Supported rhodium nanoparticles obtained by Metal Vapour Synthesis as catalysts in the preparation...

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al 339 (2008) 84–92
Applied Catalysis A: Gener
Supported rhodium nanoparticles obtained by Metal Vapour Synthesis

as catalysts in the preparation of valuable organic compounds

Claudio Evangelisti a,b, Nicoletta Panziera a,b, Maria Vitulli b,1, Paolo Pertici a,*,Federica Balzano b, Gloria Uccello-Barretta a,b, Piero Salvadori a,b

a Istituto di Chimica dei Composti Organo-Metallici, ICCOM-CNR, Sezione di Pisa, via Risorgimento 35, 56126 Pisa, Italyb Dipartimento di Chimica e Chimica Industriale, Universita di Pisa, via Risorgimento 35, 56126 Pisa, Italy

Received 23 November 2007; received in revised form 10 January 2008; accepted 14 January 2008

Available online 18 January 2008

Abstract

Rhodium nanoparticles, derived from mesitylene-solvated rhodium atoms, deposited on g-Al2O3, are excellent catalysts for the selective

hydrogenation of the double bond in a,b-unsaturated carbonyl compounds. The low metal loading Rh on g-Al2O3 catalyst, containing

trioctylamine, TOA, as stabilizing agent of the metal nanoparticles, Rh(TOA)/g-Al2O3, 0.1 wt.% Rh, showed the highest catalytic activity.

Using this catalyst 4-(6,-methoxy-2,-naphthyl)-3-buten-2-one was reduced to the anti-inflammatory drug 4-(6,-methoxy-2,-naphthyl)-butan-2-one,

NabumetoneTM, with complete selectivity and under mild reaction conditions (room temperature, atmospheric pressure of hydrogen). Similarly, 2-

acetyl-5,8-dimethoxy-3,4-dihydronaphthalene was hydrogenated with high selectivity (85%) to 2-acetyl-5,8-dimethoxy-1,2,3,4-tetrahydro-

naphthalene, precursor of antitumor anthracyclinic compounds, which was obtained chemically pure by crystallization. No leaching of rhodium

was observed. The catalyst was completely recovered and, if reused, it works without loss of activity. 1H NMR DOSY analysis of the Rh(TOA)/

mesitylene solution evidenced the presence of nanoparticles with a diameter of about 1.1 nm.

# 2008 Elsevier B.V. All rights reserved.

Keywords: Supported rhodium nanoparticles; Metal Vapour Synthesis; Selective hydrogenation; a,b-Unsaturated ketones; 1H NMR DOSY analysis

1. Introduction

A great deal of attention is currently being given to the study

of nanostructured metal catalysts [1–3]. Their chemical

behaviour is strongly related to the particle size; clusters

averaging 1–2 nm in diameter are expected to be very reactive,

not only because of the large surface area but also as a result of

the significantly different electronic structure of the small

nanoclusters [4,5].

Among the various preparative methods [1], Metal Vapour

Synthesis (MVS) provides a valuable synthetic route to very

active and selective nanostructured catalysts [6–8]. Solvent-

stabilized metal microclusters (solvated metal atoms, SMA),

obtained by co-condensation of metal vapours with weakly

* Corresponding author. Tel.: +39 050 2219224; fax: +39 050 2219260.

E-mail address: [email protected] (P. Pertici).1 Present address: Drogheria e Alimentari S.r.l., via Nilde Iotti 23, 53037

Firenze, Italy.

0926-860X/$ – see front matter # 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcata.2008.01.010

stabilizing organic ligands, are good precursors for the

preparation of very efficient catalysts, which are employed in

a wide range of reactions such as oligomerization [9,10],

hydroformylation [11], hydrosilylation [12,13], silylformylation

[14] and carbon–carbon coupling [15,16]. We recently reported

that nanosized rhodium catalysts supported on g-alumina,

prepared by deposition of rhodium particles from mesitylene-

solvated rhodium atoms stabilized by the added ligand

trioctylamine (TOA), are excellent hydrogenation catalysts

[17]. The trioctylamine (TOA) serves to quench the growth of

the metal microclusters in the mesitylene solution [18].

Encouraged by these preliminary results, we have studied the

catalytic activity of these systems for selective hydrogenation

of the a,b-unsaturated ketones 4-(60-methoxy-20-naphthyl)-3-

buten-2-one, 1, and 2-acetyl-5,8-dimethoxy-3,4-dihydro-

naphthalene, 2, to the corresponding saturated ketones. These

compounds are of interest to pharmaceutical industry. The

catalytic activity has been compared with that of catalysts

similarly prepared in the absence of TOA and with that of

commercial samples of rhodium on g-alumina. Complexing

mailto:[email protected]

http://dx.doi.org/10.1016/j.apcata.2008.01.010

C. Evangelisti et al. / Applied Catalysis A: General 339 (2008) 84–92 85

properties of TOA were investigated by 1H NMR DOSY

(Diffusion Ordered Spectroscopy) analysis.

