Selective catalytic oxidation of alcohols, aldehydes, alkanes and alkenes employing manganese catalysts and hydrogen peroxide

Article abstract of DOI:10.1002/adsc.201300275

The manganese-containing catalytic system [Mn^IV,IV ₂O₃(tmtacn)₂]²⁺ (1)/carboxylic acid (where tmtacn=N,N′,N′′-trimethyl-1,4,7-triazacyclononane), initially identified for the cis-dihydroxylation and epoxidation of alkenes, is applied for a wide range of oxidative transformations, including oxidation of alkanes, alcohols and aldehydes employing H₂O₂ as oxidant. The substrate classes examined include primary and secondary aliphatic and aromatic alcohols, aldehydes, and alkenes. The emphasis is not primarily on identifying optimum conditions for each individual substrate, but understanding the various factors that affect the reactivity of the Mn-tmtacn catalytic system and to explore which functional groups are oxidised preferentially. This catalytic system, of which the reactivity can be tuned by variation of the carboxylato ligands of the in situ formed [Mn^III,III ₂(O)(RCO₂)₂(tmtacn)₂]²⁺ dimers, employs H₂O₂ in a highly atom efficient manner. In addition, several substrates containing more than one oxidation sensitive group could be oxidised selectively, in certain cases even in the absence of protecting groups. Copyright

Full text of DOI:10.1002/adsc.201300275

FULL PAPERS
DOI: 10.1002/adsc.201300275
Selective Catalytic Oxidation of Alcohols, Aldehydes, Alkanes
and Alkenes Employing Manganese Catalysts and
Hydrogen Peroxide
Pattama Saisaha,a Lea Buettner,a Margarethe van der Meer,a Ronald Hage,b
Ben L. Feringa,a Wesley R. Browne,a,* and Johannes W. de Boerb,*
a
Stratingh Institute for Chemistry, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands
Fax : (+31)-50-363-4290; phone: (+31)-50-363-4428; e-mail: w.r.browne@rug.nl
Catexel BV, BioPartner Centre Leiden, Galileiweg 8, 2333 BD Leiden, The Netherlands
b
Fax : (+31)-71-332-2222; phone: (+31)-71-332-2221; e-mail: hans.de.boer@catexel.com
Received: April 3, 2013; Revised: July 15, 2013; Published online: September 9, 2013
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201300275.
Abstract: The manganese-containing catalytic system lytic system and to explore which functional groups
3A
tmtacn=N,N’,N’’-trimethyl-1,4,7-triazacyclononane),
initially identified for the cis-dihydroxylation and ep- carboxylato ligands of the in situ formed
oxidation of alkenes, is applied for a wide range of [MnIII,III2(O) (tmtacn)2]2+ dimers, employs
(RCO2)2A
which the reactivity can be tuned by variation of the
A
CHTUNGTRENNUNG
oxidative transformations, including oxidation of al- H2O2 in a highly atom efficient manner. In addition,
kanes, alcohols and aldehydes employing H2O2 as ox- several substrates containing more than one oxida-
idant. The substrate classes examined include pri- tion sensitive group could be oxidised selectively, in
mary and secondary aliphatic and aromatic alcohols, certain cases even in the absence of protecting
aldehydes, and alkenes. The emphasis is not primari- groups.
ly on identifying optimum conditions for each indi-
vidual substrate, but understanding the various fac- Keywords: homogeneous catalysis; manganese; oxi-
tors that affect the reactivity of the Mn-tmtacn cata- dation; peroxides; triazacyclononane
Introduction
zymes.[7,8,9,10] The first report on the use of [MnIV,IV2O3
AHCTUNGTRENNUNG
(tmtacn)2]2+ (1) (Figure 1) as a catalyst was by Hage
Oxidative transformations, in particular the oxidation et al. in 1994, where it was employed in an aqueous
of alkenes to their corresponding diols or epoxides,[1,2] carbonate buffer (pH 8–9) with H2O2 as terminal oxi-
alcohols to aldehydes, ketones or carboxylic acids and dant for clean and efficient low-temperature laundry
oxidation of alkanes are central processes in biology, bleaching as well as the epoxidation of alkenes (albeit
synthetic organic chemistry and industrial chemistry.[3] typically with 100 equiv. of H2O2 with respect to
In recent years, the development of atom-efficient alkene).[11] Since then, several research groups have
and environmentally benign catalytic systems has focused on applying this catalyst in oxidative transfor-
been a major challenge.[4] A primary goal is to devel- mations with initial efforts directed towards the use of
op systems that show high atom efficiency and mini-
mal formation of by-products, with molecular oxygen
(O2) and hydrogen peroxide (H2O2) being the most
atom economic oxidants.[5,6]
Over the last two decades several groups have fo-
cused on the use of manganese catalysts based on the
ligand
N,N’,N’’-trimethyl-1,4,7-triazacyclononane
(tmtacn) originally developed by Wieghardt and co-
workers in the late 1980s as model systems for the
water splitting component of photosystem II (PS II)
Figure 1. Complexes [MnIV,IV2O
(CCl3CO2)2A(tmtacn)2](PF6)2 2.
