Model Studies of 6,7-Indolequinone Cofactors
(ii) Reaction with Benzylamine. The reactions of 1-3 and
benzylamine were performed similarly but at 60 °C, and the
products were analyzed by ESI-MS.
(iii) Theoretical Calculations. Density-functional theory (DFT)
calculations were performed on an 8 CPU workstation (PQS,
Quantum Cube QS8-2400C-064). Geometry optimizations were
carried out with the Becke3LYP functional and 6-31G(d) basis set,42
with the restricted Hartree-Fock (RHF) formalism and as imple-
mented in the Gaussian 03 program Revision C.02.43
higher electron donor ability of the oxygen substituent in 2 as
compared to that of the sulfur substituent in 1. The introduction
of the amino group at C-4 of the quinone in 3 resulted in a
further increase in the energy barrier for the iminoquinone
formation process, thus lowering the observed second-order rate
constant kobs(2) (Table 3). Thus, the 6,7-indolequinone having
such C-4 substituents may not be able to serve as a redox
cofactor in amine dehydrogenases.
Synthesis: 4-Benzyloxy-3-methylindole-6,7-dione (2). 3-Meth-
yl-1-p-tosylindole-6,7-dione (7) was prepared from vanillin via 11
steps according to the reported procedures.29,44,45 Benzyl alcohol
(13.5 µL, 0.13 mmol) and potassium tert-butoxide (14.4 mg, 0.13
mmol) were suspended in dry CH3CN (5.0 mL) at room temperature
under N2, and the mixture was stirred for 1 h. A dry CH3CN (10.0
mL) solution of 7 (40.0 mg, 0.13 mmol) was then added to the
mixture, and the resulting solution was stirred at 25 °C for 6 h.
The reaction mixture was then diluted with water (30 mL), extracted
with ethyl acetate (20 mL × 3), and dried over MgSO4. After
removing MgSO4 by filtration, evaporation of the solvent gave a
dark brown solid, from which compound 2 was isolated as a dark
brown solid in a 23% conversion yield by SiO2 column chroma-
tography (CHCl3) under air. Single crystals of this compound were
obtained by recrystallization from CH2Cl2/hexane: IR (KBr) 1662,
Experimental Section
General. The reagents and the solvents used in this study were
commercial products of the highest available purity and were further
purified by the standard methods, if necessary.41 CTQ model
compound 1 and TTQ model compound 4 were obtained from
previous studies.26,29
X-ray Structure Determination. The single crystals were
mounted on a glass-fiber. Data of X-ray diffraction were collected
by a Rigaku RAXIS-RAPID imaging plate two-dimensional area
detector, using graphite-monochromated Mo KR radiation (λ )
0.71069 Å) to 2θmax of 55.0°. All the crystallographic calculations
were performed by using the Crystal Structure software package
of the Molecular Structure Corporation [Crystal Structure: Crystal
Structure Analysis Package version 3.5.1, Molecular Structure Corp.
and Rigaku Corp. (2003)]. The crystal structures of compounds 3
and 8 were solved by direct methods and refined by full-matrix
least-squares with SIR-92. All non-hydrogen atoms and hydrogen
atoms were refined anisotropically and isotropically, respectively.
Electrochemical Measurement. Cyclic voltammetric measure-
ments were performed on a BAS 50W potentiostat with a three-
electrode system consisting of a glassy carbon electrode with φ )
3.0 mm (Bioanalytical System), a platinum plate auxiliary electrode,
and an Ag|AgCl (saturated KC1) reference electrode. The glassy
carbon electrode was polished with 0.05 µm alumina powder,
sonicated to remove it, and washed with water. All electrochemical
measurements were carried out at 25 °C under an atmospheric
pressure of nitrogen, which was previously passed through a
solution of the same composition as the electrolysis solution. An
aliquot (5 µL) of a stock solution of compounds 1, 2, and 3 (ca.20
mM) in DMSO was injected into 1.0 mL of the electrolysis solutions
containing 20% (v/v) isopropyl alcohol. All pH values of aqueous/
organic mixed solvents were measured with a conventional pH
meter and indicated without correction.
Kinetic Analysis. The reactions of the quinones and several
amines were followed spectroscopically with a Hewlett-Packard
8453 photodiode array spectrophotometer under pseudo-first-order
conditions with excess amine in deaerated CH3OH. Typically, a
CH3OH solution of the quinone (5.0 × 10-5 M) was placed in a
UV cell (1 cm path length, sealed tightly with a silicon rubber cap)
and the solution was deaerated by bubbling N2 through it for ca.
