Oxidation by Chemical Manganese Dioxide. Part 1. Facile Oxidation of Benzylic Alcohols in Hexane

Article abstract of DOI:10.1039/a709079j

An inexpensive, stable, commercially available reagent, chemical manganese dioxide, can be used for the oxidation of a wide variety of benzylic alcohols and the C-C bond cleavage of aromatic diols in hexane under relatively mild conditions.

Full text of DOI:10.1039/a709079j

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308 J. CHEM. RESEARCH (S), 1998
J. Chem. Research (S),
1998, 308±309$
Oxidation by Chemical Manganese Dioxide. Part 1.
Facile Oxidation of Benzylic Alcohols in Hexane$
Masao Hirano,*a Sigetaka Yakabe,a Hideki Chikamori,a
James H. Clarkb and Takashi Morimotoa
aDepartment of Applied Chemistry, Faculty of Technology, Tokyo University of Agriculture
and Technology, Koganei, Tokyo 184-8588, Japan
bDepartment of Chemistry, University of York, Heslington, York YO1 5DD, UK
An inexpensive, stable, commercially available reagent, chemical manganese dioxide, can be used for the oxidation of
a wide variety of benzylic alcohols and the C0C bond cleavage of aromatic diols in hexane under relatively mild
conditions.
Recent development in organic synthesis owes its success,
in part, to the use of potent, selective oxidizing reagents
such as manganese salts.1,2 The usefulness of active
manganese dioxide (AMD),3,4 manganese(III) acetate5 and
various manganates and permanganates6 for a broad range
of synthetic reactions has been reviewed.1±6 Among the
widely used manganese reagents, AMD enjoys from a
practical point of view, remarkable advantages over others,
e.g., the reactions can be performed in aprotic solvents, viz.
under neutral and water-free conditions, being therefore
applicable to acid or base labile, or hydrolytically unstable
compounds, and the products can easily be separated from
manganese salts by ®ltration. However, it is well known that
the preparation of AMD is time-consuming and also that
the activity of AMD depends strongly on the preparation
procedures and the posttreatments of the resultant oxides.7
Scheme 1
Instead of AMD, we have used a new reagent, chemical
manganese dioxide (CMD) which has been industrially
produced mainly as a battery component8 and is now avail-
able as an inexpensive, stable laboratory reagent,9 for the
oxidation of benzylic alcohols.
fragrance and a synthetic precursor of Dopa, in quasi-
quantitative yield. It is particularly interesting that there is
no appreciable dependence of the reactivity of p-substituted
benzyl alcohols 1d±g,j,k and 1-phenylethanols 1p±s,v on
the electronic properties of the substituents, but that
oxidations of o-substituted benzylic alcohols 1b,h,n,t were
much slower than those of m- (1c,i,o,u) and p-isomers
(1d,j,p,v), suggesting that the steric environment around the
hydroxyl groups played an important role in determining
the reactivity of alcohols. A similar steric in¯uence on the
reactivity of benzylic alcohols has been reported in AMD
oxidations.10 Moreover, the CMD/MS system was useful
for cleavage of aromatic glycols 3 (Scheme 2), giving 4 in
essentially quantitative yield in each case.
Reactions were carried out simply by re¯uxing
a
heterogeneous mixture of an alcohol and CMD in hexane
under an inert atmosphere. As the reaction proceeded dark
muddy precipitates gradually appeared and adhered to the
walls of the reaction vessel, making the work-up of the
product troublesome. This phenomenon was assumed to
be due to the formation of water, eqn. (1),7 and it was
therefore considered expedient to use an absorbent to trap
water.
R1R2CH OH ‡ MnIVO2
4
In short, the CMD/MS system can be successfully used
for the oxidation of benzylic alcohols in hexane, providing a
substitute for conventional AMD procedures.3,4
R1R2CˆˆO ‡ MnIIO ‡ H2O ꢀ1†
Attempted oxidations of benzhydrol 1w with 5 equivalents
of CMD in the presence of predried molecular sieves (MS),
anhydrous Na2SO4, MgSO4, and K2CO3 ran smoothly to
100% conversion within 1 h and furnished benzophenone
2w quantitatively in every case, but the work-ups of the last
three reactions again su€ered from the formation of muddy
precipitates. On the contrary, the work-up in the reaction
with MS was accomplished without diculty.
Scheme 2
The CMD/MS system has been favourably applied to the
oxidation of benzylic alcohols 1a±x (Scheme 1), a€ording
the corresponding aldehydes or ketones in excellent (>90%)
to quantitative yields. The sul®de moiety in 1k was resistant
to CMD, and the hydrolytically labile methylenedioxy
linkage in 1l survived, giving piperonal 2l, an important
Experimental
General.ÐCommercial CMD (Wako), benzylic alcohols 1a±m,
1w±x, and glycols 3 were used as received. Molecular sieves 4A
powder (Aldrich), anhydrous Na2SO4, MgSO4, and K2CO3 were
oven-dried. 1-Phenylethanols 1n±v were prepared by reduction of
the parent acetophenones with our in situ generated alumina-
supported NaBH4 reagent.11 Hexane was dried, distilled, and stored
over molecular sieves. Mps were determined on a Yanagimoto
MP-S3 melting point apparatus and are uncorrected. Analytical
GLC was performed on a Shimadzu GC-14B instrument with a
2 m 5 mm diameter glass column packed with 5% PEG-20M
*To receive any correspondence.
$This is a Short Paper as de®ned in the Instructions for Authors,
Section 5.0 [see J. Chem. Research (S), 1998, Issue 1]; there is there-
fore no corresponding material in J. Chem. Research (M).

