Isotope effects and the mechanism of triazolinedione ene reactions. Aziridinium imides are innocent bystanders

Article abstract of DOI:10.1021/ja9933339

Kinetic isotope effects were determined for the ene reactions of 4-phenyl-1,2,4-triazoline-2,5-dione with 2-methyl-2-butene and 2-methyl-1-pentene. In each case highly inverse (k_H/k_D = 0.87-0.88) vinylic deuterium isotope effects were observed. Large ¹³C isotope effects were observed for the less-substituted olefinic carbons of each alkene and much smaller effects were found in the more-substituted olefinic carbon. Becke3LYP calculations of the mechanistic pathway for the ene reaction of 1,2,4-triazoline-2,5-dione with isobutene, 2-methyl-2-butene, and tetramethylethylene predict a new mechanism for these reactions involving an open biradical as the key intermediate. This biradical may either form the ene product or reversibly form an intermediate aziridinium imide. The validity of these calculations is supported by a comparison of experimental and theoretically predicted isotope effects. A curved Arrhenius plot of product ratios was found for the ene reaction of 2-methyl-1-pentene in the presence of methanol. This is inconsistent with a simple two-step ene mechanism involving only an aziridinium imide intermediate, but the curvature of this plot could be modeled well by the theoretical mechanism using enthalpy and entropy parameters close to theoretically predicted values. A variety of observations associated with these reactions are examined and found to support the new mechanism.

Full text of DOI:10.1021/ja9933339

J. Am. Chem. Soc. 1999, 121, 11885-11893
11885
Isotope Effects and the Mechanism of Triazolinedione Ene Reactions.
Aziridinium Imides Are Innocent Bystanders
Daniel A. Singleton* and Chao Hang
Contribution from the Department of Chemistry, Texas A&M UniVersity, College Station, Texas 77843
ReceiVed September 14, 1999
Abstract: Kinetic isotope effects were determined for the ene reactions of 4-phenyl-1,2,4-triazoline-2,5-dione
with 2-methyl-2-butene and 2-methyl-1-pentene. In each case highly inverse (kH/kD ) 0.87-0.88) vinylic
13
deuterium isotope effects were observed. Large C isotope effects were observed for the less-substituted olefinic
carbons of each alkene and much smaller effects were found in the more-substituted olefinic carbon. Becke3LYP
calculations of the mechanistic pathway for the ene reaction of 1,2,4-triazoline-2,5-dione with isobutene,
2
-methyl-2-butene, and tetramethylethylene predict a new mechanism for these reactions involving an open
biradical as the key intermediate. This biradical may either form the ene product or reversibly form an
intermediate aziridinium imide. The validity of these calculations is supported by a comparison of experimental
and theoretically predicted isotope effects. A curved Arrhenius plot of product ratios was found for the ene
reaction of 2-methyl-1-pentene in the presence of methanol. This is inconsistent with a simple two-step ene
mechanism involving only an aziridinium imide intermediate, but the curvature of this plot could be modeled
well by the theoretical mechanism using enthalpy and entropy parameters close to theoretically predicted values.
A variety of observations associated with these reactions are examined and found to support the new mechanism.
Introduction
The reactions of 1,2,4-triazoline-3,5-diones (TADs) have
attracted interest owing to their expected similarity to singlet
oxygen and their multiform high reactivity in Diels-Alder
An ene reaction is the addition of an electrophilic double or
triple bond to an alkene with concomitant transfer of an allylic
hydrogen. Despite the simple formal description of the bonding
changes in these reactions (eq 1) and the fact that they are
8
9
10
reactions, ene reactions, [2 + 2] cycloadditions, and even
11
ring-openings of strained C-C single bonds. The ene reactions
of TADs were quickly recognized as stepwise processes on the
1
0,12
basis of trapping experiments.
Deuterium KIE studies have
backed this conclusion and provided more detailed information
on the mechanistic pathway. In a Stephenson isotope effect test
on the reaction of RTAD [R ) phenyl (PTAD) or methyl
allowed pericyclic processes, these synthetically important
reactions have often been shown to involve complex mecha-
(MTAD)] with cis-, trans-, and gem-substituted tetramethyl-
etylene-d6 (1a-c), Greene and co-workers observed a significant
intramolecular KIE only with 1b and 1c (Scheme 1).13 This
observation indicates that the product-determining step involves
a choice of reaction modes between cis-oriented CH3 and CD3
groups, and was taken as implicating the intermediacy of the
aziridinium imide (AI) 2.
This two-step mechanism involving an AI has been supported
by both experimental and theoretical studies. Foote has observed
KIEs for the reaction of PTAD with cis- and trans-2-butene-d3
and, in concordance with the Green studies, observed a
substantial intramolecular KIE only with the cis isomer. An AI
1
nisms. This includes ene reactions of singlet oxygen, nitroso
2
3
4
compounds, selenium dioxide, acylium ions, and Lewis acid-
catalyzed ene reactions.5
A central component of mechanistic studies of ene reactions
has been the observation of deuterium kinetic isotope effects
(
KIEs). The observation of a large (kH/kD > ∼2) KIE is
indicative of hydrogen transfer in the rate-limiting step and has
often been taken as evidence for a concerted mechanism, while
6
the absence of a substantial deuterium KIE is the signature of
a stepwise mechanism. Stepwise ene reactions that lack an
intermolecular deuterium KIE may still exhibit an intra-
molecular KIE if a hydrogen is transferred in the product-
determining step. The observation of intramolecular KIEs has
provided information about the nature of the intermediates in
many of the stepwise ene reactions, and this is the basis for the
(7) (a) Grdina, B.; Orfanopoulos, M.; Stephenson, L. M. J. Am. Chem.
Soc. 1979, 101, 3111-2. (b) For earlier work on intramolecular isotope
effects, see: Dai, S.-H.; Dolbier, W. R., Jr. J. Am. Chem. Soc. 1972, 94,
3
946.
(8) Burrage, M. E.; Cookson, R. C.; Gupte, S. S.; Stevens, I. D. R. J.
Chem. Soc., Perkin Trans. 2 1975, 1325.
9) Pirkle, W. H.; Stickler, J. C. J. Chem. Soc., Chem. Commun. 1967,
760.
(10) von Gustorf, E. K.; White, D. V.; Kim, B.; Hess, D.; Leitich, J. J.
“
Stephenson isotope effect test”.7
(
(1) Frimer, A. A.; Stephenson, L. M. In Singlet O2; Frimer, A. A., Ed.;
CRC Press: Boca Raton, FL, 1985; pp 68-87.
