Synthesis of fragrance compounds from acyclic monoterpenes: Rhodium catalyzed hydroformylation and tandem hydroformylation/acetalization of linalool and β-citronellene

Article abstract of DOI:10.1016/j.apcata.2013.06.037

Rhodium-catalyzed hydroformylation of acyclic monoterpenic compounds, i.e., linalool and β-citronellene, was studied in toluene and ethanol solutions in the presence of PPh₃ or P(O-o-^tBuPh)₃ ligands. Although both substrates have a monosubstituted terminal double bond, they show different behavior under the hydroformylation conditions. In toluene, linalool gave almost quantitatively a cyclic hemiacetal; whereas the hydroformylation of β-citronellene resulted in two isomeric aldehydes also in a nearly quantitative combined yield. The reactions occurred approximately two times faster in ethanol than in toluene giving the corresponding acetals even in the absence of additional acid co-catalysts. In the absence of phosphorous ligands, linalool (differently from β-citronellene) was very resistant to hydroformylation probably due to the binding with rhodium through both the double bond and the hydroxyl group to form stable chelates. The P(O-o-^tBuPh)₃ ligand exerted a remarkable effect on the reactivity of both substrates accelerating the reactions by 5-20 times as compared to the system with PPh₃. Several fragrance compounds were obtained in high yields through a simple one-pot procedure starting from the substrates easily available from natural bio-renewable resources.

Full text of DOI:10.1016/j.apcata.2013.06.037

Applied Catalysis A: General 466 (2013) 208–215
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Applied Catalysis A: General
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Synthesis of fragrance compounds from acyclic monoterpenes:
Rhodium catalyzed hydroformylation and tandem
hydroformylation/acetalization of linalool and ␤-citronellene
Camila G. Vieira, Eduardo N. dos Santos, Elena V. Gusevskaya∗
Departamento de Química, Universidade Federal de Minas Gerais, 31270-901 Belo Horizonte, MG, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
Rhodium-catalyzed hydroformylation of acyclic monoterpenic compounds, i.e., linalool and ␤-
citronellene, was studied in toluene and ethanol solutions in the presence of PPh3 or P(O-o-tBuPh)3
ligands. Although both substrates have a monosubstituted terminal double bond, they show different
behavior under the hydroformylation conditions. In toluene, linalool gave almost quantitatively a cyclic
hemiacetal; whereas the hydroformylation of ␤-citronellene resulted in two isomeric aldehydes also in
a nearly quantitative combined yield. The reactions occurred approximately two times faster in ethanol
than in toluene giving the corresponding acetals even in the absence of additional acid co-catalysts. In the
absence of phosphorous ligands, linalool (differently from ␤-citronellene) was very resistant to hydro-
formylation probably due to the binding with rhodium through both the double bond and the hydroxyl
group to form stable chelates. The P(O-o-tBuPh)3 ligand exerted a remarkable effect on the reactivity of
both substrates accelerating the reactions by 5–20 times as compared to the system with PPh3. Several
fragrance compounds were obtained in high yields through a simple one-pot procedure starting from
the substrates easily available from natural bio-renewable resources.
Received 21 February 2013
Received in revised form 21 June 2013
Accepted 25 June 2013
Available online 4 July 2013
Keywords:
Acetalization
Citronellene
Hydroformylation
Linalool
Monoterpenes
Rhodium
© 2013 Elsevier B.V. All rights reserved.
1. Introduction
synthesis of various vitamins and fragrances, such as vitamin A,
vitamin E, citral and citronellol [3,4]. Linalool is found in essen-
Terpenic compounds, in general, are an important biomass-
based renewable feedstock for the industrial production of
fragrances, perfumes, flavors and pharmaceuticals as well as syn-
thetic intermediates [1–4]. These natural compounds are easily
available from essential oils of many plants and flowers. For
several years, we have been interested in catalytic transforma-
tions of terpenes to more valuable chemicals, in particular, via
hydroformylation reactions [5–9]. Rhodium or cobalt catalyzed
hydroformylation is an industrially important tool for the synthesis
of oxygen-containing compounds from alkenes [10]. The process
is usually performed in aprotic solvents and gives aldehydes as
main products, which can be further converted to other desired
compounds.
Although the hydroformylation of more abundant monoter-
penes, such as limonene, ␤-pinene, myrcene and camphene, was
quite extensively studied [11–20], only few reports appeared on
the hydroformylation of linalool [11,21–23] and ␤-citronellene
(dihydromyrcene) [11,24,25]. Linalool, a monoterpenoid allylic
alcohol with a pleasant lily odor, is a key building block for the
tial oils of several plants, e.g., Brazilian rosewood and Chinese
Ho leaf oils, but most linalool is produced synthetically from ␣-
pinene, a cheap major constituent of turpentine oils obtained from
coniferous trees [3,4,26]. The hydroformylation of allylic alcohols
represents a direct method for the synthesis of substituted tetrahy-
drofurans since primarily formed hydroxyl-aldehydes undergo a
spontaneous intramolecular cyclization to give five-membered
hemiacetals [13,27–30]. Hemiacetals can be further converted in
other useful products: acetals, lactones or dihydrofuran derivatives.
␤-Citronellene is also a valuable feedstock for the industrial syn-
thesis of various fragrance ingredients, e.g., dihydromyrcenol and
cyclodemol [3]. ␤-Citronellene is commercially produced by the
thermal isomerization of pinane obtained through the hydrogena-
tion of ␣-pinene [3,31,32]. ␣-Pinene is one of the most widespread
monoterpene hydrocarbons; its content in turpentine oils can reach
85%. Aldehydes and acetals resulting from the hydroformylation of
␤-citronellene and linolool are valuable fragrance compounds with
fresh floral and/or green citrus odors [11].
