Mo(CO)6-promoted facile deoxygenation of α,β-epoxy ketones and α,β-epoxy esters

Article abstract of DOI:10.1016/S0040-4039(03)00231-4

Deoxygenation of α,β-epoxy ketones and α,β-epoxy esters is accomplished in high yields under mild and neutral conditions by the use of Mo(CO)₆.

Full text of DOI:10.1016/S0040-4039(03)00231-4

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 44 (2003) 2355–2357
Mo(CO)6-promoted facile deoxygenation of a,b-epoxy ketones
and a,b-epoxy esters
Asit Patra, Mausumi Bandyopadhyay and Dipakranjan Mal*
Department of Chemistry, Indian Institute of Technology, Kharagpur 721 302, India
Received 27 November 2002; revised 14 January 2003; accepted 24 January 2003
Abstract—Deoxygenation of a,b-epoxy ketones and a,b-epoxy esters is accomplished in high yields under mild and neutral
conditions by the use of Mo(CO)6. © 2003 Published by Elsevier Science Ltd.
Deoxygenation of epoxides is an important synthetic
transformation in organic chemistry. It can be achieved
by a number of methods, which have been comprehen-
sively reviewed by Larock.1 The scope of the reagents
are limited in nature as few functional groups can be
tolerated and the reaction conditions are harsh. In
certain cases, the reagents required are expensive.
Methods known for the deoxygenation of a,b-epoxyke-
tones and a,b-epoxy esters are scanty in the literature.
These include Cr(OAc)2,2 sodium cyclopentadienyldi-
products through thermal rearrangement of epoxides,
we had the opportunity to investigate the reactivity of
indenone epoxides towards metal carbonyls. This study
has yielded the finding that deoxygenation of a,b-epoxy
ketones and a,b-epoxy esters can be smoothly per-
formed with the use of an equimolar amount of
Mo(CO)6 in benzene or toluene under mild conditions.
The epoxides studied were prepared by epoxidation of
the corresponding unsaturated ketones and esters using
30% H2O2 in the presence of sodium ethoxide,11 and
submitted for deoxygenation under the conditions
described below. The results are presented in Table 1. A
variety of functional groups tolerate the reaction. These
are ketones, esters, sulfones and olefins. The deoxy-
genation of epoxy esters is stereoselective. The first
example examined was epoxy furoindanone (entry 1)
which underwent smooth deoxygenation to yield the
corresponding indenone in an excellent yield. The epox-
ide derived from indenone, when submitted to similar
conditions, furnished a small amount of indenone along
with a large amount of polymeric material which might
have arisen due to the instability of indenone. With
other epoxy ketones (entries 2–5), the reactions gave the
desired products in high yields. This method is equally
applicable to epoxy esters (entries 6–7). The most strik-
ing feature of the reaction, in addition to its mildness, is
its E stereoselectivity. It may be noted that there are
very few deoxygenation methods which are stereoselec-
tive.9,12 When a 6:4 mixture of trans- and cis-ethyl
a,b-epoxy-b-phenylpropionate was subjected to the
reaction with Mo(CO)6, trans ethyl cinnamate was
obtained as the sole product. Similar was the result
with a trans sulfone substituted epoxy ester (entry 6)
and the acetate (entry 8). The cis isomers of the product
carbonylferrate,3
dimethyl
diazomalonate,4
and
Cp2TiCl2–Zn.5 Recently, thiourea dioxide6 has been
introduced as a new reagent for deoxygenation of a,b-
epoxy ketones which necessitates alkaline THF as reac-
tion medium. Alternatively, Fe(CO)5,7 among metal
carbonyls, has been reported to be useful under neutral
conditions. However, its success has been restricted to
the use of two uncommon solvents namely, tetra-
methylurea and N,N-dimethylacetamide. The reactions
fail in refluxing tetrahydrofuran as the medium. In an
earlier occasion, the reactivity of Mo(CO)6 towards
various substituted epoxides in 1,2-dimethoxyethane
was thoroughly examined by Alper et al.8 The general
trend with regard to product formation was rearrange-
ment to the corresponding aldehydes. Tungsten hexa-
carbonyl was found to be an ineffective catalyst for
such reactions. Most recently, samarium iodide has
been reported to effect E-selective deoxygenation of
a,b-epoxy esters in high yields.9
In connection with our on-going research program on
the total synthesis10 of fused ring isocoumarin natural
Keywords: molybdenum hexacarbonyl; deoxygenation; epoxides;
enones.
* Corresponding author.
1
were not traceable by H NMR.
0040-4039/03/$ - see front matter © 2003 Published by Elsevier Science Ltd.
doi:10.1016/S0040-4039(03)00231-4

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A. Patra et al. / Tetrahedron Letters 44 (2003) 2355–2357
Table 1. Deoxygenation of epoxides with Mo(CO)6
It appears that the deoxygenation becomes successful if
the epoxides contain a p-conjugative group like car-
bonyl, ester and aryl. For the cis epoxy-diacetate (entry
10), the reaction with Mo(CO)6 afforded an inseparable
mixture of products, which did not contain any of the
expected deoxygenation product. For trans stilbene
oxide (entry 9), the major product was trans stilbene
(84%), accompanied by diphenylacetaldehyde (14%) as
the minor component. When the same reaction was
performed in 1,2-dimethoxyethane,8 the product distri-

A. Patra et al. / Tetrahedron Letters 44 (2003) 2355–2357
2357
(1)
bution was entirely the opposite, diphenylacetaldehyde
was formed in 66% yield and trans stilbene in 28% yield.
Acknowledgements
This work was supported by a grant from CSIR, New
Delhi. A.P. and M.B. are thankful to CSIR, New Delhi
for their research fellowships.
Of many plausible mechanisms by which the deoxygena-
tion can take place, the one with a simple formalism is
presented below. The reaction is probably initiated by
loss of CO, followed by complexation of the epoxide
oxygen to the Mo center of Mo(CO)6 forming the
complex A (Eq. (1)), which subsequently collapses to
form the olefin. The greenish-black residue of the reac-
tion, as judged by its IR spectrum, appears to contain
an arene complex of a molybdenum oxo species.
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In conclusion, we have demonstrated that molybdenum
hexacarbonyl is an efficient reagent for deoxygenation of
a variety of epoxides under neutral and mild conditions.
Further studies on solvent effects and scope of the
reaction are in progress.
Typical experimental procedure: To a well-stirred solution
of an epoxy enone or epoxy ester (2.0 mmol) in benzene
or toluene (5.0 mL) is added molybdenum hexacarbonyl
(2.0 mmol). The mixture is heated at reflux till no starting
material is detected by TLC. The resulting dark colored
mixture is concentrated and the residue is chro-
matographed on silica gel using mixtures of ethyl acetate-
petroleum ether as eluents. The products are
11. Patra, A.; Bandyopadhyay, M.; Ghorai, S. K.; Mal, D.,
submitted.
1
12. Furstner, A.; Hupperts, A. J. Am. Chem. Soc. 1995, 117,
4468.
characterized by comparison of IR and H NMR data
with those of authentic materials.