Magtrieve: A convenient catalyst for the oxidation of alcohols

Article abstract of DOI:10.1016/j.tetlet.2014.06.055

We find that Magtrieve (CrO₂) catalyzes the oxidation of a wide variety of alcohols with periodic acid as the terminal oxidant. Mild conditions, short reaction times, and facile aqueous work-up make this a most attractive method. Olefins are not oxidized under these conditions; thus alcohols react selectively in the presence of alkenes. Conditions have been optimized with respect to catalyst loading, solvent, and co-oxidant; and the scope of the reaction includes primary and secondary benzylic, allylic, and aliphatic alcohols.

Full text of DOI:10.1016/j.tetlet.2014.06.055

Tetrahedron Letters 55 (2014) 4452–4454
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Magtrieve™: a convenient catalyst for the oxidation of alcohols
⇑
Chip S. Few, Kathryn R. Williams, Kenneth B. Wagener
Dept. of Chemistry, University of Florida, Gainesville, FL 32611, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
We find that Magtrieve™ (CrO2) catalyzes the oxidation of a wide variety of alcohols with periodic acid as
the terminal oxidant. Mild conditions, short reaction times, and facile aqueous work-up make this a most
attractive method. Olefins are not oxidized under these conditions; thus alcohols react selectively in the
presence of alkenes. Conditions have been optimized with respect to catalyst loading, solvent, and co-oxi-
dant; and the scope of the reaction includes primary and secondary benzylic, allylic, and aliphatic
alcohols.
Received 15 April 2014
Revised 10 June 2014
Accepted 13 June 2014
Available online 21 June 2014
Keywords:
Oxidation
Ó 2014 Elsevier Ltd. All rights reserved.
Alcohol
Chromium
Hypervalent iodine
Catalysis
Introduction
cat. CrO2
OH
O
H5IO6
R2
R1
R1 R2
We offer a new and convenient oxidation method for alcohols
using Magtrieve™ (CrO2) as a catalyst and periodic acid as the
terminal oxidant. Magtrieve™ is a cheap ($1/g), nontoxic, easily dis-
posed chromium(IV) oxide that can be readily removed from the
reaction due to its heterogeneous nature and ferromagnetic proper-
ties.1,2 Due to its ability to be regenerated by heating in air,1 we
wondered if Magtrieve™ could be regenerated in situ by a suitable
co-oxidant, thereby catalyzing the oxidation of a substrate.
Alcohol oxidation chemistry is not new. Many stoichiometric
methods have been reported over the years; some of the most
notable being pyridinium chlorochromate (PCC), Swern type oxi-
dations, Dess–Martin Periodinane, and Oppenauer type oxida-
tions.3 In an effort to minimize the economic and environmental
cost associated with the above methods, more recent work has
focused on catalysis.4
H
MeCN/H2O, rt
or
cat. CrO2
OH
O
a
H5IO6
R1
H
R1 OH
H
MeCN/H2O, rt
Scheme 1. R1 = Aryl, Alkyl. R2 = H, Alkyl, Aryl. aDifferent molar ratios of H5IO6 to
alcohol were used depending on the target carbonyl. Representative oxidation of
alcohols was carried by Magtrieve™ with periodic acid as oxidant. Eight examples
are reported here, though many other possibilities exist.
Reaction conditions were optimized for the conversion of benzyl
alcohol to benzaldehyde, and the scope of the reaction was studied
by oxidizing various substituted benzyl alcohols, as well as pri-
mary and secondary aliphatic alcohols, using the optimized condi-
tions (Scheme 1). Thus we present a rapid and cheap oxidation
method with an easy work-up.
Although these methods are feasible, we find that Magtrieve
catalysis is much simpler to perform. Our strategy does not require
anhydrous conditions, meaning it can be carried out open to air at
room temperature. The reaction is complete in a matter of minutes
in most cases. Inorganic byproducts are easily removed from the
carbonyl products by aqueous work-up.
This study reports the oxidation of primary and secondary
alcohols to the respective carbonyl compounds using Magtrieve™
in catalytic amounts with periodic acid as the terminal oxidant.
Results and discussion
Optimization
The key to success has been understanding the proper reaction
conditions, which were investigated using benzyl alcohol (1a) as a
⇑
Corresponding author. Tel.: +1 352 392 4666; fax: +1 352 392 9741.
E-mail address: wagener@chem.ufl.edu (K.B. Wagener).
http://dx.doi.org/10.1016/j.tetlet.2014.06.055
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.