Following a general trend in synthetic organic chemistry, we
have recently directed our efforts to involve the hydroformylation
reaction in tandem reaction sequences [8,9,33,34]. Such catalytic
processes can lead directly from alkenes to various important prod-
ucts through a simple one-pot procedure without the isolation of
∗
Corresponding author. Tel.: +55 31 34095741; fax: +55 31 34095700.
E-mail address: elena@ufmg.br (E.V. Gusevskaya).
0926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2013.06.037

C.G. Vieira et al. / Applied Catalysis A: General 466 (2013) 208–215
209
aldehydes. In particular, we integrated the hydroformylation and
aldehyde acetalization steps in one reactor by performing the pro-
cess in ethanol solutions, which allowed to obtain diethyl acetals
from various monoterpenes [9,35].
In continuation of our ongoing project on valorization of
natural ingredients of essential oils, we report herein the rhodium-
catalyzed hydroformylation of linalool (1a) and ␤-citronellene (2a)
as well as the tandem hydroformylation/acetalization of these sub-
strates under mild non-acidic conditions. All isolated products
(aldehydes, hemiacetals and acetals) have a pleasant scent and can
be useful as components of synthetic fragrances.
Data for 2,3,7-trimethyl-6(7)-octenal (2c, a mixture of two
diastereoisomers: (2R,3R) and (2S,3R), shorter GC retention time
compared to 2b): MS (70 eV, EI): m/z (%): 168 (2) [M+], 135 (19),
109 (43), 95 (76), 85 (21), 83 (34), 82 (32), 70 (25), 69 (100), 67
(43), 56 (26), 55 (57). 1H NMR (400 MHz, CDCl3, 25 ◦C, TMS): ı = 0.83
and 0.98 (d, 3J = 7.0 Hz, 3H; C1H3), 0.89 (d, 3J = 6.4 Hz, 3H; C10H3),
1.12–1.22 (m, 1H; C4HH), 1.28–1.38 (m, 1H; C4HH), 1.40–1.50 (m,
2H; C3H), 1.60 (s, 3H; C8H3), 1.68 (s, 3H; C9H3), 1.90–2.05 (m, 2H;
C5H2), 2.25–2.35 (m, 1H; C2H), 5.09 (t, 3J = 7.0 Hz, 1H; C6H), 9.68 and
9.67 ppm (br. s, 1H; CHO). 13C NMR (100 MHz, CDCl3, 25 ◦C, TMS):
ıC = 14.07 and 15.34 (C1), 17.27 (C8), 19.20 (C10), 25.63 (C5), 25.67
(C9), 32.15 and 33.40 (C3), 33.25 and 34.74 (C4), 50.51 and 51.50
(C2), 123.98 and 124.06 (C6), 131.78 and 131.80 (C7), 205.64 and
205.70 ppm (CHO).
2. Experimental
Data
for
cis-5-ethoxy-2-methyl-2-(4-methyl-3-
All chemicals were purchased from commercial sources and
used as received, unless otherwise indicated. Racemic linalool [( )-
3,7-dimethyl-1,6-octadien-3-ol] and (−)-␤-citronellene [dihy-
dromyrcene, (R)-(−)-3,7-dimethyl-1,6-octadiene] acquired from
Aldrich were used as substrates. [Rh(COD)(OMe)]2 (COD = 1,5-
pentenyl)tetrahydrofuran (cis) (1d cis): MS (70 eV, EI): m/z (%):
212 (0.5) [M+], 166 (15) [M+−C2H5OH], 129 (79), 123 (20), 122
(54), 109 (26), 108 (12), 107 (55), 101 (39), 95 (25), 86 (27), 85
(22), 83 (56), 69 (100), 67 (20), 58 (22), 55 (35). 1H NMR (400 MHz,
CDCl3, 25 ◦C, TMS): ı = 1.15–1.25 (m, 3H; CH2CH3), 1.33 (s, 3H;
C10H3), 1.40–1.50 (m, 2H; C4H2), 1.61 (s, 3H; C8H3), 1.67 (s, 3H;
C9H3), 1.75–1.90 (m, 2H; C2H2), 1.90–2.00 (m, 4H; C5H2 and
C1H2), 3.35–3.45 (m, 1H; CHHCH3), 3.70–3.80 (m, 1H; CHHCH3),
5.05–5.15 ppm (m, 2H; C6H and OCHOC2H5). 13C NMR (100 MHz,
CDCl3, 25 ◦C, TMS): ıC = 15.21 (CH2CH3), 17.55 (C8), 23.23 (C5),
25.42 (C9), 28.24 (C10), 33.29 (C1), 34.36 (C2), 41.91 (C4), 61.99
(CH2CH3), 84.17 (C3), 103.83 (OCHOC2H5), 124.45 (C6), 131.27 ppm
(C7).
cyclooctadiene) was prepared by
a published method [36].
Tris(O-tbutylphenyl)phosphite, P(O-o-tBuPh)3, was prepared as
described in [37] and purified by column chromatography (silica
gel 60) using mixture of hexane and CHCl3 as eluents. Toluene was
purified under reflux with sodium wire–benzophenone for 8 h and
then distilled under argon Ethanol was purified under reflux with
magnesium turnings and iodine crystals for 6 h and then distilled
under argon.
Catalytic experiments were carried out in homemade stainless
steal reactors with magnetic stirring. Reactions were followed by
gas chromatography (GC) by sampling the liquid phases through a
valved dip tube. The products were analyzed by gas chromatog-
raphy (GC-Shimadzu QP2010, Rtx®-5MS capillary column, FID
detector). Conversion and selectivity were determined by GC. The
GC mass balance was based on the substrate charged using dode-
cane as an internal standard. Initial turnover frequencies (TOFs)
were measured at low conversions (up to 20–40%) taking aliquots
at short reaction times for GC analysis.
