Oxidative N-debenzylation of N-benzyl-N-substituted benzylamines catalyzed by horseradish peroxidase

Article abstract of DOI:10.1002/poc.648

A report on the oxidative N-debenzylation of N-benyl-N-substituted benzylamines catalyzed by horseradish peroxidase was presented. A solution of benzylamine in benzene was added to a benzene solution of p-anisaldehyde in 100 ml flask over 10 minutes. Expulsion of proton and hydroxylation yielding α-hydroxylamines were followed by the formation of benzaldehydes and benzylamines.

Full text of DOI:10.1002/poc.648

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2003; 16: 555–558
Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/poc.648
Oxidative N-debenzylation of N-benzyl-N-substituted
benzylamines catalyzed by horseradish peroxidase†
Sung Soo Kim* and Hwan Kyu Jung
Department of Chemistry and Center for Chemical Dynamics, Inha University, Incheon 402-751, South Korea
Received 10 September 2003; revised 15 January 2003; accepted 13 March 2003
ABSTRACT: N-Benzyl-N-substituted benzylamines and compound I of horseradish peroxidase engender electron
transfer yielding the corresponding nitrogen radical cation 1, which is simultaneously converted into 2 and 3.
Subsequently, expulsion of proton and hydroxylation yielding a-hydroxylamines are followed by the formation of
benzaldehydes and benzylamines. Copyright  2003 John Wiley & Sons, Ltd.
KEYWORDS: oxidation; N-debenzylation; horseradish peroxidase; hydrogen peroxide; amines
INTRODUCTION
were observed to be kH/kD = 1, the values being constant
with variation of Y from p-OMe to p-NO2.
The mechanism of the N-dealkylation of N-dimethyl-
anilines catalyzed by heme enzymes and their analogs
has attracted considerable attention.1–17 Iodosylbenzene
(C6H5IO) catalyzed by tetraphenylporphyrinatoiron(III)
chloride [Fe(III)TPPCl]12 oxidizes N,N-dimethylbenzyl-
amines by an initial electron transfer (ET) process. The
reactions indicate a small negative r value (ꢀ = À0.41)
for Fe(III)TPPCl and marginal intermolecular kinetic
isotope effect (KIE), kH/kD = 1.3 with PhCH2NMe2 and
PhCD2NMe2. The KIE and Hammett correlations in the
oxidative N-demethylation of N,N-dimethylanilines cat-
alyzed by tetrakis(pentafluorophenyl)porphyrinatoiron
(III) chloride16 were investigated. The intramolecular
KIE of 4-Y-N-methyl-N-trideuteriomethylanilines are
much larger than intermolecular KIE with 4-Y-N,N-
dimethylanilines and 4-Y-N,N-ditrideuteriomethylani-
lines. The Hammett correlations give rise to better
RESULTS AND DISCUSSION
The reactions are shown in Schemes 1 and 2. The
competitive intramolecular N-debenzylation of N-ben-
zyl-N-substitutedbenzylamines with HRP–H2O2 has been
studied through Hammett correlations and KIE. The
relative rates caused by substituents (Y = p-OCH3, p-
CH3, H, p-Cl, m-Cl, p-CN and p-NO2) were obtained
from the [YC6H4CHO]/[C6H5CHO] ratios. The log(kY/
‡
kH) values were plotted against s and s to yield better
‡
correlation with ꢁ (ꢀ = À0.76; r = 0.997) than with ꢁ
(ꢀ = À0.51, r = 0.965).
‡
correlations with ꢁ (r = 0.995) than with ꢁ (r = 0.993).
The KIE profiles (plot of kH/kD vs the pKa of the aniline
radical cations) by lignin peroxidase–H2O2 and
5,10,15,20-tetraphenyl-21H,23H-porphine-p, p', p@, p@'-
tetrasulfonic acid iron(III) chloride–H2O2 for a number of
Scheme 1
ring-substituted
N,N-bis(dideuteriomethyl)anilines17
show bell-shaped curves. The intermolecular KIE of 4-
Y-N,N-dimethylanilines and 4-Y-N,N-trideuteriomethyl-
anilines by horseradish peroxidase (HRP) compound I15
Scheme 2
*Correspondence to: S. S. Kim, Department of Chemistry and Center
for Chemical Dynamics, Inha University, Incheon 402-751, South
Korea.
The Hammett correlations for the oxidation of N,N-
‡
E-mail: sungsoo@inha.ac.kr
dimethylanilines (ꢀ = À0.88)16 may suggest that the
†This paper is dedicated to Professor Shinjiro Kobayashi on the
occasion of his retirement.
Contract/grant sponsor: Korea Science and Engineering Foundation;
Contract/grant number: F 01-2000-6-123-01-2.
electron transfer step for the formation of radical cation is
involved with rate-determining step. The negative sign of
‡
ꢀ = À0.88 is also parallel with their oxidation potentials
Copyright  2003 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2003; 16: 555–558

