1,5-Hexadiene selective hydroformylation reaction catalyzed with Rh(acac)2/P(OPh)3 and Rh(acac)(CO)(PPh3)/PPh3 complexes

Article abstract of DOI:10.1016/0022-328X(94)87017-9

Hydroformylation of 1,5-hexadiene, catalyzed by the system Rh(acac)2/P(OPh)3 (I), leads to the formation of mono- and dialdehydes, 4-heptenal, 2-Me-3-hexenal and octane-1,8-dial, 2,5-Me2-hexanedial respectively.The yield of dialdehydes increases with temperature and pressure and reaches 100percent at 80 deg C and 10 atm of CO/H2 = 1.The reaction catalyzed by Rh(acac)(CO)(PPh3)/PPh3 (II) system produces, besides dialdehydes, monoaldehydes with a terminal double bond, namely 6-heptenal and 2-Me-5-hexenal.The migration of the double bonds in monoaldehydes dependsmainly on the donor properties of modifying ligands.Phosphine when used as a modifying ligand (II), because of its strong donor properties, restricts isomerization (double bond migration) and diminishes yield of 4-heptenal.In contrast, when less strongly donating ligands such as phosphite are used as modifying ligands (I), independently of its steric properties, 4-heptenal is the main reaction product.Key words: Hydroformylation; Rhodium; Hexadiene; Catalysis

Full text of DOI:10.1016/0022-328X(94)87017-9

A.M. Trzeciak,J.Z Zidlkowski / Hydroformylationof L5-hexadienebyRh-complexes
109
main reaction product (81% at 60°C) (Table 2). Similar
effects of total (CO + H 2) pressure are observed for
the reaction catalyzed by the Rh(acacXCOXPPh3)/
PPh 3 system. Also in this system, lower total (CO + H z)
pressure stops the hydroformylation reaction at the
stage of monoaldehyde-6-heptenal (Table 2). With
pressure changing from 10 to 6 atm the yield of 6-
heptenal increases from 25 to 85%, but isomerization
of 6-heptenal ~ 4-heptenal does not occur.
TABLE 4. Composition of the monoaldehyde fraction of the product
of the hydroformylation reaction of 1,5-hexadiene catalyzed by
Rh(acac){P(OPh)3} 2 +L ([L]:[Rh]= 2) at 10 atm, 60°C, after 3 h
L=
4-Heptenal b 6-Heptenal a v(CO)* O
(cm-1) (deg)
PEtPh2
P(p-MeC6H4)3
PPh3
P(OEt)3
P(O-o,o-Me2C6H3)3 100
30
54
40
70
45
60
0
2066.7 140
2066.7 145
2068.9 145
2076.3 109
2083.2 190
100
0
In contrast to 6-heptenal, which is an intermediate
product and may undergo further hydroformylation,
the 4-heptenal may be treated as the side product
because in practical terms it is not reactive in hydro-
formylation. Applying the phosphine based catalytic
system (II), which is not active in 4-heptenal formation,
octanedials are obtained already at 100% yield at 60°C
whereas the phosphite based system (I) requires higher
temperature (80°C).
The selectivity factor, n/iso ratio, is practically inde-
pendent of changes of reaction parameters (tempera-
ture, CO/H 2 ratio and total pressure). For phosphite
based catalytic system (I) the n/iso factor is 2.5-3.2
whereas for the system (II) (phosphine based) it is
lower (1.8-2.1) for both mono- and di-aldehydes.
Decrease of isomerization reaction yield, caused by
reaction parameter changes, is not accompanied by
n/iso increase. Similarly, in hex-l-ene hydroformyla-
tion [2,6], with increase of n/iso parameter some in-
crease (not decrease) of hex-2-ene yield was observed.
Phosphite concentration increase causes decrease of
reaction rate only, and not changes of the final reac-
tion products. At seven-fold phosphite excess versus
the rhodium complex (at 50°C and 10 atm) 23% of
4-heptenal and 77% of octanedial were obtained (a
considerably higher percentage yield of dialdehydes
was obtained with increase of phosphite concentration).