Data
for
trans-5-ethoxy-2-methyl-2-(4-methyl-3-
pentenyl)tetrahydrofuran (1d trans, shorter GC retention time
compared to 1d cis): MS (70 eV, EI): m/z (%): 212 (0.2) [M+], 166
(23) [M+−C2H5OH], 129 (73), 123 (27), 122 (80), 109 (32), 108
(20), 107 (84), 101 (37), 95 (27), 86 (22), 85 (21), 83 (57), 69 (100),
67 (25), 58 (20), 55 (38). 1H NMR (400 MHz, CDCl3, 25 ◦C, TMS):
ı = 1.15–1.25 (m, 3H; CH2CH3), 1.33 (s, 3H; C10H3), 1.62 (s, 3H;
C8H3), 1.67 (s, 3H; C9H3), 1.65–1.70 (m, 2H; C4H2), 1.90–2.00 (m,
6H; C2H2, C5H2 and C1H2), 3.35–3.45 (m, 1H; CHHCH3), 3.70–3.80
(m, 1H; CHHCH3), 5.05–5.15 ppm (m, 2H; C6H and OCHOC2H5).
13C NMR (100 MHz, CDCl3, 25 ◦C, TMS): ıC = 15.13 (CH2CH3), 17.49
(C8), 23.57 (C5), 26.07 (C9), 28.24 (C10), 32.69 (C1), 34.77 (C2), 42.78
(C4), 62.10 (CH2CH3), 84.45 (C3), 103.57 (OCHOC2H5), 124.65 (C6),
131.10 ppm (C7).
Data for 9,9-diethoxy-2,6-dimethylnon-2-ene (2d): MS (70 eV, EI):
m/z (%): 196 (6) [M+−C2H5OH], 135 (20), 109 (29), 107 (22), 103
(100), 97 (20), 96 (29), 95 (51), 85 (28), 81 (41), 75 (84), 69 (55), 57
(31). 1H NMR (400 MHz, CDCl3, 25 ◦C, TMS): ı = 0.85 (d, 3J = 6.4 Hz,
3H; C10H3), 1.18 (t, 3J = 6.8 Hz, 6H; CH2CH3), 1.10–1.20 (m, 2H; C4HH
and C2HH), 1.25–1.45 (m, 3H; C4HH, C2HH and C3H), 1.57 (s, 3H;
C8H3), 1.50–1.65 (m, 2H; C1H2), 1.65 (s, 3H; C9H3), 1.85–2.05 (m,
2H; C5H2), 3.40–3.50 (m, 2H; CHHCH3), 3.55–3.65 (m, 2H; CHHCH3),
4.43 (t, 3J = 6.0 Hz, 1H; CH(OC2H5)), 5.07 ppm (t, 3J = 7.2 Hz, 1H;
C6H). 13C NMR (100 MHz, CDCl3, 25 ◦C, TMS): ıC = 15.38 (CH2CH3),
17.54 (C8), 19.42 (C10), 25.46 (C5), 25.62 (C9), 31.02 (C1), 31.63
(C2), 32.22 (C3), 36.91 (C4), 60.67 (CH2CH3), 60.74 (CH2CH3), 103.28
(CH(OC2H5)), 124.90 (C6), 131.04 ppm (C7).
In
a typical run, toluene or ethanol (20.0 mL) contain-
ing [Rh(COD)(OMe)]2 (5.0–6.0 ␮mol), phosphorus ligand
(0–0.6 mmol), substrate (2–6 mmol), and dodecane (1–3 mmol,
internal standard) was transferred under argon into a stainless
steel autoclave, which was pressurized to 20–80 atm (CO/H2 = 1/2
to 2/1), placed in an oil bath (40–80 ◦C), and magnetically stirred.
After the reaction was carried out and cooled to room temperature,
the excess CO and H2 were slowly vented.
The products were separated by column chromatography (silica
gel 60) using mixtures of hexane and CH2Cl2 as eluents and iden-
tified by GC–MS, 1H, and 13C NMR. The assignment of 1H and 13C
NMR signals was made using bidimensional techniques. NMR spec-
tra were recorded in CDCl3 using a Bruker 400 MHz spectrometer,
with TMS as an internal standard. Mass spectra were obtained on
a Shimadzu QP2010-PLUS instrument operating at 70 eV.
For the NMR and MS data cis-5-methyl-5-(4-methyl-3-
pentenyl)tetrahydro-2-furanol (1c cis and trans) see [23].
Data for 4,8-dimethyl-7(8)-nonenal (2b): MS (70 eV, EI): m/z (%):
168 (2) [M+], 135 (26), 109 (55), 107 (20), 81 (26), 70 (20), 69
(100), 67 (27), 56 (21), 55 (45). 1H NMR (400 MHz, CDCl3, 25 ◦C,
TMS): ı = 0.89 (d, 3J = 6.4 Hz, 3H; C10H3), 1.12–1.22 (m, 1H; C4HH),
1.28–1.38 (m, 1H; C4HH), 1.40–1.50 (m, 2H; C3H and C2HH), 1.60 (s,
3H; C8H3), 1.68 (s, 3H; C9H3), 1.65–1.75 (m, 1H; C2HH), 1.90–2.05
(m, 2H; C5H2), 2.38–2.48 (m, 2H; C1H2), 5.09 (t, 3J = 7.0 Hz, 1H;
C6H), 9.77 ppm (t, 3J = 1.8 Hz, 1H; CHO). 13C NMR (100 MHz, CDCl3,
25 ◦C, TMS): ıC = 17.61 (C8), 19.21 (C10), 25.41 (C5), 25.67 (C9), 28.83
(C2), 32.00 (C3), 36.73 (C4), 41.66 (C1), 124.51 (C6), 131.37 (C7),
202.89 ppm (CHO).
Data for 8,8-diethoxy-2,6,7-trimethyloct-2-ene (2e, shorter GC
retention time compared to 2d): MS (70 eV, EI): m/z (%): 196 (2)
[M+−C2H5OH], 135 (19), 103 (100), 95 (35), 75 (83), 69 (23).