(tmtacn)]
E
1
and
and dinuclear manganese-based catalase en- [MnIII,III2O
N
N
ACHTUNGTRENNUNG
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additives to suppress the catalase-type activity (i.e., portant challenge examined is whether compounds
wasteful decomposition of H2O2) of 1 and to enhance containing multiple oxidation sensitive groups can be
its activity towards the oxidation of alkenes.[12,13,14,15] oxidised selectively without the need of protecting
De Vos et al. reported the efficient epoxidation of groups. Finally, the tolerance of common protecting
a range of alkenes using this catalyst in oxalate-buf- groups to the present system is examined.
fered aqueous acetonitrile. Under these conditions
the catalase-type activity was suppressed effectively
(typically 1.5–3 equiv. H2O2 were required).[16] Subse- Results and Discussion
quently, Berkessel and co-workers showed that a mix-
ture of l-ascorbic acid and sodium l-ascorbate in Oxidation of Secondary Alcohols to Ketones
combination with tmtacn and MnII could be used in
the epoxidation of alkenes and the oxidation of alco- Treatment of aromatic secondary alcohols (Table 1,
hols, albeit with excess H2O2 (typically 2– entries 1–3) by a slow addition of 1.45 equiv. of H2O2
2.6 equiv.).[17] The range of substrate classes that have in the presence of 0.1 mol% 1 and 1 mol% trichloro-
been examined with Mn-tmtacn-based catalysts now acetic acid in acetonitrile afforded the corresponding
includes alkanes, alkenes, primary and secondary al- ketones in excellent conversions and moderate to
cohols and allylic alcohols.[14,18] Later reports showed good isolated yields. Trichloroacetic acid was chosen
that the catalytic activity of the Mn-tmtacn system to- as co-catalyst since our earlier studies had identified
wards oxidation of alkanes and epoxidation of alkenes this carboxylic acid to provide the most active catalyst
could be enhanced dramatically in the presence of system in general.[22,23] Side reactions or side products
a large excess of acid, e.g., acetic acid (typically 40 to were not observed. Aliphatic secondary alcohols (en-
>100 mol%),[19] or aldehydes such as choral (typically tries 4–7) were oxidised to the corresponding ketones
25 mol%),[20] respectively. Importantly, in the latter
case, and in the earlier example by De Vos et al.[21]
with a heterogeneous version of this catalyst, Mn-cat-
alysed cis-dihydroxylation was observed also. Subse-
Table 1. Oxidation of secondary alcohols to ketones using
H2O2 catalysed by 1.[a]
quently, it was shown by our group that carboxylic
acids at co-catalytic levels (typically 1 mol%) were
the key to controlling the activity of 1 and enabled
Entry Substrate
Conversion Product
[%][a]
Isolated
yield [%]
the efficient use of H2O2 for the oxidation of al-
kenes.[22,23]
The recognition of the role of carboxylic acids and
other factors (e.g., solvent composition) in controlling
the reactivity of catalyst 1 in the oxidation of al-
kenes[23] raises the question as to how conversion of
other substrate classes besides alkenes can be con-
trolled and, importantly, how primary and secondary
oxidation products can affect the reactivity of the
active species formed from 1, in particular the bis-car-
1[b]
2[b]
3[b]
4[b]
full
full
full
>90
77
63
93
boxylato bridged MnIII,III dimers such as
2
2
(Figure 1).[22,23] In this contribution the focus is on the
catalytic system employing Mn-tmtacn, carboxylic
acids (as co-catalyst) and H2O2 (typically 1.05–
31[c]
1.5 equiv.) as terminal oxidant for the selective and 5[b]
atom efficient oxidation of a range of organic sub-
>95
86
6[b]
full
92
99
strate classes.
The substrate classes examined in detail are pri-
mary and secondary aliphatic and aromatic alcohols,
aldehydes, alkenes and alkanes. In all cases, the condi-
tions used for alkene oxidation in our earlier re-
ports[22] are applied initially, followed by optimisation.
The goal, however, was not primarily to identify opti-
mum conditions for each individual substrate but to
understand the various factors that affect the reactivi-
ty of the Mn-tmtacn catalytic system in each case and
to explore which functional groups are oxidised pref-
erentially by the catalytic system. Furthermore, an im-
7[b]
full
[a]
[b]
1
Conversion was determined by H NMR spectroscopy.
Reaction conditions: 0.1 mol% 1, 1.0 mol% Cl3CCO2H,
1.45 equiv. H2O2, CH3CN ([substrate]=1M), room tem-
perature.
The isolated yield of cyclohexanone is low, however, this
is due to its relatively high volatility (and accompanying
loss during work-up) and not due to poor performance of
the catalyst.
