Bimolecular kinetic studies with high-temperature gas-phase 19F NMR: Cycloaddition reactions of fluoroolefins

Article abstract of DOI:10.1021/ja0111277

A gas-phase NMR kinetic technique has been used for the first time to obtain accurate measurements of rate constants of some bimolecular, second-order cycloaddition reactions. As a test of the potential use of this technique for the study of second-order reactions, the rate constants and the activation parameters for the cyclodimerization reactions of chlorotrifluoroethylene (CTFE) and tetrafluoroethylene (TFE) were determined in the temperature range 240-340 °C, using a commercial high-temperature NMR probe. Obtaining excellent agreement of the results with published data, the technique was then applied to the reaction of 1,1-difluoroallene with 1,3-butadiene, the results of which indicate that the use of gas-phase NMR for reaction kinetics is particularly valuable when a reagent is available only in small amounts and in cases where there are several competing processes occurring simultaneously. The major processes observed in this reaction are regioselective [2+2] and [2+4] cycloadditions, whose rates and activation parameters were determined [k₂=9.3×10⁶ exp(-20.1 kcal mol^-1/RT) L/mol^-1 S^-1 and k₃=1.2×10⁶ exp(- 18.4 kcal mol^-1/RT) L/mol^-1 S^-1, respectively] in the temperature range 130-210 °C.

Full text of DOI:10.1021/ja0111277

9956
J. Am. Chem. Soc. 2001, 123, 9956-9962
19
Bimolecular Kinetic Studies with High-Temperature Gas-Phase F
NMR: Cycloaddition Reactions of Fluoroolefins†
Alexander B. Shtarov,*,‡,¶ Paul J. Krusic,*,§ Bruce E. Smart,*,§ and
William R. Dolbier, Jr.*,‡
Contribution from the Department of Chemistry, UniVersity of Florida, GainesVille, Florida 32611-7200,
and DuPont Central Research and DeVelopment, Experimental Station, Wilmington, Delaware 19880
ReceiVed May 4, 2001
Abstract: A gas-phase NMR kinetic technique has been used for the first time to obtain accurate measurements
of rate constants of some bimolecular, second-order cycloaddition reactions. As a test of the potential use of
this technique for the study of second-order reactions, the rate constants and the activation parameters for the
cyclodimerization reactions of chlorotrifluoroethylene (CTFE) and tetrafluoroethylene (TFE) were determined
in the temperature range 240-340 °C, using a commercial high-temperature NMR probe. Obtaining excellent
agreement of the results with published data, the technique was then applied to the reaction of 1,1-difluoroallene
with 1,3-butadiene, the results of which indicate that the use of gas-phase NMR for reaction kinetics is
particularly valuable when a reagent is available only in small amounts and in cases where there are several
competing processes occurring simultaneously. The major processes observed in this reaction are regioselective
[2+2] and [2+4] cycloadditions, whose rates and activation parameters were determined [k2 ) 9.3 × 106
exp(-20.1 kcal mol-1/RT) L/mol-1 s-1 and k3 ) 1.2 × 106 exp(-18.4 kcal mol-1/RT) L/mol-1 s-1, respectively]
in the temperature range 130-210 °C.
Introduction
spectra, remote viewing of the progress of the reaction, and easy
construction of kinetic reaction profiles showing the time
evolution of the concentrations of all reaction components that
contain the detected nucleus. Unlike conventional techniques
for vapor-phase kinetics, gas-phase NMR is an in situ technique,
specific for each detected molecule within the limitations of
spectral resolution, which does not require periodic sample
extractions and extensive calibrations of analytical instruments.
In essence, a sealed glass ampule containing micromolar
amounts of reagents, estimated so as not to produce excessive
pressure after complete vaporization, is placed into the probe
of the spectrometer, which is maintained at the desired reaction
temperature. Spectra are then acquired automatically at prear-
ranged time intervals to follow the chemical changes that are
taking place inside the ampule.
The major difference between liquid- and gas-phase NMR
is that NMR absorptions in the gas phase are broader than in
liquids because of shorter T1 nuclear spin relaxation times caused
by efficient spin-rotation interaction. There is, therefore, a
reduction in signal intensity for the same number of spins per
unit volume that can be easily compensated for by faster signal
averaging made possible by the shorter T1 relaxation times. For
fluorine detection, several hundred pulses in a minute are
commonly used. Since lines are broader there is no need to spin
the samples, and field homogeneity is much less important than
for high-resolution NMR. Furthermore, since acquisition times
are shorter, a distinct advantage for kinetic work, there is also
no need for deuterium field locks. The fluorines of a prototypical
CF3 group have T1 values of 30-50 ms corresponding to line
widths of about 40 Hz at a pressure of about 1 atm. Narrower
lines can be observed for other types of fluorines. A decrease
in line width is observed as the pressure and the molecular size
are increased due to the effects of intermolecular collisions.
