Copper(II) Carboxylate Promoted Intramolecular Diamination of Terminal Alkenes: Improved Reaction Conditions and Expanded Substrate Scope

Abstract

The copper(II) carboxylate promoted diamination reaction has been improved by the use of the organic soluble copper(II) neodecanoate [Cu(ND)₂]. Cu(ND)₂ allowed the less polar solvent, dichloroethane (DCE) to be used, and as a consequence, decomposition of less reactive substrates could be avoided. High diastereoselectivity was observed in the synthesis of 2,5-disubstituted pyrrolidines. Ureas, bis(anilines) and α-amido pyrroles derived from 2-allylaniline could also participate in the diamination reaction.

Nitrogen heterocycles that contain vicinal amines are privileged biologically active structures.¹ Compounds containing vicinal diamines have demonstrated a range of activity that includes anti-parasitic [e.g., (R)-Praziquantel, Oxamniquine],² anti-depressant (e.g., Mianserin),³ anti-cancer [(−)-quinocarcin],⁴ adenosine kinase⁵ and protease inhibitory activity (Fig. 1).⁶ Cyclic sulfamides, which are especially accessible via the technology described in this paper, appear frequently as components of small molecule enzyme inhibitors (Fig. 1).⁷

Figure 1.

Biologically active cyclic vicinal diamines

The synthesis of vicinal diamines via olefin diamination is an active area of research.^1,8,9 The intramolecular diamination of alkenes provides a direct entry into the cyclic vicinal diamine motif.^9b,c We recently reported a novel intramolecular alkene diamination protocol promoted by copper(II) acetate.^9b This protocol provided efficient synthesis of fused cyclic sulfamide pyrrolidines and piperidines (eq. 1). This reaction is a member of a growing class of copper(II) promoted oxidative cyclizations that also includes the intramolecular copper(II) promoted alkene carboamination reaction.¹⁰

(1).

We report herein an expansion of the diamination substrate scope to include substrates with other linked (RNHX_nNHR) bis(amino) units. In addition, the diastereoselectivity in the oxidative cyclization of alkyl-substituted pent-4-enyl sulfamides and a deuterated alkene substrate was examined. The substrate expansion was enabled by the use of an organic soluble copper(II) salt, copper(II) neodecanoate [Cu(ND)₂] and the less polar solvent dichloroethane (DCE). The level and direction of observed diastereoselectivity, and comparison to the mechanistically similar copper(II) promoted carboamination reaction, provided insight into a plausible reaction mechanism (vide infra).

The effect of solvent, copper(II) ligand and heating method (oil bath vs microwave) was systematically examined as shown in Table 1. We quickly found that the solubility of the copper(II) carboxylate in the organic solvent was important to the efficiency of the reaction. In initial experiments we used Cu(OAc)₂ with polar solvents (DMF) and additive (4 equiv DMSO, 90 °C, 48 h) (entries 1 and 2, Table 1).^9b We subsequently found that the use of more organic soluble copper(II) carboxylates, e.g., copper(II) neodecanoate [Cu(ND)₂] allowed shorter reaction times (90 °C, 24 h) and the use of less polar solvents (dichloroethane, toluene). The reaction time could be further reduced by the use of microwave heating (120 °C for 20 min, entry 4, Table 1). All of these reactions are carried out in pressure tubes.

Table 1.

Temperature, Solvent and Ligand Effects^a

entry	CuXn	solvent	temp, time	yield (%)b
1	Cu(OAc)2	DMF	90 °C, 48 h	78
2	Cu(OAc)2	DMF/DMSO	90 °C, 48 h	92
3	Cu(ND)2	DMF	90 °C, 24 h	94
4	Cu(ND)2	DMF	120 °C (μW), 20 min	90
5	Cu(OAc)2	CH3CN	90 °C, 48 h	38
6	Cu(ND)2	CH3CN	90 °C, 24 h	94
7	Cu(ND)2	DCE	90 °C, 24 h	73
8	Cu(ND)2	toluene	90 °C, 24 h	40
9	Cu(OAc)2	tert-amylOH	90 °C, 48 h	38

^aAll reactions were run in sealed tubes at 0.1 M w/r to 1. Cu(ND)₂ = copper(II) neodecanoate.

^bYield refers to amount of product isolated after purification by flash chromatography on silica gel.

