Kinetics study and theoretical modeling of the diels-alder reactions of cyclopentadiene and cyclohexadiene with methyl vinyl ketone. The effects of a novel organotungsten catalyst

Article abstract of DOI:10.1021/jo026596l

The Diels-Alder reaction rate constants of methyl vinyl ketone with cyclopentadiene and cyclohexadiene in the presence of a novel organotungsten catalyst, [P(2-py)₃W(CO)(NO)₂]²⁺, have been measured experimentally and modeled theoretically at several temperatures. The uncatalyzed systems were also studied for direct comparison. When 0.0022 M of catalyst is present at room temperature, the rate constants were found to be approximately 5.3 and 5300 times higher than the corresponding uncatalyzed reactions for cyclopentadiene and cyclohexadiene systems, respectively. Experimental data suggested that the catalyst reduced the activation energies by 5-10 kcal/mol. However, the preexponential factors showed reduction of more than 3 orders of magnitude upon catalysis due to the entropic effects. The energy barriers and the rate constants of the uncatalyzed systems were accurately modeled by correlated electronic structure and dual-level variational transition state theory calculation. The calculated endo selectivity is in good agreement with the observed product distribution. Theoretical calculation also suggested the catalyzed reactions proceeded in a highly asynchronous or even stepwise fashion.

Full text of DOI:10.1021/jo026596l

Kin etics Stu d y a n d Th eor etica l Mod elin g of th e Diels-Ald er
Rea ction s of Cyclop en ta d ien e a n d Cycloh exa d ien e w ith Meth yl
Vin yl Keton e. Th e Effects of a Novel Or ga n otu n gsten Ca ta lyst
Yaw-Shien Fu, Shih-Chung Tsai, Chun-Huei Huang, Shih-Yao Yen, Wei-Ping Hu,* and
Shuchun J oyce Yu*
Department of Chemistry and Biochemistry, National Chung Cheng University,
Min-Hsiung, Chia-Yi, Taiwan 621
chejyy@ccu.edu.tw
Received October 23, 2002
The Diels-Alder reaction rate constants of methyl vinyl ketone with cyclopentadiene and
cyclohexadiene in the presence of a novel organotungsten catalyst, [P(2-py)3W(CO)(NO)2]2+, have
been measured experimentally and modeled theoretically at several temperatures. The uncatalyzed
systems were also studied for direct comparison. When 0.0022 M of catalyst is present at room
temperature, the rate constants were found to be approximately 5.3 and 5300 times higher than
the corresponding uncatalyzed reactions for cyclopentadiene and cyclohexadiene systems, respec-
tively. Experimental data suggested that the catalyst reduced the activation energies by 5-10 kcal/
mol. However, the preexponential factors showed reduction of more than 3 orders of magnitude
upon catalysis due to the entropic effects. The energy barriers and the rate constants of the
uncatalyzed systems were accurately modeled by correlated electronic structure and dual-level
variational transition state theory calculation. The calculated endo selectivity is in good agreement
with the observed product distribution. Theoretical calculation also suggested the catalyzed reactions
proceeded in a highly asynchronous or even stepwise fashion.
In tr od u ction
mild conditions and to retain atom economy is of practical
importance. From our previous study,3 we have demon-
strated that the organotungsten complex [P(2-py)3W(CO)
(NO)2](BF4)2 (1) possesses strong Lewis acidity upon loss
of the CO ligand, and the relative Lewis acid strength of
1 is found to be comparable to that of BF3. In addition, 1
The Lewis acid-catalyzed Diels-Alder (DA) reaction1
is one of the most powerful synthetic tools for the
construction of six-membered ring systems that are
essential building subunits for various pharmaceuticals
and specialty chemicals. For various DA reactions, Lewis
acid catalyst can provide not only rate acceleration but
also better selectivity compared with the uncatalyzed
systems.2 However, most conventional Lewis acids are
generally employed in large quantities due to their
extreme water-sensitivity and their feasibility in product
binding. These Lewis acids are sometimes too active that
undesired side reactions occur. Thus, the development
of alternative strategies that use moisture- and air-stable,
low-valent organometallic Lewis acids as catalysts under
can be easily prepared in high yield from W(CO)6, and it
can be stored as a crystalline solid in air for months
without significant decomposition. In addition, we have
also demonstrated that 1 is a potent and selective Lewis
acid catalyst for various carbon-carbon formation reac-
tions such as Friedel-Crafts alkylation,3b aldehyde
cyclotrimerization,3b and DA reactions of 1,3-dienes and
R,â-unsaturated ketones and/or aldehydes.3c
* To whom correspondence should be addressed. Fax: (886) 5-272-
1040.
(1) (a) Carruthers, W. Cycloaddition Reactions in Organic Synthesis;
Pergamon Press: Oxford, 1990; pp 50-53. (b) Fringuelli, F.; Tatichi,
A. Dienes in the Diels-Alder Reactions; Wiley: New York, 1990. (c)
Oppolzer, W. Comprehensive Organic Synthesis; Fleming, I., Trost, B.
M., Eds.; Pergamon Press: Oxford, 1991; Vol. 5, pp 315-399. (d) Roush,
W. R. In Comprehensive Organic Synthesis; Fleming, I., Trost, B. M.,
Eds.; Pergamon Press: Oxford, 1991; Vol. 5, pp 513-550. (e) Santelli,
M.; Pons, J .-M. Lewis Acids and Selectivity in Organic Synthesis; CRC
Press: Boca Raton, 1996. (f) H. Yamamoto Lewis Acids in Organic
Synthesis; Wiley-VCH: Weinheim, 2000.
Although numerous new reports on DA reactions are
published in the literature each year, there are relatively
(2) For reviews see: (a) Kagan, H. B.; Riant, O. Chem. Rev. 1992,
92, 1007. (b) Pindur, U.; Lutz, G.; Otto, C., Chem. Rev. 1993, 93, 741.
(c) Li, C.-J . Chem. Rev. 1993, 93, 2023. (d) Cativiela, C.; Garcia, J . I.;
Mayoral, J . A.; Salvatella, L. Chem. Soc. Rev. 1996, 25, 209. (d) Mehta,
G.; Uma, R Acc. Chem. Res. 2000, 33, 278. (e) Kumar, A. Chem. Rev.
2001, 101, 1.
(3) (a) Kuo, C.-Y.; Fuh, Y.-S.; Shiue, J .-Y.; Lee, G.-H.; Peng, S.-M.;
Yu, S. J . J . Organomet. Chem. 1999, 588, 260. (b) Wang, H.-S.; Yu, S.
J . Tetrahedron Lett. 2002, 43, 1051. (c) Kuo, C.-Y.; Fuh, Y.-S.; Chaen,
M.-C.; Yu, S. J . Tetrahedron Lett. 1999, 40, 6451.
