Metathesis and metallotropy: A versatile combination for the synthesis of oligoenynes

Article abstract of DOI:10.1021/ja057153c

We have demonstrated that the combined use of enyne metathesis and metallotropic [1,3]-shift of the corresponding alkynyl ruthenium carbenes is a powerful synthetic tool to construct oligoenynes. In this reaction, alkynyl carbene intermediates formed from an initial ring-closing metathesis reaction (RCM) undergo repetitive [1,3]-shifts and RCMs to give the final products. Linear poly-1,3-diynes containing repeating functionality of the type -[XCH₂CCCCCH₂]_n- generated long-chain conjugated oligoenynes up to n = 5. Copyright

Full text of DOI:10.1021/ja057153c

Published on Web 12/06/2005
Metathesis and Metallotropy: A Versatile Combination for the Synthesis of
Oligoenynes
Mansuk Kim and Daesung Lee*
Department of Chemistry, UniVersity of Wisconsin, Madison, Wisconsin 53706
Received October 20, 2005; E-mail: dlee@chem.wisc.edu
Scheme 1. Sequential Metathesis and Metallotropy (M&M)
The bond reorganization processes, defined as metallotropic shift,
of various alkynyl carbene complexes with Rh,1 Mn,2 Re,3 Cr,4
Mo,4 and W4 metals have been reported. The rearrangement
involving Rh, Cr, Mo, and W is a [1,3]-shift (eq 1), while that
with Mn and Re is formally defined as a [1,1.5]-shift (eq 2).
However, the metallotropic shift of ruthenium alkynyl carbene
complexes has not been observed until recently.5 Because the
metallotropic [1,3]-shift of a transient ruthenium carbene complex
was involved in the enyne ring-closing metathesis (RCM) of diyne-
containing substrates (eq 3),6-8 we surmised that these two bond
Table 1. Enyne Metathesis and Metallotropic [1,3]-Shifta
reorganization processes may share a close mechanistic relationship.
Also, due to their tandem bond-forming and -breaking nature, both
processes can be incorporated into a synthetic design in multiple
combinations such that an efficient access to remarkably complex
networks can arise. The prerequisite for this supposition is that the
equilibrium composition or the reactivity of the equilibrating alkynyl
ruthenium carbene species in eq 1 or 3 can be controlled to allow
only one of the regioisomeric carbene complexes to react selectively.
In this vein, the controlled [1,3]-shift behavior of an alkylidene
derived from ruthenium complex 19 depending on the substituent
is highly instrumental (eq 3).10 On the basis of this observation,
we envisioned that, if properly designed, an alternating metallotropic
[1,3]-shift and enyne RCM could produce a single propagating
carbene species that would travel along a linear poly-1,3-diyne
(-[XCH2CCCCCH2]n-), generating an enediyne and π-conjugated
oligoenynes (Scheme 1).11 In this sequential enyne metathesis and
metallotropic [1,3]-shift (M&M) event, the overall catalytic process
initiated from one of the terminal alkenes would relay the metal
carbene to the proximate alkyne.12,13 Suppressed by strain from
effecting a RCM reaction, the newly formed alkynyl carbene would
undergo a [1,3]-shift, creating a new carbene effectively positioned
for a facile metathetical closure to a five-membered ring. Herein
we report one-step construction14 of enediynes and oligoenynes
enabled by the uniquely controlled repetitive metallotropic [1,3]-
shift of ruthenium carbene species and its RCM.
a With catalyst 1 (5 mol %) in CH2Cl2 (0.02 M) under reflux for 4-8 h.
b Isolated yield. c Yield under ethylene atmosphere.
of 2a with 1 (5 mol %) in CH2Cl2 (40 °C) provided the desired
product in 86% yield. The presence of ethylene during the reaction
was found to be deleterious, which is contrary to the well-known
beneficial ethylene effect in normal enyne metathesis.16 It is also
First, the planned M&M sequence was examined in a general
sense with the lowest homologue 2a (Table 1, entry 1).15 Treatment
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C O M M U N I C A T I O N S
that the electronic and geometrical constraints for both processes
are similar, yet further study is necessary to provide a more accurate
mechanistic picture for the latter.21 The reaction mechanism and
the utility of this bond-forming process will be further explored.
Acknowledgment. We thank NSF (CHE-0401783) for financial
support of this work as well as the NSF and NIH for NMR and
mass spectrometry instrumentation. We also thank Prof. Wender
for helpful discussions and Dr. Guzei for the X-ray structure
determination of compound 3c.
Figure 1. X-ray structure of 3c.
Scheme 2. M&M with an Alkyne-Terminating Group
Supporting Information Available: General procedures and
characterization of represented compounds. This material is available
free of charge via the Internet at http://pubs.acs.org.
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In conclusion, we have demonstrated that the combined use of
enyne metathesis and metallotropic [1,3]-shift of alkynyl ruthenium
carbenes is a powerful synthetic tool to construct enediynes and
oligoenynes. The facile streamlining of the enyne metathesis and
metallotropic [1,3]-shift of alkynyl ruthenium carbene may indicate
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