C O M M U N I C A T I O N S
Table 1. Hydroformylation of Internal Alkenes and 1-Octene with Both Nonencapsulated and Encapsulated Catalyst Assembliesa
entry
olefin
template
P
CO (bar)
T (°
C)
t (h)
conv (%)
iso (%)b
1 (%)
2 (%)
3 (%)
4 (%)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
trans-2-octene
trans-2-octene
trans-2-octene
trans-2-octene
trans-2-octene
trans-2-octene
trans-3-octene
trans-3-octene
trans-3-octene
trans-3-octene
trans-3-octene
trans-3-octene
1-octene
-
10
10
10
10
10
10
10
10
10
10
10
10
10
10
5
25
25
40
40
80
80
25
25
40
40
80
80
25
25
25
25
25
25
73
73
42
42
13
13
73
73
42
42
13
13
24
24
24
24
48
48
17
32
33
63
61
88
26
45
30
60
66
82
5.9
52
27
99
18
56
0.6
1.6
0.7
1.4
15.0
16.2
0.2
1.4
0.4
2.8
6.1
6.3
2.9
1.3
1.3
1.2
1.9
0.8
0
55.6
9.4
43.8
87.8
43.0
80.1
22.4
20.9
49.3
23.3
49.7
30.2
44.4
31.7
0
0
Zn-tpp
-
0.7
0.1
1.8
5.9
21.6
0
0
0.1
0
0.6
9.2
72.1
35.3
73.5
39.0
71.7
32.7
0.5
0.3
1.2
3.4
6.1
50.5
75.4
49.6
66.5
43.0
36.1
0
55.9
15.5
53.3
35.2
0
0
0.2
0.5
5.9
16.7
25.0
63.4
25.2
59.8
26.4
66.5
Zn-tpp
-
Zn-tpp
-
Zn-tpp
-
Zn-tpp
-
Zn-tpp
-
1-octene
1-octene
1-octene
1-octene
Zn-tpp
-
0
0
0
0
0
0
0
0
Zn-tpp
-
5
15
15
1-octene
Zn-tpp
0
0
a [Rh(acac)(CO)2] ) 0.70 mmol/L in toluene, PH ) 10 bar, substrate/rhodium ) 1052, [phosphorus] ) 6.4 mmol/L. b Iso denotes the total amount of
2
isomerization based on the total product distribution.
an increased conversion and a high selectivity for the formation of
2-methyloctanal in the hydroformylation of 1-octene (entries 13
References
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and 14). Experiments using various partial CO pressures (entries
13-18) indicate a negative order in CO. Interestingly, the regi-
oselectivity was retained, and the isomerization is low in all
experiments. Control experiments in which the partial hydrogen
pressure was varied showed that the reaction is zero order in
hydrogen, and that the regioselectivity and chemoselectivity were
preserved (Supporting Information).
These data suggest that, in line with literature,9 the rate-
determining step in the reaction mechanism of the hydroformylation
is either alkene coordination or migratory insertion of the hydride
to the rhodium-alkene. As the isomerization is suppressed at room
temperature, this step is likely irreversible, which is commonly
observed at low temperatures and high CO pressure.9 Since after
this point all steps are either fast or irreversible, the regioselectivity
is determined early in the cycle. Since, after alkene coordination,
still both isomers can be formed, we propose that the regioselectivity
induced by the encapsulated catalysts is determined during the
hydride migration step. During this migration, the substrate has to
rotate,10 and the steric restrictions of the metal-olefin complex
imposed by the innerside of the capsule reduce rotational free-
dom.
In conclusion, we have shown that transition metal encapsulation
using a straightforward templated ligand approach results in the
formation of high-precision catalysts. In the current example, the
catalyst shows unprecedented high selectivity in the rhodium-
catalyzed hydroformylation of internal alkenes, forming predomi-
nantly one of the branched aldehydes. In analogy to enzymes, the
cavity formed around the active site is of crucial importance. Likely,
it reduces the rotation possibilities required for the hydride migration
to the coordinated alkene. This is the first catalyst system that
is able to discriminate between carbon atoms C3 and C4 in trans-
3-octene, and the current findings can lead to new routes in
organic synthesis. The next logical step we are exploring is the
use of chiral capsules to produce the branched aldehydes in
enantiopure form.
(3) (a) Leung, D. H.; Fiedler, D.; Bergman, R. G.; Raymond, K. N. Angew.
Chem., Int. Ed. 2004, 43, 963-966. (b) Fiedler, D.; Bergman, R. G.;
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(5) The complete characterization of these encapsulated transition metal
complexes has already been reported (refs 4a and 4b).
(6) Catalysts for the hydroformylation of internal olefins to terminal alde-
hydes: (a) van der Veen, L. A.; Kamer, P. C. J.; van Leeuwen, P. W. N.
M. Angew. Chem., Int. Ed. 1999, 38, 336-338. (b) Beller, M.; Zimmer-
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(7) (a) Breit, B.; Fuchs, E. Chem. Commun. 2004, 694-695. (b) Shirakawa,
S.; Shimizu, S.; Sasaki, Y. New J. Chem. 2001, 25, 777-779.
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Acknowledgment. We kindly acknowledge NWO-CW for
financial support.
(9) van Leeuwen, P. W. N. M. Homogeneous Catalysis: Understanding the
Art; Kluwer Academic Publishers: Dordrecht, The Netherlands, 2004.
(10) (a) Decker, S. A.; Cundari, T. R. J. Organomet. Chem. 2001, 635, 132-
141. (b) Decker, S. A.; Cundari, T. R. Organometallics 2001, 20,
2827-2841.
Supporting Information Available: Experimental details and
hydroformylation of other internal olefins. This material is available
JA063294I
9
J. AM. CHEM. SOC. VOL. 128, NO. 35, 2006 11345