(2) Seymour, C. A.; Greene, F. D. J. Org. Chem. 1982, 47, 5226.
(3) Stephenson, L. M.; Speth, D. R. J. Org. Chem. 1979, 44, 4683-9.
(4) Beak, P.; Berger, K. R. J. Am. Chem. Soc. 1980, 102, 3848-56.
(5) Snider, B. B.; Ron, E. J. Am. Chem. Soc. 1985, 107, 8160.
(6) Achmatowicz, O., Jr.; Szymoniak, J. J. Org. Chem. 1980, 45, 4774.
Org. Chem. 1970, 35, 1155.
(11) Amey, R. L.; Smart, B. E. J. Org. Chem. 1981, 46, 4090.
(12) Turner, S. R.; Guilbault, L. J.; Butler, G. B. J. Org. Chem. 1971,
36, 2838.
(13) (a) Seymour, C. A.; Greene, F. D. J. Am. Chem. Soc. 1980, 102,
6384. (b) Cheng, C.-C.; Seymour, C. A.; Petti, M. A.; Greene, F. D.; Blount,
J. F. J. Org. Chem. 1984, 49, 2910.
Kwart, H.; Brechbiel, M. W. J. Org. Chem. 1982, 47, 3355. Dai, S.-H.;
Dolbier, W. R., Jr. J. Am. Chem. Soc. 1972, 94, 3953.
1
0.1021/ja9933339 CCC: $18.00 © 1999 American Chemical Society
Published on Web 12/03/1999

11886 J. Am. Chem. Soc., Vol. 121, No. 50, 1999
Singleton and Hang
Table 1. Experimental and Calculated KIEs (k12C/k13C
C) for the Ene Reactions of 2-Methyl-1-pentene and
-Methyl-2-butene with PTAD
or k /k , 25
Scheme 1
H
D
°
2
exptl: 2-methyl-1-pentene
calcd: 5
expt 1a
expt 2a
Becke3LYP
RHF
C1
C2
C3
1.036(4)
1.003(3)
1.001(2)
1.036(4)
1.003(1)
1.000(2)
1.037
1.009
1.001
1.047
1.011
1.001
C(4)
C6
1.00 (assumed)
1.002(2)
0.87(1)
1.04(1)
1.01(1)
1.000(2)
0.87(1)
1.04(1)
1.03(1)
1.001
0.862
1.026
1.026
1.001
0.841
1.016
1.016
H
vinyl
C3HD
C6H
2
D
has been directly observed spectroscopically in reactions of
14
15
exptl: 2-methyl-2-butene
calcd (all Becke3LYP)
RTAD with biadamantylidene, trans-cycloheptene, and
trans-cyclooctene.16 Interestingly, the AI intermediate formed
from trans-cycloheptene is cis-fused. Ab initio calculations by
Houk and Foote at the RHF/6-31G* level (with higher-level
energy calculations) have characterized transition states for the
reaction of alkenes with TAD and for the formation of the ene
expt 3a
expt 4a
7
8
9
C1
C2
C3
C4
C5
1.000(1)
1.014(3)
1.026(3)
1.001(1)
1.013(2)
1.025(3)
1.001
1.009
1.038
0.998
1.001
0.869
1.045
1.004
0.973
1.002
1.010
1.036
0.999
1.001
0.854
1.007
1.039
0.977
0.997
1.039
1.010
1.001
0.998
1.025
0.971
0.970
1.049
1.00 (assumed)
1.003(2)
0.87(2)
1.02(2)
1.01(2)
1.004(2)
0.88(2)
1.01(2)
1.00(2)
1
7
product, as well as the AI intermediate with various alkenes.
H
vinyl
C1H
C5H
C4H
2
2
2
D
D
D
1.00 (assumed)
a
Experiments 1-4 were taken to 66%, 70%, 75%, and 67% (all
(
3%) completion, respectively.
regioisomeric transition state. This assumption was not supported
by later results (vide infra) and we therefore also studied the
reaction of PTAD with the more unsymmetrical 2-methyl-1-
pentene. Although the reaction of 2-methyl-1-pentene forms two
products, the stereoisomeric pathways for formation of these
two products should involve very similar transition states in
the first mechanistic step.
It has at times been proposed that other intermediates play a
role in the reactions of alkenes with TADs, but the two-step
mechanism for the ene reaction of TADs with alkenes through
an AI intermediate is well-accepted. The results here conflict
with this view. We report the results of a combined experimental
and theoretical study of these reactions which suggest that the
AI is an innocent bystander in the ene reaction, and that an
alternative mechanism provides explanations for the diverse
observations associated with these reactions. These results have
implications toward the mechanisms of other reactions of TADs
and other ene reactions.
13
2
The C and H KIEs for these reactions were determined
19
combinatorially at natural abundance. Reactions of 2-methyl-
2
1
-butene on a 0.5-mol scale with PTAD were carried out in
,4-dioxane at 25 °C, and were taken to 75((3)% and 67((3)%
conversion. The unreacted 2-methyl-2-butene was then recov-
ered by vacuum transfer followed by fractional distillation. The
13
2
resulting material was analyzed by C and H NMR compared
to a standard sample of the original 2-methyl-2-butene. The
13
2
changes in C and H isotopic composition were calculated by
using the C4 methyl group as “internal standard” with the
assumption that their isotopic composition does not change.
Similarly, reactions of 2-methyl-1-pentene on a 0.4-mol scale
with PTAD were taken to 66((3)% and 70((3)% conversion
and the recovered unreacted alkene was compared to the original
Results
Experimental Isotope Effects. Our initial aim was to study
with isotope effects the synchronicity of C-N bond formation
in the reaction of PTAD with alkenes, as we presumed that the
transition state involved AI formation. This requires the use of
unsymmetrically substituted alkenes because symmetrical alk-
enes would necessarily exhibit symmetrical KIEs due to
averaging. 2-Methyl-2-butene was chosen for study because its
ene reaction with PTAD produces a single product in quantita-
tive yield,18 and it was thought that the initial addition of PTAD
to this unsymmetrical alkene would be dominated by a single
20
alkene using the C4 methylene group as standard. From the
changes in isotopic composition, the KIEs were calculated in
19
the previously reported fashion.
The results are summarized in Table 1. In accord with
previous observations, no primary deuterium KIE was observed.
The most notable features of the KIEs were the substantial
13
difference in C1 and C2 C KIEs for 2-methyl-1-pentene and
the highly inverse Hvinyl deuterium KIE.
