An old but simple and efficient method to elucidate the oxidation mechanism of NAD(P)H model 1-aryl-1,4-dihydronicotinamides by cations 2-methyl-5-nitroisoquinolium, tropylium, and xanthylium in aqueous solution

Article abstract of DOI:10.1021/jo0009696

Cations 2-methyl-5-mitroisoquinplinium (IQ⁺), tropylium (T⁺), and xanthylium (Xn⁺) were treated by an NAD(P)H model 1-(p-substituted phenyl)-1.4-dihydronicotinamide series (1) in buffered aqueous solution to give the corresponding reduced products by accepting hydride. Effects of the 4-substituents of 1 on the reaction rates were investigated. Hammett's linear free energy relationship analysis on the three reactions of 1 provides the reaction constants of -0.48, -2.2, and -1.4 with IQ⁺, T⁺, and Xn⁺ as the hydride acceptors, respectively. Comparison of the present reactions with the reaction examples whose mechanisms are well-known, such as the reaction of 1 with a one-electron oxidant Fe(CN)₆^-3, shows that the active site of 1 in the oxidation with IQ⁺ is at the 4-position on the dihydropyridine ring but that the active site of 1 in the oxidations with T⁺ and Xn⁺ is at the 1-position, which is in agreement with the results from the Bronsted-type linear analysis and the relation studies of the logarithm of the second-order rate constants with the oxidation potentials of the hydride donors. According to the dependence of the reaction mechanism on the active site of 1, a conclusion can be made that the reaction of 1 with IQ⁺ proceeds by direct one-step hydride transfer mechanism, but the reactions of 1 with T⁺ and Xn⁺ would take place via multistep hydride transfer mechanism initiated by one-electron transfer.

Full text of DOI:10.1021/jo0009696

370
J . Org. Chem. 2001, 66, 370-375
An Old bu t Sim p le a n d Efficien t Meth od to Elu cid a te th e
Oxid a tion Mech a n ism of NAD(P )H Mod el
1-Ar yl-1,4-d ih yd r on icotin a m id es by Ca tion s
2-Meth yl-5-n itr oisoqu in oliu m , Tr op yliu m , a n d Xa n th yliu m in
Aqu eou s Solu tion
Xiao-Qing Zhu,* Yang Liu, Bing-J un Zhao, and J in-Pei Cheng*
Department of Chemistry, Nankai University, Tianjin 300071, China
xqzhu@nankai.edu.cn
Received J une 27, 2000
Cations 2-methyl-5-nitroisoquinolinium (IQ+), tropylium (T+), and xanthylium (Xn+) were treated
by an NAD(P)H model 1-(p-substituted phenyl)-1.4-dihydronicotinamide series (1) in buffered
aqueous solution to give the corresponding reduced products by accepting hydride. Effects of the
4-substituents of 1 on the reaction rates were investigated. Hammett’s linear free energy relationship
analysis on the three reactions of 1 provides the reaction constants of -0.48, -2.2, and -1.4 with
IQ+, T+, and Xn+ as the hydride acceptors, respectively. Comparison of the present reactions with
the reaction examples whose mechanisms are well-known, such as the reaction of 1 with a one-
electron oxidant Fe(CN)6-3, shows that the active site of 1 in the oxidation with IQ+ is at the
4-position on the dihydropyridine ring but that the active site of 1 in the oxidations with T+ and
Xn+ is at the 1-position, which is in agreement with the results from the Brønsted-type linear
analysis and the relation studies of the logarithm of the second-order rate constants with the
oxidation potentials of the hydride donors. According to the dependence of the reaction mechanism
on the active site of 1, a conclusion can be made that the reaction of 1 with IQ+ proceeds by direct
one-step hydride transfer mechanism, but the reactions of 1 with T+ and Xn+ would take place via
multistep hydride transfer mechanism initiated by one-electron transfer.