556
S. S. KIM AND H. K. JUNG
Table 1. Kinetic data for oxidative N-debenzylations of N-benzyl-N-substituted benzylamines by HRP
p-OCH3
1.64
3.43 Æ 0.3
p-CH3
p-H
p-Cl
m-Cl
0.491
1.17 Æ 0.02
p-CN
kYH/kHH
kYH/kYD
1.30
1
0.609
0.310
‡
ꢀ (r) = À0.76 (0.997); ꢀ (r) = À0.51 (0.965)
1.34 Æ 0.06
decreasing from p-NO2 to p-OCH3. The better correlation
with s in Table 1 indicates that positive charge is
localized on the nitrogen atom. Compound 1 can be
simultaneously transformed into either 2 or 3 (Scheme 3)
since both of them are more stable than 1. The
the substituent gradually approaches electron donating,
m-Cl (kH/kD = 1.17), H (kH/kD = 1.34), p-OCH3 (kH/kD
= 3.34).
16
intramolecular KIE values for 4-Y-C6H4N(CH3)CD3
are distinct and increase from p-NO2 (kH/kD = 2.0) to p-
OCH3 (kH/kD = 3.0). This increasing trend parallels the
magnitude of pKa of the corresponding radical cation, and
suggests that there is a significant reverse electron
transfer which competes with the a-deprotonation. The
KIE for p-OCH3, kH/kD = 3.43 in Table 1, shows a similar
situation for the reversibility. In contrast, when electron
transfer is the rate-determining step, no such KIE would
be observed, that is, kH/kD = 1. The intermolecular KIE of
Y-C6H4N(CH3)2 and Y-C6H4N(CD3)2 utilizing horse-
radish peroxidase compound I15 shows an absence of
KIE. Our KIE for m-Cl, kH/kD = 1.17, may indicate that
reverse electron transfer occurs to a small extent.
Therefore, the degree of the reversibility increases as
CONCLUSIONS
N-Demethylation of DMA proceed through the formation
of the radical cation Y-C6H4N(CH3)2 The reversible
+
Á
+
Á
formation of YC6H4N(CH3)2 should be influenced by the
substituent(Y) and the kind of oxidant. The reversibility
of formation of our radical cation, YC6H4CH—NH2—
CH2C6H5, by HRP compound 1 increases as the
substituent (Y) becomes more electron donating.
+
Á
EXPERIMENTAL
Materials and methods. Benzylamine, substituted ben-
zaldehydes (Y = p-OCH3, p-CH3, H, p-Cl, m-Cl, p-CN
and p-NO2), substituted benzonitriles (Y = p-OCH3 and
m-Cl), LiAlD4 and other reagents were commercial
products. Horseradish peroxidase was of type VI and was
obtained from Sigma. A Varian Gemini 2000 NMR
spectrometer was used for the identification of the
compounds. Relative quantities of the aldehydes were
obtained using a Varian 3300 gas chromatograph with a
DB-1 column and a flame ionization detector.
Preparation of N-benzyl-N-4-methoxybenzylamine. A
solution of benzylamine (0.02 mmol) in benzene was
added to a benzene solution (15 ml) of p-anisaldehyde
(0.02 mmol) in 100 ml flask over 10 min. The mixture
was stirred for 3 h and Na2SO4 was added to eliminate
H2O. The formation of imine was confirmed when
evaporating benzene in a rotary evaporator. To the imine
dissolved in methanol in an ice-bath, 1.5 equiv of NaBH4
were added very slowly. The reaction mixture was stirred
at room temperature for 1 h. The solvent was evaporated
and the remainder was extracted with CH2Cl2–H2O. The
CH2Cl2 layer was dried with Na2SO4 to give the pure
product (4.34 g, 95% yield). NMR (CDCl3): ꢂ 3.8 (d, 7H,
CH3O, 2CH2), 6.9 (d, 2H, C6H4), 7.2 (d, 2H, C6H4), 7.3
(m, 5H, C6H5).
Other benzylamines were similarly synthesized and
their NMR data are given below.
Scheme 3
Copyright  2003 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2003; 16: 555–558

