Abstract
Use of a base-free Pd(DMSO)2(TFA)2 catalyst system enables the synthesis of six-membered nitrogen heterocycles via a Wacker-type aerobic oxidative cyclization of alkenes bearing tethered sulfonamides. Various heterocycles, including morpholines, piperidines, piperazines, and piperazinones are accessible by this method.
Nitrogen heterocycles, such as morpholines, piperazines, and piperidines, are ubiquitous in natural products and pharmaceuticals, and they have been the focus of extensive synthetic interest.1 PdII-catalyzed oxidative cyclization of alkenes represents an efficient route to heterocycles,2 and many “Wacker-type” reactions of this type are compatible with the use of O2 as the stoichiometric oxidant.3 Nevertheless, while many of these reactions are effective for the formation of five-membered rings,2 few general methods have been reported for the synthesis of six- and larger-membered nitrogen heterocycles.4,5,6 Here, we describe the application of a recently developed Pd(DMSO)2(TFA)2 catalyst system (DMSO = dimethylsulfoxide, TFA = trifluoroacetate)7,8 for the synthesis of a variety of six-membered nitrogen heterocycles under aerobic conditions.
The challenge of preparing six-membered rings via Wacker-type oxidative cyclization is illustrated with substrate 1a, a precursor to 3-vinylmorpholine derivative 2a. Cyclization of this substrate was attempted with two of the most versatile catalyst systems that have been reported previously: Pd(OAc)2 in DMSO as the solvent, introduced by Larock and Hiemstra,9 and Pd(OAc)2/pyridine in toluene, which we reported in 2002.10,11,12 These catalysts have been used to prepare diverse pyrrolidine derivatives, but they have exhibited limited success in the preparation of six-membered rings. Reactions of 1a with these catalysts afforded only trace yields of product 2a (Scheme 1).
Scheme 1.
Illustration of the Difficulty of Wacker-Type Cyclization to Afford a Six-Membered Heterocycle.
We recently identified Pd(DMSO)2(TFA)2 as an effective catalyst for the oxidative cyclization of allylic sulfamides in THF.7b Use of DMSO as a stoichiometric ligand for the Pd catalyst in 2:1 stoichiometry, rather than the solvent, resulted in significantly higher product yield, and optimal results were obtained with sodium benzoate (NaOBz) as a Brønsted base. We tested similar catalytic conditions for the cyclization of 1a. A 15% yield of 2a was obtained when 10 mol % Pd(DMSO)2(TFA)2 was used as the catalyst in THF with 1 equiv of NaOBz (Table 1, entry 1), an outcome slightly better than that observed with DMSO as the solvent (entry 2). Despite the modest yields from these reactions, the improvements relative the results in Scheme 1 prompted us to explore additional conditions. Numerous other solvents were tested (acetonitrile, dichloroethane, dimethylacetamide, dimeth-oxyethane, dimethylformamide, dioxane, nitromethane, and toluene), and ~40% yields of 2a were obtained in acetonitrile and toluene (entries 3 and 4). Replacement of NaOBz with other bases (e.g., NaOAc, NaHCO3, and Na2CO3) did not have a significant impact on the outcome of the reaction, but the yield improved to ~60% in the absence of added base (entries 5 and 6). Replacing Pd(TFA)2 with Pd(OAc)2 as a catalyst precursor led to a lower yield of 2a (32%, entry 7).
Table 1.
Evaluation of Catalytic Conditions for Intermolecular Oxidative Amination of Alkenesa
 | ||||
|---|---|---|---|---|
| entry | mol % PdII | solvent | comment | (conv) yieldb |
| 1 | 10 | THF | 1 equiv NaOBz | (43) 15 |
| 2 | 10 | DMSO | 1 equiv NaOBz | (35) 10 |
| 3 | 10 | MeCN | 1 equiv NaOBz | (70) 41 |
| 4 | 10 | toluene | 1 equiv NaOBz | (68) 38 |
| 5 | 10 | MeCN | no base | (80) 57 |
| 6 | 10 | toluene | no base | (100) 62 |
| 7 | 10 | toluene | Pd(OAc)2c | (50) 32 |
| 8 | 5 | toluene | no base | (51) 31 |
| 9 | 5 | toluene | no base, 60 psi O2d | (100) 71 |
Conditions: 1a (0.075 mmol), Pd(TFA)2 (5 or 10 mol %), DMSO (10 or 20 mol %), base, solvent (0.1 M), 3A MS (20 mg), O2 (1 atm), 60 °C, 13 h.
Conversion of 1a in parentheses; yield determined by 1H NMR spectroscopy with 1,3,5-trimethylbenzene as the int. std.
Pd(OAc)2 used instead of Pd(TFA)2
Substrate concentration of 0.032 M.
The yield of 2a decreased significantly upon reducing the catalyst loading from 10 to 5 mol % (entry 8). The presence of Pd black at the end of the reaction suggested that the low conversion under these conditions could be explained, at least in part, by decomposition of the catalyst. The yield of 2a increasing to 71% when the reaction was performed under 60 psi of O2. We have previously noted in intermolecular oxidative oxidative amination reactions that catalyst stability can be increased under elevated O2 pressures.13 The beneficial effect of the higher O2 pressure is rationalized by the mechanism in Scheme 3, whereby productive oxidation of the catalyst (kox step) can be enhanced relative to catalyst decomposition (kdec step).
Scheme 3.
