Galactose oxidase model: Biomimetic enantiomer-differentiating oxidation of alcohols by a chiral copper complex

Article abstract of DOI:10.1002/chem.200802064

An enantiopure analogue of galactose oxidase (GO) was synthesized from an easily and readily available source with a similar activity of GO's in primary alcohol oxidation and enantioselective catalysts for aerobic oxidative kinetic resolution (AORK) of racemic secondary alcohols. Enantiopure (R)-binam was treated with Cu(OTf)₂ in toluene at room temperature for one hour to give a (R)-binam-Cu complex. It is found that enantiopure benzoin is an important intermediate in the synthesis of a potent anticancer agent and can been synthesized by enzymatic hydrolysis with benzaldehyde lyase (BAL). The S enantiomer of the racemate is oxidized faster to the corresponding benzil and the slow-reacting R enentiomer of benzoin is recovered in an enantiomerically enriched form. It is also observed that ortho substitution reduces the rate of reaction, while a reduced enentioselectivity is observed in case of ortho-substituted benzoin.

Full text of DOI:10.1002/chem.200802064

DOI: 10.1002/chem.200802064
Galactose Oxidase Model: Biomimetic Enantiomer-Differentiating Oxidation
of Alcohols by a Chiral Copper Complex
Santosh Kumar Alamsetti, Sreedevi Mannam, Pandi Mutupandi, and
Govindasamy Sekar*[a]
Dedicated to Professor Vinod K. Singh on the occasion of his 50th birthday
Galactose oxidase (GO) is a copper-containing fungal
enzyme that oxidizes alcohols to their corresponding alde-
hydes with concomitant reduction of molecular oxygen.[1] Its
crystal structure reveals a unique mononuclear copper site
with two nitrogen atoms (from histidine imidazole groups)
and two oxygen atoms (from tyrosine groups) as donor
atoms, plus an exogenous water or acetate molecule in a dis-
torted square-pyramidal coordination.[2] The GO enzyme
contains chiral copper in its active site that results from the
histidine and tyrosine residues (Y495 and Y272). This
enzyme has so far inspired several research groups to design
a variety of synthetic analogues.[3,4] However, all the ligands
in the GO models reported so far have been either achiral[3]
or racemic,[4] and it is important to mention that in most
cases the ligands employed are not readily available and
have to be synthesized by tedious multi-step syntheses.
amples of ubiquitous alcohol oxidation exist. Recently,
chiral Ru,[9] Pd,[10] Mn,[11] V,[12] and Ir[13] catalysts were devel-
oped for oxidative kinetic resolution to produce optically
active secondary alcohols. However, these metal salts are
either expensive or they provide poor selectivity. Also, the
chiral ligands are not readily available and must be arduous-
ly synthesized in most cases. On the other hand, to the best
of our knowledge chiral copper-catalyzed oxidation, which is
an economic, mild, and biomimetic functional model of the
mononuclear copper enzyme GO, has not been reported in
the literature to date.
There are several problems associated with the synthetic
analogues of GO reported so far. First, most of the ligands
are not readily available, so they require laborious multi-
step syntheses and rigorous reaction conditions to form
complexes with copper salts because phenolic groups do not
coordinate well with copper due to their inherent weak-field
ligand status in the spectrochemical series. This is a disad-
vantage of the relatively harder nature of oxygen donor li-
gands. Also, in the case of oxidative kinetic resolution,
chiral metal salts are either expensive[9–10,13] or they provide
poor selectivity.[11] To combat these facts, we aimed at syn-
thesizing an enantiopure analogue of GO from an easily and
readily available source to subsequently mimic GOꢀs activity
not only in primary alcohol oxidation but also as enantiose-
lective catalysts for AOKR of racemic secondary alcohols.
As a part of our ongoing research towards copper-catalyzed
oxidation chemistry,[14] we envisioned binam (1,1’-binaphth-
yl-2,2’-diamine) as a suitable ligand for our purpose owing
to its ready availability and relative ease of coordination
compared with other oxygen-based ligands.