Resolution can thus be improved by increasing reactant con-
Rate constants for second-order gas-phase reactions have been
commonly measured by monitoring pressure changes in constant
volume systems and by concomitant chemical analyses typically
involving gas chromatography. The latter requires periodic
sample extractions causing mass loss and extensive standardiza-
tion with authentic materials, and it is time-consuming. For
carrying out quantitative vapor-phase kinetic studies, gas-phase
NMR is an efficient alternative technique, the potential of which
has been largely overlooked. The substantial literature on gas-
phase NMR1 mainly deals with physical studies (spin relaxation,
conformational analysis, hydrogen bonding, hindered rotation,
tautomerism, etc.) with little mention of reaction product analysis
or kinetics2 until our recent publications.3 The kinetic measure-
ments are simplified due to the automation of modern NMR
spectrometers that allows unattended runs, fast processing of
† DuPont contribution number 8193.
* Authors to whom correspondence should be addressed. E-mail:
Alexander.B.Shtarov@usa.dupont.com; Paul.J.Krusic@usa.dupont.com;
Bruce.E.Smart@usa.dupont.com; wrd@chem.ufl.edu.
‡ University of Florida.
§ DuPont Central Research and Development.
¶ Present address: E. I. Du Pont de Nemours and Co., Jackson
Laboratory, Chambers Works, Deepwater, NJ 08023.
(1) For reviews see: (a) Govil, G. Appl. Spectrosc. ReV. 1973, 7, 47. (b)
Harris, R. K. Org. Magn. Reson. 1983, 21, 580. (c) Armstrong, R. L. Magn.
Reson. ReV. 1987, 12, 91. (d) Jameson, C. J. Chem. ReV. 1991, 91, 1375.
(e) Jameson, C. J. In Encyclopedia of NMR; Grant, D. M., Harris, R. K.,
Eds.; Wiley: New York, 1996; Vol. 4, p 2179. (f) True, N. S. In
Encyclopedia of NMR; Grant, D. M., Harris, R. K., Eds.; Wiley: New York,
1996; Vol. 4, p 2173. (g) LeMaster, C. B. J. Prog. Nuclear Magn. Spectrosc.
1997, 31, 119.
(2) (a) Haugh, M. J.; Dalton, D. R. J. Am. Chem. Soc. 1975, 97, 5674.
(b) Costello, F.; Dalton, D. R.; Poole, J. A. J. Phys. Chem. 1986, 90, 5352.
(3) (a) Kating, P. M.; Krusic, P. J.; Roe, D. C.; Smart, B. E. J. Am.
Chem. Soc. 1996, 118, 10000. (b) Roe, D. C.; Kating, P. M.; Krusic, P. J.;
Smart, B. E. Top. Catal. 1998, 5, 133. Krusic, P. J.; Roe, D. C.; Smart, B.
E. Isr. J. Chem. 1999, 39, 117-123.
10.1021/ja0111277 CCC: $20.00 © 2001 American Chemical Society
Published on Web 09/21/2001

Cycloaddition Reactions of Fluoroolefins
J. Am. Chem. Soc., Vol. 123, No. 41, 2001 9957
centrations or by the addition of an inert buffer gas within the
limits of safe internal pressures in the ampule and complete
vaporization required for quantitative analysis. The reduced
resolution in gas-phase 19F NMR is usually of little consequence
because of the wide dispersion of the chemical shifts for fluorine
nuclei in different chemical environments.4 A wide bore magnet
allowing the use of 10 mm OD ampules is a definite advantage
for kinetic gas-phase NMR, as is a commercial high-temperature
probe with an upper temperature limit of 400 °C.5 No other
specialized equipment is required beyond a conventional vacuum
system with an accurate pressure transducer and inexpensive
ampules that are now commercially available.6
The aim of the current study was, first, to validate gas-phase
19F NMR as a technique for making reliable kinetic measure-
ments of second-order reactions and, second, to use the method
to study the kinetics of the cycloaddition reactions of 1,1-
difluoroallene with 1,3-butadiene, for which no kinetic data are
available.7,8 The latter reaction is of considerable kinetic and
mechanistic interest because [2 + 2] and [2 + 4] cycloadditions,
mechanistically quite distinct, are in competition with each other
and, to our knowledge, no experimental kinetic data or activation
parameters for allene cycloadditions are presently available.9
To carry out the validation of this kinetic technique, it was
decided to reexamine the kinetics of two prototypical cy-
clodimerization reactions involving fluoroolefins, those of
chlorotrifluoroethylene and tetrafluoroethylene, whose rates and
kinetic activation parameters have been determined quite
accurately. The kinetics of the thermal cyclodimerization of
chlorotrifluoroethylene (CTFE)10,11 and tetrafluoroethylene
(TFE)10,11b,12 have been studied by several investigators by
measuring the pressure changes during the course of the reaction
in heated vessels of 250 mL to 1 L volume. At pressures of 1
atm or less, these cyclodimerizations (eq 1) are well-behaved
homogeneous second-order reactions, with rates that are inde-
pendent of pressure and unaffected by heterogeneous wall
reactions. The reactions are believed to proceed via a mechanism
involving biradical intermediates.13,14 The cyclodimerization of
CTFE at temperatures below 410 °C is remarkably regioselec-
tive, giving almost exclusively cis- and trans-1,2-dichloro-
hexafluorocyclobutane, 1, which are formed initially at equal
Figure 1. CTFE dimerization: 19F NMR near the start of the reaction
(top); 19F NMR near the end of the reaction (bottom).
