Highly efficient rhodium-phosphite catalyzed hydroformylation of camphene and other terpenes

Full text of DOI:10.14233/ajchem.2019.21656

Asian Journal of Chemistry; Vol. 31, No. 1 (2019), 213-219
A
SIAN
J
OURNAL OF HEMISTRY
C
https://doi.org/10.14233/ajchem.2019.21656
Highly Efficient Rhodium-Phosphite Catalyzed Hydroformylation of Camphene and otherTerpenes
1,3
NITIN S. PAGAR1,2,* and RAJ M. DESHPANDE
1Homogeneous Catalysis Division, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pune-411008, India
2Post Graduate Department of Chemistry, Sir Parshurambhau College (Affiliated to Savitribai Phule Pune University), Pune-411030, India
3SABIC Technology Centre, Bangalore-562 125, India
*Corresponding author: Tel:+91 20 24331978; E-mail: nitin.pagar@spcollegepune.ac.in, nspagar@gmail.com
Received: 14 August 2018;
Accepted: 15 October 2018;
Published online: 30 November 2018;
AJC-19184
The hydroformylation of terpenes has been studied with homogeneous Rh, Co and Pt complexes. The rhodium complex, Rh(CO)2(acac)
modified with triphenyl phosphite ligand has been found to be the best catalyst system for hydroformylation of camphene and other
terpenes. The role of ligands, solvents, Rh:P ratio and reaction conditions on the activity and selectivity towards camphene hydroformylation
have been investigated.
Keywords: Homogeneous, Hydroformylation, Camphene, Terpenes, Rhodium phosphite.
33,34], (+)-R-limonene [9,18,20,33], β-pinene [33] and carvone
[20]. Rhodium complexes modified with phosphorus ligands
usually promote the preferable formation of cis-10-formyl pinane
or corresponding acetal and allow a higher chemoselectivity
in hydroformylation of β-pinene [27]. On the other hand, it has
been reported that presence of phosphite ligands favours the
formation of trans-aldehyde, but they are not efficient in supp-
ressing substrate isomerization [30]. Hydroformylation of
camphene gave linear aldehyde (exo and endo) with both modi-
fied and unmodified rhodium catalyst systems. The addition
of phosphorus ligands favours the formation of endo isomer
(exo/endo=1/1.5), whereas the exo/endo ratio is 1/1 in unmodi-
fied systems [12]. Hydroformylation of limonene using dinuc-
lear rhodium complexes [Rh(COD)Cl]2 along with various
bulky phosphite ligands e.g. tris(2-t-butylphenyl)phosphite,
tris(2-phenylphenyl)phosphite, tris(2-t-butyl 4-methyl phenyl)-
phosphite gives very high rates (TOF = 1500-4000 h-1) as comp-
ared to Rh-TPP system (TOF ≤100) [35,36]. The dinuclear
rhodium complexes [Rh2(µ-SR)2(CO)2L2], with L = PPh3, P(OPh)3
or P(OMe)3 were found to give moderate activities for hydro-
formylation of limonene, α-, β-pinene, (-)-camphene and other
terpenes [9,20]. Hydroformylation of endocyclic double bonds
in para-menthenic terpenes such as terpinolene, γ-terpinene
and α-terpinene under mild conditions has been investigated by
da Silva et al. [37]. Aqueous biphasic and ionic liquid toluene
INTRODUCTION
Terpenes represent a renewable and useful source of inexp-
ensive olefins derived from citrus and pine oils. They are a natural
and sustainable supply of building blocks for the fine chemical
industry. Hydroformylation of these naturally occurring olefinic
monoterpenes seems to be a promising synthetic route to obtain
a wide variety of aldehydes with applications in the perfume,
flavour and pharmaceutical industries [1-6]. Optically pure ter-
penes containing prochiral centers are easily available and their
stereoselective functionalization could be useful for the produ-
ction of chiral synthetic intermediates (chiral synthones) [2].