[c]
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Selective Catalytic Oxidation of Alcohols, Aldehydes, Alkanes and Alkenes Employing Manganese Catalysts
Table 2. Limitations in the oxidation of secondary alcohols to ketones using H2O2 catalysed by 1.[a]
Entry
1[b]
Substrate
Product(s)
–
Conversion [%][a]
<20
2[b]
>90 (for both)
>90 (for both)
3[b]
4[c]
5[b]
>95
<20
–
6[b]
12 (for menthol); full (for cyclohexanol)
7[c]
–
<20
[a]
1
Conversion was determined by H NMR spectroscopy.
[b]
[c]
Reaction conditions: 0.1 mol% 1, 1.0 mol% Cl3CCO2H, 1.45 equiv. H2O2, CH3CN ([substrate]=1M), room temperature.
Reaction conditions: 0.5 mol% 1, 5.0 mol% Cl3CCO2H, 1.45 equiv. H2O2, tert-BuOH/H2O (2/1 v/v) ([substrate]=1M),
room temperature.
in good yields and, importantly, cyclohexanol and cy- tial importance in limiting the reactivity. Indeed, in
clooctanol (entries 4 and 5) can be oxidised selective- tert-butyl alcohol/H2O (2/1 v/v), 5-nonanol could be
ly to cyclohexanone and cyclooctanone, respectively, converted to 5-nonanone with greater than 95% con-
without formation of Baeyer–Villiger-type oxidation version (entry 4).
products. This was confirmed by performing the reac-
The effect of steric hindrance was inferred in the
tion (entry 4) in acetonitrile-d3 followed by direct case of reaction with l-menthol and cyclohexanol,
1
analysis by H NMR spectroscopy. For longer linear which differ in the presence of an isopropyl group ad-
aliphatic alcohols, e.g., 2-octanol (entry 6) and bicyclic jacent to the hydroxy moiety. In contrast to the case
alcohols, e.g., isoborneol (entry 7), high conversions with 5-nonanol, when a 1:1 mixture of cyclohexa-
and isolated yields were obtained.
nol:l-menthol was used full conversion of cyclohexa-
Oxidation of several aliphatic secondary alcohols nol to cyclohexanone was obtained, however, only
such as 5-nonanol and l-menthol (Table 2, entries 1 10% conversion of l-menthol to menthone was ob-
and 5, respectively) was found not to proceed in ace- served (entry 6). Furthermore, in contrast to 5-nonan-
tonitrile under standard reaction conditions. Remark-
ACHTUNGTRENoNGNU l, conversion of l-menthol was not observed in tert-
ably, >90% conversion of 5-nonanol to 5-nonanone butyl alcohol/H2O (2/1 v/v) (entry 7).
was obtained when a 1:1 mole ratio mixture of 5-non-
From these examples it is clear that the current cat-
anol:cyclohexanol (entry 2) or 5-nonanol:2-octanol alyst system is intrinsically unreactive towards certain
(entry 3) was employed. This observation indicates substrates (e.g., due to steric hindrance). However,
that 5-nonanol is not intrinsically unreactive with re- for other substrates a simple change in the polarity of
spect to the catalyst system but instead a microscopic the reaction mixture can result in dramatically im-
phase separation of the substrate may be of substan- proved conversions (e.g., by adding another com-
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pound which, presumably, interferes with microscopic form acetic acid.[25,26] In the present study this aspect
phase separation or by changing the solvent system). is examined further.
Primary aliphatic alcohols, e.g., 1-octanol (Table 3,
entry 1), were converted to a mixture of the corre-
Oxidation of Primary Alcohols to Carboxylic Acids
sponding aldehyde and carboxylic acid with low to
moderate conversion using a range of reaction condi-
In earlier studies, Mn-tmtacn-based catalysts have tions. This is in agreement with reports that aliphatic
been shown to be active in the oxidation of alcohols. alcohols are less reactive than benzylic substrates.[27]
Berkessel et al. found that a primary and a secondary Moreover, selective oxidation of aliphatic alcohols is
alcohol could be oxidised to their corresponding generally more difficult due to the presence of oxida-
À
ketone and carboxylic acid when sodium ascorbate tion sensitive C H bonds.
was used as additive, albeit with ca. 2.5 equiv. of H2O2
Using 0.3 mol% instead of 0.1 mol% of 1 did not
with respect to substrate.[17] Zondervan et al. reported lead to improved conversion. As shown in the Sup-
the use of Mn-tmtacn for the oxidation of benzyl alco- porting Information, Figure S1, the product 1-octanoic
hols to their corresponding aldehydes employing acid can displace the carboxylato ligands of the
either H2O2 or TBHP and to carboxylic acids when catalyst. In general, complexes of the type
excess H2O2 (8 equiv. with respect to substrate) was [MnIII,III2O (tmtacn)2]2+ where R is a linear
ACHTNUTRGENN(UG RCO2)2ACHTUTGNRENNUGN
used.[24] Although acid co-catalysts were not added alkyl group showed lower activity than when trichlor-
deliberately in this case, it is nevertheless probable oacetato ligands are present.[23,26] Furthermore, the
that the activity observed was due to the in situ for- catalyst loses activity over time due to ligand dissocia-
mation of carboxylic acids from the substrates them- tion and/or reduction to the MnII oxidation state,[23]
selves or from decomposition of acetone by H2O2 to and hence increasing the initial catalyst loading does
not result in a significant increase in conversion.