Theoretical Calculations. These experimental KIEs sug-
gested in a qualitative fashion that the reaction of PTAD with
2-methyl-1-pentene involved extremely asynchronous C-N
bond formation, if an AI were being formed at all. The results
with 2-methyl-2-butene, particularly the less differing 13C KIEs
at the olefinic carbons, qualitatively suggested a less asynchro-
(
(
14) Nelsen, S. F.; Kapp, D. L. J. Am. Chem. Soc. 1985, 107, 5548.
15) Squillacote, M.; Mooney, M.; De Felippis, J. J. Am. Chem. Soc.
1
990, 112, 5364.
16) Poon, T. H. W.; Park, S. H.; Elemes, Y.; Foote, C. S. J. Am. Chem.
Soc. 1995, 117, 10468.
17) Chen, J. S.; Houk, K. N.; Foote, C. S. J. Am. Chem. Soc. 1997,
19, 9852.
18) Ohashi, S.; Leong, K.; Matyjaszewski, K.; Butler, G. B. J. Org.
Chem. 1980, 45, 3467.
(
(
1
(19) Singleton, D. A.; Thomas, A. A. J. Am. Chem. Soc. 1995, 117, 9357.
(
(20) C4 was chosen as standard due to an overlapping impurity in the
C NMR which prevented the accurate integration of C5.
13

Mechanism of Triazolinedione Ene Reactions
J. Am. Chem. Soc., Vol. 121, No. 50, 1999 11887
Figure 1. Becke3LYP transition structures for the reaction of TAD
with isobutene, 2-methyl-2-butene, and tetramethylethylene. RHF
distances are shown in parentheses.
Figure 2. UBecke3LYP intermediate structures for the reaction of TAD
with isobutene, 2-methyl-2-butene, and tetramethylethylene.
nous formation of the two new C-N bonds and seemed
reconcilable with the prior calculational results. However, a more
detailed interpretation of the KIEs with the aid of calculations
supports a very different picture of the reaction mechanism.
The complete mechanistic pathways for the reaction of the
unsubstituted TAD with isobutene, 2-methyl-2-butene, and
tetramethylethylene were studied in HF and Becke3LYP
calculations using a 6-31G* basis set. Transition structures for
the initial attack of TAD on these alkenes are shown in Figure
structures closely resemble that calculated by Houk and Foote
17
for reaction of TAD with propene, but the Becke3LYP
structures show significantly less sign of bond formation
between the attacking nitrogen of TAD and the distal olefinic
carbon. Most notably, examination of the atomic motion
associated with the transition vectors for 5-9 suggested that
the attacking TAD nitrogen was undergoing bond formation with
only one of the olefinic carbons: the angle of attack of the TAD
nitrogen on the olefinic carbons was in each case actually
increasing in the transition vector.
1. Only one low-energy transition structure was found with
isobutene and tetramethylethylene (5 and 6, respectively), while
three stereoisomeric/regioisomeric low-energy transition struc-
tures were found with 2-methyl-2-butene (7-9).21 All of these
first-step transition structures involve a very unsymmetrical
attack on the alkene. However, the transition structure with
isobutene differs little from those with 2-methyl-2-butene, in
contrast to what was qualitatively suggested by the experimental
KIEs above (considering isobutene as a good model for
Based on these observations, an IRC analysis of structures 7
and 8 was performed. In intriguing contrast with the standard
mechanism for these reactions, 7 and 8 did not lead to AI
intermediates. Instead, open structures in which the attacking
TAD nitrogen has bound to only one of the olefinic carbons
were formed. Corresponding to each of the first-step transition
structures, the open structures 10-14 in Figure 2 were found.
Structures 10-13 are minima in both restricted and unre-
stricted Becke3LYP calculations, while 14 could be found only
in unrestricted calculations. None of these structures correspond
to minima at the RHF level. The unrestricted structures have
2-methyl-1-pentene). A better explanation for the differing KIEs
for 2-methyl-1-pentene versus 2-methyl-2-butene will be dis-
cussed below. At the RHF level, the first-step transition
(21) A fourth regioisomeric transition structure in which N2 is anti to
2
biradical character, having an S of 0.42-0.63 and having
C4 was higher in energy (5.4 kcal/mol at the Becke3LYP level) and was
not considered further.
Mulliken spin densities of 0.6-0.74 on the unbonded of the

11888 J. Am. Chem. Soc., Vol. 121, No. 50, 1999
Singleton and Hang
Figure 4. Energetic profile predicted for the reactions of TAD with
isobutene, 2-methyl-2-butene, and tetramethylethylene, in kcal/mol
relative to starting material energies. ((U)Becke3LYP/6-31G* + ZPE).
observed ene products are predicted to be formed via transition
states 15-18. In contrast to the literature mechanism, an IRC
analysis links these transition states to the biradicals 10-13,
not to AIs 24-27. The biradicals may either form product or
close reversibly to the AIs via transition states 19-23; in the
case of 14 closure to the AI 26 is predicted to be strongly
favored (but see the discussion of 14 below) and none of the
regiochemically reversed product is formed experimentally.
A key question is whether the biradicals 10-14 correspond
to true minima in reality on the free energy surface. Considering
the biradical character of these structures, it is notable that even
the restricted Becke3LYP calculations predict 10-13 to be
minima: 10, 11, 12, and 13 are 1.8, 1.2, 0.9, and 1.8 kcal/mol
lower in energy (including ZPE) than the transition states 19,
2
0, 21, and 22, respectively, leading to AIs. The introduction
of solvent benzene by an SCICPM model with the restricted
structures lowers the barrier for ring closure slightly, from 0.9
to 0.6 kcal/mol with 12. The use of unrestricted Becke3LYP
calculations lowers the energy of 10-13 but has no impact on
19-22, so it raises the barriers to AI formation (including ZPE)
to 3.8, 1.9, 2.1, and 3.1 kcal/mol, respectively. It should be noted
that the use of unrestricted DFT calculations is problematical.
Addressing the energetics from a different direction, a CASSCF
calculation (2 electron, 2 orbital) found a 3.5 kcal/mol barrier
for AI formation from 10. Entropy is also predicted to favor
the open intermediates over their transition states for ring closure
by 3-6 eu at the (U)B3LYP level, raising the free-energy well
for 10-13 by an additional 0.9-1.8 kcal/mol. All of these
factors suggest that 10-13 correspond to real minima on the
free energy surface. The reality of biradical 14 is uncertain, as
it is predicted to exist only in the unrestricted calculations and
even then only faces a 0.6 kcal/mol barrier for ring closure to
the AI 26. The lower stability of 14, if it is real, may be the
result of involving a secondary carbon radical instead of the
tertiary carbon radicals in 10-13.