2. Experimental

2.1. General

All operations involving the MVS products were performed

under a dry argon atmosphere. The co-condensation of rhodium

and mesitylene was carried out in a static reactor previously

described [19]. The ‘‘mesitylene-solvated Rh atoms’’ solution

was worked up under argon atmosphere with use of the standard

Schlenk techniques. The amount of rhodium in the above

solution was determined by Atomic Absorption Spectrometry

in a electrochemically heated graphite furnace with a

PerkinElmer 4100ZL instrument. The limit of detection

calculated for rhodium was 2 ppb. Solvents were purified by

conventional methods, distilled and stored under argon.

Commercial g-Al2O3 (Chimet product, type 49, surface area

110 m2/g, mean particle diameter 31 mm) was dried in a static

oven before use. Commercial rhodium on g-Al2O3 (5 wt.% of

Rh, surface area 130 m2/g) was an Engelhardt product.

Trioctylamine (from Aldrich) was distilled and stored over

KOH pellets before use. 4-(60-Methoxy-2,-naphthyl)-3-buten-

2-one, 1, was prepared starting from 2-bromo-6-methoxy-

naphthalene (Aldrich product) according to the procedure

described in the literature [20]. 2-Acetyl-5,8-dimethoxy-3,4-

dihydronaphthalene, 2, was prepared by starting from

p-benzoquinone and 1,3-butadiene, methylation with dimethyl-

sulfate, isomerization with potassium t-butoxide and acylation,

according to the procedure reported in the literature [21,22].

The GLC analyses were performed on a PerkinElmer Auto

System gas chromatograph, equipped with a flame ionization

detector (FID), using a SiO2 column (DB1, 30 m � 0.52 mm,

5 mm) and helium as carrier gas. The TLC analyses were

performed with silica gel using petroleum ether/AcOEt 8/2 as

eluent. The HPLC analyses were performed on a Waters 600E

chromatograph, equipped with a Lichrosper 100 column (RP

18, 250 mm � 40 mm, 5 mm) using CH3CN/TFA 0.1% 60/40

as eluent with a flow of 1 ml/min at l = 214 nm. The GC–MS

analyses were carried out with a PerkinElmer Q-mass 910

spectrometer connected with a PerkinElmer gas chromato-

graph, equipped with a ‘‘split–splitless’’ injector, using a SiO2

capillary column and helium as carrier gas. 1H NMR and 13C

NMR spectra were measured on a Varian Gemini 200

spectrometer at 200 and 50 MHz, respectively, using chloro-

form-d as a solvent; chemical shifts are relative to internal

Si(CH3)4. NMR measurements were also performed on a

Varian INOVA 600 spectrometer operating at 600 and 150 MHz

for 1H and 13C, respectively, using a 5 mm broadband inverse

probe with z-axis gradient. The sample temperature was

maintained at 25 8C. 1H and 13C NMR chemical shifts are

referenced to TMS as external standard. The 2D NMR spectra

were obtained by using standard sequences. Proton gCOSY 2D

spectra were recorded in the absolute mode acquiring 4 scans

with a 3 s relaxation delay between acquisitions for each of 256

FIDs. DOSY experiments were performed on samples obtained

by evaporation of the solvent from the Rh(TOA)/mesitylene

solvated and further dissolution in C6D6. They were carried out

by using a stimulated echo sequence with self-compensating

gradient schemes, a spectral width of 6600 Hz and 64 K data

points. A value of 120 ms was used for D, 1.0 ms for d, and g

was varied in 30 steps (16 transients each) to obtain an

approximately 90–95% decrease in the resonance intensity at

the largest gradient amplitudes. The baselines of all arrayed

spectra were corrected prior to processing the data. After data

acquisition, each FID was apodized with 1.0 Hz line broad-

ening and Fourier transformed. The data were processed with

the DOSY macro (involving the determination of the resonance

heights of all the signals above a pre-established threshold and

the fitting of the decay curve for each resonance to a Gaussian

function) to obtain pseudo two-dimensional spectra with NMR

chemical shifts along one axis and calculated diffusion

coefficients along the other. Infrared (IR) spectra were recorded

using a PerkinElmer Spectrum GX Ft-IR. The melting points

were determined on a Reichert Thermovar instrument.

2.2. Preparation of the rhodium catalysts

2.2.1. Synthesis of the solvated Rh atoms

In a typical experiment, rhodium vapour, generated by

resistive heating of a tungsten wire surface coated with

electrodeposited rhodium (170 mg), was co-condensed at

liquid nitrogen temperature with mesitylene (45 ml) in the

glass reactor chamber of the MVS apparatus in ca. 45 min. The

reactor chamber was warmed to the melting point of the solid

matrix (ca. �30/�40 8C) and the resulting brown solution was

siphoned at low temperature in a Schlenk tube and kept in a

refrigerator at �20 8C. The content of the metal solution was

1.8 mg rhodium/ml (0.0175 mg atom/ml).

