Selective oxidation of amines to aldehydes or imines using laccase-mediated bio-oxidation

Article abstract of DOI:10.1002/adsc.201500165

An efficient and practical chemo-enzymatic aerobic oxidation in water of benzylamines to obtain aldehydes or imines is described. Laccase from Trametes versicolor was chosen as biocatalyst, and TEMPO (radical 2,2,6,6-tetramethylpiperidine 1-oxyl) as mediator. A study on the pH dependence of the aqueous medium allowed us to realise a fine tuning on product selectivity. Under our optimized reaction conditions, the bio-oxidation of a series of primary, secondary and cyclic amines has been achieved.

Full text of DOI:10.1002/adsc.201500165

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DOI: 10.1002/adsc.201500165
Selective Oxidation of Amines to Aldehydes or Imines using
Laccase-Mediated Bio-Oxidation
Paola Galletti,a,* Federica Funiciello,a Roberto Soldati,a and Daria Giacominia,*
a
Department of Chemistry “G. Ciamician”, University of Bologna, 40126 Bologna, Italy
Fax: (+39)-051-209-9456; phone: (+39)-051-209-9528; e-mail: paola.galletti@unibo.it or daria.giacomini@unibo.it
Received: February 16, 2015; Revised: April 23, 2015; Published online: May 13, 2015
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201500165.
Abstract: An efficient and practical chemo-enzymat- tuning on product selectivity. Under our optimized
ic aerobic oxidation in water of benzylamines to reaction conditions, the bio-oxidation of a series
obtain aldehydes or imines is described. Laccase of primary, secondary and cyclic amines has been
from Trametes versicolor was chosen as biocatalyst, achieved.
and TEMPO (radical 2,2,6,6-tetramethylpiperidine 1-
oxyl) as mediator. A study on the pH dependence of Keywords: aldehydes; amines; enzyme catalysis;
the aqueous medium allowed us to realise a fine imines; oxidation
Introduction
methods for amine oxidation.[2] Focusing on primary
amines, some relevant transformations should be con-
Selective oxidation of amines is an important tool to sidered: the oxidation to nitriles or carbonyl com-
get functional group interconversions in organic syn- pounds, the oxidative self-condensation of the starting
thesis. Depending on the amine, catalytic system, re- substrates to give imines, and with a second amine,
action conditions, and oxidizing agent, a panel of dif- the oxidative cross-coupling to give cross imines
ferent products could be obtained, such as, for in- (Figure 1).
stance, carbonyl compounds, amides, nitriles, and so
The oxidative dehydrogenation of amines has often
on (Figure 1).[1] Great progress has been made in been based on transition metal complexes, as cata-
recent years in developing catalytic and selective lysts, under aerobic conditions with molecular oxygen
as final oxidant[3] and in water as solvent.[4] Other ap-
proaches utilized Cu salts/N-oxyl radical systems,[5,6]
metal-organic framework solids,[7] gold catalysis,[8]
metal-free aerobic conditions[9] and, very recently,
TiO2 photocatalytic oxidation in water.[10] However,
many of these systems still require harsh reaction con-
ditions, give metal-containing wastes, and selectivity
can be difficult to achieve and control.
Biocatalysis is emerging as a valuable tool to devel-
op more benign and selective redox processes.[11] Bio-
oxidations could have higher selectivity (regio-,
chemo-, or stereo-) suitable even for fine chemicals
with complex structures and oxidation-sensitive func-
tional groups.[12] In nature, copper amine oxidases
(CAOs, EC 1.4.3.21 and EC 1.4.3.22) couple the oxi-
dation of primary amines to aldehydes with the reduc-
tion of molecular oxygen to hydrogen peroxide using
ortho-quinone cofactors.[13] By mimicking CAOs, bio-
mimetic aerobic oxidation of primary benzylamines
has been recently achieved by using quinones as cata-
Figure 1. Functional group diversity generated by amine oxi-
dation.
