Selective catalytic oxidation of benzyl alcohols to benzaldehydes with a dinuclear manganese(IV) complex

Article abstract of DOI:10.1039/a607779j

A dinuclear manganese(IV) catalyst is capable of oxidising benzylic alcohols to benzaldehydes with very high turnovers numbers (up to 1000) and high selectivities (above 99%).

Full text of DOI:10.1039/a607779j

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Selective catalytic oxidation of benzyl alcohols to benzaldehydes with a
dinuclear manganese(iv) complex
a
b
a
Charon Zondervan, Ronald Hage and Ben L. Feringa* †
a
Department of Organic and Molecular Inorganic Chemistry, Groningen Centre for Catalysis and Synthesis, University of
Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands
b
Unilever Research Laboratory Vlaardingen, PO Box 114, 3130 AC Vlaardingen, The Netherlands
A dinuclear manganese(IV) catalyst is capable of oxidising
benzylic alcohols to benzaldehydes with very high turnovers
numbers (up to 1000) and high selectivities (above 99%).
in attempting to increase the turnover number by using a very
large excess hydrogen peroxide we only observed formation of
benzoic acid (entry 2).‡ This could be overcome by adding 2000
equiv. of oxidant over a 10 h period, which cleanly gave 380
III
A key reaction in synthetic organic chemistry is the trans-
formation of alcohols to aldehydes. Many procedures have been
developed over the years, most notably the Oppenhauer and
Swern oxidations and reactions utilising high valent metal
turnovers to benzaldehyde (entry 3). The mononuclear Mn
complex 2 gives a somewhat higher conversion with benzyl
alcohol, but the bis(m-acetato)(m-oxo) complex 3 was virtually
unreactive (entries 4 and 5). Complexes such as 3 are most
probably dead ends in the catalytic cycle when, starting with 1
or 2, traces of benzoic acid are formed during the oxidation
reaction. Indeed, addition of a few equivalents of benzoic acid
with respect to 1 completely inhibits any turnover on benzylic
alcohols when the reaction is carried out under nitrogen
atmosphere (entry 6). Exposure to air eventually leads to full
conversion to benzoic acid. For practical synthetic applications
further improvements with complete conversion to benzalde-
hydes were highly desirable.
2
compounds such as MnO , ammonium perruthenates and
pyridinium chlorochromate.1 However, almost all require
stoichiometric amounts of metal compounds and sometimes co-
oxidants, with concomitant environmental problems. Only a
few examples are known in which metal complexes are used in
a catalytic fashion. Benzaldehydes are obtained from benzyl
2
alcohols with RuCl
2
(PPh
3
)
3
and ZrO(OAc)
2
using PhIO or
3
tert-butyl hydroperoxide as oxidants, and similar results were
obtained with SeO
–bis(4-anisyl) selenoxide.4 A valuable
alternative is the [NBu ][RuO ]–N-methylmorpholine oxide
system. These procedures typically require 1–10 mol% of
catalyst and 100–300 mol% oxidant. Herein we report a novel
method for the efficient and highly selective oxidation of
benzylic alcohols to benzaldehydes using an active dinuclear
manganese(iv) complex as catalyst and hydrogen peroxide or
tert-butyl hydroperoxide as oxidant.
2
4
4
5
Me
O
O
O
O
O
IV
LMn
IV
MnL
III
LMn
The excellent bleaching and epoxidation properties of 1 in
(PF6)2
(ClO4)2
water6a and acetone
6b,c
were recently reported. This complex
H2O
consists of two manganese(iv) ions held together by three oxo
Me
ligands, with both metals bound to an N,NA,NB-trimethyl-
7
1
,4,7-triazacyclononane ligand. The intriguing coordination
1
2
chemistry of this ligand and its analogues has been re-
viewed.8
Me
Me
Me
Initial results of the benzylic oxidation by using 0.6 mm 1, 5
N
N
O
O
O
III
MnL
III
LMn
2 2
mm H O and 0.6 mm benzyl alcohol in acetone in air were quite
O
(ClO4)2
L =
promising as highly selective aldehyde formation was observed,
although the catalytic activity was modest (280 turnovers; entry
, Table 1). Due to the efficient catalase activity of 1 under these
O
N
Me
Me
1
conditions, we anticipated that excess of oxidant is required, but
3
Table 1 Effects of changing various parameters on the reactivity of benzyl alcohol
Entry
Catalyst and oxidanta
Variation
Turnoverb
t/min Selectivityc
1
2
3
4
5
6
7
8
9
1 + H
1 + H
2
2
O
O
2
280
1000
380
660
16
0
0
480
360
0
100
60
600
100
60
—
—
150
150
—
> 99
> 99d
> 99
> 99
> 99
—
2
(80000 equiv.)
e
1
f
2 + H
2
O
2
g
3 + H
2
O
2
1 + H
1 + Bu O
1 + Bu O
2 2
1 + H O
2
O
2
BnOH (2 equiv.) added
t
2
H
H
—
t
2
Pretreatment with H
Pretreatment with H
No catalyst added
2
2
O
O
2
> 99
> 99
—
2
1
0
2 2
H O
a
In all cases 1000 equiv. alcohol and 8000 equiv. oxidant were used in acetone in air; the reaction was followed by GC and terminated when conversion
b
c
ceased to give benzaldehydes as product (the selectivity on aldehyde in all cases was above 99%). Turnover number in mol aldehyde per mol 1. Selectivity
d
e
in acetone were added over a 10 h period. f Ref. 9.
in mol aldehyde per mol converted substrate. Benzoic acid was the only product. 2000 equiv. H
O
2 2
g
Ref. 7b.
Chem. Commun., 1997
419