One of the major problems in hydroformylation of these terpenes
is the prolonged reaction time and severe reaction conditions
(>100 ºC, > 5 MPa syngas pressure) resulting in the lower TOF
of the catalysts [3-6] as compared to other olefins. Thus, the
low reactivity of terpenes, their isomerization tendency and
poor regio- and stereoselectivity towards hydroformylation
products are the major challenges in hydroformylation of terpenes.
Hydroformylation of monocyclic and bicyclic monoter-
penes and terpenoids such as camphene [7-14] limonene [9,
15-26], β-pinene [9,13,27-30] and carvone [20,31,32] has been
reported using homogeneous cobalt and rhodium complex
catalysts.Alternatively platinum-tin catalytic systems have also
been investigated for the hydroformylation of camphene [12,
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214 Pagar et al.
Asian J. Chem.
biphasic hydrformylation of some terpenes has been reported
recently [38,39].
sample of liquid withdrawn, and the reaction started by switc-
hing the stirrer on. The reaction was then continued at a constant
pressure by supply of CO + H2 (1:1) from the reservoir vessel.
Since, the major products formed were aldehydes, supply of
CO + H2 at a ratio of 1:1 (as per stoicheometry) was adequate
to maintain a constant composition of CO and H2 in the auto-
clave, as introduced in the beginning. This was confirmed in a
few cases by analysis of the CO content in the gas phase at the
end of the reaction. In each run, samples were withdrawn at
regular intervals of time and analysed for reactants and products
in order to check the material balance. The reproducibility of
the experiments was found to be in a range of 5-7 %.
Analytical methods: The quantitative analysis of reactant
and hydroformylation products was carried out by an external
standard method using a gas chromatographic technique. For
this purpose, HP 6890 gas chromatograph controlled by the
HP Chemstation software and equipped with an auto sampler
unit, fitted with HP-5 capillary column (30 M × 320 µm ×
0.25 µm film thickness with a stationary phase of 5 % diphenyl
95 % dimethyl polysiloxane) and FID detector was used.
The terpenes hydroformylation products were identified
on GC-MS (Agilent 6890N with MS 5973N mass selective
detector on a similar column.). The formation of the products
was confirmed by comparison with the library fragmentation
patterns available and with a > 90 % match. The aldehyde products
obtained after hydroformylation of camphene, limonene, β-
pinene, 3-carene, γ-terpinene and carvone were isolated from
the reaction mixture by distillation under reduced pressure.
They were separated by column chromatography (silica). The
products of terpenes hydroformylation were identified using
GC-MS.
exo-3,3-Dimethyl-2-norbornaneacetaldehyde (7a)
shorter GC retention time): IR (KBr, νmax, cm-1): 1726, 2713
(CHO), 1366, 1385 (mixed dimethyl). MS (m/z rel. int.): 166/4
(M+); 133/15; 122/53; 109/31; 107/52; 97/100; 83/22; 81/32;
79/41; 69/47; 67/51; 55/40; 41/44.
endo-3,3-Dimethyl-2-norbornaneacetaldehyde (7b),
longer GC retention time): IR (KBr, νmax, cm-1): 1726, 2713
(CHO), 1366, 1385 (mixed dimethyl). MS (m/z rel. int.): 166/3;
(M+); 133/11; 122/38; 109/22; 107/39; 97/100; 83/18; 81/24;
79/31; 69/37; 67/40; 55/30; 41/35.
3-(4-Methylcyclohex-3-enyl)butanal (11): MS (m/z rel.
int.): 166/7 (M+); 148/47; 133/33; 121/25; 106/34; 91/59; 93/
100; 67/57; 55/23; 41/26.
1-Methyl-4-(propan-2-ylidene)cyclohex-1-ene (12) (isom-
erized limonene): MS (m/z rel. int.): 136/86 (M+); 121/100;
105/22; 93/85; 91/43; 79/31; 77/25.
