Bis(2-ethylhexyl)amine as an Effective Organocatalyst for the Racemic Reactions of α,β-Unsaturated Aldehydes Involving an Iminium Ion

This letter is dedicated to Prof. Hisashi Yamamoto on the occasion of his 80th birthday.

Abstract

Bis(2-ethylhexyl)amine is shown to be a suitable organocatalyst to prepare racemic compounds in the reactions of α,β-unsaturated aldehydes involving an iminium ion, whereas diphenylprolinol silyl ether is a well-known chiral organocatalyst for the asymmetric versions of the same reactions. The generality and limitations of bis(2-ethylhexyl)amine are disclosed.

The organocatalyst field has developed rapidly, and many effective organocatalysts have been reported.[1] One of the privileged organocatalysts is diphenylprolinol silyl ether,[2] which was developed independently by our group[3] and Jørgensen’s group[4] in 2005. This catalyst promotes many types of reactions involving enamines[5] and iminium ions[6] as reactive intermediates.

The racemic synthesis of organic molecules is generally considered to be facile compared with the synthesis of the corresponding chiral molecules with excellent enantioselectivity. Once an asymmetric catalytic reaction has been established, the preparation of racemates from it is generally trivial. However, the situation is different in the case of diphenylprolinol silyl ethers II, III, and IV (Figure [1]). To make a racemic molecule for determining enantioselectivity, our group usually prepares the racemate by using a racemic catalyst by mixing chiral (R) and (S) enantiomers. However, it would be easier if inexpensive amines such as pyrrolidine, which is a key skeleton of chiral catalyst II, promoted the reaction, but they usually do not afford the desired products in reasonable yields. The poor yields are due to the diphenylprolinol silyl ether possessing a bulky and electron-withdrawing diphenylsiloxymethyl moiety on the pyrrolidine scaffold, and the reactivity and basicity of the diphenylprolinol silyl ether and pyrrolidine are rather different.

Table 1 The Effect of Amines on the Reaction of Cinnamaldehyde and Nitromethane^a

Entry	Amine	Time (h)	NMR yield (%)
			1	2	3
1	I	24	80b	<5	<5
2	diisobutylamine	20	72b	<5	<5
3	benzylamine	168	<5	33b	<5
4	allylamine	168	<5	45	<5
5	DBU	120	<5	<5	34
6	triethylamine	120	<5	<5	<5
7	pyrrolidine	24	20	30	<5
8	piperidine	24	31b	30b	<5
9	azepane	24	12	<5	<5
10	dihexylamine	24	34	<5	<5
11	diallylamine	15	45	<5	<5
12	dibenzylamine	28	45	<5	<5

^a Unless otherwise noted, reactions were performed by employing cinnamaldehyde (0.50 mmol), nitromethane (1.50 mmol), amine (0.050 mmol), and benzoic acid (0.10 mmol) in MeOH (1.0 mL) at room temperature for the indicated time.

^b Isolated yield.

In our study, to develop a formal [3+3] cycloaddition reaction, we examined the reaction of isopropylidenemalononitrile and 3-phenyl-5-trimethylsilyl-pent-2-en-4-ynal in the presence of diphenylprolinol silyl ether II. We found that a domino reaction afforded substituted quinolines (Scheme [1], eq. 1).[7] As the product does not possess a stereogenic center, it was not necessary to use a chiral catalyst. Thus, we examined an inexpensive amine catalyst and found that bis(2-ethylhexyl)amine (I), which is commercially available and fairly inexpensive,[8] was efficient (Scheme [1], eq. 2). Herein, we describe the generality of bis(2-ethylhexyl)amine (I) for the synthesis of racemic molecules, in which the corresponding chiral molecules are synthesized with excellent enantioselectivities by utilizing diphenylprolinol silyl ether.

First, we chose the Michael reaction of nitromethane and cinnamaldehyde. This reaction was catalyzed by diphenylprolinol silyl ether II in MeOH at room temperature with excellent enantioselectivity (Scheme [2], eq. 3).[9] We subsequently investigated several amines as catalysts under the same reaction conditions (MeOH, room temperature) (Table [1]). According to the amine catalyst, several products were generated. Aldehyde 1 is the Michael product, diene 2 would be generated by iminium ion formation and an aza-Henry reaction, followed by elimination of the amine catalyst, or via a Henry reaction, followed by elimination of water, and alcohol 3 is the Henry reaction product. Primary amines such as benzylamine and allylamine afforded 2, whereas DBU gave 3 in 34% yield (entries 3–5). No reaction was observed when utilizing triethylamine (entry 6). Cyclic secondary amines such as pyrrolidine, piperidine and azepane gave both 1 and 2 in low yields (entries 7–9). Other linear secondary amines, such as dihexylamine, diallylamine, and dibenzylamine, afforded product 1 in low yields (entries 10–12). However, bis(2-ethylhexyl)amine (I) and diisobutylamine afforded only the Michael product 1 in good yield (entries 1 and 2). Linear secondary amines with sterically bulky substituents, such as bis(2-ethylhexyl)amine (I), were found to be effective amine catalysts in this reaction. Steric bulkiness prevents the 1,2-addition reaction (aza-Henry reaction).

