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E. Amadio et al. / Journal of Organometallic Chemistry 745-746 (2013) 115e119
Â
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Quite interestingly, in the presence of a high concentration of
PPh3 (PPh3/Pd ¼ 6/1), the new ethyl species [Pd(Et)(PPh3)3](TsO)
(V0) was detected during the catalysis. Thus, in the presence
of relatively high concentration of PPh3, catalysis might be
inhibited also because the insertion of CO into the PdeEt bond is
retarded.
PdðCOEtÞðTsOÞðPPh3Þ2 þMeOH þ PPh3/
II
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(3)
PdHðPPh3Þ3 ðTsOÞ þMP
IV
3.2.3. Reactivity of II with MeOH and PPh3, in the presence of TsOH
and subsequent admission first of ethene and then of CO (reactivity
of IV first with ethene and then with CO and with MeOH)
It is interesting to note that only the ethyl complex requires an
larger excess of PPh3 in order to give a tricoordinatedePPh3 com-
plex, which suggests that the ethyl ligand has a higher trans-in-
fluence that the hydride, propionyl and carbomethoxy ligands.
The above experiment was repeated till all II was transformed
into IV at 293 K. Then the solution was cooled to 193 K and ethene
(0.3 MPa) was admitted. At 293 K, IV starts to react with ethene to
give trans-[Pd(Et)(TsO)(PPh3)2] (V) (50% conversion after 50 min
between 293 K and 323 K). V has been already reported [30].
Starting form 303 K, the 1H signals of the ethyl ligand
broaden indicating that the protons slowly interchange. Above 323 K
the interchange is fast compared to the NMR scale, which causes the
appearance of just one signal at 0.7 ppm. At 333e343 K all IV dis-
appears and V starts to decompose with formation of some Pd metal.
The experiment was repeated till most of IV gave V at room
temperature. The solution was then cooled to 193 K, washed with
CO several times in order to take away dissolved ethene and then
pressurized with CO (0.3 MPa). At 223 K, V reacts with CO giving II0.
At 298 K II0 is converted into II, which reacts with MeOH generating
MP and IV, as found in Section 3.2.2.
3.3. Reactivity of III
Although all the above reported evidences are in favour of the
PdeH mechanism, the occurrence of the carbomethoxy mechanism
cannot be excluded. In order to ascertain whether this mechanism
plays any role, the reactivity of III has been investigated.
3.3.1. Reactivity of III with ethene in the presence of PPh3
III and 2 equivalents of PPh3 were dissolved in CD2Cl2/MeOH at
193 K. The solution was pressurized with 0.4 MPa of ethene. In
place of the signals of III, those of III0 appeared. The NMR spectrum
remained unchanged up to 313 K. Above this temperature, III0
started to convert into III, which was the only complex present for
ca. 30 min at 343 K. Note that in this experiment MP could have
been formed, but none of it was detected. In conclusion, ethene
insertion into the PdeCOOMe bond does not occur even at 343 K,
which is well above the temperature at which ethene inserts into
the PdeH bond of IV (293 K, cfr. Section 3.2.3) and catalysis occurs
(293 K, cfr. Section 3.1.3).
It can be concluded that the insertion of ethene into the PdeH
bond of IV is slow compared to the insertion of CO into the PdeEt
bond of V. However, it should be pointed out that the latter com-
plex presents an easily accessible coordination site, on the contrary
of IV in which case ethene must displace a PPh3 ligand.
On the contrary, using the diphosphino complexes [Pd(COO-
Me)(L)(PeP)](OTf) (L ¼ CO, CH3CN; PeP ¼ 1,3-bis(di-iso-butyl-
phosphino)propane and 1,3-bis(diphenylphosphino)propane), the
insertion of ethene into the PdeCOOMe bond occurs at tempera-
ture as low as 243 K under 0.1 MPa of ethene [5].
3.2.4. Reactivity of II with MeOH, ethene and CO in the presence of
TsOH but in the absence of PPh3
The experiment described in Section 3.2.2 was repeated, but in
the absence of PPh3 and admitting both CO and ethene (1/1,
0.6 MPa total pressure) at 193 K from the beginning. Upon rising the
temperature, II started to react with MeOH to give MP (1 TON in ca.
100 min between 253 and 323 K). During this time the intensity of
the signals of II lowered till to disappear, without the appearance of
signals in the hydride region. At the end of the experiment Pd metal
was visible. MP was formed in a stoichiometric amount, no catalysis
occurred even though both gases were present.
These observations suggest that after MP is formed decompo-
sition to inactive Pd metal occurs. By comparing these results with
those described just above and in Section 3.1.3, it appears that PPh3
stabilizes the hydride that is formed in the MP generating step.
3.3.2. Reactivity of III with H2O in the presence of PPh3, TsOH and
MeOH
It should be underlined that the above experiment was carried
out in the absence of TsOH and H2O, which are key components in
order to observe significant catalytic activity [19]. Hereafter, it is
shown how they modify the picture depicted in Section 3.3.1.
III, 2 equivalents of PPh3 and 6 equivalents of TsOH$H2O were
dissolved in CD2Cl2/MeOH at 193 K under argon. At this tem-
perature the 31P{1H} NMR spectrum did not show the presence of
III nor of III0, but of I0 only and of PPh3 in the ratio I0/PPh3 ¼ 1/1.
Thus, in the presence of 6 equivalents of H2O and TsOH the car-
bomethoxy group is not stable even at 193 K and reaction (1) is
practically reversed. Note that, in their absence, complex III is
stable up to 313 K (cfr. Section 3.3.1) and that, in their presence,
but under 3 bar of CO, III0 is stable up to 0 ꢂC (cfr. Section 3.1.2).
Therefore, we made this experiment to proceed further as
follows.
After cooling the solution to 193 K, CO was admitted (0.3 MPa).
At this temperature, most of I0 was converted to III0, at 273 K the
conversion is complete. Upon increasing the temperature up to
293 K in 80 min the NMR signals of III0 gradually disappear,
replaced by the signals of IV (100% conversion). The solution was
cooled again to 193 K and CO was replaced with a mixture of CO/
ethene (1/1,0.4 MPa). Upon raising the temperature up to 293 K, IV
converted partially into II and II0 and catalysis was observed as
described in Section 3.1.3.
3.2.5. Reactivity of II with MeOH, ethene and CO in the presence of
TsOH and of variable amounts of PPh3
The above experiment was repeated in the presence of variable
amounts of PPh3. Using PPh3 and II in the ratio 2/1, 4/1 and 6/1. MP
was formed at ca. 293, 298 and 303 K, respectively. Since a pre-
requisite for the formation of MP is the coordination of MeOH to the
metal [33], these observations suggest that PPh3 competes with
MeOH for the coordination, thus inhibiting the methanolysis of the
PdeCOEt moiety.
In addition, it was observed that the catalytic activity measured
in 30 min between 303 and 333 K passes through a maximum upon
increasing the PPh3/Pd ratio, being ca. 4, 6 and 1.5 mol of MP/Pd
when the ratio is 2/1, 4/1 or 6/1, respectively. Thus, the presence of
PPh3 is essential in order to observe catalysis, because it stabilizes
hydride IV and avoids decomposition to inactive Pd metal. How-
ever, an excess of ligand inhibits the catalysis. A similar PPh3 in-
fluence has been observed carrying out the catalysis in an autoclave
under standard conditions [19].
In conclusion, in the presence of H2O and TsOH, the carbome-
thoxy moiety is not stable and is converted into a hydride initiator,
as found in Sections 3.1.2 and 3.1.3.
Amadio