10.1021/jo026596l CCC: $25.00 © 2003 American Chemical Society
Published on Web 03/12/2003
3068
J . Org. Chem. 2003, 68, 3068-3077

Reactions of Cyclopentadiene with Methyl Vinyl Ketone
few detailed kinetics studies,4-7 especially those that
involve catalysis. In this paper, we report a comprehen-
sive kinetics study of both the uncatalyzed and the
corresponding organotungsten Lewis acid 1-catalyzed DA
reactions of cyclopentadiene (CP) with methyl vinyl
ketone (MVK) and cyclohexadiene (CH) with MVK at
several temperatures. In the current study, the reaction
mechanism of the catalyzed system is assumed to be
simplified model systems were used to assess the changes
in the transition-state structures, reaction pathways, and
reaction energy barriers accompanied by the action of the
catalyst. It is hoped that by combining the results of
experimental kinetics measurements and theoretical
modeling, one can gain better insight in both the uncata-
lyzed and the organometallic Lewis acid-catalyzed DA
reactions. In the current study, for the computational
resource consideration, we chose [(NH3)3W(NO)2]2+ as our
model catalyst in which the tripodal ligand, P(2-py)3 of
catalyst 1, was replaced by three monodentate NH3
molecules. This simplification would certainly reduce the
reliability of the modeling to some degree. However, the
current theoretical study concerns only the primary
catalytic effects exerted by the central metal atom. The
simplified model does make the semiquantitative theo-
retical calculation feasible, and important knowledge can
be learned from this model system.
MVK + Cat-CO f Cat-MVK + CO
(1)
diene + Cat-MVK f [Cat-Ts] f Cat-product (2)
MVK + Cat-product f product + Cat-MVK (3)
and then (2), (3), and so on. The symbol Cat-CO is used
as an abbreviation for the catalyst precursor before its
loss of CO ligand, and TS is the symbol for the transition
state of diene + MVK. Note that step 1 is based on the
experimental observation3a that the evolution of CO took
place upon the addition of organic Lewis bases such as
ketones and aldehydes to the solution of catalyst 1. Step
2 was assumed to be the rate-determining step.
The results of the current kinetic studies would enable
us to obtain quantitative information regarding the rate
constants, catalyst efficiencies, and various activation
parameters, which are extremely important in under-
standing the action of catalysis.
The current experimental study was further comple-
mented by theoretical dynamics modeling. Theoretical
reaction dynamics calculation can often assist experi-
mental study in understanding the nature of the reaction
systems. For example, calculation of the transition states
or even the reaction pathways can elucidate the detailed
reaction mechanism and can, at least qualitatively,
explain the branching ratios of the products if multiple
reaction pathways are possible. The causes of the tem-
perature dependence and the extents of the solvation
effects can also be probed by using modern electronic
structure and reaction dynamics methods. Consequently,
in addition to the experimental kinetics measurements
in the present study, we also performed various electronic
structure calculations on the molecules involved in the
reactions. Dual-level variational transition state theory
calculation8,9 with multidimensional tunneling correction
(VTST/MT)10 was also carried out to model the observed
reaction rate constants. For the catalyzed reactions,
Exp er im en ta l Section
Gen er a l Meth od s. Cyclopentadiene was freshly cracked
prior to use. Methyl vinyl ketone was dried over KOH and
distilled under nitrogen. Cyclohexadiene was also dried and
distilled over NaBH4 under nitrogen. Nitromethane was
purified over CaCl2 and dried over CaSO4. The organotungsten
Lewis acid [P(2-py)3W(CO)(NO)2](BF4)2 (1) was prepared by
following literature procedures.3a Syringe techniques were
employed in the transfer of all liquid reactants. 1H NMR
spectra for kinetics measurements and product identification
were recorded on a 400 MHz NMR spectrometer. A GC/MS
spectrometer was used for the determination of the product
identities and the endo/exo ratios.
Kin etics. The kinetics was studied in 10 mL of CH3NO2
for both the uncatalyzed and 1-catalyzed reaction systems. To
obtain the activation parameters, the kinetics study were
conducted at three different temperatures within 0-100 °C
range. Three sets of reactant concentrations were applied to
study the reaction orders. See the Supporting Information for
experimental details.
Com p u t a t ion a l Met h od s. (1) Un ca t a lyzed Syst em .
Geometry optimization of the reactants, products, and the
transition states of the uncatalyzed system in the gas phase
was performed using the Hartree-Fock (HF),11 B3LYP hybrid
density-functional theory,12 and Møller-Plesset second-order
perturbation theory (MP2)11,13 with a 6-31+G* basis set.11
Transition states for four possible pathways (endo-cis, endo-
trans, exo-cis, and exo-trans) were calculated. For comparison
with the catalyzed systems, the LANL2DZ14 basis set was also
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therein. (c) Gugelchuk M. M.; Doherty-Kirby, A. L. J . Org. Chem. 1996,
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1998, 63, 6503. (e) Buback, M.; Heiner, T.; Hermans, B.; Kowollik, K.;
Kozhushkov, S. I.; de Meijere, A. Eur. J . Org. Chem. 1998, 107, 7. (f)
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J . Org. Chem, Vol. 68, No. 8, 2003 3069

Fu et al.
TABLE 1. Exp er im en ta l Ra te Con sta n ts (in cm 3
Molecu le-1 s-1) of th e Diels-Ald er Rea ction s of CP w ith
MVK a t Differ en t Tem p er a tu r es (°C) a n d In itia l Rea cta n t
Con cen tr a tion s (M)
TABLE 2. Exp er im en ta l Ra te Con sta n ts (in cm 3
Molecu le-1 s-1) of th e Diels-Ald er Rea ction s of CH w ith
MVK a t Differ en t Tem p er a tu r es (°C) a n d In itia l Rea cta n t
Con cen tr a tion s (M).
a
a
T
[CP]
[MVK]
kexpt
kcalc
T
[CH]
[MVK]
kexpt
kcalc
0
1.00
1.00
0.50
0.50
1.00
1.00
3.23 ( 0.32b (-26)c
3.72 ( 0.33 (-26)
4.01 ( 0.29 (-26)
4.04 (-26)
80
2.50
2.50
1.25
1.25
2.50
2.50
4.32 ( 0.44b (-27)c
5.09 ( 0.72 (-27)
4.39 ( 0.41 (-27)
4.83 (-27)
29
1.00
0.50
0.50
0.50
0.50
1.00
3.64 ( 0.50 (-25)
4.07 ( 0.20 (-25)
3.89 ( 0.20 (-25)
3.65 (-25)
1.44 (-24)
90
2.50
2.50
1.25
1.25
2.50
2.50
9.82 ( 0.50 (-27)
1.04 ( 0.06 (-26)
9.96 ( 0.68 (-27)
9.70 (-27)
1.88 (-26)
50
0.50
0.50
0.25
0.25
0.50
0.50
1.40 ( 0.20 (-24)
1.61 ( 0.22 (-24)
1.32 ( 0.10 (-24)
100
2.50
2.50
1.25
1.25
2.50
2.50
1.90 ( 0.21 (-26)
1.98 ( 0.18 (-26)
1.74 ( 0.16 (-26)
a
a
From theoretical modeling, see the text and the Supporting
From theoretical modeling, see the text and the Supporting
Information. Error in 90% confidence limit. c 3.23(-26) means
Information. Error in 90% confidence limit. c 4.32(-27) means
b
b
3.23 × 10-26
.
4.32 × 10-27
.