Further, enantiopure alcohols are very important structur-
al units for the synthesis of a wide range of natural products,
chiral ligands, and biologically active compounds.[5] General-
ly, enantiopure secondary alcohols are synthesized by the
enzymatic kinetic resolution of racemic secondary alcohols
through acylation/deacylation reactions,[6] nonenzymatic
ACHTUNGTRENNUNG
kinetic resolution,[7] or enantioselective reduction of ke-
tones.[8] The aerobic oxidative kinetic resolution (AOKR) of
secondary alcohols is a feasible alternative and ranks high
amongst the easiest methods to synthesize enantiopure sec-
ondary alcohols. Although excellent catalytic methods are
available for the achiral alcohol oxidation process, it is sur-
prising that relatively very few catalytic enantioselective ex-
[a] S. K. Alamsetti, S. Mannam, P. Mutupandi, Dr. G. Sekar
Department of Chemistry
Enantiopure (R)-binam was treated with CuACTHNURGTNEUNG(OTf)2 in tol-
Indian Institute of Technology Madras
Chennai, Tamil Nadu, 600036 (India)
Fax : (+91)44-2257-4202
uene at room temperature for one hour to give a (R)-
binam–Cu complex. The X-ray crystal structure of this syn-
thetic complex is reported here (Figure 1). [Cu{(R)-
E-mail: gsekar@iitm.ac.in
binam}2]ACTHNUGTRNENG[U OTf]2 has a distorted octahedral geometry in which
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200802064.
CuII is coordinated by four nitrogen atoms (from the two
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Chem. Eur. J. 2009, 15, 1086 – 1090

COMMUNICATION
To mimic GO, oxidation of p-methoxybenzylic alcohol by
[Cu{(ꢀ)-binam}2]
ACHTUNGRTENN[UNG OTf]2 with 5 mol% of 2,2,6,6-tetramethyl-
piperidin-1-oxyl (TEMPO) in nitromethane under an O2 at-
mosphere was carried out and the reaction proceeded
smoothly at room temperature to give an isolated yield of
95% of the corresponding benzaldehyde (Table 1, entry 4).
Table 1. (ꢀ)-binam–Cu
ACHTUNGRTENUN(NG OTf)2-catalyzed oxidation of primary alcohols.
Entry Alcohol
Product
Time
[h]
Yield[a]
[%]
Figure 1. X-ray structure of the GO enzyme model, [CuII{(R)-binam}2],
(CCDC 677060) contains the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Ellipses were drawn at the 30% probability level). Ethyl acetate is in the
sixth position and shows a weak van der Waals interaction (2.566 ꢂ) with
the central copper.
1
2
3
4
5
6
7
8
C6H5CH2OH
C6H5CHO
22
24
25
20
32
22
19
17
71
71
60
95
81
86
50
84
o-NO2-C6H4CH2OH
p-Cl-C6H4CH2OH
p-OMe-C6H4CH2OH
p-NO2-C6H4CH2OH
m-NO2-C6H4CH2OH
m-OMe-C6H4CH2OH
3,4,5-(OMe)3-
o-NO2-C6H4CHO
p-Cl-C6H4CHO
p-OMe-C6H4CHO
p-NO2-C6H4CHO
m-NO2-C6H4CHO
m-OMe-C6H4CHO
3,4,5-(OMe)3-
C6H2CH2OH
C6H2CHO
(R)-binam molecules), one water molecule occupies an axial
position, and the other axial position is occupied by an eth-
ylacetate molecule through a weak van der Waals interac-
tion (the bond length between CuII and ethylacetate oxygen
is 2.566 ꢂ[15]). The complex crystallizes in a triclinic crystal
system with space group P1 and unit cell dimensions of a=
10.1523(4), b=13.6844(5), c=18.4478(9) ꢂ; a=111.692(2),
[a] Isolated yield.
The reaction is catalytic, only 5 mol% of [Cu{(ꢀ)-binam}2]-
[OTf]2 complex was required for the complete conversion of
AHCTUNGTRENNUNG
the alcohol. Here, molecular oxygen is used as the sole oxi-
dant and water is obtained as the ultimate byproduct. This
makes our process eco-friendly and green as well. To investi-
gate the scope of our catalyst, the oxidation reaction was ex-
tended to several other primary alcohols, which were oxi-
dized to the corresponding aldehydes at room temperature
and the results obtained are summarized in Table 1. Oxida-
tion here is very selective because over-oxidized product
(carboxylic acids) was not observed in any case.
b=96.544(3),
g=92.518(2)8;
Z=1,
cell
volume=
2355.60(17) ꢂ3. The solution-state EPR spectrum of (R)-
binam–Cu at 77K supports a distorted octahedral structure
because its gk value (2.17) is significantly larger than its g?
