Diastereoselective synthesis of pinacols from aromatic aldehydes using the system pentacarbonyliron-hexamethylphosphoric triamide. Dual reactivity of pentafluorobenzaldehyde

Article abstract of DOI:10.1134/S1070428007040057

The system Fe(CO)₅-HMPA effectively promotes diastereoselective reductive dimerization of aromatic aldehydes to the corresponding pinacols having preferentially or exclusively dl configuration. In the reaction with pentafluorobenzaldehyde, the product structure strongly depends on the presence of traces of moisture in the reaction system. Nauka/Interperiodica 2007.

Full text of DOI:10.1134/S1070428007040057

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2007, Vol. 43, No. 4, pp. 518–521. © Pleiades Publishing, Ltd., 2007.
Original Russian Text © A.B. Terent’ev, T.T. Vasil’eva, O.V. Chakhovskaya, N.E. Mysova, K.A. Kochetkov, 2007, published in Zhurnal Organicheskoi Khimii,
2007, Vol. 43, No. 4, pp. 521–524.
Diastereoselective Synthesis of Pinacols from Aromatic Aldehydes
Using the System Pentacarbonyliron–Hexamethylphosphoric
Triamide. Dual Reactivity of Pentafluorobenzaldehyde
A. B. Terent’ev†, T. T. Vasil’eva, O. V. Chakhovskaya, N. E. Mysova, and K. A. Kochetkov
Nesmeyanov Institute of Organometallic Compounds, Russian Academy of Sciences,
ul. Vavilova 28, Moscow, 119991 Russia
e-mail: terent@ineos.ac.ru
Received July 21, 2006
Abstract—The system Fe(CO)5–HMPA effectively promotes diastereoselective reductive dimerization of
aromatic aldehydes to the corresponding pinacols having preferentially or exclusively dl configuration. In the
reaction with pentafluorobenzaldehyde, the product structure strongly depends on the presence of traces of
moisture in the reaction system.
DOI: 10.1134/S1070428007040057
In the preliminary communication [1] we showed
that the system Fe(CO)5–HMPA can be successfully
used to promote diastereoselective reductive dimeriza-
tion of aromatic aldehydes to give the corresponding
1,2-dihydrobenzoins (pinacols) on heating the reac-
tants for 3–4 h in boiling benzene. This procedure is
more advantageous than the known methods [2–5] in
several aspects which were considered in detail in [1].
Decafluoroethane-1,2-diol formed as a by-product in
the addition of bromoform and methyl bromopropi-
onate to pentafluorobenzaldehyde in the presence of
Fe(CO)5 [6] was a mixture of dl and meso diastereo-
isomers which were isolated and characterized. Only
the use of Fe(CO)5–HMPA ensured preparation of that
diol exclusively as dl diastereoisomer.
we have found that diols like I can be obtained with
a good yield and high diastereoselectivity in the system
Fe(CO)5–HMPA–aldehyde (Scheme 1) at room tem-
perature in 3–4 days; therefore, the use of chiral co-
catalysts in this reaction could give rise to enantiosel-
ective processes. Pentafluorobenzaldehyde (see table,
run no. 1) and 4-chlorobenzaldehyde (run no. 2) react-
ed most effectively to produce exclusively dl isomers
of diols like I, while in the reaction with benzaldehyde,
the dl-to-meso isomer ratio was 80:20 (run no. 3).
Scheme 1.
OH
O
Fe(CO)5, HMPA
Ar
Ar
Ar
H
OH
Further studies in this line revealed some interest-
ing specific features of the above reaction. Primarily,
Ar = C6F5, 4-ClC6H4, Ph, 4-MeOC6H4.
Yields and diastereoisomer ratios of diols I in the reductive
dimerization of aromatic aldehydes
The structure, purity, configuration, and crystal
lattice parameters of diol I (Fig. 1) were determined on
the basis of a combination of data obtained by GLC
Run no.
Ar
Yield of diol I,a % Ratio dl:mesob
1
analysis, H NMR spectroscopy, and X-ray analysis.
1
2
3
C6F5
92
91
50
51
>99:10
>99:10
>80:20
>80:20
As shown in Fig. 2, molecules of diol I in crystal are
linked to dimers via intermolecular hydrogen bonds.
The X-ray diffraction data for the dl isomer of I are
well consistent with those reported for D-1,2-diphenyl-
ethane-1,2-diol [7].
4-ClC6H4
Ph
4
4-MeOC6H4
a
Isolated and purified product.
According to the 1H NMR data.
b
The formation of pinacol-like structures may be
rationalized by a scheme including one-electron trans-
†
Deceased.
518