OXIDATIVE N-DEBENZYLATION OF SUBSTITUTED BENZYLAMINES
557
N-Benzyl-N-4-methylbenzylamine. NMR (CDCl3): ꢂ 2.4
(s, 3H, CH3), 3.8 (d, 4H, 2CH2), 7.2–7.4 (m, 9H, C6H5,
C6H4).
1000 ml) was incubated at 37°C for 30 min with vigorous
stirring. The reaction mixture was then cooled with an
ice-bath and 2 ml of 5% HCl solution were added to make
the salt of substituted benzylamines. 1,4-Dibromoben-
zene (0.02 mmol) was added to the reaction mixture as an
internal standard. CH2Cl2 (3 ml  3) was added to extract
the organic layer. This was dried with anhydrous Na2SO4
and concentrated to 20 ml for GC analysis.
N-Benzyl-N-4-chlorobenzylamine. NMR (CDCl3): ꢂ 3.8
(d, 4H, 2CH2), 7.2–7.4 (m, 9H, C6H5, C6H4).
N-Benzyl-N-3-chlorobenzylamine. NMR (CDCl3): ꢂ 3.8
(d, 4H, 2CH2), 7.2–7.4 (m, 9H, C6H5, C6H4).
Kinetic isotope effects. These were determined indi-
rectly as follows. kYH/kYD = kYH/kHHÁkHH/kYD was used
when Y = p-OCH3 and p-Cl, and kHH/kHD = kHH/kmÀCl
HÁkm-Cl H/kHD can be obtained when Y = H using m-
ClC6H4CH2 as an internal standard.
N-Benzyl-N-4-cyanobenzylamine. NMR (CDCl3): ꢂ 3.8
(d, 4H, 2CH2), 7.2–7.6 (m, 9H, C6H5, C6H4).
N-Benzyl-N-4-nitrobenzylamine. NMR (CDCl3): ꢂ 3.8
(d, 4H, 2CH2), 7.2–7.4 (m, 5H, C6H5), 7.5 (d, 2H, C6H4),
8.2 (d, 2H, C6H4).
Preparation of p-CH3OC6H4CD2NHCH2C6H5. 4-Meth-
oxybenzonitrile (0.03 mol) in 20 ml of THF was added
very slowly to an LiAlD4 (0.045 mol) solution of THF in
an ice-bath. The reaction mixture was then stirred for 1
day under nitrogen at room temperature. After reaction,
5% HCl solution was added slowly until the reaction
mixture became acidic. The aqueous layer was separated
by addition of benzene. To the aqueous layer, 3 M NaOH
solution was added to make a basic solution and the
amine layer was separated with benzene. 4-Methoxy(a,a-
dideuterio)benzylamine (0.023 mol, 77%) was then
obtained by evaporation of benzene. N-Benzylidene-4-
methoxy(a,a-dideuterio)benzylamine was prepared by
reaction
of 4-methoxy(a,a-dideuterio)benzylamine
(0.023 mol) and benzaldehyde (0.023 mol). This was
reduced with NaBH4 to give p-CH3OC6H4CD2
NHCH2C6H4 (4.98 g, 72% yield). NMR (CDCl3): ꢂ 3.8
(s, 5H, OCH3, CH2), 6.9 (d, 2H, C6H4), 7.2–7.4 (m, 7H,
C6H5, C6H4).
Other deuterated benzylamines were similarly pre-
pared and their NMR spectra are listed below.
Acknowledgements
We warmly thank the Korean Science and Engineering
Foundation for financial support through F 01-2000-6-
123-01-2.
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J. Phys. Org. Chem. 2003; 16: 555–558