2.2.2. Preparation of rhodium on g-Al2O3 catalysts

The Rh/mesitylene reaction solution (5.6 ml, 10 mg Rh) was

added to a suspension of g-Al2O3 (10 g) in mesitylene (25 ml).

The mixture was stirred for 12 h at room temperature. The

colourless solution was removed and the light-brown solid,

containing 0.1 wt.% Rh, was washed with n-pentane and dried

under reduced pressure. The Rh/g-Al2O3 catalysts, containing

1 wt.% Rh, was prepared similarly using 1 g of g-Al2O3.

2.2.3. Preparation of rhodium–trioctylamine (TOA)

on g-Al2O3 catalysts

(a) Preparation of Rh(TOA)/mesitylene solution. Trioctylamine

(TOA) (0.56 ml, 1.31 mmol) was added to the MVS Rh/

mesitylene solution (25 ml, 0.44 mg atom) and the mixture

was stirred for 1 h at room temperature. The resulting

brown Rh(TOA)/mesitylene solution was stable at room

temperature for a long time (about 3 months).

(b) P
reparation of the Rh(TOA)/g-Al2O3 catalysts. The brown
Rh(TOA)/mesitylene solution (5.6 ml, 10 mg Rh) was

added to a suspension of g-Al2O3 (10 g) in mesitylene

(25 ml). The mixture was stirred for 12 h at room

temperature. The colourless mesitylene was removed and

the light-brown solid, containing 0.1 wt.% Rh, was washed

Tabl

Hyd

hydr

Run

1

2

3d

4

5

6

7

8e

9g

Reaa Db Sc Md Re Rf Rg R

C. Evangelisti et al. / Applied Catalysis A: General 339 (2008) 84–9286

with pentane and dried under reduced pressure. The

Rh(TOA)/g-Al2O3 catalyst containing 1 wt.% Rh was

prepared similarly using 1 g of g-Al2O3.

2.3. Catalytic reactions

2.3.1. Hydrogenation of 4-(60-methoxy-2,-naphthyl)-3-

buten-2-one, 1, to 4-(60-methoxy-20-naphthyl)-butan-2-one,

1a, NabumetoneTM

In a typical experiment (runs 1–7), toluene (20 ml), 4-(6,-

methoxy-2,-naphthyl)-3-buten-2-one, 1 (1 g, 4.42 mmol) and

the rhodium catalyst (4.42 � 10�3 mg atom Rh) were intro-

duced under nitrogen atmosphere into a 100 ml flat-bottomed,

three necked flask equipped with a stirring magnetic bar, a

silicon stopper and connected to a gas-volumetric burette

containing hydrogen. The nitrogen was removed under vacuum

and hydrogen was introduced. The mixture was stirred (ca.

1200 rev/min) at room temperature and under atmospheric

pressure of hydrogen. The progress of the reaction was

monitored by the absorption of hydrogen in the burette and the

composition of the reaction mixture was determined by GLC

analysis of liquid samples taken from the stoppered side neck

with a syringe. The products were identified by comparison of

their GLC retention times with those of authentic samples and

by GC–MS analysis. The reaction was interrupted at the time

indicated in Table 2.

The runs 8 and 9, performed with a large amount of 1 in

toluene or ethanol 95% as reaction medium, respectively, have

been carried out introducing the solvent (100 ml), 1 (50 g,

0.221 mol) and the catalyst [Rh(TOA)/g-Al2O3 (0.1 wt.% Rh),

22.7 g, 0.221 � 10�3 g atom Rh] in a 1l flat-bottomed, three

necked flask equipped with a stirring magnetic bar, a silicon

stopper and a glass cannula, connected to the hydrogen

reservoir, for the gas bubbling. The nitrogen was removed

under vacuum and hydrogen was introduced. The mixture was

stirred (ca. 1200 rev/min) at room temperature and hydrogen

e 1

rogenation of 4-(60-methoxy-20-naphthyl)-3-buten-2-one, 1, with MVS-rhodium

ogen

Catalyst Time (min)

Rh/g-Al2O3 (1 wt.% Rh) 120

Rh(TOA)/g-Al2O3 (1 wt.% Rh) 42

Rh/g-Al2O3 commercial (5 wt.% Rh) 720

Rh(TOA)/g-Al2O3 (0.1 wt.% Rh) 24

Rh/g-Al2O3 (0.1 wt.% Rh) 180

Rh(TOA)/g-Al2O3 (1 wt.% Rh) recovered from run 2 42

Rh(TOA)/g-Al2O3 (0.1 wt.% Rh) recovered from run 4 24



ction conditions: compound 1 = 1 g (4.42 mmol); catalyst [Rh] = 4.42 � 10�3

etermined by GLC analysis.

ee Scheme 2 for the assignment of the products.

oles of converted 1 per gram atom rhodium per minute.

eaction performed with a molar ratio 1/Rh = 100.

eaction performed with: 1 = 50 g (0.221 mol); catalyst = 22.7 g (0.1 wt.% Rh

ecovered by crystallization.