10 min. Then, an anaerobic stock solution of the amine was added
with use of a microsyringe to start the reaction. The pseudo-first-
order rate constant was determined from the rate of a decrease in
intensity of the absorption due to the quinone or an increase in
intensity of the absorption due to the product. The nonlinear curve-
fitting program was used to determine the rate constants when the
final value of the absorbance (A∞) was obscured by the follow-up
reaction.
1
1631 cm-1 (CdO); H NMR (CDCl3) δ 2.20 (s, 3 H), 5.13 (s, 2
H), 5.59 (s, 1 H), 6.88 (s, 1 H), 7.26-7.43 (m, 5 H); HRMS (FAB)
m/z 268.0968 ([M + 1]+) calcd for C16H13NO3 268.0974. Anal.
Calcd for C16H13NO3 + (1/4)H2O: C, 70.71; H, 5.01; N, 5.15.
Found: C, 70.68; H, 4.85; N, 5.02.
4-(N-Methylbenzylamino)-3-methyl-1-p-tosylindole-6,7-di-
one (8). To a dry CH3CN solution (6.0 mL) of 7 (30.0 mg, 0.095
mmol) was added N-methylbenzylamine (12 µL, 0.095 mmol) at
room temperature under N2, and the resulting mixture was stirred
at room temperature for 2 day. The reaction mixture was then
diluted with water (30 mL), extracted with ethyl acetate (20 mL ×
3), and dried over MgSO4. After MgSO4 was removed by filtration,
evaporation of the solvent gave a purplish red solid, from which
compound 8 was isolated as a red solid in a 95% conversion yield
by SiO2 column chromatography (CHCl3) under air. Single crystals
of this compound were obtained by recrystallization from CH2Cl2/
hexane: IR (KBr) 1680, 1616 (CdO), 1375 and 1178 cm-1 (SO2);
1H NMR (CDCl3) δ 2.30 (s, 3 H), 2.42 (s, 3 H), 2.72 (s, 3 H), 4.40
(s, 2 H), 5.55 (s, 1 H), 7.16-7.34 (m, 7 H), 7.69 (s, 1 H), 8.09 (d,
J ) 8.3 Hz, 2 H); HRMS (FAB) m/z 435.1382 ([M + 1]+) calcd
for C24H22N2O4S 435.1378. Anal. Calcd for C24H22N2O4S + (1/
3)H2O: C, 65.44; H, 5.19; N, 6.36. Found: C, 65.16; H, 5.02;
N, 6.40.
4-(N-Methylbenzylamino)-3-methylindole-6,7-dione (3). To a
suspension of 8 (40.0 mg, 0.092 mmol) in dry CH3CN (15.0 mL,
deoxygenated by bubbling N2 for 10 min) was added methylhy-
drazine (45 µL, 0.855 mmol, 9.3 equiv), and the reaction mixture
was stirred for 2 days under anaerobic conditions (N2). Removal
of the solvent under reduced pressure gave a yellow solid material
(8H2), to which 3 N NaOH in H2O-EtOH (1.1 mL, 9:5, v/v) was
added. The mixture was then stirred at room temperature for 10
min and then extracted with ethyl acetate (15 mL × 6). After the
extract was dried over MgSO4, evaporation of the solvent gave a
dark red residue, from which compound 3 was isolated in 72%
yield by column chromatography (SiO2, CHCl3): IR (KBr) ∼3275
Product Analysis: (i) Reaction with Cyclopropylamine. A
methanol solution of 1 (5.0 × 10-5 M) was placed in a UV cell (1
cm pass length, sealed tightly with a silicon rubber cap) and the
solution was deaerated by bubbling N2 through it for ca. 10 min.
Then cyclopropylamine (50 equiv) was introduced into the solution
with a microsyringe to start the reaction at 30 °C. The reaction
products were analyzed by ESI-MS.
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L.; Schleyer, P. v. R.; Pople, J. A. Ab Initio Molecular Orbital Theory;
Wiley: New York, 1986.
(43) Frisch, M. J.; et al. Gaussian 03, Revision C.02; Gaussian, Inc.:
Wallingford CT, 2004.
(44) Benington, F.; Morin, R. D.; Clark, L. C., Jr. J. Org. Chem. 1959,
24, 917-919.
(45) Magnus, P.; Gazzard, L.; Hobson, L.; Payne, A. H.; Rainey, T. J.;
Westlund, N.; Lynch, V. Tetrahedron 2002, 58, 3423-3443.
(41) Perrin, D. D.; Armarego, W. L. F.; Perrin, D. R., Purification of
Laboratory Chemicals, 4th ed.; Pergamon Press: Elmsford, NY, 1996.
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