lysts.[14]
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Selective Oxidation of Amines to Aldehydes or Imines
Laccases belong to the multi-copper family of oxi- the possibility of a further oxidation of the aldehyde
dases (EC 1.10.3.2); they contain four copper centres to the corresponding para-methoxybenzoic acid
per protein molecule and catalyse the oxidation of (3a),[19] but only the aldehyde was recovered (Table 1,
electron-rich aromatic substrates, usually phenols or entry 3). Use of a lower amount of the enzyme was
aromatic amines using oxygen as the electron accept- evaluated and it was observed that the efficiency of
or.[15] Water being the only by-product, laccases are the bio-oxidation was maintained until 0.1 mg (1 U)
ideal catalysts for sustainable chemical and technolog- of Laccase Tv, (Table 1, entry 6), even if after 24 h the
ical processes. In fact, they can be used in organic reaction was not complete (conversion 83%) and
synthesis, have industrial applications, and offer great a considerable amount of the imine 4a was isolated
applications in environmental biotechnology.[16]
after work-up (Table 1, entry 6). On extending the re-
Although the natural substrates of laccases are phe- action time to 7 days aldehyde 2a was quantitatively
nolic residues of lignin, the use of mediators in the recovered (Table 1, entry 7). Our attention was then
laccase-mediator system (LMS) makes accessible the focused on the aqueous reaction medium, and we ob-
oxidation of non-phenolic substrates.[17]
served that on lowering the concentration of acetate
Application of LMS in the bio-oxidation of alcohols buffer from 0.5M to 0.2M, a complete conversion
is well documented in the literature.[18] Recently, we was reached in a shorter reaction time (Table 1,
reported an application of LMS in the oxidation of entry 8 versus 2). To stoichiometrically buffer the
some primary alcohols to the corresponding alde- amine basicity and use the lowest amount of acetate,
hydes and carboxylic acids. Moreover, we succeeded unbuffered H2O with 1 equiv. of acetic acid as addi-
in a relevant application, developing the LMS-oxida- tive was tested on 1a, and an efficient oxidation to al-
tion of 2-arylpropanols (profenols) to the correspond- dehyde 2a was achieved (Table 1, entry 9). Under
ing 2-arylpropionic acids (profens), in high yields and these conditions amine and acetic acid formed the
with a complete retention of configuration.[19] Con- corresponding ammonium salt and the amount of free
cerning the oxidation of amines only few applications amine in the aqueous solution depends on the hydrol-
of laccases were reported, mainly on anilines.[20]
ysis constant of the salt. On consuming the free amine
Herein we describe a selective oxidation of amines by oxidation, the pH of the reaction solution is low-
employing laccase from Trametes versicolor (Laccase ered by the increasing release of acetic acid from the
Tv) as the enzyme, TEMPO (2,2,6,6-tetramethylpiper- salt hydrolysis. At neutral pH, phosphate buffer pH 7,
idine 1-oxyl radical) as mediator and O2 as oxidant, in conversions of 50% and 83% were reached in 24 and
buffered water as solvent. We found that, depending 48 h, respectively. It is interesting to note that at this
on the reaction conditions, the bio-oxidation could be pH a complete selectivity towards the imine 4a was
selectively driven to give the corresponding aldehydes obtained after work-up (Table 1, entries 10–12).
or imines in good yields.
TEMPO, the redox mediator in the oxidation with
laccase, is needed in sub-stoichiometric amounts be-
cause laccases constantly restore the oxamonium ion
responsible for the oxidation of the substrate.[22] We
then tested the reaction on lowering the amount of
Results and Discussion
The bio-oxidation of amines was initially investigated TEMPO: from 20 to 10 mol% at pH 4.5 the reaction
starting from the reaction conditions optimized for al- proceeded well with complete conversion and total
cohol oxidation, as we previously reported:[19] Laccase product selectivity towards the aldehyde, whereas
Tv (Sigma–Aldrich, 5 mg, 50 units), 2,2,6,6-tetrame- using 5 and 2.5 mol% lower conversion and selectivity
thylpiperidine 1-oxyl (TEMPO; free radical) 20 mol% were obtained (Table 1, entries 13, 14 and 17). On
in water (6 mL) at room temperature with O2 bubbled lowering the TEMPO mol% at pH 7, the reaction
into the reaction vessel for 30 sec. In a preliminary at- was slowed down and the conversion was poor but
tempt para-methoxybenzylamine (1a, 0.5 mmol), after the work-up only the imine 4a was recovered
chosen as a model substrate, did not react (Table 1, (Table 1, entries 15, and 16). On extending the reac-
entry 1). The use of unbuffered water as reaction tion time to 4 days with TEMPO 2.5 mol%, the con-
medium, which gave good results with alcohols,[19] in version was complete and the imine 4a was isolated in
this case failed maybe due to inactivation of Laccase 95% yields (Table 1, entry 18).