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Table 2 Reactivity of substituted benzyl alcohols using two different procedures
Method A
Turnoverb
Method B
Entry
Substratea
t/min Turnoverb
t/min
1
2
3
4
5
6
7
8
9
Benzyl alcohol
360
710
300
—
700
490
80
530
580
240
340
360
60
45
60
480
1000
1000
1000
1000
1000
—
1000
1000
1000
600
150
45
20
20
25
40
4-Methoxybenzyl alcohol
3,4-Dimethoxybenzyl alcohol
4-(Dimethylamino)benzyl alcohol
4-Chlorobenzyl alcohol
2-Chlorobenzyl alcohol
2,6-Dichlorobenzyl alcohol
4-Nitrobenzyl alcohol
3-Nitrobenzyl alcohol
2-Nitrobenzyl alcohol
4-Trifluoromethylbenzyl alcohol
Octan-2-ol
45
45
90
15
20
15
210
100
45
25
35
50
90
1
1
1
0
1
2
360
a
In all cases 1000 equiv. alcohol were used in acetone under inert atmosphere; the reaction was followed by GC and terminated when conversion ceased
to give benzaldehydes as product (the selectivity on aldehyde in all cases was above 99%). b Turnover number in mol aldehyde per mol 1.
To our surprise we found that the activity is significantly
increased by employing a modified procedure. First the catalyst
is mixed with 3000 equiv. of hydrogen peroxide in acetone for
whereas a blank reaction, i.e. without 1, gave no reaction at all.
-Nitrobenzaldehyde is not oxidised by 1 in air, or by 1 and H under
turnover conditions.
From the 16 line spectrum obtained from EPR measurements at 77 K we
3
2 2
O
§
3
0 min during which all hydrogen peroxide is decomposed
III
IV
inferred that 1 is reduced to a dinuclear Mn –Mn mixed valent species
(probably via catalase activity of 1).§ Addition of an aliquot of
instantaneously when mixed with excess hydrogen peroxide in acetone,
substrate and hydrogen peroxide (1000 and 8000 equiv.,
respectively) to the thus pretreated catalyst leads to quantitative
formation of benzoic acid, but under an inert atmosphere to
selective formation of benzaldehyde (360 turnovers, entry 9).
For substituted benzyl alcohols generally 400–700 turnovers to
the corresponding benzaldehydes are found with over 99%
selectivity with or without inert atmosphere conditions (Table 2,
method A).
Although compound 1 with tert-butyl hydroperoxide (TBHP)
is not active in the benzylic oxidation (Table 1, entry 7), the use
of TBHP as oxidant in the second step, i.e. after pretreating 1
with hydrogen peroxide for 30 min, was found to have a highly
beneficial influence on both turnover number and reaction rate
6a
although we do not know if this reaction goes to full completion. Within
2
0 min the spectrum gradually changes to ultimately give a 6 line spectrum,
indicative of MnII species.
¶ A general experimental procedure is as follows: 1.0 ml of a 0.6 mm stock
solution of crystallised 1 in acetone and 0.5 ml of hydrogen peroxide (30
wt%) are mixed for 30 min at room temperature under a nitrogen