The diastereomeric excess (d.e.) achieved for exo and endo
aldehyde product in the hydroformylation of camphene with
both rhodium and platinum complexes bearing either achiral
or chiral ligands is relatively low [33,34]. Although hydrofor-
mylation of various terpenes with different transition metals
complex catalysts has been studied previously, their rates of
hydroformylation were generally very poor [13].The most studied
terpenes were limonene and β-pinene. The highest rates were
observed for monocyclic terpene such as limonene [18,35] but for
bicyclic terpenes like β-pinene and camphene poor rates (TOF
< 100 h-1) were observed due to their steric properties.
The use of phosphite ligands for rhodium complex catalyzed
hydroformylation of terpenes has also not been investigated
in detail. In this work, we have demonstrated hydroformylation
of camphene and other terpenes using a highly efficient Rh(CO)2-
(acac)/P(OPh)3 catalyst for the first time. The effect of different
parameters on activity and selectivity in the hydroformylation
of camphene using Rh(CO)2(acac)/P(OPh)3 catalyst was studied.
EXPERIMENTAL
Rhodium(III) chloride trihydrate (RhCl3·3H2O, 40 % Rh),
cobalt(II) chloride hexahydrate (CoCl2·6H2O) and chloroplatinic
acid(H2[PtCl6]) were obtained from Hindustan Platinum (Mumbai,
India) and used as received. (-)-Camphene, R-(+)-limonene,
(-)-β-pinene, γ-terpinene, 3-(-)-carene, α-pinene, R-(-) carvone,
myrcene, citral, ( )-β-citronellol, P(OPh)3, P(OBu)3, P(OEt)3,
bis(diphenylphosphino)ethane (dppe), bis(diphenylphosphino)-
propane (dppp), bis(diphenylphosphino)butane (dppb), 1,5-
cyclooctadiene and cobalt(II) acetate tetrahydrate, were procured
from Sigma-Aldrich, USA and used without further purifi-
cation. Triphenyl phosphine (PPh3), acetyl acetone, potassium
acetate, KOH, HCHO solution (40 % w/w), NaBH4, dimethyl-
formamide (DMF), acetic acid were purchased from Loba Chemie
India. The solvents ethanol, methanol, methyl ethyl ketone, 1,2-
dichloroethane (DCE), dichloromethane (DCM), xylene, toluene,
hexane, cyclohexane, petroleum ether, ethyl acetate obtained
from Merck, India were freshly distilled and dried prior to
use. Hydrogen and nitrogen gas supplied by Indian Oxygen,
Mumbai and carbon monoxide (> 99.8 % pure, Matheson Gas
USA) were used directly from the cylinders. The syngas mixture
(CO + H2) 1:1 was prepared by mixing H2 and CO in a reservoir
vessel. The complexes HRh(CO)(PPh3)3 [40], Rh(CO)2(acac)
[41], [Rh(COD)Cl)]2 and [Rh(µ-OAc)(COD)]2 [42], [Rh(µ-
OMe)(COD)]2 [43], Co2(CO)8 [44] and cis-PtCl2(PPh3)2 [45]
were prepared using literature procedures.
Experimental setup and procedure:All the hydroformy-
lation experiments were carried out either in a 50 mL autoclave,
made of stainless steel, supplied by Amar Instruments India
Pvt. Ltd. or a 300 mL Parr reactor (Parr instruments, USA). The
reactor setup was similar to that reported earlier [40].
In a typical experiment, known quantities of catalyst,
ligand, olefin (terpene), and the solvent were charged into the
autoclave and the reactor was flushed with nitrogen. The
contents were then flushed with a mixture of CO and H2 and
heated to a desired temperature. A mixture of CO and H2, in
the required ratio (1:1), was introduced into the autoclave, a
cis-10-Formylpinane (13a ), longer GC retention time):
MS (m/z rel. int.): 166/2 (M+); 151/18; 133/17; 122/90; 107/52;
95/31; 93/30; 81/57; 79/100; 69/69; 67/71; 55/80; 41/82.
trans-10-Formylpinane (13b), shorter GC retention time):
MS (m/z rel. int.):166/3 (M+), 151/23; 133/21; 122/91; 107/54;
93/32; 81/62; 79/100; 69/68; 67/68; 55/86; 41/80.
