HETEROCYCLES

Paper | Special issue | Vol. 79, No. 1, 2009, pp. 851-863
Received, 30th September, 2008, Accepted, 28th November, 2008, Published online, 3rd December, 2008.
DOI: 10.3987/COM-08-S(D)54

■ Development of Cyclic Hydrazine and Hydrazide Type Organocatalyst ⎯ Mechanistic Aspects of Cyclic Hydrazine/Hydrazide-Catalyzed Diels-Αlder Reactions

Ichiro Suzuki,* Ai Hirata, and Kei Takeda

Divison of Medicinal Chemisry, Graduate School of Biomedical Sciences, Hiroshima University, 1-2-3 Kasumi, Minami-ku, Hiroshima 734-8551, Japan

Abstract

Some hydrazines and hydrazides were prepared and screened for their catalytic efficiencies in Diels-Alder reactions. 1H-NMR studies and ab initio calculations revealed that catalytic efficiencies of these catalysts are greatly dependent on the release of the catalysts from the Diels-Alder adducts.

INTRODUCTION
In the early 1970s, two industrial research groups reported the first examples of (S)-proline-catalyzed enantioselective intramolecular aldol reactions. These reactions, so-called Hajos–Parrish–Eder–Sauer–Wiechert reactions, attracted little attention for about 30 years despite their potential values.1 In the early 2000s, List and coworkers reported (S)-proline-catalyzed intermolecular asymmetric aldol reactions.2 Thereafter, as well as proline, related five-membered cyclic secondary amine derivatives have also been applied to many reactions that proceed through enamine or iminium ion intermediates.3 It is well established that a nitrogen atom that is directly bonded to an atom with one or more lone pairs, such as H2N-NH2 and H2N-OH, tends to be a stronger nucleophile than would otherwise be expected. The nucleophic enhancement is called “α-heteroatom effect”,4 and such α-nucleophiles seem to be more advantageous as nitrogen-based heterocyclic organocatalysts than simple secondary amine catalysts, but there are not many reported examples. This type of organocatalyst was first reported to promote Diels-Alder reactions efficiently via iminium ions in 2003,5 and camphor-based chiral hydrazide catalysts were also reported.6 Recently, we have investigated Biginelli reaction catalyzed by cyclic hydrazine-type organocatalysts as shown in Figure 1, and we have reported that pyrazolidine dihydrochloride 1a could catalyze Biginelli reactions very efficiently under mild conditions.7 These results promoted us to screen several cyclic hydrazines and hydrazides for their catalytic activities in other reactions, including Diels-Alder reactions; however, during these studies, we became aware of ambiguities in the catalytic activities of the hydrazine-type organocatalysts.

For example, N-Cbz pyrazolidine hydrochloride 1c accelerated Diels-Alder reactions more efficiently than did catalyst 1a, which showed a significantly higher level of catalytic activity than did 1c in Biginelli reactions as shown in Scheme 1.

In order to develop chiral hydrazine-type organocatalysts superior to conventional pyrrolidine-based organocatalysts, it is important to elucidate these ambiguities, and we have therefore carried out the mechanistic studies on hydrazine/hydrazide-catalyzed Biginelli and Diels-Alder reactions. In this paper, we present the results of experiments and ab initio calculations on hydrazine- and hydrazide-catalyzed Diels-Alder reactions. These results showed that not the formation of reactive hydrazonium ions but the regeneration of hydrazine/hydrazide catalysts was more important in the catalyzed Diels-Alder reactions.

RESULTS AND DISCUSSION
All hydrazines 1a, 1b and hydrazides 1c – 1e were prepared by methods reported in the literature8-11 as depicted in Scheme 2. For catalysts 1a and 1b, we first tried to isolate them in acid-free forms, but all attempts failed because they are prone to immediately suffer from air oxidation under neutral or basic conditions to give only polymeric materials. In contrast to 1a and 1b, hydrazides 1c – 1e were relatively stable in acid-free forms when stocked in a refrigerator for a few weeks.

