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This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
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Cite this: DOI: 10.1039/c0xx00000x
ARTICLE TYPE
Chemoenzymatic one-pot synthesis in aqueous medium: Combination of metal-catalysed allylic alcohol isomerisation–asymmetric bioamination† Nicolás Ríos-Lombardía,a Cristian Vidal,b María Cocina,a Francisco Morís,a Joaquín García-Álvarezb,* and Javier González-Sabína,* 5
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Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x The ruthenium-catalysed isomerisation of allylic alcohols was coupled, for the first time, with an asymmetric bioamination in a one-pot process in aqueous medium. In those cases involving proquiral ketones, the -TA exhibited excellent enantioselectivity, identical to that observed in the single step. As a result, amines were obtained from allylic alcohols with high overall yields and excellent enantiomeric excesses.
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Multi-step one-pot processes are an exciting and emerging opportunity in sustainable chemistry, fitting some of the crucial points to consider when a catalytic green chemical process is designed.1,2 In this sense, performing multi-step syntheses in a concurrent fashion enables the bypass of costly and tedious intermediate downstream and purification steps, thus improving both the ecological footprint and the economic efficiency of a process.2 Moreover, the choice of a safe, non-toxic, biorenewable and cheap solvent, is one of the major goals in organic synthesis (from a green chemistry point of view).3 In this sense, water can be considered as an attractive solvent to deliver truly greener processes.4 Historically, chemists have combined reactions belonging to a particular technology, that is, chemocatalysis (mainly in organic solvent) or biocatalysis (mainly in water).5 However examples of metal catalysts (or organocatalysts) working hand in hand with enzymes are still very scarce, especially in aqueous media.6 Actually, the assembly of these two worlds imposes several challenges, namely: i) catalyst compatibility and stability; ii) cross-reactivity; iii) shifting the reaction equilibrium to the product side, and iv) scalability. In spite of these facts, the pool of metallic catalysts able to operate in water has expanded enormously in the last years,7 and provides novel opportunities to combine with enzymes which have water as their natural environment. Some successful examples include the assembly of a Wacker oxidation8 or Suzuki and Heck crosscouplings with bioreductions,9 and olefin metathesis followed by enzymatic hydrolysis or hydroxylation.10 Likewise, the use of artificial metalloenzymes and supramolecular host-guest complexes have enabled the use of sensitive metal complexes in water also in a tandem fashion with biocatalysts.11 Within the past few years, enzymes such as -transaminases (-TAs), which catalyse the transfer of an amino group from an amino donor onto a carbonyl moiety using pyridoxal-5’phosphate (PLP) as co-factor, have emerged as a powerful This journal is © The Royal Society of Chemistry [year]
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alternative to hydrolases and proteases for chiral amine preparation.12 Thus, the increased availability of -TAs and the advancement of enzyme engineering make the enzymatic bioamination a feasible and attractive manufacturing option for chiral amine synthesis in pharmaceuticals.13 Notwithstanding, and as far as we are concerned, there are no reports dealing about the combination of -TAs and metal-catalysed reactions in a concurrent fashion. Since the natural substrates of these enzymes are both aldehydes and ketones, we envisaged that a process such the metal-catalysed isomerisation of allylic alcohols, which delivers carbonyl groups, could be implemented into a cascade fashion (Scheme 1). Actually, this widely-studied catalytic isomerisation has recently been performed in a variety of nonconventional solvents [like water, ionic liquids and Deep Eutectic Solvents (DESs)],14,15 which open the way to a combined process in aqueous media.
Scheme 1 Synthesis of chiral amines combining metal-catalysed allylic alcohol isomerisation and enzymatic amination in a one-pot process.
Despite being two water-compatible reactions, a major concern to settle a feasible coupled process is the compatibility of the different reaction conditions of both transformations. Thus, while enzyme inhibition by heavy metals is a known phenomenon; the required buffer solutions to accomplish the biotransformation could also deactivate the metal catalyst.6 Furthermore, parameters such as pH, temperature or concentration should be also optimised to address both reactions efficiently. Accordingly, we designed some preliminary experiments aimed at verify the compatibility of the two reactions. Initially, and as the metalcatalysed transformation proceeds first, we evaluated three highly efficient ruthenium(IV) catalysts (1-3) in the allylic isomerisation of -vinylbenzyl alcohol (4a, Table 1),14 but changing water by the reaction medium used in enzymatic transaminations; that is a phosphate buffer (100 mM, pH 8.5) containing iPrNH2 (1M).16 Similarly, the temperature was lowered to 50 ºC, meeting one of the key points to avoid the erosion of the -TAs. Pleasantly, the three complexes were active and selective catalysts (in the absence of any co-catalyst), yielding the desired ketone (4b) [journal], [year], [vol], 00–00 | 1
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Table 2. Isomerisation of the allylic alcohols 4a-10a catalysed by the Ru(IV) complex 3 in a phosphate buffer 100 mM pH 8.5 (1M iPrNH2).a
under the aforementioned conditions and with a catalyst loading of 1 mol% (entries 1-3), being complex 3 by far the most efficient catalyst. At this point, it is important to note that the vast majority of Ru-catalysts previously reported in the literature are not able to promote the isomerisation reaction below 80-100 ºC in pure water as solvent.15 Notably, 4a could be isomerised even on smoother conditions such as 35 ºC, which is a more desirable temperature for biotransformations, although longer reactions are required to achieve quantitative conversions (entry 4). Finally, the use of catalytic amounts of complex 3 is mandatory, as in its absence, no reaction was observed (entry 5).
