Tetrahedron Letters
Osmium (VI) catalyzed chemoselective oxidation of allylic
and benzylic alcohols q
Shekaraiah Devari a,b, Ramesh Deshidi a,b, Manjeet Kumar b, Arvind Kumar a,b, Simmi Sharma b,
Masood Rizvi c, Manoj Kushwaha d, Ajai Prakash Gupta d, Bhahwal Ali Shah a,b,
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a Academy of Scientific and Innovative Research, India
b Natural Product Microbes, CSIR-Indian Institute of Integrative Medicine, Canal Road, Jammu-Tawi, J&K 180001, India
c Department of Chemistry, University of Kashmir, Hazaratbal, J&K, India
d Quality Assurance and Quality Control Division, CSIR-Indian Institute of Integrative Medicine, Canal Road, Jammu-Tawi, J&K 180001, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 27 July 2013
Revised 4 September 2013
Accepted 11 September 2013
Available online 18 September 2013
A mild and highly chemoselective approach to oxidation of allylic, electron rich/deficient benzylic, and
heterocyclic alcohols employing catalytic quantities of K2[OsO2(OH)4] (3 mol %) and chloramine-T
(50 mol %) is described. The protocol offers short reaction times (25 min–2 h), controlled oxidation,
and tolerance to a variety of substrates. A systematic mechanistic study based on the LC-ESI-MS/MS
reveals the presence of imidotriooxoosmium species which further reacts with alcohol to give the oxi-
dized product.
Keywords:
Oxidation
Ó 2013 Elsevier Ltd. All rights reserved.
K2[OsO2(OH)4]
Chloramine-T
Allylic alcohols
Chemoselective
One of the key transformations in organic synthesis is the
chemoselective oxidation of alcohols to corresponding carbonyl
compounds. Over the years numerous oxidation methods such as
TEMPO,1 Swern,2 Dess-Martin periodinane,3 IBX,4 MnO2,5 etc., have
been reported. Precious metal catalysts such as Pd,6 Ru,7 Au,8 and
Pt9 have also been employed for the same, which generally involve
relatively large amounts of the catalyst and most of the time re-
sults in over oxidation. Despite many advances in the oxidation
methods, chemoselective oxidation of alcohols particularly allylic
and benzylic alcohols still remains challenging. To address this is-
sue several methods like DDQ/NaNO2,10 DDQ/(MnOAc)3,11 NBS/
thiourea,12 and vanadium complexes13 have been developed in re-
cent years. Osmium (VI), used routinely for dihydroxylation of ole-
fins,14 synthesis of triols from allyl alcohols,15 and amino
hydroxylation with chloramine-T,16 has also been employed in
the oxidation of allylic alcohols by activating it with either quinu-
clidine and copper salts17 or DABCO.18 However, the reactions are
not chemoselective for allylic/benzylic alcohols and they use large
quantities of ligands and take long time to complete i.e., >24 h. We
reasoned that if quinuclidine or DABCO can be used as ligand for
oxidation with K2[OsO2(OH)4], a chloramine-T catalyzed process
could also be established. In this Letter, we report a highly chemo-
selective oxidation of allylic and benzylic alcohols catalyzed by Os
(VI) and chloramine T at rt in neutral reaction medium and in the
presence of air. The simplicity of the protocol, short reaction times
(25 min–2 h), tolerance to a variety of substrates specially electron
deficient benzyl alcohols, and controlled oxidation are some of the
added advantages over the existing methods.
Initial investigation of Os (VI) and chloramine-T catalyzed oxi-
dation was studied using benzyl alcohol as a model substrate to
establish the reaction parameters. The reaction of benzyl alcohol
in the presence of 10 mol % K2[OsO2(OH)4] and 0.5 equiv chlora-
mine-T in tBuOH:H2O (1:1) for 45 min gave benzaldehyde in 96%
yields (Table 1, entry 1). To show the utility of the present reaction
system for the oxidation of alcohols we focussed our attention on
optimizing the reaction conditions by decreasing the loading of
expensive metal catalyst (Table 1). The lowering of catalyst,
K2[OsO2(OH)4] from 10 to 3 mol %, did not significantly alter the
reaction yields, however, though inconsequential, there was a
slight increase in reaction times (Table 1, entries 2 and 3). Never-
theless, further decreasing the amount of catalyst to 1 mol % led
to a significant drop in the yields (63%) (Table 1, entry 4) with long-
er reaction time. Increasing the amount of chloramine-T from 0.5
to 2 equiv showed no significant effect on the yield or time (Table 1,
entries 5 and 6). Thus, catalyst loading of 3 mol % with 0.5 equiv of
q
CSIR-IIIM Communication No. IIIM/1590/2013.
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Corresponding author. Tel.: +91 (191) 2569000 010; fax: +91 (191) 2569333/
2569017.
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