Volume 4,Issue 10
Understanding the influence of Lewis acids in Diels-Alder reaction of alkyl
2-p-tolylsufinylacrylates : A DFT study
2010/03
Tsung-Fan Teng, Jyh-Chiang Jiang*
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HIGH PERFORMANCE COMPUTING
Introduction:
The cycloaddition reaction is a powerful pericyclic process, leading to the
formation of two C-C bonds in a single transformation. Pericyclic transformations,
such as ene or Diels-Alder reactions, with chiral alkyl sulfoxides give particularly
high diastereomeric excess and often are utilized in natural product synthesis. In
addition, the development of chiral dienophiles has been a major target in the field of
asymmetric Diels-Alder reactions since the past decade. Chiral sulfoxides can be
utilized as powerful chiral auxiliaries because they can easily be prepared in high
enantiopurity. However, the stereoselectivity in the Diels-Alder reaction has been
discussed by invoking many factors. In particular, secondary orbital interactions (SOI)
between the non-bonded atoms in a diene and a dienophile, suggested by Hoffmann
and Woodward, have attracted the attention of chemists for a long time. In modern
synthetic chemistry, the stereochemistry can be controlled by using various Lewis
acids. Therefore, we were highly interested in the structure and properties of Lewis
acid-sulfoxide complexes.
In this work, three commonly-used Lewis acids, boron trifluoride (BF3), znic
chloride (ZnCl2) and aluminium trichloride (AlCl3) were employed to describe the
catalyzed chiral dienophile-cyclopentadiene in Diels-Alder reaction. The effects of
Lewis acids on the cycloaddition paths were investigated by ab initio calculations.
Fig1: Diels-Alder reaction is between CP and alkyl 2-p-tolylsufinylacrylates with
Lewis Acid catalysts
LA3
O
LA2
O
Lewis acid
S
*
OC2H5
LA1
H3C
endo
BF3
ZnCl2
AlCl3
SOTol
*
COOC2H5
exo
Computational Section:
Calculations were performed with the Gaussian 03 suites of computational program.
Because the chiral dienophile is a big molecular, we chose 6-31+G* as basis sets and
used Hartree-Fock (HF) theory level. All energy minima are confirmed to have no
imaginary frequencies while the transition state structures have one single imaginary
P.1
Understanding the influence of Lewis acids in Diels-Alder reaction of alkyl
2-p-tolylsufinylacrylates : A DFT study
Tsung-Fan Teng, Jyh-Chiang Jiang*
frequency. The shown energies have been corrected by zero-point vibration energy
(ZPVE) by the same level of theory.
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HIGH PERFORMANCE COMPUTING
Results:
The presence of LA catalysts has been considered in three coordination modes:
single chelating with esterified oxygen, single chelating with carbonyl oxygen and
double chelating with oxygen in sulfoxide oxygen and carbonyl oxygen.
Unfortunately, the double chelating work had not yet succeeded. In table 1, the
activation energies had obvious differences between in absent and in present of LA
catalysts in Diels-Alder reaction. The results had an agreement with the secondary
orbital interaction theory. In addition, the LA chelating positions had differences in
activation energies and endo/exo ratios. If AlCl3 chelated with oxygen in ester, it had
better endo/exo ratio but high activation energy than AlCl3 chelated with oxygen in
sulfoxide.
Table1: The endo/exo ratio between absent or present Lewis acid
complex
transition
product
△E≠(HF/6-31+G*)
K1/K2
endo/exo
10.9100
91.6/8.4
20.697
95.4/4.6
9.64
90.6/9.4
no-Lewis acid
endo
-1277.0672
-1277.0049
-1277.0825
39.0719
exo
-1277.0671
-1277.0026
-1277.0816
40.4719
AlCl3-O
endo
-2897.6581
-2897.5972
-2897.6534
38.1978
exo
-2897.6581
-2897.5943
-2897.6526
39.9975
endo
-2897.7136
-2897.6646
-2897.7294
30.8045
exo
-2897.7136
-2897.6624
-2897.7291
32.1473
AlCl3-SO
Fig2: Geometries of transition states of the DA reactions between cyclopentadiene
and alkyl 2-p-tolylsufinylacrylates with AlCl3 as LA
Endo(COOEt)
Exo(COOEt)
Conclusion:
In our results, we realized the present catalyst reactions had low activation energies
and large endo selectivities. We will still focus on the influences of different Lewis
acids as BF3 and ZnCl2 in Diels-Alder reactions and double chelating with Lewis acid
and dienophile.
P.2