(η2-alkyne)methyl(dioxo)rhenium complexes as aldehyde-olefination catalysts

Article abstract of DOI:10.1021/ja021315c

Complexes CH₃ReO₂L (L = 2-butyne, 3-hexyne, diphenylacetylene) are catalysts for the olefination of aldehydes, using 4-nitrobenzaldehyde (4-nba) as the standard aldehyde and ethyldiazoacetate (eda) as the diazo compound. Spectroscopic studies including in situ ³¹P, ¹⁷O, ¹³C, and ¹H NMR spectroscopy are used to elucidate the mechanism and the nature of the active species. One of the key steps of the mechanism is the rapid formation of phosphazine at the beginning of the cycle and its subsequent reaction with the metal dioxide complex to form the catalytically active carbene species. Copyright

Full text of DOI:10.1021/ja021315c

Published on Web 02/08/2003
(η2-Alkyne)methyl(dioxo)rhenium Complexes as Aldehyde-Olefination
Catalysts
Ana M. Santos,† Carlos C. Roma˜o,*,† and Fritz E. Ku¨hn*,‡
Instituto de Tecnologia Qu´ımica e Biolo´gica da UniVersidade NoVa de Lisboa, Quinta do Marqueˆs, EAN, Apt 127,
2781-901 Oeiras, Portugal, and Anorganisch-chemisches Institut der Technischen UniVersita¨t Mu¨nchen,
Lichtenbergstrasse 4, D-85747 Garching bei Mu¨nchen, Germany
Received October 31, 2002 ; E-mail: ccr@itqb.unl.pt; fritz.kuehn@ch.tum.de
The olefination of aldehydes and ketones is a very important
transformation in organic synthesis.1 Although the Wittig reaction
as well as its modified versions provide a highly effective and
general method, they still have several drawbacks.2 Useful orga-
nometallic stoichiometric reagents based on titanium and zinc are,
however, expensive, difficult to handle, and not completely devoid
of unwanted side reactions.3 The search for reagents that carry out
the olefination reaction in a catalytic manner has produced some
promising results with Ru,2c Rh,2c Fe,4 Mo,5 and Re complexes6 as
catalysts (eq 1). In the Re case, both complexes ReOCl3(PPh3)2
and CH3ReO3 (MTO) show a remarkable catalytic activity, but the
reaction mechanism remains speculative.7 In the MTO-catalyzed
reaction, it was proposed that the first step is oxygen atom
abstraction by PPh3 to form “CH3ReO2” (MDO). This then reacts
with eda (ethyldiazoacetate) to form a carbene complex, CH3-
ReO2(CHCO2Et), which reacts further with the aldehyde, forming
the olefin and regenerating MTO. Accordingly, Re(V) MDO
derivatives should be suitable precursors for catalytic aldehyde
olefination.
To elucidate the mechanism, spectroscopic studies were per-
formed using in situ 31P, 17O, 13C, and 1H NMR spectroscopy (see
Supporting Information). Compound 1 was selected as the catalyst
for these tests. For the NMR measurements, the amounts of catalyst
used were higher than those used in the catalytic runs, to enable
the observation of the changes of the catalytic active species during
the course of the reaction. Because the catalytic reaction involves
four different reagents (1, PPh3, eda, 4-nba), we considered it
necessary to identify possible initial reactions between sets of two
and three of the reagents involved.
As mentioned above, the olefination reaction requires both 1 and
PPh3. In the absence of 1, no olefin is formed, but a slow reaction
takes place under formation of azine and OPPh3 (eq 2).
4-O2NC6H4CHO + PPh3 +
N2CHCO2Et f 4-O2NC6H4CHdN-NdCHCO2Et +
OdPPh3 (2)
This reaction is already known to take place when PPh3, an
aldehyde, and eda are reacted in benzene under reflux conditions.5
Most important, however, is the fact that this azine does not react
with 1. Addition of 1 to a mixture of eda, 4-nba, and PPh3, which
was previously allowed to react for some hours, does not change
the NMR spectra of the organic product compounds (azine and
OPPh3) and does not lead to evolution of N2. Therefore, the reaction
presented in eq 2 can be envisaged as a side reaction with respect
to the catalytic reaction in eq 1. It consumes aldehyde and PPh3
but does not lead to olefination products and does not involve 1.
Aldehyde (4-nba) and PPh3 do not react, and no OPPh3 is formed
(31P NMR). Catalyst 1 also does not react with PPh3 (31P, 17O NMR
evidence). Catalyst 1 furthermore does not react with eda (13C, 1H,
17O NMR evidence). This is a surprising observation because the
formation of carbene complexes has been postulated upon similar
reactions between MTO and eda in olefination catalysis.6c,9 In fact,
the 13C, 1H, and 17O NMR spectra of a 1:1 mixture of eda and 1 in
This is indeed the case for adducts of MDO with acetylenes,
CH3ReO2(RtR) (R ) C6H5 (1), R ) C2H5 (2), R ) CH3 (3)).
Spectroscopic studies clarifying the mechanism and the nature of
the active species as well as a new mechanism for oxygen
abstraction by PPh3 are presented.
The catalytic activity of the Re(V) compounds 1-38a was tested,
using 4-nitrobenzaldehyde (4-nba), eda, and PPh3. The reactions
were conducted at room temperature in THF with the reaction ratios
of PPh3:eda:4-nba:catalyst 1.1:1.2:1.0:0.05.8b The reaction is clearly
catalytic because no olefination products are formed in the absence