Under the new reaction conditions [Cu(ND)₂, (CH₂Cl)₂], the reactions of substrates 3, 5 and 7 were significantly improved (Table 2). At 120 °C or above, these less reactive, more entropically challenging substrates tended to undergo decomposition (removal of the sulfamide) using the Cu(OAc)₂, DMSO, DMF reaction conditions. It is possible DMF or its decomposition product (Me₂NH) could promote sulfamide decomposition. Under the new conditions, the isoindoline adduct 4 was obtained in 81% yield. The unsubstituted, aliphatic sulfamide 5 cyclized efficiently to provide pyrrolidine 6 in 86% yield. The N-2-γ-butenyl-3-methylphenyl-N′-benzylsulfamide (7) cyclized to form the tetrahydroquinoline adduct 8 in 81% yield.

Table 2.

Diamination of Challenging Substrates

entry	substrate	product	conditionsa	yield (%)b
1	3	4	A	56
2			B	81
3	5	6	A	43
4			B	86
5			C	55
6			D	48
7	7	8	A	decomp
8			B	81

^aReaction conditions A: 3 equiv Cu(OAc)₂, 2 equiv K₂CO₃, DMF (0.1 M), DMSO (10 equiv), 120 °C, 48 h, sealed tube; Conditions B: 3 equiv Cu(ND)₂, 2 equiv K₂CO₃, DCE (0.1 M), 120 °C, 48 h, sealed tube. Conditions C: Same as B except Cu(OAc)₂ was used instead of Cu(ND)₂. Conditions D: Same as B except DMF was used instead of DCE.

^bYield refers to amount of product isolated after purification by flash chromatography on silica gel.

The use of Cu(ND)₂ and dichloroethane as solvent was especially important in the case of monosubstituted N-pent-4-enyl-N′-benzyl sulfamides 9, 11 and 13 (Table 3). Good to excellent levels of diastereoselectivity were observed with these substrates (Table 3). Reactions of substrates 9 with substitution alpha to the sulfamide unit were highly diastereoselective, generating the cis-pyrrolidine core 10 with >20:1 selectivity (entries 1–3, Table 3). The high degree of cis-pyrrolidine selectivity is similar to that observed in the copper(II) carboxylate promoted carboamination reaction.^10b Pent-4-enyl sulfonamide 11 containing an allylic stereocenter also provided a diastereoselective reaction (dr = 3:1) favoring the trans diastereomer (entry 4, Table 3). The 2-substituted pent-4-enyl sulfamide 13 afforded pyrrolidines cis-14 and trans-14 in a 1 : 1 ratio of diastereomers (entry 5, Table 3). Discussion of the reaction diastereoselectivity is provided in eq 2 (vide infra) and in the Supporting Information. We have previously demonstrated that sulfur dioxide can be reductively removed to reveal the diamine if desired.^9b

Table 3.

Diastereoselectivity in Cyclizations of Pent-4-enyl Sulfamides^a

entry	substrate	Product	yield (%)b (selectivity)c
1	9a	10a	65 (dr >20 : 1)
2	9b	10b	60 (dr >20 : 1)
3	9c	10c	67 (dr > 20 : 1)
4	11	trans-12 cis-12	88 (trans : cis = 3 : 1)
5	13	cis/trans-14	83 (trans : cis = 1 : 1)

^aReaction conditions: 3 equiv Cu(ND)₂, 2 equiv K₂CO₃, DCE (0.1 M), 120 °C, 48 h, sealed tube.

^bYield refers to amount of product isolated upon purification by flash chromatography on silica gel.

^cSelectivity determined by analysis of the crude ¹H NMR spectrum.

The generality of the intramolecular diamination protocol was further examined as illustrated in Table 4. While the N-2-allylphenyl-N′-benzyl sulfamide 1 reacted at the lowest temperature of all the substrates examined (90 °C, Table 1), the reaction could be extended at higher temperature (120 °C) to analogous substrates with different diamine units such as ureas, bis(anilines) and α-amidopyrroles. Diamination with the urea substrates 15a–c produced bicyclic ureas 16a–c while cyclization of the α-amidopyrrole 17 and the bis(aniline) 19 produced 1,4-diazines 18 and 20. Cyclic ureas and 1,4-diazines are common components in biologically active compounds. The N-tosyl urea 15d and aliphatic γ-pentenylureas were unreactive under the reaction conditions.

Table 4.