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J. CHEM. RESEARCH (S), 1998 309
on Chromosorb WAW-DMCS and interfaced with a Shimadzu
Chromatopac C-R6A integrator, with temperature programming.
Typical Oxidation Procedure.ÐBenzhydrol 1w (1 mmol; 0.184 g),
predried MS (0.5 g), hexane (10 ml), and CMD (5 mmol; 0.435 g)
were mixed in a 30 ml round-bottom ¯ask. After ¯ushing the ¯ask
with dry argon, the resultant mixture was vigorously stirred for 1 h
under gentle re¯ux. After cooling to room temperature, the solvent
was decanted and the residual precipitates were thoroughly washed
with dry diethyl ether by decantation (5 Â10 ml). Removal of
the combined solvent gave satisfactorily pure (1H NMR, GC, and
TLC) benzophenone 2w quantitatively (0.181 g; mp 48±49 8C, lit.12
48.5±49 8C).
2 H. O. House, Modern Synthetic Reactions, W. A. Benjamin,
California, 1972, ch. 5; J. Iqbal, B. Bhatia and N. K. Nayyar,
Chem. Rev., 1994, 94, 519; G. G. Melikyan, Synthesis, 1993,
833.
3 (a) A. J. Fatiadi, in Organic Syntheses by Oxidation with Metal
Compounds, ed. W. J. Mijs and C. R. H. I. de Jonge, Plenum
Press, New York, 1986, ch. 3; (b) A. J. Fatiadi, Synthesis, 1976,
65 (Part 1) and 133 (Part 2); (c) A. H. Haines, Methods for the
Oxidation of Organic Compounds. Alcohols, Alcohol Derivatives,
Alkyl Halides, Nitroalkanes, Alkyl Azides, Carbonyl Compounds,
Hydroxyarenes, and Aminoarenes, Academic Press, London,
1988, ch. 2, pp. 47±54.
The other alcohols and diols (1 mmol) were similarly oxidized by
using CMD (5 mmol, except for 1x, 10 mmol), for given times
(see Scheme 1) determined on the basis of their reactivities and the
product yields. Products thus obtained were known compounds
identi®ed by spectroscopic comparisons (1H NMR and IR) with
authentic commercial samples and/or physical (either mp or bp)
comparisons with literature data.12 A large-scale oxidation of 1w
(30 mmol) using CMD (150 mmol), MS (4 g), and hexane (40 ml)
was also successful, a€ording, after 1 h reaction followed by usual
work-up (ether washing, 5Â 30 ml), 2w in 98% yield.
4 M. Hudlicky, Oxidations in Organic Chemistry, American
Chemical Society, Washington, DC, 1990, pp. 32±33.
5 W. J. de Klein, in Ref. 3(a), ch. 4.
6 R. Stewart, in Ref. 1, ch. 1; Ref. 3(c), pp. 44±47; Ref. 4,
pp. 33±37.
7 R. J. Gritter, G. D. Dupre and T. J. Wallace, Nature (London),
1964, 202, 179; I. M. Goldman, J. Org. Chem., 1969, 34, 1979.
8 See Encyclopedia of Chemical Technology, Wiley, New York,
3rd edn., 1981, vol. 14, pp. 863±889.
9 See Y. Hamada, M. Shibata, T. Sugiura, S. Kato and T. Shioiri,
J. Org. Chem., 1987, 52, 1252; J. Matsubara, K. Nakao,
Y. Hamada and T. Shioiri, Tetrahedron Lett., 1992, 33, 4187;
N. Irako, Y. Hamada and T. Shioiri, Tetrahedron, 1992, 48,
7251; H. Ogawa, T. Aoyama and T. Shioiri, Synlett, 1994, 461;
Heterocycles, 1996, 42, 75.
One of us (J.H.C.) would like to thank the RAEng/
EPSRC for a Clean Technology Fellowship.
Received, 18th December 1997; Accepted, 2nd February 1998
Paper E/7/09079J
10 E. F. Pratt and J. E. Van de Castle, J. Org. Chem., 1961, 26,
2973.
11 S. Yakabe, M. Hirano, J. H. Clark and T. Morimoto, J. Chem.
Res. (S), 1998, 322.
12 Dictionary of Organic Compounds, Chapman and Hall, London,
6th edn., 1996.
References
1 W. A. Waters and J. S. Littler, in Oxidation in Organic
Chemistry, ed. K. B. Wiberg, Academic Press, New York, 1965,
Part A, ch. 3.