Table 3. Mn-tmtacn-catalysed oxidation of primary alcohols to carboxylic acids using H2O2.
Entry
1[b]
Substrate
Conversion [%][a]
54
Product
Isolated yield [%]
37
2[c]
3[d]
4[d]
5[e]
full
full
full
91
>95
94
95
81
6[e]
<50
22
[a]
1
Conversion was determined by H NMR spectroscopy.
[b]
Reaction conditions: 0.4 mol% 1 (stepwise addition), 1.0 mol% Cl3CCO2H, 3 equiv. H2O2, CH3CN/H2O (9/1 v/v) ([sub-
strate]=1M), room temperature.
Reaction conditions: 0.1 mol% 1, 1.0 mol% Cl3CCO2H, 2.05 equiv. H2O2, CH3CN ([substrate]=1M), room temperature.
Reaction conditions: 0.1 mol% 1, 1.0 mol% salicylic acid, 2.95 equiv. H2O2, CH3CN/H2O (4/1 v/v) ([substrate]=0.2M),
room temperature.
[c]
[d]
[e]
Reaction conditions: 0.4 mol% 1 (stepwise addition), 1.0 mol% salicylic acid, 3 equiv, H2O2, CH3CN/H2O (4/1 v/v) ([sub-
strate]=0.2M), room temperature.
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Selective Catalytic Oxidation of Alcohols, Aldehydes, Alkanes and Alkenes Employing Manganese Catalysts
However, adding the catalyst in small portions over recycling of starting material would be necessary to
time (see the Experimental Section) improved the provide higher overall yields of benzaldehyde.
conversion of 1-octanol to 54% with a 37% yield of
the corresponding carboxylic acid (Table 3, entry 1).
This system is also efficient in the oxidation of
benzyl alcohol derivatives, i.e., 4-methylbenzyl alco-
The same reaction conditions (0.1 mol% of 1/ hol and 4-chlorobenzyl alcohol (entries 3 and 4, re-
1.0 mol% trichloroacetic acid and 3 equiv. H2O2) were spectively) were converted to their corresponding car-
applied in the oxidation of benzyl alcohol. However, boxylic acids in excellent conversions and isolated
only 60% conversion of benzyl alcohol to a mixture yields using only 0.1 mol% of 1 and 1.0 mol% of sali-
of benzaldehyde and benzoic acid (1:2 ratio) was ach- cylic acid as co-catalyst.
ieved initially (data not shown). Salicylic acid, 2,6-di-
For certain substituted benzyl alcohols a higher cat-
chlorobenzoic acid and benzoic acid itself were tested alyst loading was required, i.e., 4-nitrobenzyl alcohol
as co-catalyst and all provided full conversion of gave 91% conversion and 81% isolated yield of car-
benzyl alcohol to benzoic acid in excellent isolated boxylic acid after applying 0.4 mol% of catalyst in
yields (96–98%). These results are in contrast to the total with stepwise addition of the catalyst (entry 5).
previous report by Zondervan and co-workers in However, 4-methoxybenzyl alcohol (entry 6), in par-
which benzaldehyde was the sole product obtained.[24] ticular, gave less than 50% conversion even with
To investigate this difference further, online monitor- higher catalyst loadings. In this case it should be
ing of the oxidation of benzyl alcohol catalysed by noted that the reaction mixture turned black, which
1 and salicylic acid was performed using Raman spec- indicates oxidation of the substrate to a redox active
troscopy (Figure 2). Formation of benzaldehyde and quinone-type species that may lead to catalyst decom-
benzoic acid as well as the presence of H2O2 in the re- position.
action are plotted against time. Over the first hour,
the rate of benzaldehyde formation is higher than
that of benzoic acid, however, after that period both Oxidation of Aldehydes to Carboxylic Acids
rates are comparable. The highest yield of benzalde-
hyde is obtained at 3 h and conversion to benzoic acid As expected (vide supra) aliphatic aldehydes (Table 4,
occurs eventually. This confirms the observation of entries 1–3) were found to be oxidised to the corre-
oxidation of benzyl alcohol to benzoic acid over 16 h sponding carboxylic acids in high conversions and
under the present conditions and that selective alde- good isolated yields. The aromatic aldehydes, benzal-
hyde formation was not observed. If the aldehyde is dehyde and 4-fluorobenzaldehyde (entries 4 and 5)
the desired product, performing the reaction with were converted fully to their corresponding carboxylic
shorter reaction times (such as stopping the reaction acids in 97% and 89% isolated yield, respectively. By
within 2 h), followed by separation of the product and contrast, phenylacetaldehyde (entry 6) showed very
low conversion and the formation of cleavage prod-
ucts (benzaldehyde and benzoic acid) was observed.