Formation of Cyclic Ureas and 1,4-Diazines

entry	substrate	product	conditionsa	yield (%)b
1	15a, R = Bn	16a	A	69
2	15b, R = Ph	16b	B	68
3	15c, R = Pr	16c	A	59
4	15d, R = Ts	16d	A	no rxn
5	15d, R = Ts	16d	B	decomp
6	17	18	B	61
7	19	20	B	61c

^aReaction conditions A: 3 equiv Cu(ND)₂, 2 equiv K₂CO₃, DCE (0.1 M), 120 °C, 48 h, sealed tube. Conditions B: same as A except DMF was used as solvent instead of DCE.

^bYield refers to amount of product isolated after purification by flash chromatography on silica gel.

^c11% of the carboamination product was also formed (see Supporting Information).

Based upon the high diastereoselectivity of the diamination reactions with alpha substitutents, wherein the cis-pyrrolidines are highly favored, we propose that the first C-N bond is formed via a syn aminocupration (e.g., transition state 21), in analogy to the mechanistically similar copper(II) promoted intramolecular carboamination reaction (eqs. 2 and 3).^10b By comparing the direction and degree of diastereoselectivity in the carboamination reaction to the preferences found in analogous reactions, we argued that a syn aminocupration mechanism (analogous to transition state 21) best accounted for the observed diastereoselectivity.

(2).

(3).

To probe the mechanism of the second C-N bond-forming step, we submitted the trans-deuteroalkene 25 to the diamination conditions (eq. 4). Partial conversion to diamination adduct 26 allowed for the recovery and examination of the remaining starting material 25. We found that adduct 26 is formed in a 1:1 ratio of diastereomers. This is in contrast to the analogous study by Muniz and co-workers, who found this bond is formed stereospecifically in their palladium-catalyzed diamination reaction (eq 5).^9c

(4).

(5).

The proposed reaction mechanism for the copper(II) carboxylate promoted intramolecular alkene diamination is illustrated in Scheme 1. The stereorandom formation of deuterated diamination adducts 26 (eq. 4) indicates the presence of an intermediate with an sp² hybridized deuterium-substituted carbon, possibly a primary radical (e.g., 29, Scheme 1).¹¹ Thus, ligand exchange in the reaction of 9a with Cu(ND)₂ would provide for N-Cu bond formation (cf. 9a → 27, Scheme 1). Syn aminocupration would occur in stereoselective fashion via transition state 21, forming the cis-pyrrolidine. The unstable organocopper(II) intermediate 28 would undergo C-Cu bond homolysis, generating primary radical 29. Organocopper(II) species are known to be unstable due to the paramagnetic nature of copper(II).^12,13 The primary radical does not revert back to the starting material, as indicated by the fact that the recovered deuterated alkene 25 can be isolated without olefin isomerization (vide supra, eq 4). Because another electron must be lost from the substrate in this net two-electron oxidation process, it seems necessary that copper be involved in the second C-N bond forming process. One likely scenario would involve combination of the primary radical with Cu(ND)₂. The affinity of carbon radicals for Cu(II) has previously been studied.¹³ The resulting Cu(III) intermediate 30 could then undergo ligand exchange and reductive elimination or S_N2 to provide the observed product. Prior coordination of Cu(ND)₂ to the second sulfamide nitrogen and intramolecular delivery to the carbon radical may also be operative. Since copper carboxylate salts can easily disproportionate, an adequate amount of Cu(II) can be provided for the entire process.

Scheme 1.

Proposed Diamination Mechanism

An alternative mechanism would involve ligand exchange and reductive elimination or S_N2 of organocopper(II) intermediate 28, but the stereorandom formation of the second C-N bond would still have to be accounted for. Although a mechanism involving a primary carbocation intermediate could also account for the stereorandom second C-N bond formation, such a species seems unlikely as no rearrangement or elimination products are observed. Also, Kochi has previously observed that copper(II) salts do not promote carbocation formation unless a stable carbocation can be formed.^13b

In summary, we have identified milder reaction conditions that allow an expanded substrate scope in the copper(II) carboxylate promoted intramolecular diamination of terminal alkenes. Stereochemical probes have been used to identify a probable reaction mechanism. The copper(II) carboxylate-promoted protocol has demonstrated the highest levels of diastereoselectivity among intramolecular alkene diaminations to date.

Supplementary Material

si20070319_121. Supporting Information Available.

Proceedures and characterization data and NMR spectra for all new products. This material is available free of charge via the Internet at http://pubs.acs.org