C O M M U N I C A T I O N S
Table 1. Cyclic Enol Ethers through a Tandem Ring-Closing
Metathesis-Olefin Isomerization Sequence
to determine the specific catalytic species responsible for the olefin
isomerization activity, as well as to introduce new ruthenium-
catalyzed tandem processes, are underway.
Acknowledgment. Studies on related isomerizations by Ale-
jandro D. Alonso are greatly appreciated. We are grateful to the
NSF (CHE-0132221) for financial support. In addition, we thank
the Merck Chemistry Council and GlaxoSmithKline for additional
funding.
Supporting Information Available: Experimental procedures and
data on new compounds (PDF). This material is available free of charge
References
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isolated yield for at least two reactions.
a 4:1 mixture of benzyl ethers 24 and 25 were obtained from either
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protocol (eq 6), the resulting enol ether 4 was generated without
loss of enantiomeric purity, indicating that the isomerization does
not proceed through the achiral enol ether 27.
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(13) Previously, such an approach has been conducted in separate steps utilizing
1 for the RCM and Wilkinson’s catalyst for the olefin isomerization.
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(14) The 95:5 nitrogen:hydrogen mixture (Forming Gas) is typically used for
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The specific ruthenium catalyst responsible for the isomerization
1
is still in question. 31P and H NMR studies were inconclusive;
treatment of 2 with either H2 or 95:5 N2:H2 on a time scale similar
to that used during the reactions (1-3 min) revealed neither a new
Ru-hydride, nor an observable change in the ruthenium-alkylidene
signal (based on an internal standard). Attempts to “preform” an
active Ru-hydride species from 2 by prolonged exposure to H2 (in
the absence of an olefin substrate) revealed a new 31P NMR signal
at 48 ppm;18 however, this pretreatment did not generate an active
isomerization catalyst. Nevertheless, with purified alkylidene 2, only
metathesis products are obtained until a small amount of H2 is
introduced into the reaction, suggesting that an active, but yet to
be characterized, ruthenium hydride is responsible for the isomer-
ization activity.
In summary, an efficient entry into cyclic enol ethers from readily
available starting materials utilizing a tandem RCM-olefin isomer-
ization protocol is presented. Given the notable range of reactions
catalyzed by ruthenium complexes, we anticipate this two-step
process will represent an early example of tandem sequences
utilizing metathesis-active ruthenium alkylidenes. Further studies
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