TABLE 3. Exp er im en ta l Ra te Con sta n ts (in cm 3
Molecu le-1 s-1) of th e 1-Ca ta lyzed a Diels-Ald er
Rea ction s of CP w ith MVK a t Differ en t Tem p er a tu r es
(°C) a n d In itia l Rea cta n t Con cen tr a tion s (M)
used with the MP2 and B3LYP methods. The counterpoise
calculation15 was performed on the MP2 and B3LYP results
to obtain more reliable relative energies. Polarized continuum
model (PCM)16 was used to estimate the solvation energy in
nitromethane. Dual-level VTST/MT calculation at the CVT/
µOMT level8,9,17 was performed for the endo-cis pathways to
model the experimental reaction rate constants. (The endo-
cis pathway is chosen because it is believed to be the dominant
pathway in the uncatalyzed systems. In reality, all four
pathways contribute to the total rate constants.) A detailed
dynamics approach is described in the Supporting Information.
The transition states of the CP dimerization reaction were also
calculated at the MP2/6-31+G* and B3LYP/6-31+G* levels for
comparison.
(2) Ca ta lyzed System . For the catalyzed system, a simpler
model catalyst [(NH3)3W(NO)2]2+ was used. The solvation is
expected to have significant effects on the energies and
molecular structures in the charged-catalyzed systems. The
geometries of CP, CH, the Cat-MVK complex, and the Cat-
TS complex were optimized at the HF/LANL2DZ and B3LYP/
LANL2DZ levels including the solvation effects with the
Onsager model.18 Single-point energy calculations at MP2/
LANL2DZ and B3LYP/LANL2DZ levels was performed with
the Onsager and PCM models at the optimized geometries.
The counterpoise correction was also performed on the single-
point energies mentioned above. The same reaction dynamics
approach was applied to model the experimental reaction rate
constants as in the uncatalyzed systems. We further assumed
that the role of the catalyst was only to lower the energy
barrier and thus the catalyst was not considered explicitly.
This assumption was later found to be inadequate as will be
discussed in the next section.
b
T
[CP]
[MVK]
kexpt
kcalc
0
0.20
0.10
0.10
0.10
0.10
0.20
4.88 ( 0.21 (-25)
5.05 ( 0.36 (-25)
4.46 ( 0.44 (-25)
2.65 (-25)
27
0.10
0.10
0.05
0.05
0.10
0.10
1.98 ( 0.06 (-24)
1.94 ( 0.06 (-24)
1.80 ( 0.12 (-24)
1.77 (-24)
7.08 (-24)
50
0.10
0.10
0.05
0.05
0.10
0.10
4.31 ( 0.31 (-24)
4.27 ( 0.29 (-24)
4.27 ( 0.45 (-24)
a
b
The concentration of catalyst 1 is 0.0022 M. From theoretical
modeling
TABLE 4. Exp er im en ta l Ra te Con sta n ts (in cm 3
Molecu le-1 s-1) of th e 1-Ca ta lyzed a Diels-Ald er
Rea ction s of CH w ith MVK a t Differ en t Tem p er a tu r es
(°C) a n d In itia l Rea cta n t Con cen tr a tion s (M)
b
T
[CH]
[MVK]
kexpt
kcalc
0
0.25
0.25
0.50
0.50
0.25
0.25
6.02 ( 0.24 (-26)
5.42 ( 0.37 (-26)
5.64 ( 0.70 (-26)
3.22 (-26)
27
0.25
0.25
0.50
0.50
0.25
0.25
2.52 ( 0.54 (-25)
2.57 ( 0.32 (-25)
2.74 ( 0.36 (-25)
2.35 (-25)
1.00 (-24)
50
0.25
0.25
0.50
0.50
0.25
0.25
6.58 ( 0.57 (-25)
6.69 ( 0.52 (-25)
6.14 ( 0.81 (-25)
Resu lts a n d Discu ssion
(1) Exp er im en ta l Ra te Con sta n ts. The measured
initial rate constants and the estimated errors are shown
in Tables 1-4 (see the Supporting Information for details
of data analysis). Arrhenius fits to the experimental rate
constants for CP/MVK systems are depicted in Figures
a
b
The concentration of catalyst 1 is 0.0022 M. From theoretical
modeling
1 and 2. (Arrhenius plots for CH/MVK systems are
included in the Supporting Information.) In one of our
previous studies,3c the uncatalyzed DA reaction of CH
and MVK proceeded extremely slowly at temperatures
below 70 °C. Only less than 5% conversion was detected
after 24 h at room temperature, and about 7% conversion
was obtained after 6 h at 70 °C. On the other hand, the
uncatalyzed DA reaction of CP and MVK was observed
to proceed much faster (∼83% in 20 h at room temper-
ature). This is consistent with our current kinetics study
as shown in Tables 1 and 2 that the reaction rate
(15) (a) Morokuma, K.; Kitaura, K. In Chemical Applications of
Atomic and Molecular Electronic Potential; Poltizer, P., Truhlar, D.
G., Eds.; Plenum: New York, 1981; p 215. (b) Boys, S. F.; Bernardi, F.
Mol. Phys. 1970, 13, 440.
(16) (a) Miertus, S.; Scrocco, E.; Tomasi, Chem. Phys. 1981, 55, 117.
(b) Miertus, S.; Tomasi, J . Chem. Phys. 1982, 65, 239. (c) Cossi, M.;
Barone, V.; Cammi, R.; Tomasi, J . Chem. Phys. Lett. 1996, 255, 327.
(17) Huang, C.-H.; You, R.-M.; Lian, P.-Y.; Hu, W.-P. J . Phys. Chem.
A. 2000, 104, 7200.
(18) Wong, M. W.; Frisch M. J .; Widberg, K. B. J . Am. Chem. Soc.
1991, 113, 4776.
3070 J . Org. Chem., Vol. 68, No. 8, 2003

Reactions of Cyclopentadiene with Methyl Vinyl Ketone
sponding uncatalyzed CP/MVK and CH/MVK systems,
respectively. Under similar conditions, the catalyzed rate
constants at 50 °C were found to be 2.9 and 1650 times
higher than the corresponding uncatalyzed systems. (The
extrapolated rate constants are used for the uncatalyzed
CH/MVK reaction system.) The extent of acceleration
decreases as the temperature increases, which is as
expected when the catalyst lowers the activation ener-
gies. The rate constant of the 1-catalyzed CP/MVK
reaction system is only about 10 times higher than that
of the 1-catalyzed CH/MVK system at room temperature.
This is also consistent with the earlier observation3c that
both 1-catalyzed reactions were found to proceed ap-
preciably within 1 h at temperatures between 0 and 70
°C. Apparently, the catalyst accelerates the slower CH/
MVK system more significantly.
The current results can be compared to the recent work
by Engbert and co-workers6 where they studied the DA
reactions of 3-phenyl-1-(2-pyridyl)-2-propen-1-ones with
CP using 10 mM (∼5 times catalyst loading of the current
study) of Cu(NO3)2 as the catalyst. They observed an
∼30000-fold increase in rate constants in acetonitrile at
25 °C. With the same catalyst loading, our catalyst 1 and
Engbert’s catalyst is estimated to provide similar rate
acceleration in polar organic solvents. Strukul and co-
workers20 have studied the DA reactions of CP and CH
with MVK, acrolein, and methacrolein using cationic Pd-
(II) and Pt(II) complexes as catalysts. They found that
with the same dienophile the reaction of CH is much
slower than that of CP, which is consistent with our
results.