α-Pinene (14) (isomerized product of β-pinene): MS
(m/z rel. int.): 136/10 (M+); 121/13; 105/10; 93/100; 77/25;
41/9.
2-Caranecarbaldehyde (15) or (3,7,7-trimethylbicyclo
[4.1.0]heptane-2-carbaldehyde): MS (m/z rel. int.): 166/6 (M+);

Vol. 31, No. 1 (2019)
Highly Efficient Rhodium-Phosphite Catalyzed Hydroformylation of Camphene and other Terpenes 215
151/22; 137/94; 123/9; 109/34; 105/27; 95/81; 93/39; 81/100;
69/42; 67/42; 55/43; 41/40.
3-Caranecarbaldehyde (16) or (3,7,7-trimethylbicyclo-
[4.1.0]heptane-3-carbaldehyde): MS (m/z rel. int.): 166/19
(M+); 151/31; 135/77; 123/48; 109/33; 105/27; 93/100, 81/
94; 67/69; 55/48; 41/63.
CO/H2
+
CHO
CHO
Catalyst
7a
7b
1
3-Isopropyl-6-methylcyclohex-3-enecarbaldehyde (17)
(major): MS (m/z rel. int.): 166/43 (M+); 151/17; 137/61; 123/26;
109/24; 93/100; 81/63; 67/26; 55/26; 43/30.
6-Isopropyl-3-methylcyclohex-3-enecarbaldehyde (18)
(minor): MS (m/z rel. int.): 166/43 (M+); 151/17; 137/61; 123/26;
109/24; 93/100; 81/63; 67/26; 55/26
1-Isopropyl-4-methylcyclohex-1-ene (19): MS (m/z rel.
int.): 138/32 (M+); 123/24; 95/100; 81/69; 6729; 55/14; 41/15.
4-Isopropyl-1-methylcyclohex-1-ene (20) MS (m/z rel.
int): 138/34 (M+); 179/34; 95/100; 81/27; 68/40; 67/44; 55/15;
41/16.
3-Isopropyl-6-methylcyclohexa-1,4-diene (21) (isomer-
ized product): MS (m/z rel.int): 136/54 (M+);121/100; 105/17;
93/75; 91/36; 79/21; 77/25.
3-(4-Methylcyclohex-4-en-3-onyl)butanal (22)(major):
MS (m/z rel.int.): 180/3 (M+); 162/5: 136/65; 109/100; 82/40.
5-(4-Hydroxybutan-2-yl)-2-methylcyclohex-2-enone
(23): MS (m/z rel. int.): 182/22 (M+); 138/47; 122/33; 109/100;
94/30; 81/27; 69/39; 55/33; 41/42.
3-(4-Methyl-3-oxocyclohexyl)butanal (24): MS (m/z rel.
int.): 182/1 (M+); 164/6; 138/100; 111/67, 95/33, 81/24, 69/35;
55/70; 41/41.
8
9
10
Scheme-I: Hydroformylation of camphene
The Wilkinson's catalyst showed poor catalytic activity
(TOF = 14 h-1) in the hydroformylation of camphene (entry 1,
Table-1) and gave linear aldehydes along with hydrogenated
and isomerized products (~9 %). On the other hand, the unmo-
dified rhodium complex catalysts such as Rh(CO)2(acac) and
[Rh(COD)Cl)]2 showed improved activity over Wilkinson's
catalyst (entry 2, Table-1). When phosphite ligand was added
to this catalyst, the activity and selectivity increased dramati-
cally (entry 4, Table-1). The Rh(CO)2(acac)/P(OPh)3 catalyst
system gave the highest catalytic activity (TOF =186 h-1) with
99 % selectivity to linear aldehyde (with d.e. = 9 % to endo).