also been previously measured. The dimerization of TFE to
octafluorocyclobutane also proceeds quite cleanly at pressures
of less than 1 atm, showing signs of reversibility at around 500
°C. Above this temperature the dimer dissociates into other
products, including hexafluoropropylene, by complex carbene
mechanisms.
Results and Discussion
CTFE and TFE Cyclodimerizations. Figure 1 (top) shows
the 19F NMR spectrum that was obtained a few minutes after
inserting a sealed, 3.8 mL internal-volume ampule containing
100 µmol of CTFE into the probe at 350 °C. The initial
concentration of CTFE is 0.026 mol/L and the approximate
initial pressure, calculated by the ideal gas law at 350 °C, is
1.3 atm.15 Tests demonstrated that temperature equilibration in
these experiments requires no more than 3 min, a time that is
very short compared to the time scale of the reactions that are
being considered here. The three major resonances with partially
resolved spin-spin coupling for the three vinylic fluorines agree
well with published data in solution. In this case, rather
exceptionally, the NMR spectrum was also studied in the gas
phase to provide information on the 19F nuclear magnetic
shielding for the three different vinylic environments in
rates. Equilibration favoring the thermodynamically more stable
trans isomer occurs at higher temperatures. Above 420 °C the
dimers begin to dissociate into starting CTFE at rates that have
(4) T1 values are longer for protons in the gas phase with typical line
widths of a few hertz.
(5) NALORAC Corporation, 841A Arnold Drive, Martinez, CA 94553.
E-mail: sales@nalorac.com.
(6) New Era Enterprises, P.O. Box 425, Vineland, NJ 08360-0425.
E-mail: fbosco@newera-nmr.com.
(7) Dolbier, W. R., Jr.; Piedrahita, C. A.; Houk, K. N.; Strozier, R. W.;
Gandour, R. W. Tetrahedron Lett. 1978, 26, 2231-2234.
(8) Dolbier, W. R., Jr.; Burkholder, C. R. J. Org. Chem. 1984, 49, 2381-
2386.
(9) For relevant computational studies see, e.g.: (a) Rastelli, A.; Bagatti,
M.; Gandolfi, R. J. Am. Chem. Soc. 1995, 117, 4965-4975. (b) Halevi, E.
A.; Wolfsberg, M. J. Chem. Soc., Perkin Trans. 2 1993, 1493-6.
(10) Lacher, J.; Tompkin, G. W.; Park, J. D. J. Am. Chem. Soc. 1952,
74, 1693-1696.
(12) (a) Atkinson, B.; Trenwith, A. B. J. Chem. Soc. 1953, 2082. (b)
Drennan, G. A.; Matula, R. A. J. Phys. Chem. 1968, 72, 3462-3468. (c)
Buravtsev, N. N.; Grigor’ev, A. S.; Kolbanovskii, Yu. A. Kinet. Katal. 1985,
26 (1), 7-14.
(13) Bartlett, P. D. Science 1968, 159, 833.
(14) Buravtsev, N. N.; Kolbanovskii, Y. A.; Ovsyannikov, A. A.
MendeleeV Commun. 1994, 48-50. Buravtsev, N. N.; Kolbanovsky, Y. A.
J. Fluorine Chem. 1999, 96, 35-42.
(15) Attention must be paid not to create excessive pressures in sealed
gas-phase ampules at elevated temperatures. We routinely calculate the
pressures that would be generated at the reaction temperature by the ideal
gas law from the total moles of reactants that were sealed in the ampule
taking into account the stoichiometry of the gas-phase reactions. We consider
that our ampules, now availble commercially (see ref 5), can safely withstand
∼3 atm internal pressure since they did not explode in barricaded tests
with a calculated 10 atm of CF4.