Entry
Substrate
Temp.
mol% [Ru]
Time
Conversionc
1
4a
50 ºC
1
3h
>99
2
5a
50 ºC
1
16 h
>99
3
6a
50 ºC
1
12 h
>99
4
7a
50 ºC
1
17 h
>99
5
8a
50 ºC
1
3h
>99
6
d
9a
50 ºC
1
4h
>99
7
10a
75 ºC
10
15 h
>99
Table 1. Isomerisation of -vinylbenzyl alcohol catalysed by the Ru(IV)
a
complexes 1-3 in a typical aqueous medium of enzymatic aminations.a Entry
Complex
mol% [Ru]
Time
Temp.
Conversionc
1
1
1
5h
50 ºC
>99
2
2
1
24 h
50 ºC
90
3
3
1
3h
50 ºC
>99
4
3
1
8.5 h
35 ºC
>99
5
-
-
24 h
50 ºC
1
45
50
Reaction conditions: -Vinylbenzyl alcohol (1 mmol) under Ar atmosphere at 50 ºC, 1 mol% Ru(IV) catalyst, in 5 mL of KH2PO4 buffer 100 mM (1M iPrNH2) pH 8.5. Previously, buffer solution was degassed with Ar. c Calculated by GC. a
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Then, once identified the prerequisites for the first catalytic step, we extended the study to some allylic alcohols which could fulfill the structural features to undergo a metal-catalysed isomerisation and subsequent bioamination (4a-10a; Table 2). Under identical conditions to those described in Table 1, complex 3 is also very efficient in the isomerisation of a variety of allylic alcohols. Thus, both arylic (4a-7a; entries 1-4) and aliphatic (8a, entry 5) monosubstituted allylic alcohols could be smoothly converted into their corresponding carbonyl compounds. Regarding 1,2-disubstituted allylic alcohols (9a-10a; entries 6-7), these substrates had previously proven to be challenging, and it was necessary to increase temperature or catalyst loading to reach complete conversion.14 Next, we turned our attention to the second step of the overall process, namely the bioamination of the carbonyl compounds. Thus, the previously obtained substrates 4b-10b were screened with a series of 28 commercially available -TAs, the Codex® Transaminase Screening Kit, which are able to work using isopropylamine in molar excess as amino donor (Table 3). After this enzymatic screening (see Tables S1-S7 in the ESI for further details), in the case of the prochiral ketones 4b-8b we identified highly efficient -TAs which gave access, after 24 h, to both enantiomers of each amine with high conversion and excellent enantiomeric excess. (entries 1-20, Table 3). 2 | Journal Name, [year], [vol], 00–00
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Reaction conditions: Allylic alcohol (1 mmol) under Ar atmosphere, catalyst 3 (variable mol%), in 5 mL of KH2PO4 buffer 100 mM (1M iPrNH2) pH 8.5. Previously, buffer solution was degassed with Ar. c Calculated by GC. d Complex 1 (1 mol%) was used as catalyst.
Interestingly, meanwhile the para-substitution with methyl (5b) and bromine groups (7b) has a negligible effect on the bioamination with the unsubstituted 4b, the para-methoxy derivative 6b exhibited very low conversion with most -TAs, which could be attributed to electronic rather than steric effects. Similarly, the aliphatic ketone 8b delivered the amine 8c with good conversion and enantiomeric excess, although the (R)selective -TAs were more effective than the (S)-selective counterparts (entries 17-20). Regarding the achiral aldehyde 9b, reactions proceeded much faster and reached complete conversion with all the -TAs in 5 h (entries 21-22). Similarly, the bioamination of cyclohexanone (10b) was quantitative with most of the enzymes (entries 23-24). It is worth noting that in the optimization process we found optimal temperatures up to 60 ºC in some cases, which is not trivial since -TAs suffer from stability issues Likewise, and more importantly, after running several biotransformations in the presence of catalytic amounts of the metal complex 3, we discarded any kind of inhibition. Table 3. Enzymatic amination of 4b-10b catalysed by -TAs.a
Entry
Substrate
Product
1 2 3
(4c)
-TA
T (º C)
c (%)b
ee (%)c
ATA-251
50
83
>99 (S)
ATA-260
60
84
>99(S)
ATA-024
60
84
>99 (R)
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ATA-033
60
85
>99 (R)
5
ATA-237
50
80
>99 (S)
ATA-251
60
85
98 (S)
7
ATA-025
60
86
>99 (R)
8
ATA-024
60
85
98 (R)
9
ATA-237
35
64
>99 (S)
ATA-251
60
70
98 (S)
11
ATA-025
60
72
>99 (R)
12
ATA-024
35
80
>99 (R)
13
ATA-237
50
95
>99 (S)
14
ATA-251
60
85
>99 (S)
6
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DOI: 10.1039/C5CC03298A
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same buffer mixture (Table 4). Gratifyingly, the bioamination worked exceptionally well, leading to conversions and enantioselectivities identical to those measured in the single step. As a result, and after a simple extraction protocol, the corresponding amines 4c-8c were isolated without the need for chromatographic purification with overall yields ranging from 70-88%. Finally, the reaction with 4a was scaled up to 150 mg (entry 2), obtaining high yield (82%) and complete selectivity (ee >99%) of the amine 4c.