Amines are known to not only react with α,β-unsaturated aldehydes via 1,2-addition reactions to form iminium ions, but also in 1,4-addition reactions.[10] Landge, Young and co-workers reported that the 1,4-addition of ⁱ PrNH₂ to cinnamaldehyde in D₂O/AcOD resulted in the deuteration of the α-position of the formyl group.[11] In order to investigate the possibility of the 1,4-addition reaction of the amine I toward cinnamaldehyde, a mixture of cinnamaldehyde, the amine I and benzoic acid in CD₃OD was treated with NaBH₄. Deuterated amine 4-D and deuterated cinnamyl alcohol 5-D were obtained in 60% and 23% yields, respectively, indicating that the deuteration occurred in good yield (Scheme [3], eq. 5). On the other hand, when a mixture of cinnamaldehyde and amine I in MeOH was treated with NaBH₄, not only amine 4 (43%) and alcohol 5 (50%) but also a small amount of the MeOH Michael addition product 6 (7%) were obtained (eq. 6).[12] These results indicate that there is a process of Michael and retro Michael reactions of MeOH, and α-deuteration of the aldehyde would occur by this process, although we cannot eliminate the possibility of the Michael reaction of the amine I for the α-deuteration of the aldehyde in the present reaction.

As bis(2-ethylhexyl)amine (I) was found to be an effective amine catalyst in the Michael reaction of nitromethane, we investigated the efficiency of catalyst I in several other reactions catalyzed by diphenylprolinol silyl ether.

The formal carbo [3+3] cycloaddition reaction of cinnamaldehyde and dimethyl 3-oxopentanedioate was catalyzed by diphenylprolinol silyl ether II in 75% yield with 95% ee,[13] and this reaction was also catalyzed by bis(2-ethylhexyl)amine (I) to afford the desired product in 58% yield (Scheme [4], eq. 7).

The reaction of cinnamaldehyde and isopropylidenemalononitrile is known to be catalyzed by diphenylprolinol silyl ether III, in which acetone was used as a solvent.[14] Formal [3+3] cycloaddition, followed by dehydration, proceeded in a single-pot operation to afford the cyclohexene derivative in good yield (Scheme [5], eq. 8). When we used bis(2-ethylhexyl)amine (I) as the catalyst, toluene gave a better result than acetone in the first step, and a higher yield was obtained when we removed catalyst I prior to the dehydration step. Although this is a two-pot process, a good yield was obtained (71%, eq. 9).

The Michael reaction of cinnamaldehyde and cyclopentadiene was reported to be catalyzed by diphenylprolinol silyl ether catalyst IV in good yield after 20 hours (Scheme [6], eq. 10).[15] The same reaction was catalyzed by bis(2-ethylhexyl)amine (I) within a much shorter time (5 h) to afford the Michael product in 78% yield.

The Michael reaction of crotonaldehyde and 4-methyl-2-oxazoline-5-one was catalyzed by diphenylprolinol silyl ether IV to afford the Michael product in good yield and with excellent diastereo- and enantioselectivity (Scheme [7], eq. 11).[16] Bis(2-ethylhexyl)amine (I) also catalyzed the same reaction to afford the product in 65% yield.

Córdova and co-workers reported that the epoxidation reaction of cinnamaldehyde and hydrogen peroxide catalyzed by diphenylprolinol silyl ether II afforded the corresponding epoxide with excellent diastereo- and enantioselectivities using CHCl₃ as the solvent (Scheme [8], eq. 12).[17] Jørgensen and co-workers reported an asymmetric epoxidation using the diarylprolinol silyl ether V possessing a trifluoromethyl substituent in CH₂Cl₂, and good yields and excellent diastereo- and enantioselectivities were obtained.[18] When bis(2-ethylhexyl)amine (I) was employed as the catalyst, ^t BuOH was a suitable solvent, and the epoxide was obtained in 66% yield with decreased diastereoselectivity.