Figure 3. Intermediates, transition structures, and reaction pathways
for the ene reactions of TAD with isobutene, 2-methyl-2-butene, and
tetramethylethylene. Complete structures are given in the Supporting
Information.
The energetics for the mechanistic pathways (not including
9, 14, and 23) at the (U)Becke3LYP(+ZPE) level are sum-
marized in Figure 4. In each case the first step is predicted to
be rate limiting and the barriers for the remaining steps are low.
The biradicals are predicted to be roughly comparable in energy
to the AIs. Entropy is predicted to favor biradical 10 over AI
original olefinic carbons, and we will describe these structures
as biradicals. The restricted structures (see Supporting Informa-
-
tion) exhibit slightly greater charge separation, with ≈0.5 e
2
4 substantially (5.2 eu), though the entropy differences in the
charge transfer from the 2-methyl-2-butene to the TAD in a
Mulliken analysis. The moderate zwitterionic character exhibited
by the restricted structures would likely be enhanced in solution
reactions, but in the calculations falls well short of the degree
of charge transfer expected in a zwitterion.
other cases are small. The relative enthalpic barrier for product
formation versus AI formation from the biradical intermediates
depends on the starting alkene. Based on the predicted energies
here, much of the ene product from isobutene would form
without the intermediacy of an AI intermediate, while tetra-
methylethylene would be expected to repeatedly visit the AI
side path before product formation.
The remainder of the reaction pathway is illustrated in Figure
3
(and shown in detail in the Supporting Information). The

Mechanism of Triazolinedione Ene Reactions
J. Am. Chem. Soc., Vol. 121, No. 50, 1999 11889
or k /k , 25
Table 2. Weighted-Average Calculated KIEs (k12C/k13C
°
The barrier for N1-C1 or C1-C2 rotation in the open
biradicals 10-14 is of interest owing to implications regarding
the intramolecular isotope effects. The barrier for conformational
isomerization in biradicals, such as the conversion of 12 into
H
D
C) for the Ene Reaction of 2-Methyl-2-butene with TAD
weighted av weighted av
Aa
Ba
Aa
Ba
13, has often been assumed to be low. However, Houk and Foote
C1
C2
C3
C4
C5
1.001
1.010
1.036
0.998
1.001
1.000
1.017
1.030
0.999
1.000
H
C1H
C5H
vinyl
0.865
1.020
1.026
0.978
0.907
1.013
1.007
0.996
found that the transition structure for isomerization of aziri-
dinium imide intermediates with propene was 7.4 kcal/mol
2
D
D
2
(
UB3LYP + spin correction).17 This is substantially higher than
2
C4H D
the barrier for formation of the ene products. The search for
the rotational transition structures with more alkyl groups on
the alkene proved difficult: in many attempts to find a transition
structure for the isomerization of 12 and 13 the search invariably
fell into transition structures for the formation of the ene product,
the diazetidine, or an oxazoline from C2-O1 bond formation.
However, the transition structure 28 was located for conforma-
tional isomerization in 10. Structure 28 has Cs symmetry and
results from rotation about the C1-N1 bond in 10. The barrier
for this conformational isomerization is 3.9 kcal/mol (UB3LYP
a
Weighted averages A and B are based on a Boltzmann distribution
of 7, 8, and 9 using energies at the Becke3LYP/6-31G* +ZPE level
for A and the MP4(SDQ)/6-31G//Becke3LYP/6-31G*+ZPE level for
B, including a -T∆S correction. See the Supporting Information for
weighted-average predictions based on energies calculated in other
ways.
lower than the predicted KIE of 1.009. The RHF structure, which
shows greater bond formation to C2, results in less accurate
prediction of the experimental KIEs.
The prediction of KIEs for the reaction of 2-methyl-2-butene
is more complicated. The calculations of Houk and Foote had
predicted that the TAD addition to propene would be extremely
regioselective, with attack at the more substituted olefinic carbon
+
zpe), which is 1.0 kcal/mol higher than the barrier predicted
for formation of the ene product (cf. Figure 4). The correspond-
ing Cs-symmetric structure 29 for rotation about the C1-N1 bond
in 11 is a shallow minimum (ν for rotation ) 16 cm ) which
is 5.5 kcal/mol above 11. If conformational isomerization in
-
1
disfavored by 8.9-9.3 kcal/mol at the MP2/6-31G*//RHF/6-
1
1 must proceed through 29 (our search of the energy surface
17
3
1G* level. In contrast, we find here that transition state 9 is
has not been sufficient to establish this), then the barrier would
be predicted to be at least 2.5 kcal/mol higher than the barrier
for formation of the ene product.
comparable in energy to 7 and 8 and should contribute to the
observed KIEs. The degree of this contribution affects the
predicted KIEs tremendously. A large contribution from 9 would
result in C2 and Hvinyl KIE predictions that are too high and a
C3 KIE prediction that is too low, opposite in direction from
the errors in the predictions for 7 and 8. In principle a total
prediction can be based on a Boltzmann distribution of 7, 8,
and 9. In practice a problem is that small changes in the
predicted relative energies of 7, 8, and 9 have large effects on
the predicted KIEs and these predicted energies vary substan-
tially with the theoretical level of the calculation. Table 2 shows
two sets of predictions based on energies at theBecke3LYP/6-
3
3
1G*+ZPE level and at a MP4(SDQ)/6-31G*//Becke3LYP/6-
1G*+ZPE level including a correction for the relative entropies
of 7, 8, and 9 to approximate their relative free energy. The
predictions for these two sets nearly bracket the experimental
values. However, the predicted KIEs vary with basis set,
calculational level, and whether ZPE, thermal factors, and
entropy are included. Nonetheless, it appears that a combination
of 7, 8, and 9 can result in a reasonable agreement of the
predicted KIEs with the experimental values. In contrast, RHF
calculations, which predict direct formation of the aziridinium
imides intermediates, result in KIE predictions which are in
weak agreement with the experimental values at any weighting
of contribution from the transition state isomers (see the
Supporting Information.)