In tr od u ction
perimental results collected are not in line to support
each other: some evidence supports that the hydride
transfer occurs by a direct one-step mechanism,9 but
others support that the hydride transfer takes place via
multistep sequence of e-H+-e or e-H•.10 So, the mechanism
of the hydride transfer is still obscure. Systematic
examination of past publications on this subject shows
that various research methods such as kinetic isotope
effect,11 isotope trace,12 thermodynamics,4 photochemis-
try,13 electrochemistry,14 computer-chemistry,15 reaction
intermediate trapping,16 analogue simulator,17 and ster-
The reduced form of the nicotinamide adenine dinucle-
otide coenzyme (NAD(P)H), with the biologically impor-
tant 1,4-dihydropyridine partial structures,1 plays an
important role in many bioreductions by transferring a
hydride ion or an electron to the surrounding substrates.2
The mechanism of the hydride transfer has been a very
interesting subject and has been drawing much more of
the attention of many researchers in the world. 1-Benzyl-
1,4-dihydronicotinamide (BNAH), Hantzsch 1,4-dihydro-
pyridine (HEH), 10-methyl-9,10-dihydroacridine (AcrH2),
and many other 1,4-dihydropyridine derivatives have
been used as NAD(P)H models to probe the mechanistic
details of the hydride transfer.3-8 However, many ex-
(8) (a) Zhu, X.-Q.; Liu, Y.-C. J . Org. Chem. 1998, 63, 2786. (b) Zhu,
X.-Q.; Liu, Y.-C.; Wang, H.-Y.; Wang W. J . Org. Chem. 1999, 64, 8983.
(c) Zhu, X.-Q.; Zou, H.-L.; Yuan, P.-W.; Liu, Y.; Cao, L.; Cheng, J .-P.
J . Chem. Soc., Perkin Trans. 2 2000, 1857. (d) Zhu, X.-Q.; Liu, Y.-C.;
Li, J .; Wang, H.-Y. J . Chem. Soc., Perkin Trans. 2 1997, 2191. (e) Zhu,
X.-Q.; Wang, H.-Y.; Wang, J .-S.; Liu, Y.-C. J . Org. Chem. 2000, in press
(J O001434F). (f) Zhao, B.-J .; Zhu, X.-Q.; He, J .-Q.; Xia, Z.-Z.; Cheng,
J .-P. Tetrahedron Lett. 2000, 41, 257. (g) Liu, Y.-C.; Li, B.; Guo, Q.-X.
Tetrahedron 1995, 51, 9671.
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1984, 106, 3850. (b) Carlson, B. W.; Miller, L. L. J . Am. Chem. Soc.
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T. J Am. Chem. Soc. 1987, 109, 305. (d) Sinha, A.; Bruice, T. C. J .
Am. Chem. Soc. 1984, 106, 7291. (e) Lai, C. C.; Colter, A. K. J . Chem.
Soc., Chem. Commun. 1980, 1115. (f) Fukuzumi, S.; Kondo, Y.; Tanaka,
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Ishikawa, M.; Tanaka, T. J . Chem. Soc., Perkin Trans. 2 1989, 1811.
(10) (a) Powell, M. F.; Bruice, T. C. J . Am. Chem. Soc. 1982, 104,
5834. (b) Powell, M. F.; Bruice, T. C. J . Am. Chem. Soc. 1983, 105,
1014. (c) Verhoeven, J . W.; van Gerresheim, W.; Martiens, F. M.; van
der Kerk, S. M. Tetrahedron 1986, 42, 975. (d) Carlson, B. W.; Miller,
L. L. J . Am. Chem. Soc. 1985, 107, 479. (e) Ostovic, D.; Roberts, R. M.
G.; Kreevoy, M. M. J . Am. Chem. Soc. 1983, 105, 7629. (f) Ostovic, D.;
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10.1021/jo0009696 CCC: $20.00 © 2001 American Chemical Society
Published on Web 12/30/2000

1-Aryl-1,4-dihydronicotinamides
J . Org. Chem., Vol. 66, No. 2, 2001 371
Sch em e 1
eochemistry,18 etc. are widely used. But, to our best
knowledge, very little attention has been paid to the
dependence of the active sites of 1,4-dihydropyridine on
the mechanisms of the hydride transfer. As is well-
known, there are two active sites on the 1,4-dihydropy-
ridine ring responsible for the NAD(P)H reductions, one
of which is at the 1- or N-position on the ring, the other
is at the 4-position (see Scheme 1). It is conceivable that
if the hydride transfer takes place by a direct one-step
mechanism, the active site of 1,4-dihydropyridine would
be at the 4-position due to the absence of hydrogen at
the N-position. If the hydride transfer is initiated by one-
electron transfer, the active site would be at the N-
position rather than the 4-position, since it is only the
nitrogen atom at 1-position that can lose an electron in
the initial reaction step. So it is a facile and effective
method to elucidate the oxidation mechanisms of NAD-
(P)H model reactions by determining the active center
position of 1,4-dihydropyridine during the reaction coor-
dination. This idea induced us to design and synthesize
a series of 1-(p-substituted phenyl)-1.4-dihydronicotina-
mides (1) as NAD(P)H models and choose 2-methyl-5-
nitroisoquinolinium (2), tropylium (3), and xanthylium
(4) cations as NAD(P)+ models (Scheme 2) to determine
the reaction center of 1 in the oxidations with 2, 3, and
4 by using Hammett’s linear free energy correlation
method.