F IGURE 1. Arrhenius plot of the experimental and calculated
rate constants (cm3 molecule-1 s-1) of CP with MVK. The solid
line is the linear least-squares fit to the experimental values.
It is well-known that this type of DA reaction can also
be accelerated by using solvents such as water and
methanol due to the hydrophobic and hydrogen-bonding
effects.19,21,22 However, most of the organic reagents are
not soluble in water. In our present study, with the
employment of a very low concentration of catalyst 1
(<0.1 mol %), the DA reactions of CH and MVK proceed
much more efficiently than in any known solvent sys-
tems. For example, when the uncatalyzed DA reactions
was carried out in methanol instead of in nitromethane,
a 6-fold increase in the rate constant was observed for
the CH/MVK system at 50 °C. The difference of the CH/
MVK rate constants in the two solvents is consistent with
Breslow’s results on CP/MVK.19 However, the rate con-
stant of the 1-catalyzed DA reaction of CH and MVK at
50 °C is estimated to be 1650 times larger than the
corresponding uncatalyzed reaction in nitromethane.
In the 1-catalyzed systems, the “apparent” rate con-
stants are expected to be proportional to the concentra-
tion of the catalyst. This has been carefully checked up
in both CP/MVK and CH/MVK systems at three different
temperatures by doubling the catalyst concentration from
0.0022 to 0.0044 M. To a good approximation, the
bimolecular rate constants thus obtained were all doubled.
When the catalyst concentrations were reduced to 0.0011M
at 50 °C, we found the rate constants for both CP/MVK
F IGURE 2. Arrhenius plot of the experimental and calculated
catalyzed rate constants of CP with MVK.
constant of CP/MVK at 29 °C is about 2 orders of
magnitude higher than that of CH/MVK at 80 °C. If the
rate constant of the CH/MVK system is extrapolated to
29 °C, it is 7000 times smaller than the rate constant of
the CP/MVK system. The rate constants of the CP/MVK
system have been measured by Breslow et al.19 in various
solvent systems at 293 K. They have shown that the rate
constants are very sensitive to the choice of solvents. For
example, their rate constant in isooctane is about half of
our value obtained in the nitromethane solvent system,
while their value measured in methanol is about six
times larger than ours.
(20) Pignat, K.; Vallotto, J .; Pinna F.; Strukul, G. Organometallics
2000, 19, 5160.
(21) (a) Breslow, R. Acc. Chem. Res. 1991, 24, 159. (b) Breslow, R.;
Guo, T. J . Am. Chem. Soc. 1988, 110, 5613.
(22) (a) Blake, J . F.; J orgensen, W. L. J . Am. Chem. Soc. 1991, 113,
7430. (b) Blake, J . F.; Lim, D.; J orgensen, W. L. J . Org. Chem. 1994,
59, 803. (b) Furlani, T. R.; Gao, J . J . Org. Chem. 1996, 61, 5492. (c)
Meijer, A.; Otto, S.; Engberts, J . B. F. N. J . Org. Chem. 1998, 63, 8989.
When 0.0022 M of catalyst 1 is present in the solution,
the rate constants at 27 °C were found to be ap-
proximately 5.3 and 5300 times higher than the corre-
(19) Rideout, D. C.; Breslow, R. J . Am. Chem. Soc. 1980, 102, 7816.
J . Org. Chem, Vol. 68, No. 8, 2003 3071

Fu et al.
TABLE 5. Exp er im en ta l Ra te Con sta n ts (cm 3
TABLE 7. Ca lcu la ted DA Rea ction En er giesa (k ca l/m ol).
Molecu le-1 s-1) a t Differ en t Ca ta lyst Con cen tr a tion sa ,b
CP + MVK
CH + MVK
CP + MVK
[Cat] )
CH + MVK
[Cat] ) [Cat] )
endo
exo
endo
exo
[Cat] )
HF/6-31+G*
-14.2
-16.9
-14.3
-17.2
-18.8
-16.5
-22.5
-24.8
-22.5
-22.8
-25.0
-22.7
T (°C) 0.0022 M 0.0044 M ratio 0.0022 M 0.0044 M ratio
MP2/6-31+G*b
B3LYP/6-31+G*
0
27
50
5.05 (-25) 1.01 (-24) 2.00:1 5.42 (-26) 1.25 (-25) 2.31:1
1.94 (-24) 3.62 (-24) 1.87:1 2.57 (-25) 5.29 (-25) 2.06:1
4.27 (-24) 8.48 (-24) 1.99:1 6.69 (-25) 1.27 (-24) 1.90:1
a
Born-Oppenheimer energy, not including ZPE, relative to
b
the trans-MVK and the diene. Including counterpoise correction
a
In the CP/MVK system, the concentrations of the reactants
for basis set superposition errors (BSSE).
are both 0.100 M; in the CH/MVK system the concentrations of
b
the reactants are both 0.250 M. We have also measured the rate
constants with 0.0011 M catalyst at 50 °C. The rate constants are
2.00 (-24) and 3.68 (-25) for the CP/MVK and CH/MVK systems,
respectively.
activation entropies. Hence, the DA catalysis by 1 is
suppressed by the unfavorable change in the activation
entropy. Since the 1-catalyzed reactions require very
specific spatial arrangement between the two reactants
and the bulky catalyst in the transition state, the current
results are reasonable.
TABLE 6. Exp er im en ta l P r eexp on en tia l F a ctor s (cm 3
Molecu le-1 s-1), Activa tion En er gies (k ca l/m ol),
Activa tion En th a lp ies (k ca l/m ol), a n d Activa tion
En tr op ies (ca l/m ol K)
(3) En d o-Exo Selectivity. On the basis of the NMR
spectra of the products from the CP/MVK systems, the
majority of the DA product was identified as in the endo
form, and the endo/exo ratios were apparently enhanced
in the catalyzed system. However, for both the uncata-
lyzed and catalyzed CH/MVK systems, the endo form is
the only detectable product from the NMR spectroscopic
analysis. Precise endo/exo product ratios were subse-
quently obtained for both the uncatalyzed and catalyzed
CP/MVK systems at three temperatures by GC/MS
techniques. For the uncatalyzed system, the product was
found to contain 93.0%, 91.0%, and 87.2% of the endo
form at 0, 29, and 50 °C, respectively. These ratios were
consistent with the integration ratios obtained from the
NMR spectra. For the catalyzed system, the endo distri-
butions are 99.2%, 98.5%, and 95.8% at 0, 27, and 50 °C,
respectively. That is, the endo selectivity is approximately
6-8% higher for the catalyzed system. These product
ratios translate approximately to 1.2-1.4 kcal/mol and
2.0-2.6 kcal/mol in the endo-exo activation energy dif-
ferences for the uncatalyzed and catalyzed systems,
respectively. For the CH/MVK systems, we were not able
to further resolve the endo/exo ratios by the GC/MS
methods, presumably because there was only insignifi-
cant amount of exo product. This is also consistent with
the large endo/exo selectivity in the CH/MVK system
obtained from theoretical modeling as will be discussed
later in this section.