Tv at the basic pH generated by amine dissolution in
Under standard conditions benzylamine 1b in 24 h
H2O.[21] The use of acetate buffer at pH 4.5 then re- (TEMPO 20 mol% and acetate buffer, Table 1,
sulted in a successful reaction and 1a was quantita- entry 19) gave quantitatively benzaldehyde 2b, on ex-
tively converted in the corresponding aldehyde 2a in tending the reaction time to 10 days, as well as on
24 h (Table 1, entry 2). The product 2a was easily iso- lowering the buffer concentration to 0.2M, considera-
lated in quantitative yields from the reaction mixture ble amounts of benzoic acid were detected in the re-
by a simple solvent extraction (see Experimental Sec- action mixture (Table 1, entries 20 and 21).
tion). We extended the reaction time to 7 days to test
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Table 1. Optimization of reaction conditions.[a]
Entry Substrate Buffer, pH, conc.
Enzyme TEMPO Time[b] Conversion Selectivity Product
[mg]
[mol%]
[%][c]
2/3/4[d]
(Yield [%])[e]
1
2
3
4
5
6
7
8
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1a
1b
1b
1b
1b
1b
1b
1b
1a
1a
unbuffered water, r.t.
acetate, pH 4.5, 0.5M
acetate, pH 4.5, 0.5M
acetate, pH 4.5, 0.5M
acetate, pH 4.5, 0.5M
acetate, pH 4.5, 0.5M
acetate, pH 4.5, 0.5M
acetate, pH 4.5, 0.2M
1 equiv. acetic acid
phosphate, pH 7, 0.5M
phosphate, pH 7,0.5M
phosphate, pH 7, 0.5M
acetate, pH 4.5, 0.5M
acetate, pH 4.5, 0.5M
phosphate, pH 7, 0.5M
phosphate, pH 7, 0.5M
acetate, pH 4.5, 0.5M
phosphate, pH 7, 0.5M
acetate, pH 4.5, 0.5M
acetate, pH 4.5, 0.5M
acetate, pH 4.5, 0.2M
acetate, pH 4.5, 0.5M
acetate, pH 4.5, 0.5M, 508C
phosphate, pH 7,0.5M
phosphate, pH 7,0.5M
acetate, pH 4.5, 0.5M
phosphate, pH 7, 0.5M
5
5
5
2.5
1
0.1
0.1
5
5
5
5
0.1
5
5
5
5
5
5
5
5
5
5
5
5
5
–
–
20
20
20
20
20
20
20
20
20
20
20
20
10
5
8 d
24 h
7 d
24 h
24 h
24 h
7 d
3.5 h
2.5 h
24 h
48 h
24 h
24 h
24 h
24 h
24 h
24 h
4 d
24 h
10 d
10 d
3 d
10 d
2 d
0
–
–
>99
>99
>99
>99
83
>99
>99
>99
50
83
15
>99
62
45
100/0/0
100/0/0
100/0/0
100/0/0
60/0/40
100/0/0
100/0/0
100/0/0
0/0/100
0/0/100
0/0/100
100/0/0
60/0/40
0/0/100
0/0/100
80/0/20
0/<5/95
100/0/0
57/43/0
61/39/0
75/0/25
100/0/0
0/0/100
0/<5/95
–
2a (>99)
2a (>99)
2a (>99)
2a (>99)
–
2a (96)
2a (95)
2a (>99)
4a (32)
4a (80)
4a (8)
2a (>99)
–
4a (24)
4a (20)
–
4a (95)
2b (98)
–
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
5
2.5
2.5
2.5
20
20
20
2.5
20
2.5
2.5
20
20
35
44
>99
>99
>99
>99
90
>99
40
>99
0
–
–
2b (>99)
4b (36)
4b (92)
–
4 d
4 d
7 d
0
–
–
[a]
[b]
[c]
[d]
[e]
Reaction conditions: Laccase Tv, substrate (0.5 mmol), aqueous buffer (6 mL), bubbled O2 in closed vial.
Reaction time reported in days (d) or hours (h).
Conversion determined on the crude reaction mixture after the work-up.
Ratio between aldehyde, acid, and imine has been evaluated by 1H NMR (see Experimental Section).
Yields determined on the crude by 1H NMR after solvent extraction and evaporation (see Experimental Section).