Undoubtedly, the catalytic activity of each system
depends on the electronic and steric effects of the
ligands coordinated to the metal. To judge which domi-
nates in the phosphite system the content of the
mono-aldehydes fraction in the reaction catalyzed by
Rh(acac){P(OPh)3}2+ L (L -- PPh 3 or P(OR)3) was de-
termined. The data collected in Table 4 indicate the
prevailing electronic effect of the ligands applied and
only when the ligands have stronger donor properties
(according to Tolman scale [7]) does some 6-heptenal
appear in reaction products. This suggests that isomer-
ization was partially limited only by stronger donor
ligands. We failed to obtain the mixture of 4- and
6-heptenals in reaction catalyzed by Rh(acac)(CO)-
(PPh a) modified with P(OPh)3 or bulky P(O-2,4,6-
MeaC6H3)3. Even at [L]:[Rh] = 1, 4-heptenal was the
only monoaldehyde identified in both reaction mix-
tures. The results discussed above allow us to distin-
P(O-o-MeC6H4)3
P(OPh)3
100
100
0
0
2084.1
2085.3 130
141
* v(CO) (A1) of Ni(CO)3L in cn2c12.
a,b See Table 2.
guish some aspects of the behaviour of catalytic sys-
tems (I and II) revealed by studies using different
phosphorus ligands (e.g. PPh 3 and P(OPh)3) used for
their modification. Two catalytic systems of practically
identical content: Rh(acac){P(OPh)3}2/PPh 3 and
Rh(acac)(COXPPh3)/P(OPh)3 act differently as is
demonstrated by the reaction products. In the first
case, the presence of free phosphine causes retardation
of isomerization whereas in the second system only
4-heptenal is obtained, similarly to the system with only
the phosphite ligand.
To explain these experimental facts 31p-NMR stud-
ies were performed. The results of the measurements
were the following:
a) For the system Rh(acac){P(OPh3}2+ PPh3' re-
gardless of the PPh 3 concentration and reaction time,
only the Rh(acac){P(OPh)3}2 complex was observed (8
123.1 ppm, J(Rh-P) 307 Hz);
b) in the system Rh(acac)(CO)(PPh3)+ p(OPh)3
([P(OPh)3]: [Rh] = 1) immediately after mixing, the
bands typical for Rh(acacXCO)(PPh3) complex (8 48.6
ppm, J(Rh-P) 178 Hz) disappear and the bands, as-
signed for Rh(acac){P(OPh)3}2 appear. After a couple
of hours an equilibrium is established and three com-
plexes Rh(acac)(CO)(Pph3)' Rh(acac){P(OPh)3}2 and
Rh(acac)(CO){P(OPh)3} (8 121 ppm, J(Rh-P) 293 Hz)
are found in the solution;
c) in the system: Rh(acac)(CO)(PPh3)+ P(O-o-Me-
C6H4)3, ([P(O-o-MeC6H4)3]: [Rh] = 2), in spite of the
significant steric hindrance of this phosphite ligand
(O = 141°) it substitutes immediately for both PPh 3
and CO and the only product identified is Rh(acac){P-
(O-o-MeC6H4)a}2 (8 123 ppm, J(Rh-P) = 305 Hz).