F igu r e 1. Time dependence of the electronic absorption
spectra of the following reactions at room temperature: (a)
IQ+ + 1 (X ) H), [IQ+] ) 12.64 × 10-3 M, ∆t ) 1 min; (b) T+
+ 1 (X ) CH3), [T+] ) 3 × 10-3 M, ∆t ) 1 min; (c) Xn+ + 1
(X ) H), [Xn+] ) 2.2 × 10-3 M, ∆t ) 1 min.
Sch em e 2
nitroisoquinolinium cation by 1 in 30% acetonitrile/70%
H2O gave 1,2-dihydro-5-nitroisoquinoline (eq 1) which can
Resu lts
Red u ction of 5-Nitr oisoqu in olin iu m Ca tion (2) by
1 in Aqu eou s Solu tion . Treatment of N-methyl-5-
(11) (a) Ohno, A.; Yamamoto, H.; Oka, S. J . Am. Chem. Soc. 1981,
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9, 1637-1640.
be identified by UV-vis, MS, and 1H NMR spectra
analysis (see Experimental Section). Since 1,2-dihydro-
5-nitroisoquinoline is the only species in eq 1 which
absorbs at 450 nm, the kinetics of this reaction may be
conveniently monitored by following the time dependent
absorbance at this wavelength. The increase in absor-
bance at 450 nm was kinetically first order in dihydroni-
cotinamide under conditions in which the hydride accep-
tor was in 164- to 336-fold excess (Figure 1a). Pseudo-
first-order rate constants, kobs, for five X are tabulated
in Table 1 (Supporting Information). The second-order
rate constants (k2) obtained from the kobs as a function
of the concentration of the hydride acceptor are sum-
marized in Table 2.
(13) (a) Nesterenko, A. M.; Buryak, N. I.; Polumbrik, O. M.;
Yasnikov, A. A.; Dopov Akad. Nauk Ukr. RSR. Ser. B: Geol. Khim.
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S. H.; Binder, D. A.; King, G. W. J . Am. Chem. Soc. 1988, 110, 524-
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Y.-D.; Lai, D. K. W. J . Am. Chem. Soc. 1995, 117, 4100. (d) Nonkebel,
S. T.; Orszulid, S. T.; Suckling, C. T. J . Chem. Soc., Chem. Commun.
1982, 146. (e) Lauries, D.; Lucas, E.; Nonkebel, D. C.; Suckling, C. T.
Tetrahedron 1986, 42, 1035. (f) Merjer, L. H. P.; Van Niel, J . C. G.;
Pandit, U. K. Tetrahedron 1984, 40, 5185.
(16) (a) Li, B.; Liu, Y.-C.; Guo, Q.-X. J . Photochem. Photobiol. A:
Chemistry 1997, 103, 101-104. (b) Deng, G.; Xu, H. J .; Chen, D. W. J .
Chem. Soc., Perkin Trans. 2, 1990, 7, 1133.
(17) Meijer, L. H. P.; Pandit, U. K. Tetrahedron 1985, 41, 467.
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1976, 755.
Red u ction of Tr op yliu m (3) a n d Xa n th yliu m (4)
Ca tion s by 1 in Aqu eou s Solu tion s. Tropylium and

372 J . Org. Chem., Vol. 66, No. 2, 2001
Zhu et al.
