Solvent dependent leaving group fluorine kinetic isotope effect in a nucleophilic aromatic substitution reaction

Article abstract of DOI:10.1021/ja952003v

Fluorine kinetic isotope effects (F KIEs) have been determined using the accelerator-produced short-lived radionuclide ¹⁸F in combination with natural ¹⁹F. The solvent dependence of the leaving group F KIE was investigated for the nu

Full text of DOI:10.1021/ja952003v

20
J. Am. Chem. Soc. 1996, 118, 20-23
Solvent Dependent Leaving Group Fluorine Kinetic Isotope
Effect in a Nucleophilic Aromatic Substitution Reaction
Jonas Persson, Svante Axelsson, and Olle Matsson*
Contribution from the Institute of Chemistry, Uppsala UniVersity, P.O. Box 531,
S-751 21 Uppsala, Sweden
ReceiVed June 20, 1995X
Abstract: Fluorine kinetic isotope effects (F KIEs) have been determined using the accelerator-produced short-lived
radionuclide 18F in combination with natural 19F. The solvent dependence of the leaving group F KIE was investigated
for the nucleophilic aromatic substitution reaction (SNAr) of 2,4-dinitrofluorobenzene with piperidine at 22 oC. The
F KIE was determined to be 1.0262 ( 0.0007 in tetrahydrofuran (THF) and 0.9982 ( 0.0004 in acetonitrile. The
significant F KIE in THF suggests that, in this solvent, departure of the leaving group is the rate-limiting step. On
the other hand, the almost vanishing KIE in acetonitrile is consistent with addition of the nucleophile to the aromatic
substrate being the rate-limiting step. A full account of the previously communicated (Matsson et al. J. Am. Chem.
Soc. 1993, 115, 5288-5289) experimental method for determination of fluorine KIEs is also given.
Introduction
Scheme 1
Displacement reactions on activated aromatic molecules have
been the subject of considerable mechanistic interest over the
years. A great deal of attention has been directed to nitro
aromatics with halogen or other leaving groups.1 The influence
of nucleophile, solvent, leaving group, and the presence of base
catalysis on kinetic parameters are some of the system variations
that have been employed in these mechanistic investigations.
For several reasons the nucleophilic substitution of 2,4-
dinitrofluorobenzene (DNFB) with secondary amines appeared
to be a good candidate as a model system for the demonstration
of a fluorine KIE. We recently reported2 the first example of
22 °C
THF
+
+
H*F
O2N
*F
NO2
N
H
O2N
N
NO2
= 18 or 19
*
Scheme 2
F
18F
NO2
NO2
CH3CN
+ Kryptofix K+ 18F-
+ Kryptofix K+F-
RT, 2 min
NO2
NO2
a
18F/19F leaving group KIE for such a reaction.
The generally accepted mechanism for nucleophilic aromatic
The kinetic method is based on HPLC fractionation of
reactants and products and is similar to the method for
determination of KIEs using short-lived 11C in combination with
14C which was developed earlier in our laboratory.6 Measure-
ment of the 18F radioactivity was performed by liquid scintil-
lation counting. Accurate measurements of 19F, regarded as
equivalent to the total DNFB,7 were done by careful analysis
using the integrated signal from the UV detector. The KIEs
were calculated from the isotopic fractions of reaction deter-
mined for a series of samples where the extents of reaction were
predetermined by adding different amounts of piperidine to an
excess of DNFB.
substitution (the SNAr mechanism) is an addition-elimination
and involves the formation of a Meisenheimer type of inter-
mediate.3 Whether the rate-limiting step of this mechanism is
the formation of the intermediate or expulsion of the leaving
group has been found to depend on the character of the
nucleophile and the leaving group as well as on the solvent.
The existence of a significant leaving group F KIE for the
reaction of DNFB with piperidine in tetrahydrofuran (THF) at
22 °C (Scheme 1), observed by us earlier, unequivocally
demonstrates that C-F bond cleavage is rate limiting in that
system. Therefore we thought it would be of interest to know
if the rate-limiting step is affected by change of solvent, as
reported by Nudelman et al.,4 and if this could be confirmed
by determining the leaving group F KIE.
