New insights into the asymmetric Diels–Alder reaction: the endo- and S-selective retro-Diels–Alder reaction

Abstract

The endo- and S-selective retro-Diels–Alder reactions in an imidazolethione-catalyzed asymmetric Diels–Alder reaction were verified and investigated, and account for the low ee values in a CH₃CN–H₂O catalytic system. This reverse process could be controlled by forming the dimethyl acetal of the aldehyde product with methanol. Both the exo- and endo-isomers were obtained in the CH₃OH–H₂O system in high yields with good to excellent enantioselectivities.

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Asymmetric Diels–Alder reactions have received substantial attention over the past few decades due to their capacity to form an enantiomerically enriched cyclohexene moiety, and their wide applicability in the synthesis of natural products.¹ In 2000, MacMillan first reported the imidazolidinone-catalyzed enantioselective Diels–Alder reaction.² Since then, several organocatalysts³ such as chiral diamines, hydrazides, sulfonyl hydrazides, binaphthyl-based chiral amine, and diaryprolinol silyl ethers have been reported to mediate asymmetric Diels–Alder reactions. The retro-Diels–Alder reaction (rDA),⁴ as a well-known reverse process of Diels–Alder cycloaddition (DA), has also been widely applied to synthetic chemistry⁵ and materials science.⁶ In addition to practical applications, the theoretical studies of rDA reactions have been carried out by many groups,⁷ such as the mechanisms of the rDA process, and the influence of some reaction parameters or carbon–carbon bond distance on the rate of rDA reaction, etc.

Despite a wide range of studies on asymmetric Diels–Alder reactions, the rDA reaction in organocatalytic asymmetric Diels–Alder reactions has rarely been reported. In 2006, Ogilvie and co-workers reported hydrazide-catalyzed enantioselective Diels–Alder reactions and verified the presence of an rDA reaction by deracemization experiments and NMR analysis.^7f However, to the best of our knowledge, the different cycloreversion activity of four Diels–Alder adducts (2S-endo-, 2R-endo-, 2S-exo- and 2R-exo-isomers) and the influence of the rDA reaction on the relative ratio of endo/exo diastereomers or R/S enantiomers in the asymmetric Diels–Alder reaction have not been explored until now. As part of our continuous research on imidazolethione⁸-catalyzed asymmetric reactions, we herein apply imidazolethione catalysts to the asymmetric Diels–Alder reaction between α,β-unsaturated aldehydes and cyclopentadiene. Investigation into the stereoselectivity of the rDA reaction during the catalytic cycle and its impact on dr or ee values is expected to help further improve the enantioselectivity in Diels–Alder reactions by controlling the reaction conditions.

Several imidazolethione catalysts with different steric constraints, 1a–1d, were synthesized and initially examined in the asymmetric Diels–Alder reaction between (E)-cinnamaldehyde and cyclopentadiene (Table 1). After comprehensively comparing the yield and ee values of cycloadducts, imidazolethione 1a was selected as the catalyst for further investigation (Table S1 ESI†). Interestingly, a significant difference in enantioselectivity was observed between CH₃CN–H₂O and CH₃OH–H₂O catalytic systems when we optimized the reaction conditions. As shown in Table 1, a reaction with 10 mol% catalyst 1a and CF₃COOH as the co-catalyst in CH₃CN–H₂O provided adducts in high yield but only with moderate ee values (Table 1, entry 1). However, good ee values in both exo- and endo-isomers were detected when the solvent system was replaced with CH₃OH–H₂O (Table 1, entry 2). Other acid additives like TfOH, HBF_4, p-TSA and HCl were subsequently examined in these two solvent systems. An improvement in enantioselectivity was observed with other acids in CH₃CN–H₂O, but the ee values were still lower when compared to the corresponding CH₃OH–H₂O catalytic system (Table 1, entries 2–10). Notably, the ee values of endo-isomers were generally lower than exo-isomers in the CH₃CN–H₂O solvent system. In contrast, the reactions proceeded efficiently and showed the highest ee values when using HCl as the co-catalyst in the CH₃OH–H₂O system (Table 1, entry 10). With regard to the above experiment phenomenon, we speculated that it was the rDA reaction during the catalytic cycle which resulted in the low ee values in the CH₃CN–H₂O solvent system.