Calculated versus Experimental Isotope Effects. The
comparison of experimental and calculated KIEs has been found
to provide a sensitive measure of the transition state geometry
and the accuracy of calculated transition structures. Toward that
end, KIEs were predicted for each of the transition structures 5
22
and 7-9 by the method of Bigeleisen and Mayer from the
scaled theoretical vibrational frequencies,23 and tunneling cor-
rections were applied using the one-dimensional infinite para-
bolic barrier model.24 The results are shown in Table 1.
Using isobutene as a calculational model for 2-methyl-1-
pentene, the Becke3LYP structure of 5 provides very good
predictions of the experimental KIEs. The large C1 and Hvinyl
KIEs are predicted within experimental error, and only the C2
KIE prediction differs substantially from the experimental value.
The error at C2 is larger than errors observed in previous
Methanol Trapping of Intermediates. Triazolinedione ene
reactions carried out in the presence of methanol result in the
formation of methanol adducts (e.g., 30) in addition to the
regular ene product. Except when a highly stable cation could
be formed, an anti addition of the TAD and methoxy groups is
observed. This has been concluded to result from methanol
13
25-27
predicted C KIEs based on Becke3LYP transition structures,
but it is notable that the experimental value of 1.003 for C2 is
28
attack on an aziridinium imide intermediate and is evidence
(25) Beno, B. R.; Houk, K. N.; Singleton, D. A. J. Am. Chem. Soc. 1996,
(
22) (a) Bigeleisen, J.; Mayer, M. G. J. Chem. Phys. 1947, 15, 261. (b)
Wolfsberg, M. Acc. Chem. Res. 1972, 5, 225.
23) The calculations used the program QUIVER (Saunders, M.; Laidig,
118, 9984-5.
(26) Singleton, D. A.; Merrigan, S. R.; Liu, J.; Houk, K. N. J. Am. Chem.
Soc. 1997, 119, 3385-6.
(
K. E.; Wolfsberg, M. J. Am. Chem. Soc. 1989, 111, 8989) with Becke3LYP
frequencies scaled by 0.9614 (Scott, A. P.; Radom, L. J. Phys. Chem. 1996,
(27) DelMonte, A. J.; Haller, J.; Houk, K. N.; Sharpless, K. B.; Singleton,
D. A.; Strassner, T.; Thomas, A. A. J. Am. Chem. Soc. 1997, 119, 9907-
9908.
(28) Smonou, I.; Khan, S.; Foote, C. S.; Elemes, Y.; Mavridis, I. M.;
Pantidou, A.; Orfanopoulos, M. J. Am. Chem. Soc. 1995, 117, 7081.
1
00, 16502).
24) Bell, R. P. The Tunnel Effect in Chemistry; Chapman & Hall:
London, 1980; pp 60-63.
(

11890 J. Am. Chem. Soc., Vol. 121, No. 50, 1999
Singleton and Hang
2
∆
∆
.3 eu (favoring formation of 31). The best-fit values were
q
∆H ) -4.6 kcal/mol (favoring the methanol adduct) and
q
q
∆S ) -17 eu for the partitioning of 31 and ∆∆H ) 0.3
kcal/mol for the partitioning of 30 (favoring ene product
formation). The last value may be compared with a predicted
energy difference of 0.8 kcal/mol for 15 versus 19.
Discussion
The calculational results predict a new mechanism for the
ene reaction of alkenes with TADs as shown in Figure 3. In
this mechanism, the ene product is formed via the open biradical
intermediates 10-14, and the aziridinium imides 24-27 are
Figure 5. Logarithmic plot of the ratio of 32:(3 + 4) versus
temperature, and theoretical curve modeled on the mechanism of
Scheme 2 (see text).
“
innocent bystanders” and not on the direct pathway between
starting materials and products. However, the aziridinium imides
can be formed because the overall barrier for their formation
from 10-14 is comparable to the barrier for product formation.
In this way the aziridinium imides can have an impact on the
experimental observations (vide infra).
Scheme 2
The ultimate test of this mechanism is the degree to which it
can reconcile the diverse experimental observations in these
reactions. We discuss here in turn each of these observations
and their consistency with the mechanism of Figure 3.
Intermolecular KIEs. The isotope effects observed with
2-methyl-1-pentene are qualitatively consistent with the forma-
tion of a biradical intermediate, and are inconsistent with a
transition state involving synchronous or moderately asynchro-
nous formation of two C-N bonds of an aziridinium imide.
The good correspondence of calculated and experimental KIEs
supports the proposition that the Becke3LYP calculations
provide a reasonably accurate description of the first step in
the reaction pathway. With 2-methyl-1-pentene the only sig-
nificant error is at C2. The predicted KIE for C2 (1.009) is
greater than the observed value (1.003), and the direction of
this error is suggestive of less bond formation to C2 in the actual
transition state than depicted in the calculated structure 5. This
error thus adds support for the formation of a biradical
intermediate.
against the initial involvement of a zwitterion. Although a
concerted AdE3-type addition of TAD and methanol might also
explain the anti addition, we have noted that the reaction of
PTAD with tetramethylethylene is qualitatively faster in CH2-
Cl2 than in methanol (complete in 4 min versus 13 min
respectively at -78 °C). This argues against methanol involve-
ment in the rate-limiting step. Because the transition state for
methanol attack would be entropically disfavored relative to the
unimolecular formation of the ene product, there is a strong
dependence of the product distribution on temperature with the
For 2-methyl-2-butene the agreement between the predicted
and experimental KIEs is harder to evaluate due to the difficulty
of accurately weighing the contributions of three possible
transition structures 7-9. No weighting of the isotope effects
2
9
methanol adducts being favored at low temperature.
1
3
calculated for 7-9 perfectly predicts the C KIEs observed
for 2-methyl-2-butene: at best the experimental and weighted
calculated KIEs differ by 0.3-0.4%. However, this problem
seems related to the incorrect prediction of the C2 KIE above
with 2-methyl-1-pentene. Allowing for an over-prediction for
If these reactions involve as predicted an intermediate
biradical that may form either the ene product or an aziridinium
imide (Scheme 2), one might expect that a portion of the ene
product not formed via the aziridinium imide would not be
intercepted by methanol. To test the idea, the reaction of
1
3
7
-9 of the C KIEs at the distal olefinic carbons by replacing
2
-methyl-1-pentene with PTAD was studied in a 1:1 mixture
of methanol:CH2Cl2. As the temperature is decreased from 40
C to -78 °C, the methanol adduct 32 increases from 15% to
5% of the total product. However, there is a definite leveling
off or break in the curve in a logarithmic plot of the ratio of
the predicted KIEs of 1.009-1.010 with 1.003, a 70:30 mixture
of (7 + 8):9 would give KIEs of 1.014 and 1.026 for C2 and
C3 of 2-methyl-2-butene, respectively, in perfect agreement with
the experimental results. This exercise in numerology is hardly
convincing. However, it does show that the KIEs observed with
°
5
3
2:(3 + 4) as the temperature is decreased (Figure 5).
