3A02
Stereoselective Isomerization of Phenylpropenoids by in situ Generated
Nickel(0)NitrilePhosphine
PERMANA, Yessi*1; SAPUTRA, Leo1; GUSTINI, Nunik,1 Arifin*2; IRLE, Stephan2;
PATAH, Aep1; MARTOPRAWIRO, Muhamad A.1
1
Chemistry Department, Faculty of Mathematics and Natural Sciences, Institut Teknologi
Bandung, Indonesia
2
Institute of Transformative Bio-Molecules, Nagoya University, Japan
E-mail address: [email protected]
[Introduction]
Phenylpropenoids, such as eugenol, methyleugenol, and estragole are readily available chemicals
from nature. Geometric isomerism of phenylpropenoids is one of the interesting chemical
transformations (Figure 1) because the applicability of the isomerized products, for example
isoeugenol, in perfumeries, cosmetics, food products, medicines, and insecticide. However, the
stereoselectivity of the isomerization product is the most important factor for the application, where
only the trans-product are marketed due to the toxic character of the corresponding cis-product.
R2
+
R2
1
R
phenylpropenoid
1
R
trans-product
R2
R1
cis-product
Figure 1. Isomerization of phenylpropenoids to trans- and cis-isomerized products.
We have performed the isomerization for eugenol and other phenylpropenoids to its
trans-products by the Ni(0) phosphine as the catalyst. In this work, Ni(0) phosphine was generated in a
simpler way from inexpensive Ni(II) salts, simple tertiary phosphines, and Zn powder as a reductant in
only 1:2:1 mol ratio. During the reaction, addition of acetonitrile (MeCN) is a critical factor to get the
high yield. A small addition of MeCN (100 μL) can dramatically increase the catalytic activity.
Density functional theory (DFT) calculations were performed to analyze the role of MeCN and the
reaction mechanism.
[Method]
Experimental procedure:
In our typical reactions, a 50 mL Schlenk tube was charged with NiCl2(PPh3), Zn powder, PPh3
with the ratio is 1:10:1. The Schlenk tube was purged by nitrogen gas (1 atm), followed by addition of
3 mL of MeCN. The mixture was heated to 353 K and stirred for 2 min until the solution changed the
color from blue to red. Eugenol was charged via syringe to the solution and the mixture was heated
and stirred for 1 h. After the reaction has completed, oxygen was purged into the solution, resulting in
rapid loss of the red color.
Computational details:
The calculations were done in the gas phase, where PBE0-D3 DFT functional was carried out for
all calculation.1 The Stuttgart RSC 1997 with ECP basis set was employed for Ni atom. 6-31+G(d)
was used for nitrogen, oxygen, and phosphorus atoms, 6-31G(d) and 6-31G(d,p) basis sets were
applied for the carbons and hydogens of alkene, respectively. For all other atoms, 6-31G basis set was
used. All calculations were performed using Gaussian 09 version D.01.
[Results and discussions]
We have performed the isomerization for eugenol and other phenylpropenoids, i.e. methyl
eugenol, and estragole, to its trans-products with the conversions are 99% and trans-selectivities are
95-97%. Kinetic studies showed that the reaction was first order to eugenol, with a rate
constant (k) of 0.252 min-1 at 353 K. A negative entropy of activation (S‡= 22 e.u.) might be
attributed to an associative pathway taken by the complex with eugenol. The DFT results show that
MeCN stabilize intermediate and provide the lower energy pathway (Figure 2). The free energy
barriers are 104.8 and 140.0 kJ/mol for the reaction with and without addition of MeCN, respectively.
More details will be discussed in the oral presentation.
Figure 2. Free energy profile of eugenol isomerization to trans-products.
[References]
(1)
Steinmetz, M.; Grimme, S. Benchmark Study of the Performance of Density Functional Theory
for Bond Activations with (Ni,Pd)-Based Transition-Metal Catalysts. ChemistryOpen 2013, 2,
115–124.