C. S. Few et al. / Tetrahedron Letters 55 (2014) 4452–4454
4453
Table 1
Optimization of reaction conditions for room temperature oxidation of benzyl alcohol (5.0 mmol)
catalyst/
oxidant
OH
O
O
+
OH
Solvent
1a
2a
3a
Entry
Solvent
Oxidant
Catalyst
None
Time
1
2
3
1
2
3
4
5
6
7
8
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
DCM
Toluene
92:8 CH3CN/H2O
60:40 CH3CN/H2O
80:20 CH3CN/H2O
H5IO6 (7.5 mmol)
H5IO6 (7.5 mmol)
H5IO6 (7.5 mmol)
H5IO6 (7.5 mmol)
H5IO6 (7.5 mmol)
H5IO6 (7.5 mmol)
H5IO6 (5.5 mmol)
Air
H5IO6 (7.5 mmol)
H5IO6 (7.5 mmol)
H5IO6 (5.5 mmol)
H5IO6 (5.5 mmol)
H5IO6 (7.5 mmol)
36 h
36 h
1 h
76
65
20
23
34
79
>99
>99
>99
>99
>6
<1
<1
<1
<1
CrO2 (0.05 mmol)
CrO2 (0.25 mmol)
CrO2 (0.5 mmol)
CrO2 (1.0 mmol)
CrO2 (0.5 mmol)b
CrO2 (0.5 mmol)
CrO2 (0.5 mmol)
CrO2 (0.5 mmol)
CrO2 (0.5 mmol)
CrO2 (0.5 mmol)
CrO2 (0.5 mmol)
CrO2 (0.5 mmol)
1 h
NDa
ND
ND
ND
93
15 min
30 min
20 min
3.5 h
16 h
16 h
1 h
ND
ND
ND
ND
ND
19
9
22
78
10
11
12
13
37
44
2c
94c
87c
90c
4c
1 h
1 h
12c
6.5c
<1c
3.5c
Crude ratios determined by GC–MS peak integration ratios unless noted otherwise.
a
ND = Not detected.
0.5 mmol hydroquinone was added to the reaction.
Percent of crude as determined by GC–FID peak integration.
b
c
Table 2
Substrate scope
Entry
1
1
2
3
H5IO6 (equiv)
1.5
Time
1
2
3
OH
O
O
OH
O
1 h
6.5
90
3.5
1a
2a
3a
OH
O
OH
2
1.1
15 min
28.5
69.5
2
Cl
Cl
Cl
3b
1b
2b
3
4
1b
2b
3b
2.2
1.5
5 h
1 h
NDa
3.5
12
88
OH
O
NAb
96.5
NA
2c
1c
OH
O
5
6
NA
1.5
1.1
15 min
1 h
3
5
97
95
NA
NA
2d
1d
OH
O
NA
NA
1e
2e
7
1f
7
2f
7c
8
1.5
2.2
1 h
1 h
3.5
4d
96.5
4d
NA
OH
O
OH
OH
O
O
OH
92d
7
1g
7
7
3g
O
2g
OH
3
1h
3
3
9
3.0
1 h
1.5
1.5
97
O
2h
3h
Five mmol alcohol and 80:20 CH3CN/H2O used unless noted otherwise. Crude ratio determined by GC–FID peak integration ratios unless noted otherwise. Product identities
verified according to retention times of pure standards or by 1H and 13C NMR analysis of the crude product.
a
ND = Not detected.
NA = Not applicable.
b
c
Used 2.4 mmol starting alcohol.
d
Calculated from 1H NMR peak integration.
test substrate (Table 1). We found that 1a converts to benzalde-
hyde (2a) using 1.5 equiv of periodic acid and 10 mol %
Magtrieve™ in an 80:20 acetonitrile/water solution. Completion
of this transformation was reached in less than 1 h (Table 1, entry
13 and Table 2, entry 1) while the control experiment (without cat-
alyst) yielded only 23% conversion after 36 h (Table 1, entry 1).