atmosphere. Simultaneously 0.6 mmol substrate and 0.5 ml hydrogen
peroxide (30 wt%) (method A) or 0.6 mmol substrate and 0.5 ml of aqueous
tert-butyl hydroperoxide (70 wt%) (method B) are added and the reaction is
followed by GC until the maximum conversion is reached (usually within
1
h).
References
(Table 1, entry 8). Nearly all substituted benzylic alcohols were
fully converted to the corresponding benzaldehydes with
excellent selectivities within an hour at room temperature using
1
Organic Synthesis by Oxidation with Metal Compounds, ed. W. J. Mijs
and C. R. H. I. De Jonge, Plenum, New York, 1986; E. J. Corey and
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K. Oshima, S. Matsubara, K. Takai and H. Nozaki, Tetrahedron Lett.,
0
.1 mol% of catalyst 1 (Table 2, method B).¶
From the results with method A, we see a small but distinct
2
steric effect as ortho-substituted substrates react more slugg-
ishly than either meta- or para-substituted benzyl alcohols. In
fact 2,6-disubstituted substrates are largely unreactive (entry 7).
Substrates with electron-donating properties, e.g. (di)methoxy
and dimethylamine benzyl alcohol (entries 2, 3 and 4), also
appear to react faster under the conditions of method A than
those with electron-withdrawing substituents such as nitro and
trifluoromethyl groups (entries 8 and 11). The latter one even
failed to give full conversion under the conditions of method B.
As expected, secondary alcohols are selectively converted to
ketones although the outcome is indifferent towards the
procedure used. A typical example is given in entry 12.
1
983, 24, 2185.
3
4
K. Kaneda, Y. Kawanishi and S. Teranishi, Chem. Lett., 1984, 1481.
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833.
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6
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3
In conclusion, we have demonstrated that manganese com-
plex 1 is an excellent catalyst or catalyst precursor in novel
procedures employing either H
O or TBHP as oxidant for the
2 2
7
(a) K. Wieghardt, U. Bossek, B. Nuber, J. Weiss, J. Bonvoisin,
M. Corbella, S. E. Vitols and J. J. Girerd, J. Am. Chem. Soc., 1988, 110,
7
398; (b) J. H. Koek, S. W. Russell, L. Van der Wolf, R. Hage,
highly selective and very fast conversion of substituted benzyl
alcohols to benzaldehydes. Further studies to elucidate the role
of the pretreatment procedure and the exact nature of the
catalytic species are in progress.
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9
Footnotes
†
‡
E-mail: feringa@chem.rug.nl
A control experiment showed that 1 is capable of oxidising benzaldehyde
to benzoic acid in air. In 90 min, 125 turnovers were found in acetone,
Received, 18th November 1996; Com. 6/07779J
420
Chem. Commun., 1997