Ta ble 2. Th e Secon d -Or d er Ra te Con sta n ts, F r ee En er gy Ch a n ges of On e-Electr on Tr a n sfer fr om th e NAD(P )H Mod els
to th e Ca tion s, a n d Rela tive Red ox P oten tia ls of th e NAD(P )H Mod els
k2 (M-1‚s-1
)
S
a
b
c
d
e
IQ+ a
T+ b
9.86 × 10-2
3.79 × 103
2.22 × 107
7.18 × 10-2
1.35 × 103
1.98 × 107
5.80 × 10-2
8.01 × 102
1.06 × 107
4.93 × 10-2
2.76 × 102
0.49 × 107
5.01 × 10-2
2.52 × 102
0.48 × 107
Xn+ c
oxidation potentials of the NAD(P)H models
0.313 0.363
Eox
0.281
0.395
0.400
free energy changes of one-electron transfer from the NAD(P)H models to the cations (∆G(eT)) d
IQ+ e
T+ f
27.8
19.2
11.2
28.6
20.0
11.7
29.7
21.1
12.4
30.5
21.9
13.4
30.6
22.0
13.3
Xn+ g
a
b
d
Standard deviation < ( 0.24 × 10-2
.
Standard deviation < ( 0.31 × 103. c Standard deviation < ( 0.32 × 107. Units: kcal/mol;
E
red(IQ+) ) -0.926 (V vs Fc+/Fc in acetonitrile), Ered(T+) ) -0.553 (V vs Fc+/Fc in acetonitrile), Ered(Xn+) ) -0.293 (V vs Fc+/Fc in
acetonitrile). e Standard deviation < ( 1.5. f Standard deviation < ( 1.5. Standard deviation < ( 2.0.
g
xanthylium cations are very active species and easily
hydrolyze in aqueous solutions to give their correspond-
ing hydroxide adducts. In acidic aqueous solutions, the
tropylium and xanthylium cations exist in equilibrium
with their corresponding hydroxide adducts and the
equilibrium constant was measured to be 4.78619 and
-0.83,20 respectively (see eqs 2 and 4). Obviously, the
F igu r e 2. Hammett σ-F analysis for
a series of 1-(4-
substituted phenyl)-1,4-dihydronicotinamides and their rates
of (9) IQ+-, (b) Fe(CN)6-3-, (2) T+-, and (1) Xn+-mediated
oxidations.
plots of ln(At - At+∆) vs t (Table 1, Supporting Informa-
app
tion). The apparent second-order rate constants (k2
)
were evaluated from the slopes of kobs vs ([R+] + [ROH]).
The pH-dependent second-order rate constants were
evaluated from eq 6.
k2 ) k2/(1 + KR+/[H+])
app
(6)
app
If KR+ . [H+], eq 6 simplifies to k2 ) k2[H+]/KR+, i.e.,21
app
log k2 ) log k2 + pKR+ - pH
(7)
concentrations of cations 3 and 4 in acidic aqueous
solutions can be calculated from the concentration of the
corresponding alcohol and the pH value of the buffered
aqueous solutions, respectively.
The second-order rate constants obtained from eq 7 are
summarized in Table 2.
The reductions of 3 and 4 by 1 in aqueous solutions
gave the corresponding reduced products by hydride
transfer from 1 to 3 and 4 (eqs 3 and 5, respectively),
which were verified by checking their spectral properties
against those of the authentic samples. The kinetics
studies of the reactions were carried out in a buffered
aqueous solution containing 30% acetonitrile under
pseudo-first-order conditions in 20- to 70- or 20- to 43.4-
fold excess of the hydride acceptor at constant pH by
measuring the maximum absorption of 1 (see Figure 1b
and c). The pseudo-first-order rate constants (kobs) were
evaluated at six concentrations of the cations from the
Discu ssion
The reduction rates of cations 2, 3, and 4 by 1 collected
in Tables 1 and 2 show that all the three reactions obey
second-order kinetics with first-order dependence on each
reactant concentration. A Hammett-type free energy
analysis on the three reactions provides three excellent
lines of log k2 against the σ constant of the substituents
X with reaction constant F values of -0.48 ( 0.07, -2.2
( 0.2, and -1.4 ( 0.1 for IQ+, T+, and Xn+ as the hydride
acceptors, respectively (Figure 2). The negative F values
indicate the developing positive charge on the 1,4-
dihydropyridine ring in the rate-limiting step. The mag-
(19) Ritchie, C. D.; Fleischhauer, H. J . Am. Chem. Soc. 1972, 94,
3481.
(20) Martin, J . C.; Smith, R. G. J . Am. Chem. Soc. 1964, 86, 2252.