Table 1 Optimization of reaction conditions in the CH₃CN–H₂O and CH₃OH–H₂O system^a

Entry	Solvent	Acid	Yield^b (%)	Exo/endo^c	ee^d (%)
					Exo	Endo
a Reaction condition: trans-cinnamaldehyde (1.0 mmol), cyclopentadiene (5.0 mmol), organic solvent (1.9 mL), H2O (0.1 mL), catalyst 1a (10 mol%), acid (10 mol%), r.t., 12 h.b Isolated yield.c Exo/endo selectivity was determined by ^1H NMR analysis.d Enantiomeric excess was determined by HPLC analysis.
1	CH3CN/H2O	TFA	92	1.3 : 1	59	56
2	CH3OH/H2O	TFA	95	1.2 : 1	88	87
3	CH3CN/H2O	TfOH	90	1.2 : 1	75	73
4	CH3OH/H2O	TfOH	93	1.1 : 1	87	86
5	CH3CN/H2O	HBF4	87	1.2 : 1	75	71
6	CH3OH/H2O	HBF4	90	1.2 : 1	84	83
7	CH3CN/H2O	p-TSA	85	1.1 : 1	82	78
8	CH3OH/H2O	p-TSA	89	1.1 : 1	83	83
9	CH3CN/H2O	HCl	92	1.2 : 1	80	79
10	CH3OH/H2O	HCl	96	1.2 : 1	95	94

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To verify the presence of an rDA reaction during the catalytic cycle, deracemization experiments were carried out (Table 2). Racemic aldehyde adducts were subjected to standard reaction conditions in the CH₃CN–H₂O and CH₃OH–H₂O catalytic system. As anticipated, in the newly generated cinnamaldehyde slight changes in dr and ee values were observed confirming that the rDA reaction had in fact taken place in the CH₃CN–H₂O catalytic system (Table 2, entries 1 and 2). The extent of the rDA reaction increased to different degrees when a little more catalyst and acid were added, or the reaction temperature was raised to 50 °C (Table 2, entries 2 to 7). An increased ratio of exo-isomers, a slight excess of (2R)-adducts, and a larger excessive of (2R)-adducts in endo-isomers than in exo-isomers were detected after this deracemization process (Table 2, entries 2 to 6), demonstrating that the rDA reaction might occur in the endo- and S-adducts selectively. The endo- and S-selective rDA reaction caused the isomerization of endo-adducts to exo-adducts and the conversion of (2S)-adducts to (2R)-adducts, which resulted in the low ee values in the CH₃CN–H₂O catalytic system, especially in endo-adducts.

Table 2 Deracemization of cycloadducts^a

Entry	Cat. 1a (equiv.)	TFA (equiv.)	Exo/endo^b	2R : 2S (%)
				Exo (ee^c)	Endo (ee^c)
a Reaction condition: racemic aldehyde adducts (1.0 mmol), 1.9 mL CH3CN, 0.1 mL H2O, r.t., 48 h.b Exo/endo selectivity was determined by ^1H NMR analysis of a crude reaction mixture.c ee values of (2R)-adducts determined by HPLC analysis.d 1.9 mL CH3OH, 0.1 mL H2O.e Reaction performed at 50 °C.f Reaction performed at −10 °C.
1^d	0.2	0.2	1.2 : 1	50 : 50 (0)	50 : 50 (0)
2	0.2	0.2	1.4 : 1	51 : 49 (2)	53 : 47 (6)
3	—	0.2	1.2 : 1	50 : 50 (0)	50 : 50 (0)
4	0.2	1.0	1.7 : 1	54 : 46 (8)	55 : 45 (10)
5	1.0	1.0	1.8 : 1	54 : 46 (8)	56 : 44 (12)
6^e	0.2	1.0	1.8 : 1	65 : 35 (30)	70 : 30 (40)
7^f	0.2	1.0	1.2 : 1	50 : 50 (0)	50 : 50 (0)