Diels-Alder reactions of cinnamaldehyde and cyclopentadiene were performed in the presence of 10 mol% of catalysts 1a – 1e at room temperature, and the results are summarized in Table 1. When the reaction was carried out in the presence of 1a, the reaction was very sluggish, affording an inseparable mixture of endo/exo isomers of dimethylacetals (endo/exo)-712 in 25% yield after 24 h (entry 1). The yield was slightly improved when the reaction time was extended to 48 h (entry 2). Catalyst 1b showed a level of catalytic activity similar to that of catalyst 1a (entries 3 and 4). On the other hand, N-Cbz pyrazolidine hydrochloride 1c showed a higher level of catalytic activity and the reaction was completed within 11 h, giving acetals (endo/exo)-7 in 70% yield with 6% aldehydes (endo/exo)-813 (entry 5). The reaction proceeded in an endo selective manner in the presence of catalyst 1c, though catalysts 1a and 1b accelerated the reactions exo-selectively. These reactions were constituted by three reversible processes: (1) hydrazonium ion formation, (2) Diels-Alder reaction, and (3) solvolysis of Diels-Alder adducts. This mechanistic complexity make it difficult to clearly explain the obtained endo/exo ratios by simple kinetic and/or thermodynamic reasons. Interestingly, N-acetyl pyrazolidine hydrochloride 1d is much less effective than 1c, and the reaction did not complete even after 46 h, with acetals 7 and aldehydes 8 being obtained in 63% and 6% yields, respectively accompanied by 29% recovery of cinnamaldehyde (entry 6). Hydrazides 1e could also catalyze the reaction, but the catalytic efficiency of 1e was unexpectedly low, giving Diels-Alder adducts in a total 32% yield even after 48 h (entries 7 and 8).

Since aqueous MeOH and water were employed for the reaction solvent in reported examples,5, 6 we also examined the reactions in these solvents. When the reactions were carried out in MeOH containing 10 v/v% of water, the reactions became inhomogeneous and very sluggish and acetals 7 were also obtained (entries 9 – 12). For instance, after 49 h, the starting aldehyde did not disappear even in the reaction using the most effective catalyst 1c (entry 11). Furthermore, when water was used as a reaction solvent, catalysts worked less efficiently as in MeOH, giving miserable results except for 1c (entries 13 – 15).
In addition, Diels-Alder reactions of cinnamaldehyde and cyclopentadiene in MeOH, MeOH-H2O (9:1) and water were carried out in the absence of catalyst. In these reactions, Diels-Alder adducts were obtained in only low yields (entries 16 – 18). From these results, it became clear that catalytic efficiency of 1a - 1e is in the following order in MeOH;

These catalysts would be in equilibria with acid-free forms in solutions and the catalysts should work as acid-free forms. In considering catalytic activities of these catalysts, it is reasonable to assume that pyrazolidine is more nucleophilic than N-acylpyrazolidines and pyrazolidinones and catalyst 1c therefore shows a higher level of catalytic activities than do other catalysts; however, the experimentally obtained order did not reflect this expected order.14 Then, we investigated the reasons why more nucleophilic pyrazolidine showed only limited catalytic activity in Diels-Alder reactions by using ab initio calculations and 1H-NMR techniques.
Diels-Alder reaction is thought to include three processes as shown in Figure 2: (1) hydrazonium ion formation from a catalyst hydrazine and an aldehyde, (2) cycloaddition of diene to the hydrazonium ion, and (3) hydrolysis of the resulting hydrazonium ion to release hydrazine.

In a cycloadditon step, the second step, Diels-Alder reaction is generally considered to be an orbital-controlled reaction, and therefore the facility of the reaction must be dependent on LUMO levels of the hydrazonium ions. We therefore calculated LUMO levels of cinnamaldehyde, iminium ion 9, and hydrazonium ions 10a – 10e.15 To avoid conformational ambiguity, we employed 10c and 10e as computational models. For hydrazonium ions 10a – 10e, both E and Z isomers were calculated. All geometries were optimized at the B3LYP/6-31+G(d) level of theory, followed by calculations of LUMO levels and energies at the B3LYP/6-311+G(d, p) level16, and the results are shown in Table 2.