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Table 4. Metal-catalysed and Chemoenzymatic one-pot process.a
(5c)
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(7c)
Overall yield (%)b
ee (%)c
1
4a
4c
ATA-033
80
>99 (R)
2
d
4a
4c
ATA-033
82
>99 (R)
95
99 (R)
3
5a
5c
ATA-251
80
>99 (S)
16
ATA-024
50
95
>99 (R)
4
6a
6c
ATA-024
70
>99 (R)
17
ATA-237
35
87
89 (S)
5
7a
7c
ATA-237
88
>99 (S)
6
8a
8c
ATA-033
75
97 (R)
18
ATA-260
60
68
92 (S)
19
P2-A07
35
65
>99 (R)
20
ATA-033
35
82
97 (R)
ATA-200
35
>99
---
ATA-237
35
>99
---
ATA-200
35
>99
---
ATA-237
35
>99
---
a
(9c) 22 23
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(10c) 24 a
Reaction conditions: Substrate (20 mM) in KH2PO4 buffer 100 mM pH 8.5 (500 L) with PLP (1 mM) and iPrNH2 (1M), -TA (2 mg), DMSO (10% v/v), for 24 h at 250 rpm. b Measured by GC or HPLC. c Measured by chiral HPLC.
20
-TA
60
21
15
Product
ATA-025
40
10
Substrate
15
(8c)
5
Entry
Having in hand the two optimised steps, we planned their assembly in the one-pot process but taking into consideration a critical parameter such as the concentration of both catalytic systems. Actually, the allylic isomerisation proceeded 10 times more concentrated (200 mM) than the bioamination (20 mM). In this regard, isomerisations conducted at a lower concentration exhibited a significant drop in conversion as well as several byproducts. Similarly, higher concentrations had a strong negative impact on enzyme activity. Satisfactorily, we were able to tackle the concentration hurdles just by simple dilution (10 times) of the reaction mixture with the buffer, when the fastest metal-catalysed step was finished. As a proof of concept, we selected one overall reaction for substrates 4a-8a, according to the optimised conditions for each step (Tables 2 and 3). Thus, allylic alcohols 4a-8a were converted, in a one-pot procedure, into their corresponding primary amines 4c-8c by the combined action of the metal catalyst 3 and the -TA (in the presence of PLP) in the This journal is © The Royal Society of Chemistry [year]
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Reaction conditions: 4a-8a (200 mM) under Ar atmosphere in KH2PO4 buffer 100 mM pH 8.5, 1 mol% of 3 and 50 ºC (buffer solution previously degassed with Ar). Once completed (GC), addition of -TA, DMSO (10% v/v) and PLP dissolved in the buffer until a 10 times-dilution and stirring at 250 rpm (T and time depending on optimised conditions of Table 3). b After isolation of the amine by acid-base extraction. c Measured by chiral HPLC. d Experiment performed to 150mg scale.
In summary, we have designed an efficient, stereoselective and operationally simple chemoenzymatic protocol for obtaining chiral amines from allylic alcohols via an unprecedented one-pot process involving the Ru(IV)-catalysed isomerisation of a variety of allylic alcohols (yielding quantitatively the desired carbonyl compounds), and the concomitant bioamination promoted by TA (under the same reaction conditions and in aqueous medium), delivering the desired amines in very high yields and with excellent enantiomeric excesses. Thus, this methodology represents an important contribution to the almost unexplored field of one-pot processes involving both metal- and biocatalysed reactions in water. Further efforts devoted to the development of new bio- and metal catalytic systems active in water are currently underway. We are indebted to the MINECO of Spain (Projects CTQ201014796/BQU and CTQ2013-40591-P), the Gobierno del Principado de Asturias (Project GRUPIN14-006) and the COST action SIPs-CM1302 for financial support. J.G.-A. thanks the MINECO and the European Social Fund for the award of a “Ramón y Cajal” contract.