3. Results and discussion
We have studied the behavior of linalool (1a) and ␤-citronellene
(2a) under the hydroformylation conditions in toluene and ethanol
solutions using [Rh(COD)(OMe)]2 as a catalyst precursor in the
presence of PPh3 or tris(O-tbutylphenyl)phosphite, P(O-o-tBuPh)3,

210
C.G. Vieira et al. / Applied Catalysis A: General 466 (2013) 208–215
Table 1
Rhodium catalyzed hydroformylation of linalool (1a).a
Run
Ligand
P/Rh
Time (h)
Conversion (%)
TOF (h−1
)
Selectivity for 1c (%) (cis/trans)
1b
PPh3
10
10
10
20
50
5
1
–
5
6.0
0.5
1.5
2.0
3.0
1.5
1.5
6.0
21.0
100
96
100
99
100
98
94
38
385
400
233
183
350
300
10
94 (93/7)
87 (92/8)
98 (89/11)
99 (87/13)
99 (85/15)
99 (91/10)
91 (94/6)
85 (30/70)
89 (20/80)
2b,c
3d
P(O-o-tBuPh)3
P(O-o-tBuPh)3
P(O-o-tBuPh)3
P(O-o-tBuPh)3
P(O-o-tBuPh)3
P(O-o-tBuPh)3
None
4d
5d
6d
7d
8d,e
9d
12
47
PPh3
33
a
For most of the runs the reaction time is given for nearly complete conversions. Conversion and selectivity are based on the substrate reacted. TOF – initial turnover
frequency (mol of the substrate converted per mol of Rh per hour).
b
Conditions: linalool (0.10 M), [Rh(COD)(OMe)]2 (0.25 mM), ligand (5.0 mM), 50 ◦C, 80 atm (CO/H2 = 1/1), toluene.
c
Average TOF is given as the reaction was too fast.
d
Conditions: linalool (0.30 M), [Rh(COD)(OMe)]2 (0.30 mM), 40 ◦C, 20 atm (CO/H2 = 1/1), toluene.
e
50 ◦C.
as P-donor auxiliary ligands. The hydroformylation of linalool in
toluene with the Rh/PPh3 system has been described in our previ-
ous publication [23]; however, we have included some runs in the
tables for comparison.
for both substrates, with GC yields for some individual products
being nearly quantitative under optimized conditions.
3.1. Hydroformylation of linalool in toluene solutions
Under the hydroformylation conditions in toluene, both sub-
strates gave one or two major products derived from the direct
carbonylation of their terminal double bonds. No reactivity of the
internal double bond was observed even in the system with the
bulky P(O-o-tBuPh)3 ligand. When the reactions were performed
in ethanol, the corresponding acetals appeared in the reaction
mixtures due to the in situ acetalization of the primarily formed
products. It is important to point out that the formation of acetals
occurred without using any additional acid co-catalyst. The regio-
and stereoselectivity of the reaction steps was remarkably depend-
ent on the reaction variables and the nature of the auxiliary
phosphorous ligand. The substrates and corresponding products
are numbered in the present communication by the same number
with the addition of the letter “a” to indicate the substrate and other
letters to indicate the products derived from this substrate. In all
products, we have maintained a carbon numbering conventionally
used for starting monoterpene molecules. The total selectivity for
the hydroformylation products in most of the runs exceeded 95%
Hydroformylation of linolool in the presence of PPh3 gave hemi-
acetal 1c as the main product (Table 1, run 1, Scheme 1). This
product formally results from the intramolecular cyclization of
the primarily formed hydroxy aldehyde 1b, which we have never
detected in the reaction solutions. Hemiacetal was formed almost
exclusively with a cis configuration of the methylpentenyl and
hydroxyl groups with respect to the tetrahydrofuran ring. The use
of the P(O-o-tBuPh)3 ligand instead of PPh3 remarkably increased
the reactivity of linalool (Table 1, run 2 vs. run 1). The reaction in run
2 was too fast for measuring the initial rate by GC being completed
for 30 min. However, even the average turnover frequency (TOF) in
the run with P(O-o-tBuPh)3 was more than ten times higher than
the initial TOF in the reaction performed with PPh3 under the same
conditions. In a further study we decreased the temperature and
the pressure to slow down the reactions.
The hydroformylation of linalool with the Rh/P(O-o-tBuPh)3 sys-
tem at 40 ◦C and 20 atm resulted in nearly quantitative yield of
OH
OH
O
*
O
*
OH
OH
CHO
*
*
CO/H2
*
*
+
1b
1a
1c
1c ( )
trans
(
cis
)
-H2O
C2H5OH
OC2H5
+
OC2H5
O
*
O
*
*
*
1d
1d ( )
cis
(
)
trans
Scheme 1. Hydroformylation/acetalization of linalool (1a).

C.G. Vieira et al. / Applied Catalysis A: General 466 (2013) 208–215
211
100
100
80
60
40
20
0
80
60
P/Rh
t
P(O-o- BuPh)
3
1
5
40
20
0
10
20
50
PPh3
0,0
0,5
1,0
1,5
2,0
2,5
3,0
0
1
2
3
4
5
6
7
8
Time (h)
Time (h)
Fig. 1. Hydroformylation of linalool catalyzed by Rh/P(O-o-tBuPh)3: effect of the
P/Rh ratio. Conditions: linalool (0.30 M), [Rh(COD)(OMe)]2 (0.30 mM), 40 ◦C, 20 atm
(CO/H2 = 1/1), toluene.