(1.99; see Figure 2).
Our main objective was to use our enantiopure GO
model as an efficient catalyst in the synthesis of biologically
important organic molecules through oxidation reactions.
Enantiopure benzoin (2-hydroxy-1,2-diphenylethanone) is
an important intermediate in the synthesis of a potent anti-
cancer agent[16] and has generally been synthesized by enzy-
matic hydrolysis with benzaldehyde lyase (BAL) or benzoyl-
formate decarboxylase (BFD) enzyme.[17] Other methods in-
clude enantioselective benzoin condensations by using poly-
cyclic triazolium salt or its analogues as catalysts, which
again has the drawback of having to be synthesized in many
steps.[18] By avoiding these problems, we have synthesized
enantiomerically enriched benzoins for the first time to the
best of our knowledge by nonenzymatic kinetic resolution
with [Cu
action.
At the outset, (ꢀ)-benzoin was subjected to AOKR with
5 mol% of enantiopure (R)-binam, 5 mol% of Cu(OTf)2,
ACHUTGTNRNE(NUG binam)2]ACHTUNGTREN[NUGN OTf]2 as the catalyst in the oxidation re-
ACHTUNGTRENNUNG
Figure 2. Solution-state EPR spectrum of [CuII{(R)-binam}2] in acetone at
and 5 mol% of TEMPO in toluene at 608C. In this reaction,
77K.
molecular oxygen is used as a stoichiometric oxidant. The
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G. Sekar et al.
Table 2. The AOKR of (ꢀ)-benzoins by [Cu{(R)-binam}2]
ACHTUNGTRENNUNG[OTf]2.
reaction proceeded well and gave isolated yields of 84% for
the ketone product and 15% for recovered benzoin, which
was obtained in 90% enantiomeric excess (ee) in 7 h. A
wide range of copper salts were screened with (R)-binam in
toluene, and (R)-binam–CuACHTNUTRGENNG(U OTf)2 turned out to be the com-
plex of choice for oxidative kinetic resolution because the
other copper salts gave poor results. Then the reaction was
screened in several solvents to obtain the recovered alcohol
with good yield and very high optical purity. Among the sol-
vents screened, toluene turned out to be the best solvent.
Next, we studied the effect of the ratio of (R)-binam and
Entry (ꢀ)-Benzoin
Time Benzil
[h]
Recovered benzoin
Yield[a] Yield[a] ee[b,c] Config.[d]
[%]
[%]
[%]
1
2
5
8
65
83
33
15
92
98
R
Cu
ACHTUNGTRENNUNG
binam and 5 mol% of CuAHCTUNGTRENNUNG
yields of 65% for benzil and 33% for recovered benzoin in
92% ee in 5 h (Scheme 1). This result clearly shows that an
3
4
5
0.5 63
28
25
29
98
93
84
R
R
R
12
14
65
60
6
144 68
25
28
R
R
7
8
10
12
60
71
37
20
76
95
Scheme 1. The AOKR of (ꢀ)-benzoin by (R)-binam–Cu.
9
10
4
6
61
77
34
17
86
98
R
optimised 2:1 ratio of (R)-binam and CuACTHNUTRGENUG(N OTf)2 is the appro-
priate combination for effective catalytic activity and this
ratio unequivocally corresponds to the ratio obtained from
our crystal structure of GO model. In all reactions in this
AOKR, the S enantiomer of racemic benzoin was oxidized
faster to the corresponding benzil and the slower-reacting R
enantiomer was recovered in a highly enantiomerically en-
riched form.
11
12
13
16
4
78
76
70
20
24
24
95
90
84
R
R
R
After optimizing the reaction conditions for the AOKR of
racemic benzoin, we initiated detailed investigations into
8
the scope of the [Cu{(R)-binam}2]ACTHNUTRGNEUNG[OTf]2-catalyzed AOKR
[a] Isolated yield. [b] The ee was determined by using HPLC on a chiral
column-Diacel ChiralPAK AS-H (see the Supporting Information).