(21) Bunting, J . W.; Conn, M. M. Can. J . Chem. 1990, 68, 537-54.

1-Aryl-1,4-dihydronicotinamides
J . Org. Chem., Vol. 66, No. 2, 2001 373
nitude of the F values is a measure of the sensitivity of
the three reactions to the effects of the substituent X
changes.22
Comparing the magnitude of the second-order reaction
rates among the three reactions, it is found that the
magnitude order of the rate constants is k2 (IQ+) , k2
(T+) , k2 (Xn+) at the same temperature. This result
indicates that if the three reactions proceeded by an
identical or similar pathway, the absolute value magni-
tude of the reaction constants F would decrease in the
following order: |F(IQ+)| > |F(T+)| > |F(Xn+)| on the basis
of the reactivity-selectivity principle (RSP).23 However,
the fact is that the absolute value of F in the case of IQ+
as the hydride acceptor (F ) -0.48) is much smaller than
those in the cases of T+ and Xn+ as the hydride acceptor
(F ) -2.2 and -1.4, respectively), which means that the
three reactions took place via different mechanisms.
Obviously, the active site of 1 in the former reaction is
farther away from the source of the substituent X
perturbation than those in the latter two reactions. This
result allows us to make a proposal that, for the reaction
with IQ+, the active site of 1 would be at the 4-position
on the 1,4-dihydronpyridine ring, whereas, for the reac-
tions with T+ and Xn+, the active site of 1 would be at
the 1- rather than the 4-position on the ring, which
means that the reaction of IQ+ by 1 proceeds by direct
one step hydride transfer, but the other two reactions
would be initiated by one-electron transfer.
F igu r e 3. Plots of the logarithm of the second-order rate
constants for the reductions of the cations (9) IQ+, (b)
Fe(CN)6 ,
-3 25 (2) T+, and (1) Xn+ with 1-(p-substituted phenyl)-
1,4-dihydronicotinamides against the oxidation potentials
of 1.
mide ring and unambiguously indicates that the reaction
of 1 with IQ+ proceeded by the direct hydride transfer
mechanism. Similar analysis can also be done on the
comparison of the oxidations of 1 by T+ and Xn+ with
the ferricyanide-mediated oxidation. As shown in Figure
2, for the three reactions of 1 with T+, Xn+, and
Fe(CN)6-3, the magnitude order of the rate constants is
k2(Fe(CN)6-3) < k2(T+) < k2(Xn+), and in agreement with
the absolute value order of the reaction constants
|F(Fe(CN)6-3)| > |F(T+)| > |F(Xn+)| in terms of RSP,23 if
the three reactions take place by an identical or similar
pathway. In other words, this result indicates that the
oxidations of 1 by T+ and Xn+, as the ferricyanide-
mediated oxidation, would be initiated by one-electron
transfer from the nitrogen atom on the 1,4-dihydropyri-
dine ring.
To further confirm the different active sites of 1 in the
three oxidations, it is necessary to compare the present
reactions with the well-known reaction examples of the
Hammett correlation. From Figure 2, the F-value of the
oxidation of 1 by IQ+ (F ) -0.48) is close to that of the
following known reaction: ArCOCH3 + Br2 f ArCOCH2-
Br + HBr (F ) -0.46).24 This indicates that the active
site of 1 in the oxidation could not be at the 1-position.
However, the F-values of the oxidations of 1 by T+ (F )
-2.2) and Xn+ (F ) -1.4) are similar to those of the
known reactions X-ArNH2 + ArCOCl f X-ArNHCOAr
(F ) -2.78) and ArNH2 + HCO2H f ArNHCHO + H2O
(F ) -1.43), respectively.24 This implies that the active
site of 1 in the two present oxidations would be at the
heterocyclic nitrogen. It is more efficient to compare the
oxidations of 1 by IQ+, T+, and Xn+ with the oxidation of
To support the above proposal, the dependence of the
rate constants (k2) of the oxidations of 1 by IQ+, Fe(CN)6
,
-3
T+, and Xn+ on the oxidation potentials (Eox) of the NAD-
(P)H models 1 were examined. Figure 3 shows the plots
of log k2 against Eox of 1 with the slopes of -2.08 (r )
0.99), -12.12 (r ) 0.99), -9.32 (r ) 0.99), and -6.00 (r
) 0.98), respectively. Since the slope value represents
only the pure electron loss component of the rate con-
stants, the greater dependence of the rate on the Eox of
the NAD(P)H models indicates that the reaction is more
favorable to take place by the initial one-electron-transfer
mechanism. From Figure 3, even though the rate con-
stants for the oxidation of 1 by T+ and Xn+ are much
larger than that for the oxidation of 1 by IQ+, the absolute
values of the slope for the former two reactions are quite
larger than that for the latter reaction. This result shows
that the former two reactions are largely affected by the
energy of the electron escape from the nitrogen atom, but
a small tendency was observed for the latter reaction,
which indicates that the former two reactions proceed
by initial one-electron transfer, however the other reac-
tion would take place via hydride one-step direct transfer.