The radiochromatograms showed only the reactant peak
(retention time, r.t., 2.45 min). In the UV chromatogram,
toluene (internal standard; r.t. 4.48 min) and the product N-(2,4-
dinitrophenyl)piperidine (r.t. 5.78 min) were observed in
addition to the DNFB peak (see Figure 1).
Results
The 18F labeled DNFB was prepared in an exchange reaction
between unlabeled DNFB and [18F]fluoride (Scheme 2).5 The
optimal yield was obtained after 2 min at room temperature;
prolonged reaction times or heating resulted in a lower yield of
DN18FB.
The KIEs obtained from the kinetic experiments in THF and
acetonitrile were 1.0262 ( 0.0007 and 0.9982 ( 0.0004,
respectively. The results from the different experiments per-
formed in THF and acetonitrile are shown in Table 1.
* To whom correspondence should be addressed. e-mail: ollem@kemi.
uu.se. FAX: +46 18 508542. Phone: +46 18 183797.
X Abstract published in AdVance ACS Abstracts, November 15, 1995.
(1) Terrier, F. Nucleophilic Aromatic Displacement. The Influence of the
Nitro Group; VCH Publishers: Weiheim, 1991.
(2) Matsson, O.; Persson, J.; Axelsson, B. S.; Långstro¨m, B. J. Am. Chem.
Soc. 1993, 115, 5288-5289.
(3) See e.g. ref 1, Chapter 1.
(5) A study of 18F incorporation in DNFB has been reported earlier:
Korguth, M. L.; DeGrado, T. R.; Holden, J. E.; Gatley, S. J. J. Lab. Comp.
Radiopharm. 1988, 25, 369.
(6) (a) Axelsson, B. S.; Långstro¨m, B.; Matsson, O. J. Am. Chem. Soc.
1987, 109, 7233-7235. (b) Axelsson, B. S.; Matsson, O.; Långstro¨m, B. J.
Phys. Org. Chem. 1991, 4, 77-86. (c) Axelsson, B. S.; Matsson, O.;
Långstro¨m, B. J. Am. Chem. Soc. 1990, 112, 6661-6668.
(7) The relative amount of labeled DNFB is approximately 0.01% in
the reaction solution.
(4) Nudelman, N.; Mancini, P. M. E.; Martinez, R. D.; Vottero, L. R. J.
Chem. Soc., Perkin Trans. 2 1987, 951- 954.
0002-7863/96/1518-0020$12.00/0 © 1996 American Chemical Society

SolVent Dependent LeaVing Group F KIEs
J. Am. Chem. Soc., Vol. 118, No. 1, 1996 21
Scheme 3
+
F
NR1R2
NHR1R2
NO2
F
NO2
NO2
k2
k1
+ HF
–
+ NHR1R2
k-1
k3[B]
NO2
NO2
NO2
18
DN FB
A leaving group KIE is expected to monotonously increase with
increasing degree of bond breaking between the isotopic leaving
group atom and the R-carbon in the transition state of the rate-
limiting step. Thus sulfur, oxygen, and chlorine leaving group
KIEs have been reported.10 Nucleophilic reactions on activated
aromatic substrates are particularly attractive as model systems
for fluorine kinetic isotope effects since the reaction is quick
enough to work with short-lived isotopes. Fluorine has also
been employed as the leaving group in many other mechanistic
studies of this reaction type. Nucleophilic aromatic displace-
ment on activated substrates has recently been reviewed in a
very useful monograph by Terrier.1
DNFB
Figure 1. Radiochromatogram (right) and UV chromatogram for the
HPLC analysis of the reaction solution. Toluene is the internal standard.
The product peak was eluted.
The current mechanistic view of the SNAr reaction for neutral
nucleophiles such as primary or secondary amines is an
addition-elimination type of mechanism (see Scheme 3) in
which the nucleophile is first added to the aromatic substrate
forming a zwitterionic intermediate.3 Decomposition of the
intermediate may be base catalyzed as indicated in Scheme 3
(k3[B]). Either the nucleophile or some added base acts as the
catalyst. The observation of base catalysis has been used as a
mechanistic criterion of whether the formation or the decom-
position of the intermediate is rate limiting. The rate expression
for this mechanism, assuming steady state conditions for the
intermediate, is given by eq 1.