Notes and references 65
a
EntreChem SL, Edificio Científico Tecnológico, Campus El Cristo, 33006 Oviedo, Spain. E-mail: [email protected] b Laboratorio de Compuestos Organometálicos y Catálisis (Unidad Asociada al CSIC), Departamento de Química Orgánica e Inorgánica,
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(IUQOEM), Centro de Innovación en Química Avanzada (ORFEOCINQA), Facultad de Química, Universidad de Oviedo, E-33071 Oviedo, Spain. E-mail: [email protected] † Electronic Supplementary Information (ESI) available: Experimental procedures and full panel of experiments. 1 The 12 Principles of Green Chemistry have become a widely accepted set of criteria for the rapid assessment of the greenness of a given chemical reaction: P. T. Anastas and J. C. Warner, in Green Chemistry Theory and Practice, Oxford University Press, Oxford, 1998, p 30. 2 I. W. C. E. Arends, R. A. Sheldon and U. Henefeld, in Green Chemistry and Catalysis, Wiley-VCH, Weinheim, 2007. 3 A recent editorial in Organic Process Research and Development discourages chemists to use solvents that are either known to be toxic, dangerous for large scale preparations or expensive to dispose as waste. T. Laird, Org. Process Res. Dev., 2012, 16, 1. 4 (a) C.-J. Li and T. H. Chan, in Organic Reactions in Aqueous Media, John Wiley & Sons, New York, 1997; (b) U. M. Lindström, Chem. Rev., 2002, 102, 2751. 5 (a) A. Bruggink, R. Schoevaart and T. Kieboom, Org. Process Res. Dev., 2003, 7, 622; (b) O. Pamies and J. E. Bäckvall, Chem. Rev., 2003, 103, 3247. 6 H. Gröger and W. Hummel, Curr. Opin. Chem. Biol., 2014, 19, 171. 7 Metal-Catalyzed Reactions in Water, eds. P. H. Dixneuf and V. Cadierno, Wiley-VCH, Weinheim, 2013. 8 H. Sato, W. Hummel and H. Gröger, Angew. Chem. Int. Ed., 2015, 54, 4488. 9 (a) E. Burda, W. Hummel and H. Gröger, Angew. Chem. Int. Ed., 2008, 47, 9551; (b) A. Boffi, S. Cacchi, P. Ceci, R. Cirilli, G. Fabrizi, A. Prastaro, S. Niembro, A. Shafir and A. Vallribera, ChemCatChem., 2011, 3, 347. 10 (a) K. Tenbrink, M. Sebler, J. Schatz and H. Gröger, Adv. Synth. Catal., 2011, 353, 2363; (b) C. A. Denard, H. Huang, M. J. Bartlett, L. Lu, Y. Tan, H. Zhao and J. F. Hartwig, Angew. Chem. Int. Ed., 2014, 53, 465. 11 (a) V. Köhler, Y. M. Wilson, M. Dürrenberger, D. Ghislieri, E. Churakova, T. Quinto, L. Knörr, D. Häussinger, F. Hollmann, N. J. Turner and T. R. Ward, Nat. Chem., 2013, 5, 93; (b) Z. J. Wang, K. N. Clary, R. G. Bergman, K. N. Raymond and F. D. Toste, Nat. Chem., 2013, 5, 100. 12 (a) W. Kroutil, E.-M. Fischereder, C. S. Fuchs, H. Lechner, F. G. Mutti, D. Pressnitz, A. Rajagopalan, J. H. Sattler, R. C. Simon and E. Siirola, Org. Process Res. Dev., 2013, 17, 751; (b) R. C. Simon, N. Richter, E. Busto and W. Kroutil, ACS Catal., 2014, 4, 129. 13 (a) C. K. Savile, J. M. Janey, E. C. Mundorff, J. C. Moore, S. Tam, W. R. Jarvis, J. C. Colbeck, A. Krebber, F. J. Fleitz, J. Brands, P. N. Devine, G. W. Huisman and G. J. Hughes, Science, 2010, 329, 305; (b) M. Girardin, S. G. Ouellet, D. Gauvreau, J. C. Moore, G. Hughes, P. N. Devine, P. D. O’Shea and L.-C. Campeau, Org. Process Res. Dev., 2013, 17, 61; (c) C. K. Chung, P. G. Bulger, B. Kosjek, K. M. Belyk, N. Rivera, M. E. Scott, G. R. Humphrey, J. Limanto, D. C. Bachert and K. M. Emerson, Org. Process Res. Dev., 2014, 18, 215. 14 (a) V. Cadierno, S. E. García-Garrido, J. Gimeno, A. Varela-Álvarez and J. A. Sordo, J. Am. Chem. Soc., 2006, 128, 1360; (b) J. GarcíaÁlvarez, J. Gimeno and F. J. Suárez, Organometallics, 2011, 30, 2893; (c) C. Vidal, F. J. Suárez and J. García-Álvarez, Catal. Commun., 2014, 44, 76. 15 For recent reviews and book chapters covering this topic see: (a) P. Lorenzo-Luis, A. Romerosa and M. Serrano-Ruiz, ACS Catal., 2012, 2, 1079; (b) N. Alhsten, A. Bartoszewicz and B. Martín-Matute, Dalton Trans., 2012, 41, 1660; (c) J. García-Álvarez, S. E. GarcíaGarrido, P. Crochet and V. Cadierno, Curr. Top. Catal., 2012, 10, 3556. See also ref 7. 16 Ru-catalysed isomerisation of allylic alcohols in different buffer solutions has been previously reported: (a) T. Campos-Malpartida, M. Fekete, F. Joó, A. Kathó, A. Romerosa, M. Saoud and W. Wojtków, J. Organomet. Chem., 2008, 693, 468; (b) B. González, P. Lorenzo-Luis, M. Serrano-Ruiz, E. Papp, M. Fekete, K. Csépkec, K. Ősz, A. Kathó, F. Joó and A. Romerosa, J. Mol. Catal. A: Chem., 2010, 326, 15; (c) M. Serrano-Ruiz, P. Lorenzo-Luis, A. Romerosa and A. Mena-Cruz, Dalton Trans., 2013, 42, 7622.
4 | Journal Name, [year], [vol], 00–00
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5
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DOI: 10.1039/C5CC03298A
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ChemComm Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: N. Ríos Lombardía, C. Vidal, M. Cocina, F. Moris, J. García-Álvarez and J. González-Sabín, Chem. Commun., 2015, DOI: 10.1039/C5CC03298A.
