3
3
configuration intermediate between reactants and products.
This is followed by the second step in which the hydride is trans-
ferred, probably accompanied by tunnelling. In the third step,
the products are stabilized by further solvent and heavy-atom
reorganization. This description is somewhat consistent with
the mechanism shown in Scheme 2 provided that the steps are
considered to involve the formation of intermediates as well as
three distinct transition states rather than a single transition
pared as described in the literature. 1-Benzyl-3-cyanoquinol-
inium perchlorate was prepared by ion exchange of 1-benzyl-3-
cyanoquinolinium bromide obtained from the reaction of
17
3-cyanoquinoline with benzyl bromide. The bromide salt was
dissolved in dry acetonitrile in the presence of a 50-fold excess
of sodium perchlorate. The solvent was evaporated until a small
amount of sodium iodide precipitated. The precipitate was
removed by filtration. After evaporation of the solvent the
residue was washed with water and collected by filtration. The
process was repeated twice to ensure complete exchange. The
resulting solid was recrystallized from absolute ethanol to give
the perchlorate salt (mp 198–200 ЊC, lit. 202 ЊC). It should
be noted that organic percholorates are explosive and must
be handled with care. Acetonitrile was refluxed over KMnO4
and K CO and twice distilled from P O under a nitrogen
17
state as implied.
A detailed study of potential energy surfaces for hydride
transfer using variational transition state theory along with
linearized Marcus theory led to the conclusion that hydride
17
15
tunnelling is of importance in these reactions. The calcu-
Ϫ1
lations led to the conclusion that about 1 kcal mol of the
reaction barrier is evaded by hydride tunneling.
2
3
2
5
Our resolution of the kinetics of the reaction between
atmosphere.
ϩ
MAH(D) and BQCN in acetonitrile provides support for
17,15
many of the conclusions from previous work
based on the
Kinetic experiments
apparent rate constants. We now extend these conclusions with
more direct experimental evidence for tunnelling including
large KIEreal and Arrhenius parameters for the hydride transfer
step which indicate significant hydride tunnelling. Our data
suggest that as much as 9 kcal mol of the reaction barrier for
hydride transfer is circumvented by tunnelling.
ϩ
The kinetics of the reactions between MAH(D) and BQCN
ϩ
were followed by the appearance of absorbance due to MA at
36 nm using an HP8452A Diode Array Spectrometer housed
in a glove box ([O ] < 1 ppm). The cuvettes were tightly stop-
pered in order to avoid evaporation of acetonitrile solvent. The
temperature was controlled to ±0.5 K. Digital absorbance–time
data were smoothed using the 15-point least-squares procedure
4
Ϫ1
2
Since we observe the EDA complex as a possible inter-
mediate in the hydride transfer reaction, the possibility that the
34
of Savitsky and Golay before conversion into extent of
reaction takes place by an inner-sphere electron transfer (K )
et
ϩ
reaction ([MA ]/[MAH(D)] )–time profiles.
31
0
mechanism must be considered. However, since K is equal to
et
31
K /K
and KEDA is close to unity over the entire temperature
3
EDA
Determination of equilibrium constants for the reaction of MAH
range K ≈ K . Thus the inner-sphere electron transfer mechan-
et
3
؉
with BQCN in acetonitrile
ism is ruled out by the arguments presented in the Introduction
to rule out the outer-sphere electron transfer mechanism.
ϩ
A solution of MAH (2.97 mM) and BQCN (3.13 mM) in
acetonitrile was sealed in a cuvette and placed in the glove box
at about 20 ЊC for 14 days. The cuvette was placed in the
spectrometer and the temperature adjusted to the desired value.
The spectrum was recorded periodically until no further change
took place as equilibrium was approached. Repeat values at
different temperatures gave good correspondence and indicated
that no decomposition occurs under the conditions of the
experiments. Equilibrium constants were calculated from the
absorbance and the initial concentrations of reactants.
Conclusions
The facts that (i) extent of reaction–time profiles for the reac-
tions between MAH and BQCN deviate significantly from
those expected for the irreversible second-order mechanism (5),
ϩ
(
ii) significant extent of reaction dependent apparent kinetic
isotope effects are observed and (iii) and an intermediate EDA
complex is observed rule out mechanism (5) for the hydride
transfer reaction. All of the data are consistent with the
reversible consecutive second-order mechanism illustrated in
Scheme 2. Needless to say, our data do not conclusively prove
the latter mechanism for the hydride transfer reaction between
Determination of the equilibrium constant for the formation of
؉
the EDA complex between MAH and BQCN in acetonitrile
Equal volumes of solutions of MAH (0.025–0.1 M) and
ϩ
MAH and BQCN . What we have shown with certainty is
ϩ
BQCN (0.01 M) were mixed at the appropriate temperature
that the reversible consecutive second-order mechanism pro-
vides an appropriate model for the discussion of the reaction
mechanism.
Our overall conclusion is that hydride transfer reactions join
the growing list of organic reactions which follow the reversible
consecutive second-order mechanism, as illustrated for proton
transfer by eqn. (4) and for hydride transfer by eqn. (6) or
Scheme 2. Reactions which have been reported to follow the
reversible consecutive second-order mechanism include the pro-
and the absorbance at 526 nm was recorded as a function of
time using the Hi-Tech Scientific SF-61 stopped-flow spec-
trometer. Initial concentrations were such that [MAH]0/
[
BQCN] were equal to 25, 50, 75 and 100. The absorbance–
0
time curves were effectively flat at short times and [EDA Com-
plex] were determined by averaging 50 data points gathered at
about 10 ms after mixing. The data were treated according to
eqn. (11) to determine KEDA and ε526. The time dependence of
[EDA complex] over longer time intervals is illustrated by the
21–23
ton transfer reactions of methylanthracene radical cations
data in Fig. 7.
24
and those of nitroalkanes. It is anticipated that further studies
will reveal other classes of organic reactions which conform to
this general mechanism.
Acknowledgements
This research was supported by the National Science Found-
ation (CHE-0074405). We gratefully acknowledge this support.
Experimental
Materials
References
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one. 10-Methyl-9,10-dihydroacridine was prepared by reduc-
tion of N-methylacridinium iodide using sodium borohydride
in dry methanol, followed by recrystallization from absolute
1
(a) J. W. Bunting, Bioorg. Chem., 1991, 19, 456; (b) S. Yausi and
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2
A. Anne, J. Moiroux and J. M. Savéant, J. Am. Chem. Soc., 1993,
1
15, 10224.
32
ethanol. 10-Methyl-9,10-dihydroacridine-10,10-d2 was pre-
3 J.-P. Cheng, Y. Lu, X. Zhu and L. Mu, J. Org. Chem., 1998, 63, 6108.
1
80
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 1 7 3 – 1 8 1
Lu, Yun