(11) (a) Atkinson, B.; Stedman, M. J. Chem. Soc. 1962, 512-519. (b)
Atkinson, B.; Tsiamis, C. Int. J. Chem. Kinet. 1979, 11, 585-593. (c)
Ivanova, S. M.; Zemlyanskaya, N. V.; Volkov, G. V.; Boikov, Yu. A.;
Barabanov, V. G.; V’yunov, K. A. Kinet. Katal. 1986, 27 (5), 1236-1237.

9958 J. Am. Chem. Soc., Vol. 123, No. 41, 2001
ShtaroV et al.
Figure 2. CTFE dimerization: second-order plots of rate data (top);
Arrhenius plot (bottom).
CTFE.16,17 Already quite visible are the narrower lines belonging
to a 1:1 mixture of cis- and trans-1,2-dichlorohexafluorocy-
clobutane whose solution spectra have also been studied be-
fore.11a,18 These lines grow at the expense of the CTFE reso-
nances, as can be seen graphically by viewing the set of spectra
obtained automatically according to a prearranged time schedule.
A spectrum near the end of the reaction is shown in Figure 1
(bottom). The CTFE multiplets at each end of the spectrum have
no overlap with the product spectra and can be integrated for
the entire set of acquired spectra by using the automation
features available in modern spectrometers. After scaling the
resulting integrals by the initial concentration, a plot of 1/(con-
centration, mol/L) vs time gives a good straight line representing
the disappearance of CTFE, the slope of which is the rate
constant. The experiment was repeated for several temperatures
between 240 and 340 °C, each leading to similar linear plots
(Figure 2), and the resulting rate constants were used to construct
the Arrhenius plot that is shown in Figure 2.19
Figure 3. TFE dimerization: 19F NMR spectrum deconvolution at 250
(top) and at 325 °C (bottom).
signal intensity of each reaction component. Figure 3 shows
the fits for two kinetic points at two different temperatures.
Obviously, a complication of this kind is time-consuming since
each kinetic point requires a computer fit, and it is fortunate
that in practice such situations do not occur very frequently.
Most molecules, unlike TFE and octafluorocyclobutane, have
more than one NMR resonance that can be used to follow the
kinetics. The 1/[C] (mol/L) vs time plot for the disappearance
of TFE at 225 °C, giving the rate constant as the slope,19 is
shown in Figure 4 (top) together with the Arrhenius plot for
the dimerization of TFE in the temperature range 220-300 °C
(Figure 4, bottom).
An analogous procedure was followed with TFE (100 µmol
in a 3.8 mL ampule). The only fluorine resonances that can be
detected are two single resonances belonging to TFE and
octafluorocyclobutane (δ ∼ -134 ppm relative to CFCl3).
Unfortunately, they overlap substantially and ordinary integra-
tion cannot be used as above to follow the disappearance of
TFE with time. The chemical shift difference of the two
resonances is also slightly temperature dependent, tending to
decrease resolution at higher temperatures (∆δ ) 0.7 ppm at
25 °C and 0.3 ppm at 325 °C). Recourse can be made to spectral
deconvolution that is available in the software of modern
spectrometers: the overall absorption is fitted by least-squares
methods as a superposition of two Lorentzian curves to obtain
their integral area values, which are proportional to the NMR
The Arrhenius activation parameters extracted from the
Arrhenius plots of Figures 2 and 4 are compared with previously
published values in Table 1. The agreement is excellent,
particularly as regards the Arrhenius activation energies, despite
the widely different reactors used with widely different surface-
to-volume ratios: 250 mL to 1 L in the previous work vs
ampules of less than 4 mL internal volume in this study.20 To
complete the comparison, the entropy and heats of activation
for the two reactions were determined as in ref 10a,b, using the
transition-state theory expression for the rate constant of eq 2
with a transmission coefficient of unity. R1 ) 0.08205 atm L
K-1 mol-1, R ) 1.987 cal K-1 mol-1, and the standard states
are at 1 atm of pressure.21,22 Using the logarithmic form of eq
2 with ∆nq ) -1, a plot of ln(kh/kBT(R1T)) vs 1/T gives a
straight line. The activation enthalpy is obtained from the slope
and the activation entropy from the intercept. The resulting
(16) The chemical shifts in solution and in the gas phase are not identical
but the differences are generally small. We calibrate the chemical shift scale
by adding to the ampule about 10 mol % of Freon11 or chemically more
inert CF4 (δ ) -63.5 ppm).
(17) Jameson, C. J.; Jameson, A. K.; Oppusunggu, D. J. Chem. Phys.
1985, 83, 5420-5424.
(18) Gazzard, V. J.; Harris, R. K. Org. Magn. Reson. 1974, 6, 404-
406.
(19) See Supporting Information for rate constant data.
(20) We often test for wall catalysis by carrying out the NMR kinetic
runs in the presence of crushed ampule glass.
