Theoretical and experimental study of reactions of esters of P(IV) acids with pentachloroethane

Full text of DOI:10.1007/s10631-005-0034-1

Doklady Chemistry, Vol. 401, Part 2, 2005, pp. 66–68. Translated from Doklady Akademii Nauk, Vol. 401, No. 6, 2005, pp. 769–771.
Original Russian Text Copyright © 2005 by M. Gazizov, Zverev, Karimova, K. Gazizov, Sinyashin.
CHEMISTRY
Theoretical and Experimental Study
of Reactions of Esters of P(IV) Acids with Pentachloroethane
M. B. Gazizov, V. V. Zverev, R. F. Karimova,
K. M. Gazizov, and Corresponding Member of the RAS O. G. Sinyashin
Received December 23, 2004
Data on the dehydrochlorinating reactivity of esters does not participate directly in the reaction, retains the
of P(IV) acids have not been reported in the literature. initial conformation. As a result, the gg conformer of
Since the phosphoryl and ester oxygens of these com- phosphonate Ia is transformed into the –gc conforma-
pounds have lone electron pairs, we suggested that tion of III, while the gt conformer is converted into the
esters of P(IV) acids would exhibit basic properties gg conformation of phosphonate III.
under appropriate conditions and participate in dehy-
The calculations show that the first step of the reac-
drochlorination.
tion starts with the formation of the prereaction H-com-
In this paper, we report on theoretical and experi-
mental evidence of the dehydrochlorinating reactivity
of P(IV) esters. The study was mainly concerned with
the reaction of O,O-dimethyl methylphosphonate Ia
with pentachloroethane II. We assumed that the reac-
tion occurs in two steps. In the first step, the reactants Ia
and II are converted into intermediate phosphorane VI,
while, in the second step, the phosphorane is trans-
formed into monomethyl methylphosphonate III and
methyl chloride V.
Theoretical predictions were made using quantum-
chemical methods, namely, density functional theory
(DFT/PBE/TZ2P) with the ab initio Perdew–Burke–
Ernzerhof exchange correlation potential [1] and a tri-
ple-ζ basis set augmented with two polarization func-
tions (TZ2P) according to the PRIRODA program [2]
and the PM3 semiempirical method to analyze the
structure of phosphonate Ia; the ab initio Hartree–Fock
method at the HF/6-31G* level was also used.
These three methods—DFT/PBE/TZ2P, HF/6-
31G*, and PM3—show that phosphonate Ia has two
stable conformations, with gg and gt orientations of the
methoxy groups relative to the P=O bond, whereas
phosphonate III has a larger number of stable conform-
ers, those with –gc and gg orientations of the methoxy
and hydroxy groups being the most favorable. The reac-
tion route of the gg and gt conformers of phosphonate
Ia that we consider involves the g-oriented methoxy
group, whereas the other methoxy group (g or t), which
plex VI, which is transformed via transition state TS1
(Fig. 1a) into the intermediate complex VII with a for-
mation energy of about 1 kcal/mol. The complex
decomposes to give the phosphorane intermediate VIII
(Fig. 1b) and tetrachloroethene IV.
MeP(O)(OMe)2 + CCl3CHCl2
(Ia)
(II)
CCl3CCl2–H…O=P(OMe)2Me
(VI)
TS1
MePCl(OMe)2OH ⋅ CCl2=CCl2
(VII)
MePCl(OMe)2OH + CCl2=CCl2.
(VIII)
(IV)
The second step of the reaction may follow two
routes. According to one route, intermediate VIII rear-
ranges into complex IX via transition state TS2
(Fig. 1c). Complex IX decomposes to give reaction
products III and V.
VIII
TS2
MeP(O)(OMe)OH ⋅ CH3Cl
(IX)
→ III + V.
The other route involves decomposition of the phos-
phorane intermediate VIII to complex X, protonated at
the phosphoryl oxygen, and the chloride anion XII.
This is followed by the nucleophilic substitution of the
chloride anion for the phosphonate group via transition
state TS3. Since the heats of formation of TS2 and TS3
proved to be similar (–162.29 and –160.91 kcal/mol for
TS2(gg) and TS2(g – g) and –163.41 and –162.53 kcal/mol
for TS3(–gg) and TS3(–gt), respectively), both routes are
equally probable if only decomposition of phosphorane
VIII does not face obstacles.