Possible Mechanisms for the Formation of Allylic Sulfonamide Products
The optimized oxidative amination conditions were examined with a number of different alkene substrates bearing tethered sulfonamides (Table 2). The reactions were performed at 0.3 mmol scale under 60 psi O2 in a Parr pressure vessel. A higher yield of 2a was obtained (76% isolated yield) relative to the smaller-scale screening reaction, and the o-nitrobenzenesulfonamide (Ns) derivative 1b and the cis-alkene substrate cis-1a also reacted in good yields (86% in both cases). The trisubstituted alkene derivative 1c reacted to afford a product with a quaternary carbon center adjacent to nitrogen (entry 4, 73% yield), and the reaction of the substrates with substituents on either side of the oxygen atom, 1d–1g, afforded the desired products with good diastereoselectivities (entries 5–8). N-tosyl benzomorpholine derivatives 2h and 2i formed in near-quantitative yield from 1h and 1i, respectively (entries 9 and 10).
Table 2.
Pd-Catalyzed Intramolecular Aerobic Oxidative Amination reactiona
 | ||||
|---|---|---|---|---|
| entry | substrate | product | yieldb (%) | |
| 1 |
|
Z = Ts (1a) |
|
2a: 76% |
| 2 | Z = 2-Ns (1b) | 2b: 86% | ||
| 3 | cis-1a | 2a: 86% | ||
| 4 |
|
1c |
|
2c: 73% |
| 5 |
|
R = Me (1d) |
|
2d: 72% |
| 6 | R = iPr (1d) | 2e: 59% (dr = >10:1) | ||
| 7 |
|
1f |
|
2f: 66% |
| 8 |
|
1g |
|
2g: 50% (dr = 7:1) |
| 9 |
|
1hd: R = H |
|
2h: 96% |
| 10 | 1id: R = OMe | 2i: 96% | ||
| 11 |
|
1j |
|
2j: 60% 3j: 9% |
| 12 |
|
1kc |
|
2k: 92% |
| 13 |
|
1ld |
|
2l: 71% |
| 14 |
|
1mc |
|
2m: 92% |
| 15 |
|
1ne |
|
2n: 85%f |
Conditions: 1 (0.3 mmol), Pd(TFA)2 (5 mol %), DMSO (10 mol %), toluene (0.032 M), 3A MS (80 mg), O2 (60 psi), 60 °C, 24 h.
Isolated yield.
E/Z = 2/1.
E/Z = 6/1.
E/Z = 4/1.
Reaction conditions: Pd(DMSO)(TFA)2 (15 mol%), [1n] = 0.01 M
The reaction of 1j with a terminal alkene led to product 2j, which probably forms via isomerization of the double bond of an initial product 2j′ containing an exo-methylene substituent. Alkene isomerization could be mediated by a PdII–H intermediate (cf. Scheme 2).14 A five-membered cyclic aminal 3j, isolated from this reaction in low yield, formally reflects an allylic amination product. However, in light of the alkene isomerization evident in the formation of 2j, this product probably arises from PdII–H mediated migration of the double bond of 1j to afford a vinyl ether derivative, followed by Wacker-type oxidative cyclization of the isomeric substrate (see below for further mechanistic analysis).
Scheme 2.
Proposed Mechanism of Palladium-Catalyzed Intramolecular Aerobic Oxidative Amination of Alkenes.
The same catalyst and reaction conditions were applied to the preparation of other heterocyclic rings, including piperidine, piperazine, piperazinone and seven-membered heterocycles 2k – 2n (entries 12–15). These products were formed in good yield, although formation of the seven-membered heterocycle 2n in good yield required the use of elevated catalyst loading (15 mol %).
The predominant formation of allylic amide products, including the formation of 3j from 1j, raises the possibility that these reactions could proceed via an allylic C–H activation pathway, rather than via aminopalladation of the alkene (Scheme 3). The allylic amination reactions reported by White et al., which employ PdII catalysts in DMSO, are of particular relevance in this context.15 To distinguish between the two mechanisms, the homoallyl ether substrate 1o was subjected to the oxidative cyclization conditions (eq 1), and the reaction mixture was analyzed by 1H NMR spectroscopy. The seven-membered ring 2o, expected from an aminopalladation pathway, was the major product of the reaction. Only a small amount of the allylic C–H activation product 2a was detected; however, the formation of this product probably arises from aminopalladation of 1a, which forms in situ via isomerization of 1o. Collectively, these observations suggest that product formation arises predominantly from the aminopalladation pathway.
 |
(1) |
The use of high pressures of O2 with an organic solvent is potential safety hazard. This issue can be addressed by diluting the O2 with an inert gas, such as N2, to ensure the mixture remains outside of flammability limits for the solvent.16 Such conditions were implemented in a one-gram scale oxidative cyclization of 1i, which was featured in the five-step synthesis of the unprotected benzomorpholine product from commercially available starting materials (Scheme 4). The aerobic oxidative cyclization step proceeds in quantitative yield under these modified conditions.
Scheme 4.
Gram-Scale Oxidative Cyclization Reaction Using Diluted O2
Recent studies have shown that Pd(DMSO)2(TFA)2 is an important variant of previously known PdII catalyst systems for aerobic oxidation reactions.7 Its utility has been demonstrated for diverse methods, including directed oxidative arylation of C–H bonds,7a oxidative heterocyclization reactions of alkenes,7b and α,β-dehydrogenation of cyclic ketones.7c The results presented here show that Wacker-type aerobic oxidative cyclization reactions can provide efficient access to six-membered nitrogen heterocycles, and the Pd(DMSO)2(TFA)2 catalyst system is critical to the success of these reactions.
Supplementary Material
Acknowledgments
We are grateful to Dr. Charlie Fry (UW-Madison) for assistance in NMR characterization of heterocyclic products. We thank the NIH (R01 GM67173) for financial support of this work and Eli Lilly and Johnson-Matthey for generous donations of Pd salts.
Footnotes
Supporting Information Available. Experimental procedures and characterization data for new compounds. This material is available free of charge via the Internet at http://pubs.acs.org.
References
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