Fig. 2. Rhodium catalyzed hydroformylation of linalool: ligand effect. Condi-
tions: linalool (0.30 M), [Rh(COD)(OMe)]2 (0.30 mM), ligand (3.0 mM), 40 ◦C, 20 atm
(CO/H2 = 1/1), toluene.
period in the Rh/PPh3 system supports the chelation hypothesis:
at the beginning of the reaction, inactive rhodium–linalool chelate
complexes should be opened by the PPh3 coordination [23]. A great
advantageof using P(O-o-tBuPh)3 consists in a high efficiencyof this
ligand to prevent the linalool chelation and/or to break up rhodium-
linalool chelates. Even in the reaction with small amounts of P(O-
o-tBuPh)3, no induction period was observed on the kinetic curves.
The effect of the ligand nature on the stereochemistry of the
hemiacetal deserves a special attention. On changing the ligand
from P(O-o-tBuPh)3 to PPh3 under the same reaction conditions,
the stereoselectivity of cyclization was virtually switched from cis
to trans (Table 1, runs 6 and 9). This result contradicts with the
reaction path shown in Scheme 1, in which two reaction steps
occur consecutively with no participation of rhodium in the cycliza-
tion step. A spontaneous cyclization of aldehyde 1b should give
preferably the thermodynamically more stable trans isomer of 1c
and stereochemistry should not depend on the ligands on the
rhodium species. A strong ligand effect argues for the suggestion
that the interaction between the hydroxyl and carbonyl groups
leading to the intramolecular cyclization could also occur in the
rhodium acyl intermediate before its hydrogenolysis to give alde-
hyde 1b [23]. The cyclization of the rhodium acyl intermediate
should give preferably a less hindered cyclic rhodium intermediate
in which a bulky rhodium/P(O-o-tBuPh)3 fragment and methylpen-
tenyl group are at the opposite sides of the tetrahydrofuran ring.
The hydrogenolysis of the trans cyclic rhodium intermediate will
give hemiacetal 1c with the methylpentenyl and hydroxyl groups
in a cis position.
The inversion of stereochemistry of hemiacetal from cis to trans
can also be achieved by the increase in the hydrogen pressure
as higher hydrogen concentrations favor the hydrogenolysis of
the rhodium acyl intermediate before its cyclization. At 20 atm
(CO/H2 = 1/1), the cis isomer accounted for ca. 90% of the mass bal-
ance; whereas at 80 atm, both isomers were formed in comparable
amounts (Table 2, run 1 vs. run 3). Moreover, increasing only the
hydrogen pressure allowed to obtain the trans isomer in nearly 80%
yield (Table 2, run 4). On the other hand, the increase in the reaction
temperature switched stereoselectivity again to cis isomer (cf. run 3
in Table 2 and run 2 in Table 1). Thus, through the choice of reaction
variables we can control stereochemistry of the hydroformylation
product derived from linalool. Each of two isomers of hemiacetal
1c can be obtained in 75–90% individual yields under appropriate
conditions.
hemiacetal 1c (ca. 90% cis, Table 1, runs 3–5). Kinetic curves for
the runs with various P/Rh ratios (runs 2–7 in Table 1) are pre-
sented in Fig. 1. A slight acceleration can be seen when the P/Rh
ratio increases from 1 to 10, whereas further addition of the ligand
decreases the reaction rate. The kinetic curves are nearly straight
lines up to 80–90% conversions even at P/Rh = 50, indicating that
the substrate competes successfully with the ligand for the coordi-
nation sites on rhodium. In other words, the substrate reacts with
rhodium quite readily and a majority of the metal centers, even at
high conversions and high ligand concentrations, contain strongly
coordinated linalool or fragments derived from linalool. In most of
the runs, the cis isomer of 1c predominated (85–95%), with its rela-
tive amounts being only slightly decreased with the increase in the
ligand concentration.
An unexpected accelerating effect of PPh3 on the reactivity of
linolool has been reported in our previous publication [23]. In the
present work, we have found that the acceleration is much more
pronounced with the P(O-o-tBuPh)3 ligand. In the absence of the P
ligand, only a 12% conversion was observed for 6 h, while with small
amounts of P(O-o-tBuPh)3 the reaction was nearly completed in
1.5 h even at lower temperature (cf. runs 7 and 8, Table 1). In most
modified rhodium systems, the hydroformylation rate decreases
with the increase in the P-ligand concentration due to the compe-
tition between the ligand and the substrate for coordination sites
on rhodium. The unusual effect observed at the hydroformylation
of linalool can be explained by the chelation of the substrate on
rhodium through the coordination of both the double bond and
the hydroxyl group [23]. The increase in the P-ligand concentra-
tion favors the cleavage of the inactive chelates allowing the proper
positioning of the C C double bond for the hydride migration. It
is remarkable that the presence of only 0.2 mol% of P(O-o-tBuPh)3
(P/Rh = 1) is sufficient to overcome the chelation problem and to
perform the hydroformylation of linolool rapidly (TOF = 300 h−1
and selectively under mild conditions (Table 1, run 7).
)
The strong effect of the ligand nature on the hydroformylation of
linalool is illustrated in Fig. 2, which shows the kinetic curves for the
reactions with PPh3 and P(O-o-tBuPh)3 under the same conditions
(Table 1, runs 6 and 9). Only 20% of linalool was converted for 8 h in
the presence of PPh3, with the 3-h induction period being observed
on the reaction profile. Nevertheless, the reaction in the presence
of P(O-o-tBuPh)3 occurred more than ten times faster without any
induction period (TOF = 350 vs. 33). The existence of the induction

212
C.G. Vieira et al. / Applied Catalysis A: General 466 (2013) 208–215
Table 2
Rhodium catalyzed hydroformylation of linalool (1a): effect of pressurea
Run
P (CO) (atm)
P (H2) (atm)
TOF (h−1
)
Selectivity for 1c (%) (cis/trans)
1
2
10
25
40
10
40
10
25
40
40
10
400
350
330
1830
100
98 (89/11)
97 (86/14)
95 (42/58)
96 (23/77)
97 (91/9)
3
4b
5
a
Conditions: linalool (0.30 M), [Rh(COD)(OMe)]2 (0.30 mM), P(O-o-tBuPh)3 (6.0 mM), P/Rh = 10, 40 ◦C, toluene. Selectivity at nearly complete conversions is given and is
based on the substrate reacted. TOF – initial turnover frequency (mol of the substrate converted per mol of Rh per hour).
b
Average TOF is given as the reaction was too fast.