Kazan State Technological University,
ul. Karla Marksa 68, Kazan,
420015 Tatarstan, Russia
Arbuzov Institute of Organic and Physical Chemistry,
Kazan Research Center, Russian Academy of Sciences,
ul. akademika Arbuzova 8, Kazan,
420083 Tatarstan, Russia
0012-5008/05/0004-0066 © 2005 Pleiades Publishing, Inc.

THEORETICAL AND EXPERIMENTAL STUDY
67
(‡)
C
(b)
(c)
Cl
Cl
Cl
Cl
H
Cl
H
H
H
C
C
H
C
H
H
H
H
H
O
H
O
C
O
Cl
P
O
O
Cl
H
H
C
H
H
H
P
P
H
C
O
H
H
H
C
O
H
H
H
O
C
C
O
H
H
H
C
H
H
H
H
Fig. 1. Gauche–gauche conformers of (a) TS1, (b) phosphorane, and (c) TS2.
MeP+(OH)(OMe)2 + Cl–
(X) (XI)
and PO bonds are located close to each other, and lH…OP
equals 1.78 Å.
VIII
The DFT and PM3 calculations of the thermody-
namic balance indicate that the reaction absorbs heat
(about 1 kcal/mol). The relative reaction energies for
the separate steps are 14 and –13 kcal/mol (DFT) and
−7 and 8 kcal/mol (PM3). Apparently, the semiempiri-
cal PM3 method overestimates the energy of the inter-
mediate, which accounts for these results.
TS3
MeP(O)(OMe)OH + CH3Cl.
(III) (V)
The total energies of compounds and complexes I, II,
III, IV, V, VI, VIII, IX, and X calculated at the
DFT/PBE/TZ2P level are equal to –686.429957,
−2376.911837,
–647.173509,
–1916.259650,
–1147.059169,
−499.901650,
–3063.350141,
−1147.086915, and –686.776758 au, respectively.
Our experiments fully confirmed the conclusion that
O,O-dimethyl methylphosphonate Ia exhibits dechlori-
nating reactivity with respect to pentachloroethane II,
The spatial structures of the gg and gt conformers of
intermediate VIII were optimized by the
DFT/PBE/TZ2P methods. In phosphorane intermediate based on quantum-chemical calculations. Indeed, this
VIII, four atoms (phosphorus, the methyl carbon atom,
and the hydroxyl and methoxyl oxygen atoms) lie in the
equatorial plane, while chlorine and the second meth-
oxy group lie above and below this plane. The length of
the phosphorane P–Cl bond is about 2.4 Å and the
ël…C distance between chlorine and the carbon atom
being attacked is about 3.10 Å, while this distance in
TS2 is shortened to 2.20 Å.
process is endothermic; hence, it takes place on long-
term heating of a mixture of these compounds at
150−170°ë. Tetrachloroethylene IV and acid ester III
were obtained in good yields. Other P(IV) esters,
namely, trimethyl phosphate Ib, O-methyl diethylphos-
phinate Ic, and O,O-diethyl methylphosphonate Id,
were also found to exhibit dehydrochlorinating ability
and were also converted into the corresponding acids or
acid esters.
We studied the internal reaction coordinates, which
relate the transition state to the reactants and the prod-
ucts. It was found that TS1 and TS2 are shifted toward
the intermediate. The transition state TS1 is essentially
asymmetric, the C–H bond being almost cleaved
(1.63 Å) and the C–Cl bond being substantially loos-
ened (2.09 Å). The transition state TS3 is also shifted
toward intermediate VIII along the reaction coordinate;
1
According to H NMR data, for completion of the
reaction, a 1 : 1 mixture should be heated for 10 h at
150°ë (phosphinate Ic) or for 20 h at 160 or 170°ë
(phosphonate Ia and phosphate Ib, respectively). The
successive replacement of the electron-withdrawing
MeO groups by alkyl groups in compounds I, i.e., the
the C–O and C–Cl bond lengths are 1.672 and 2.294 Å, transition Ib
Ia
Ic, is known to result in a
and the carbon hybridization is close to sp2, which is
decrease in the first ionization potentials, which are equal
to 10.81, 10.53, and 10.01 (10.20, R1 = R2 = Me) eV,
respectively [3]. The parallel increase in reactivity in
the same series attests to the electron-donating behavior
of P(IV) esters in the reaction under study. The nucleo-
philic attack of the phosphoryl oxygen on the hydrogen
atom of pentachloroethane yields prereaction complex VI,
usual for SN2 processes at a tetrahedral carbon atom.