Table 1. The 18F/19F KIEs Determined for the Reaction of Labeled
DNFBa with Piperidine in THF and Acetonitrile at 22 °C
expt
no.
point
KIE ( std dev
no. of
points
KIEs obtained in
expt #2
1.0257 (2.91)b
1.0158 (2.47)b
1.0250 (1.65)b
1.0339 (0.859)b
solvent
THF
THF
THF
acetonitrile
acetonitrile
1
2
3
4
5
1.0274 ( 0.0053
1.0251 ( 0.0074
1.0260 ( 0.0064
0.9978 ( 0.0061
0.9985 ( 0.0071
5
4
2
5
2
a The concentration of DNFB was ca. 3.3 10-3 M. The last column
shows the individual KIEs obtained in experiment #2. b Starting
concentration of piperidine (mM).
d[Ar]/dt ) kA[Ar][Nuc];
kA ) (k1k2 + k1k3[B])/(k-1 + k2 + k3[B]) (1)
Discussion
Variations in the relative rates for the different elementary steps
yield situations where formation or breakdown of the intermedi-
ate is rate limiting. Three different cases have been experi-
mentally demonstrated:
(i) k2 + k3[B] . k-1. Base catalysis is not observed. This
occurs when the first elementary reaction step i.e., formation
of the intermediate, is rate limiting. The rate expression
simplifies to
In mechanistic investigations fluoride is commonly employed
as a leaving group in elimination and substitution reactions.
The determination and interpretation of kinetic isotope effects
provide one of the most important tools in physical organic and
bioorganic chemistry. It therefore seemed worthwhile to try to
include fluorine among the heavy atoms used for kinetic isotope
effect measurements. Since natural fluorine consists of 100%
of the isotope 19F and since no long-lived radioisotopes are
available, the only way to accomplish determination of F KIEs
is to use a short lived radionuclide. Among these the accelera-
tor-produced 18F has a convenient half-life of 110 min and is
routinely produced in many laboratories; the isotope 18F is used
in the labeling of radiopharmaceuticals and other compounds
used for biomedical research and clinical diagnosis utilizing the
PET-imaging technique.8 Nucleophilic as well as electrophilic
labeling reagents are available and quite a number of compounds
have been labeled with 18F; these include carbohydrates, alkyl
halides, fatty acids, and steroids.9
kA ) k1
(2)
(ii) k2 + k3[B] , k-1. The intermediate is formed in a
preequilibrium and decomposition of the intermediate is rate
limiting. Base catalysis is observed and the rate constant
depends linearly on the concentration of base (eq 3).
kA ) k1/k-1(k2 + k3[B]) ) k′ + k′′[B]
(3)
(iii) k2 + k3[B] ≈ k-1. Base catalysis is observed but in this
case kA depends on [B] in a curvlinear fashion.
The base catalysis in reactions where decomposition of the
intermediate is rate limiting or partially rate limiting may operate
Via two different mechanisms: The specific base-general acid
(SB-GA) mechanism or the rate-limiting proton transfer mech-
anism (RLPT).3 In both cases the base deprotonates the initially
formed zwitterionic intermediate ZH (Scheme 4) yielding the
anionic intermediate Z-.
In the SB-GA mechanism a rapid base catalyzed proton
abstraction from ZH takes place, followed by rate limiting
general acid catalyzed expulsion of the leaving group from the
anionic intermediate Z-.12 Evidence for this mechanism have
been obtained in dipolar aprotic media.13 The situation in media
Leaving group KIEs are fairly easy to interpret and have been
utilized for a long time in mechanistic investigations of
nucleophilic aliphatic substitution10 and elimination reactions.11
(8) (a) Greitz, T.; Ingvar, D. H.; Wide´n, L., Eds, The Metabolism of the
Human Brain Studied with Positron Emission Tomography; Raven Press:
New York, 1985. (b) Phelps, M.; Mazziotta, I.; Schelbert, H., Eds. Positron
Emission Tomography and Autoradiography: Principles and Applications
for the Brain and Heart; Raven Press: New York, 1986.