This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
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Journal Name
ChemComm
View Article Online DynamicDOI: Article Links ► 10.1039/C5CC03298A
Cite this: DOI: 10.1039/c0xx00000x
ARTICLE TYPE
Chemoenzymatic one-pot synthesis in aqueous medium: Combination of metal-catalysed allylic alcohol isomerisation–asymmetric bioamination† Nicolás Ríos-Lombardía,a Cristian Vidal,b María Cocina,a Francisco Morís,a Joaquín García-Álvarezb,* and Javier González-Sabína,* 5
10
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x The ruthenium-catalysed isomerisation of allylic alcohols was coupled, for the first time, with an asymmetric bioamination in a one-pot process in aqueous medium. In those cases involving proquiral ketones, the -TA exhibited excellent enantioselectivity, identical to that observed in the single step. As a result, amines were obtained from allylic alcohols with high overall yields and excellent enantiomeric excesses.
50
55
15
20
25
30
35
40
45
Multi-step one-pot processes are an exciting and emerging opportunity in sustainable chemistry, fitting some of the crucial points to consider when a catalytic green chemical process is designed.1,2 In this sense, performing multi-step syntheses in a concurrent fashion enables the bypass of costly and tedious intermediate downstream and purification steps, thus improving both the ecological footprint and the economic efficiency of a process.2 Moreover, the choice of a safe, non-toxic, biorenewable and cheap solvent, is one of the major goals in organic synthesis (from a green chemistry point of view).3 In this sense, water can be considered as an attractive solvent to deliver truly greener processes.4 Historically, chemists have combined reactions belonging to a particular technology, that is, chemocatalysis (mainly in organic solvent) or biocatalysis (mainly in water).5 However examples of metal catalysts (or organocatalysts) working hand in hand with enzymes are still very scarce, especially in aqueous media.6 Actually, the assembly of these two worlds imposes several challenges, namely: i) catalyst compatibility and stability; ii) cross-reactivity; iii) shifting the reaction equilibrium to the product side, and iv) scalability. In spite of these facts, the pool of metallic catalysts able to operate in water has expanded enormously in the last years,7 and provides novel opportunities to combine with enzymes which have water as their natural environment. Some successful examples include the assembly of a Wacker oxidation8 or Suzuki and Heck crosscouplings with bioreductions,9 and olefin metathesis followed by enzymatic hydrolysis or hydroxylation.10 Likewise, the use of artificial metalloenzymes and supramolecular host-guest complexes have enabled the use of sensitive metal complexes in water also in a tandem fashion with biocatalysts.11 Within the past few years, enzymes such as -transaminases (-TAs), which catalyse the transfer of an amino group from an amino donor onto a carbonyl moiety using pyridoxal-5’phosphate (PLP) as co-factor, have emerged as a powerful This journal is © The Royal Society of Chemistry [year]
60
65
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75
80
alternative to hydrolases and proteases for chiral amine preparation.12 Thus, the increased availability of -TAs and the advancement of enzyme engineering make the enzymatic bioamination a feasible and attractive manufacturing option for chiral amine synthesis in pharmaceuticals.13 Notwithstanding, and as far as we are concerned, there are no reports dealing about the combination of -TAs and metal-catalysed reactions in a concurrent fashion. Since the natural substrates of these enzymes are both aldehydes and ketones, we envisaged that a process such the metal-catalysed isomerisation of allylic alcohols, which delivers carbonyl groups, could be implemented into a cascade fashion (Scheme 1). Actually, this widely-studied catalytic isomerisation has recently been performed in a variety of nonconventional solvents [like water, ionic liquids and Deep Eutectic Solvents (DESs)],14,15 which open the way to a combined process in aqueous media.
Scheme 1 Synthesis of chiral amines combining metal-catalysed allylic alcohol isomerisation and enzymatic amination in a one-pot process.
Despite being two water-compatible reactions, a major concern to settle a feasible coupled process is the compatibility of the different reaction conditions of both transformations. Thus, while enzyme inhibition by heavy metals is a known phenomenon; the required buffer solutions to accomplish the biotransformation could also deactivate the metal catalyst.6 Furthermore, parameters such as pH, temperature or concentration should be also optimised to address both reactions efficiently. Accordingly, we designed some preliminary experiments aimed at verify the compatibility of the two reactions. Initially, and as the metalcatalysed transformation proceeds first, we evaluated three highly efficient ruthenium(IV) catalysts (1-3) in the allylic isomerisation of -vinylbenzyl alcohol (4a, Table 1),14 but changing water by the reaction medium used in enzymatic transaminations; that is a phosphate buffer (100 mM, pH 8.5) containing iPrNH2 (1M).16 Similarly, the temperature was lowered to 50 ºC, meeting one of the key points to avoid the erosion of the -TAs. Pleasantly, the three complexes were active and selective catalysts (in the absence of any co-catalyst), yielding the desired ketone (4b) [journal], [year], [vol], 00–00 | 1
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Table 2. Isomerisation of the allylic alcohols 4a-10a catalysed by the Ru(IV) complex 3 in a phosphate buffer 100 mM pH 8.5 (1M iPrNH2).a
under the aforementioned conditions and with a catalyst loading of 1 mol% (entries 1-3), being complex 3 by far the most efficient catalyst. At this point, it is important to note that the vast majority of Ru-catalysts previously reported in the literature are not able to promote the isomerisation reaction below 80-100 ºC in pure water as solvent.15 Notably, 4a could be isomerised even on smoother conditions such as 35 ºC, which is a more desirable temperature for biotransformations, although longer reactions are required to achieve quantitative conversions (entry 4). Finally, the use of catalytic amounts of complex 3 is mandatory, as in its absence, no reaction was observed (entry 5).