For hydrazonium ions 10a, 10b and 10e, Z isomers were more stable than E isomers by 0.77 kcal/mol, 1.5 kcal/mol and 1.2 kcal/mol, respectively (entries 3 – 6, 11 and 12). On the other hand, for 10c and 10d, E isomers were more stable than Z isomers by 1.1 kcal/mol and 1.2 kcal/mol, respectively (entries 7 – 10). Since LUMO energy levels of E and Z isomers for 10a – 10e were essentially the same, 17 the preference for E or Z isomer should not affect the net reactivity of each type of hydrazonium ions. The LUMO level of cinnamaldehyde was -2.55 eV and the levels of iminium ion 9 and hydrazonium ions 10a – 10e were in the range of -6.81 to -6.39 eV. These results indicate that hydrazonium ions 10a – 10e are more active for Diels-Alder reactions than is the parent aldehyde, as well as iminium ion 9; however, we could not find out any remarkable differences in LUMO energy levels that are consistent with catalytic activities of 10a – 10e.
Since we could not rationalize the tendency of the reactivity of catalysts 1a – 1e only by LUMO levels of intermediate hydrazonium ions, we next investigated the formation of hydrazonium ions (the first step in Figure 2). Cinnamaldehyde was treated with 1 equivalent of catalyst 1a – 1e in CD3OD, and the reactions were monitored by 1H-NMR, and the results are summarized in Table 3. All reactions proceeded to give hydrazonium ions 10a – 10e and/or acetal 11 without any by-products.

When catalyst 1a was reacted with cinnamaldehyde, exclusive formation of hydrazonium ion 10a was observed within 10 min at 25 ˚C, giving a mixture of E- and Z-isomers (24:76). On the other hand, the reaction of catalyst 1b and 1c with cinnamaldehyde gave equilibrium mixtures containing hydrazonium ion 10b (73%), 10c (59%) and acetal 11 within 10 min. Additionally, in the reaction of catalyst 1d and the aldehyde, hydrazonium ions 10d and acetal 11 were formed similarly; however, the equilibrium between hydrazonium ions and the acetal was shifted to the acetal formation and the yield of hydrazonium ions 10d was reduced to 21% with increasing the formation of acetal 11 to 68% yield. While we could not observe noticeable differences in the rates of hydrazonium ion formations in our 1H-NMR experiments, it is noteworthy that the ratios of hydrazonium ions to the acetal 11 changed from 64:36 (for 10c) to 24:76 (for 10d). These results can rationalize the observed catalytic activities of catalysts 10c and 10d. In the catalyzed reactions, the formation of hydrazonium ions, which are reactive intermediates, is essential, and catalyst 1c yielded much more hydrazonium ion 10c than did 1d, being reflected in the higher level of catalytic activity of 1c than that of 1d. On the other hand, although the reaction of cinnamaldehyde and catalyst 1a afforded reactive intermediary hydrazonium ions 10a exclusively, 1a catalyzed the Diels-Alder reactions less efficiently than did catalyst 1c. This result implies that the formation of hydrazonium ions is not necessarily a rate-determining step. In this case, we must additionally consider release of a catalyst 1a from Diels-Alder adducts (Figure 2, the third step). In this step, hydrazine/hydrazide moieties work as leaving groups, and hence pKa values of their conjugate acids should be taken into account.14, 18 Since acyl hydrazines, such as catalyst 1c, are considered to be less basic than hydrazines 1a and 1b by 4~5 pka units, it is reasonable that catalysts 1c worked as a leaving group more efficiently than did 1a and 1b, leading to enhanced net catalytic efficiency. Although catalyst 1e gave hydrazonium ion 10e and acetal 11 in a ratio similar to that of 10c and their acidities are considered to be almost same, catalytic efficiency of 1e was considerably inferior to catalyst 1c. It is plausible that the steric repulsion between the alkene part and the acyl moiety in hydrazonium ion 10c might be more severe than the steric repulsion in hydrazonium ion 10e as shown in Figure 3. This severe steric repulsion would facilitate the hydrolytic release of 1c from hydrazonium ion 10c.

