Manganese(IV) oxamato-catalyzed oxidation of secondary alcohols to ketones by dioxygen and pivalaldehyde

Article abstract of DOI:10.1039/a800930i

A new manganese(IV) oxamato complex possessing a bis(μ-oxo)dimanganese core has been synthesized, magnetically and structurally characterized, and found to catalyze the aerobic oxidation of secondary alcohols to ketones with co-oxidation of pivalaldehyde to pivalic acid with good yields and high selectivities.

Full text of DOI:10.1039/a800930i

View Article Online / Journal Homepage / Table of Contents for this issue
Manganese(iv) oxamato-catalyzed oxidation of secondary alcohols to ketones
by dioxygen and pivalaldehyde
Rafael Ruiz,a Ally Aukauloo,a Yves Journaux,*a† Isabel Ferna´ndez,b Jose´ R. Pedro,*b Antonio L. Rosello´,b
Beatriz Cervera,c Isabel Castroc and M. Carmen Mun˜ozd
a Laboratoire de Chimie Inorganique, URA 420, CNRS, Universite´ de Paris-Sud, 91405 Orsay, France
b Departament de Qu´ımica Orga`nica, Facultat de Qu´ımica, Universitat de Vale`ncia, 46100 Burjassot, Vale`ncia, Spain
c Departament de Qu´ımica Inorga`nica, Facultat de Qu´ımica, Universitat de Vale`ncia, 46100 Burjassot, Vale`ncia, Spain
d Departamento de F´ısica Aplicada, Universidad Polite´cnica de Vale`ncia, 46071 Vale`ncia, Spain
A new manganese(IV) oxamato complex possessing a bis(m-
oxo)dimanganese core has been synthesized, magnetically
and structurally characterized, and found to catalyze the
aerobic oxidation of secondary alcohols to ketones with co-
oxidation of pivalaldehyde to pivalic acid with good yields
and high selectivities.
between the amide nitrogen donor set. The phenylene linker
forces a twist of the N(1)–C(5) and N(2)–C(10) bonds by
125.1(5) and 127.0(5)°, respectively, resulting in a dihedral
angle between the mean planes of the oxamato groups of the
opba ligand of 71.7(2)°. This distortion is also reflected in the
Mn–N(amide) bond distances [1.986(5) and 2.006(5) Å] which
are slightly longer than the Mn–O(carboxylate) ones [1.970(4)
and 1.965(4) Å]. This unprecedent situation contrasts with that
found for the related mononuclear manganese(iii) complex with
the 4,5-dichloro-opba derivative where the Mn–N(amide) and
Mn–O(carboxylate) bond distances average 1.95 and 1.98 Å,
respectively.6 Within this complex the oxamato ligand adopts
an almost planar configuration, and the coordination scheme is
the familiar tetradentate N2O2 one with the ligand occupying the
equatorial plane about the manganese atom (trans isomer).
We have investigated the capability of this novel dinuclear
manganese(iv) complex towards oxidative catalytic trans-
formations of various organic substrates by the combined use of
dioxygen and an aldehyde as oxidant.7 The results obtained for
some representative primary and secondary alcohols, both
aromatic and aliphatics, are detailed in Table 1. Complex 1
selectively catalyzes the oxidation of 1-phenylethanol to the
corresponding ketone, acetophenone, by dioxygen plus piva-
laldehyde in dichloromethane solution with good yields, i.e.
Interest in high valent manganese coordination chemistry stems
largely from the fundamental role that manganese ion in
relatively high oxidation states plays in biological dioxygen
activation.1 Manganese interacts with dioxygen and its reduced
derivatives in a variety of enzymes from mononuclear (man-
ganese superoxide dismutase) and dinuclear (manganese cata-
lase or manganese ribonucleotide reductase) to tetranuclear
(water-oxidizing complex in photosystem II), making use of the
MnII, MnIII, MnIV and, probably, MnV oxidation states. Once it
was recognized that the active site in catalase and the water-
oxidizing complex contains oxo-bridged dimanganese cores,
considerable effort was devoted to obtaining dimeric man-
ganese complexes which could be structural as well as
functional models for this class of metalloproteins.2 During the
last two decades several bis(m-oxo)dimanganese complexes
with different ligand types have been reported, but compar-
atively scarce are the studies concerning their reactivities and,
particularly, their use as oxidation catalysts.3,4 In fact, the
manganese complex in the water-oxidizing center also exhibits
oxidation chemistry in its lower oxidation states.5 Here we
report the synthesis and physical characterization,‡ and the
crystal and molecular structure§ of the manganese complex
C(7)
C(8)
C(9)
C(6)
C(5)
C(10)
O(6)
O(3)
[PPh4]4[Mn(opba)(O)]2·4H2O
1,
where
opba
=
C(3)
C(4)
o-phenylenebis(oxamato) ligand, as well as a preliminary
investigation of its catalytic oxidation properties with dioxy-
gen.