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The different reversion activities were evaluated between (2S)-adducts and (2R)-adducts to prove the S-selectivity in the rDA reaction. The isolated (2S)-adducts and (2R)-adducts were performed under standard reaction conditions in the CH₃CN–H₂O system (Fig. 1). As predicted, the ee values of the (2S)-adducts decreased obviously within 48 h (95% to 73%, Fig. 1, ■), but the ee values of the (2R)-adducts were maintained above 90% (Fig. 1, ▲). These results indicate that the (2S)-adducts were more prone to an rDA reaction occuring, and the newly generated cinnamaldehyde and cyclopentadiene yielded corresponding (2S)- and (2R)-adducts again. After numerous reversible processes, (2S)-adducts converted to (2R)-adducts and the (2R)-adducts accumulated gradually, which could account for the slight excess of (2R)-adducts after the deracemization process and the low ee values in the Diels–Alder reaction, when using CH₃CN–H₂O as the reaction medium. The conversion from (2S)-adducts to (2R)-adducts was also verified by following the changes in ee values of both adducts in the direct D–A reaction mixture. The ee value decreased from 70% to about 40% within 36 h (Fig. S1 ESI†) in CH₃CN–H₂O, which further proves the above conclusions.

Fig. 1 Different reversion activity between the (2S)- and (2R)-adducts in the CH₃CN–H₂O system: (2S)-adducts (95% ee in endo-isomers) and (2R)-adducts (95% ee in endo-isomers), 20 mol% 1a, 50 mol% TFA, CH₃CN (1.9 mL), H₂O (0.1 mL), 40 °C, 48 h.

The endo-selectivity in the rDA reaction was verified by examining the stability of two diastereomers in the CH₃CN–H₂O and CH₃OH–H₂O catalytic system. Aldehyde adducts were subjected to standard reaction conditions in a different solvent system for 72 h (Fig. 2). In the CH₃CN–H₂O catalytic system, the ee values of the endo-isomers decreased obviously with increasing time (95% to 75% ee Fig. 2, ●), however, the ee values of the exo-isomers were maintained at good levels (92% to 94% ee, Fig. 2, ◆). In the CH₃OH–H₂O system, both endo- and exo-isomers could maintain ee values between 91% and 95%, even in 72 hours (Fig. S2 ESI†). These results imply that the rDA reaction preferentially occurred in the endo-isomers, and the partial conversion of (2S)-endo-adducts to (2R)-endo-adducts caused a gradual decrease in ee values. Furthermore, both the endo- and exo-isomers were stable and could maintain good ee values in the CH₃OH–H₂O system, which might be due to the formed dimethyl acetal of aldehyde adducts with methanol. It should be mentioned that the endo-selectivity of the rDA reaction was consistent with the following – a larger excess of (2R)-adducts in endo-isomers than in exo-isomers after the deracemization process (Table 2), and the generally lower ee values in endo-isomers than exo-isomers in asymmetric Diels–Alder reactions with CH₃CN–H₂O as the solvent (Table 1).

Fig. 2 The stability of isolated aldehyde adducts in the CH₃CN–H₂O system: aldehyde products (1 mmol, 94% ee in 2S-endo-isomers, 95% ee in 2S-exo-isomers), 20 mol% 1a, 100 mol% TFA, CH₃CN (1.9 mL), H₂O (0.1 mL), r.t., 72 h.

The impact on the low ee values from the rDA reaction was further examined with other co-catalyst acids in the CH₃CN–H₂O system. As shown in Table 3, with the extent of rDA reaction increasing, the ee values in the asymmetric Diels–Alder reaction decreased accordingly. These results further proved the correlations between the rDA reaction during the catalytic cycle and the low ee values in the asymmetric Diels–Alder reaction in the CH₃CN–H₂O system.

Table 3 Retro-Diels–Alder reactions mediated by different acids^a

Entry	Acids	ee^b (%)		ee^c (%)
		(2S)-exo	(2S)-endo	(2R)-exo	(2R)-endo
a Reaction condition: trans-cinnamaldehyde (1.0 mmol), cyclopentadiene (3.0 mmol), CH3CN (1.9 mL), H2O (0.1 mL), catalyst 1a (5 mol%), acid (5 mol%), r.t., 12 h.b Enantiomeric excess was determined using HPLC analysis.c Racemic cinnamaldehyde adducts (1.0 mmol), CH3CN (1.9 mL), H2O (0.1 mL), catalyst 1a (20 mol%), acid (100 mol%), r.t., 48 h, ee values of (2R)-adducts were determined using HPLC analysis.
1	HCl	80	79	1	1
2	p-TSA	82	79	1	3
3	HBF4	75	72	1	3
4	TfOH	75	73	1	4
5	TFA	59	56	8	10