eaction performed with: 1 = 50 g (0.221 mol); catalyst = 22.7 g (0.1 wt.% Rh

was bubbled through the suspension. The progress of the

reaction was monitored as above reported. At complete

conversion of 1 the reaction was stopped (see Table 1). The

solution was filtered and the catalyst was washed with the

reaction solvent. The solutions were united and the solvent was

removed in vacuo giving a crude solid which was recrystallized

from methanol to give pure 1a (49.5 g, 0.216 mol, yield 98%).

mp = 81–83 8C. MS m/z: 228 (M+). IR n(C O) = 1705 cm�1.1H NMR, d: 2.17 (s, 3H, CH3CO); 2.85 (t, 2H, J = 7.2 Hz,

ArCH2CH2); 3.05 (t, 2H, CH2CH2CO); 3.93 (s, 3H, CH3O);

7.11–7.75 (m, 6H, H arom). 13C NMR, d: 198.1 (CO); 151.1

(Ar); 136.2 (Ar); 133.1 (Ar); 128.9 (Ar); 128.4 (Ar); 127 (Ar);

126.3 (Ar); 125.1 (Ar), 118.9 (Ar); 105.7 (Ar); 55.4 (OMe);

45.3 (CH2CO); 32.7 (CH2Ar); 30.3 (COCH3).

2.3.2. Hydrogenation of 2-acetyl-5,8-dimethoxy-3,4-

dihydronaphthalene, 2, to 2-acetyl-5,8-dimethoxy-1,2,3,4-

tetrahydronaphthalene, 2aIn a typical experiment (runs 10–15), toluene (20 ml),

2-acetyl-5,8-dimethoxy-3,4-dihydronaphthalene, 2 (1 g,

4.3 mmol) and the rhodium catalyst (4.3 � 10�2 mg atom

Rh) were introduced under nitrogen atmosphere into a 100 ml

flat-bottomed, three necked flask equipped with a stirring

magnetic bar, a silicon stopper and connected to a gas-

volumetric burette containing hydrogen. The nitrogen was

removed under vacuum and hydrogen was introduced. The

mixture was stirred (ca. 1200 rev/min) at room temperature and

under atmospheric pressure of hydrogen. The progress of

the reaction was monitored by the absorption of hydrogen in the

burette and by TLC analysis of liquid samples taken from

the stoppered side neck with a syringe. The composition of the

reaction mixture was determined by HPLC analysis by

comparing the retention times with those of authentic samples.

The reaction was interrupted at the time indicated in Table 1.

The runs 16 and 17, performed with a large amount of 2 in

toluene or ethanol 95% as reaction medium, respectively, have

on g-alumina derived catalysts at room temperature and atmospheric pressure of

Conversiona (%) Selectivity (%)a,b Specific activityc (SA) (min�1)

1a 1b 1c

100 98 – 2 8.3

100 100 – – 23.8

100 90 – 10 0.3

100 100 – – 41.7

10 100 – – 0.6

100 100 – – 23.8

100 100 – – 41.7

100 100 (98)f – – 41.7

100 100 (98)f – – 55.6

mg atoms; solvent = toluene (20 ml); T = 25 8C; P(H2) = 1 atm.

, 0.221 � 10�3 g atoms); solvent = toluene (100 ml).

, 0.221 � 10�3 g atoms) solvent = ethanol 95% (100 ml).

Table 2

Hydrogenation of 2-acetyl-5,8-dimethoxy-3,4-dihydronaphthalene, 2, with MVS-rhodium on g-alumina derived catalysts at room temperature and atmospheric

pressure of hydrogen

Run Catalyst Time (h) Conversiona (%) Selectivity (%)a,b Specific activityc (SA) (h�1)

2a 2b 2c

10 Rh/g-Al2O3 (1 wt.% Rh) 24 100 81 8 11 4.1

11 Rh(TOA)/g-Al2O3 (1 wt.% Rh) 24 100 86 3 11 4.1

12 Rh/g-Al2O3 commercial (5 wt.% Rh) 24 85 74 16 10 3.5

13 Rh(TOA)/g-Al2O3 (0.1 wt.% Rh) 3.5 98 89 3 8 19.6

5 100 90 2 8 20

14 Rh/g-Al2O3 (0.1 wt.% Rh) 5 Traces –

15 Rh(TOA)/g-Al2O3 (0.1 wt.% Rh) recovered from run 13 5 100 90 2 8 20

16d Rh(TOA)/g-Al2O3 (0.1 wt.% Rh) 5 100 90 (84)e 2 8 20

17f Rh(TOA)/g-Al2O3 (0.1 wt.% Rh) 2.5 100 85 (80)e 5 10 40

Reaction conditions: compound 2 = 1 g (4.3 mmol); catalyst [Rh] = 4.3 � 10�2 mg atoms; solvent = toluene (20 ml); T = 25 8C; P(H2) = 1 atm.a Determined by TLC and HPLC analyses.b See Scheme 6 for the assignment of the products.c Moles of converted 2 per gram atom rhodium per hour.d Reaction performed with: 2 = 40 g (0.172 mol); catalyst = 177 g (0.1 wt.% Rh, 0.172 � 10�2 g atoms); solvent = toluene (300 ml).e Recovered by crystallization.f Reaction performed with: 2 = 40 g (0.172 mol); catalyst = 177 g (0.1 wt.% Rh, 0.172 � 10�2 g atoms); solvent = ethanol 95% (300 ml).