The possible reason for the observed rate enhancement could
be the facile dissociation of phosphite ligand [18,35,46-48]
which facilitates faster coordination of olefins to the metal center.
This results in better stabilization of the intermediate catalytic
species. Similar observation of rate enhancement was reported
for linear as well as internal olefin hydroformylation using Rh-
phosphite complex catalysts [46]. The hydrogenation and isome-
rized camphene products (8, 9 and 10) also reduced substan-
tially with the phosphite containing catalyst. This suggests that
the presence of phosphite ligand suppresses the side reactions
like isomerization and hydrogenation.
Dimeric rhodium complexes such as [Rh(COD)Cl)]2, [Rh(µ-
OMe)(COD)]2 , [Rh(µ-OAc)(COD)]2 in the presence of phosphite
ligand showed lower activity compared to the Rh(CO)2(acac)/
P(OPh)3 complex catalyst (entry 5-7; Table-1). The dissociation
of the dimer to monomeric species is known to be a slower step
[49], and hence results in lower activity. No reaction was observed
using complexes of cobalt and platinum under similar reaction
conditions (entry 8-11; Table-1). This is probably due to the fact
that these catalysts are effective at higher syngas pressure (> 9
MPa) compared to the existing conditions [7,8,33,34].
5-Isopropyl-2-methylcyclohex-2-enone (25): MS (m/z rel.
int.): 152/100 (M+); 137/32; 109/60; 95/25; 81/65; 69/70; 41/41.
RESULTS AND DISCUSSION
Catalyst screening studies: As a preliminary investiga-
tion, hydroformylation of camphene was carried out in toluene
at 373 K in the presence of the various Rh, Co and Pt complexes
and the results are presented in Table-1. The products found in
all cases were the terminal aldehyde. Hydroformylation of
camphene (1) is found to give only the terminal aldehyde 3,3-
dimethyl-2-norbornaneacetaldehyde, as a diastereomeric mixture
of exo (7a) and endo (7b) isomers. A small quantity of bypro-
ducts namely tricyclene (8) and isocamphane (9) are formed
by hydrogenation side reaction while fenchene (10) was obtained
as a byproduct by isomerization reaction (Scheme-I).
TABLE-1
SCREENING OF CATALYST FOR HYDROFORMYLATION OF CAMPHENE
Conversion
(%)
Aldehyde
selectivity (%)
iso/hd.
camphene (%)
Run No.
Catalyst
HRhCO(PPh3)3
Rh(CO)2 (acac)
[Rh(COD)Cl)]2
Rh(CO)2(acac)/P(OPh)3
[Rh(COD)Cl)]2/P(OPh)3
[Rh(µ-OAc)COD]2)/P(OPh)3
[Rh(µ-OMe)COD]2)/P(OPh)3
Co(acetate)2 / PPh3
Co2(CO)8
Time (h)
exo/endo
TOF (h-1)
1
2
3
4
5
24.9
18.1
16.0
1.8
5.0
4.0
4.0
8.3
9.1
7.0
98.3
97.6
96.7
94.3
99.1
81.6
96.2
91.8
92.7
97.9
98.8
98.7
99.4
99.5
9.2
7.3
2.1
1.2
1.3
1.18
1.24
1.22
0.84
0.89
0.64
0.65
14
16
23
186
57
72
6a
7a
8b
9b
10b
11b
0.6
0.5
85
No reaction
No reaction
Co(acetate)2/ P(OPh)3
PtCl2(PPh3)2
Catalyst formation was not observed.
Platinum metal precipitates out.
7.0
Reaction conditions: Camphene: 0.64 kmol/m3, catalyst: 1.23 × 10-3 kmol/m3, ligand: 7.4 × 10-3 moles (Rh:P = 1:6), T: 373 K, agitation speed : 16.6
Hz, PCO + H2: 4.14 MPa, solvent: toluene, total charge: 8.1 × 10-5 m3 a = Solvent: MEK, b = PCO+H2 : 8.6 MPa

216 Pagar et al.
Asian J. Chem.