(9) Kilbourn, M. R. Fluorine-18 Labeling of Radiopharmaceuticals;
Nuclear Science Series NAS-NS-3203; National Academy Press, Wash-
ington, DC, 1990.
(10) Shiner, V. J., Jr.; Wilgis, F. P. In Isotopes in Organic Chemistry;
Elsevier: Amsterdam, 1992; Vol. 8, Chapter 6.
(11) Melander, L.; Saunders, W. H., Jr. Reaction Rates of Isotopic
Molecules; John Wiley & Sons: New York, 1980; Chapter 9.2.
(12) Reference 1, Chapter 1.6.1, and references therein.

22 J. Am. Chem. Soc., Vol. 118, No. 1, 1996
Persson et al.
Scheme 4
inverse according to the argument concerning the KIE for this
step discussed above for case (i). In fact the maximal secondary
KIE for the addition step should be equal to this EIE i.e., the
limiting value obtained for a TS with a structure identical to
the intermediate ZH. The third factor in eq 6 is the EIE for the
proton transfer reaction ZH + B a Z- + BH+. This factor is
expected to be very close to unity, since the force field for the
isotopic heavy atom is hardly changed at all in the proton
transfer process. Thus the main contributor to the leaving group
fluorine KIE for case (ii) is the second factor in eq 6. This
factor corresponds to the KIE for the general acid assisted
departure of the fluorine from the anionic intermediate Z-. A
maximum value for this KIE was earlier estimated to be
approximately 3% using a simple diatomic C-F model.2
H
L
+NR1R2
NO2
NR1R2
NO2
L
L
NO2
k3[B]
k1
–
–
+ R1R2NH
k-3[BH]
k-1
NO2
ZH
NO2
Z-
NO2
k4[BH]
k2
NR1R2
NR1R2
NO2
B
H
L
NO2
–
Our observation of a significant leaving group fluorine KIE
(2.6%) for the reaction run in THF thus clearly demonstrates
that departure of the leaving group is rate limiting in this solvent,
i.e., case (ii) of the SB-GA mechanism above holds. This is
consistent with Nudelmans finding4 that the reaction is subject
to base catalysis in THF. The close to maximal value for the
fluorine KIE determined suggests that breaking of the carbon-
fluorine bond is well advanced in the rate limiting TS. In fact,
the KIE should be even somewhat larger than the observed value
since it is multiplied by the slightly inverse EIE for the addition
step (eq 6).
NO2
NO2
of low permittivity14 is sometimes complicated by existence of
dimers of the nucleophile15 and by formation of complexes
between the substrate and the nucleophile or base catalyst.14,16
In the RLPT mechanism,17 base promoted deprotonation of
the zwitterionic intermediate ZH is rate limiting. This mech-
anism is operative mainly in protic media.
The expression for the observed rate constant for the SB-
GA mechanism in Scheme 4, based on the steady state
assumption for the intermediates, is given by eq 4,
In acetonitrile no significant fluorine KIE is observed,
although the small inverse value determined might be attributed
to the very small secondary effect expected for rate limiting
formation of the intermediate ZH (case (i)). Again, the leaving
group KIE is consistent with the conclusions based on variation
of the concentration of base: the first reaction step is rate
limiting since no base catalysis was found for the reaction run
in acetonitrile.4,18 This change of rate limiting step between
the solvents THF and acetonitrile has been attributed to their
different hydrogen bond donating (HBD) abilities.4 Solvent
HBD acidity may be estimated using the R-parameters by
Kamlet and Taft.19 Acetonitrile has a finite albeit small R-value
(0.270), whereas THF has an R-value of zero. Accordingly,
acetonitrile being a better HBD solvent would act as a general
acid to assist departure of the fluoride resulting in a diminished
need for catalysis by the conjugate acid of the base. The F
KIE would, however, be smaller in acetonitrile even if expulsion
of fluoride was rate limiting or partially rate limiting in this
solvent since the interaction between the departing fluoride and
a solvent molecule tends to decrease the isotope effect. The
importance of solvation effects on chlorine KIEs has been
experimentally demonstrated by Cromartie and Swain.20
Except for a few studies of deuterium KIEs,21,22 the literature
is lacking in reports of isotope effects for SNAr reactions. In a
study by Hart and Bourns23 the 18O leaving group KIE for the
nucleophilic aromatic substitution reaction of 1-phenoxy-2,4-
dinitrobenzene with piperidine in aqueous dioxane was deter-
mined. In that investigation k16/k18 was found to decrease with
increasing concentration of hydroxide ion added; this was
interpreted as evidence for a change from rate-limiting decom-
position of the intermediate to a rate-limiting addition step.