Table 3
Rhodium catalyzed hydroformylation of ␤-citronellene (2a).a
Run
Ligand
P/Rh
P (CO) (atm)
P (H2) (atm)
Time (h)
TOF (h−1
)
Selectivity (%)
2b
2c
1
2
3
4
5
6
7
8
9
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
PPh3
None
2
10
20
40
10
10
10
–
40
40
40
40
20
40
20
40
40
40
40
40
40
40
40
20
20
40
40
40
40
40
4.0
7.0
9.0
10.0
6.0
9.0
4.0
1.0
0.75
0.75
3.0
54
42
36
34
44
26
80
200
520
480
9
74
84
84
84
84
83
84
70
68
68
55
21
15
15
15
15
16
15
21
31
32
10
P(O-o-tBuPh)3
P(O-o-tBuPh)3
P(OPh)3
10
20
10
10
11b
a
Conditions: ␤-citronellene (0.10 M), [Rh(COD)(OMe)]2 (0.25 mM), 50 ◦C, toluene. Conversion and selectivity are based on the substrate reacted; the reaction time is given
for nearly complete conversions. TOF – initial turnover frequency (mol of the substrate converted per mol of Rh per hour).
b
14% conversion for 3 h.
A concerned to the effects of the carbon monoxide and hydrogen
pressure on the hydroformylation of linalool in the Rh/P(O-o-
tBuPh)3 system, the general trends are similar to those observed in
the Rh/PPh3 system [23]: a nearly first positive order in hydrogen
and also nearly first negative order in carbon monoxide (Table 2).
The increase in the total pressure decreased only slightly the
reaction rates (Table 2, runs 1–3), reflecting a net result of the oppo-
site kinetic effects of the gas reagents. The accelerating effect of
hydrogen suggests that the oxidative addition of hydrogen to the
rhodium acyl intermediate seems to be a rate-determining step at
the linalool hydroformylation.
3.2. Hydroformylation of ˇ-citronellene in toluene solutions
The hydroformylation of ␤-citronellene (2a) occurred smoothly
in toluene solutions in the presence of PPh3, with selectively being
high even at low P/Rh ratio (Table 3, run 1). Two aldehydes resulted
from the hydroformylation of the terminal double bond were
10
3
10
1
3
R
R
R
2
CHO
4
5
4
*
*
CHO
*
CO/H2
R, S
2
*
5
+
1
6
7
6
7
8
9
8
9
2a
2b
2c
-H2O
C2H5OH
10
3
1
R
CH(OC2H5)2
4
5
*
CH(OC2H5)2
2
+
6
7
9
8
2d
2e
Scheme 2. Hydroformylation/acetalization of ␤-citronellene (2a).

C.G. Vieira et al. / Applied Catalysis A: General 466 (2013) 208–215
213
Table 4
100
80
60
40
20
0
Hydroformylation of ␤-citronellene (2a) catalyzed by Rh/P(O-o-tBuPh)3.a
Run
P/Rh
P (CO)
(atm)
P (H2)
(atm)
Time (h)
TOF (h−1
)
Selectivity
(%)
t
P
(
O-o- BuPh
)
3
2b
2c
PPh
3
1
2
3
4
5
6
10
20
30
10
10
10
40
40
40
20
20
40
40
40
40
20
40
20
1.5
1.5
1.5
1.5
0.75
2.0
240
200
240
250
360
150
68
66
66
68
67
66
31
33
33
31
32
32
none
a
Conditions: ␤-citronellene (0.10 M), [Rh(COD)(OMe)]2 (0.25 mM), 40 ◦C,
toluene. Conversion and selectivity are based on the substrate reacted; the reac-
tion time is given for nearly complete conversions. TOF – initial turnover frequency
(mol of the converted substrate per mol of Rh per hour).
0
1
2
3
Time (h)
runs 2 and 7; Table 4, cf. runs 1 and 5). The variation in the total
pressure of the equimolar gas mixture had no influence on the reac-
tion rates due to the opposite kinetic effects of the gas reagents
(Table 3, cf. runs 2 and 5; Table 4, cf. runs 1 and 4). The positive
order in hydrogen observed at relatively high pressures suggests
that the hydrogenolysis of the rhodium acyl intermediate seems to
be the most likely rate-determining step in the hydroformylation
of ␤-citronellene.
Fig. 3. Rhodium catalyzed hydroformylation of ␤-citronellene: ligand effect. Condi-
tions: ␤-citronellene (0.10 M), [Rh(COD)(OMe)]2 (0.25 mM), ligand (5.0 mM), 50 ◦C,
80 atm (CO/H2 = 1/1), toluene.
formed with a 95% combined selectivity (Scheme 2), the linear iso-
mer (2b) accounting for 74% of the mass balance. The branched
aldehyde (2c) was formed as a mixture of two diastereoisomers,
(2R,3R) and (2S,3R), detected in nearly equal amounts as the
molecule of starting ␤-citronellene has R configuration at asym-
metric carbon C-3. The diastereoisomers were not GC separable
under the conditions used; however, their molecules showed dif-
ferent NMR spectra.
The increase in P/Rh to 10 expectedly decreased the reaction
rate and improved selectivity for aldehydes, which were formed
in a nearly quantitative combined yield. A further increase in the
phosphine concentration did not affect significantly the reaction.