∆qH°
∆qS°
Aa
Ea
(300 K)b (300 K)c
a
CP + MVK 7.4 × 10-16 12.8 ( 0.7d
12.2
7.1
17.8
8.0
-130
-144
-129
-145
catalyzed CP + MVK 7.5 × 10-19
catalyzed CH + MVK 4.2 × 10-19
7.7 ( 0.4
CH + MVK 1.2 × 10-15 18.4 ( 1.4
8.6 ( 0.4
a
From least-squares linear fit to the Arrhenius equation, eq 4
in the text. From eq 5 ∆qH° ) Ea - RT, same uncertainties as
b
Ea. c Using eq 5 in the text. Error in 90% confidence limit.
d
and CH/MVK systems were almost halved. Thus in the
range of 1.1 to 4.4 mM of catalyst loading the measured
rate constants are indeed proportional to the concentra-
tion of the catalyst. These results are shown in Table 5.
(2) Activa tion P a r a m eter s. The Arrhenius plots of
the experimental rate constants for the CP/MVK systems
are shown in Figures 1 and 2. Each reaction was
measured at three different temperatures and three sets
of initial concentrations. These nine data points were
used to obtain the linear least-squares fits shown in the
Figures. Table 6 lists the preexponential factors, activa-
tion energies, activation enthalpies, and the activation
entropies calculated according to the Arrhenius
k ) Ae-E /RT
(4)
a
and Eyring equations.23
kBT
h
kBT
h
∆qS°/Re-E /RT
)
e
∆qS°/Re-∆qH°/RT (5)
a
k ) e
e
(4) Th eor etica l Mod elin g. (i) Un ca ta lyzed Sys-
tem s. En er getics. Table 7 lists the calculated reaction
energies in the gas phase at various levels of theory. All
energies are relative to the reactants CP or CH and
s-trans-MVK. All calculations predicted very exoergic
reactions. For the CP/MVK system, the exo-DA product
was found to be more stable by 2-3 kcal/mol while for
the CH/MVK system both endo- and exo-DA products
were predicted to be very similar in energies. At the MP2
level, the CH/MVK reaction was predicted to be 6-8 kcal/
mol more exoergic than the CP/MVK system. Tables 8
and 9 show the calculated barrier heights at various
levels in the gas phase and in nitromethane. The HF/6-
31+G* method predicted very high energy barriers while
the corresponding MP2 and B3LYP values with the same
basis set are lower by more than 20 kcal/mol. It is known
that the electron correlation is very important for calcu-
For the uncatalyzed systems, the observed activation
energies are 12.8 and 18.4 kcal/mol for the CP/MVK and
CH/MVK reactions, respectively. As also seen in Table
6, the catalyst lowers the activation energy by ap-
proximately 5 kcal/mol for the CP/MVK system and 10
kcal/mol for the CH/MVK system. However, 5-10 kcal/
mol of reduction in the activation energy suggests much
larger increases in the reaction rate constants than those
that were experimentally observed. As seen in Table 6,
along with the reduction in activation energies, there are
also dramatic decreases (3-4 orders of magnitude) in the
preexponential factors, and this corresponds to an ap-
proximately 14-16 cal/mol‚K reduction in the standard
(23) Laidler, K. J . In Chemical Kinetics; Harper & Row: New York,
1987; p 114.
3072 J . Org. Chem., Vol. 68, No. 8, 2003

Reactions of Cyclopentadiene with Methyl Vinyl Ketone
TABLE 8. Ca lcu la ted En er gy Ba r r ier sa (k ca l/m ol) of th e Diels-Ald er Rea ction s betw een CP a n d MVK.
gas phase
PCM in CH3NO2
endo-cis
endo-trans
exo-cis
exo-trans
endo-cis
endo-trans
exo-cis
exo-trans
HF/6-31+G*
36.6
18.4
39.2
21.6
22.5
(24.4)
12.8
(14.9)
20.3
37.2
18.1
21.0
(22.8)
10.8
(12.9)
17.5
41.6
23.1
24.6
(26.4)
15.1
(17.1)
21.8
B3LYP/LANL2DZ
17.3
20.5
(22.2)
9.8
(11.8)
16.3
(18.2)
19.3
20.9
(22.8)
11.0
(13.1)
18.1
(20.2)
17.3
20.3
(22.2)
10.4
(12.5)
16.9
(18.9)
21.2
23.2
(25.0)
13.7
(15.7)
19.9
(21.9)
MP2/LANL2DZ//B3LYP/LANL2DZ
21.0
(22.7)b
10.2
MP2/6-31+G*
(12.1)c
17.4
B3LYP/6-31+G*
(19.3)c
(22.4)
(19.5)
(23.8)
a
b
Born-Oppenheimer energy, not including ZPE. The MP2 and B3LYP results include the counterpoise correction for BSSE. Including
unscaled ZPE calculated at B3LYP/LANL2DZ level. c Including unscaled ZPE calculated at B3LYP/6-31+G* level.
TABLE 9. Ca lcu la ted En er gy Ba r r ier sa (k ca l/m ol) of th e Diels-Ald er Rea ction s betw een CH a n d MVK
gas phase
PCM in CH3NO2
endo-cis
endo-trans
exo-cis
exo-trans
endo-cis
endo-trans
exo-cis
exo-trans
HF/6-31+G*
44.1
22.4
46.1
25.3
25.6
(27.2)
18.5
(20.4)
24.7
45.8
23.2
26.0
(27.5)
17.9
(19.6)
23.2
49.3
27.5
28.0
(29.7)
21.0
(22.9)
27.1
B3LYP/LANL2DZ
21.6
24.4
(25.9)
17.1
(18.8)
21.3
(23.1)
23.6
24.3
(26.0)
18.9
(20.8)
23.2
(25.1)
23.5
26.2
(27.7)
19.7
(21.4)
23.0
(24.8)
26.2
27.1
(28.8)
22.1
(24.0)
26.0
(27.9)
MP2/LANL2DZ//B3LYP/LANL2DZ
24.8
(26.3)b
16.0
MP2/6-31+G*
(17.7)c
22.3
B3LYP/6-31+G*
(24.0)c
(26.6)
(25.0)
(29.0)
a
b
Born-Oppenheimer energy, not including ZPE. The MP2 and B3LYP results include the counterpoise correction for BSSE. Including
unscaled ZPE calculated at B3LYP/LANL2DZ level. c Including unscaled ZPE calculated at B3LYP/6-31+G* level.
lating the energy barriers of DA reactions.24 All the
calculations with the 6-31+G* basis set predicted that
the endo-cis pathways have the lowest energy barriers.
