Communications
Experimental Section
alkylimine
General procedure: Controlled-potential electrolysis was carried out
in a cylindrical, three-electrode, divided cell (9 cm diameter), using an
electronic potentiostat. In the main compartment, a platinum grid
(area = 60 cm2) served as the anode (working electrode). A platinum
sheet was placed in the concentric cathodic compartment (counter-
electrode), which was separated from the main compartment with a
glass frit. The reference electrode was an aqueous saturated calomel
electrode (SCE), which was isolated from the bulk solution in a glass
tube with a fine-porosity frit. The electrolyte solution (0.04 molLÀ1
lithium perchlorate or tetraethylammonium perchlorate in methanol)
was poured into the anodic and the cathodic compartments, as well as
into the glass tube that contained the SCE electrode. 3,4-Amino-
phenol[15] 1red (0.1 mmol) and an excess of primary aliphatic amine
(5mmol) were added to the solution in the main compartment
(250 mL), and the resulting solution was then oxidized under nitrogen
at room temperature at + 600 mV vs. SCE (initial current 30–
40 mA), that is, at a potential following the anodic peak observed in
cyclic voltammetry, characteristic of the two-electron oxidation of 1red
to 1ox. After exhaustive electrolysis (6–10 h, n = 8–56), that is, when a
negligible value of the current was recorded (0.5–1.0 mA), the
solution was worked up by addition of 2.5mmol of 2,4-dinitrophe-
nylhydrazine reagent (in 5mL of H 2SO4, 15mL of EtOH and 5mL of
water) because 5mmol of primary amine gave 2.5mmol of alkylimine
(Scheme 2). After 1 h, the resulting solution was concentrated to
40 mL. The solid was collected by filtration, washed with water, and
dried in a vacuum desiccator. The identity and purity of DNPH was
confirmed by thin-layer chromatography (TLC) and 1H NMR
spectroscopy, after comparison with an authentic sample. In the
case of n-hexylamine or phenylpropylamine, the solid was purified by
chromatography on silica gel to separate DNPH from 1,2-bis-DNPH.
Control studies indicated that the amount of aldehyde produced
either from simple autoxidation or from electrochemical oxidation of
the starting amine in the absence of catalyst was negligible.
Isolation of 2 and 3: After exhaustive electrolysis (6 h, n = 9 or
10), the electrolysis solution was poured into an acetic acid buffered
aqueous solution of pH 4.8 (1 molLÀ1, 50 mL). The resulting hydro-
alcoholic solution was concentrated to remove methanol. The yellow
solid, identified as the 1,4-benzoxazine derivative, was collected by
filtration, washed with water, and dried in a vacuum desiccator.
2: M.p. 1628C; 1H NMR (300 MHz, CDCl3): d = 2.25(m, 1H),
2.75(t, J = 6 Hz, 2H), 3.15(m, 2H), 3.75 (s, 3H), 3.90 (s, 3H), 5.90 (d,
J = 8 Hz, 1H), 6.55 (d, J = 9 Hz, 1H), 6.75(d, J = 8 Hz, 2H), 6.95(d,
J = 8 Hz, 2H), 7.05(d, J = 8 Hz, 2H), 7.45(d, J = 9 Hz, 1H), 7.55 (m,
3H), 7.70 (d, J = 8 Hz, 2H), 8.00 (d, J = 8 Hz, 2H), 13.10 ppm (s, 1H);
13C NMR (75MHz, CDCl 3): d = 35.7, 46.2, 55.2, 55.3, 81.2, 108.3,
113.9, 114.2, 122.7, 127.9, 128.3, 128.9, 129.2, 129.6, 131.2, 131.6, 133.2,
138.2, 150.7, 154.5, 158.1, 160.3, 161.9, 202.2 ppm; DCI (desorption
chemical ionization) MS: m/z: 509 [MH+].
7)
H
O
O
Ar
HN
O
Ar
N
H
8)
O
H
H
O
O
O
H
N
–2e–,–2H+
Ar
N
O
Ar
HN
9)
HN
O
Ar
Ar
2: Ar = 4-MeO-C6H4
3: Ar = 3,4-MeO-C6H3
Scheme 3. Inactivation of modelcofactor 1ox by ring-substituted
phenylethylamine substrates, leading to 1,4-benzoxazine derivatives 2
and 3.
which seems favored when starting amines are not sterically
encumbered by b and g branching.
With ring-substituted phenylethylamines, the catalytic
process ceased after roughly 2 turnovers (Table 1, entries 12
and 13). Close inspection of the exhaustively oxidized
solution revealed that electrogenerated 3,4-iminoquinone
1ox was trapped with the tautomeric enamine form of the
alkylimine extruded during the catalytic process (Scheme 3,
step 7) through an inverse-electron-demand Diels–Alder
reaction,[12] affording unstable 1,4-benzoxazine derivatives
(Scheme 3, step 8). However, these compounds could be
isolated as stable products 2 and 3 in 50 and 65% yields,
respectively, after a subsequent two-electron oxidation reac-
tion (Scheme 3, step 9). Interestingly, this multistep one-pot
electrochemical procedure could lead to novel neuroprotec-
tive agents as a consequence of the structural similarity of 2
and 3 with earlier reported 1,4-benzoxazine derivatives.[13]
In summary, we have demonstrated for the first time that
unactivated primary aliphatic amines can be efficiently
oxidized by a synthetic model cofactor of amine oxidases in
the absence of a metal ion. The catalyst 1ox exhibits the same
substrate specificity as the copper amine oxidases themselves,
that is, poor reactivity with a-branched primary amines and
no reactivity toward secondary amines. The reaction displays
two features that are most often associated with enzymatic
systems: a) the reaction is likely enhanced through the
participation of neighboring substituents, as they prevented
the competing formation of Michael adducts (in this case, the
benzoyl and 2-hydroxy groups); b) the presence of an active
hydroxy proton very probably constitutes an essential com-
ponent of the catalytic activity (in this case, the 2-hydroxy
proton, analogous to that of TPQ).[14] We are currently
developing further analogues to confirm the substantial role
of the 2-hydroxy group to convert a catalytic inert species into
a highly effective enzymatic prosthetic group.
Received: October 14, 2002 [Z50356]
[1] a) J. P. Klinman, Chem. Rev. 1996, 96, 2541 – 2561; b) J. P.
Klinman, J. Biol. Chem. 1996, 271, 27189 – 27192; c) J. A.
Stubbe, W. A. Van der Donk, Chem. Rev. 1998, 98, 705– 776;
d) J. P. Klinman, Proc. Natl. Acad. Sci. USA 2001, 98, 14766 –
14768; e) M. A. Halcrow, Angew. Chem. 2001, 113, 358 – 362;
Angew. Chem. Int. Ed. 2001, 40, 346 – 349.
[2] Y.-L. Hyun, V. L. Davidson, Biochemistry 1995, 34, 816 – 823,
and references therein.
[3] S. X. Wang, N. Nakamura, M. Mure, J. P. Klinman, J. Sanders-
Loehr, J. Biol. Chem. 1997, 272, 28841 – 28844.
[4] M. Mure, S. A. Mills, J. P. Klinman, Biochemistry 2002, 41, 9269 –
9278.
[5] a) S. A. Mills, J. P. Klinman, J. Am. Chem. Soc. 2000, 122, 9897 –
9904; b) B. Schwartz, A. K. Olgin, J. P. Klinman, Biochemistry
1028
ꢀ 2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1433-7851/03/4209-1028 $ 20.00+.50/0
Angew. Chem. Int. Ed. 2003, 42, No. 9