Thermoregulated Microemulsions
COMMUNICATION
Table 1. Conversion (C; %), Selectivity (S; %), linear/branched ratio (l/
b) for the hydroformylation of various substrates in the presence of imi-
modification of the micelleꢁs environment due to the rhodi-
um sequestration by the imidazolium charge and the partial
inclusion of the olefin in the CDꢁs cavity (DH=
À15 kcalmolÀ1 for the complex 1/a-CD).[21] The modifica-
tion of the bromide anion give similar conversions and se-
lectivities, but the l/b ratio decrease up to 2.7 in the case of
the triflate anion (see Supporting Information). This is due
to a lack of exchange between the TPPTS ligand and the tri-
flate compared with the bromide salt. The use of [C16MIM]
[Br] with or without a-CD gives a quantitative conversion
of 1 with very high selectivities. Moreover, a high regioselec-
dazolium surfactants with or without a-CD additives[a]
.
Entry
Substrate
Surfactant
a-CD[a]
C[b]
S[c]
l/b[d]
1
2
3
4
5
6
7
8
1
1
1
1
1
1
2
2
2
2
2
3
3
3
3
3
–
–
ACHTUNGTRENNUNG
À
+
À
+
3
30
49
86
80
87
96
49
89
79
85
91
87
93
94
94
89
3.1
3.6
4.0
3.9
4.6
5.2
3.3
4.9
1.4
2.0
2.3
0.3
0.3
0.3
2.1
0.2
10
78
87
41
100
74
25
51
73
47
21
38
99
69
100
ACHTUNGTRENNUNG
[e]
T
+
N
+
–
–
C
À
[e]
+
9
À
[e]
tivity is obtained with the couple a-CD/ACHTUNTRGNEUNG[C16MIM][Br] sur-
factant and the l/b ratio enhances from 4.6 to 5.2.
10
11
12
13
14
15
16
E
+
[e]
E
+
–
–
G
À
For substrate 2, the control experiment without imidazoli-
um surfactant or CD, gives a better conversion, due to the
more hydrophilic character of this olefin (entry 7). The se-
lectivity is not high (49%) and the l/b ratio is in the same
range as 1 (entries 7 and 1). In the presence of a-CD, the
conversion decreases strongly to 25% but the selectivity, as
well as the l/b ratio, increase (entries 8 and 9). We believe
that in the presence of a-CD, the formation of inclusion
complexes occurs by complexation of the allyl side chain in
the hydrophobic cavity. The encapsulation of the olefin in-
hibits the hydroformylation reaction. In the presence of
[C12MIM][Br] the conversion increases to 51%, the selectiv-
ity is good, but the l/b ratio decreases to 1.4 (entry 9). The
combination of [C12MIM][Br] and a-CD gives a better con-
version (73%) and a good selectivity (85%; entry 10), but
the l/b ratio is smaller (2.0). The higher l/b ratio is obtained
for this substrate by using [C12MIM][Br] and a-CD, even if
the conversion is smaller (entry 15).
[e]
+
À
[e]
R
+
[e]
A
+
[a] [RhACHTUNGTRENNUNG(acac)(CO)2] (3.5 mm), TPPTS (18 mm), H2O (11.5 mL), substrate
(2.91 mmol), a-CD (5 mm, except for [e]), imidazolium surfactant (8 mm),
n-undecane (4 mmol, GC internal standard), 1150 rpm, T=808C, CO/H2
1:1: 50 atm. [b] Conversion calculated with respect to the starting olefin.
[c] S: % of the converted olefins. The side product was mainly internal
olefins. [d] l/b: linear/branched ratio (GC). [e] a-CD (25 mm). In all cases
the b1/b2 ratio was smaller than 1. All reported values were the mean of
at least three runs. The standard deviation of the mean never deviated
Æ1.5 %. For the calculation methods see the Supporting Information.
As a control experiment, we performed the hydroformyla-
tion of each substrate in the presence of [Rh(CO)2ACTHNUTRGNEU(GN acac)]/
TPPTS without imidazolium surfactant and without a-CD
(Table 1). A second control experiment was realized in the
presence of imidazolium surfactant without the a-CD. For 1,
the two control experiments gave similar results: 3 and 10%
conversion and an l/b ratio of 3.1 and 3.6, respectively (en-
tries 1 and 2). The weak conversion increase in the presence
of a-CD is probably due to the formation of a weak inclu-
sion complex between the a-CD and 1. This observation is
also corroborated by the increase of the l/b ratio. In both
control experiments the selectivities are in the same range
(about 40%) due, generally, to the formation of the hydro-
genated products. The use of [C12MIM][Br] without a-CD
gives higher conversion (78%) and enhances greatly the se-
lectivity of the reaction (86%), as well the l/b ratio (4.0),
due to the constraint of the rhodium catalyst at the micelleꢁs
interface (i.e., anion exchange between the TPPTS ligand
and the bromide; DH=À36 kcalmolÀ1 for PM3 calculation
of the anionic metathesis see Supporting Information).[21]
The decrease of the conversion in the presence of a-CD
may be due to the partial complexation of the imidazolium
surfactant by a-CD which results in a decrease of the
number of micelle (DH=À45 kcalmolÀ1 for 2:1 complex be-
tween [C12MIM][Br]/a-CD, see Table S2, Supporting Infor-
mation).[21] The selectivity and the l/b ratio is the same with
or without a-CD. If the a-CD concentration is increased,
the conversion and the selectivity are in the same range, but
the l/b ratio increase to 4.6 (Table 1, entry 5). We believe
that this can be attributed to a a-CD effect, that is, the
For substrate 3, the use of [C12MIM][Br] increases the re-
action selectivity, but the combination of a-CD/ACTHNUTRGNE[UNG C12MIM]
[Br] results in the decrease of the conversion due to the
competition between the incorporation of the substrate into
the micelle and its complexation by the CD (entries 13–15).
Previously, Monflier et al. have described the biphasic hy-
droformylation of styrene by a per(2,6-di-O-methyl)-a-CD
as mass transfer promoter, where the reaction rates were
good (100% conversion after 2 h vs. 65% without CD).[22]
In our study the enhancement of the reaction rate is due to
micelle formation and styrene transfer, as in the case of the
use of CD. However, when [C16MIM][Br] is used in combi-
nation with the a-CD, a complete conversion of the olefin is
obtained but the l/b ratio is still low. The reaction may take
place in the case of the styrene, both at the micelle interface
and in the CDꢁs cavity.
To summarize the catalytic process, the imidazolium sur-
factants can exchange their bromides by the TPPTS sulfo-
nates. This exchange creates a high rhodium micro-concen-
tration in direct micelle proximity.[23] The aldehydes selectiv-
ities observed in the reaction are lower than in the case of
the previous use of the cyclodextrins as mass promoter.
When the olefin is inside the hydrophobic interior of the mi-
celle, the formation of the isomerization species leading to
internal olefins by b-hydride elimination is less prohibited
by the steric hindrance, as in the case of CDs.[24] This obser-
Chem. Eur. J. 2009, 15, 6327 – 6331
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