Effect of solvent: Solvents are known to play a major role
selectivity of homogeneous catalytic reactions [51]. The influ-
ence of phosphite ligands on the activity and selectivity of the
Rh(CO)2(acac) catalyst was investigated for the hydroformylation
of camphene in methyl ethyl ketone solvent at 373K. The results
are presented in Table-2. Triphenylphosphite [P(OPh)3], triethyl-
phosphite [P(OEt)3] and tri-(n-butyl)phosphite [P(OBu)3]
promoted Rh catalyst systems were found to be active for the hydro-
formylation of camphene. However, Rh/triphenyl phosphite
catalyst was by far the most active catalyst for camphene hydrofor-
mylation. This observation can be explained on the basis of the
electronic factor i.e. higher electronegativity (χ) value of P(OPh)3
with respect to P(OBu)3 and P(OEt)3. The high electronegativity
of the ligand makes rhodium centre electron deficient, and
induces a fast replacement of CO ligand, which promotes faster
coordination of olefin to rhodium centre [52]. The high exo/endo
ratio observed is due to the larger cone angle of P(OPh)3 (128º)
compared to P(OEt)3, (123º) and P(OBu)3 (109º). No reaction
was observed when bidentate ligands such as diphenylphosphi-
noethane (dppe), diphenylphosphinopropane (dppp) and diphenyl-
phosphinobutane (dppb) were used under similar reaction condi-
tions. These bidentate ligands are known to be less active even
for the hydroformylation of linear olefins as compared to the
monodentate ligands [53], and hence their activity for a hindered
olefin like camphene is expected to be much less.
Effect of P/Rh ratio:The dependence of activity and product
distribution on the P/Rh ratio for camphene hydroformylation
was studied using Rh(CO)2(acac) in presence of additional
P(OPh)3 ligand. The results are presented in Fig. 2. The effect
of Rh to phosphite ratio on activity was studied in the initial
range of olefin conversion (~ 35 %). A strong negative effect
of P(OPh)3 concentration on activity of camphene hydroformy-
lation was observed. The activity increases with the concen-
tration of ligand upto Rh:P of 1:3. Thereafter, it decreases on
further phosphite addition from a Rh:L ratio of 1:3 (TOF =
1029 h-1) to 1:12 (TOF = 273 h-1). The aldehyde selectivity,
however, remains unaffected (> 99 %). The highest activity
was observed at a P/Rh ratio of three as seen in Fig. 2. At low
P/Rh ratio, the phosphite ligand is partially replaced by a
carbonyl ligand resulting in a higher rate. As per the hydro-
formylation mechanism, the phosphite dissociation must occur
to form the coordinatively unsaturated intermediate. This disso-
ciation is suppressed by increased P(OPh)3 concentration, which
serves to reduce the concentration of active Rh species in the
catalytic cycle. This observation of inhibition in rate also sup-
ports the hydroformylation mechanism using phosphite ligand
[54].
in the activity as well as selectivity in hydroformylation reac-
tions. To understand the role of solvents in hydroformylation
of camphene, reactions were taken in a number of solvents
using Rh(CO)2(acac)/P(OPh)3 catalyst. It was observed that the
solvents have a prominent influence on the selectivity and
activity of the catalyst. The rates of hydroformylation were higher
in polar protic (ethanol, methanol, methyl ethyl ketone) solvents
and lower in non-polar solvents (cyclohexene, hexane). Methanol
and ethanol gave higher rates (TOF 486 and 488 h-1, respec-
tively) but acetals (96.6 and 35.7 %, respectively) were formed
by condensation of aldehyde with alcohol solvents. The best
activity with 100 % selectivity to aldehydes was observed in
methyl ethyl ketone wherein a TOF of 417 h-1 was observed.