been carried out introducing the solvent (300 ml), 2 (40 g,

0.172 mol) and the catalyst [Rh(TOA)/g-Al2O3 (0.1 wt.% Rh),

177 g, 0.172 � 10�2 g atom Rh] in a 1l flat-bottomed, three

necked flask equipped with a stirring magnetic bar, a silicon

stopper and a glass cannula, connected to the hydrogen

reservoir, for the gas bubbling. The nitrogen was removed

under vacuum and hydrogen was introduced. The mixture was

stirred (ca. 1200 rev/min) at room temperature and hydrogen

was bubbled through the suspension. The progress of the

reaction was monitored as above reported. At complete

conversion of 2 the reaction was stopped (see Table 2). The

solution was filtered and the catalyst was washed with the

reaction solvent. The solutions were collected and the solvent

was removed in vacuo giving a crude product which was

recrystallized from isopropyl alcohol to give pure 2-acetyl-5,8-

dimethoxy-1,2,3,4-tetrahydronaphthalene, 2a (yield = 80–

84%, see Table 2). mp = 81–83 8C. MS m/z: 234 (M+). IR

n(C O) = 1708 cm�1. 1H NMR (600 MHz, CDCl3, 25 8C): d

1.63 (m, 1H, H-d), 2.16 (m, 1H, H-d0), 2.25 (s, 3H, COMe), 2.54

(ddd, J = 17.3, 11.7, 5.9 Hz, 1H, H-e), 2.59 (ddd, J = 16.6, 11.6,

1.5 Hz, 1H, H-b), 2.65 (m, 1H, H-c), 2.94 (ddd, J = 17.3, 5.4,

3.0 Hz, 1H, H-e0), 3.04 (ddd, J = 16.6, 4.6, 1.8 Hz, 1H, H-b0),3.77 (s, 3H, OMe), 3.78 (s, 3H, OMe), 6.63 (s, 2H, H-a). 13C

NMR (150 MHz, CDCl3, 25 8C): d 211.5 (CO), 151.3

(quaternary C), 151.2 (quaternary C), 126.2 (quaternary C),

125.4 (quaternary C), 106.9 (CH–Ar), 106.8 (CH–Ar), 55.6

(OMe), 47.3 (CH), 28.1 (Me), 25.4 (CH2), 24.4 (CH2), 23.1

(CH2).

3. Results and discussion

3.1. Preparation of the catalysts: (i) rhodium on

g-alumina, Rh/g-Al2O3, and (ii) rhodium, containing

trioctylamine (TOA), on g-alumina, Rh(TOA)/g-Al2O3

The catalysts containing rhodium on g-alumina, Rh/g-

Al2O3, and Rh(TOA)/g-Al2O3, were prepared by MVS, as

shown in Scheme 1.

The solution of mesitylene-solvated Rh atoms (Scheme 1,

solvated A), obtained from rhodium and mesitylene vapours,

is stable at low temperature (�40 8C). On warming to room

temperature in the presence of g-Al2O3, it furnishes metal

nanoparticles which deposit homogeneously on the support

with formation of the Rh/g-Al2O3 catalyst. Addition of the

organic ligand trioctylamine, TOA, to solvated A stabilizes

the microclusters against agglomeration, giving rise to a

solution of mesitylene-solvated Rh atoms which is stable at

r.t. (Scheme 1, solvated B). On addition of g-Al2O3 to

solvated B, rhodium nanoparticles, containing TOA, deposit

homogeneously on the support with formation of the

Rh(TOA)/g-Al2O3 catalyst.

The rhodium particle size distribution, determined by

HRTEM analyses [17], gave a mean diameter dm = 2.1

� 0.25 nm for the Rh/g-Al2O3 catalyst and a mean diameter

dm = 1.1 � 0.25 nm for the Rh(TOA)/g-Al2O3 catalyst. The

structural characterization of Rh(TOA)/g-Al2O3 catalyst,

performed by HRTEM and EXAFS analyses and by IR

spectroscopy, after treatment with CO, demonstrated the

presence of homogeneously distributed metal particles with

very small diameters (about 1.1 nm). The effect of TOA was to

prevent the aggregation of atoms/clusters to form large particles

thus making available more Rh atoms for the catalysis. In

addition, TOA stabilizes the nanoparticles towards poisoning

and hinders their extensive oxidation by the solid oxide support

Fig. 1. DOSY map (600 MHz, C6D6, 25 8C) of solvated Rh(TOA)/mesitylene.