(21) Cf.: Benson, S. W. Thermochemical Kinetics, 2nd ed.; J. Wiley and
Sons, Inc.: New York, 1976.

Cycloaddition Reactions of Fluoroolefins
J. Am. Chem. Soc., Vol. 123, No. 41, 2001 9959
at 145 °C in Figure 5 that also shows the appropriate assign-
ments. Preliminary work indicated that the dimerization of DFA,
giving 5 and 6, was competitive with DFA’s [2 + 2] and [2 +
4] cycloadditions as shown in Scheme 1. Moreover, 6 underwent
further Diels-Alder reaction rapidly with butadiene to give 7,
a result that obscured the [2 + 2] and [2 + 4] cycloadditions of
interest.
The measured integral of the strong singlet at -99.5 ppm
corresponding to the endocyclic -CF2- group of dimer 5 is
significantly greater than expected for the 2F which correspond
to the dCF2 (doublets at -83 and -91 ppm) of dimer 5.
Therefore, we consider that this signal must consist of two
overlapping signals that are not resolved in the gas-phase 19F
NMR spectrum. Indeed, when the 19F NMR for the mixture
was measured in CDCl3 solution after the gas-phase reaction at
145 °C, two signals (1.8:1) were able to be resolved. In CDCl3,
the larger peak at -97.88 (tt, 4JF-H ) 4.2, 3.1 Hz) was assigned
to the exocyclic dCF2 group of 1-difluoromethylene-3-vinyl-
cyclobutane [2 + 2] adduct 8, which has magnetically equivalent
fluorines with small 4JF-H couplings. The smaller, adjacent peak
at -98.37 ppm (tt, J ) 10.1, 2.7 Hz) is consistent the -CF2-
signal for head-to-tail dimer 5. In the GC/MS examination of
the product mixture, products 3, 4, and 8 apparently eluted close
to one another, with one major and one satellite peak (95:5)
having similar mass spectra. The concentration of adduct 8 in
the gas-phase reactions was, therefore, derived by subtracting
the 2F integral of dCF2 (-82.5 and -91 ppm) of dimer 5 from
the integral of the -99.5 ppm signal (Figure 5). According to
19F NMR, in the 1:10 DFA-butadiene reaction, adduct 8 is
formed in 5-9% yield.
Using a 10-fold molar excess of butadiene reduced the
importance of the DFA dimerization to 10-17% of the reac-
tion, but it also required much smaller amounts of DFA to en-
sure safe internal pressures and to ensure that all components
were in the vapor phase. Thus the initial concentration of
DFA, under these conditions, was only 6.3 × 10-3 M. Although
the signal-to-noise factor was consequently substantially re-
duced, the kinetic profiles were nevertheless quite adequate for
kinetic analysis, albeit showing more scatter than customary.
A typical reaction profile obtained under these conditions (Fig-
ure 6, top) shows the decay of DFA as a result of the three
major reactions that use it and the formation of the products
of interest, 3, 4, and 8, at 132 °C. These products contribute
81-86% to the total mass balance, with 8 contributing only
5-9%.
The decay of DFA follows pseudo-first-order kinetics over
the entire temperature range used as can be seen in Figure 6
(middle) by the linearity of the ln(C/C0) vs time plots.23
The slopes of the linear plots yield values for k1 that are
needed below for the determination of k3 and k4 (k8 will be
neglected henceforward).19 As shown in the Supporting Infor-
mation section, k3 and k4 were obtained as the slopes of the
linear plots obtained by plotting the cycloadduct concentrations
vs [C4H6]{[DFA]0 - [DFA]}/k1 at six different temperatures.
The resulting rates were used to construct the very satisfactory
Arrhenius plots of Figure 6 (bottom), yielding log A and the
activation energy Ea. Application of the transition state theory
equation (eq 2) afforded linear plots yielding the activation
enthalpies and activation entropies listed in Table 2. Note that
the formation of the [2 + 2] DFA-butadiene adduct 4 prevails
over the formation of the [2 + 4] adduct 3 at temperatures above
160 °C, whereas the [2 + 4] adduct 3 is preferred at lower
temperatures.
Figure 4. TFE dimerization: typical second-oder plot derived from
deconvolution (top); Arrhenius plot (bottom).
values (Table 1) compare in a very satisfactory manner with
previous work, from which we conclude that gas-phase NMR
∆Hq ∆Sq
RT RT
kBT
nq
k (L mol-1 s-1) )
(R1T)-∆ exp -
(2)
(
)(
)
h
is a very attractive alternative to conventional techniques for
quantitative vapor-phase chemical kinetics of second-order
reactions, requiring minute amounts of materials and much less
tedious work.