In general, solvents with a higher polarity gave better activity
over those with a lower polarity in hydroformylation of olefins
[50]. A comparison of the relative polarity of solvents with
the activity and exo/endo ratio of the products (Fig. 1) for the
hydroformylation of camphene. It was also observed that with
decrease in polarity of solvents, the distereoselectivity towards
exo product increases.
TOF
exo/endo ratio
1.0
0.8
0.6
0.4
0.2
0
500
400
300
200
100
0
0.06 0.2 2.4
2.5 3.5 4.7 5.1
Relative polarity
5.2
Fig. 1. A plot of activity and exo/endo ratio vs. relative polarity; Reaction
conditions: camphene: 0.64 kmol/m3, Rh(CO)2(acac): 1.23 × 10-3
kmol/m3, P(OPh)3: 7.4 ×10-3 kmol/m3 (Rh:P = 1:6), T: 373 K,
agitation speed: 16.6 Hz, PCO + H : 4.14 MPa, total charge: 8.1 × 10-5
2
m3. % (hexane 0.06, cyclohexane 0.2, toluene 2.4, xylene 2.5, 1,2
DCE 3.5, MEK 4.7, methanol 5.1, ethanol 5.2)
Screening of ligands: The type and nature of ligands
is known to have a dramatic influence on the activity and
TABLE-2
LIGAND SCREENING STUDIES FOR HYDROFORMYLATION OF CAMPHENE
Conversion
(%)
Aldehyde
selectivity (%)
iso./hd.
camphene (%)
S. No.
Ligand
exo/endo
TOF (h-1)
Time (h)
1
2
3
4
5
6
P(OPh)3
P(OEt)3
P(OBu)3
dppe
dppp
dppb
99.2
20.7
74.9
99.4
99.2
99.5
0.6
0.8
0.5
0.87
0.71
0.69
419
20
29
1.3
9.0
9.0
7.0
7.0
7.0
No reaction
No reaction
No reaction
No reaction
No reaction
No reaction
No reaction
No reaction
No reaction
No reaction
No reaction
No reaction
No reaction
No reaction
No reaction
Reaction conditions: Rh(CO)2(acac): 1 × 10-3 kmol/m3, ligand: 3 × 10-3 kmol/m3, camphene: 0.37 kmol/m3, Rh:P = 1:3, MEK: 2.82 × 10-5 m3, total
charge : 3.0 × 10-5 m3 , T: 373 K PCO + H2: 4.14 MPa, agitation speed : 16.6 Hz

Vol. 31, No. 1 (2019)
Highly Efficient Rhodium-Phosphite Catalyzed Hydroformylation of Camphene and other Terpenes 217
1200
CO/H2
+
1000
800
Catalyst
CHO
2
11
12
Scheme-II: Hydroformylation of limonene
600
400
200
CHO
CHO
CO/H2
+
+
Catalyst
3
13a
13b
14
Scheme-III: Hydroformylation of β-pinene
0
1:00
1:02
1:03
1:06
1:09
1:12
Rh:P ratio
CHO
Fig. 2. A plot of initial activity vs. Rh:P ratio; Reaction conditions:
CHO
Rh(CO)2(acac) : 1 × 10-3 kmol/m3, camphene: 0.37 kmol/m3, T: 373
CO/H2
K, PCO + H : 4.14 MPa, MEK: 2.82 × 10-5 m3, total charge: 3.0 × 10-5
2
+
m3, agitation speed: 16.6 Hz
Catalyst
Hydroformylation of various terpenes and terpenoids
using Rh(CO)2(acac)/P(OPh3)3 catalyst: The highly active
Rh(CO)2(acac)/P(OPh)3 catalyst was tested for the hydroformy-
lation of other terpenes at a total pressure of 4.14 MPa (CO/
H2 = 1), Rh(CO)2(acac): 1 × 10-3 kmol/m3, P(OPh)3: 3 × 10-3
kmol/m3, and terpene concentration of 0.37 kmol/m3 at 373
K. The results are presented in Table-3. In addition to hydrofor-
mylation of camphene (1), the catalyst was found to be very