Although we have several successful results using bis(2-ethylhexyl)amine (I) as a catalyst, there are reactions that are not well catalyzed by I catalyst. For instance, cyclohexanone is a suitable Michael donor in the reaction of cinnamaldehyde catalyzed by diphenylprolinol silyl ether III with a combination of 4-hydroxyproline.[19] However, this Michael reaction did not proceed in the presence of bis(2-ethylhexyl)amine (I) and 4-hydroxyproline. While the domino Michael/Michael reaction proceeded between cinnamaldehyde and 3-hexene-2,5-dione catalyzed by diphenylprolinol silyl ether II,[20] this reaction was not promoted by bis(2-ethylhexyl)amine (I). We employed a domino Michael/Michael reaction of an α,β-unsaturated aldehyde and ethyl 4-oxo-2-pentenoate as a key step for the time-economical synthesis of a Corey lactone (Scheme [9], eq. 14).[21] However, this reaction did not proceed well in the presence of bis(2-ethylhexyl)amine (I), in which aldol condensation was the main reaction (eq. 13). The aldol condensation reaction was found to proceed when a mixture of cinnamaldehyde and an alkynyl methyl ketone was treated with bis(2-ethylhexyl)amine (I) (eq. 15), whereas the asymmetric Michael reaction proceeded with excellent enantioselectivity in the presence of diphenylprolinol silyl ether II (eq. 16).[22]

Finally, we investigated the efficiency of catalyst I in the Michael reaction of propanal and trans-nitrostyrene,[3] [23] in which catalyst I reacts with an aldehyde to generate an enamine intermediate (Scheme [10], eq. 17). Not only catalyst I but also pyrrolidine catalyzed this reaction, which indicates that there might be no specificity for using catalyst I in reactions involving an enamine as an intermediate.

In summary, bis(2-ethylhexyl)amine (I) is an effective organocatalyst for reactions involving an iminium ion generated from an α,β-unsaturated aldehyde, which showed a very different reactivity compared with the other amine catalysts. Diphenylprolinol silyl ether is a well-known organocatalyst for the enantioselective versions of the same reactions, which proceed with excellent enantioselectivity. When we prepare racemic compounds in these reactions, bis(2-ethylhexyl)amine (I) is a suitable catalyst, although there are some limitations, which have been disclosed in this article.

Conflict of Interest

The authors declare no conflict of interest.

• References and Notes


Selected reviews on organocatalysis:
• 1a Asymmetric Organocatalysis 1 . List B. Thieme; Stuttgart: 2012
  Search in Google Scholar
• 1b Comprehensive Enantioselective Organocatalysis: Catalysts, Reactions, and Applications. Dalko PI. Wiley-VCH; Weinheim: 2013
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• 2d Jensen KL, Dickmeiss G, Jiang H, Albrecht L, Jørgensen KA. Acc. Chem. Res. 2012; 45: 248
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• 2f Donslund BS, Johansen TK, Poulsen PH, Halskov KS, Jørgensen KA. Angew. Chem. Int. Ed. 2015; 54: 13860
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• 9a Gotoh H, Ishikawa H, Hayashi Y. Org. Lett. 2007; 9: 5307
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• 9b Zu L.-S, Xie H.-X, Li H, Wang J, Wang W. Adv. Synth. Catal. 2007; 349: 2660
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The 1,4-addition reaction of an amine to α,β-unsaturated carbonyl compounds is a key step of the Baylis–Hillman reaction:
• 10a Baylis AB, Hillman ME. D. German Patent 2155113, 1972 , Chem. Abstr. 1972, 77, 34174q.
  PubMed