To force the formation of the carboxylic acid, the
Therefore, we performed time course experiments
reaction was kept at 508C for longer times but only for the oxidation of amine 1a under three different
the aldehyde was recovered (Table 1, entry 23). At medium conditions: acetate buffer pH 4.5, 0.5M and
pH 7 and TEMPO 2.5 mol%, conversion and selectiv- 0.2M, or H2O with 1 equivalent of acetic acid as addi-
ity towards the imine 4b were complete, and only tive (Figure 2). The reaction mixture was analysed via
traces of a further oxidation to benzoic acid were ob- HPLC by sampling until complete conversions.
served after 4 days (Table 1, entries 24 and 25). As
In acetate buffer 0.5M the reaction required 24 h
control experiments the reaction was conducted in to give the aldehyde 2a, whereas on decreasing the
the absence of laccase, with TEMPO 20 mol% and O2 buffer concentration (0.2M) or in the presence of
at pH 4.5 and pH 7 (Table 1, entries 26 and 27), as ex- acetic acid in H2O the reaction was faster and com-
pected the oxidation did not proceed.
plete in 3–4 h. It is indeed likely that a high buffer
From the initial screening, pH conditions emerged concentration could be detrimental, to some extent,
as a parameter to control kinetic and selectivity of the for TEMPO oxidation or for solubility of reactants in
bio-oxidation.
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Selective Oxidation of Amines to Aldehydes or Imines
1
2a appeared. Figure 3 reports a time course H NMR
analysis performed in D2O with 1 equivalent of acetic
acid as additive. In detail, in an NMR tube, 1a
(0.05 mmol), D2O (0.6 mL) and acetic acid
(0.05 mmol) were mixed, then TEMPO (0.01 mmol)
and Laccase Tv (0.5 mg, 5 U) were added and finally
O2 was bubbled for 30 seconds, the spectrum was re-
corded (upper spectrum, Figure 3), and the NMR
tube was then stirred on an orbital shaker at 150 rpm
at 308C. After 60 min the tube was re-analysed and
a second spectrum was obtained (Figure 3). The
NMR analysis showed that from the very beginning
1
together with H signals of the starting amine (a–d),
traces of the aldehyde 2a appeared (signals e–h).
After 60 min the aldehyde was as expected much
more abundant (about 30%) and noteworthy
1H NMR analysis showed that during the reaction
course no other detectable intermediates or by-prod-
ucts were present in the reaction mixture.
Figure 2. Time course of enzymatic oxidation: amine 1a con-
sumption (filled tag), and aldehyde 2a formation (empty
tag) depending on buffer concentration or additive.
the aqueous medium and that this resulted in a wor-
sening of the whole process.
With a similar NMR protocol, we analysed the re-
action under the conditions to selectively get the
The bio-oxidation reaction is quite clean and its imine. Amine 1a, Laccase Tv/TEMPO in buffer phos-
1
progress could be easily monitored by H NMR spec- phate at pH 7 in D2O were mixed, then O2 was bub-
troscopy: as the amine 1a was consumed the aldehyde bled for 30 seconds.
Figure 3. 1H NMR time course analysis for Laccase Tv/TEMPO oxidation of amine 1a into aldehyde 2a in D2O and 1 equiv.
of AcOH as additive. Upper spectrum: reaction mixture at t=2 min, signals of starting amine 1a tagged with a, b, c, d;
bottom spectrum: reaction mixture at 60 min, signals of aldehyde 2a appear, tagged as e, f, g, h.
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Paola Galletti et al.
Figure 4. 1H NMR time course analysis for Laccase Tv/TEMPO oxidation of amine 1a in deuterated phosphate buffer
(pH 7) in D2O in an NMR tube. Upper spectrum: initial time, signals of starting amine 1a, tagged as a, b; central spectrum:
reaction mixture after 4 days, a mixture of amine 1a and aldehyde 2a is present (no imine signals), aldehyde signals tagged
as c, d, e; bottom spectrum: reaction mixture after work-up, only signals of imine 4a are present, tagged as f, g, h, i, l.