The barriers calculated in the current MP2/6-31+G*
calculation for the CP/MVK system are slightly higher
(0.2-1 kcal/mol) than the MP2/6-31G* values reported
by J orgensen et al.25 Furthermore, in the gas phase, the
inclusion of diffuse functions on the heavy atoms in-
creases the energy difference between the endo-cis and
exo-cis transition states from 0.29 kcal/mol25 to 0.64 kcal/
mol in the CP/MVK system. The MP2/6-31+G* calcula-
tion predicted endo-cis barriers (including vibrational
zero-point energies) of 11.8 and 18.8 kcal/mol in ni-
tromethane for CP/MVK and CH/MVK systems, respec-
tively. These values are in good agreement with the
experimental activation energies shown in Table 6. The
B3LYP method predicted 4-6 kcal/mol higher barriers
and similar (∼0.7 kcal/mol) endo selectivity. This is
compared to the experimentally estimated (by GC/MS
data) 1.2-1.4 kcal/mol endo selectivity for the CP/MVK
system mentioned above. The calculated endo selectivity
for the CH/MVK system by the MP2/6-31+G* method is
∼2 kcal/mol higher than the CP/MVK system. This is
consistent with the experimental result that essentially
the entire product is in the endo form. As also seen in
Tables 8 and 9, within the polarized continuum model,
the effect of solvation is not significant in the uncatalyzed
reaction, with changes in barrier heights of approxi-
mately 0-2 kcal/mol. However, it should be realized that
experimental results have shown that the reaction rate
constants in many cases correlate positively with polarity
of the solvents.2d,3c,21b That is, the solvents can indeed
modify the energy barriers to some extent. The specific
interaction between the polar solvent molecules and the
reacting systems cannot be accurately modeled by the
continuum methods such as PCM. Under the experimen-
tal conditions, CP may undergo self-dimerization reac-
tion. The calculated gas-phase barrier heights (not
including the vibrational zero-point energies) for CP
dimerization are 12.2 and 16.8 kcal/mol at MP2/6-31+G*
level and are 21.1 and 24.5 kcal/mol at the B3LYP/6-
31+G* level for the endo and exo pathways, respectively.
That is, the barrier of the endo dimerization pathway is
∼2 kcal/mol higher than the endo-cis pathway for the DA
reaction of CP with MVK. Similar results have been
reported by J orgensen et al.25 This is also consistent with
the current experimental results that no detectable
amount of dimerization product was observed.
Str u ctu r es. The calculated transition state structures
are depicted in Figures 3 and 4. Table 10 lists the
calculated bond lengths of the two forming C-C bonds
of the uncatalyzed transition states at various levels. It
can be seen that the bond formations are all asynchro-
nous, which is as expected for the unsymmetrical dieno-
phile (MVK). In all cases, the bond connecting the diene
and the terminal olefinic carbon of MVK is predicted to
form earlier. The bond formations of the endo-cis and exo-
cis transition states are the most asynchronous. This is
consistent with several earlier studies.25-29 The MP2/6-
31+G* method predicted bond lengths of 2.149 and 2.522
Å for the forming C-C bonds in the endo-cis TS of the
(26) Houk, K. N.; Loncharich, R. J .; Blake, J . F.; J orgensen W. L. J .
Am. Chem. Soc. 1989, 111, 9172.
(24) (a) Houk, K. N.; Gonzalez, J .; Li, Y. Acc. Chem. Res. 1995, 28,
81. (b) Huang, C.-H.; Tsai, L.-C.; Hu, W.-P. J . Phys. Chem. A 2001,
105, 9945.
(25) J orgensen, W. J .; Lim, D. L.; Blake, J . F. J . Am. Chem. Soc.
1993, 115, 2936.
(27) Birney, D. M.; Houk, K. N. J . Am. Chem. Soc. 1990, 112, 4127.
(28) Garc´ıa, J . I.; Mart´ınez-Merino, V.; Mayoral, J . A.; Salvatella,
L. J . Am. Chem. Soc. 1998, 120, 2415.
(29) Avalos, M.; Rabiano, R.; Bravo, J . L.; Cintas, P.; J ime´nez, J .
L.; Palacios, J . C.; Silva, M. A. J . Org. Chem. 2000, 65, 6613.
J . Org. Chem, Vol. 68, No. 8, 2003 3073

Fu et al.
TABLE 10. Ca lcu la ted Bon d Len gth s (Å) of th e
F or m in g C-C Bon d s in th e TS of th e Un ca ta lyzed
Rea ction sa
CP + MVK
MP2/6-31+G*
B3LYP/6-31+G*
endo-cis
endo-trans
exo-cis
2.149, 2.522
2.179, 2.358
2.162, 2.464
2.168, 2.381
1.990, 2.690
2.070, 2.430
2.020, 2.580
2.066, 2.460
exo-trans
CH + MVK
MP2/6-31+G*
B3LYP/6-31+G*
endo-cis
endo-trans
exo-cis
2.152, 2.596
2.207, 2.448
2.182, 2.554
2.219, 2.437
2.007, 2.720
2.101, 2.490
2.020, 2.730
2.100, 2.501
exo-trans
a
The HF/6-31+G* and B3LYP/LANL2DZ results are included
in the Supporting Information.
kcal/mol) and with the MP2/6-31+G* results (11.8 and
18.8 kcal/mol). That is, a good fit to the experimental rate
constants can be achieved by merely adjusting the
calculated barrier height by 1.3 and 0.9 kcal/mol for CP/
MVK and CH/MVK systems, respectively. There were
only negligible variational effects9 at all temperatures.
However, we found that the tunneling effects increase
the rate constants by 63% and 70% at room temperatures
for the CP/MVK and CH/MVK systems, respectively.
Here the tunneling means that the whole system tunnels
through a potential energy barrier (from the reactant side
to the product side) between two isoenergetic geometries
along the reaction path. The geometry changes (or
tunneling coordinates) are dominated by the two forming
C-C bonds. Even through carbon atoms are relatively
heavy; however, the high barriers of the DA reactions
make the tunneling effects not negligible at and below
room temperature. It can be clearly seen in the Arrhenius
plots that even though the rate constants were fitted to
a single temperature, the experimental temperature
dependence was precisely modeled by the dual-level
dynamics calculation.
F IGURE 3. Transition-state structures of CP + MVK.
It can be seen from Tables 6, 8, and 9 that the large
difference in reactivity between CP and CH is due to the
6-7 kcal/mol difference in reaction energy barrier. We
noted that the activation energies of the uncatalyzed CP/
MVK and CH/MVK systems are significantly lower than
that of the parent DA reactions (27.5 kcal/mol).24 Both
the nature of the dienophiles and the magnitude of the
ring strains of the cyclic dienes play important roles in
the reduction of the DA energy barriers. Thus, one
possible explanation to the difference in the energy
barriers between the two systems is that CP is a more
strained molecule than CH, and thus in the CP/MVK
system the energy of the reactants is closer to that of
the DA transition state. For the DA products in the
current study, in addition to the six-membered ring
formed by the [4 + 2] cycloaddition, another five-
membered (for CP/MVK) or six-membered (for CH/MVK)
ring is also formed at the same time. The more exoergic
CH/MVK reaction (see Table 7) implies that, if everything
else being equal, the formation of the new six-membered
ring releases 6-8 kcal/mol more energy than the five-
membered ring. This is also consistent with the above
explanation.
F IGURE 4. Transition-state structures of CH + MVK.
CP/MVK system and 2.152 and 2.596 Å for the CH/MVK
system. These values are compared to 2.279 Å for the
parent DA reaction (1,3-butadiene + ethylene) calculated
at the same level of theory.24b The B3LYP method
predicted even more asynchronous TS structures in all
cases.