active for the hydroformylation of R(+)-limonene (2) (Scheme-
II) and α-pinene (3) (Scheme-III). Less hydroformylation
activity was observed for 3-carene (4) (Scheme-IV) and γ-ter-
pinene (5) (Scheme-V). The hydroformylation of limonene
(2) and β-pinene (3) gives 3-(4-methylcyclohex-3-enyl)butanal
(11) and cis- (13a) and trans- (13b) 10-formylpinane by hydro-
formylation and isomerized limonene (12) and α-pinene (14) as
isomerization byproducts, respectively. The hydroformylation
of 3-carene (4) gives 2-caranecarbaldehyde (15) and 3-carane-
carbaldehyde (16) in equal proportion. For γ-terpinene isomeri-
zation and hydrogenation activity was more as compared to
hydroformylation activity. γ-Terpinene (5) on hydroformyl-
4
15
16
Scheme-IV: Hydroformylation of 3-carene
CHO
CO/H2
Catalyst
+
OHC
5
17
18
+
+
19
20
21
Scheme-V: Hydroformylation of γ-terpinene
TABLE-3
SCREENING OF TERPENES USING Rh(CO)2(acac)/P(OPh)3 IN METHYL ETHYL KETONE
Aldehyde
selectivity (%)
iso/hd. terpenes
(%)
exo/endo or
cis/trans ratio
Run No.
Terpene
Conversion (%)
TOF (h-1)
Time (h)
1
2
3
4
5
6
7
8
(-)-Camphene
(R)-Limonene
β-Pinene
99.2
99.3
78.9
45.5
10.7
94.4
99.4
99.4
70.0
28.0
98.3
92.7
0.6
0.6
30.0
72.0
1.7
0.87
-
0.88
-
-
-
419
526
56.6
9.2
9.5
688
1.3
0.7
3.3
5.0
4.0
0.5
4.0
7.3
γ-Terpinene
3-Carene
R-(-) Carvone*
α-Pinene
1.0
No reaction
Myrcene
No reaction, only isomerization is observed
*Reaction in toluene as substrate is insoluble in MEK, alcohol selectivity of 6.3 %; Reaction conditions: Rh(CO)2(acac): 1 × 10-3 kmol/m3, P(OPh)3:
3 × 10-3 kmol/m3, Rh:P = 1:3, terpene: 0.37 kmol/m3 , T: 373 K, PCO + H2: 4.14 MPa, agitation speed: 16.6 Hz, MEK: 2.82 × 10-5 m3, total charge: 3.0
× 10-5 m3

218 Pagar et al.
Asian J. Chem.
ation gives 3-isopropyl-6-methylcyclohex-3-ene carbaldehyde
(17) as a major and 6-isopropyl-3-methylcyclohex-3-ene carb-
aldehyde (18) as a minor aldehyde product. The other products
formed are hydrogenated products such as (1-isopropyl-4-methyl-
cyclohex-1-ene (19) and 4-isopropyl-1-methylcyclohex-1-ene
(20) and isomerized product (3-isopropyl-6-methylcyclohexa-
1,4-diene (21) by hydrogenation and isomerization side reaction
(Scheme-V). No reaction was observed with α-pinene and
myrcene. In case of myrcene only isomerization was observed.
The catalyst was found to be more active for hydroformy-
lation of carvone (6) (Scheme-VI) as compared to camphene
and limonene and a TOF of 688 h-1 was obtained. The major
product for carvone hydroformylation is 3-(4-meth1cyclohex-
4-en-3-onyl)butanal (22). The other minor products include
3-(4-methyl-3-oxocyclohexyl)butanal (23) and alcohol namely
5-(4-hydroxybutan-2-yl)-2-methylcyclohex-2-enone (24) by
aldehyde hydrogenation. Small extents of isomerized and hydro-
genated carvone (25) products were also observed (Scheme-VI).
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