Entry
Substrate
Temp.
mol% [Ru]
Time
Conversionc
1
4a
50 ºC
1
3h
>99
2
5a
50 ºC
1
16 h
>99
3
6a
50 ºC
1
12 h
>99
4
7a
50 ºC
1
17 h
>99
5
8a
50 ºC
1
3h
>99
6
d
9a
50 ºC
1
4h
>99
7
10a
75 ºC
10
15 h
>99
Table 1. Isomerisation of -vinylbenzyl alcohol catalysed by the Ru(IV)
a
complexes 1-3 in a typical aqueous medium of enzymatic aminations.a Entry
Complex
mol% [Ru]
Time
Temp.
Conversionc
1
1
1
5h
50 ºC
>99
2
2
1
24 h
50 ºC
90
3
3
1
3h
50 ºC
>99
4
3
1
8.5 h
35 ºC
>99
5
-
-
24 h
50 ºC
1
45
50
Reaction conditions: -Vinylbenzyl alcohol (1 mmol) under Ar atmosphere at 50 ºC, 1 mol% Ru(IV) catalyst, in 5 mL of KH2PO4 buffer 100 mM (1M iPrNH2) pH 8.5. Previously, buffer solution was degassed with Ar. c Calculated by GC. a
15
55
20
25
30
35
40
Then, once identified the prerequisites for the first catalytic step, we extended the study to some allylic alcohols which could fulfill the structural features to undergo a metal-catalysed isomerisation and subsequent bioamination (4a-10a; Table 2). Under identical conditions to those described in Table 1, complex 3 is also very efficient in the isomerisation of a variety of allylic alcohols. Thus, both arylic (4a-7a; entries 1-4) and aliphatic (8a, entry 5) monosubstituted allylic alcohols could be smoothly converted into their corresponding carbonyl compounds. Regarding 1,2-disubstituted allylic alcohols (9a-10a; entries 6-7), these substrates had previously proven to be challenging, and it was necessary to increase temperature or catalyst loading to reach complete conversion.14 Next, we turned our attention to the second step of the overall process, namely the bioamination of the carbonyl compounds. Thus, the previously obtained substrates 4b-10b were screened with a series of 28 commercially available -TAs, the Codex® Transaminase Screening Kit, which are able to work using isopropylamine in molar excess as amino donor (Table 3). After this enzymatic screening (see Tables S1-S7 in the ESI for further details), in the case of the prochiral ketones 4b-8b we identified highly efficient -TAs which gave access, after 24 h, to both enantiomers of each amine with high conversion and excellent enantiomeric excess. (entries 1-20, Table 3). 2 | Journal Name, [year], [vol], 00–00
60
65
Reaction conditions: Allylic alcohol (1 mmol) under Ar atmosphere, catalyst 3 (variable mol%), in 5 mL of KH2PO4 buffer 100 mM (1M iPrNH2) pH 8.5. Previously, buffer solution was degassed with Ar. c Calculated by GC. d Complex 1 (1 mol%) was used as catalyst.
Interestingly, meanwhile the para-substitution with methyl (5b) and bromine groups (7b) has a negligible effect on the bioamination with the unsubstituted 4b, the para-methoxy derivative 6b exhibited very low conversion with most -TAs, which could be attributed to electronic rather than steric effects. Similarly, the aliphatic ketone 8b delivered the amine 8c with good conversion and enantiomeric excess, although the (R)selective -TAs were more effective than the (S)-selective counterparts (entries 17-20). Regarding the achiral aldehyde 9b, reactions proceeded much faster and reached complete conversion with all the -TAs in 5 h (entries 21-22). Similarly, the bioamination of cyclohexanone (10b) was quantitative with most of the enzymes (entries 23-24). It is worth noting that in the optimization process we found optimal temperatures up to 60 ºC in some cases, which is not trivial since -TAs suffer from stability issues Likewise, and more importantly, after running several biotransformations in the presence of catalytic amounts of the metal complex 3, we discarded any kind of inhibition. Table 3. Enzymatic amination of 4b-10b catalysed by -TAs.a
Entry
Substrate
Product
1 2 3
(4c)
-TA
T (º C)
c (%)b
ee (%)c
ATA-251
50
83
>99 (S)
ATA-260
60
84
>99(S)
ATA-024
60
84
>99 (R)
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4
ATA-033
60
85
>99 (R)
5
ATA-237
50
80
>99 (S)
ATA-251
60
85
98 (S)
7
ATA-025
60
86
>99 (R)
8
ATA-024
60
85
98 (R)
9
ATA-237
35
64
>99 (S)
ATA-251
60
70
98 (S)
11
ATA-025
60
72
>99 (R)
12
ATA-024
35
80
>99 (R)
13
ATA-237
50
95
>99 (S)
14
ATA-251
60
85
>99 (S)
6
10
DOI: 10.1039/C5CC03298A
25
same buffer mixture (Table 4). Gratifyingly, the bioamination worked exceptionally well, leading to conversions and enantioselectivities identical to those measured in the single step. As a result, and after a simple extraction protocol, the corresponding amines 4c-8c were isolated without the need for chromatographic purification with overall yields ranging from 70-88%. Finally, the reaction with 4a was scaled up to 150 mg (entry 2), obtaining high yield (82%) and complete selectivity (ee >99%) of the amine 4c.