Interestingly, the heteroaromatic aldehyde, thio-
phene-2-carbaldehyde (entry 8), was oxidised to thio-
phene-2-carboxylic acid as the sole product selective-
ly, with further oxidation[28] to the corresponding sulf-
oxide or sulfone not observed. However, oxidation of
furan-2-carbaldehyde did not reach completion even
with 0.5 mol% of 1, with mainly substrate being re-
covered and only trace amounts of furan-2-carboxylic
acid being obtained (entry 9).
Oxidation of Alkenes
Previously, the carboxylic acid-promoted cis-dihydrox-
ylation and epoxidation of alkenes catalysed by
[MnIII,III2O (tmtacn)2]2+ employing H2O2 as ox-
ACHTNUTRGENN(UG RCO2)2ACHTUTGNRENNUGN
idant was reported by our group.[22,29,30] This system
shows highest activity for electron-rich alkenes, i.e.,
cyclooctene, cyclohexene, cis- and trans-heptene, and
shows relatively little activity with electron-deficient
alkenes such as dimethyl fumarate and dimethyl mal-
eate. A general trend is that the use of the ligand 2,6-
Figure 2. Online monitoring of oxidation of benzyl alcohol
(with 3 equiv. of H2O2) in CH3CN catalysed by 1 (0.1 mol%)
with salicylic acid (1.0 mol%) under air monitored by
Raman spectroscopy (lexc =785 nm).
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Table 4. Oxidation of aldehydes to carboxylic acids with H2O2 catalysed by 1.
Entry
1[b]
Substrate
Conversion [%][a]
93
Product
Isolated yield [%]
61
2[b]
3[b]
79
56
66
full
4[b]
full
97
5[b]
6[b]
7[b]
8[b]
full
89
n.d.[c]
full
–
–
n.d.
94
92
77
9[b]
n.d.
n.d.
[a]
1
Conversion was determined by H NMR spectroscopy.
[b]
[c]
Reaction conditions: 0.1 mol% 1, 1.0 mol% Cl3CCO2H, 1.45 equiv. H2O2, CH3CN ([substrate]=1M), room temperature.
Not determined (n.d.).
dichlorobenzoic acid leads to higher selectivity to-
wards the cis-dihydroxylation products and that the
use of salicylic acid leads to higher selectivity for the
epoxide products.
To expand the scope of alkene substrates, gem-di-
ACHTUNGTRENNUNGsubstituted and trisubstituted alkenes were examined.
For 2,4-dimethyl-1-heptene and 2-methyl-2-pentene,
full conversion and good yields (73% and 72%, re-
spectively) of epoxide were obtained employing sali-
cylic acid as co-catalyst and 1.1 equiv. of H2O2
(Scheme 1).
Scheme 1. Oxidation of gem-disubstitued and trisubstituted
alkenes. Reaction conditions: 0.1 mol% 1, 1.0 mol% salicylic
acid, 1.1 equiv. H2O2, 0.5 equiv. 1,2-dichlorobenzene, CD3CN
([substrate]=1M), 08C to room temperature.
A key question regarding the control of selectivity
through the use of various carboxylic acid co-catalysts Oxidation of Alkanes
in the oxidation of less electron-rich alkenes was in-
vestigated with ethyl cinnamate, in which the alkene Although the primary focus of the present study is on
is polarised. A notable difference in selectivity is ob- oxidative transformations of functional groups, the ac-
served in its oxidation with salicylic acid compared to tivity of 1 towards alkane oxidation has been noted
2,6-dichlorobenzoic acid as co-catalyst. However, sub- previously.[19a,22] The good to excellent selectivity to-
stantial further oxidation of the cis-diol product to the wards the oxidation of alkenes, alcohols and alde-
corresponding a-hydroxy ketone was observed also hydes indicates that alkane oxidation is not the pre-
(Table 5).
ferred mode of action for this catalyst system, howev-
er, in the absence of such functional groups activity is
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Selective Catalytic Oxidation of Alcohols, Aldehydes, Alkanes and Alkenes Employing Manganese Catalysts
Table 5. Oxidation of ethyl cinnamate with catalyst 1 and various carboxylic acids.
Entry
Co-catalyst
Conversion [%][a]
Product ratio[a] A:B:C
1
2
salicylic acid
2,6-dichlorobenzoic acid
full
60
0.9:1.6:1
1.2:0.7:1
[a]
1
Conversion and product ratio were determined by H NMR spectroscopy.
Table 6. Oxidation of alkanes.