2
-methyl-2-butene are consistent with the picture painted by
This plot was found to be modeled well by the mechanism
the 2-methyl-1-pentene KIEs of a TAD attack on alkenes that
is highly unsymmetrical, and plausibly more unsymmetrical than
the Becke3LYP calculated structures.
Several other studies have reported intermolecular KIEs or
intramolecular KIEs that effectively represent intermolecular
q
q
of Scheme 2, varying the ∆∆H and ∆∆S for the partitioning
q
q
of 31 and the ∆∆H for partitioning of 30, fixing the ∆∆S for
the partitioning of 30 at the predicted value for 15 versus 19 of
(29) Elemes, Y.; Orfanopoulos, M. Tetrahedron Lett. 1991, 32, 2667.

Mechanism of Triazolinedione Ene Reactions
J. Am. Chem. Soc., Vol. 121, No. 50, 1999 11891
KIEs. In the reaction of MTAD with â,â-dimethylstyrene (33)
a significant inverse kH/kD of 0.955 per deuterium was observed
within the methyl groups.30 This may be compared with the
deuterium KIEs of 0.970 and 0.971 predicted for the C1 and
C5 methyl groups in transition state 9, assuming that the MTAD
would preferably attack the â carbon to form the more stable
open biradical. The ene product could then be formed after ring
closure to the aziridinium imide and ring opening to the opposite
biradical. A recent report of an overall kH/kD of 0.98 ( 0.02 in
the ene reaction of MTAD with 34 attributed the isotope effect
to a mixture of rate-limiting steps with a counterbalancing
inverse isotope effect for a step forming the aziridinium imide
The isotope effect of 1.25 for 35 is notable because it is
significantly greater than the unity value expected for a
mechanism involving direct formation and reaction of an
aziridinium imide. Houk and Foote explained such isotope
effects based on a hydrogen-bond-like interaction between a
TAD nitrogen and the methyl groups present at the transition
3
1
and a normal isotope effect for the hydrogen-transfer step.
A
much simpler explanation is that the transition state for reaction
of 34 resembles 7 or 8: the predicted KIEs for the combination
of vinylic and C1/C5 deuterium substitution in 7 and 8 are 1.00
and 0.98, respectively, in perfect agreement with the KIE in
17
state for TAD addition to the alkene. Such an interaction
appears to be present in 5-9, but the predicted isotope effect
in 35, when based solely on the transition structure 5 for the
first mechanistic step, is only 1.12 for the RHF structure and
32
3
4.
1
.15 for the Becke3LYP structure. A plausible explanation for
Intramolecular KIEs. Outside of providing some support
the higher observed isotope effect is a small amount of rotation
in 10, as 28 is predicted to be only 1.0 kcal/mol higher than the
transition structure 15 for ene product formation. Based on the
energy of 29, rotation in tetramethylethylene should be negli-
gible. In keeping with this, the isotope effect seen with 1a is
only 1.1.
for the accuracy of the Becke3LYP calculations, the inter-
molecular KIEs here provide no direct information about the
rest of the reaction pathway. The intramolecular KIEs of Greene
for 1 were originally interpreted as excluding biradical inter-
mediates such as 10-14. The same is true with the KIEs
observed by Foote for butene isomers 35-37. The assumption
in this interpretation is that a biradical intermediate such as 38
or 39 would have no choice of isotopes in the product-
determining step and so exhibit a small intramolecular KIE,
while 40 or 41 could choose between reaction of H and D and
so exhibit large intramolecular KIEs. This is opposite of the
experimental observations. However, this idea assumes that the
reactions of a singlet biradical including the 1,2 shift of the
TAD ring will be slower than a relatively hindered bond rotation,
which is contrary to the theoretical predictions here.
The intramolecular isotope effect for the reaction of 1c with
PTAD was intriguingly found by Orfanopoulos to exhibit
activation parameters indicative of substantial tunneling.33 The
isotope effects and contribution of tunneling appeared to vary
little in solvents ranging from CCl4 to acetonitrile. These
observations would be somewhat surprising if the last step in
the ene reaction occurred directly by proton transfer in the
aziridinium imide. It has been noted previously that a relatively
small proportion of reactions involving the redistribution of
charge, such as any proton transfer, exhibit significant tunneling
34
effects. This is thought to result from the importance of solvent
3
5
reorganization in such reactions, and the effect of solvent and
steric bulk on tunneling in proton transfers has been discussed.34
The significant tunneling with little effect of solvent is readily
consistent with hydrogen atom transfer within a biradical.
The Observation of Aziridinium Imides. The prior spec-
troscopic observations of aziridinium imides establishes incon-
trovertibly that aziridinium imides can be formed in the reactions
of RTAD with alkenes. The spectroscopically observed aziri-
dinium imides must be more stable than the corresponding open
biradicals but the experimental observations associated with
these reactions suggest the importance of open intermediates
as well. This is most obvious for the formation of the cis
aziridinium imide 47 from the reaction of trans-cycloheptene
The results here highlight an important limitation to the
interpretation of the results of a Stephenson isotope effect test.
If the equilibrium between open biradical intermediates 42 and
15
with MTAD. Although the initial intermediacy of a trans-
bicyclic aziridinium imide intermediate cannot be excluded, it
would have to open rapidly at -135 °C to biradical intermediate
44 is more rapid than product formation or C-N bond rotation,
4
5 to explain the exclusive observation of the cis structure 47.
then the Stephenson isotope effect test will show results similar
to those observed for 1, and will appear to implicate an
intermediate with the symmetry of 43, without 43 being on the
direct pathway to product.
With trans-cyclooctene the resulting aziridinium imide is trans,
but the multiple products observed in this case were proposed
16
to result from an open zwitterionic intermediate. The ultimate
product in the reaction of adamantylideneadamantane with
MTAD is the diazetidine 50. A consistent explanation for the
formation of 50 from the observed intermediate aziridinium
imide 48 is that it too reacts via a thermally accessible open
structure 49 which ring closes to form a four-membered ring.
(
30) Stratakis, M.; Orfanopoulos, M.; Foote, C. S. J. Org. Chem. 1998,
3, 1315.
31) Vassilikogiannakis, G.; Stratakis, M.; Orfanopoulos, M.; Foote, C.