30
Table 4. Metal-catalysed and Chemoenzymatic one-pot process.a
(5c)
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(7c)
Overall yield (%)b
ee (%)c
1
4a
4c
ATA-033
80
>99 (R)
2
d
4a
4c
ATA-033
82
>99 (R)
95
99 (R)
3
5a
5c
ATA-251
80
>99 (S)
16
ATA-024
50
95
>99 (R)
4
6a
6c
ATA-024
70
>99 (R)
17
ATA-237
35
87
89 (S)
5
7a
7c
ATA-237
88
>99 (S)
6
8a
8c
ATA-033
75
97 (R)
18
ATA-260
60
68
92 (S)
19
P2-A07
35
65
>99 (R)
20
ATA-033
35
82
97 (R)
ATA-200
35
>99
---
ATA-237
35
>99
---
ATA-200
35
>99
---
ATA-237
35
>99
---
a
(9c) 22 23
45
(10c) 24 a
Reaction conditions: Substrate (20 mM) in KH2PO4 buffer 100 mM pH 8.5 (500 L) with PLP (1 mM) and iPrNH2 (1M), -TA (2 mg), DMSO (10% v/v), for 24 h at 250 rpm. b Measured by GC or HPLC. c Measured by chiral HPLC.
20
-TA
60
21
15
Product
ATA-025
40
10
Substrate
15
(8c)
5
Entry
Having in hand the two optimised steps, we planned their assembly in the one-pot process but taking into consideration a critical parameter such as the concentration of both catalytic systems. Actually, the allylic isomerisation proceeded 10 times more concentrated (200 mM) than the bioamination (20 mM). In this regard, isomerisations conducted at a lower concentration exhibited a significant drop in conversion as well as several byproducts. Similarly, higher concentrations had a strong negative impact on enzyme activity. Satisfactorily, we were able to tackle the concentration hurdles just by simple dilution (10 times) of the reaction mixture with the buffer, when the fastest metal-catalysed step was finished. As a proof of concept, we selected one overall reaction for substrates 4a-8a, according to the optimised conditions for each step (Tables 2 and 3). Thus, allylic alcohols 4a-8a were converted, in a one-pot procedure, into their corresponding primary amines 4c-8c by the combined action of the metal catalyst 3 and the -TA (in the presence of PLP) in the This journal is © The Royal Society of Chemistry [year]
50
55
60
Reaction conditions: 4a-8a (200 mM) under Ar atmosphere in KH2PO4 buffer 100 mM pH 8.5, 1 mol% of 3 and 50 ºC (buffer solution previously degassed with Ar). Once completed (GC), addition of -TA, DMSO (10% v/v) and PLP dissolved in the buffer until a 10 times-dilution and stirring at 250 rpm (T and time depending on optimised conditions of Table 3). b After isolation of the amine by acid-base extraction. c Measured by chiral HPLC. d Experiment performed to 150mg scale.
In summary, we have designed an efficient, stereoselective and operationally simple chemoenzymatic protocol for obtaining chiral amines from allylic alcohols via an unprecedented one-pot process involving the Ru(IV)-catalysed isomerisation of a variety of allylic alcohols (yielding quantitatively the desired carbonyl compounds), and the concomitant bioamination promoted by TA (under the same reaction conditions and in aqueous medium), delivering the desired amines in very high yields and with excellent enantiomeric excesses. Thus, this methodology represents an important contribution to the almost unexplored field of one-pot processes involving both metal- and biocatalysed reactions in water. Further efforts devoted to the development of new bio- and metal catalytic systems active in water are currently underway. We are indebted to the MINECO of Spain (Projects CTQ201014796/BQU and CTQ2013-40591-P), the Gobierno del Principado de Asturias (Project GRUPIN14-006) and the COST action SIPs-CM1302 for financial support. J.G.-A. thanks the MINECO and the European Social Fund for the award of a “Ramón y Cajal” contract.