DIASTEREOSELECTIVE SYNTHESIS OF PINACOLS FROM AROMATIC ALDEHYDES
519
F3
F7
fer from Fe(0) to aldehyde to give radicals A in the
iron coordination sphere; their dimerization, followed
by hydrolysis, yields diol I (Scheme 2).
F4
C6
F8
C11
F2
C7
F6
C12
C13
Scheme 2.
C8
C3
C5
C10
C9
F5
O
OFe
Fe(CO)5, HMPA
C4
F1
F9
·
C6F5
H
C6F5
H
C14
F10
H1
A
C2
C1
OH
H2
C6F5
O2
H20
O1
H10
C6F5
OH
I
Fig. 1. Structure of the molecule of 1,2-bis(pentafluoro-
phenyl)ethane-1,2-diol (I) with atom numbering according to
the X-ray diffraction data .
The reactions performed in anhydrous solvents
(benzene and toluene) at room temperature resulted in
the same composition of the reaction mixtures, and the
yields of the corresponding diols were similar to those
obtained using solvents without preliminary dehydra-
tion. Unexpected results were obtained in the dimeriza-
tion of pentafluorobenzaldehyde in anhydrous benzene
and toluene at 80°C. The reaction mixture contained
decafluorostilbene II as the major product, 1,2-bis-
(pentafluorophenyl)ethanone (III), and 2,3-bis(penta-
fluorophenyl)oxirane (IV), while no diol I was detect-
ed (Scheme 3). When the reaction was carried out in
a dilute solution (the substrate concentration was lower
by a factor of 3 to 5), stilbene II was the only product.
a
0
b
The reaction with benzaldehyde in anhydrous sol-
vent gave only the corresponding diol, i.e., the pres-
ence of moisture did not affect the reaction direction.
In order to elucidate the role of water, we carried out
the reaction with pentafluorobenzaldehyde in anhy-
drous benzene in the presence of an equimolar amount
of water. In this case, the product was diol I, whereas
stilbene II was almost absent. The formation of stil-
benes from aldehydes in the presence of metal com-
plexes was reported previously [8, 9], but no details of
the reaction mechanism were given.
c
Fig. 2. Packing of 1,2-bis(pentafluorophenyl)ethane-1,2-diol
(I) molecules in crystal (projection along the a axis).
The fact that stilbene II is formed as the only prod-
uct in the reaction with pentafluorobenzaldehyde under
3–5-fold dilution indicates inner-sphere formation of
stilbene II (path b) and intermolecular mechanism of
formation of adducts III and IV (path c). The observed
considerable difference in the results of the reactions
with pentafluorobenzaldehyde and benzaldehyde in
anhydrous solvents is likely to reflect effect of polar
substituents in the benzene ring of the substrate. Un-
like the phenyl group in unsubstituted benzaldehyde,
electron-withdrawing pentafluorophenyl group favor
stabilization of complex B. Thus pentafluorobenzalde-
We presumed that the reaction performed by heat-
ing in anhydrous solvents includes the following trans-
formations (Scheme 3): formation of metal carbenoid
B, followed by inner-sphere dimerization to stilbene II
(path b); and reaction of carbenoid complex B with
aldehyde via insertion into the C–H or C=O bond to
give ketone III or oxirane IV, respectively (path c).
Presumably, the presence of moisture hampers forma-
tion of metal carbenoid B, so that the major product is
diol I (path a).
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 43 No. 4 2007