Scheme 1.


[17]. All these factors account for the high activity exhibited by

the Rh(TOA)/g-Al2O3 catalysts.

3.2. Characterization via 1HNMR and 1HNMR-DOSY of

mesitylene-solvated rhodium atoms solution stabilized by

TOA (solvated B, Scheme 1)

DOSY technique represents a very powerful tool for the

detection of size-dependent phenomena [23,24]. As a matter of

fact, it allows to measure translational diffusion coefficients

(D), which are correlated to molecular sizes by means of

hydrodynamic radius (RH) on the basis of Stokes-Einstein

equation (Eq. (1)):

D ¼ kT

6ph R H

(1)

where k is the Boltzmann constant, h is the solvent viscosity and

T is the temperature in K. Eq. (1) strictly holds for spherical

systems in non-viscous solutions, whose viscosity can be

approximated to that one of the solvent. Thus we recorded

DOSY maps of pure TOA in C6D6 and of its rhodium solvated

system. For the pure compound we measured a diffusion

coefficient of 7.0 � 10�10 m2 s�1 to be compared to the value

of 5.9 � 10�10 m2 s�1 (Fig. 1) corresponding to the solvated

species, the diameter of which was calculated as about 1.1 nm

(Eq. (1)). A remarkable decrease of the amine diffusion coeffi-

cient occurred as a consequence of metal complexation.

The diameter of the solvated species in solution is

comparable to that found by HRTEM analysis of the

Rh(TOA)/g-Al2O3 sample [17] reported in Section 3.1,

pointing out that no aggregation process of the nanoparticles

happens during their deposition on the support.

Interestingly, the comparison of 1H NMR spectra in C6D6 of

the pure amine and of the solvated species solution (Fig. 2a and

b, respectively) pointed out differences both in the linewidth

and chemical shifts, these last being mainly relevant for b- and

g-methylene groups of the amine, low-frequency shifted by

�0.025 and �0.055 ppm, respectively, These results are in

agreement to those reported by Richard and co-workers [25]

regarding colloidal Rh(TOA) systems obtained from reduction

of RhCl3, in which amine aliphatic chains are mainly involved

in the formation of metal aggregates. They highlighted a

dynamic interaction between the doublet of the amine nitrogen

and the surface of the rhodium particle. This interaction gives

rise to a spatial distribution of the aliphatic chains in which the

a-CH2 groups are far from the metal surface while the b-CH2

and the g-CH2 groups are located near to this surface, justifying

their chemical shifts with respect to the free amine.

3.3. Catalytic studies with Rh/g-Al2O3 and

Rh(TOA)/g-Al2O3 systems

3.3.1. Selective hydrogenation of the a,b-unsaturated

ketones 4-(60-methoxy-20-naphthyl)-3-buten-2-one, 1, and

2-acetyl-5,8-dimethoxy-3,4-dihydronaphthalene, 2, to the

corresponding saturated ketones 4-(60-methoxy-20-naphthyl)-butan-2-one, 1a, and 2-acetyl-5,8-dimethoxy-

1,2,3,4-tetrahydronaphthalene, 2aConsidering the interesting catalytic properties of Rh/g-

Al2O3 and of Rh(TOA)/g-Al2O3 exhibited in the selective

hydrogenation of cinnamaldehyde to give the hydrocinnamal-

dehyde in high yield [17], these catalysts have been employed

in the selective reduction of the double bond in a,b-unsaturated

ketones such as 4-(60-methoxy-20-naphthyl)-3-buten-2-one, 1,

and 2-acetyl-5,8-dimethoxy-3,4-dihydronaphthalene, 2, which

Fig. 2. 1H NMR (600 MHz, C6D6, 25 8C) spectra of (a) trioctylamine, TOA and

(b) solvated Rh(TOA)/mesitylene.


are precursors to compounds of considerable value in fine

chemistry.

3.3.1.1. Hydrogenation of 4-(6,-methoxy-2,-naphthyl)-3-bu-

ten-2-one, 1. The 4-(60-methoxy-20-naphthyl)-3-buten-2-one,

1, furnishes by hydrogenation of the double bond the saturated

ketone 4-(60-methoxy-20-naphthyl)-butan-2-one, 1a, commer-

cially called nabumetoneTM, which is an important non-

steroidal anti-inflammatory drug. Other products which can be

obtained in the course of the hydrogenation of 1 are the a,b-

unsaturated alcohol, 1b, and the saturated alcohol, 1c

(Scheme 2).

The formation of 1a by the catalytic hydrogenation of 1 is

the final step of a synthetic procedure industrially used for the

preparation of nabumetone. Such procedure starts from 2-

bromo-6-methoxynaphthalene and, via the reaction between 6-

methoxy-2-naphthalenecarbaldehyde and acetone, furnishes 1

in 50% yield [20,26]. By catalytic hydrogenation of 1 in the

Scheme

Scheme

presence of 10% Pd on carbon at room temperature and

atmospheric pressure of hydrogen 1a is obtained in 70% yield

(Scheme 3).