In conclusion, we investigated the mechanistic features of hydrazine/hydrazide-catalyzed Diels-Alder reactions. 1H-NMR studies and ab initio calculations revealed that catalytic efficiencies of these catalysts are greatly dependent on the release of the catalysts from the Diels-Alder adducts.

EXPERIMENTAL
General Remarks
1H-NMR spectra were measured in CDCl3, DMSO-d6 or CD3OD solutions and referenced to CHCl3 (7.26 ppm), DMSO-d6 (2.54 ppm) and CD3OD (3.30 ppm) using JEOL Delta-500 (500 MHz) spectrometer. 13C-NMR spectra were measured in CDCl3, DMSO-d6 or CD3OD solutions and referenced to CDCl3 (77.0 ppm), DMSO-d6 (39.7 ppm) and CD3OD (49.0 ppm) using JEOL Delta-500 (125 MHz) spectrometers. Column chromatography was performed on silicagel, KANTO KAGAKU N-60. Thin-layer chromatography was performed on precoated plates (0.25 mm, silicagel Merck Kieselgel 60 F254). All solvents were distilled prior to use. All reactions were performed in oven-dried glassware under positive pressure of nitrogen, unless otherwise noted.

Pyrazolidine dihydrochloride (1a).8
1H-NMR (500 MHz, DMSO-d6) δ 3.06 (4H, t, J = 7.4 Hz), 1.98 (2H, quint, J = 7.4 Hz); 13C-NMR (125 MHz, DMSO-d6) δ 46.5, 26.1.

1,2-Piperazine dihydrochloride (1b).9
1H-NMR (500 MHz, DMSO-d6) δ 2.99 (4H, m), 1.67 (4H, m); 13C-NMR (125 MHz, DMSO-d6) δ 45.2, 21.7.

Benzyl pyrazolidine-1-carboxylate hydrochloride (1c).10
1H-NMR (500 MHz, CD3OD) δ 7.50-7.30 (5H, m), 5.28 (2H, s), 3.76 (2H, t, J = 6.9 Hz), 3.59 (2H, t, J = 6.9 Hz), 2.36 (2H, quint, J = 6.9 Hz); 13C-NMR (125 MHz, CD3OD) δ 153.6, 135.6 and 135.2, 128.5, 128.4, 128.3, 128.2, 68.9 and 68.0, 46.4 and 46.2, 24.3.

N-Acetyl Pyrazolidine hydrochloride (1d).10
1H-NMR (500 MHz, CD3OD) δ 3.90 (2H, t, J = 6.5 Hz), 3.57 (2H, t, J = 6.5 Hz), 2.50-2.40 (2H, m); 2.19 (3H, s); 13C-NMR (125 MHz, CD3OD) δ 168.6, 46.6 and 46.3, 45.4, 25.6 and 25.0, 19.9.

5-Methylpyrazolidin-3-one (1e).11
1H-NMR (500 MHz, DMSO-d6) δ 4.11 (1H, ddd, J = 6.9, 8.0 and 8.7 Hz), 3.59 (2H, brs), 2.70 (1H, dd, J = 8.0 and 16.7 Hz), 2.34 (1H, dd, J = 8.7 and 16.7 Hz), 1.38 (3H, d, J = 6.9 Hz); 13C-NMR (125 MHz, CD3OD) δ 173.8, 55.3, 35.3, 15.6.
General procedure for catalyzed Diels-Alder reaction
To a solution of (E)-cinnamaldehyde (126 μL, 1.0 mmol) and cyclopentadiene (247 μL, 3.0 mmol), which was distilled by cracking dicyclopentadiene at 180 ˚C prior to use, in distilled MeOH (333 μL) was added N-Cbz pyrazolidine hydrochloride 1c (24 mg, 0.1 mmol), and the resulting mixture was stirred at rt until the reaction was judged to be completed by TLC analysis. The reaction was quenched with 1N HCl and the mixture was extracted twice with Et2O. The combined organic layers were washed successively with saturated aqueous solution of NaHCO3 and brine, and dried over MgSO4. After evaporation, the residue was purified with silica gel chromatography (5% AcOEt in hexane was used as an eluent) to give the desired adducts as a mixture of acetals 7 (colorless oil, 171 mg, 70%, endo/exo = 33:67) and aldehydes 8 (colorless oil, 12 mg, 6%).