TERENT’EV et al.
520
Scheme 3.
OH
O
80°C, aq. PhH
C6F5
+
L(CO)nFe
C6F5
a
C6F5
H
OH
I
PhH, 80°C H2O
C6F5CH=CHC6F5
b
II
[L(CO)nFe=CHC6F5]x
O
O
C6F5CHO
c
B
C6F5
+
C6F5
C6F5
C6F5
III
IV
L = (Me2N)3PO.
determination and refinement. The structure was
solved by the direct method; non-hydrogen atoms were
localized by the difference Fourier syntheses and were
refined in anisotropic approximation. The positions of
hydrogen atoms H(C) were calculated (hydroxy hydro-
gen atoms were localized by the difference syntheses)
and refined in isotropic approximation using the riding
model. The final divergence factors were R1 = 0.0448
[calculated by Fhkl from 2806 reflections with I > 2σ(I)]
hyde exhibits dual reactivity in the reductive dimeriza-
tion, depending on the presence of even traces of water
in the reaction system.
EXPERIMENTAL
The mass spectra were recorded on a Finnigan
MAT Magnum GC–MS system (25-m Ultra-2 column,
oven temperature programming from 30 to 220°C at
a rate of 2.5 deg/min). GLC analysis was performed on
an LKhM-80 chromatograph equipped with a thermal
conductivity detector and a 1300×3-mm steel column
packed with 15% of SKTFT-50Kh on Chromaton-
N-AW; carrier gas helium, flow rate 60 ml/min; oven
temperature programming from 50 to 250°C at a rate
2
and wR2 = 0.1350 (calculated by Fhkl from all 3460
independent reflections); GOF 1.004; 251 independent
parameters. The coordinates of atoms and their tem-
perature factors were deposited to the Cambridge Crys-
tallographic Data Center (entry no. CCDC 637471).
General procedure for reductive dimerization of
aromatic aldehydes. A solution of 1 mmol of the cor-
responding aldehyde, 2 mmol of Fe(CO)5, and 4 mmol
of HMPA in 1 ml of benzene was heated for 4 h at
80°C and was then left to stand for 3 days at room
temperature. The mixture was diluted with benzene,
washed with dilute hydrochloric acid and water, and
dried. The solvent was distilled off, and the residue
was analyzed by GC–MS and GLC using authentic
samples as references; crystalline products were pur-
ified by recrystallization or by column chromatography
on silica gel using hexane–ethyl acetate (4:1) as eluent.
1
of 6 deg/min. The H and 13C NMR spectra were
measured on a Bruker WP-200 instrument (200 MHz
1
for H) relative to TMS; CDCl3 was used as solvent.
X-Ray analysis was performed at the X-Ray Analysis
Laboratory (Nesmeyanov Institute of Organometallic
Compounds, Russian Academy of Sciences).
All organic reagents were purified by distillation;
commercial Fe(CO)5 with a purity of 97% (from
Fluka) was used without additional purification; the
solvents were dehydrated over CaCl2 and were then
distilled over metallic sodium.
X-Ray diffraction data for 1,2-bis(pentafluoro-
phenyl)ethane-1,2-diol (I). C14H4F10O2, M 394.17;
monoclinic crystals with the following unit cell
parameters (120 K): a = 8.950(1), b = 5.6865(9), c =
25.834(4) Å, β = 93.606(3)°; V = 1312.3(4) Å3; Z = 4;
dcalc = 1.995 g/cm3; space group P21/c. Total of 10706
reflections were measured on a Bruker SMART 1000
CCD diffractometer at 120 K. After averaging of
equivalent reflections, 3460 independent reflections
(Rint = 0.0184) were obtained and used in the structure
1,2-Bis(pentafluorophenyl)ethane-1,2-diol (I).
a. The reaction was carried out at 80°C (4 h) in 1 ml of
benzene (without preliminary dehydration) [1]. Yield
~90%, mp 185°C (from benzene). According to the
1H NMR data, only one isomer was formed and was
1
identified as dl by X-ray analysis. H NMR spectrum,
δ, ppm: 5.38 d and 5.40 d (1H each, CH, J = 6 Hz; the
signals are split due to nonequivalence of those protons
in that isomer), 2.50 d and 2.52 d (1H each, OH, J =
6 Hz). Found, %: C 42.94; H 1.20; F 46.81; OH 9.75.
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 43 No. 4 2007

DIASTEREOSELECTIVE SYNTHESIS OF PINACOLS FROM AROMATIC ALDEHYDES
521
C14H4F10O2. Calculated, %: C 42.6; H 1.0; F 48.2;
OH 8.6.
δC 62.4 ppm (CHO). Mass spectrum, m/z (Irel, %): 376
[M]+ (1), 358 [M – H2O]+ (100), 195 [C6F5CO]+ (15).
1,2-Diphenylethane-1,2-diol, 1,2-bis(4-chlorophen-
yl)ethane-1,2-diol, and 1,2-bis(4-methoxyphenyl)-
ethane-1,2-diol [1] were obtained in a similar way.
b. The reaction was carried out at 80°C (4 h) in
5 ml of anhydrous benzene. Yield 20%. No adducts III
and IV were present in the reaction mixture.
b. The reaction was carried out at 20°C (3 days)
using 1 ml of anhydrous or non-dehydrated benzene.
Yield of I ~90% (65% in anhydrous benzene),
de 100%.
This study was performed under financial support
by the Russian Foundation for Basic Research (project
no. 06-03-32820) and by the Federal Science and
Innovation Agency (state contract no. 02.435.11.3005).
The authors are sincerely grateful to Dr. Z.A. Starikova
for performing the X-ray analysis and interpreting the
results.
1,2-Diphenylethane-1,2-diol (yield 30%, de 80%)
and 1,2-bis(4-chlorophenyl)ethane-1,2-diol (yield 75%,
de 100%) were obtained in a similar way.
c. The reaction was carried out at 80°C (4 h) in 1 ml
of anhydrous benzene containing 1 mmol of water.
Yield of I 60%, de 100%.
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1
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