Entry
1[c]
Substrate
Conversion [%][a]
80
Product
Isolated yield [%]
61[b]
2[d]
60
n.d.[e]
3[d]
4[d]
5[d]
70
37
30
40
69
30–40
70
6[d]
70
[a]
Conversion of the substrate was determined by Raman spectroscopy (lexc =785 nm) using 1,2-dichlorobenzene as internal
standard.
[b]
[c]
1
Yield was determined by H NMR spectroscopy.
Reaction conditions: 0.1 mol% 1, 1.0 mol% Cl3CCO2H, 4.5 equiv. H2O2, CH3CN ([substrate]=1M), 08C to room temper-
ature.
[d]
[e]
Reaction conditions: 0.1 mol% 1, 1.0 mol% Cl3CCO2H, 1.45 equiv. H2O2, CH3CN ([substrate]=1M), 08C to room tem-
perature.
Not determined (n.d.).
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Table 7. Oxidation of citronellol under various reaction conditions.
Entry
Co-acid
Equiv. of H2O2
Conversion [%][a]
Isolated yield [%]
1
2
3
2,6-dichlorobenzoic acid
salicylic acid
salicylic acid
1.05
1.05
1.15
50
65–80
>95
n.d.[b]
30
62
[a]
1
Conversion was estimated by H NMR spectroscopy of crude material after work-up.
Not determined (n.d.).
[b]
expected. The activity of 1 with trichloroacetic acid in
the selective oxidation of alkanes was examined.
The bifunctional substrate, citronellol, which con-
tains a trisubstituted electron-rich alkene and a pri-
Remarkably, aliphatic cyclic alkanes, such as cyclo- mary alcohol, was oxidised with 1.05 equiv. of H2O2
octane (Table 6, entry 1), can be converted with good catalysed by 1 together with either 2,6-dichlorobenzo-
selectivity to the corresponding mono-ketone prod- ic acid or salicylic acid (Table 7). Good conversion of
ucts. This is attributed to the fact that the system is citronellol (50–80%) to the corresponding epoxy alco-
more active in oxidation of the initially formed alco- hol as the major product was obtained in each case,
hol to the corresponding ketone, than towards oxida- with salicylic acid providing better conversion. The
tion of alkanes (vide supra).
corresponding diol-, epoxide-, and alkene-aldehyde
For aromatic alkanes, the benzylic protons are the were also obtained as by-products. With 1.15 equiv. of
most sensitive to oxidation and indeed good conver- H2O2, >95% conversion of citronellol was achieved
sion and yield of the mono-ketone products could be and despite the instability of the product during pu-
achieved for several substrates. In the case of ethyl- rification (silica gel as stationary phase for column
benzene (entry 3), however, the sensitivity of the chromatography), the epoxy alcohol could be isolated
product to subsequent aldol reactions may have had in 62% yield. Similar preference for epoxidation over
a substantial impact on the yield obtained. Propylben- alcohol oxidation has been observed before by De
zene (entry 4) seems unreactive under the reaction Vos et al.[31] for allyl alcohol and 3-buten-1-ol when
conditions used (trichloroacetic acid as co-catalyst), using oxalic acid as additive with 2–3 equiv. of H2O2.
even increasing the catalyst loading and adjusting the
In the case of 3-vinylbenzaldehyde, where the
rate of H2O2 addition did not improve on the conver- highly oxidation sensitive aldehyde group is present,
sion. The system 1/CCl3CO2H showed good selectivity the catalysed oxidation was examined with salicylic
and efficiency for oxidation at the benzylic position in acid and trichloroacetic acid as co-catalyst (Table 8).
case of indane and tetraline (entries 5 and 6, respec-
tively). Moreover, by controlling the amount of H2O2
(1.5 equiv.) added, a-tetralone (the mono-ketone
product) can be obtained in high yield (entry 6).
Table 8. Oxidation of 3-vinylbenzaldehyde under various re-
action conditions.
Oxidation of Bifunctional Substrates
A key challenge in modern oxidation catalysis is the
selective oxidative transformation of one functional
group in the presence of a second oxidation sensitive
functional group. As discussed previously, the selec-
tivity of the catalyst, with respect to product formed
upon oxidation of an alkene, can be controlled to
a significant extent by varying the nature of the car-
boxylic acid co-catalyst.[22] Therefore, in a study of
functional group selectivity, it is interesting to investi-
gate whether or not the co-catalyst employed exerts
similar levels of control.
Entry
Co-acid
Equiv. of H2O2
Product[a]
1
salicylic acid
salicylic acid
salicylic acid
CCl3CO2H
CCl3CO2H
1.50
0.80
0.80
1.50
0.80
E
E
D
E
D
2
3[b]
4
5[b]
[a]
1
Determined by H and 13C NMR spectroscopy.
[b]
Reaction was performed under N2 and in CD3CN to
enable direct analysis by 1H NMR spectroscopy.