kA ) (k1k2 + k1k4K3[B])/(k-1 + k2 + k4K3[B])
(4)
where K3 is the equilibrium constant for deprotonation of ZH
to Z- and k4 is the rate constant for the general acid catalyzed
expulsion of the leaving group from Z-. Again, as for the
simplified mechanism in Scheme 3, there are two limiting
cases: (i) fully rate limiting formation of the zwitterionic
intermediate, i.e. rate-limiting addition of the nucleophile to the
substrate, and (ii) rate-limiting departure of the leaving group
in a catalyzed step. The kinetic isotope effect for case (i) is
given by eq 5,
KIEobs ) k1/k1*
(5)
where the asterisk indicates the heavy isotope. For the present
case with isotopic fluorine as a leaving group a secondary KIE
may be observed for k1. The hybridization for the carbon bound
to fluorine changes from sp2 to sp3 when going from the reactant
to the transition state. This is associated with a small increase
of the force field for the isotopic atom since one normal mode
out-of-plane bending vibration is transformed to a valence angle
bending vibration possessing a somewhat higher frequency.
Thus, the leaving group fluorine isotope effect for the addition
step is expected to be very small and inverse.
For case (ii) the expression for the KIE is more complex and
made up of three different contributions (eq 6), assuming that
the uncatalyzed path k2 is negligible as compared to the
catalyzed decomposition k4.
KIEobs ) (K1/K1*)(k4/k4*)(K3/K3*)
(6)
The first factor (K1/K1*) in eq 6 is the equilibrium isotope effect
(EIE) for the addition step. This EIE should be small and
(13) (a) Orvik, J. A.; Bunnett, J. F. J. Am. Chem. Soc. 1970, 92, 2417-
2427. (b) Bunnett, J. F.; Garst, R. H. J. Am. Chem. Soc. 1965, 87, 3879-
3884. (c) de Rossi, R. H.; Rossi, R. A. J. Org. Chem. 1974, 39, 3486-
3488. (d) Bamkole, T. O.; Hirst, J.; Onyido, I. J. Chem. Soc., Perkin Trans.
2 1982, 889-893.
(14) Hirst, J. J. Phys. Org. Chem. 1994, 7, 68-79 and references therein.
(15) Nudelman, N. S. J. Phys. Org. Chem.1989, 2, 1-14.
(16) Forlani, L.; Bosi, M. J. Phys. Org. Chem . 1992, 5, 429-434 and
references therein.
(18) Ayediran, D.; Bamkole, T. O.; Hirst, J. J. Chem. Soc., Perkin Trans.
2 1976, 1396-1398.
(19) Kamlet, M. J.; Taft, R. W. J. Chem. Soc., Perkin Trans. 2 1979,
349-356.
(20) (a) Cromartie, T. H.; Swain, C. G. J. Am. Chem. Soc. 1976, 98,
545-550. (b) Cromartie, T. H.; Swain, C. G. J. Am. Chem. Soc. 1976, 98,
2962-2965.
(21) For a review see: Zollinger, H. In AdVances in Physical Organic
Chemistry; Gold, V., Ed.; Academic Press: London, 1964; Vol. 2, pp 163-
200.
(17) (a) Bunnett, J. F.; Randall, J. J. J. Am. Chem. Soc. 1958, 80, 6020-
6030. (b) Bernasconi, C. F.; de Rossi, R. H.; Schmid, P. J. Am. Chem. Soc.
1977, 99, 4090-4101.
(22) Hawthorne, M. F. J. Am. Chem. Soc. 1954, 76, 6358-6360.