For selected reviews, see:
• 10b Basavaiah D, Rao A, Satyanarayana T. Chem. Rev. 2003; 103: 811
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• 10c Masson G, Housseman C, Zhu J. Angew. Chem. Int. Ed. 2007; 46: 4614
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• 12 In eq. 5, a small amount of the Michael addition product of CD₃OD might be generated, but we were unable to isolate it after reduction.
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  CrossrefPubMed
• 15a Gotoh H, Masui R, Ogino H, Shoji S, Hayashi Y. Angew. Chem. Int. Ed. 2006; 45: 6853
  CrossrefPubMedSearch in Google Scholar
• 15b Gotoh H, Ogino H, Ishikawa H, Hayashi Y. Tetrahedron 2010; 66: 4894
  CrossrefSearch in Google Scholar
• 16 Hayashi Y, Obi K, Ohta Y, Okamura D, Ishikawa H. Chem. Asian J. 2009; 4: 246
  CrossrefPubMedSearch in Google Scholar
• 17 Sundén H, Ibrahem I, Córdova A. Tetrahedron Lett. 2006; 47: 99
  CrossrefSearch in Google Scholar
• 18 Marigo M, Franzén J, Poulsen TB, Zhuang W, Jørgensen KA. J. Am. Chem. Soc. 2005; 127: 6964
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• 19 Hayashi Y, Umekubo N. Angew. Chem. Int. Ed. 2018; 57: 1958
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• 20 Umekubo N, Iwata R, Hayashi Y. Chem. Lett. 2020; 49: 867
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• 21a Umekubo N, Suga Y, Hayashi Y. Chem. Sci. 2020; 11: 1205
  CrossrefPubMedSearch in Google Scholar
• 21b Hayashi Y. J. Org. Chem. 2021; 86: 1
  CrossrefPubMedSearch in Google Scholar
• 22 Umekubo N, Hayashi Y. ACS Org. Inorg. Au 2022; 2: 245
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• 23a Patora-Komisarska K, Benohoud M, Ishikawa H, Seebach D, Hayashi Y. Helv. Chim. Acta 2011; 94: 719
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• 23b Seebach D, Sun X.-Y, Ebert M.-O, Schweizer WB, Purkayastha N, Beck AK, Duschmalé J, Wennemers H, Mukaiyama T, Benohoud M, Hayashi Y, Reiher M. Helv. Chim. Acta 2013; 96: 799
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Corresponding Author

Publication History

Received: 23 June 2023

Accepted after revision: 21 September 2023

Accepted Manuscript online:
21 September 2023

Article published online:
31 October 2023

© 2023. Thieme. All rights reserved

Georg Thieme Verlag KG
Rüdigerstraße 14, 70469 Stuttgart, Germany

• References and Notes


Selected reviews on organocatalysis:
• 1a Asymmetric Organocatalysis 1 . List B. Thieme; Stuttgart: 2012
  Search in Google Scholar
• 1b Comprehensive Enantioselective Organocatalysis: Catalysts, Reactions, and Applications. Dalko PI. Wiley-VCH; Weinheim: 2013
  Search in Google Scholar
• 2a Palomo C, Mielgo A. Angew. Chem. Int. Ed. 2006; 45: 7876
  CrossrefPubMedSearch in Google Scholar
• 2b Mielgo A, Palomo C. Chem. Asian J. 2008; 3: 922
  CrossrefPubMedSearch in Google Scholar
• 2c Xu L.-W, Li L, Shi Z.-H. Adv. Synth. Catal. 2010; 352: 243
  CrossrefSearch in Google Scholar
• 2d Jensen KL, Dickmeiss G, Jiang H, Albrecht L, Jørgensen KA. Acc. Chem. Res. 2012; 45: 248
  CrossrefPubMedSearch in Google Scholar
• 2e Gotoh H, Hayashi Y. Diarylprolinol Silyl Ethers: Development and Application as Organocatalysts. In Sustainable Catalysis: Challenges and Practices for the Pharmaceutical and Fine Chemical Industries. Dunn J, Hii KK, Krische MJ, Williams MT. John Wiley & Sons; Hoboken: 2013: 287
  CrossrefSearch in Google Scholar
• 2f Donslund BS, Johansen TK, Poulsen PH, Halskov KS, Jørgensen KA. Angew. Chem. Int. Ed. 2015; 54: 13860
  CrossrefPubMedSearch in Google Scholar
• 2g Reyes-Rodriguez GJ, Rezayee NM, Vidal-Albalat A, Jørgensen KA. Chem. Rev. 2019; 119: 4221
  CrossrefPubMedSearch in Google Scholar
• 3 Hayashi Y, Gotoh H, Hayashi T, Shoji M. Angew. Chem. Int. Ed. 2005; 44: 4212
  CrossrefPubMedSearch in Google Scholar
• 4 Marigo M, Wabnitz TC, Fielenbach D, Jørgensen KA. Angew. Chem. Int. Ed. 2005; 44: 794
  CrossrefPubMedSearch in Google Scholar
• 5 Mukherjee S, Yang J.-W, Hoffmann S, List B. Chem. Rev. 2007; 107: 5471
  CrossrefPubMedSearch in Google Scholar
• 6 Erkkilä A, Majander I, Pihko PM. Chem. Rev. 2007; 107: 5416
  CrossrefPubMedSearch in Google Scholar
• 7 Hayashi Y, Han X, Mori N. Chem. Eur. J. 2023; 29: e202301093
  CrossrefPubMedSearch in Google Scholar
• 8 Amine I is a mixture of meso- and dl-isomers, but their ratio could not be determined by NMR analysis or by HPLC analysis after Cbz protection.
• 9a Gotoh H, Ishikawa H, Hayashi Y. Org. Lett. 2007; 9: 5307
  CrossrefPubMedSearch in Google Scholar
• 9b Zu L.-S, Xie H.-X, Li H, Wang J, Wang W. Adv. Synth. Catal. 2007; 349: 2660
  CrossrefSearch in Google Scholar
• 9c Wang Y.-C, Li P.-L, Liang X.-M, Zhang TY, Ye J.-X. Chem. Commun. 2008; 1232
  CrossrefPubMed