5-(Dimethoxymethyl)-6-phenylbicyclo[2.2.1]hept-2-ene (endo-7).12
1H-NMR (500 MHz, CDCl3) δ 7.37-7.12 (5H, m), 6.34 (1H, m), 6.15 ( 1H, dd, J = 2.8 and 5.7 Hz), 3.93 (1H, d, J = 9.2 Hz), 3.37 (3H, s), 3.13 (3H, s), 2.96 (1H, brs), 2.87 (1H, brs), 2.54 (1H, m), 2.40 (1H, dd, J = 1.5 and 4.8 Hz), 1.77 (1H, d, J = 8.7 Hz), 1.55 (1H, m); 13C-NMR (125 MHZ, CDCl3) δ 145.1, 138.5, 135.1, 128.3, 127.9, 125.7, 108.3, 52.7, 52.4, 49.4, 46.7, 47.3, 44.5.

5-(Dimethoxymethyl)-6-phenylbicyclo[2.2.1]hept-2-ene (exo-7).12
1H-NMR (500 MHz, CDCl3) δ 7.37-7.12 (5H, m), 6.35 (1H, m), 5.94 (1H, dd, J = 2.8 and 5.7 Hz), 4.36 (1H, d, J = 8.3 Hz), 3.38 (3H, s), 3.12 (1H, dd, J = 3.4 and 5.1 Hz), 3.07 (3H, s), 3.00 (1H, brs), 2.90 (1H, dd, J = 1.4 and 4.9 Hz), 2.03 (1H, ddd, J = 1.6, 5.1 and 8.2 Hz), 1.68 (1H, d, J = 8.6 Hz), 1.49 (1H, m); 13C-NMR (125 MHZ, CDCl3) δ 144.3, 137.4, 135.3, 128.1, 127.9, 125.9, 108.0, 53.5, 52.4, 49.4, 49.1, 47.6, 45.3.

3-Phenylbicyclo[2.2.1]hept-5-ene-2-carboxaldehyde (endo-8).13
1H-NMR (500 MHz, CDCl3) δ 9.60 (1H, d, J = 2.3 Hz), 7.13–7.34 (5H, m), 6.35 (1H, dd, J = 3.2 and 5.5 Hz), 6.10 (1H, dd, J = 3.2 and 5.8 Hz), 3.26 (1H, brs), 3.06 (1H, brs), 3.02 (1H, dd, J = 1.6 and 4.9 Hz), 2.91 (1H, ddd, J = 2.3, 3.5 and 4.9 Hz), 1.74 (1H, m), 1.62 (1H, m); 13C-NMR (125 MHZ, CDCl3) δ 203.6, 143.7, 139.3, 133.9, 128.7, 127.5, 126.3, 60.9, 48.5, 47.2, 45.8, 45.3.

3-Phenylbicyclo[2.2.1]hept-5-ene-2-carboxaldehyde (exo-8).13
1H-NMR (500 MHz, CDCl3) δ 9.85 (1H, d, J = 2.1 Hz), 7.13–7.34 (5H, m), 6.27 (1H, dd, J = 3.5 and 5.8 Hz), 6.00 (1H, dd, J = 3.5 and 5.8 Hz), 3.65 (1H, dd, J = 3.5 and 4.9 Hz), 3.15 (2H, m), 2.52 (1H, ddd, J = 2.3, 3.5 and 4.9 Hz), 1.53–1.59 (2H, m); 13C-NMR (125 MHZ, CDCl3) δ 202.9, 143.7, 136.6, 136.4, 128.2, 128.0, 126.4, 59.5, 48.5, 47.7, 45.59, 45.56.
General procedure for 1H-NMR experiments
A solution of cinnamaldehyde (15 μL, 0.12 mmol) in CD3OD (0.4 mL) was mixed with pyrazolidine dihydrochloride 1a (17 mg, 0.12 mmol) in an NMR tube at rt. The reaction was monitor by 1H-NMR .

