C O M M U N I C A T I O N S
Table 2. OsO4/Oxone Promoted Oxidative Cleavagea
Scheme 2
26, which is subsequently attacked by the same to yield intermediate
27. Fragmentation of 27 regenerates OsO4 and produces two
aldehydes, which can undergo further oxidation to yield carboxylic
acids (Scheme 2). We believe that the nucleophilicity of Oxone,
and the fact that it contains an excellent leaving group (i.e.,
bisulfate) drives the reaction forward. On the other hand, as
compared to other routinely used co-oxidants such as NaIO4 and
Cr oxides, Oxone does not oxidize alcohols or diols independently.13
To highlight the utility of this reaction, the oxidative cleavage
of 2 was successfully scaled to 50 mmol (9 g), and the amount of
OsO4 required was greatly reduced from 1 to 0.02 mol % (5000
turnovers). The isolated yield of this reaction remains high at 95%
(88% after crystallization from chloroform).
During the course of our investigation we have been able to show
that a simple, mild, and efficient oxidative cleavage of olefins takes
place with OsO4 and Oxone in DMF, to provide ketones or
carboxylic acids. Modification of the reaction to deliver aldehydes
exclusively is in progress. This reaction can be considered as an
alternate to ozonolysis.
Acknowledgment. Generous support was provided in part by
Michigan State University Startup funds to B.B. and a grant from
the Michigan Economic Development Corporation (GR-183).
Supporting Information Available: Experimental procedures and
spectral data for compounds 12a-23a (PDF). This material is available
a All reactions were performed with olefin (1 equiv), Oxone (4 equiv),
and OsO4 (0.01 equiv) in DMF (0.2 M) for 3 h at rt. b GC yield. c 4 equiv
NaHCO3. d Only 2 equiv of Oxone was used.
compounds and is likely the operative route in the latter oxidation.8
1,2-Cyclohexanedione, subjected to the same reaction conditions
(without OsO4) was also oxidized to adipic acid (see Supporting
Information), thus demonstrating the lability of the R-dicarbonyl
functionality. In a similar fashion, (+)-pulegone (22) yielded the
dicarboxylic acid 22a via the intermediacy of an R-diketone.
Treatment of nootkatone (23) containing dissimilar olefins under
standard conditions furnished ketone 23a showing that selectivity
in oxidation is also obtainable. Last, alkyne 24 was subjected to
the cleavage conditions; however, it proved immune to oxidation,
and the starting material was recovered, thus indicating selectivity
for alkenes versus alkynes.
Oxone, a monopotassium peroxysulfate salt, is known to be an
effective oxidant for numerous transformations. For instance, Oxone
is well-known in the preparation of sulfones or sulfoxides from
sulfides,9 oxides of both phosphorus10 and nitrogen,11 and several
reports have shown that Oxone can also be used to oxidize
aldehydes to carboxylic acids.12 We believe that in this system
Oxone functions in three distinct oxidizing roles: (1) oxidizes the
initially formed osmate back to Os(VIII), (2) promotes the oxidative
cleavage to an intermediate aldehyde, and (3) independently
oxidizes the aldehyde to the carboxylic acid.
We are not certain as to the mechanism of the oxidative cleavage;
however, we do propose the intermediacy of an osmate ester which
undergoes the cleavage. We do not believe that 1,2-diols are
intermediates in this reaction for the following two reasons: (i)
The oxidation of olefins with the OsO4/Oxone system proceeds just
as well under anhydrous conditions, that is there is no hydrolysis
of the osmate ester. (ii) Submission of 1,2-diols such as styrene
glycol to this reaction does not yield products, and in fact starting
diol is recovered quantitatively.13 Scheme 2 depicts our proposed
mechanism, in which osmate 25 is oxidized by Oxone to furnish
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