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Selective Catalytic Oxidation of Alcohols, Aldehydes, Alkanes and Alkenes Employing Manganese Catalysts
1
Figure 3. Direct H NMR analysis of oxidation of 3-vinylbenzaldehyde a) at t0 b) after 7 h in CD3CN (from Table 8, entry 3).
1
Conversion and product distribution were confirmed
Direct H and 13C NMR analyses showed that both
1
by H NMR spectroscopic analysis after work-up. For reactions from Table 8, entries 3 and 5 provided only
both co-catalysts only the epoxide product was ob- epoxy-benzaldehyde as the product. No epoxy-benzo-
tained with no diol product detected by 1H NMR ic acid and only trace amounts of diol-benzaldehyde
1
spectroscopy. However, almost full conversion of sub- were observed in both cases (see example of H NMR
strate to product bearing both oxidised functional analysis in Figure 3). This means that the alkene
groups (epoxide and carboxylic acid) was obtained moiety of this compound is more reactive than the al-
even when less than 1 equiv. of H2O2 was used dehyde as high selectivity of alkene oxidation was ob-
(Table 8, entry 2). It was postulated that the oxidation served with the present system and that the oxidation
of the aldehyde to the carboxylic acid could occur of the aldehyde unit observed earlier was due to aero-
due to aerobic oxidation during the reaction or work- bic oxidation during work-up.
up and not from the catalytic system itself. To study
To confirm this observation, Raman spectroscopy
this in more detail, reactions were performed under was used to monitor the reaction online (Table 8,
a nitrogen atmosphere in acetonitrile-d3 (degassed) to entry 3) by following the intensity of the bands at
allow for direct analysis by 1H NMR spectroscopy 1635 cmÀ1 (C=C stretching mode) and 1705 cmÀ1 (C=
(entries 3 and 5, example of analysis is shown in O stretching mode) (Figure 4). Plotting the intensity
Figure 3). Online monitoring by Raman spectroscopy of each band against time shows a lag time lasting the
was also used to follow the conversion of each func- first hour. After that period the intensity of the band
tional group in real time (for entry 3, see Figure 4 and due to the alkene moiety decreased rapidly, while the
Figure 5).
intensity of the band due to the aldehyde did not
change substantially (Figure 5). The increase in the
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FULL PAPERS
Scheme 2. Oxidation of the diene carvone. Conversion and
product yield were determined by 1H NMR spectroscopy
using 1,2-dichlorobenzene (0.5 equiv.) as internal standard.
Reaction conditions: 0.1 mol% 1, 1.0 mol% salicylic acid,
1.1 equiv. H2O2, CH3CN ([substrate]=1M), 08C to room
temperature).
the catalyst is electrophilic. For carvone, 57% yield of
epoxide from the electron-rich alkene and 5% from
electron-deficient alkene were obtained with some
side products such as the diepoxide.
Figure 4. Online reaction monitoring of 3-vinylbenzaldehyde
with catalyst 1, salicylic acid and H2O2 under N2 using
Raman spectroscopy (lexc =532 nm).
Protecting Group Tolerance
The stability of protecting groups under the reaction
conditions employed is an important characteristic of
the catalyst system. Silyl-based protecting groups, e.g.,
tert-butyldiphenylsilyl (TBDPS) were found to be un-
stable under the reaction conditions due to the sensi-
tivity of TBDPS groups to H2O2. TBDPS-protected
primary aliphatic alcohol and benzyl alcohol were
converted to their corresponding aldehyde and car-
boxylic acid in low yield, and with many by-products
from the oxidation of the unprotected alcohol
(Table 9, entries 1 and 2 and see also Supporting In-
formation for more details).
The benzyl ether group (Bn) proved to be an un-
suitable protecting group with the current catalytic
system as well (Table 9, entry 3). Oxidation of benzyl-
protected alcohol employing various reaction condi-
tions, i.e., varying the co-acids (trichloroacetic acid,
salicylic acid and oxalic acid) and equivalents of H2O2
Figure 5. Plot of remaining amount of alkene and aldehyde
against time from online monitoring using Raman spectros-
copy (lexc =532 nm) of the oxidation of 3-vinylbenzaldehyde
with catalyst 1, salicylic acid and H2O2 under N2 in CD3CN.
full-width at half maximum of the carbonyl stretching (1.0–3.0 equiv.), gave similar results (by 1H NMR
band (at 1635 cmÀ1) of the aldehyde was due to the spectroscopy) with only differences in product ratios.
steady increase in water content (concomitant with 1H NMR spectroscopic data demonstrated that not
addition of aqueous H2O2) (Figure 4). The changes at only was the primary alcohol group oxidised to the
1705 cmÀ1 are not due to transformation to the car- corresponding aldehyde and carboxylic acid, but also
1
boxylic acid. These data support the results from H the methylene unit (CH2) of the benzyl ether group
and 13C NMR spectroscopic data (Figure 3).