A similar analysis can also be done on the dependence
of the logarithms of the second-order rate constants on
the pKa’s of the corresponding substituted anilines26 for
the four reactions of 1 with IQ+, Fe(CN)6-3, T+, and Xn+.
-3
1 by Fe(CN)6 in aqueous solutions.25 Since the ferri-
cyanide is a well-known one-electron oxidant, the ferri-
cyanide-mediated oxidation of 1 must be initiated by one-
electron transfer, and the active site of 1 in this reaction
should be at the 1-(N)-position. Examination of the
oxidation of 1 by IQ+ and the oxidation of 1 by Fe(CN)6
-3
shows that the rate constant of the former reaction is
obviously smaller than that of the latter reaction, but the
absolute value of the reaction constant F is quite larger
than that of the latter. This result strongly supports that
the active center of 1 in the oxidation with IQ+ is at the
4- rather than the 1-position on the 1,4-dihydronicotina-
(22) Isaacs, N. S. Physical Organic Chemistry, 1st ed.; J ohn Wiley
& Sons: New York, 1987; Chapter 7.
(23) Schmidt, M. W.; Baldrige, K. K.; Boatz, J . A.; Elbert, S. T.;
Gordon, M. S.; J ensen, J . H.; Koseki, S.; Matsunnage, N.; Nguyen, K.
A.; Su, S. J .; Windus, T. L.; Dupouis, M.; Montgomery, J . A. J . Comput.
Chem. 1993, 14, 1347.
(24) Page, M.; Williams, A. Organic and Bio-organic Mechanisms,
1st ed.; Addison-Wesley Longman Ltd.: London, 1997; p 54.
-3
(25) Second-order rate constants for the reduction of Fe(CN)6 by
(26) pKa’s of anilines are taken from: Kotake, M. Constants of
Organic Compounds; Asakury Publishing Co., Ltd.: Tokyo, 1963; pp
593-595.
1 are obtained from: Brewster, M. E.; Simay, A.; Czako, K.; Winwood,
D.; Farag, H.; Bodor, N. J . Org. Chem. 1989, 54, 3721-3726.

374 J . Org. Chem., Vol. 66, No. 2, 2001
Zhu et al.
with T+ and Xn+ is at the 1-position, from which a
conclusion was naturally deduced that the reaction of 1
with IQ+ proceeds by direct one-step hydride transfer,
whereas the reactions of 1 with T+ and Xn+ take place
via multistep hydride transfer initiated by one-electron
transfer. This is an old but simple and efficient method
to elucidate the mechanism of NAD(P)H mimic reactions,
which, to our knowledge, appears to be the first work in
the mechanism investigation of NAD(P)H mimic reac-
tions.28
Exp er im en ta l Section
Ma ter ia ls. 2-Methyl-5-nitroisoquinolinium iodide was pre-
pared according to the literature method29,30 and recrystallized
from methanol/2-proanol before use. Tropylium tetrafluorobo-
rate was purchased from Aldrich Chemical Co. and recrystal-
lized from ethanol prior to use. 9-Hydroxylxanthene (9-
xanthydrol) was synthesized from the corresponding xanthenone
by reduction with NaBH4 in water. 1-(4-Substituted phenyl)-
1,4-dihydronicotinamides were prepared according to the
following general method: 1 mmol of the appropriate aniline
dissolved in dry methanol was added to a solution of 1 mmol
of 1-(2,4-dinitrophenyl)nicotinomide chloride in 100 mL of
methanol. The resulting red solution was then heated gently
overnight or until the red color faded to yellow, indicating the
formation of 2,4-dinitroaniline. The solution was cooled, and
the precipitated side product was removed by filtration. The
filtrate was then evaporated in a vacuum, and the residue was
dissolved in 100 mL of H2O. The aqueous phase was then
exhaustively washed with ethyl ether. The water layer was
then evaporated under reduced pressure to give a crude
product, which was recrystallized from methanol-ether. Re-
duction of the pyridilium salt was performed in aqueous basic
sodium dithionite to give the appropriate reduced pyridine
derivatives.