Ra te Con sta n ts. In the theoretical modeling, the
classical barrier heights were adjusted to reproduce the
experimental rate constants for the CP/MVK system at
29 °C and for the CH/MVK systems at 90 °C. The barriers
thus determined plus the zero-point energies are 13.1 and
17.9 kcal/mol for the CP/MVK and CH/MVK systems,
respectively. These values are in very good agreement
with the experimental activation energies (12.8 and 18.4
3074 J . Org. Chem., Vol. 68, No. 8, 2003

Reactions of Cyclopentadiene with Methyl Vinyl Ketone
TABLE 11. Ca lcu la ted En er gy Ba r r ier sa (in k ca l/m ol) of th e Ca ta lyzed Rea ction of CP a n d MVK
solvation ) PCMb
endo-cis
endo-trans
exo-cis
exo-trans
HF/LANL2DZ
B3LYP/LANL2DZ
15.3
2.0
13.4
3.0
(4.8)
11.2
(13.0)
10.5
3.7
(5.4)
10.2
(11.8)
12.6
2.4
(4.0)
11.0
(12.6)
(3.7)c
8.2
MP2/LANL2DZ//B3LYP/LANL2DZ
(9.9)c
a
b
Born-Oppenheimer energy, not including ZPE. The MP2 and B3LYP results include the counterpoise correction for BSSE. The
results using the Onsager solvation model are included in the Supporting Information. c Including ZPE calculated at B3LYP/LANL2DZ
level.
TABLE 12. Ca lcu la ted En er gy Ba r r ier sa (in k ca l/m ol) of th e Ca ta lyzed Rea ction of CH a n d MVK
solvation ) PCMb
endo-cis
endo-trans
exo-cis
exo-trans
HF/LANL2DZ
B3LYP/LANL2DZ
18.3
6.0
16.0
4.4
(5.9)
14.3
(15.8)
13.9
7.2
(8.6)
19.6
(21.0)
18.2
6.0
(7.4)
18.6
(20.1)
(7.7)c
15.5
MP2/LANL2DZ//B3LYP/LANL2DZ
(17.2)c
a
b
Born-Oppenheimer energy, not including ZPE. The MP2 and B3LYP results include the counterpoise correction for BSSE. The
results using the Onsager solvation model are included in the Supporting Information. c Including ZPE calculated at B3LYP/LANL2DZ
level.
(ii) Ca ta lyzed System s. En er getics. Even with the
simplified model catalyst, modeling the catalyzed systems
is very challenging due to their large sizes. Tables 11 and
12 show the calculated energy barriers for the catalyzed
systems. Here, the barrier is defined as the difference
between the energy of the Cat-TS and the sum of the
energies of Cat-MVK complex and the diene. Due to the
much larger sizes of the catalyzed systems, the smaller
LANL2DZ basis set was used in the geometry optimiza-
tion instead of the 6-31+G* set used in the uncatalyzed
systems.
catalysis was calculated to be 10-12 kcal/mol while the
experimental activation energies showed a reduction of
6-10 kcal/mol. At the MP2 level the endo selectivity was
calculated to be significantly higher (1-2 kcal/mol) than
that of the uncatalyzed systems, which is consistent with
the current experimental results for the CP/MVK system.
Interestingly, in the current systems the combined
effects of the organotungsten Lewis acid catalyst and
solvation on the energy barrier reduction (∼10 kcal/mol)
were found to be similar to earlier experimental and
theoretical results on the Diels-Alder reactions catalyzed
We found that the Onsager solvation model signifi-
cantly underestimated the energy barriers while the
PCM model predicted more reasonable barrier heights.
Thus, the following discussion will be based on the PCM
results. For the CP/MVK system, at the MP2 level
(B3LYP/LANL2DZ geometry) the endo-cis pathway was
found to have the lowest barrier of 9.9 kcal/mol that is
about 2 kcal/mol higher than the experimental activation
energy. The calculated endo selectivity (∼2.0 kcal/mol)
at the MP2 level is in good agreement with the experi-
mentally estimated value of 2.0-2.6 kcal/mol mentioned
earlier in this section. The B3LYP calculation also
predicted endo-cis to be the lowest energy pathway but
with barriers approximately 4 kcal/mol too low and a very
small endo selectivity (0.3 kcal/mol). For the CH/MVK
system, at the MP2 level the endo-trans pathway was
found to be the one with the lowest barrier of 15.8 kcal/
mol that is about 7 kcal/mol higher than the experimental
activation energy. The calculated endo selectivity is 4.3
kcal/mol at the MP2 level. The high endo selectivity is
also consistent with the results obtained from both the
NMR and GC/MS analyses. The B3LYP calculation also
predicted endo-trans to be the lowest energy pathway but
with barriers approximately 3 kcal/mol too low and a
smaller endo selectivity (1.6 kcal/mol). The overestima-
tion of the barriers by the MP2 calculation is expected
because of the lower quality of the atomic basis set
applied compared to the uncatalyzed systems. At the
MP2/LANL2DZ (PCM) level, the barrier reduction by the
, BF3, and AlCl3.27-30
by simple Lewis acids such as BH3
Str u ctu r es. The calculated structures of the Cat-TS
are depicted in Figures 5 and 6 and the calculated bond
lengths of the two forming C-C bonds are shown in Table
13. In some cases, for a particular pathway more than
one TS differing on the relative orientations between the
model catalyst and the diene/MVK units were found.
Their energies are all within 2 kcal/mol. Only the one
with the lowest energy is considered here. Note that the
bond lengths of the two forming C-C bonds in the TS
are now significantly different. In particular, at the
B3LYP/LANL2DZ level the differences between the two
bond lengths are 1.1-1.2 Å in the CP/MVK system and
are 1.4-2.2 Å in the CH/MVK system. These are com-
pared with the 0.2-0.4 and 0.4-0.9 Å differences in the
uncatalyzed systems predicted by MP2/6-31+G* and
B3LYP/LANL2DZ calculation, respectively. Apparently,
in the catalyzed DA reactions the bond formations are
much more asynchronous than the uncatalyzed reactions,
especially for the CH/MVK system. This raised the
question whether the catalyzed reactions might even be
stepwise. We were able to locate a second transition state
(and the corresponding complex between the two transi-
tion states) corresponding to the formation of the second
C-C bond at HF/LANL2DZ level in the gas-phase for all
four pathways for the CP and CH systems. The calculated
structure of the second TS for the endo-cis pathway of
(30) Inukai, T.; Kojima, T.; J . Org. Chem. 1967, 32, 872.
J . Org. Chem, Vol. 68, No. 8, 2003 3075

Fu et al.
F IGURE 6. Transition-state structures of CH + MVK with
the model catalyst.
F IGURE 5. Transition-state structures of CP + MVK with
the model catalyst.