From the above experiments, one may conclude that
any phosphite ligand used practically eliminates the
phosphine ligand from the coordination sphere of the
rhodium catalyst precursor at the very beginning of the
reaction. The rhodium catalyst precursor with coordi-

A.M. Trzeciak,J.Z Zi61kowski/Hydroformylationof 1,5-hexadiene,byRh-complexes
110
such a system can ultimately be attributed to rhodium-
phosphite species;
iii) a quite insignificant effect is observed when
rhodium phosphite base catalyst precursor is modified
with excess of free phosphine ligand. The final effect
recorded represents an average impact of the two
nated phosphite ligand is, however, not changed under
influence of excess of free phosphine. In the further
stages of the reaction, when HRh(CO)L3, an active
form of catalyst, is produced, its interaction with the
modifying ligand may play an essential role. In our
earlier studies [8] the HRh(CO){P(OPh)3}2(PPh3) com-
plex was identified as a product of reaction of HRh-
(CO){P(OPh)3}3 with a 3-fold excess of PPh 3. Tri-
phenylphosphine substitutes only one of the three co-
ordinated phosphites. The reverse reaction, substitu-
tion of PPh 3 by P(OPh)3 was studied earlier with the
1H-NMR method [4] and HRh(CO){P(OPh)3}2(PPh3)
and HRh(CO){P(OPh)3}3 were identified in solution.
These results are confirmed by 31p-NMR studies of the
solution containing HRh(CO)(PPh3)3 and 3 P(OPh)3.
The bands which originated from HRh(CO)(PPh3)3 (8
38.4 ppm, J(Rh-P) 158 Hz) disappear immediately
after mixing and next, the lines characteristic for HRh-
(CO){P(OPh)3}2(PPh3) (8 143.7 ppm, d of d, J(Rh-P)
250 Hz, 8 35.8 ppm, d of t, J(Rh-P) 150.4 Hz, J(P-P)
141.6 Hz) and for HRh(CO){P(OPh)3}3 (8 138.6 ppm,
J(Rh-P) 238 Hz) appear. Also the more bulky phos-
phite P(O-o-MeC6H4)3, used in excess, substitutes two
of the three phosphines coordinated in HRh(CO)-
(PPh3)3. The reaction product was identified by 31p_
NMR as HRh(CO){P(O-o-MeC6H4)3}2(PPh3) complex
(8 135 ppm, d of d, J(Rh-P) 252 Hz, 8 38 ppm d of t
J(Rh-P) 146 Hz). At higher excess of phosphite also
HRh(CO){P(O-o-MeC6H4)3}3 is formed (8 132 ppm,
J(Rh-P) 240 Hz).
ligands.
2.2. Hydroformylation of 1,7-octadiene
To obtain more general information about hydro-
formylation of dienes we extended our studies to hy-
droformyiation of 1,7-octadiene (Table 5). Similarly to
1,5-hexadiene, the rhodium phosphite system (I) ap-
plied in the case of 1,7-octadiene manifested higher
activity in the isomerization reaction of mono-al-
dehydes. Among the reaction products, besides dialde-
hydes, only 6-nonenal was identified (Table 5). The
isomerization reaction yield depends less on the
(CO/H 2) pressure. The extremely high yield of 6-non-
enal compared with that of 4-heptenal in the hydro-
formylation of 1,5-hexadiene must be noticed.
Small amounts of 6-nonenal appear even in the
reaction catalyzed by Rh(acacXCO(PPh3) in which iso-
merization does not normally occur. This difference
can only be caused by the reactivities of the substrates
themselves differing.
3. Experimental section
Rhodium complexes were obtained according to
methods described in the literature: Rh(acacXCO)-
(PPh 3) [9], Rh(acac){P(OPh)3}2 [10], HRh(COXPPh3)3
These results allow the following conclusions:
i) the phosphine ligand is always more easily substi-
tuted by any phosphite ligand than viceversa;
ii) at any stage of catalytic reaction with the rhodium
phosphine base catalyst precursor it may be modified
with phosphite ligand and the catalytic properties of
[11].
Toluene, 1,5-hexadiene and 1,7-octadiene were dis-
tilled before use. Hydroformylation was carried out in
a thermostated autoclave of 40 ml volume with mag-
TABLE 5. The products of hydroformylation reaction of 1,7-octadiene catalyzed by Rh(acac){P(OPh)3}z/P(OPh)3 (I) and
Rh(acacXCOXPPh3)/PPh 3 (II)
Catalytic
system
Temp.