F igu r e 4. Plots of the logarithm of the second-order rate
constants for the reductions of the cations (9) IQ+, (b)
Fe(CN)6 ,
-3 24 (2) T+, and (1) Xn+ with 1-(p-substituted phenyl)-
1,4-dihydronicotinamides against the pKa of the corresponding
arylamine.26
The Brønsted-type linear plots were shown in Figure 4
with the slopes of 0.168, 1.00, 0.77, and 0.51 for the
oxidations by IQ+, Fe(CN)6-3, T+, and Xn+, respectively.
Reacting with the oxidants Fe(CN)6-3, T+, and Xn+, the
reactivity of 1 is largely influenced by the lone-pair
electron density on the ring nitrogen, but reacting with
IQ+, the reactivity is weakly affected, which also indi-
cated that the active site of 1 in the oxidation with IQ+
is at the 4-position of the 1,4-dihydropyridine, however
the active site of 1 is at the 1-position in the oxidations
with Fe(CN)6-3, T+, and Xn+.
On the other hand, distinction between the one-step
and the multistep hydride transfer mechanisms may also
be made on the basis of the thermodynamic point of
view.27 For this purpose, electrochemical measurement
of the each reactant was performed and showed that the
free energy changes of electron transfer from 1 to IQ+,
T+, and Xn+ is 27.8-30.6 kcal/mol, 19.2-22.0 kcal/mol,
and 11.2-13.3 kcal/mol, respectively. According to the
free energetic criteria proposed for differentiating the
formal hydride transfer mechanism of the NAD(P)H
model-mediated reactions,27 if the free energy change of
one-electron transfer is larger than 23.06 kcal/mol, the
reaction is advantageous to taking place by one-step
hydride transfer mechanism; if the case is reverse, the
reaction proceeds in a direction favorable to one-electron
transfer. Obviously, the reaction of 1 with IQ+ is advan-
tageous to carrying out by direct one-step hydride
transfer, but the reactions of 1 with T+ and Xn+ proceed
in a direction favorable to one-electron transfer.
P r od u ction s of th e Rea ction of 1 by IQ+. To 4 mL of a
0.2 M solution of 1 (X ) H) in acetonitrile was added 10 mL of
a
0.08 M solution of IQ+I- in pH 7.0 phosphate buffer
containing 2 M KCl and 6.0 mL of H2O to make a total reaction
volume of 20 mL. The mixture was stirred for 1 h, and the
dark red product was extracted into dichloromethane. After
removal of dichloromethane on a rotator, 1,2-dihydro-2-methyl-
5-isoquinoline was obtained: UV-vis λmax ) 450 nm; MS m/z
1
) 190; H NMR (90 MHz, CDCl3) δ ) 2.76 (3H, s), 4.20 (2H,
s), 5.93 (1H, d), 6.27 (1H, d), 6.77 (1H, m), 7.02 (1H, dd), 7.73
(1H, dd). The product in the aqueous layer was evaporated
under vacuum to give the cation of 1.
P r od u cts fr om th e Oxid a tion of 1 by T+ (or Xn +). A
mixture of 1 mmol 1 (X ) H) and 1.5 mmol T+ (or Xn+) in 20
mL 30% acetonitrile/70% water was magnetically stirred at
room temperature in the dark for 24 (or 16) hours. The organic
layer was extracted with ether (3 × 20 mL), and the combined
organic layers were washed with water and dried with MgSO4
and concentrated. The crude material was chromatographed
to give cycloheptatriene (or xanthene). The product in the
aqueous layer was treated to give the cation of 1.
Cycloheptatriene: 1H NMR (90 MHz, CDCl3) δ ) 2.2-2.4
(2H, m), 5.15-5.6 (2H, m), 6.1-6.35 (2H, m), 6.45-6.62 (2H,
m); MS m/z ) 92 (M+•), 91 (100%). Xanthene: 1H NMR (90
MHz, CDCl3) δ ) 4.0 (2H, s), 6.9-7.3(8H, m); MS m/z ) 182
(M+•), 181 (100%).