Compound 1 is obtained also in high yield (90%) but under

severe reaction conditions (autoclave, 130 8C) by the palla-

dium-catalyzed C–C coupling of 2-bromo-6-methoxynaphtha-

lene and 3-buten-2-one, according to the methodology of the

Heck–Mizoroki reaction (Scheme 4) [27].

Recent publications describe improved methods in the

preparation of 1a by hydrogenation of 1. A patent [28] reports

that rhodium-based catalytic systems heterogenized on an

organic or mineral support, prepared by reduction of Rh(III)

salts with hydrogen in the presence of lipophilic tertiary

amines, tertiary amides or quaternary ammonium salts, resulted

very efficient in the selective reduction of 1 to 1a (yield � 98%,

hydrogen pressure 5–15 atm, T = 50–75 8C). Ravasio et al. [29]

report that copper on silica catalysts, prepared by reduction of

Cu(II) salts with hydrogen at 270 8C, give 1a by hydrogenation

of 1 in high yield (98%, hydrogen pressure 5 atm, T = 90 8C).

The results obtained in the hydrogenation of 1 with the

catalysts Rh/g-Al2O3 and Rh(TOA)/g-Al2O3 prepared by MVS

are reported in Table 1. The reactions were performed under

mild conditions (room temperature, atmospheric pressure of

hydrogen) using a molar ratio substrate/Rh = 1000.

The Rh/g-Al2O3 (1 wt.% Rh) catalyst showed high activity

and selectivity in the reduction of 1 to 1a (run 1,

SA = 8.3 min�1, sel. = 98%). Even better results were obtained

with the Rh(TOA)/g-Al2O3 (1 wt.% Rh) catalyst, which

reduces 1 completely to 1a with high catalytic activity (run

2, SA = 23.8 min�1, sel. = 100%). In contrast, the commercial

catalyst showed a lower activity and a lower selectivity than

the previous reported catalysts (run 3, SA = 0.3 min�1,

sel. = 90%). The MVS-derived catalyst with a low rhodium

loading, Rh(TOA)/g-Al2O3 (0.1 wt.% Rh), exhibited the best

2.

3.

Scheme 4.


catalytic properties, reducing completely 1 to 1a with a very

high efficiency (run 4, SA = 41.7 min�1, sel. = 100%). The Rh/

g-Al2O3 (0.1 wt.% Rh) catalyst containing no TOA furnished

complete selectivity in 1a but displayed low activity (run 5,

SA = 0.6 min�1, sel. = 100%). It is worth noting that, as far as

the activity of the catalysts having a low rhodium loading is

concerned, a similar result has been previously observed also in

the hydrogenation of cinnamaldehyde [17]. No evidence of

catalytic deactivation was found for Rh(TOA)/g-Al2O3

catalysts containing either 1 wt.% Rh or 0.1 wt.% Rh. Such

catalysts, recovered from the runs 2 and 4, respectively, and

reused, showed the same catalytic activity (runs 6 and 7).

Because of the high efficiency shown by Rh(TOA)/g-Al2O3

(0.1 wt.% Rh) catalyst in the hydrogenation of 1 to 1a, the

process was scaled to 50 g, under the experimental conditions

of run 4; the yield of 1a was the same (run 8). Excellent results

were also obtained in the environmentally friendly solvent 95%

ethanol; conversion to 1a was complete and the reaction was

faster than in toluene (run 9, SA = 55.6 min�1, sel. = 100%).

No leaching of rhodium from the catalyst to the solution was

observed in the runs 8 and 9. In fact, the recovered catalyst

maintains the same catalytic activity and it contains 0.1 wt.%

Rh, as in the starting material, within the interval �2%. The

behaviour of the mother liquor was also investigated. First,

AAS revealed no rhodium content. In addition, as a further test

for the presence of active metal in the recovered solution, no

hydrogenation was observed adding fresh 1 for up to 24 h.

Finally, AAS revealed no rhodium content in 1a.

Scheme

The mild reaction conditions (room temperature, atmo-

spheric pressure of hydrogen), the complete selectivity to

NabumetoneTM, the stability of the Metal Vapour Synthesis-

derived Rh(TOA)/g-Al2O3, 0.1 wt.% Rh, make this catalyst an

useful alternative to the very active catalysts recently reported

[28,29].

3.3.1.2. Hydrogenation of 2-acetyl-5,8-dimethoxy-3,4-dihy-

dronaphthalene, 2. The interest in the selective reduction of

the double bond in the 2-acetyl-5,8-dimethoxy-3,4-dihydro-

naphthalene, 2, is related to the importance of the corresponding

saturated carbonyl compound 2-acetyl-5,8-dimethoxy-1,2,3,4-

tetrahydronaphthalene, 2a, in fine chemistry. In fact 2a is a

precursor of the so-called ‘‘Wong’s tetraline’’ [30], 3, which is

the main building block of antitumor anthracyclinic compounds,

4, currently produced by pharmaceutical industries (Scheme 5)

[31,32].