S. J. Org. Chem. 1999, 64, 4130.
32) A slightly differing allylic alcohol in ref 30 exhibited a kH/kD of
6
(
(
1
.15 ( 0.02, attributed to reversible formation of the aziridinium imide
(33) Elemes, Y.; Orfanopoulos, M. J. Am. Chem. Soc. 1992, 114, 11007.
(34) Caldin, E. F.; Mateo, S. J. Chem. Soc., Faraday Trans. 1975, 71,
1876.
followed by rate-limiting hydrogen abstraction. This is rather surprising;
considering the magnitude of the observed intramolecular isotope effects
(
3.7 or greater), the presence of only a very small amount of reversibility
(35) (a) Kurz, J. L.; Lurz, L. C. J. Am. Chem. Soc. 1972, 94, 4451. (b)
Borgis, D.; Hynes, J. T. J. Phys. Chem. 1996, 100, 1118.
in the first step would account for the slightly elevated kH/kD.

11892 J. Am. Chem. Soc., Vol. 121, No. 50, 1999
Singleton and Hang
The intermediacy of open biradicals also provides a simple
explanation for the many other observations of diazetidines in
the reactions of alkenes with RTAD.
Figure 6. Regioselectivity in some ene reactions of PTAD with
alkenes. The position from which a hydrogen has been abstracted in
the ene product is shown. Results are taken from refs 18 and 38.
going from benzene to acetonitrile. The Becke3LYP predictions
are thus in better accord with the experimental observations.
Regiochemistry. The ene reactions of PTAD with alkenes
follow several distinctive regiochemical trends (Figure 6).1
Simple trisubstituted alkenes react with exclusive hydrogen
abstraction from the more substituted end of the double bond.
For 1,2-disubstituted alkenes there is high selectivity for
hydrogen abstraction from the bulkier alkyl group. However,
if a methyl group is geminal to the bulky group, there is a strong
preference for abstraction at the geminal methyl group.
The preference for reaction at the bulkier group in cis-1,2-
disubstituted alkenes has been rationalized as the result of
nonbonding interactions between the bulky group and the TAD
ring in the transition state for the disfavored hydrogen abstrac-
tion from the less bulky group. The origin of this substantial
steric interaction is subtle if there is direct formation of the ene
product from an aziridinium imide, as in 56, but any steric
interaction would be expected to be greater in a transition state
from an open biradical.
8,38
Reactivity. Ene reactions of simple alkenes with RTAD
exhibit a characteristic reactivity trend with increasing alkyl
substitution in which each additional alkyl group, in any
position, increases the rate of reaction. It is sufficient to note
here that the activation energy for formation of the Becke3LYP
transition structures leading to the open intermediates reasonably
reflects the experimental trends. It was found previously that
Becke3LYP single-point energies for the RHF transition struc-
tures in the reaction of TAD with propene, trans-butene, cis-
butene, and tetramethylethylene also reproduced the trend in
the observed reactivities. Thus, the relative reactivity of alkenes
would seem to not be a good diagnostic of whether aziridinium
imides or open structures are the initial intermediates.
Solvent Effects. The general observation in reactions of
RTAD with alkenes is that there is little effect of solvent
polarity.1 There is no simple correlation of reaction rates with
solvent polarity scales and, in fact, reactions are generally faster
in CH2Cl2 or benzene than in acetonitrile. Solvent effects on
the ene reaction with 3-hexene, diazetidine formation with
3,36
The regiochemistry of the reactions of 54 and 55 seems
particularly difficult to rationalize by the standard aziridinium
imide mechanism. In either case, formation of the “wrong”
aziridinium imides 57 and 58 in an irreversible first step would
necessarily result in regioisomeric products that are not ob-
served. There is no obvious potential origin for such selectivity
in the formation of the aziridinium imides, particularly in light
of the fact that the disfavored aziridinium imides 57 and 58 are
opposite in orientation of the TAD ring with respect to the
neopentyl group. However, the intermediacy of an open biradical
would allow the regioselectivity of these reactions to be
expressed at the stage of the product-forming step, with selective
formation of open biradicals such as 59 having the TAD ring
away from the neopentyl group. Having no favorable alternative
except reversible formation of 57, 59 would eventually undergo
the C-N bond rotation that allows the formation of the observed
product.
chloroethyl vinyl ether, and the Diels-Alder reaction with 1,3-
cyclooctadiene are all very similar.1
3,36,37
The lack of a
substantial rate increase with solvent polarity has been taken
as evidence against the direct formation of a 1,4-dipolar
intermediate. Surprisingly the same observation has not been
taken as incongruous with direct formation of an aziridinium
imide, describable as either a 1,2- or 1,4-dipole.
One approach to interpreting the solvent polarity observation
is to compare the predicted effect of a polar solvent on the
relative stability of the RHF transition structures (which lead
directly to an aziridinium imide) with the effect predicted for
the Becke3LYP transition structures (which lead to the biradical
intermediates). Using an SCICPM model, the barrier for reaction
of TAD with 2-methyl-2-butene via 8 is predicted to increase
slightly (0.1 kcal/mol) going from benzene to acetonitrile. In
contrast, the barrier via the corresponding RHF transition
structure is predicted to decrease significantly (1.3 kcal/mol)
(38) (a) Orfanopoulos, M.; Elemes, Y.; Stratakis, M. Tetrahedron Lett.
1
990, 31, 5775. (b) Orfanopoulos, M.; Stratakis, M.; Elemes, Y. J. Am.
(
(
36) Hall, J. H.; Jones, M. L. J. Org. Chem. 1983, 48, 822.
37) Isaksen, H.; Snyder, J. P. Tetrahedron Lett. 1977, 889.
Chem. Soc. 1991, 113, 3180. (c) Elemes, Y.; Stratakis, M.; Orfanopoulos,
M. Tetrahedron Lett. 1989, 30, 6903.

Mechanism of Triazolinedione Ene Reactions
J. Am. Chem. Soc., Vol. 121, No. 50, 1999 11893
isotope effects observed with 1 is in the former category: the
direct formation and reaction of aziridinium imides would
provide a simpler explanation for the Stephenson isotope effect
results, but the predicted difficulty of rotation of the open
biradicals allows this mechanism to account for the intra-
molecular isotope effects. The biradical mechanism is favored
by observations of solvent effects, regioselectivity, the formation
of the cis aziridinium imide 47 from trans-cycloheptene, and
tunneling in the intramolecular isotope effects for 1c. Finally,
the curved Arrhenius plot in the trapping of intermediates by
methanol appears inconsistent with the simple two-step mech-
anism through an aziridinium imide, and provides the strongest
classical experimental support for the biradical mechanism.