was oxidised. The latter provided the intermediate
Oxidation of a substrate containing dissimilar formation of a hemi-acetal, which is unstable under
double bonds, i.e., a substrate containing an electron- the reaction conditions employed. Some intermedi-
rich alkene and an electron-deficient alkene in the ates were oxidised further to obtain a benzoyl group
same molecule, was investigated. When salicylic acid and some underwent cleavage to afford many unde-
was used as co-catalyst, selectivity for epoxidation of sired products such as pentanol, pentanal, pentanoic
the electron-rich alkene was observed for carvone acid, benzyl alcohol, benzaldehyde and benzoic acid
(Scheme 2). This is in agreement with previous re- (see details in the Supporting Information). Their for-
sults[22] that the system is more reactive towards elec- mation was confirmed by GC-MS analysis.
tron-rich alkenes over electron-deficient alkenes, i.e.,
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Selective Catalytic Oxidation of Alcohols, Aldehydes, Alkanes and Alkenes Employing Manganese Catalysts
Table 9. Oxidation of protected organic compounds.
Entry
Substrate
Conversion [%][a]
Products
Isolated yield [%]
1[b]
2[b]
n.d.[f]
n.d.
many products
many products
n.d.
n.d.
3[c]
4[d]
n.d.
80
many products
n.d.
70
5[e]
75
80
60
63
6[e]
7[e]
full
full
quant.
86
8[e]
[a]
1
Conversion was determined by H NMR spectroscopy.
[b]
[c]
[d]
See the Supporting Information, Scheme S1 for reaction conditions and more details.
See the Supporting Information, Scheme S2 for reaction conditions and more details.
Reaction conditions: 0.1 mol% 1, 1.0 mol% salicylic acid, 3 equiv. H2O2, CH3CN/H2O (4/1 v/v) ([substrate]=0.2M), room
temperature.
[e]
[f]
Reaction conditions: 0.1 mol% 1, 1.0 mol% salicylic acid, 1.1 equiv. H2O2, CH3CN ([substrate]=0.5M), 08C to room tem-
perature.
Not determined (n.d.).
To avoid the cleavage of the protecting group version, 63% isolated yield) and minor products were
under the reaction conditions used, the benzoyl group diol and its overoxidation product, a-hydroxy ketone
(Bz) was used successfully for protecting the alcohol (entry 6). Phthalimide (Phth) is also an alternative
moiety instead (Table 9, entries 4 and 5). The combi- protecting group that was found to be suitable with
nation of 1 and 1.0 mol% salicylic acid with 3 equiv. the present system.[32] A phthalimide-protected allylic
H2O2 gave 80% conversion of 5-hydroxypentyl ben- aldehyde and an alkene were converted to their cor-
zoate to the corresponding carboxylic acid in 70% responding carboxylic acid and epoxide, respectively,
(1H NMR) yield (entry 4). Epoxide products (two dia- both with high conversion and in high isolated yield
stereomers in a ratio of 1:1) of protected allylic alco- (entries 7 and 8, respectively). Deprotected species
hol were obtained in 60% isolated yield (75% conver- were not observed in either case.
sion; entry 5). Moreover, deprotected substrate or
1
products were not detected by H NMR spectroscopy
in either case.
Conclusions
tert-Butoxycarbonyl (Boc)-protected alcohols were
found to be stable under these reaction conditions as A practical catalytic system employing Mn-tmtacn,
well. The epoxide product of a Boc-protected allylic with carboxylic acids as co-catalyst and H2O2 as ter-
alcohol was obtained as the major product (80% con- minal oxidant for the oxidation of organic substrates
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Pattama Saisaha et al.
FULL PAPERS
bearing oxidation sensitive functional groups, i.e., al- Acknowledgements
cohols, aldehydes, alkenes and alkanes, has been de-
scribed.
The current catalytic system is capable of oxidation
The authors thank Dr. Paul L. Alsters for useful discussion,
the Netherlands Organisation for Scientific Research (VIDI
Grant 700.57.428, to WRB), the European Research Council
(Starting Investigator Grant 279549, to WRB), the University
of Groningen (Ubbo Emmius studentship, to PS), Erasmus
(to LB, MvdM), NRSC-C (to BLF) and the Foundation for
Technology and Science (STW Grant No. 11059, to WRB,
JWdB) for financial support. COST action CM1003 is ac-
knowledged for discussion.
of a wide variety of oxidisable groups. For substrates
containing more than one oxidation sensitive moiety,
selective oxidation of only one functional group could
be attained, as exemplified in the selective epoxida-
tion of, e.g., citronellol and 3-vinylbenzaldehyde. The
sensitivity of the Mn-tmtacn/carboxylic acid system
towards the electronic and steric properties of the
substrate can be used to obtain selective oxidation in
bifunctional substrates, such as carvone and citronel-
lol. Furthermore, the reactivity of the catalytic system
can be fine-tuned by varying, for example, the nature
of the carboxylic acid co-catalyst (which are ligands in
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