In the absence of the P-ligand, the hydroformylation of ␤-
citronellene occurred faster than in the Rh/PPh3 system, albeit
with lower selectivity (Table 3, cf. run 8 and run 2). This result
is expected for simple non-functionalized non-conjugated alkenes
due to the competition between the phosphine and the substrate
for the coordination sites on rhodium. On the other hand, a sig-
nificant accelerating effect of the phosphite ligand, P(O-o-tBuPh)3,
as compared to the unmodified system is much less expected. The
reaction in the presence of P(O-o-tBuPh)3 was 2.5 times faster than
in the absence of the P-ligand and more than ten times faster than
with PPh3 (Table 3, runs 2, 8 and 9; Fig. 3).
The results of the kinetic study are in agreement with the
observed accelerating effect of P(O-o-tBuPh)3 in the hydroformyla-
tion of ␤-citronellene as compared to the unmodified system. It is
reasonable to expect that a higher steric hindrance in the active
rhodium species containing the bulky P(O-o-tBuPh)3 ligand will
result in lower reaction rates. However, the reaction occurs much
faster in the presence of P(O-o-tBuPh)3 implying that electronic
rather than steric parameters dominate in this system. Consider-
ing that P(O-o-tBuPh)3 is more basic ligand than CO, the rhodium
center should be more electron rich as compared to the center with
only carbonyl ligands. Therefore, the most likely rate-determing
step which involves the oxidative addition of hydrogen should be
accelerated by the presence of the phosphite ligand on rhodium.
It is also important that in the Rh/P(O-o-tBuPh)3 system (dif-
ferently from the Rh/PPh3 system) the formation of less active
bis(phosphorous ligand) rhodium species is highly disfavored by
the ligand bulkiness. Bulky phosphite ligands usually show much
better performance in hydroformylation compared to phosphine
ligands due to the combination of steric and electronic effects
[39,40].
The accelerating effect of P(O-o-tBuPh)3 as compared to PPh3
can be explained by the large cone angle of the former, which dis-
favors the formation of bis(phosphorous ligand) rhodium species.
At the same P/Rh ratios, for the Rh/P(O-o-tBuPh)3 system a larger
part of rhodium exists as mono(phosphorous ligand) species, which
are more active in hydroformylation. Furthermore, the weaker ␴-
basicity combined with the stronger ␲-acidity of P(O-o-tBuPh)3
(ꢀ-value of 30.50 vs. 13.25 for PPh3) favors the more facile CO dis-
sociation and the stronger alkene association with rhodium [38]. In
the presence of phosphite, the initial reaction rates depended only
slightly on the ligand concentration (Table 4, runs 1–3). Thus, the
P/Rh ratio of 10 is enough to keep most of rhodium coordinated to
one P(O-o-tBuPh)3 and, even at P/Rh = 30, bis(phosphorous ligand)
species are not formed in appreciable amounts. On the other hand,
the reaction with less bulky phosphite, P(OPh)3, was much slower
than that with P(O-o-tBuPh)3 (Table 3, run 11).
3.3. Hydroformylation of linalool and ˇ-citronellene in ethanol
solutions
The hydroformylation of linalool and ␤-citronellene was per-
formed also in ethanol, a renewable, low cost and environmentally
friendly solvent, to substitute oil industry derivatives such as
toluene, commonly used in this reaction. The solvent promoted a
significant kinetic effect on the reactions and, under certain con-
ditions, changed the product nature. In general, the reactions with
both substrates were ca. two times faster in ethanol than in toluene
and showed nearly the same combined selectivity for the hydro-
formylation products.
Linalool was virtually inactive in ethanol solutions in the
absence of phosphorous ligand, similarly to what was found in
toluene, probably due to the formation of chelate complexes
between linolool and rhodium (Table 5, run 1). The addition of
PPh3 induced the reactivity of linalool toward hydroformylation
by cleavage of chelates, with a maximum activity being observed
at P/Rh = 10–20 (Table 5, runs 2–6). The reaction gives almost quan-
titatively the cyclic acetal 1d, which formally results from the
etherification of hemiacetal 1c with ethanol (Scheme 1). In most
We have also studied the effects of the carbon monoxide
and hydrogen pressure on the hydroformylation of ␤-citronellene
(Tables 3 and 4). The general trends are similar for both ligands:
a positive order in hydrogen (Table 3, cf. runs 2 and 6; Table 4, cf.
runs 1 and 6) and an inverse order in carbon monoxide (Table 3, cf.

214
C.G. Vieira et al. / Applied Catalysis A: General 466 (2013) 208–215
Table 5
Rhodium catalyzed hydroformylation of linalool (1a) in ethanol.a
Run
Ligand
P/Rh
Time (h)
TOF (h−1
)
Selectivity for 1d (%) (cis/trans)
1
2
3
None
PPh3
PPh3
PPh3
PPh3
PPh3
–
2
5
10
20
50
2
–
12
4
4
4
4
2
0.8
0.5
0.5
0.5
<0.5
50
150
190
185
84
200
520
99 (48/52)
98 (48/52)
97 (73/27)
98 (74/26)
98 (80/20)
96 (95/5)
97 (62/38)
97 (66/34)
97 (77/23)
97 (85/15)
97 (85/15)
4
5
6b
7
P(O-o-tBuPh)3
P(O-o-tBuPh)3
P(O-o-tBuPh)3
P(O-o-tBuPh)3
P(O-o-tBuPh)3
8
5
9c
10c
11c
10
20
30
1000
1000
1000
a
Conditions: linalool (0.30 M), [Rh(COD)(OMe)]2 (0.30 mM), 50 ◦C, 20 atm (CO/H2 = 1/1), ethanol. For most of the run the reaction time is given for nearly complete
conversions. Conversion and selectivity are based on the substrate reacted. TOF – initial turnover frequency (mol of the substrate converted per mol of Rh per hour).
b
57% conversion for 4 h.
