Mechanism of Acetal Formation
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synthesized according to literature report [15]. The
4-(diphenylphosphino)-DL-phenylglycine (1) was synthe-
sized by a published method [16]. Other reagents were
obtained from commercial sources. The hydroformylation–
acetalization was carried out in a homemade stainless steel
autoclave with magnetic stirring under an argon atmo-
sphere using standard Schlenk techniques. The solvents
and reagents were rigorously deoxygenated prior to use.
The n-nonanal was distilled to remove the n-nonoic acid
formed from oxidation of n-nonanal prior to use. The
conversion and selectivity were determined by GC with an
OV-101 capillary column. The products have been identi-
fied by GC–MS (Agilent 6890/5973 GC–MS apparatus
with a DB-35MS capillary column).
Although the effects of different Rh precursors on the
selectivity of acetals in tandem hydroformylation–acetal-
ization have been studed [11, 12], the mechanism of acetal
formation is still not fully clear. Therefore, it is necessary
to further determine the mechanisms of acetal formation in
the presence of different Rh precursors.
To begin with, we evaluated the hydroformylation and
acetalization efficiency in the one-pot hydroformylation–
acetalization of 1-octene using different Rh precursors with
triphenylphosphine (PPh3) as the model ligand (Table 1).
The conversion of 1-octene and Soxo indicate the hydro-
formylation efficiency, while Eace indicates the acetaliza-
tion efficiency. The homogeneous hydroformylation
reaction was performed at 80 °C under 5.0 MPa 1:1 CO/H2
in methanol.
2.2 General Procedure for Hydroformylation–
Acetalization of 1-Octene Using Different
Rhodium Catalyst Precursors in MeOH
It can be seen from Table 1 that the excellent hydrofor-
mylation efficiency with high conversion of 1-octene and
selectivity for oxo products was obtained using [Rh(COD)2]
BF4 and [Rh(COD)Cl]2 precursors (Table 1, entries 1 and 2),
and the acetalization efficiency reaches 32 % when using the
ionic Rh precursor [Rh(COD)2]BF4 and 87 % when using the
non-ionic Rh precursor [Rh(COD)Cl]2, indicating that the
acetalization reaction is slightly slower than hydroformyla-
tion. Interestingly, for RhCl3Á3H2O, despite the high acetal-
ization efficiency (87 %), the conversion of 1-octene is only
12 % within 2 h (Table 1, entry 3) and 1-octene converted
nearly complete only until 4 h (Table 1, entry 4). This
observation suggests that when using RhCl3Á3H2O as the Rh
precursor, the formation of the catalytically active species is
remarkably inhibited. Note that using the chlorine-free
Rh(acac)(CO)2 as the precursor (Table 1, entry 5) resulted in
high aldehyde selectivity with almost no formation of acetal
(about 1 %). Even after prolonged reaction time of 10 h, the
acetalization efficiency reached only 57 % (Table 1, entry 6),
indicating that acetalization is much slower than hydrofor-
mylation when using Rh(acac)(CO)2 as the precursor.
Rhodium catalyst precursors (3.87 9 10-3 mmol), phos-
phine ligands (3.87 9 10-2 mmol), 1-octene (0.6 mL,
3.82 mmol), internal standard (cyclohexane, 0.1 mL) and
MeOH were transferred into a stainless steel autoclave.
Then the reactor was pressurized with syngas (H2/CO) to
5.0 MPa, and the reaction system was heated to 80 °C.
After 2–10 h the autoclave was rapidly cooled with ice, and
the conversion and selectivity were analysed by GC.
2.3 General Procedure for Acetalization of n-Nonanal
in MeOH
Rhodium catalyst precursors (3.87 9 10-3 mmol), phos-
phine ligands (3.87 9 10-2 mmol), n-nonanal (3.87 mmol)
and MeOH were transferred into a stainless steel autoclave.
Then the reactor was pressurized with syngas (H2/CO) to
5.0 MPa, and the reaction system was heated to 80 °C.
After 2 h the autoclave was rapidly cooled with ice and the
conversion was analysed by GC.
According to the hydroformylation mechanism and
through analysing the above experimental results, we
believe that the acetalization under hydroformylation
condition is catalyzed by the Brønsted acid that formed
in situ during the formation of catalytically active species.
For [Rh(COD)2]BF4 and [Rh(COD)Cl]2 precursors, the
formation of catalytically active species is accompanied by
the generation of equimolar HBF4 (Scheme 1, (1)) [14] and
HCl (Scheme 1, (2)), whereas using RhCl3Á3H2O gives a
three-fold amount of HCl (Scheme 1, (3) and (4)).
According to the Wilkinson mechanism [19], in the
reversible reaction that forms catalytically active species
and H?, higher H? concentration will inhibit the forma-
tion of catalytically active species, which explains the
reduced hydroformylation efficiency (Table 1, entry 3)
when using RhCl3Á3H2O as the catalyst precursor. Besides,
the formation of catalytically active species from the
3 Results and Discussion
Recently, several acid-free Rh-catalyzed tandem hydro-
formylation–acetalization reactions have been reported
[11–14], and some transition metal complexes have suc-
cessfully been used for catalyzing the acetalization reac-
tions [17], however, these reported systems cannot rule out
the possibility of Brønsted acid catalysis, since H? formed
in situ may exist according to hydroformylation mecha-
nism. We are interested in whether Rh-phosphine com-
plexes can also catalyze the transformation of aldehyde
into acetal under Rh-catalyzed hydroformylation condition
without acid co-catalysts.
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