6802
V. Martin, S. Blakey / Tetrahedron Letters 49 (2008) 6800–6803
Ph
iPr
PPh3
iPr
Ph
N
O
NtBu
Os
+
9
O
tBuN
O
toluene, r.t.
iPr
iPr
iPr
iPr
4a
12
10, 40%
NtBu
NtBu
Os
H
O
Ar
O
Os
8
tBuN
tBuN
Ar
Ph
O
N
Ph
O
N
Ph
Ph
11
9
Scheme 3. Proposed mechanism for Os(VIII) alkylidene reaction with a nitrone.
decomposition reaction have yet to be identified. To date, our
attempts to purify alkylidene 7 have only accelerated the decay.
The complex is sensitive to chromatography (even under inert
conditions in a glove box), and the slow decay has prevented us
from growing crystals for structural characterization.
ing that these species may be useful for C–C bond formation via
[3+2] reaction with olefins, we have yet to observe this experimen-
tally. In our hands, alkylidene decay to release the corresponding
aldehyde is too rapid. However, the alkylidenes do undergo [3+2]
reaction with reactive nitrones, leading to C–C bond formation,
and ultimately to olefin synthesis.
Despite these limitations, we have explored the parameters that
facilitate the formation of this unprecedented high-oxidation state
alkylidene. Use of the sterically encumbered trisimidooxoosmi-
um(VIII) complexes 5a and 5b severely retards the reaction with
the phosphorous ylide, and no alkylidene formation is observed.
At the other extreme, use of less sterically demanding substituents
on the imido ligands in the bisimidobisoxo series, or on the phos-
phorous ylide itself leads to very rapid ylide oxidation. Again, no
Os(VIII) alkylidene species are observed. To date, we have observed
Os(VIII) alkylidene formation using sterically demanding
bisimidobisoxoosmium(VIII) species reacting with sterically
demanding phosphorous ylides, as outlined in Table 1. In all cases,
the alkylidene is readily identified by 1H NMR, and is accompanied
by initial simultaneous formation of aldehyde, corresponding to
the starting phosphorous ylide.
Acknowledgments
We thank Syed Hussaini for performing preliminary experimen-
tal investigations. We are grateful to Emory University College of
Arts and Sciences for financial support.
Supplementary data
Synthetic procedures and spectral data for new compounds.
Supplementary data associated with this article can be found, in
References and notes
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