C(1)
C(2)
N(2)
O(1)
N(1)
Mn
I
O(5 )
I
Mn( )
O(7)
O(4)
The structure of 1 consists of centrosymmetric bis(m-
oxo)dimanganese(iv) complex anions, [Mn2(opba)2(O)2]42
(Fig. 1), tetraphenylphosphonium cations and crystallization
water molecules. The two crystallographically equivalent
manganese atoms adopt a distorted octahedral geometry formed
by two amide nitrogen and two carboxylate oxygen atoms from
two symmetric related opba ligands (I = 2x, 2y, 2z), each
donating one of its NO donor set from oxamato to each of the
manganese atom at cis sites; the coordination sphere of the
manganese atoms being completed by two oxygen atoms from
the two cis-bridging oxo groups. The short Mn–O(1) and Mn–
O(1I) bonds of 1.799(4) and 1.797(4) Å, respectively, and the
acute O(1)–Mn–O(1I) angle of 85.9(2)°, are entirely consistent
with a di(m-oxo)-bridged manganese(iv) complex.2b Within the
planar Mn2O2 rhomb, the Mn···MnI and O···OI distances are
equal to 2.631(2) and 2.453(3) Å, respectively. The most
interesting structural feature of 1 is, however, the bis-bidentate
dinucleating coordination mode of the opba ligand. In fact, the
oxamato groups of each opba ligand clamp the two manganese
atoms, which are in turn tethered by the phenylene backbone
O(2)
O(5)
I
O(1 )
I
N(1 )
Fig. 1 Perspective view of the anionic dinuclear unit of 1 with the atom-
numbering scheme (thermal ellipsoids are at the 30% probability level and
hydrogen atoms have been omitted for clarity). Selected bond distances (Å)
and angles (°) with standard deviations in parentheses: Mn–O(1) 1.799(4),
Mn–O(1I) 1.797(4), Mn–O(2) 1.970(4), Mn–O(5I) 1.965(4), Mn–N(1)
1.986(5), Mn–N(2I) 2.006(5), Mn–MnI 2.631(2); O(1)–Mn–O(1I) 85.9(2),
O(1)–Mn–O(2) 173.0(2), O(1)–Mn–O(5I) 93.6(2), O(1)–Mn–N(1) 91.1(2),
O(1)–Mn–N(2I) 94.9(2), O(1I)–Mn–O(2) 93.8(2), O(1I)–Mn–O(5I)
172.8(2), O(1I)–Mn–N(1) 94.5(2), O(1I)–Mn–N(2I) 91.1(2), O(2)–Mn–
O(5I) 87.6(2), O(2)–Mn–N(1) 81.9(2), O(2)–Mn–N(2I) 92.2(2), O(5I)–Mn–
N(1) 92.7(2), O(5I)–Mn–N(2I) 81.8(2), N(1)–Mn–N(2I) 172.1(2), Mn–
O(1)–MnI 94.1(2) (symmetry code: I = 2x, 2y, 2z).
Chem. Commun., 1998
989

View Article Online
Table 1 Results for the oxidation of alcohols by dioxygen and piv-
alaldehyde catalyzed by 1a
la Generalitat Valenciana (Spain) for grants. We would also like
to express our gratitude to Professor J. J. Girerd and Dr G.
Blondin for fruitful discussions and continuous interest in this
work.
Entry
Alcohol
t/h
Yield (%)b,c
1
2
3
4
5
6
7
1-Phenylethanol
24
24
24
24
24
48
12
70
75
68
65
60
50
95d
Notes and References
1-(p-Methoxyphenyl)ethanol
1-(p-Bromophenyl)ethanol
1-(p-Trifluoromethylphenyl)ethanol
1-(p-Nitrophenyl)ethanol
4-(tert-Butyl)cyclohexanol
p-Methoxybenzyl alcohol
† E-mail: jour@icmo.u-psud.fr
‡ Synthesis and selected data for 1: the diethyl ester derivative of the opba
ligand (1.54 g, 5 mmol) was dissolved in deoxygenated MeOH (100 cm3),
NMe4OH at 25% in MeOH (8 cm3, 20 mmol) was added to the solution and
the resulting mixture was stirred at 60 °C for 15 min under N2. A
deoxygenated MeOH solution (50 cm3) of Mn(ClO4)2·6H2O (1.79 g, 5
mmol) was then added dropwise via a dropping funnel under N2, and a
gelatinous light yellow precipitate [presumably a MnII complex] rapidly
formed, together with the crystalline white precipitate of NMe4ClO4.