(°C)
Pressure
(atm)
Time
(h)
Products (% mol)
8-Nonenal a
6-Nonenal e
Decanedial f
I
II
I
50
50
50
6
6
10
2.5
2
3
-
28
-
37
7
27
-
63
65
73
100
II
I
50
60
10
6
3
2
-
-
48
6
53
-
52
94
47
100
II
I
60
60
6
10
5
3
-
-
II
60
10
3
-
d Total yield of 8-nonenal + 2-Me-7-octenal.
e Total yield of 6-nonenal + 2-Me-5-octenal.
f Total yield of decane-l,10-dial +2,7-Me2-octane-l,7-dial.

A.M. Trzeciak, J.J. Zi6tkowski / Hydroformylation of 1,5-hexadiene by Rh-complexes
111
netic stirrer. The reagents were introduced to the
autoclave under a nitrogen atmosphere and it was
filled with HE/CO mixture to the required pressure.
After reaction the autoclave was cooled and samples
collected for ~H-NMR and GC-MS measurements. For
each experiment 3.2.10 -5 mol of catalyst (Rh(acac)-
(CO)(PPh3) or Rh(acac){P(OPh)a} 2) and 9.4- 10-5 mol
PPh3 or 6.8.10 -5 mol P(OPh)3 were used. In both
catalytic systems under study, Rh(acac){P(OPh)3}E/P-
(OPh)3 (I) and Rh(acac)(CO)(PPha)/PPh 3 (II), the ra-
tio [L]: [Rh] was 4.
2-Me-5-octenal: 1H-NMR 6(CHO)9.2; 6(CH=CH)
5.2 ppm; MS 122(5) 112(25) 84(24) 83(27) 82(71) 71(24)
69(39) 68(78) 67(82) 58(26) 56(24) 55(100) 41(45).
8-Nonenal: 1H-NMR 6(CHO)9.2; 8(---CH2) 4.8;
8(--CH) 5.5 ppm; MS 122(1) 111(8) 107(6) 98(17) 93(15)
81(37) 67(37) 55(100) 54(52) 41(65).
2-Me-7-octenal: 1H-NMR 6(CHO)9.2; 6(=CH2) 4.8
8(=CH) 5.5 ppm; MS 125(3)111(10) 98(11) 97(13) 82(23)
71(23) 69(42) 58(100) 55(65) 48(39).
Decane-l,10-dial: 1H-NMR 8(CHO) 9.3 ppm; MS
152(2) 109(79) 98(28) 83(39) 82(44) 81(39) 68(31) 67(94)
57(100) 55(93) 41(58).
2,7-MeE-octane-l,7-dial: 1H-NMR 8(CHO) 9.2 ppm;
MS 152(2) 109(10) 95(22) 81(23) 67(22) 58(100) 55(42)
43(23) 41(26).
1H-NMR and 31p-NMR spectra were measured on
Tesla BS 567A (100 MHz) and BS 587 A (32.35 MHz)
spectrometers respectively. GC-MS analysis was made
using a GC-MS Hewlett-Packard instrument.
3.1. Reaction product identification (1H-NMR and MS
data)
References
4-Heptenal: ~H-NMR 6(CHO)9.3; 6(CH=CH)5.2
ppm; MS 112(M+ 1.7) 94(14) 84(6) 68(100) 67(48)
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2-Me-3-hexenal: 1H-NMR 6(CHO)9.2; 8(CH--CH)
5.2 ppm; MS 112(M+ 14) 97(15) 94(9) 69(17) 55(100)
43(39) 41(36).
6-Heptenal: 1H-NMR 8(CHO)9.2; t~(=CHE) 4.8;
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68(100) 67(54) 57(34) 55(54) 41(76).
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4.8; 8(--CH) 5.5 ppm; MS 97(3) 94(7) 79(3) 68(4) 58(100)
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MS 124(3) 95(37) 81(22) 71(27) 69(41) 58(100) 57(65)
55(34) 43(55) 41(54).
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ppm; MS 122(24) 98(29) 96(21) 93(29) 83(30) 81(41)
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