Chemoenzymatic one-pot synthesis in an aqueous medium: combination of metal-catalysed allylic alcohol isomerisation-asymmetric bioamination

Article abstract of DOI:10.1039/c5cc03298a

The ruthenium-catalysed isomerisation of allylic alcohols was coupled, for the first time, with asymmetric bioamination in a one-pot process in an aqueous medium. In the cases involving prochiral ketones, the ω-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.

Full text of DOI:10.1039/c5cc03298a

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Lombardía, C. Vidal, M. Cocina, F. Moris, J. García-Álvarez and J. González-Sabín, Chem. Commun.,
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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
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
alternative to hydrolases and proteases for chiral amine
preparation.12 Thus, the increased availability of -TAs and the
50 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
55 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
60 isomerisation has recently been performed in a variety of non-
conventional solvents [like water, ionic liquids and Deep Eutectic
Solvents (DESs)],14,15 which open the way to a combined process
in aqueous media.
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
10 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.
Multi-step one-pot processes are an exciting and emerging
15 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
20 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
25 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,
30 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
35 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 cross-
couplings with bioreductions,9 and olefin metathesis followed by
40 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
Scheme 1 Synthesis of chiral amines combining metal-catalysed allylic
65 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
70 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
75 compatibility of the two reactions. Initially, and as the metal-
catalysed 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
80 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)
Within the past few years, enzymes such as -transaminases
45 (-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
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DOI: 10.1039/C5CC03P29a8gAe 2 of 4
under the aforementioned conditions and with a catalyst loading
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
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
5
10 catalytic amounts of complex 3 is mandatory, as in its absence,
no reaction was observed (entry 5).
Entry
Substrate
4a
Temp.
50 ºC
50 ºC
50 ºC
50 ºC
50 ºC
50 ºC
75 ºC
mol% [Ru]
Time
3 h
Conversionc
>99
Table 1. Isomerisation of -vinylbenzyl alcohol catalysed by the Ru(IV)
1
2
1
1
5a
16 h
12 h
17 h
3 h
>99
3
6a
1
>99
4
7a
1
>99
5
8a
1
>99
6d
7
9a
1
4 h
>99
10a
10
15 h
>99
a
Reaction conditions: Allylic alcohol (1 mmol) under Ar atmosphere, catalyst 3
complexes 1-3 in a typical aqueous medium of enzymatic aminations.a
(variable mol%), in 5 mL of KH2PO4 buffer 100 mM (1M iPrNH2) pH 8.5.
45 Previously, buffer solution was degassed with Ar. c Calculated by GC. d Complex 1
(1 mol%) was used as catalyst.
Entry
Complex
mol% [Ru]
Time
5 h
Temp.
50 ºC
50 ºC
50 ºC
35 ºC
50 ºC
Conversionc
1
2
3
4
5
1
2
3
3
-
1
1
1
1
-
>99
90
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
50 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
55 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
60 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.
24 h
3 h
>99
>99
1
8.5 h
24 h
a Reaction conditions: -Vinylbenzyl alcohol (1 mmol) under Ar atmosphere at 50
15 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.
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
20 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
25 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
65 Table 3. Enzymatic amination of 4b-10b catalysed by -TAs.a
30 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
35 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
40 enantiomeric excess. (entries 1-20, Table 3).
Entry
Substrate
Product
-TA
T
(º C)
c
ee
(%)b
(%)c
1
2
3
ATA-251
ATA-260
ATA-024
50
60
60
83
84
84
>99 (S)
>99(S)
>99 (R)
(4c)
2
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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.
4
ATA-033
ATA-237
ATA-251
ATA-025
ATA-024
ATA-237
ATA-251
ATA-025
ATA-024
ATA-237
ATA-251
ATA-025
ATA-024
ATA-237
ATA-260
P2-A07
60
50
60
60
60
35
60
60
35
50
60
60
50
35
60
35
35
35
35
35
35
85
80
>99 (R)
>99 (S)
98 (S)
>99 (R)
98 (R)
>99 (S)
98 (S)
>99 (R)
>99 (R)
>99 (S)
>99 (S)
99 (R)
>99 (R)
89 (S)
92 (S)
>99 (R)
97 (R)
---
5
As
a result, and after a simple extraction protocol, the
25 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.
6
85
(5c)
(6c)
(7c)
(8c)
7
86
8
85
30 Table 4. Metal-catalysed and Chemoenzymatic one-pot process.a
9
64
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
70
72
35
80
Entry
Substrate
Product
-TA
Overall yield
(%)b
ee (%)c
95
1
2d
3
4a
4a
5a
4c
4c
5c
ATA-033
ATA-033
ATA-251
80
82
80
>99 (R)
>99 (R)
>99 (S)
85
95
4
6a
6c
ATA-024
70
>99 (R)
95
5
6
7a
8a
7c
8c
ATA-237
ATA-033
88
75
>99 (S)
97 (R)
87
68
a
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
40 dissolved in the buffer until a 10 times-dilution and stirring at 250 rpm (T and time
65
b
depending on optimised conditions of Table 3). After isolation of the amine by
ATA-033
ATA-200
ATA-237
ATA-200
ATA-237
82
c
d
acid-base extraction. Measured by chiral HPLC. Experiment performed to 150-
mg scale.
>99
>99
>99
>99
(9c)
In summary, we have designed an efficient, stereoselective and
45 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 -
50 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
55 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 CTQ2010-
14796/BQU and CTQ2013-40591-P), the Gobierno del
60 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.
---
---
(10c)
---
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.
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
10 exhibited a significant drop in conversion as well as several by-
products. 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
15 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
20 the metal catalyst 3 and the -TA (in the presence of PLP) in the
5
Notes and references
a
65
EntreChem SL, Edificio Científico Tecnológico, Campus El Cristo,
33006 Oviedo, Spain. E-mail: jgsabin@entrechem.com
Laboratorio de Compuestos Organometálicos y Catálisis (Unidad
b
Asociada al CSIC), Departamento de Química Orgánica e Inorgánica,
This journal is © The Royal Society of Chemistry [year]
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DOI: 10.1039/C5CC03P29a8gAe 4 of 4
(IUQOEM), Centro de Innovación en Química Avanzada (ORFEO-
CINQA), Facultad de Química, Universidad de Oviedo, E-33071 Oviedo,
Spain. E-mail: garciajoaquin@uniovi.es
† Electronic Supplementary Information (ESI) available: Experimental
5 procedures and full panel of experiments.
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4
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