Notes and references 65
a
EntreChem SL, Edificio Científico Tecnológico, Campus El Cristo, 33006 Oviedo, Spain. E-mail: [email protected] b Laboratorio de Compuestos Organometálicos y Catálisis (Unidad Asociada al CSIC), Departamento de Química Orgánica e Inorgánica,
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(IUQOEM), Centro de Innovación en Química Avanzada (ORFEOCINQA), Facultad de Química, Universidad de Oviedo, E-33071 Oviedo, Spain. E-mail: [email protected] † Electronic Supplementary Information (ESI) available: Experimental procedures and full panel of experiments. 1 The 12 Principles of Green Chemistry have become a widely accepted set of criteria for the rapid assessment of the greenness of a given chemical reaction: P. T. Anastas and J. C. Warner, in Green Chemistry Theory and Practice, Oxford University Press, Oxford, 1998, p 30. 2 I. W. C. E. Arends, R. A. Sheldon and U. Henefeld, in Green Chemistry and Catalysis, Wiley-VCH, Weinheim, 2007. 3 A recent editorial in Organic Process Research and Development discourages chemists to use solvents that are either known to be toxic, dangerous for large scale preparations or expensive to dispose as waste. T. Laird, Org. Process Res. Dev., 2012, 16, 1. 4 (a) C.-J. Li and T. H. Chan, in Organic Reactions in Aqueous Media, John Wiley & Sons, New York, 1997; (b) U. M. Lindström, Chem. Rev., 2002, 102, 2751. 5 (a) A. Bruggink, R. Schoevaart and T. Kieboom, Org. Process Res. Dev., 2003, 7, 622; (b) O. Pamies and J. E. Bäckvall, Chem. Rev., 2003, 103, 3247. 6 H. Gröger and W. Hummel, Curr. Opin. Chem. Biol., 2014, 19, 171. 7 Metal-Catalyzed Reactions in Water, eds. P. H. Dixneuf and V. Cadierno, Wiley-VCH, Weinheim, 2013. 8 H. Sato, W. Hummel and H. Gröger, Angew. Chem. Int. Ed., 2015, 54, 4488. 9 (a) E. Burda, W. Hummel and H. Gröger, Angew. Chem. Int. Ed., 2008, 47, 9551; (b) A. Boffi, S. Cacchi, P. Ceci, R. Cirilli, G. Fabrizi, A. Prastaro, S. Niembro, A. Shafir and A. Vallribera, ChemCatChem., 2011, 3, 347. 10 (a) K. Tenbrink, M. Sebler, J. Schatz and H. Gröger, Adv. Synth. Catal., 2011, 353, 2363; (b) C. A. Denard, H. Huang, M. J. Bartlett, L. Lu, Y. Tan, H. Zhao and J. F. Hartwig, Angew. Chem. Int. Ed., 2014, 53, 465. 11 (a) V. Köhler, Y. M. Wilson, M. Dürrenberger, D. Ghislieri, E. Churakova, T. Quinto, L. Knörr, D. Häussinger, F. Hollmann, N. J. Turner and T. R. Ward, Nat. Chem., 2013, 5, 93; (b) Z. J. Wang, K. N. Clary, R. G. Bergman, K. N. Raymond and F. D. Toste, Nat. Chem., 2013, 5, 100. 12 (a) W. Kroutil, E.-M. Fischereder, C. S. Fuchs, H. Lechner, F. G. Mutti, D. Pressnitz, A. Rajagopalan, J. H. Sattler, R. C. Simon and E. Siirola, Org. Process Res. Dev., 2013, 17, 751; (b) R. C. Simon, N. Richter, E. Busto and W. Kroutil, ACS Catal., 2014, 4, 129. 13 (a) C. K. Savile, J. M. Janey, E. C. Mundorff, J. C. Moore, S. Tam, W. R. Jarvis, J. C. Colbeck, A. Krebber, F. J. Fleitz, J. Brands, P. N. Devine, G. W. Huisman and G. J. Hughes, Science, 2010, 329, 305; (b) M. Girardin, S. G. Ouellet, D. Gauvreau, J. C. Moore, G. Hughes, P. N. Devine, P. D. O’Shea and L.-C. Campeau, Org. Process Res. Dev., 2013, 17, 61; (c) C. K. Chung, P. G. Bulger, B. Kosjek, K. M. Belyk, N. Rivera, M. E. Scott, G. R. Humphrey, J. Limanto, D. C. Bachert and K. M. Emerson, Org. Process Res. Dev., 2014, 18, 215. 14 (a) V. Cadierno, S. E. García-Garrido, J. Gimeno, A. Varela-Álvarez and J. A. Sordo, J. Am. Chem. Soc., 2006, 128, 1360; (b) J. GarcíaÁlvarez, J. Gimeno and F. J. Suárez, Organometallics, 2011, 30, 2893; (c) C. Vidal, F. J. Suárez and J. García-Álvarez, Catal. Commun., 2014, 44, 76. 15 For recent reviews and book chapters covering this topic see: (a) P. Lorenzo-Luis, A. Romerosa and M. Serrano-Ruiz, ACS Catal., 2012, 2, 1079; (b) N. Alhsten, A. Bartoszewicz and B. Martín-Matute, Dalton Trans., 2012, 41, 1660; (c) J. García-Álvarez, S. E. GarcíaGarrido, P. Crochet and V. Cadierno, Curr. Top. Catal., 2012, 10, 3556. See also ref 7. 16 Ru-catalysed isomerisation of allylic alcohols in different buffer solutions has been previously reported: (a) T. Campos-Malpartida, M. Fekete, F. Joó, A. Kathó, A. Romerosa, M. Saoud and W. Wojtków, J. Organomet. Chem., 2008, 693, 468; (b) B. González, P. Lorenzo-Luis, M. Serrano-Ruiz, E. Papp, M. Fekete, K. Csépkec, K. Ősz, A. Kathó, F. Joó and A. Romerosa, J. Mol. Catal. A: Chem., 2010, 326, 15; (c) M. Serrano-Ruiz, P. Lorenzo-Luis, A. Romerosa and A. Mena-Cruz, Dalton Trans., 2013, 42, 7622.
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