The 1,4-addition reaction of an amine to α,β-unsaturated carbonyl compounds is a key step of the Baylis–Hillman reaction:
• 10a Baylis AB, Hillman ME. D. German Patent 2155113, 1972 , Chem. Abstr. 1972, 77, 34174q.
  PubMed

For selected reviews, see:
• 10b Basavaiah D, Rao A, Satyanarayana T. Chem. Rev. 2003; 103: 811
  CrossrefPubMedSearch in Google Scholar
• 10c Masson G, Housseman C, Zhu J. Angew. Chem. Int. Ed. 2007; 46: 4614
  CrossrefPubMedSearch in Google Scholar
• 10d Declerck V, Martinez J, Lamaty F. Chem. Rev. 2009; 109: 1
  CrossrefPubMedSearch in Google Scholar
• 11 Landge VG, Shrestha KK, Grant AJ, Young MC. Org. Lett. 2020; 22: 9745 ; as for the mechanism, see the Supporting Information
  CrossrefPubMedSearch in Google Scholar
• 12 In eq. 5, a small amount of the Michael addition product of CD₃OD might be generated, but we were unable to isolate it after reduction.
• 13 Hayashi Y, Toyoshima M, Gotoh H, Ishikawa H. Org. Lett. 2009; 11: 45
  CrossrefPubMedSearch in Google Scholar
• 14 Umekubo N, Han X, Mori N, Hayashi Y. Eur. J. Org. Chem. 2022; e202200603
  CrossrefPubMed
• 15a Gotoh H, Masui R, Ogino H, Shoji S, Hayashi Y. Angew. Chem. Int. Ed. 2006; 45: 6853
  CrossrefPubMedSearch in Google Scholar
• 15b Gotoh H, Ogino H, Ishikawa H, Hayashi Y. Tetrahedron 2010; 66: 4894
  CrossrefSearch in Google Scholar
• 16 Hayashi Y, Obi K, Ohta Y, Okamura D, Ishikawa H. Chem. Asian J. 2009; 4: 246
  CrossrefPubMedSearch in Google Scholar
• 17 Sundén H, Ibrahem I, Córdova A. Tetrahedron Lett. 2006; 47: 99
  CrossrefSearch in Google Scholar
• 18 Marigo M, Franzén J, Poulsen TB, Zhuang W, Jørgensen KA. J. Am. Chem. Soc. 2005; 127: 6964
  CrossrefPubMedSearch in Google Scholar
• 19 Hayashi Y, Umekubo N. Angew. Chem. Int. Ed. 2018; 57: 1958
  CrossrefPubMedSearch in Google Scholar
• 20 Umekubo N, Iwata R, Hayashi Y. Chem. Lett. 2020; 49: 867
  CrossrefPubMedSearch in Google Scholar
• 21a Umekubo N, Suga Y, Hayashi Y. Chem. Sci. 2020; 11: 1205
  CrossrefPubMedSearch in Google Scholar
• 21b Hayashi Y. J. Org. Chem. 2021; 86: 1
  CrossrefPubMedSearch in Google Scholar
• 22 Umekubo N, Hayashi Y. ACS Org. Inorg. Au 2022; 2: 245
  CrossrefPubMedSearch in Google Scholar
• 23a Patora-Komisarska K, Benohoud M, Ishikawa H, Seebach D, Hayashi Y. Helv. Chim. Acta 2011; 94: 719
  CrossrefSearch in Google Scholar
• 23b Seebach D, Sun X.-Y, Ebert M.-O, Schweizer WB, Purkayastha N, Beck AK, Duschmalé J, Wennemers H, Mukaiyama T, Benohoud M, Hayashi Y, Reiher M. Helv. Chim. Acta 2013; 96: 799
  CrossrefSearch in Google Scholar

• Supplementary Material