Addition of 33% aq. H2O2 (1 cm3, 10 mmol) caused immediate darkening
of the solution with concomitant disappearance of the yellow precipitate.
The reaction mixture was further stirred at 60 °C for 30 min under N2. The
dark-brown solution was filtered to eliminate the solid NMe4ClO4, and
reduced to a final volume of 10 cm3 on a rotatory evaporator. The
concentrated solution was treated successively with diethyl ether and
acetone to give a black solid which was recuperated in warm water (50 cm3).
The resulting mixture was filtered to eliminate solid particles (mainly
MnO2), and an excess of PPh4Cl (3.75 g, 10 mmol) dissolved in the
minimum amount of water was then added dropwise to the dark-brown
solution under gentle warming. Slow evaporation of the filtered solution in
air afforded, after a few days, well shaped large prismatic dark-brown
crystals of 1 suitable for X-ray analysis which were filtered on paper and air-
dried (60%). Satisfactory chemical analyses obtained (C, H, N, P, Mn).
nmax/cm21 (KBr) 3477vs (O–H) from H2O, 1672 (sh), 1647vs, 1617vs
(CNO) and 1403s, 1306s (C–O) from opba ligand, and 643m (Mn–O) from
Mn2O2 ring. lmax/nm 390 (e/dm3 mol21 cm21 10800), 440 (sh) (7080) and
605 (1170) (MeCN). Variable-temperature magnetic susceptibility (Fara-
a Reactions were carried out at room temp. by adding a CH2Cl2 solution (0.2
cm3) of alcohol (0.11 mmol) to a stirred mixture of metal catalyst (6.5 3
1023 mmol) and pivalaldehyde (0.33 mmol) in CH2Cl2 (0.2 cm3) under O2
atmosphere. Consumption of alcohol and formation of ketone during the
reaction were monitored by TLC. Obtained ketone and unreacted alcohol
were separated by flash column chromatography on silica gel. b Yields refer
to isolated and pure compounds (column chromatography on silica gel). All
compounds exhibited spectral data consistent with their structures. c In the
absence of metal catalyst some extension of oxidation was observed.
d Reaction product was exclusively p-methoxybenzoic acid.
70% after 24 h (entry 1), with formation of pivalic acid as a
coproduct. Moreover, for the series of para-substituted phenyl
derivatives a small but non-negligible electronic effect is
observed, as the substrate with the electron-donating methoxy
substituent gives a somewhat higher yield of ketone than that
with electron-withdrawing substituents such as trifluoromethyl
or nitro groups, e.g. 75 vs. 60% after 24 h (entries 2 and 5,
respectively). For all secondary benzyl alcohols, however,
1
ketones were the only oxidation products as confirmed by H
day balance, 80–300 K): J = 2158.0 cm21 (H = 2J S1·S2, S1 = S2
3/2).
=
NMR spectroscopy. Notably, for the oxidation of 4-(tert-
butyl)cyclohexanol only 4-(tert-butyl)cyclohexanone was ob-
tained, and no traces were detected of the corresponding
Baeyer–Villiger oxidation product, 4-(tert-butyl)caprolactone
(entry 6).8 This observation suggests that the acylperoxy
radicals generated in situ from the auto-oxidation of the
aldehyde are not directly involved as potential oxidizing agents.
As expected, under the same reaction conditions used for the
oxidation of secondary alcohols to ketones, the primary
alcohols give mixtures of both the aldehyde and the acid
oxidation products in variable amounts depending on the
reaction time, as exemplified by p-methoxybenzyl alcohol
which leads to almost quantitative formation of
p-methoxybenzoic acid, i.e. 95% after 12 h (entry 7).