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The endo-selectivity was consistent with the stability of the cycloadducts. The endo-isomer is the kinetic product and the exo-isomer is the thermodynamic product. During the reverse reaction, the endo compound is the first to be unblocked, and the isomerization from the endo to the exo diastereomer is preceded by a retro-Diels–Alder reaction.⁷ⁱ The DFT calculations further confirm the S-selectivity in this rDA process. The length of the two bonds, which break in the cycloreversion reaction, were compared in these four adducts. The carbon–carbon bond length in the S-isomers is longer than in R-isomers, which is easier for retro-Diels–Alder reactions^7j (the molecular model and calculation results are in the ESI†). These results clearly demonstrate the endo- and S-selective rDA process in this imidazolethione-catalyzed asymmetric Diels–Alder reaction when using CH₃CN–H₂O as the reaction medium.

Continuous efforts were paid to optimize the conditions of the asymmetric Diels–Alder reaction in the CH₃OH–H₂O solvent system (Table S3 ESI†) and evaluate the catalytic activity of imidazolethione 1a with various α,β-unsaturated aldehyde and cyclopentadienes. As revealed in Table 4, the reactions proceeded efficiently and provided the desired products in a high yield with good to excellent enantioselectivities in most cases (Table 4, entries 1–7). Cinnamaldehydes with electron-donating and electron-withdrawing groups were both confirmed to be competent substrates for the present asymmetric organic transformations (Table 4, entries 2–7). However, the reaction with furanyl substituted α,β-unsaturated aldehydes proceeded relatively slowly and a good yield with high ee values was achieved in 24 h (Table 4, entry 8). (2E)-Hexenal reacted smoothly, affording the desired products in less than 4 h, but good ee values could be obtained by lowering the reaction temperature to 0 °C (Table 4, entry 9). Overall, in contrast to the use of an imidazolidinone catalyst which was previously reported by MacMillan,² similar yield and enantioselectivity in both exo- and endo-isomers were achieved by utilizing the more rigid and ‘stiffer’ imidazolethione 1a in the CH₃OH–H₂O system.

Table 4 Application of the optimum conditions to various substrates^a

Entry	R	Yield^b (%)	Exo/endo^c	ee^d (%)
				Exo	Endo
a Reaction condition: α,β-unsaturated aldehyde (1.0 mmol), cyclopentadiene (3.0 mmol), CH3OH (1.9 mL), H2O (0.1 mL), catalyst 1a (5 mol%), HCl (5 mol%).b Isolated yield.c Exo/endo selectivity was determined by ^1H NMR analysis.d Enantiomeric excess was determined by HPLC analysis.e 10 mol% catalyst, 24 h.f Reaction performed at 0 °C, 4 h.
1	Ph	95	1.2 : 1	95	94
2	m-MeC6H4	95	1.1 : 1	93	93
3	o-OMeC6H4	96	1.2 : 1	96	94
4	p-OMeC6H4	95	1.1 : 1	95	94
5	p-FC6H4	93	1.1 : 1	93	93
6	p-ClC6H4	92	1.1 : 1	93	92
7	m-ClC6H4	93	1 : 1	92	90
8^e	Furyl	84	1 : 1	93	90
9^f	n-Pr	92	1.2 : 1	93	92

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Conclusions

In summary, we confirmed the presence of an rDA reaction in an imidazolethione-catalyzed asymmetric Diels–Alder reaction, and proposed the endo- and S-selectivity in the rDA reaction for the first time. This accounted for the low ee values in the CH₃CN–H₂O system. High enantioselectivity in both exo- and endo-isomers were obtained in the CH₃OH–H₂O system through controlling the rDA reaction by the formation of dimethyl acetal. These findings provide a new idea about racemization in asymmetric reactions, and stereoselective cycloreversion of the Diels–Alder cycloadducts may become a potential and promising methodology in chiral organic synthesis and polymer chemistry. Further studies and applications of stereoselective rDA reactions are underway in our laboratories.

Acknowledgements

We acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 21206148 and No. 21406201).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details and characterization data. See DOI: 10.1039/c5ra17788j

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