On direct sampling and NMR analysis of the reac-
tion mixture, after 2 min, only the starting amine 1a
was detected (Figure 4), after 4 days we found a 1:1
mixture of amine 1a and aldehyde 2a and no imine
present, while after the work-up, imine 4a was the
only product detected (Figure 4). As a control experi-
ment, equimolar amounts of the commercially avail-
able aldehyde 2a and the amine 1a were mixed in
buffer phosphate at pH 7 in D2O but in 24 h no traces
of the corresponding imine 4a were detected. It could
be then concluded that the imine is not a reaction in-
Figure 5. Proposed mechanism for the bio-oxidation of ben-
zylamines by Laccase Tv and TEMPO.
termediate and its quantitative obtainment occurred
during work-up (solvent extraction and evaporation)
when the starting amine underwent condensation with
the aldehyde just formed in the bio-oxidation. It was amine supported by observation of an easier oxida-
already reported in the literature that a direct mixing tion of benzylamines than alkylamines, together with
of aldehyde 2a and amine 1a without solvent (neat) ammonia elimination to give the aldehyde.
or in methanol gave the corresponding imine without
If the medium conditions slow down the reaction
any catalyst.[23] Having explored the reaction condi- rate at a 50% conversion in the work-up procedure
tions for amines 1a and 1b, the optimal conditions to condensation of the residual starting amine with the
selectively obtain aldehydes (2a, b) or imines (4a, b) aldehyde gave the imine.
were then established as follows: buffer acetate
Noteworthy, the aerobic oxidative homo-coupling
pH 4.5 at shorter reaction time for obtaining alde- of amines to imines is going to attract great attention
hydes, and eventually carboxylic acids at longer reac- as a valuable alternative to the traditional amine–car-
tion time, buffer phosphate pH 7 for obtaining imines. bonyl condensation,[7a,24] and this is the first result on
Concerning the mechanism of the amine oxidation such an oxidation obtained with enzymatic catalysis.
by laccase-TEMPO, a tentative hypothesis could be
formulated starting from the ionic route proposed for tion, the scope of the reaction was extended to
LMS oxidation of alcohols (Figure 5).[19]
a series of primary amines (Table 2), under the opti-
To study the applicability of the amine bio-oxida-
The effectiveness of the process could depend on mized conditions for a controlled product selectivity.
the facility of hydrogen abstraction on the starting Octylamine (1c), as a model of aliphatic amines, did
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Selective Oxidation of Amines to Aldehydes or Imines
Table 2. Bio-oxidation of primary amines.[a]
Entry Substrate
Buffer, pH, conc.
TEMPO Time[b] Conv. Selectivity Product
[%]
[%][c] 2/3/4[d]
(Yield [%])[e]
1
2
3
4
5
6
7
8
octylamine (1c)
acetate, pH 4.5, 0.5M 20
acetate, pH 4.5, 0.5M 20
acetate, pH 4.5, 0.5M 20
acetate, pH 4.5, 0.5M 20
acetate, pH 4.5, 0.5M 20
acetate, pH 4.5, 0.2M 20
phosphate, pH 7, 0.5M 5
acetate, pH 4.5, 0.5M 20
acetate, pH 4.5, 0.5M 20
acetate, pH 4.5, 0.2M 20
phosphate, pH 7, 0.5M 5
acetate, pH 4.5, 0.5M 20
acetate, pH 4.5, 0.5M 20
acetate, pH 4.5, 0.2M 20
7 d
24 h
7 d
24 h
7 d
10 d
7 d
24 h
7 d
10 d
5 d
24 h
7 d
10 d
7 d
7 d
24 h
4 d
24 h
7 d
10 d
7 d
5 d
24 h
7 d
10 d
7 d
24 h
4 d
0
>99
–
–
–
–
–
–
–
[f]
3,4-dihydroxybenzylamine (1d)
pyridine-3yl-methanamine (1e)
2-methoxybenzylamine (1f)
2-methoxybenzylamine (1f)
2-methoxybenzylamine (1f)
2-methoxybenzylamine (1f)
3,4-dimethoxybenzylamine (1g)
3,4-dimethoxybenzylamine (1g)
3,4-dimethoxybenzylamine (1g)
3,4-dimethoxybenzylamine (1g)
para-methylbenzylamine (1h)
para-methylbenzylamine (1h)