§ X-Ray crystal structure analysis: Enraf-Nonius CAD-4 diffractometer,
Mo-Ka, l = 0.71073 Å, graphite monochromator, 293 K. Lorentz and
polarization effects but not absorption correction (m = 3.78 cm21). Data
collection, solution and refinement: w–q, standard Patterson methods with
subsequent full-matrix least-squares refinement. SHELX86, SHELX93.10
¯
C116H96Mn2N4O18P4, triclinic, space group P1,
a
=
13.245(3),
77.98(2),
b
=
13.964(3), c
=
14.835(3) Å, a
=
73.59(2), b
=
g = 84.60(2)°, U = 2572.6(10) Å3, Z = 2, Dc = 1.33 g cm23, 1 @ q @ 25°,
crystal 0.15 3 0.15 3 0.10 mm. 6681 reflections measured, 4485 assumed
as observed with I ! 2s(I). Refinement on F2 of 651 variables with
anisotropic thermal parameters for all non-H atoms gave R = 0.060 and
Rw = 0.150 with S = 0.936 (obs. data). CCDC 182/809.
1 V. L. Pecoraro, M. J. Baldwin and A. Gelasco, Chem. Rev., 1994, 94,
807.
2 (a) K. Wieghardt, Angew. Chem., Int. Ed. Engl., 1989, 28, 1153; (b)
J. E. McGrady and R. Stranger, J. Am. Chem. Soc., 1997, 119, 8512 and
references therein.
3 R. Hage, Recl. Trav. Chim. Pays-Bas, 1996, 115, 385 and references
therein; W. Ruttinger and G. C. Dismukes, Chem. Rev., 1997, 97, 1.
4 R. Hage, J. E. Iburg, J. Kerschner, J. H. Koek, E. L. M. Lempers,
R. J. Martens, U. S. Racherla, S. W. Russell, T. Swarthoff, M. R. P. Van
Vliet, J. B. Warnaar, L. Van der Wolf and B. Krijnen, Nature, 1994, 369,
637; C. Zondervan, R. Hage and B. L. Feringa, Chem. Commun., 1997,
419.
5 W. F. Beck, J. Sears, G. W. Brudvig, R. J. Kulawiec and R. H. Crabtree,
Tetrahedron, 1989, 45, 4903.
6 M. Fettouhi, L. Ouahab, A. Boukhari, O. Cador, C. Mathoniere and
O. Kahn, Inorg. Chem., 1996, 35, 4932.
7 R. Ruiz, M. Triannidis, A. Aukauloo, Y. Journaux, I. Ferna´ndez,
J. R. Pedro, B. Cervera, I. Castro and M. C. Mun˜oz, Chem. Commun.,
1997, 2283.
8 K. Kaneda, S. Ueno, T. Imanaka, E. Shimotsuma, Y. Nishiyama and
Y. Ishii, J. Org. Chem., 1994, 59, 2915.
Although it is premature to discuss the precise role of the
metal complex in the catalytic mechanism at the present stage,
it is noteworthy that 1 alone does not lead to alcohol oxidation
under stoichiometric conditions and, consequently, involve-
ment of a bis(m-oxo)dimanganese(iv) species as the active
oxidizing agent can also be ruled out. In a typical experiment,
complex 1 (0.11 mmol) in dichloromethane (20 cm3) does
not
react
with
the
more
reactive
substrate
1-(p-methoxyphenyl)ethanol (0.11 mmol) even after a period of
three days under stirring at room temperature with or without
oxygen atmosphere conditions. That being so, manganese(iv)–
acylperoxo or higher valent metal intermediate species, such as
manganese(v)–oxo, derived from the oxidation of the bis(m-
oxo)manganese(iv) dimer by the combination of dioxygen and
pivalaldehyde, are considered more likely to be responsible for
the oxidation in our system. More interestingly, stable man-
ganese(v)–oxo monomeric complexes with amido-containing
ligands analogous to that used herein have been isolated and
structurally characterized.9 Attempts to isolate these reactive
intermediate species using transition metal ions with more
accessible high-valent oxidation states such as chromium are in
progress.
9 T. J. Collins, R. D. Powell, C. Slebodnick and E. S. Uffelman, J. Am.
Chem. Soc., 1990, 112, 899; F. M. McDonnell, N. L. P. Fackler, C. Stern
and T. V. O’Halloran, J. Am. Chem. Soc., 1994, 116, 7431.
10 G. M. Sheldrick, SHELX. A Program for Crystal Structure Determina-
tion, University of Go¨ttingen, Germany, 1990; G. M. Sheldrick,
SHELXL93. Program for the Refinement of Crystal Structures,
University of Go¨ttingen, Germany, 1993.
This work was supported by the DGICYT, Ministerio de
Educacio´n y Ciencia (Spain) through projects PB94-0985 and
PB94-1002. R. R. and B. C. thank the Ministerio de Educacio´n
y Ciencia (Spain) and the Conselleria de Educacio´ i Cie`ncia de
Received in Basel, Switzerland, 3rd February 1998; 8/00930I
990
Chem. Commun., 1998