[c] The data given are from single experiments. [d] The configuration of
recovered benzoins was determined by comparing the optical rotation
value with the literature value (see the Supporting Information).
reaction with other benzoins, and the results obtained are
summarized in Table 2. Again, the S enantiomer of the race-
mate was oxidized faster to the corresponding benzil and
the slow-reacting R enantiomer of benzoin was recovered in
an enantiomerically enriched form. The reactivity of the
benzoins in our AOKR process was increased if the benzoin
had an electron-withdrawing group, such as a chloro group
at the para position. If the benzoin had one chloro group it
took 4 h for a 70% conversion, whereas the presence of two
chloro groups reduced the conversion time to only 0.5 h
(Table 2, entries 1 vs. 3 vs. 12). On the contrary, the reactivi-
ty decreased when benzoins with electron-donating groups,
such as methyl, methoxy, and ethoxy groups at the para po-
sition (Table 2, entry 1 vs. 4, 5, and 7). However, if the elec-
tron-donating group was in the meta position of benzoin, the
decreased reactivity due to the para effect was suppressed
(Table 2, entries 4 vs. 9 vs. 1). We observed that ortho substi-
tution reduces the rate of reaction drastically and only in
the case of ortho-substituted benzoin was poor enantioselec-
tivity observed (Table 2, entry 6). We attribute this to the
steric hindrance created at the ortho position. The ee of re-
covered benzoins was determined by HPLC on a chiral sta-
tionary phase (see the Supporting Information for full de-
tails). Although the exact role of TEMPO in this AOKR is
unclear, we have a strong insight that TEMPO might act as
a hydrogen acceptor during the catalytic cycle. A detailed
mechanistic study is in progress.
Thus, we have developed for the first time an enantiopure
model of the GO enzyme from commercially available com-
pounds in a single step. This in situ-prepared GO model,
[Cu{(R)-binam}2]ACHTNUTRGNENUG[OTf]2, can be used directly as an efficient
catalyst for the oxidation of primary alcohols to the corre-
sponding aldehydes by using molecular oxygen as the sole
oxidant and with water being only byproduct. This enzyme
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Chem. Eur. J. 2009, 15, 1086 – 1090

Galactose Oxidase Model
COMMUNICATION
model has been extensively used for an enantioselective oxi-
dation process (AOKR) to synthesize highly enantiomeri-
cally enriched benzoins. This result demonstrates the first
chiral copper-catalyzed oxidative kinetic resolution of alco-
hols (benzoins) and the simplest method for the synthesis of
highly important chiral benzoins. We are continuing to ex-
plore the enormous potential of chiral binam–CuII in the
synthesis of other biologically important alcohol-containing
molecules, and mechanistic studies of AOKR and the de-
tailed results of these investigations will be reported in due
course.
Acknowledgements
This work was supported by the DST and CSIR, New Delhi, India. S.K.A
and S.M. thank CSIR, India and P.M. thanks UGC, India for research
AHCTUNGERTGfNNUN ellowships. We also thank Dr. Babu Varghese (SAIF IIT, Madras) for
XRD measurements.
Keywords: oxidation · chiral benzoins · enantioselectivity ·
enzyme models · kinetic resolution
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Experimental Section
General: Please see the Supporting Information for details of the materi-
als and equipment used in this study.