para-methylbenzylamine (1h)
para-methylbenzylamine (1h)
para-methylbenzylamine (1h)
>99 100/0/0
>99 90/10/0
>99 61/39/0
>99 <5/0/95
>99 100/0/0
>99 88/12/0
>99 61/39/0
>99 0/<5/95
>99 100/0/0
>99 77/23/0
>99 43/57/0
>99 66/34/0
2f (98)
–
–
4f (95)
2g (98)
–
–
4g (80)
2h (95)
–
–
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
1 equiv acetic acid
20
–
phosphate, pH 7, 0.5M 5
57
>99 57/0/43
59 0/0/100
0/0/100
4h (44)
–
4i (44)
2j (99)
–
3j (88)
–
4j (92)
2k (97)
–
3,5-bis(trifluoromethyl)benzylamine (1i) acetate, pH 4.5, 0.5M 20
3,5-bis(trifluoromethyl)benzylamine (1i) phosphate, pH 7, 0.5M 5
ortho-chlorobenzylamine (1j)
ortho-chlorobenzylamine (1j)
ortho-chlorobenzylamine (1j)
ortho-chlorobenzylamine (1j)
ortho-chlorobenzylamine (1j)
para-fluorobenzylamine (1k)
para-fluorobenzylamine (1k)
para-fluorobenzylamine (1k)
para-fluorobenzylamine (1k)
para-nitrobenzylamine (1l)
para-nitrobenzylamine (1l)
acetate, pH 4.5, 0.5M 20
acetate, pH 4.5, 0.5M 20
acetate, pH 4.5, 0.2M 20
>99 100/0/0
>99 50/50/0
>99 <5/95/0
>99 10/90/0
>99 <5/0/95
>99 100/0/0
>99 75/25/0
>99 59/41/0
1 equiv. acetic acid
20
phosphate, pH 7, 0.5M 5
acetate, pH 4.5, 0.5M 20
acetate, pH 4.5, 0.5M 20
acetate, pH 4.5, 0.2M 20
phosphate, pH 7, 0.5M 5
acetate, pH 4.5, 0.5M 20
phosphate, pH 7, 0.5M 5
–
94
>99 50/0/50
50 0/0/100
0/0/100
4k (92)
–
4l (36)
[a]
[b]
[c]
[d]
[e]
[f]
Reaction conditions: substrate (0.5 mmol), aqueous buffer (6 mL), enzyme (5 mg), bubbled O2 in closed vial.
Reaction time reported in days (d) or hours (h).
Conversions were evaluated on crude after work-up.
Ratio between aldehyde, acid, and imine has been evaluated by 1H NMR (see Experimental Section).
Yields determined on isolated products (see Experimental Section).
Polymerization products.
not react (Table 2, entry 1). This result is indeed con- in good yields depending on the selected reaction
sistent with the proposed mechanism of a laccase-me- conditions. Electronic effects associated with electron-
diator system (LMS), which supported an ionic hydro- donating and electron-withdrawing substituents on
gen abstraction route for oxidation with TEMPO[25,17] the phenyl ring have little effect on the efficiency of
(Figure 5): in this case the acidity of the a-proton of the oxidation reaction. The amines 3,5-bis(trifluoro-
1c was not adequate for an efficient hydrogen abstrac- methyl)benzylamine (1i) and p-nitrobenzylamine (1l)
tion. On the other hand dihydroxybenzylamine 1d at pH 4.5 gave a 1:1 mixture of the corresponding al-
and pyridine-3-yl-methanamine 1e were completely dehydes and imines (Table 2, entries 17 and 28),
converted but in a complex mixture of by-products whereas at pH 7 the selectivity for the imines 4i and
(Table 2, entries 2 and 3). Actually, laccases are well 4l was successfully obtained, albeit with lower yields
known to oxidize phenolic compounds, which are (Table 2, entries 18 and 29). Product selectivity to-
their natural substrates in lignin, or anilines.[20b]
wards the carboxylic acids was obtained with buffer
Most of the benzylamines 1f–1l were selectively acetate 0.2M and a significant amount of acid was ob-
converted into the corresponding aldehydes or imines tained with most benzylamines and, remarkably,
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Paola Galletti et al.
Scheme 1. Oxidation of a-substituted benzylamines with
Laccase Tv-TEMPO to give ketones.
Scheme 3. Oxidation of N-benzylamino esters with Laccase
Tv-TEMPO.
Scheme 2. Oxidation of secondary dibenzylamines with Lac-
case Tv-TEMPO to give aldehydes.
ortho-chlorobenzylamine (1j) in 10 days was convert-
ed in the corresponding acid 3j in 88% yield (Table 2,
entry 21).