(23) Hart, C. R.; Bourns, A. N. Tetrahedron Lett. 1966, 2995-3002.

SolVent Dependent LeaVing Group F KIEs
J. Am. Chem. Soc., Vol. 118, No. 1, 1996 23
of the solvent in Vacuo. The procedure of addition and distillation of
acetonitrile was repeated three times after which a final 1 mL of
acetonitrile was added. To this solution was added 1 mL of a 0.1 M
solution of DNFB in acetonitrile. After 2 min of reaction at room
temperature the solution was introduced onto a silica column (3 mL,
neutral, washed with 15 mL of acetonitrile) and the product was eluted
with acetonitrile. After evaporation of the solvent in Vacuo, a 5-mL
solution of THF or acetonitrile containing toluene as an internal standard
was added yielding a final concentration of DNFB of approximately
0.02 M. The concentration of DNFB was determined by HPLC (UV)
using the calibration curve.
Kinetic Procedure. Five vials containing 1.00-mL solutions of
different concentrations of piperidine in THF or acetonitrile were
prepared. The vials were capped and thermostated at 22.00 °C after
which the reaction was started by addition of 0.200 mL of the DNFB
solution Via a thermostated syringe. The DNFB was in excess in all
vials. The amounts of piperidine were chosen to yield extents of
reaction ranging from 0% up to 70%. The concentration of the
piperidine was usually between 8.3 × 10-4 and 2.5 × 10-3 M in the
reaction samples. After complete reaction, each sample was diluted
with acetonitrile (the amount determined by weight) so that a linear
UV response was obtained. The radioactive fluoride formed in the
reaction was removed by passing the reaction solution through a silica
column or by direct injection on the HPLC column. Since no difference
could be detected, the simpler method of direct injection was followed.
The analysis of the eluent did not reveal any significant amount of
radioactive fluorine passing through the column. Careful analysis of
the background radioactivity was performed during the entire experi-
ment to ensure that no radioactive fluorine was leaking out of the
column. Each sample was analyzed five times by HPLC and the DNFB
fraction was collected in scintillation bottles containing 15 mL of
scintillation liquid. The 18F radioactivity of the fraction bottles was
immediately measured by liquid scintillation counting and the fractions
of least radioactivity being measured first. The 18F-CPM (counts per
minute) were corrected for background and half-life. The half-life
corrections were made according to eq 7:
Conclusions
A method has been developed which permits the determina-
tion of fluorine KIEs accurate enough to be mechanistically
significant. This should be of general use in the study of
incoming and leaving group KIEs in organic and bio-organic
reaction systems.
For the SNAr reaction of DNFB with piperidine a significant
F KIE is observed when the solvent is THF. When acetonitrile
is used as the solvent no such F KIE is observed. These
observations are consistent with a switch in rate-limiting step
from the addition step (in acetonitrile) to the elimination step
(in THF).
Experimental Section
General. The 18F was obtained as [18F]fluoride (specific activity
approximately 180 GBq/µmol) in 30-90% 18O-enriched water in a 18O-
(p,n)18F nuclear reaction24 using the Scanditronix MC-17 cyclotron at
the Uppsala University PET Centre. A proportional regulating
thermostat (HETO) with a regulating accuracy of (0.005 °C was used.
The temperature of the water in the thermostat never deviated more
than (0.02 °C from the reported value during a kinetic run. The
glassware used was washed in chromic acid and then with water and
ethanol, dried at 150 °C, and kept over silica gel in a desiccator under
nitrogen atmosphere. The syringes were washed with 2.0 M hydro-
chloric acid, 2.0 M sodium hydroxide, distilled water, ethanol, and
finally dry ether. The syringes were kept over silica gel in a desiccator
under a nitrogen atmosphere until used.