Typical experimental procedure for primary alcohol oxidation: A mixture
of (ꢀ)-binam (28.4 mg, 0.1 mmol) and Cu
ACHTUNGRTEN(NUNG OTf)2 (18.05 mg, 0.05 mmol)
in nitromethane (2 mL) was stirred at RT for 10 min, then TEMPO
(7.82 mg, 0.05 mmol) was added to the reaction mixture. After stirring
for 5 min, para-methoxybenzylACHTNUTRGNEaNUG lcohol (138 mg, 1 mmol) was added and
the mixture was stirred under an O2 atmosphere (by using an O2 balloon)
for 20 h at RT. After complete disappearance of para-methoxybenzylal-
cohol (monitored by TLC), the reaction mixture was concentrated under
vacuum and the resulting residue was purified by column chromatogra-
phy on silica gel (eluent hexanes/ethyl acetate) to obtain para-methoxy-
benzaldehyde as a colorless liquid (130 mg, 95%). Rf =0.67 (hexanes/
ethyl acetate, 80:20); 1H NMR (400 MHz, CDCl3): d=9.87 (s, 1H), 7.82
(d, J=8.8 Hz, 2H), 6.98 (d, J=8.4 Hz, 2H), 3.87 ppm (s, 3H); 13C NMR
(100 MHz, CDCl3): d=190.7, 164.6, 131.9, 129.9, 114.3, 55.6 ppm; IR
(neat): n˜ =2840, 2741, 1679 cmꢁ1; HRMS: m/z calcd for C8H9O2: 137.0603
[M+H]+; found: 137.0598.
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(28.4 mg, 0.1 mmol) and CuACHTUNTRGNEUNG(OTf)2 (18.05 mg, 0.05 mmol) in toluene
(2 mL) was stirred at RT for 10 min, then TEMPO (7.82 mg, 0.05 mmol)
was added to the reaction mixture. After stirring for 5 min, benzoin
(212 mg, 1 mmol) was added, then the mixture was heated to 608C under
an O2 atmosphere (by using an O2 balloon) for 5 h. After cooling to RT,
the reaction mixture was diluted with ethyl acetate and then washed with
dilute HCl followed by water. The organic layer was dried over sodium
sulfate, concentrated under vacuum, and the resulting residue was puri-
fied by silica gel column chromatography (eluent: hexanes/ethyl acetate)
to give the benzil (137 mg, 65%) and the recovered benzoin (70 mg,
33%).
Benzoin: Rf =0.49 (hexanes/ethylacetate, 80:20); [a]D25 =ꢁ76.0 (c=1 in
acetone); 1H NMR (400 MHz, CDCl3): d=7.92–7.96 (m, 2H), 7.52–7.57
(m, 1H), 7.39–7.44 (m, 2H), 7.34–7.37 (m, 4H), 7.27–7.33 (m, 1H), 5.98
(d, J=6 Hz, 1H), 4.58 ppm (d, J=5.6 Hz, 1H); 13C NMR (100 MHz,
CDCl3): d=199.1, 139.1, 134.0, 133.6, 129.3, 129.2, 128.8, 128.7, 127.9,
76.4 ppm; IR (neat): n˜ =3418, 1679, 1261, 1068 cmꢁ1; HRMS: m/z calcd
for C14H12O2Na1: 235.0735 [M+Na]+; found: 235.0727. The ee was deter-
mined to be 92% by using HPLC on a ChiralPAK AS-H column (5%
iPrOH/hexanes, 1 mLminꢁ1, 220 nm): minor retention time: 11.2 min,
major retention time: 18.1 min.
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Benzil: Rf =0.70 (hexanes/ethylacetate, 80:20); 1H NMR (400 MHz,
CDCl3): d=7.89–7.94 (m, 4H), 7.57–7.62 (m, 2H), 7.42–7.47 ppm (m,
4H); 13C NMR (100 MHz, CDCl3): d=194.7, 135.0, 133.3, 130.1,
[15] Procedure for obtaining a single crystal of [Cu{(R)-binam}2]
ACHTUNGTRENNUNG
A
mixture of (R)-binam (50 mg, 0.176 mmol) and CuACHTUNGTRENNUNG
129.2 ppm; IR (neat): n˜ =3064, 1656 cmꢁ1
C14H10O2Na1: 233.0578 [M+Na]+; found: 233.0585.
; HRMS (m/z): calcd for
(63.5 mg, 0.176 mmol) in toluene (3 mL) was stirred at RT for 1 h.
The solvent was removed by rotary evaporator and the resulting res-
idue was dissolved in ethyl acetate (2 mL). Single crystals of
[Cu{(R)-binam}2]ACTHNUTRGEN[UNG OTf]2 were obtained from this solution after 15 d
by using the diffusion method with toluene.
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Received: October 6, 2008
Published online: December 23, 2008
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Chem. Eur. J. 2009, 15, 1086 – 1090