The most complex structure was found for transition
state TS2 for the intramolecular nucleophilic substitu-
tion; the C–O and C–Cl bond lengths in TS2 are 1.96
and 2.20 Å and the Cl–O and Cl–P distances are 2.54
and 2.59 Å, respectively. The attacking atom and the
leaving group form an acute angle equal to 72°, the OH which is described above by quantum-chemical data.
DOKLADY CHEMISTRY Vol. 401 Part 2 2005

68
GAZIZOV et al.
Thus, the dehydrochlorinating reactivity of P(IV)
For ë2ç7é4ê anal. calcd. (%): P, 24.57.
esters predicted by modern quantum-chemical tech-
niques was confirmed experimentally.
Found (%): P, 24.23.
1H NMR (CDCl3, δ, ppm): 3.51 (d, 6H, MeO, 3JPH
=
12.5 Hz), 12.1 (s, 1H, POH); δê 1 ppm.
Tetrachloroethylene (1.40 g, 69%) was obtained,
bp 122–123°ë, n2D0 1.5025.
EXPERIMENTAL
1H NMR spectra were recorded on a Tesla BS-567A
spectrometer operating at 100 MHz. The proton chem-
ical shifts are referred to TMS. 31P NMR spectra were
measured on an RYa-2303 instrument at 21 MHz using
85% ç3êé4 as the external standard.
c. A mixture of O-methyl diethylphosphinate
(25 mmol) and compound II (25 mmol) was heated at
150°ë for 10 h. This gave diethylphosphinic acid (1.89 g,
62%), bp 115–116°ë (0.05 mm Hg), n2D0 1.4535.
The reaction between compounds I and II. A 1 : 1
mixture of compounds I and II was heated in a sealed
tube until the signal for the methine proton (δ 6.5 ppm)
For ë4ç11é2ê anal. calcd. (%): P, 25.37.
Found (%): P, 25.44.
1
disappeared from the H NMR spectrum. The volatile
products were recondensed in a high vacuum into a liq-
uid nitrogen trap. Distillation of the residue and the
condensate from the trap gave products V and III.
1H NMR (CDCl3, δ, ppm): 1.13 (dt, 6H, ëç2ëç3,
3
2JPH = 20 Hz, JHH = 7.2 Hz), 1.66 (dq, 4H, ëç2ëç3,
3
2JPH = 14 Hz, JHH = 7.2 Hz), 12.1 (s, 1H, POH);
a. A mixture of O,O-dimethyl methylphosphonate
(39 mmol) and compound II (39 mmol) was heated at
160°ë for 20 h. This gave methyl hydrogen meth-
ylphosphonate III (2.19 g, 50%), bp 98–99°ë
δê 55 ppm.
Product V was obtained in a yield of 3.09 g (74.5%),
bp 122–123°ë, n2D0 1.5025.
(0.03 mm Hg), n2D0 1.4223.
For ë2ç7é3ê anal. calcd. (%): P, 28.15.
ACKNOWLEDGMENTS
Found (%): P, 28.01.
This work was supported by the Russian Foundation
for Basic Research, project no. 03–03–32547.
1H NMR (CDCl3, δ, ppm): 1.27 (d, 3H, Me, 2JPH
=
18 Hz), 3.63 (d, 3H, POMe, 3JPH = 12 Hz), 12.1 (s, 1H,
POH).
Tetrachloroethylene (4.73 g, 72%) was obtained,
bp 122–123°ë, n2D0 1.5020.
REFERENCES
1. Perdew, J.P., Burke, K., and Ernzerhof, M., Phys. Rev.
Lett., 1996, vol. 77, no. 15, pp. 3865–3868.
b. A mixture of trimethyl phosphate (12 mmol) and
compound II (12 mmol) was heated at 170°ë for 20 h.
This gave dimethyl hydrogen phosphate (0.81 g, 53%),
bp 128–129°ë (0.4 mm Hg), n2D0 1.4065.
2. Laikov, D.N., Chem. Phys. Lett., 1997, vol. 281, no. 1,
pp. 151–153.
3. Zverev, V.V., Zh. Org. Khim., 1984, vol. 54, no. 6,
pp. 1265–1269.
DOKLADY CHEMISTRY Vol. 401 Part 2 2005