TABLE 13. Ca lcu la ted a ,b Bon d Len gth s (Å) of th e
F or m in g C-C Bon d s in th e TS of th e Ca ta lyzed DA
Rea ction s.
the CH/MVK system is shown in Figure 7. However, the
energies of the second TS are significantly lower than
the first TS. For example, in the CP(CH)/MVK system
the second TS on the endo-cis pathway is 9.5 (11.6), and
6.0 (11.1) kcal/mol lower than the first TS at the HF/
LANL2DZ and MP2/LANL2DZ (PCM) levels, respec-
tively. Thus, even though the second TS implies that the
catalyzed reactions might actually be stepwise, the
second step would be very fast, and the first step is the
rate-determining step. Actually the second TS are only
resolvable in the HF calculation. When the correlation
energies are included (MP2, B3LYP methods) either in
geometry optimization or single point energy calculation,
the second TS disappeared. (That is, the energies along
the reaction path were found to decrease monotonically
from the first TS to the product.) We did not find any
low-lying [2 + 4] hetero-DA transition states that are
sometimes artifacts of low-level Hartree-Fock calcula-
tion.28,29,31,32 Recent studies by Silva et al.,33 Tanaka and
Kanemasa,34 and Roberson et al.35 also suggested that
Lewis acid catalyzed cycloaddition reactions might occur
CP + MVK
CH + MVK
endo-cis
endo-trans
exo-cis
(2.115, 3.170)
(2.053, 3.143)
(2.150, 3.230)
(2.050, 3.240)
(2.019, 3.407)
(2.094, 3.691)
(1.968, 4.167)
(1.986, 3.474)
exo-trans
a
Solvent effects were considered with the Onsager model.
Geometry calculated at B3LYP/LANL2DZ level. Results of HF/
b
LANL2DZ level are listed in the Supporting Information.
through a stepwise mechanism. However, their semiem-
pirical and Hartree-Fock calculation very likely overes-
timated the energy barriers of the second TS due to the
implicit treatment or omission of the electron correlation
energies. On the other hand, recent high-level theoretical
calculations36-38 showed that the radical cation DA
reactions proceed through stepwise mechanisms. The
theoretical study over recent years seem to reach a
consensus that most uncatalyzed Diels-Alder reactions
with asymmetric diene/dienophile proceed through con-
certed asynchronous pathways. When the DA reaction
is catalyzed by a Lewis acid, the electron density is drawn
out of the system and the reaction pathways become
(31) Yamabe, S.; Dai, T.; Minato, T. J . Am. Chem. Soc. 1995, 117,
10994.
(32) Garc´ıa, J . I.; Mayoral, J . A.; Salvatella, L. J . Am. Chem. Soc.
1996, 118, 11680.
(33) (a) Alves, C. N.; Camilo, F. F.; Cruber, J .; Silva, A. B. F.
Tetrahedron 2001, 57, 6877. (b) Alves, C. N.; Silva, A. B. F.; Mart´ı, S.;
Moliner, V.; Oliva, M.; Andre´s, J .; Domingo, L. R. Tetrahedron 2002,
58, 2695.
(34) Tanaka, J .; Kanemasa, S. Tetrahedron 2001, 57, 899.
(35) Roberson, M.; J epsen, A. S. J .; J orgensen, K. A. Tetrahedron
2001, 57, 907.
(36) Haberl, U.; Wiest, O.; Steckhan, E. J . Am. Chem. Soc. 1999,
121, 6730.
(37) (a) Hofmann, M.; Schaefer, H. F., III. J . Am. Chem. Soc. 1999,
121, 6719. (b) Hofmann, M.; Schaefer, H. F., III. J . Phys. Chem. A.
1999, 103, 8895.
(38) Saettel, N.; Wiest, O.; Singleton, D. A.; Meyer, M. P. J . Am.
Chem. Soc. 2002, 124, 11552.
3076 J . Org. Chem., Vol. 68, No. 8, 2003

Reactions of Cyclopentadiene with Methyl Vinyl Ketone
demonstrated that it is inappropriate to model the
catalyzed reaction as just a lower barrier uncatalyzed
reaction. The reason for the overestimation of the barrier
height or the temperature dependence is because the
entropies of activation for the catalyzed reactions are
much more negative, as seen in Table 6. As a result, the
preexponential factors for the catalyzed reaction are
much smaller than the uncatalyzed reactions. Thus, a
realistic modeling of the catalyzed reaction rates should
take the entropic factors into account. Otherwise, the
energy barrier and the temperature dependence of the
rate constants cannot be modeled accurately.
Su m m a r y
Kinetics measurements have been made on the Diels-
Alder reactions between methyl vinyl ketone with cyclo-
pentadiene and cyclohexadiene as well as on the same
reactions with the presence of an organotungsten Lewis
acid catalyst. The catalysis resulted in very significant
acceleration, especially for the cyclohexadiene/methyl
vinyl ketone system. Theoretical modeling was performed
on the uncatalyzed and catalyzed systems and the results
are consistent with the experimental activation energies,
endo selectivity, the temperature dependence of the
uncatalyzed reaction rate constants, and the barrier
reductions upon catalysis. The quantitative experimental
data and the semiquantitative theoretical modeling
information obtained in the current study might be useful
for gaining better insights in the transition-metal cata-
lyzed chemical reactions and for future theoretical mod-
eling of catalyzed DA reactions.
F IGURE 7. Structure of the second endo-cis transition state
of the catalyzed CH/MVK system at HF/LANL2DZ level.
highly asynchronous. In the extreme cases of very strong
Lewis acid catalysis or in the radical cation DA reactions,
the pathways become stepwise.
It is also noted that since in our model systems
ammonia molecules were used in place of pyridines in 1,
some additional errors were introduced. For example, the
difference in the coordinating strengths to the metal
would certainly affect the Lewis acidity of 1 and thus the
catalytic efficiency. Also, the P(2-py)3 unit of 1 makes it
more rigid than our model catalyst, and this would cause
some differences in the structures. In addition, the
pyridine is a potential π acid and might further modify
the electron density on the metal atom.
Ra te Con sta n ts. We have also tried to model the
experimental rate constants of the catalyzed systems
using the dual-level VTST/MT approach. We used the
same reaction-path information as in the uncatalyzed
systems except adjusting the classical barrier heights to
fit the measured rate constants at 27 °C for the CP/MVK
and CH/MVK systems. The resulting effective barriers
(including the zero-point energies) of 11.8 and 12.4 kcal/
mol are approximately 4 kcal/mol too high as compared
to the experimental activation energies. The calculated
and experimental rate constants of the catalyzed CP/
MVK system are plotted in Figure 2. (A figure for the
CH/MVK system is in the Supporting Information.) As
seen in these figures, the calculated temperature depen-
dence of the rate constants is too large. This clearly
Ack n ow led gm en t. This research is supported by
the National Science Council of Taiwan, Grant Nos.
NSC 89-2113-M-194-021 and NSC 88-2113-M-194-018.
We are grateful to the National Center for High-
Performance Computing of Taiwan for providing part
of the computational resources. We would also like to
thank Mr. Ming-Cheng Chen and Ms. Li-Chao Tsai for
very helpful discussions early in this project.
Su p p or tin g In for m a tion Ava ila ble: Detailed description
of the experimental conditions/procedures and the dual-level
reaction dynamics calculation, Arrhenius plots for the CH/
MVK systems, calculated bond lengths and energy barriers,
and the Cartesian coordinates and absolute energies of the
calculated transition states. This material is available free of
charge via the Internet at http://pubs.acs.org.
J O026596L
J . Org. Chem, Vol. 68, No. 8, 2003 3077