Kin etic Mea su r em en ts. All rate data were obtained in
buffered aqueous solutions containing 20% or 30% acetonitrile
(v/v) (see below) at 25 °C or 45 °C and ionic strength 1.0 or
0.5 (KCl). Stock solution of reagents for kinetic studies were
Con clu sion s
In the present work, we elucidate the reaction mech-
anisms of NAD(P)H models 1-(4-substituted phenyl)-1,4-
dihydronicotinamides with IQ+, T+ and Xn+ by deter-
mining the active site of 1 in the oxidations using
Hammett’s linear relation analysis. On the basis of the
reaction constant comparison of the present reactions
with the well-known reaction examples of Hammett
correlation, it is found that the active site of 1 in the
oxidation with IQ+ is at the 4-position on the dihydro-
pyridine ring, but the active site of 1 in the oxidation
(28) In the previous paper,11b a λ-shaped correlation of kinetic (or
product) isotope effect (kH/kD or YH/YD) with σ was reported to support
a stepwise hydride transfer mechanism. Since the correlation of isotope
effect with σ is not due to Hammett’s LFER, the slope of the lines is
not reaction constant F, which is different from our method in nature.
(29) Bunting, J . W.; Norris, D. J . J . Am. Chem. Soc. 1977, 99, 1189.
(30) Bunting, J . W.; Kabir, S. H. J . Org. Chem. 1978, 43, 3662.
(27) Cheng, J .-P.; Lu, Y.; Zhu, X.-Q.; Mu, L. J . J . Org. Chem. 1998,
63, 6108.

1-Aryl-1,4-dihydronicotinamides
J . Org. Chem., Vol. 66, No. 2, 2001 375
kept in the dark when not in use and discarded after a
maximum of 24 h. Care was taken to ensure that complete
solubility of the reactants and products was maintained before
and during all kinetic runs. Solubility limitation was the major
consideration in defining the highest accessible concentration
of the excess component in each reaction system. The reaction
of Xn+ and 1 proved to be incompatible with 20% acetonitrile
containing phosphate buffer, although solubility was main-
tained under these conditions in 30% acetonitrile.
an argon atmosphere as described previously.31 n-Bu4NPF6
(0.1M) was employed as the supporting electrolyte. A standard
three-electrode assembly consisting of a Pt disk as the working
electrode, AgNO3/Ag as reference, and a platinum wire as
counter electrode was used in CV measurements. All sample
solutions were 1.5 mM. The ferrocenium/ferrocene redox couple
(Fc+/Fc) was taken as an internal standard. Reproducibility
is generally within 5 mV.
Ack n ow led gm en t. This project was supported by
the Natural Science Foundation of China (Nos. 29972028
and 29702005) to whom the authors want to express
their sincere gratitude. We also thank the reviewers for
valuable suggestions on this work.
All reactions were followed on a U-3000 spectrophotometer
with digitized absorbance data being recorded on Pentium-
586 computer under pseudo-first-order condition with 20- to
336-fold excesses of the acceptor species over the hydride donor
([1] ) 0.05 mM). Data were collected for at least 95% of each
reaction. The pH was measured at the end of each kinetic run
on a Beckman Φ 72 pH meter.
Pseudo-first-order rate constants were calculated by fitting
the absorbance (A) at time (t) to the relationship A ) Af + (Ai
- Af) exp(-kobst) via the Marquardt algorithm, treating the
initial absorbance (Ai), the final absorbance (Af), and the
pseudo-first-order rate constant as evaluated parameters.
Su p p or tin g In for m a tion Ava ila ble: Pseudo-first-order
rate constants for the reactions of IQ+, T+, and Xn+ by the
NAD(P)H models 1 [X ) OCH3 (a), CH3 (b), H (c), Cl (d), and
Br (e)] in buffered aqueous solutions (Table 1). This material
is available free of charge via the Internet at http://pubs.acs.org.
J O0009696
Mea su r em en t of Red ox P oten tia ls. All electrochemical
measurements were carried out in dry CH3CN solution under
(31) Zhu, X.-Q.; Xian, M.; Wang, K.; Cheng, J .-P. J . Org. Chem. 1999,
64, 4187.