The substantial reduction in entropy of the order of 30 entropy
units in going from reactants to the transition state for these
cyclodimerization reactions, as seen in Table 1, is a reflection
not only of the obvious associative nature of these reactions
but also of the fact that for the olefins to react they must be
properly oriented with respect to each other.
Reaction of Difluoroallene with 1,3-Butadiene. 1,1-Dif-
luoroallene (DFA) has been shown to react with a number of
1,3-dienes by competing [2 + 4] and [2 + 2] reactions, the
Diels-Alder cycloaddition mechanism being presumably con-
certed, whereas the [2+2] cycloaddition apparently proceeds
via a diradical intermediate.7,8
Previous studies of such reactions were carried out by using
the time-honored sealed tube method, which required laborious
product analyses. No quantitative kinetic information was
reported in these studies. Our technique also involves sealed
tubes but provides immediate analytical and kinetic information
with micromolar amounts of materials. Preliminary experiments,
aided by the analytical information from previous work,
including expected NMR chemical shifts,7,8 indicated that the
major processes in the reaction between 1,3-butadiene and DFA
can be represented by Scheme 1 in the temperature range 100-
210 °C. The major reaction components have simple 19F NMR
spectra in the vapor phase as illustrated for a single kinetic point
(22) Modern usage prefers a treatment of the transition state theory based
on standard states at 1 mol L-1 (cf.: Maskill H. The Physical Basis of
Organic Chemistry; Oxford University Press: Oxford, UK, 1985; Chapter
6). Slightly different values for the activation entropy and enthalpy are
obtained with this approach.
(23) See Supporting Information for a more detailed kinetic analysis.

9960 J. Am. Chem. Soc., Vol. 123, No. 41, 2001
ShtaroV et al.
Table 1. Activation Parameters for CTFE and TFE Dimerizations
activation parameters
reaction
log A
Ea, kcal/mol
∆Hq, kcal/molc
∆Sq, cal/kmolc
TFE + TFE
8.38 ( 0.40b
26.5 ( 1.0
(25.4)12a
23.7 ( 1.0a
-34.0 ( 1.8a,b
(8.01)12a
(22.2)12a
(-38.4)12a
(8.22)10
(26.3)10
CTFE + CTFE
7.40 ( 0.42b
(7.63)11a
26.5 ( 1.0 ( 0.3
23.7 ( 1.0a
-38.4 ( 1.9a,b
(-38.6)11a
(26.6)11a
(23.9)11a
(7.55)10
(26.3)10
a Enthalpies and entropies of activation are calculated for the standard state of 1 atm at 700 K. b The log A and ∆Sq are derived from the rate of
q
q
dimer formation: k(formation) ) k(disappearance)/2. c The same ∆Hp and ∆Sp values for the standard state of 1 atm can be obtained according
to ref 21, pp 8-11, 146: since ∆Sqp ) ∆Sqc + R ∆nq ln(R′T), Ac ) (kBT/h)(R′T)-∆nqe2 exp(∆Sp /R), and ∆Hp ) ∆Hcq + ∆nqRT, where ∆nq ) -1
for the second-order reaction and Ac is expressed in concentration units (M-1 s-1). For 1 mol/L standard state Ac ) (kBT/h)e2 exp(∆Scq/R) and ∆Hcq
) Ea - RT applies. Therefore, for 1 mol/L standard state for TFE: ∆Hcq ) 25.1 kcal/mol, ∆Scq ) -25.9 eu, and for CTFE: ∆Hcq ) 25.1 kcal/mol
and ∆Scq ) -30.4 eu.
q
q
indicate that diradicals such as 9 in Scheme 1 are not reversibly
formed, that is they do not revert to allene (DFA) in competition
with their cyclization processes (i.e., formation of 4 and 8).
Therefore, the rate-determining steps in DFA’s reaction with
1,3-butadiene are the initial, bimolecular processes, and they
are different for the [2 + 4] and [2 + 2] reactions.
The lack of regiospecificity exhibited in the [2 + 2] process
combined with earlier isotope effect evidence24 are consistent
with a probable stepwise mechanism for this reaction, involving
extended diradical intermediate 9. The activation enthalpy for
formation of [2 + 2] adduct 4 (-38.7 cal/deg) is consistent
with values generally obtained for [2 + 2] cycloaddition
processes, including the values of -34.0 and -38.4 cal/deg,
Figure 5. DFA reaction with 1,3-butadiene: 19F NMR spectrum of
product mixture.
which we obtained for the dimerizations of TFE and CTFE,
respectively.
On the other hand, the evidence for a concerted mechanism
for the [2 + 4] process is by no means definitive. The somewhat
larger entropy of activation for formation of [2 + 4] adduct 3
is consistent with values that have been observed for other Diels
Alder reactions. However, the enthalpy of activation for this
supposedly “concerted” process is very similar to that of the
acknowledged diradical [2 + 2] process that forms product 4.
