polycyclic tetrahydroquinolines 6 that utilizes the in situ oxidation of readily available N-aryl amines 8.

Scheme 1. Variants of the Povarov Reaction.

The oxidative C–H functionalization of amines has a venerable history and was greatly popularized by the pioneering studies of the Murahashi¹¹ and Li¹² groups, who advanced the applicability of catalytic approaches. Oxidative reactions in which a C–H bond is replaced with a functional group are now widely referred to as cross-dehydrogenative coupling reactions (CDC reactions).^13,14 A typical CDC reaction involves the oxidation of a tertiary amine such as 8a to an iminium ion (e.g., 9), followed by capture of 9 with a nucleophilic species (NuH) to form product 10 (Scheme 2).^15,16 The vast majority of these transformations employ N-aryl tetrahydroisoquinolines and lead to the monofunctionalization of the benzylic α-position of these substrates. Few transformations of N-aryl amines have been reported that, in addition to α-functionalization, simultaneously result in the functionalization of an ortho aryl C–H bond. An example of such a process is the oxidation of 11 to radical 12 which subsequently engages an electron-deficient olefin to give product 14 via the oxidation of radical intermediate 13.¹⁷ With regard to the proposed oxidative Povarov reaction, to our knowledge such a process has only been realized with N,N-dimethylanilines (e.g., 11),¹⁸ giving relatively simple N-alkyl tetrahydroquinoline derivatives 16 (via iminium ion 15), products that are also accessible via classic Povarov reactions that utilize N-methyl aniline as the substrate.^2,4,19

Scheme 2. Examples of Oxidative Amine C–H Functionalizations.

The title reaction was first evaluated using N-phenyl 1,2,3,4-tetrahydroisoquinoline (8a) and 1-vinylpyrrolidin-2-one (17a) as model substrates (Table 1). A preliminary survey of various methods for amine oxidation (including aerobic conditions) led to the identification of tert-butyl hydroperoxide (TBHP) as the most promising terminal oxidant.²⁰ Out of a number of copper salts that were tested as catalysts in reactions performed in acetonitrile solution, copper(I) bromide provided the best results (entry 2). With regard to product yield, 1,2-dichloroethane was found to be superior to acetonitrile and other solvents such as tetrahydrofuran, dioxane, methanol, chloroform, and toluene. A reaction performed under neat conditions resulted in a dramatically reduced yield of 18a (entry 13). Decreasing the amount of 17a from two to 1.1 equiv had no adverse effects on the reaction outcome (entry 14). Under these optimized conditions, 18a was obtained in 62% isolated yield and with a dr of 6:1 in favor of the endo-product.

Table 1. Reaction Development^a.

entrya	catalyst	solvent	time [h]	drb	yieldc (%)
1	CuCl	CH3CN	24	10:1	60
2	CuBr	CH3CN	24	8.7:1	65
3	CuBr2	CH3CN	24	10:1	50
4	CuCl2	CH3CN	10	12:1	27
5	CuI	CH3CN	24	7:1	49
6	Cu(OTf)2	CH3CN	24	1:1	26
7	CuBr	THF	24	5:1	49
8	CuBr	C2H4Cl2	24	5.8:1	81
9	CuBr	dioxane	24	5.2:1	59
10	CuBr	MeOH	32	4.6:1	49
11	CuBr	CHCl3	32	3:1	60
12	CuBr	PhMe	32	2.7:1	20
13	CuBr	–	24	ND	13
14d	CuBr	C2H4Cl2	24	6:1	80 (62e)

^aReactions were performed with 0.2 mmol of 8a, 0.4 mmol of 17a, 0.02 mmol of catalyst, and 0.24 mmol of tBuOOH (5.5 M in decane) in 1 mL of solvent.

^bThe dr was determined by ¹H NMR of the crude reaction mixture.

^cYield was determined by ¹H NMR with an internal standard.

^d1.1 equiv of 17a was used.

^eIsolated yield.

The scope of the oxidative Povarov reaction was evaluated on a set of amine/dienophile combinations (Scheme 3). A range of tetrahydroisoquinolines with different substituents on the N-aryl ring readily underwent cycloaddition with 1-vinylpyrrolidin-2-one (17a) to produce the corresponding products 18 in moderate to good yields. However, ortho-substituents on the N-aryl ring proved problematic, presumably due to developing A_1,3-type strain in the transition state of this reaction. As a consequence, product 18e was isolated in only a 26% yield. Although low-yielding, N-phenyl tetrahydroazepine also engaged in a reaction with 17a to form polycyclic product 18g. To our knowledge, this represents the first example of a tetrahydroazepine derivative undergoing a CDC-type reaction.

Scheme 3. Scope of the Reaction.

Reactions were performed with 0.5 mmol of the amine, 0.55 mmol of the dienophile, 0.05 mmol of CuBr, and 0.6 mmol of tBuOOH (5.5 M in decane) in 2.5 mL of C₂H₄Cl₂. All yields are combined isolated yields of both diastereomers.

In order to explore the generality of the method, two acyclic aniline derivatives were tested. N,N-Dimethylaniline, upon reacting with 17a, provided the expected product 18h in 43% yield. An N-phenylglycine ester was also found to be a suitable substrate, giving cyclic amino acid derivative 18i, in 61% yield. This illustrates the potential utility of this method in the direct C–H functionalization of peptide derivatives.²¹ Finally, the scope of the oxidative Povarov reaction was explored with regard to the dienophile. Various acyclic and cyclic enol ethers and enamides readily underwent the title reaction to provide polycyclic products in moderate to good yields. The diastereoselectivity for some of these reactions was rather poor.

An attempt to extend the scope of the oxidative Povarov reaction to N-phenyl pyrrolidine (19) as the amine initially only led to the formation of trace amounts of the expected product 20 (Scheme 4). Instead, oxidation of 19 in the presence of 17a resulted in the formation of 21 as a 1.3:1 mixture of diastereomers in 33% yield. The yield of 21 increased to 40% when the reaction was conducted in the absence of 17a. This substrate dimerization (i.e., via 22) is easily rationalized, as the iminium ion resulting from the oxidation of 19 is expected to exist in equilibrium with its corresponding enamine.²² As was observed previously with cyclic enecarbamates, the diastereoselectivity of this process was found to be low.

Scheme 4. Oxidative Functionalization of N-Phenyl Pyrrolidine.

In order to obtain the desired product 20, a number of other conditions were evaluated. Gratifyingly, 20 was obtained, albeit in only 19% yield, in a reaction that was conducted in methanol, using copper(II) chloride dihydrate as the catalyst and air as the terminal oxidant. In this instance, dimerization product 21 was obtained in less than 10% yield. This change in product distribution may be rationalized on the basis that the intermediate iminium ion can be captured by methanol to form the corresponding N,O-acetal which in turn could act as a reservoir for the iminium ion, thus reducing the concentration of the N-phenyl pyrrolidine enamine.

In summary, we have reported oxidative Povarov reactions of various N-aryl amines as a method to rapidly access polycyclic amines. These reactions feature the dual functionalization of both a C(sp³)–H and a C(sp²)–H bond and are likely amenable to enantioselective catalysis.

Supporting Information Available

Experimental procedures and characterization data, including an X-ray crystal structure of product 18k (CIF). This material is available free of charge via the Internet at http://pubs.acs.org.

The authors declare no competing financial interest.

Funding Statement

National Institutes of Health, United States