10.1002/cctc.201901988
ChemCatChem
FULL PAPER
[Pd-H]+ species,[6] was observed initially while the temperature
was increased to 50 C. As the temperature further increasing
reaction over PdCl2(MeCN)2/Xantphos-Al(OTf)3 system, there
was nearly no change for the yield of the diester in comparison to
the result obtained with phenylacetylene and methanol containing
trace of water (Entries 2 vs 1), which ruled out the possibility of
water as the potential hydride-source. In addition, it was found
that the use of CD3OD instead of CH3OH corresponded to the
comparable yield of the target diester (74%) (Entries 3 vs 2). The
1H NMR spectra of the non-deuterated diester and deuterated
diester (Figure 3) also showed that in the deuterated diester, the
peak area of Da and Db reduced nearly unobservable, which
implied that Ha and Hb in non-deuterated diester came from
methanol rather than other H-donors like H2O in this bis-
methoxycarbonylation. These control experiments were the
supportive evidences that Al(OTf)3 really behaved as an
independent catalyst to polarize MeOH (due to its oxophilicity) to
release proton (H+ + Pd0 → [PdII-H]+), which was similar to the
observations in Al(OTf)3-catalyzed cyclization of alkenols or the
ring opening reactions of epoxides].[11, 20]
o
from 50 to 120 oC, this absorption peak gradually grew and then
reached the maximum intensity at 120 oC. Concurrently the
carbonyl vibration at 1730 cm-1 assigned to the target product of
dimethyl 2-phenylsuccinate (the standard FT-IR spectrum was
given in S. Figure 2 of SI) was observed with steadily increased
intensity, accompanied by the universal co-existence of α,β-
unsaturated esters (methyl 2-phenylacrylate and methyl
cinnamate, 1722 cm-1) (the standard FT-IR spectra were given
in S. Figure 3 and S. Figure 4 of SI). To further confirmed the
adsorption band at ca. 1944 cm-1 as observed in Figure 2-A was
truly ascribed to the stabilized [Pd–H]+ species and CH3OH was
the hydride-source, CD3OD was used instead of CH3OH in the
evolving processes of PdCl2(MeCN)2/Xantphos-Al(OTf)3 under
same conditions (Figure 2-B). Since the vibration frequency is
inversely proportional to the square root of the mass, the vibration
frequency of Pd-D bond is theoretically expected to redshift to ca.
~ 1370 cm-1 when D replaces H in [Pd-H]+ species (Pd-H 1944 cm-
1).[18] As observed in Figure 2-B, the absorption bands of [Pd-H]+
species at 1944 cm-1 completely disappeared whereas the
evolving vibrations at 1363 cm-1 were observed concurrently. The
latter was consistent to the theoretical vibration frequency of Pd-
D bond (Pd-D ~1370 cm-1). The simultaneous absence of the
peaks of 1944 cm-1 and the concurrent presence of 1363 cm-1
(Figure 2-B, Pd-D) with the involvement of CD3OD supportively
verified that the observed vibration at 1944 cm-1 was indeed
attributed to the formation of [Pd–H]+ species and that methanol
truly participated its formation as a hydride-donor. In comparison,
as for PdCl2(MeCN)2/Xantphos system without the involvement of
Al(OTf)3 (Figure 2-C), the continuous derivation of the [Pd–H]+
species with characteristic peak at 1944 cm-1 was barely
observed during the overall monitoring process, indicating the
transient [Pd–H]+ species in the Xantphos-PdCl2(MeCN)2 system
was barely detectable by the FT-IR spectroscopic technique.
Meanwhile, only methyl 2-phenylacrylate and methyl cinnamate
Table 3. The control experiments for bis-methoxycarbonylation using
CH3OH and CD3OD.[a]
Conv.
(%)[b]
Sel.L-ester
(%)[b]
Sel.B-ester
(%)[b]
Sel.diester
(%)[b]
Entry
Alcohol
1
MeOH
MeOH
CD3OD
100
100
100
18
20
26
0
0
0
82
80
74
2[c]
3[d]
[a] Standard reaction conditions: PdCl2(MeCN)2 0.05 mmol (Pd 1 mol%),
Xantphos 0.05 mmol (P/Pd = 2 molar ratio), phenylacetylene 5 mmol, MeOH
3 mL, Al(OTf)3 0.1 mmol (2 mol%), CO 1 MPa, 120 oC, 20 h. [b] Determined
by GC. Sel.L-ester represents selectivity to linear methyl cinnamate. Sel.B-ester
represents selectivity to branched methyl 2-phenylacrylate. Sel.diester
represents selectivity to dimethyl 2-phenylsuccinate. [c] Phenylacetylene
was dried by anhydrous sodium sulfate, methanol was dried by 3A
molecular sieves. [d] Phenylacetylene was dried by anhydrous sodium
sulfate.
derived
from
the
first-step
methoxycarbonylation
of
phenylacetylene was found with carbonyl characteristic vibration
at ca. 1723 cm-1. The obtained observations from in situ high-
pressure FT-IR analysis revealed the following facts: (1) The
stability of the generated [Pd-H]+ species, the active catalytic
species of the bis-methoxycarbonylation, could be facilitated by
the presence of Al(OTf)3 as the co-catalyst (Figure 1 vs Figure 3);
(2) Only the stabilized active [Pd–H]+ species promoted by
Al(OTf)3, whose longevity was long enough to be captured by
the routine FT-IR spectroscopic technique, could inherently
guarantee the success of second-step methoxycarbonylation (of
the electron-deficient α, β-unsaturated esters) to afford the target
diester; (3) Methanol upon activation by Al(OTf)3 through Lewis
acid-base pair interaction could serve as an ideal hydride-source
for the formation of [Pd–H]+ species (Figure 1 and Figure 2).
Knowingly, the non-coordinating ability of OTf- confers Al(OTf)3
the Lewis super-acidity.[10] The synergetic activation of MeOH by
Al(OTf)3 upon Lewis-acid pair interaction makes MeOH be an
available hydride-source.[12, 19]
Figure 3. The 1H NMR spectra of the non-deuterated diester (A) and deuterated
diester (B).
In addition, the activation of MeOH by Al(OTf)3, the Lewis
super-acid, was clearly demonstrated in 1H/13C NMR spectra
(Figure 4). It was indicated that, in the mixture of MeOH and
Al(OTf)3, the signal of methyl’s proton low-field shifted by 0.31
ppm along with the complete disappearance of the signal of
hydroxyl’s proton, in comparison to that of MeOH itself. Likewise,
In order to further confirm that MeOH rather than water served
as the original hydride-source upon activation by Al(OTf)3, a set
of control experiments labelled by CH3OH and CD3OD were
carried out respectively under non-aqueous condition. As shown
in Table 3, when the absolutely anhydrous reactants of
phenylacetylene and methanol were applied to repeat the
5
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Guo, Wen-Di