Overall, the new mechanism is both supported by theory and
provides the most consistent explanation for the diverse
observations associated with these reactions.
Trapping of Intermediates. The logarithmic plot of a ratio
of products versus temperature should be linear or nearly linear
whenever the product ratio is determined by a simple competi-
39
tion between two transition states. Thus the partitioning of an
aziridinium imide intermediate between forming the methanol
adduct 32 and the ene products 3 and 4 would be expected to
lead to a linear plot in Figure 5 if the aziridinium imide is formed
directly. A linear plot would also be expected if formation of
the aziridinium imide from an open biradical intermediate is
much faster than formation of the ene product. However, when
aziridinium imide and ene product formation are competitive
the curvature of Figure 5 is the expected result. The excellent
match-up of the experimentally observed product ratios with
the theoretically modeled curve shows that the mechanism in
Scheme 2 is consistent with these observations. It should be
noted that other complex mechanisms might also explain the
curvature in this Arrhenius-type plot: the basic requirements
being that there be an intermediate that is not trapped by
methanol (this weighs against a zwitterionic species) and that
at least three transition states are important in determining the
product ratio. However, it is striking that this curvature may be
Experimental Section
Ene Reactions of PTAD. As an example procedure, 72 g (0.41 mol)
of freshly prepared40 PTAD in 450 mL of 1,4-dioxane was added slowly
to a mixture of 36.94 g (0.528 mol) of 2-methyl-2-butene, 15.92 g
(0.161 mol) of 1,2-dichloroethane (used as GC internal standard), 5.035
g (36.5 mmol) of 1,4-dimethoxybenzene (used as NMR internal
standard), and 385 mL of 1,4-dioxane in a water bath at 25 °C. After
2
h the reaction was found to be 75 ( 3% complete by NMR. (The
percent conversion determined by GC agreed with this result within
the standard error.) Vacuum transfer of the volatiles from the reaction
mixture using a water aspirator followed by fractional distillation using
a 10-cm Vigreux column afforded 4.8 g of 2-methyl-2-butene (bp 38-
4
0 °C, >98% pure by GC).
q
q
An analogous reaction of 2-methyl-2-butene taken to 67 ( 3%
reproduced with ∆∆H and ∆∆S values for the partitioning of
conversion afforded 6.5 g of the starting alkene. Analogous reactions
of 2-methyl-1-pentene were taken to 70 ( 3% and 66 ( 3% completion,
and afforded 8.1 and 11.9 g, respectively, of recovered 2-methyl-1-
pentene (>98% purity by GC in each case).
3
1
0 that are close to those theoretically predicted in the model
0.
Conclusions
NMR Measurements. NMR measurements were taken on neat
samples of 2-methyl-2-butene or 2-methyl-1-pentene in 10-mm NMR
tubes filled to a constant height of 5 cm. A T1 determination by the
inversion-recovery method was carried out for each NMR sample,
and the T1 for each NMR signal remained constant within experimental
error from sample to sample.
The theoretical results here predict a new mechanism for ene
reactions of triazolinediones involving an open biradical as the
key intermediate. This biradical may either form the ene product
or reversibly form an intermediate aziridinium imide. The
formation of the aziridinium imide would thus be a shunt off
the main ene mechanistic pathway.
The isotope effects here support the basic features of
Becke3LYP-predicted transition structures for these reactions.
If the predicted structures err, the isotope effects suggest they
err toward underpredicting the asymmetry of the attack of PTAD
on alkenes. It should be recognized that these intermolecular
isotope effects pertain only to the transition state for the first
step in the reaction pathway and provide no direct information
about the rest of the mechanism. However, the comparison of
experimental and predicted isotope effects suggests that RHF
calculations which predict the direct formation of aziridinium
imides are inadequate for the these reactions, while supporting
the greater accuracy of the Becke3LYP structures.
The 13C spectra were recorded at 100.58 MHz with inverse gated
decoupling, using 175 s delays between calibrated 45° pulses for
2
-methyl-2-butene and 120 s delays between calibrated 45° pulses for
2-methyl-1-pentene. For 2-methyl-2-butene an acquisition time of 5.499
s was used and 197440 points were collected, and for 2-methyl-1-
pentene an acquisition time of 6.278 s was used and 262144 points
2
were collected. The H spectra were recorded at 61.395 MHz with
calibrated 45° pulse widths, using an acquisition time of 7.951 s and a
1
0.0-s delay between pulses for 2-methyl-2-butene, and using an
acquisition time of 4.983 s and a 9.0-s delay between pulses for
2-methyl-1-pentene. Integrations were determined numerically using
a constant region for each peak that was ≈5 times the peak width at
half-height on either side of the peak. A zeroth-order baseline correction
was generally applied, but in no case was a first-order (tilt) correction
applied. The results for all reactions are summarized in the Supporting
Information.
The other experimental observations with triazolinedione ene
reactions may be divided into categories depending on whether
they are merely consistent with the intermediacy of a biradical
or else favor the biradical mechanism over the direct involve-
ment of aziridinium imides. The pattern of intramolecular
Acknowledgment. We thank NIH grant No. GM-45617 and
The Robert A. Welch Foundation for support and NSF grant
No. CHE-9528196 and the Texas A&M University Supercom-
puting Facility for computational resources.
(
39) When one of the products is formed by a process that involves
substantial tunneling, as possible here (see ref 33), the tunneling could lead
to curvature in such an Arrhenius plot over a very wide temperature range
Supporting Information Available: Energies and full
geometries of all calculated structures and NMR integration
results for all reactions (PDF). This material is available free
of charge via the Internet at http://pubs.acs.org.
JA9933339
(see: Jonsson, T.; Glickman, M. H.; Sun, S.; Klinman, J. P. J. Am. Chem.
Soc. 1996, 118, 10319). However, Arrhenius plots for hydrogen-transfer
reactions are usually linear over a normal temperature range, and a plot of
ln kH/kD versus 1/T from 25 to -60 °C for reactions of 1c in CH2Cl2 (using
data from ref 33) shows no significant curvature. Therefore tunneling in
the formation of the ene product is unlikely to account for the large curvature
seen in Figure 5.
(40) Adams, C. E.; Aguilar, D.; Hertel, S.; Knight, W. H.; Paterson, J.
Synth. Comm. 1988, 18, 2225.