Thus, there is no evidence of a significant “enthalpy of concert”
for this reaction. Such enthalpy of concert is often invoked as
convincing evidence for concerted, pericyclic mechanisms.26 To
play the devil’s advocate in this particular case, one could readily
attribute the slightly more entropically demanding, rate-
determining step of the [2 + 4] reaction to initial formation of
the isomeric, cisoid diradical 9b, which could rapidly cyclize
to product.
Scheme 1
It is generally accepted that the [2 + 4] cycloadditions of
1,1-difluoroallene and other allenes are mechanistically con-
certed, pericyclic processes, whereas their [2 + 2] cycloadditions
are considered to be nonconcerted reactions, proceeding through
diradical intermediates. Although the smaller activation enthalpy
and larger negative entropy of activation for the [2 + 4]
cycloadditions than those observed for the [2 + 2] cycloaddi-
tions of DFA can be considered consistent with such proposed
mechanistic differences, there is nevertheless no way that such
small differences in activation parameters can be considered
conVincing evidence for the respective concerted and stepwise
mechanisms.
Because the observed activation parameters for the [2 + 4]
and [2 + 2] reactions are clearly different, what can be said
with some certainty is that the two mechanisms do not haVe a
common rate-determining step, nor can they inVolVe a common
intermediate. The latter conclusion derives from our extensive
earlier studies of allene cycloadditions,24 including those involv-
ing interconversion of [2 + 2] adducts such as 4 and 8,25 which
The major argument against such an explanation involving
“competing diradical formation” is the observed total, con-
trathermodynamic regiospecificity that is observed for the [2
+ 4] reaction, which results in exclusive formation of 3. Such
regiospecificity is consistent with computational models of
the concerted reaction,7,27 and it is not readily rationalized
otherwise.
(24) (a) Dolbier, W. R., Jr. Acc. Chem. Res. 1991, 24, 63-69. (b) Dolbier,
W. R., Jr.; Dai, S. H. J. Am. Chem. Soc. 1972, 94, 3946.
(25) Dolbier, W. R., Jr.; Piedrahita, C. A.; Al-Sader, B. H. Tetrahedron
Lett. 1979, 2957.
(26) Roth, W. R.; Lennartz, H.-W.; Doering, W. v. E.; Birladeanu, L.;
Guyton, C. S.; Kitagawa, T. J. Am. Chem. Soc. 1990, 112, 1722.

Cycloaddition Reactions of Fluoroolefins
J. Am. Chem. Soc., Vol. 123, No. 41, 2001 9961
Figure 7. Diagrams of the gas-phase NMR ampule attached to a
vacuum system before being sealed off with a propane torch (left) and
seated in a commercial high-temperature NMR probe (right).
4] reaction of DFA with 1,3-butadiene proceeding via a con-
certed mechanism.
Conclusions
In conclusion, a novel gas-phase NMR methodology has been
used to obtain accurate kinetic measurements for three bimo-
lecular, second-order cycloaddition reactions. The derived
activation parameters obtained for the thermal dimerizations of
chlorotrifluoroethylene and tetrafluoroethylene were consistent
with those that had previously been reported in the literature.
The activation parameters that were obtained for the competing
[2 + 2] and [2 + 4] cycloadditions of difluoroallene with 1,3-
butadiene are to our knowledge the first such parameters
obtained for a cycloaddition of an allene species. They indicate
highly restricted transition states for both processes, with the
[2 + 4] cycloaddition being somewhat more entropy demanding,
while being less enthalpy demanding.
Figure 6. DFA reaction with 1,3-butadiene: concentration of com-
ponents versus time (top); pseudo-first-order plot of decay of DFA
(middle); Arrhenius plots for major cycloaddition products (bottom).
Experimental Section
Chlorotrifluoroethylenene and 1,3-butadiene were obtained from
Aldrich and TFE was a DuPont product contained in a small cylinder
at low pressure. 1,1-Difluoroallene was prepared according to published
procedure.28 It was found to be 90-97% pure by 1H NMR containing
3-10% of n-butane from the BuLi reagent used in the synthesis.
The NMR reaction vessel is an ampule made from a short section
of a 10 mm NMR tube with a coaxial 5 mm tube extension to allow
quick attachment to a high-vacuum system via an O-ring adapter for
sealing NMR tubes (Figure 7). These gas-phase NMR ampules are now
available commercially at low cost.6 Gases are metered using the ideal
gas law by isolating a 53 mL bulb with the appropriate pressure of the
gaseous reagent, measured with an MKS Baratron capacitance pressure
Table 2. Activation Parameters for DFA/1,3-Butadiene
Cycloaddition Reactions
activation parametersa,b
∆Ea,
kcal/mol
∆Hq,
∆Sq,
adduct
log A
kcal/molc
cal/kmolc
4, [2 + 2] 6.97 ( 0.11 20.1 ( 0.2
18.3 ( 0.2
-38.7 ( 0.5
3, [2 + 4] 6.08 ( 0.12 18.4 ( 0.25 16.6 ( 0.25 -42.7 ( 0.6
a Enthalpies and entropies of activation are calculated for the standard
state of 1 atm at 450 K. b See Table 4 in the Supporting Information
for rate constants. c ∆Hq and ∆Sq are calculated as described in ref 23.