4454
C. S. Few et al. / Tetrahedron Letters 55 (2014) 4452–4454
OH
O
Substrate scope
Results of the oxidation of several alcohols can be found in
Table 2. Except in one case, high conversion percentages were
achieved within one hour at room temperature and open to air.
In addition to very effectively oxidizing primary benzylic alcohols
to the benzaldehydes (Table 2, entries 1–3), our method converts
secondary alcohols—aliphatic, benzylic, and allylic—cleanly and
quickly to the corresponding ketones (Table 2, entries 4–7). As
mentioned above, olefins remain unreacted during alcohol oxida-
tions (Table 2, entries 7–8). Also, primary aliphatic alcohols are
converted to the carboxylic acids (Table 2, entries 8 and 9). To con-
vert 4-chlorobenzyl alcohol (1b, Table 2, entries 2 and 3) to 4-chlo-
robenzoic acid (3b), 2.2 equiv of periodic acid and a 5 h reaction
time were used, indicating that the extent of oxidation can be con-
trolled. Product identification was confirmed either by GC reten-
tion time compared to purchased compounds or by 1H and 13C
NMR compared to literature values.7
1a
2a
-
IO4
(MagtrieveTM
CrO2
Cr(VI)
Cr(III)
)
-
IO3
-
-
IO3
IO4
Conclusions
Scheme 2. Proposed catalytic cycle for the oxidation of benzyl alcohol (1a) to
benzaldehyde (2a).
Magtrieve™ is an inexpensive catalyst for oxidations of alcohols
to the corresponding aldehydes, carboxylic acids, or ketones. Reac-
tions are generally complete within one hour at room temperature
exposed to air, and are selective for the oxidation of alcohols in the
presence of olefins. We believe this oxidation method surpasses
previous methods due to its convenience, chemoselectivity, time
to completion, and ease of work-up.
When 20 mol % Magtrieve™ was used, 1a was completely
converted after 15 min, but over-oxidation to benzoic acid (3a)
began to take place after an hour (Table 1, entry 5). Over-oxidation
also occurred using 10 mol % Magtrieve™ but only after 12 h.
Approximately 80% conversion to 2a occurred after 1 h
when 5 mol % Magtrieve™ was used (Table 1, entry 3), but
1 mol % proved ineffective toward the oxidation of 1a (Table 1,
entry 2). A catalyst loading of 10 mol % was used in subsequent
oxidations.
Solvent choice is an important. In fact, a combination of water
and acetonitrile (ACN) leads to the best result. We attempted oxi-
dations in toluene and dichloromethane (DCM), but achieved the
highest conversion in the least time using acetonitrile (see Table 1,
entries 9 and 10). To maximize periodic acid solubility and avoid
the precipitation of Cr(IO3)3, we added water to the reaction
mixture.5,6
Acknowledgments
This material is based upon work supported by the United
States National Science Foundation under Grant No. DMR-
0703261. Any opinions, findings, and conclusions or recommenda-
tions expressed in this material are those of the authors and do not
necessarily reflect the views of the United States National Science
Foundation. The authors would also like to thank Dr. Jon Stewart
from the University of Florida Department of Chemistry for the
use of GC–MS.
Formation of insoluble Cr(IO3)3(s) effectively blocks the alcohol
oxidation by periodate,6 and was avoided using 80:20 ACN/H2O
(Table 2, entry 1). Decreasing the water content to 8% resulted in
precipitate formation (Table 1, entry 11). The oxidation of 1a pro-
ceeded well in 60:40 ACN/H2O (Table 1, entry 12), but to ensure
solubility of less polar substrates, 80:20 ACN/H2O was used for
the remaining studies.
Hydroquinone was added in catalytic amounts (10 mol %) to
study its effect on over-oxidation, but it had no noticeable effect
on the reaction (Table 1, entry 6). An attempt to use air as the
co-oxidant failed (Table 1, entry 8), as the reaction was sluggish
for the first 3 h with air bubbling through the reaction mixture,
and when allowed to bubble overnight, the solvent had evaporated
by the next morning.
Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.tetlet.2014.06.
055.
References and notes
1. Lee, R. A.; Donald, D. S. Tetrahedron Lett. 1997, 38, 385.
2. (a) Bogdal, D.; Lukasiewicz, M.; Pielichowski, J.; Miciak, A.; Bednarz, S.
Tetrahedron 2003, 59, 649; (b) Liu, Y.-H. Synlett 2008, 1103.
3. Tojo, G.; Fernandez, M. Oxidation of Alcohols to Aldehydes and Ketones: A Guide to
Current Common Practice; Springer: New York, NY, 2006.
4. (a) Parmeggiani, C.; Cardona, F. Green Chem. 2012, 14, 547; (b) Khusnutdinov, R.
I.; Bayguzina, A. R.; Dzhemilev, U. M. J. Org. Chem. 2012, 48, 309; (c) Maurya, M.
R. Curr. Org. Chem. 2012, 16, 73; (d) Dhakshinamoorthy, A.; Alvaro, M.; Garcia, H.
Catal. Sci. Technol. 2011, 1, 856; (e) Vinod, C. P.; Wilson, K.; Lee, A. F. J. Chem.
Technol. Biotechnol. 2011, 86, 161.
5. (a) Schmieder-Van De Vondervoort, L.; Bouttemy, S.; Padron, J. M.; Le Bras, J.;
Muzart, J.; Alsters, P. L. Synlett 2002, 243; (b) Kassim, A. Y.; Sulfab, Y. Inorg. Chem.
1981, 20, 506.
Mechanistic considerations
To gain insight into an aspect of the mechanism, Magtrieve™
was reacted with excess periodic acid in water. A yellow solution
formed, and microchemical tests with Ag(I) suggested the presence
of CrO24À. Thus, it seems that CrO2 is initially oxidized to Cr(VI),
which accepts electrons from the alcohol to form Cr(III). The Cr(III)
must be reoxidized by the periodic acid to complete the catalytic
cycle (Scheme 2).
6. In their study of catalysis of periodate oxidation scatalyzed by CrO3, Zhao et al.
noted that Cr(III) species were formed during the reaction and had to be
reoxidized to Cr(VI) by periodate, which is the ultimate electron sink. Zhao, M.;
Li, J.; Song, Z.; Desmond, R.; Tschaen, D. M.; Grabowski, E. J. J.; Reider, P. J.
Tetrahedron Lett. 1998, 39, 5323.
7. 1H and 13C NMR spectra of ketone product 2f matched literature values:
Molander, G. A.; Jean-Gerard, L. J. Org. Chem. 2009, 74, 1297.