Average TOF is given as the reaction was too fast.
c
of the runs, small to medium amounts of 1c were detected in the
reaction solutions at incomplete linalool conversions.
runs 3 and 4). However, a really remarkable kinetic effect was pro-
moted by P(O-o-tBuPh)3: the initial reaction rates at the same P/Rh
ratio and under the same conditions were ca. 20 times higher with
P(O-o-tBuPh)3 than with PPh3(Table 6, runs 2 and 3 vs. runs 4
and 5). At P/Rh = 5 the aldehydes were obtained in nearly quan-
titative combined yield, with only small amounts of corresponding
acetals being observed at the end of the reaction (Table 6, run 4). A
complete substrate conversion occurred in 1 h. The increase in the
P(O-o-tBuPh)3 concentration (runs 5 and 6) and in reaction tem-
perature (runs 7 and 8) enhanced the acetalization activity of the
catalyst. At 80 ◦C, a substantial part of the primarily formed alde-
hydes 2b and 2c was converted into corresponding acetals 2d and
2e. The acetalization step was much slower than the hydroformyla-
tion as relative amounts of acetals increased with the reaction time.
However, we could not find the reaction conditions to convert com-
pletely the aldehydes to the acetals. At longer reaction times the
content of acetals did not increase suggesting a thermodynamic
equilibrium, whereas at higher temperature the combined selec-
tivity for the hydroformylation products became to decrease due
to the formation of numerous unidentified compounds. Thus, the
hydroformylation of ␤-citronellene in ethanol solutions can result,
depending on the reaction temperature, either almost exclusively
in the aldehyde products or in a mixture of aldehydes and acetals
with different organoleptic properties.
The use of P(O-o-tBuPh)3 instead of PPh3 remarkably acceler-
ated the hydroformylation of linalool in ethanol solutions (Table 5,
runs 7–11). The reactions in the presence of P(O-o-tBuPh)3 were
ca. 5 times faster than in the presence of PPh3 under the same
conditions. Similarly to the Rh/PPh3 system, the increase in the
P(O-o-tBuPh)3/Rh ratio up to 10 resulted in higher reaction rates.
The stereoselectivity of the cyclization step also strongly depended
on the concentration of phosphorous ligand: cis and trans isomers
of acetal were formed in comparable amounts at low P/Rh ratios,
whereas at high ligand concentrations the cis isomer became the
major or the almost exclusive reaction product. Similar trends in
stereoselectivity of the cyclization step were for both solvents and
both ligands used in the present work. The cis isomers of cyclic
hemiacetal 1c and its ethyl ether 1d arise from the less hindered
cyclic rhodium complex (proposed as the reaction intermediate)
and are preferred products in the solutions with relatively high
concentrations of bulky phosphorous ligands. It is important to
note that the efficient etherification of hemiacetal 1c occurs in
the ethanol solutions under the hydroformylation conditions in the
absence of any additional acid co-catalyst.
The hydroformylation of ␤-citronellene also readily occurs in
ethanol solution giving aldehydes 2b and 2c and corresponding
acetals 2d and 2e in a nearly quantitative combined yield (Table 6,
Scheme 2). In the unmodified system, the reaction at 40 ◦C was very
slow showing ca. 50% conversion in 24 h (Table 6, run 1). The intro-
duction of PPh3 accelerated the reaction, which was completed in
24 h, and promoted a partial acetalization of the aldehydes (Table 6,
It has been previously reported that RhCl3 in the com-
bination with P(OPh)3 is an efficient catalyst for the
hydroformylation–acetalization of alkenes without the addition of
acid co-catalysts [41,42]. Intermediate rhodium-chloro-phosphite
complexes having Lewis acidity were suggested as active species
Table 6
Hydroformylation of ˇ-citronellene (2a) in ethanol.a
Run
Ligand
P/Rh
T (◦C)
Time (h)
TOF (h−1
)
Selectivity (%)
Aldehydes
Acetals
Aldehydes/acetals
2b
2c
2d
2e
1b
2
3
4
5
None
PPh3
PPh3
40
40
40
40
40
40
60
24
24
24
1.00
1.25
1.25
0.25
4.00
0.25
1.00
18
32
29
540
550
360
1600
70
63
62
66
63
61
73
48
39
26
25
12
11
28
27
22
23
20
19
15
5
24
24
6
8
15
4
24
39
50
–
1
2
–
1
2
95/5
5
10
5
10
20
5
75/25
73/26
94/6
90/9
83/17
96/4
68/26
58/42
41/56
P(O-o-tBuPh)3
P(O-o-tBuPh)3
P(O-o-tBuPh)3
P(O-o-tBuPh)3
6
7c
2
3
6
8c
P(O-o-tBuPh)3
5
80
1600
a
Conditions: ␤-citronellene (0.20 M), [Rh(COD)(OMe)]2 (0.25 mM), 20 atm (CO/H2 = 1/1), ethanol. Conversion and selectivity are based on the substrate reacted; the reaction
time is given for nearly complete conversions. TOF – initial turnover frequency (mol of the converted substrate per mol of Rh per hour).
b
55% conversion for 24 h.
Average TOF is given as the reaction was too fast.
c

C.G. Vieira et al. / Applied Catalysis A: General 466 (2013) 208–215
215
in the conversion of alkene into aldehyde followed by acetal-
ization. We have tested the RhCl3/P(O-o-tBuPh)3 system in the
hydroformylation of linalool and ␤-citronellene in ethanol solu-
tions. Both reactions gave mainly etherification or acetalization
products (ethyl ether 1d from linalool and acetals 2d and 2e
from ␤-citronellene); however, substrate conversions occurred
much slower than with Rh(COD)(OMe)]2 as the catalyst precur-
sor. Under the same conditions, when a complete conversion
of linalool (Table 5, run 7) and ␤-citronellene (Table 6, run 8)
with Rh(COD)(OMe)]2 occurred for 2 and 0.25 h, respectively; the
reactions with RhCl3 required much longer reaction times (22 and
2 h, respectively).
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uct distribution. The reactions occurs approximately two times
faster in ethanol than in toluene and up to twenty times faster in
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