The values for 1 mol/L standard state are different: for the formation
of [2 + 2] product ∆Hcq ) 19.2 kcal/mol, ∆Scq ) -31.4 eu, and for
[2 + 4] adduct ∆Hcq ) 17.5 kcal/mol and ∆Scq ) -35.5 eu.
(27) (a) Domelsmith, L. N.; Houk, K. N.; Dolbier, W. R., Jr.; Piedrahita,
C. A. J. Am. Chem. Soc. 1978, 100, 6908. (b) Manoharan, M.; Venuvanal-
ingam, P. J. Fluorine Chem. 1995, 73, 171-174. (c) Manoharan, M.;
Venuvanalingam, P. J. Chem. Soc., Perkin Trans. 2 1996, 1423-1427.
(28) Dolbier, W. R. J.; Burkholder, C. R.; Piedrahita, C. A. J. Fluorine
Chem. 1982, 20, 637.
All in all, we consider the available evidence, both experi-
mental and computational, to be most consistent with the [2 +

9962 J. Am. Chem. Soc., Vol. 123, No. 41, 2001
ShtaroV et al.
transducer ((0.1 Torr), for the desired number of micromoles followed
by bulb-to-bulb transfer into the ampule. Alternatively, the number of
moles of gas transferred can be obtained from the pressure drop in the
manifold of known volume when the latter is opened to the evacuated
compartment containing the ampule. When all gaseous reactants are
loaded, the ampule kept in liquid nitrogen is sealed off with a flame at
its neck so as to leave a short stub that facilitates the attachment by
means of a short piece of heat-shrink Teflon tubing (Figure 7) to a
matching stub of a 10 mm NMR tube attached to the ceramic head
that seals the high-temperature probe. This method of holding the
ampule in the probe is surprisingly effective even at probe temperatures
close to 400 °C where the Teflon connector softens but continues to
hold the ampule. An internal volume of 3.8 mL can be obtained
reproducibly by sealing the ampules at the same height of the 5 mm
extension. The main purpose of a short ampule is to minimize
temperature gradients by restricting the sample volume to the thermo-
stated region of the probe. We measure temperature gradients of 2 to
3° inside the ampule (vide infra). Great attention must be paid not to
create excessiVe pressures in sealed ampules at eleVated temperatures.
A safe upper limit of 250 µmol of volatiles is obtained from the ideal
gas law with an ampule volume of 3.8 mL and with a safe internal
pressure of 3 atm at 400 °C.
compartment and which accepts a flanged 10 mm NMR tube. The
temperature was calibrated by positioning a thin thermocouple in the
center of a dummy ampule identical to those used for actual kinetic
runs except for a small hole at the end of the 5 mm stub. Repeated
calibrations indicate that the temperature in the ampule is defined within
1 to 2 °C. To initiate a kinetic run, the Macor head carrying the sample
assembly is lowered into the probe kept at the desired temperature with
a string. Automatic NMR data acquisition starts as soon as the sample
is seated in the probe according to a script that defines the time steps
in the repetitive spectral sampling.
19F NMR spectra were obtained with a GE Omega 300 MHz
spectrometer. No field lock was used, and the ampules were not spun.
A small flip angle of the magnetization vector (typically 10 to 15°)
was used for a more uniform power distribution over the wide 19F NMR
spectral widths. Very short pulse delays of 50 ms allowed the acquisition
of 512 transients in about 1 min.
Acknowledgment. Support of this research in part by the
National Science Foundation is acknowledged with thanks, and
A.B.S. wishes to thank Professor R. J. Hanrahan for helpful
discussions regarding calculations of thermochemical quantities.
The high-temperature probe is a Nalorac 10 mm probe for a wide
bore magnet with an upper temperature limit of 400 °C. Heating is
achieved by flowing into the probe hot nitrogen gas whose temperature
is regulated by a controller. Another nitrogen flow in an anular space
is used for cooling the probe externals. The probe comprises also a
cylindrical Macor ceramic head that thermally seals the heated probe
Supporting Information Available: Derivation of kinetic
expressions and tables of kinetic data (PDF). This material is
available free of charge via the Internet at http://pubs.acs.org.
JA0111277