The compounds which can be formed in the hydrogenation

of 2 are, in addition to the saturated ketone 2a, the a,b-

unsaturated alcohol, 2b, and the saturated alcohol, 2c

(Scheme 6).

The results obtained in the hydrogenation of 2 with the

catalysts Rh/g-Al2O3 and Rh(TOA)/g-Al2O3 prepared by MVS

are reported in Table 2. The reactions were performed at room

temperature and atmospheric pressure of hydrogen, using a

molar ratio substrate/Rh = 100.

In the presence of the Rh/g-Al2O3 catalyst (1 wt.% Rh), the

conversion of 2 is complete in 24 h with high selectivity in 2a

5.

Scheme 6.


(run 10, SA = 4.1 h�1, sel. = 81%). Similar results were obtained

with the Rh(TOA)/g-Al2O3 catalyst (1 wt.% Rh) (run 11,

SA = 4.1 h�1, sel. = 86%). The commercial catalyst Rh/g-Al2O3

(5 wt.% Rh) is less active and less selective to 2a than the MVS-

derived Rh catalysts (run 12, SA = 3.5 h�1, sel. = 74%). A very

high catalytic activity and selectivity to 2a was obtained using the

Rh(TOA)/g-Al2O3 (0.1 wt.% Rh) catalyst having a low rhodium

loading (run 13, SA = 20 h�1, sel. = 90%), as previously

observed in the hydrogenation of 4-(60-methoxy-20-naphthyl)-

3-buten-2-one, 1. Under the same conditions the Rh/g-Al2O3

(0.1 wt.% Rh) exhibits a very low catalytic activity (run 14). The

stability of the most active Rh(TOA)/g-Al2O3 catalyst (0.1 wt.%

Rh) was confirmed using the material recovered from run 13

which showed the same catalytic activity as that of the starting

material (run 15). With this catalyst, identical results in terms of

kinetics and selectivity were obtained when the amounts of 2

were increased to 40 g (run 16, SA = 20 h�1, sel. = 90%). In the

environmental friendly solvent 95% ethanol the catalyst is more

active but somewhat less selective than in toluene (run 17,

SA = 40 h�1, sel. = 85%). 2a was obtained pure from runs 16 and

17 very easily by crystallization of the residue from isopropyl

alcohol, following the detailed procedure described in the

experimental section. No leaching of rhodium from the catalyst

to the solution was observed in the runs 16 and 17 as proved

performing the tests and the AAS measurements foregoing

reported in Section 3.3.1.1.

4. Conclusions

It appears from the reported data that the Rh(TOA)/g-Al2O3

samples obtained by MVS are valuable catalysts for the

selective hydrogenation of the a,b-unsaturated ketones 4-(60-methoxy-20-naphthyl)-3-buten-2-one, 1, and 2-acetyl-5,8-

dimethoxy-3,4-dihydronaphthalene, 2, to the corresponding

saturated ketones 4-(60-methoxy-20-naphthyl)-butan-2-one, 1a

(NabumetoneTM) and 2-acetyl-5,8-dimethoxy-1,2,3,4-tetrahy-

dronaphthalene, 2a, respectively, which are compounds of

considerable value in fine chemistry.

The Rh(TOA)/g-Al2O3 samples are more convenient than

catalysts similarly prepared in the absence of TOA and they are

considerably more active than analogous commercial samples.

The role of TOA, which is crucial in the generation of these

very active catalysts, is to prevent aggregation of the rhodium

particles in the mesitylene-solvated rhodium solutions. As

indicated by DOSY-NMR analysis, the hydrodynamic diameter

of the TOA-stabilized rhodium particles is very small, about

1.1 nm including the shell of the stabilizer. It accounts for the

presence on the g-alumina of rhodium nanoparticles of

comparable size, still containing the TOA stabilizer, surround-

ing the metal particles [17]. The presence of TOA allows these

catalysts to be very active even at low rhodium loading

(0.1 wt.% Rh), the NR3 molecule protecting the rhodium atoms

from oxidation by the solid oxide support. Under the same

reaction conditions Rh/g-Al2O3 samples prepared in absence of

TOA and containing 0.1 wt.% Rh are quite inactive.

The fact that aliphatic amines bearing large alkyl groups

bind to transition metal atoms and thereby enhance the catalytic

activity of these atoms in the catalytic hydrogenation of olefins

was observed some years ago by Frolov [33]. The results here

reported are an additional example of the potential of these

systems as hydrogenation catalysts, which can be regarded as

new-generation hydrogenation catalysts.

Acknowledgements

We thank the Consiglio Nazionale delle Ricerche (CNR) for

financial support of this research. FB and GUB wish to thank

Ministero dell’Universita e Ricerca (MUR, grant 2005037725),

for financial support of the NMR experiments. The authors thank

Dr. Emanuela Pitzalis (IPCF-CNR, Pisa, Italy) for AAS analyses.

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