cross metathesis reaction of 18 with methyl acrylate was
achieved using 0.4 mol% of Caz-1+.
We developed the first type of cationic ruthenium complex
that was proven to be highly active in RCM, CM and enyne
metathesis (0.1–0.4 mol% Ru). The high activity of the
catalyst and thermal stability of the active species allowed
the full conversion of difficult substrates in only 15 minutes.
The synthetic accessibility of Caz-1+ and its thermal stability
are imputed to the presence of the phosphite ligand. This
system also led us to the isolation of a very rare example of
Ru(III) 13-electron complex. The mechanism by which complexes
are obtained remains unclear and studies aiming to shed light on
these matters are ongoing in our laboratories.
Fig. 5 Performance of Caz-1+ vs. Caz-1, Hov-II, G-II and Ind-II.
Conditions: 7 (0.25 mmol), cat. (0.1 mol%), xylene (1.5 mL), 140 1C.
was necessary to activate Caz-1+ and 80% conversion was
reached after only 10 min. Despite the previously reported
thermal stability of one cationic Ru complex by Caulton
et al.,13c the exceptional robustness of Caz-1+ was unexpected.
The reasons behind this high thermal stability are still unclear.
The unsuccessful attempts to abstract Cl from the non-phosphite
based Ind-II and Ind-III point to an important role of the P(OR)3
ligand in the complex formation and stabilisation. The exact
mechanism at play here is being examined but the necessary
thermal activation is in sharp contrast with the reactivity displayed
by the distorted trigonal pyramid complex reported by Piers et al.
that exhibits high activity at rt.15a The catalytic potential of Caz-1+
was next examined in RCM of a range of dienes and enynes and in
cross metathesis (Fig. 6).
The cationic system Caz-1+ is highly active for a wide range of
substrates in RCM as essentially quantitative yields are obtained
within 15 min using a catalyst loading of 0.1 or 0.2 mol%. Of
note is the fact that sterically hindered substrates are efficiently
ring closed leading to 5- and 6-membered rings bearing a
tetra-substituted double bond. However, the ring closure of
the tetra-substituted malonate derivative proved to be more
problematic. Caz-1+ was also tested in the enyne metathesis of
16 using 0.2 mol% of catalyst. Within 15 minutes, enyne 16
was fully converted to the desired cyclic diene 17. Finally, the
We thank the EC through the 7th framework program (grant
CP-FP 211468-2 EUMET) and the Royal Society (University
Research Fellowship to CSJC) for financial support.
Notes and references
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16 Isolation and characterisation of the intermediates not successful.
17 See spectra in ESIw.
18 Decomposition was observed when stored at À25 1C in a glovebox.
19 Despite numerous efforts higher quality crystals could not be
obtained.
Fig. 6 Catalytic performance of Caz-1+ in RCM, enyne metathesis
and CM.
c
1268 Chem. Commun., 2012, 48, 1266–1268
This journal is The Royal Society of Chemistry 2012