Hydrazonium ion 10a.
Major isomer: 1H-NMR (500 MHz, CD3OD) δ 8.22 (1H, d, J = 10.3 Hz), 7.80-7.70 (2H, m), 7.65 (1H, d, J = 15.6 Hz), 7.50-7.40 (3H, m), 7.37 (1H, dd, J = 10.3 and 15.6 Hz), 4.38 (2H, m), 3.60 (2H, m), 2.50-2.30 (2H, m); minor isomer: 1H-NMR (500 MHz, CD3OD) δ 8.36 (1H, d, J = 10.6 Hz), 7.80-7.70 (2H, m), 7.60 (1H, d, J = 15.4 Hz), 7.50-7.40 (3H, m), 7.24 (1H, dd, J = 10.6 and 15.4 Hz), 4.38 (2H, m), 3.51 (2H, m), 2.50-2.30 (2H, m).

Hydrazonium ion 10b.
Major isomer: 1H-NMR (500 MHz, CD3OD) δ 8.47 (1H, d, J = 10.1 Hz), 8.00-7.20 (7H, m), 4.12 (2H, m), 3.29 (2H, m), 2.05 (2H, m), 1.91 (2H, m); minor isomer: 1H-NMR (500 MHz, CD3OD) δ 8.50 (1H, d, J = 10.8 Hz), 8.00-7.20 (7H, m), 4.26 (2H, m), 3.15 (2H, m), 1.80 (4H, m).

Hydrazonium ion 10c.
Major isomer: 1H-NMR (500 MHz, CD3OD) δ 9.13 (1H, d, J = 10.3 Hz), 8.00-7.20 (6H, m), 7.24 (1H, dd, J = 10.3 and 17.9 Hz), 5.24 (2H, s), 4.53 (2H, m), 4.07 (2H, m), 2.70-2.50 (2H, m); minor isomer: 1H-NMR (500 MHz, CD3OD) δ 8.97 (1H, d, J = 10.8 Hz), 8.40-7.20 (7H, m), 7.08 (1H, dd, J = 10.8 Hz, and 17.9 Hz), 5.30 (2H, s), 4.39 (2H, m), 4.07 (2H, m), 2.70-2.50 (2H, m).

Hydrazonium ion 10d.
Major isomer: 1H-NMR (500 MHz, CD3OD) δ 9.18 (1H, d, J = 10.8 Hz), 8.40-7.20 (7H, m), 4.51 (2H, m), 4.18 (2H, m), 2.60-2.30 (2H, m), 2.34 (3H, s); minor isomer: 1H-NMR (500 MHz, CD3OD) δ 8.90 (1H, d, J = 10.3 Hz), 8.40-7.20 (7H, m), 4.37 (2H, m), 4.33 (2H, m), 2.60-2.30 (2H, m), 2.37 (3H, s).

Hydrazonium ion 10e.
Major isomer: 1H-NMR (500 MHz, CD3OD) δ 9.44 (1H, d, J = 10.1 Hz), 7.90-7.20 (7H, m), 5.00 (1H, ddd, J = 5.0, 6.9 and 9.0 Hz), 3.43 (1H, dd, J = 9.0 and 17.5 Hz), 2.89 (1H, dd, J = 5.0 and 17.5 Hz), 1.69 (3H, d, J = 6.9 Hz); minor isomer: 1H-NMR (500 MHz, CD3OD) δ 8.58 (1H, d, J = 11.0 Hz), 7.90-7.20 (7H, m), 5.38 (1H, ddd, J = 2.1, 6.7 and 9.0 Hz), 3.53 (1H, dd, J = 9.0 and 17.2 Hz), 2.84 (1H, dd, J = 2.1 and 17.2 Hz), 1.61 (3H, d, J = 6.7 Hz)
Cinnamaldehyde dimethylacetal-d6 (11).
1H-NMR (500 MHz, CD3OD) δ 7.60-7.20 (5H, m), 6.71 (1H, d, J = 16.1 Hz), 6.14 (1H, dd, J = 5.1 and 16.1 Hz), 4.92 (1H, d, J = 5.1 Hz).

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