The HPLC analyses were performed on a Hewlett Packard 1084
HPLC with a â+-flow detector in series with the UV detector of the
instrument. The HPLC was equipped with a fraction collector (Hewlett
Packard 79825 A). The fraction collector was slightly modified by
removing the Teflon insert. The HPLC analyses were performed on a
column, 200 × 4.6 mm, packed with Nucleosil RP C-18, 5 µm. The
mobile phase was 0.05 M ammonium formate, pH 3.5, and methanol
gradient flow 2.00 mL/min (0-4 min, 60% MeOH; 4-5 min, 60-
80% MeOH; 5-7 min, 80% MeOH; 7-8 min, 80-60% MeOH). The
wavelength used was 254 nm using 430 nm as a reference. A UV
calibration curve was obtained by plotting the integrated UV area versus
different concentrations of DNFB. Usually 10 samples were used to
construct the calibration curve.
The radioactive counting was performed using a liquid scintillation
counter LKB 1214 with the energy window set to 1-2000 keV. The
counting time was 1-2 min depending on the amount of radioactivity
present. The radioactive HPLC fractions (usually 4 mL) were collected
in scintillation bottles containing 15 mL of scintillation liquid (Zinsser
Quickzint 1). Analytical GC was run on a Varian 3400 gas chromato-
graph, equipped with a flame-ionization detector and a Varian 4270
integrator, using a DB5-30W capillary column. 1H NMR spectra were
obtained with a Varian Unity 400 spectrometer. The evaporations were
performed using a Bu¨chi Rotavapor M connected to an Edwards High
Vacuum Pump.
)
1/2
CPMcorr ) Z/{0.5(t/t
}
(7)
where CPMcorr and Z are the uncorrected and the decay-corrected 18F
CPM values, respectively, t is the elapsed time in seconds, and t1/2 is
the 18F half-life. Each sample was counted five times and the mean
value from these measurements was used in the calculations. The dead-
time in liquid scintillation counting is dependent on the total amount
of radioactivity present in the samples. Dead-time corrections made
by the instrument were checked and found to be accurate up to 15%
but no data with dead-times over 6% were used. At a dead-time of
5% the CPM values were approximately between 70000 and 150000
CPM. The KIE was calculated as the mean value of the KIE at each
point according to eq 8 which is valid when the reaction is of first
order in the labeled reactant.25
k18/k19 ) ln(1 - f18)/ln(1 - f19)
(8)
The fraction of reaction for DN18FB, f18, was calculated as the ratio {1
- C(x)}/{1 - C(0)} for each sample, where C(x) is the corrected CPM
value for unreacted DN18FB at x% reaction and C(0) is the corrected
CPM value at 0% reaction (no piperidine added). The fraction of
reaction of DN19FB, regarded as equivalent to the total fraction of
reaction,7 was determined by dividing the ratio of the reactant and
internal standard UV areas for each sample with that obtained for 0%
reaction. The quench error was low in these experiments since the
composition of the mobile phase was identical for all the fraction bottles.
Materials. 2,4-Dinitrofluorobenzene (from Aldrich) was used as
bought without further purification and was stored over silica gel in a
desiccator under a nitrogen atmosphere. The purity (99%) was
determined prior to use by 1H NMR and HPLC. Tetrahydrofuran (THF)
and acetonitrile, predried with molecular sieves, were freshly distilled
from calcium hydride and kept over 3 Å molecular sieves under a
nitrogen atmosphere. Piperidine predried with molecular sieves was
freshly distilled from calcium hydride in a Fischer Spaltrhor distillation
apparatus and kept under a nitrogen atmosphere. The purity (>99%)
Acknowledgment. We thank Professor Bengt Långstro¨m
and his co-workers for putting the excellent facilities of the PET-
center, Uppsala University at our disposal. We thank Professor
Go¨ran Bergson for valuable discussions. The project is
financially supported by the Swedish Natural Science Research
Council (NFR).
1
was determined using H NMR and GC.
2,4-Dinitro[18F]fluorobenzene. To an aqueous solution (0.5-1 mL)
of [18F]fluoride (usually 0.5-1.9 GBq) was added Kryptofix[2.2.2]
(4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane, 6.2 mg,
0.0166 mmol), potassium carbonate (2.3 mg, 0.0166 mmol), and 1 mL
of acetonitrile. The water was removed azeotropically by evaporation
JA952003V
(24) 13N was formed as a side product in the nuclear reaction 16O(p,R)-
13N. This radionuclide has a half-life of 10 min.
(25) Reference 11, Chapter 4.2.1