We also applied the laccase/TEMPO system in the
oxidation of a-substituted benzylamines such as 1-ary-
lethylamines 5a, b (Scheme 1): in both cases the cor-
responding acetophenones 6a, b were obtained in
good conversions and yields. The bio-oxidation was
successful also with secondary benzylamines 7 and 8,
Scheme 4. Oxidation of isoindoline 11 and tetrahydroisoqui-
noline 13 with Laccase Tv-TEMPO.
as reported in Scheme 2. Both symmetrical or unsym- lone 12 was the only product obtained. In the case of
metrical substituted benzylamines were readily oxi- tetrahydroisoquinoline 13, conversion was complete
dized to the corresponding aldehydes.
in 7 days, but in the reaction mixture, after the work-
The easy bio-oxidation of benzylamines suggested up, three oxidation products were detected: among
a tentative exploration in selective oxidation of un- them 3,4-dihydro-isoquinoline 14 was the main com-
symmetrical secondary amines such as N-benzylamino ponent, as detected by NMR on crude.
esters in which the oxidation resulted in the elimina-
tion of the benzyl groups as in deprotection steps
(Scheme 3). Two substrates were tested, the N-benzyl- Conclusions
valine methyl ester 9 and the N-benzyl beta-alanine
ethyl ester 10. Notwithstanding a total conversion of Development of more benign and selective redox pro-
the starting material, products were difficulty recov- cesses is an urgent need to get more sustainable
ered as ammonium salt (9a) or after derivatization as chemical transformations, especially at the industrial
tert-butyloxycarbonylamino derivative (10a).
level. In this context the use of commercially avail-
Finally, two heterocyclic amine, 2,3-dihydroisoin- able enzymes such as oxidases, could represent a val-
dole 11 and tetrahydroisoquinoline 13, were tested uable greener alternative. We found that Laccase Tv-
(Scheme 4). Oxidation of dihydroisoindole 11 was not TEMPO is an effective catalytic system for the aero-
efficient, giving poor conversion and yields after 7 bic oxidation of benzylamines in aqueous medium.
days of reaction, but was selective: dihydroisoindo- The product selectivity toward aldehydes or imines
1846
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Adv. Synth. Catal. 2015, 357, 1840 – 1848

FULL PAPERS
Selective Oxidation of Amines to Aldehydes or Imines
ganic phases were dried over Na2SO4, filtered, concentrated
depends on the reaction conditions and specifically
from pH of buffer solution. Under optimized reaction
conditions, an efficient and practical bio-oxidation of
a series of primary, secondary and cyclic amines was
developed.
1
under vacuum, and analysed by H NMR and 13C NMR (see
the Supporting Information).
Acknowledgements
Experimental Section
We are grateful to the University of Bologna for financial
support and to CINMPIS (Consorzio Interuniversitario Na-
zionale Metodologie e Processi Innovativi di Sintesi) for
a grant to F.F. ; Dr. M. Pori, Mr. A Ballardini and Mrs. Giulia
Martelli are also acknowledged for technical assistance and
useful discussions.
General
Commercial reagents were used as received without addi-
1
tional purification. H and 13C NMR spectra were recorded
with an INOVA 400 instrument with a 5 mm probe. TLC:
Merck 60 F254 plates. HPLC-MS: Agilent Technologies
HP1100 instrument, equipped with a ZORBAX-Eclipse
XDB-C8 Agilent Technologies column; mobile phase: H2O/
CH3CN, 0.4 mLminÀ1, gradient from 30 to 80% of CH3CN
in 8 min, 80% of CH3CN until 25 min, coupled with an Agi-
lent Technologies MSD1100 single-quadrupole mass spec-
trometer, full scan mode from m/z=50 to 2600, scan time
0.1 s in positive ion mode, ESI spray voltage 4500 V, nitro-
gen gas 35 psi, drying gas flow 11.5 mLminÀ1, fragmentor
voltage 20 V. Starting amines 1a–1l, 5a, b, 7, 11, 13, laccase
from Trametes versicolor and TEMPO were purchased from
Sigma–Aldrich (Sigma 51639, 10 U/mg), benzylamines 8, 9
and 10 were prepared by alkylation with benzyl bromide
(see the Supporting Information for details). All obtained
products were known and their spectroscopic data (see the
Supporting Information for details) were consistent with
those reported in the literature and in NMR databases
(Reaxys and AIST SDBS).
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Adv. Synth. Catal. 2015, 357, 1840 – 1848
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
asc.wiley-vch.de
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ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2015, 357, 1840 – 1848