of 1-3. Moreover, PPh3 is also necessary, and no olefination
products are formed in its absence even with 1-3 being present.
Additional catalytic runs, with new charges of substrate, lead to
the same product yields, indicating that the catalyst is stable under
catalytic conditions. Complexes 1-3 show a very similar behavior
in the catalytic runs: after 2 h of reaction time, there is no more
formation of olefin, and the yields are around 75%, the cis/trans
selectivity is ca. 5/95, and the TOFs are up to 150 mol/(mol × h).
The aldehyde conversion is in all cases 100%. The azine (4-NO2-
C6H4CHdN-NdCH(CO2Et)) is the only significant byproduct.
Excess alkyne in the catalytic reaction mixture leads to less than
one-half of the usual olefin yield. The reaction proceeds quickly,
and, in all cases, the yield after the first 5 min is over 25% of the
total yield after 2 h.
CDCl3 showed no indication of any reaction. Most notably, no 13
C
peak was observed around δ(13C) ) 300 ppm (RedC carbene
signal).10 Furthermore, no evidence was found for a metallacyclo-
propene oxide species postulated as an alternative possible inter-
mediate for aldehyde olefination with MTO-derived catalysts.7a,11
PPh3 and eda react rapidly to form the phosphazine according to
eq 3.
When PPh3 is reacted with eda at room temperature, the
instantaneous and quantitative formation of the corresponding
† Universidade Nova de Lisboa.
‡ Technische Universita¨t Mu¨nchen.
9
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J. AM. CHEM. SOC. 2003, 125, 2414-2415
10.1021/ja021315c CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1
formed at the beginning of the reaction and reacts with the metal
dioxide complex. The R-carbon atom attacks the electron-deficient
metal center, presumably setting free the η2-alkyne ligand. As the
R-carbon atom of the phosphazine interacts with the Re atom, the
electron-deficient phosphorus atom interacts with one of the terminal
oxygen atoms of the catalyst, forming a six-membered metallacycle
intermediate A. This process leads to an abstraction of one of the
terminal oxygen atoms of the Re complex and in due course to the
liberation of both OPPh3 and N2. (Using 17O-labeled 1 leads to the
formation of 17O-labeled OPPh3.) Control experiments have shown
that 1 does not exchange oxygen with any reaction partner. The
intramolecular rearrangement is very fast (vigorous liberation of
N2). 17O NMR experiments show that the intermediate compound
B has one terminal RedO bond (δ(17O) ) 676 ppm). In the absence
of aldehyde, B decomposes quickly. The carbene carbon RedC
atom is observed at δ(13C) ) 323 ppm. However, in the presence
of aldehyde, B recaptures oxygen (forming an oxo-metallacyclobu-
tane species C) and then liberates the olefin and 1 (1H, 17O NMR
evidence), thus completing the catalytic cycle. The generality of
this mechanism for other rhenium-oxo catalysts is currently being
investigated.
Acknowledgment. This work was financed by FCT and FEDER
through project POCTI 37726/QUI/01 and a postdoctoral research
fellowship (A.M.S.).
phosphazine12,13 is observed. This strongly suggests that the
formation of phosphazine precedes and triggers the catalytic
olefination reaction because this is the only fast reaction that ensues
between the reagents in the beginning of the olefination reaction.
In accordance with this hypothesis, addition of a substoichiometric
amount of 1 to a mixture of PPh3 and eda (phosphazine) results in
immediate, vigorous liberation of N2 with a significant temperature
rise and the formation of OPPh3. It can be concluded that OPPh3
is formed by the reaction of phosphazine (not PPh3) with 1. This
was confirmed by separate 17O NMR experiments with 17O labeled
1 in a stoichiometry of phosphazine:1 ) 1.2:1. A Re intermediate
with a 17O NMR signal at 676 ppm is formed. This chemical shift
indicates a compound with a terminal, not a bridging, oxygen
ligand.14 The 17O NMR signal of 1 (δ ) 731 ppm) disappears
completely. After one of the terminal oxygens is abstracted from
the catalyst without replacement (no other oxygen source such as
the aldehyde is present), the intermediate decomposes quickly (the
Re-CH3 signal disappears as well as the remaining terminal oxygen
signal), and no further phosphazine is consumed. During the
decomposition, a dark brownish precipitate is formed (EA shows
high Re and O content), while the OPPh3 signal is not significantly
increasing (31P, 17O NMR evidence). The amount of catalyst
originally present is therefore equivalent (within the measurement
error) with the formed OPPh3, meaning that only one, not two,
oxygen of 1 is transferred to the PPh3.
Supporting Information Available: Selected NMR spectra (PDF).
This material is available free of charge via the Internet at http://
pubs.acs.org.
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in Scheme 1 for the olefination reaction: Phosphazine is rapidly
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