Palladium-catalyzed oxidation of bicyclic monoterpenes by hydrogen peroxide

Palladium-catalyzed oxidation of bicyclicmonoterpenes by hydrogen peroxide

Elena Gusevskaya*, Patricia A. Robles-Dutenhefner, VinõÂcius M.S. Ferreira

Departamento de QuõÂmica-ICEx, Universidade Federal de Minas Gerais, 31270-901 Belo Horizonte ± MG, Brazil

Received 20 January 1998; received in revised form 15 May 1998; accepted 17 May 1998

Abstract

The activity of the PdCl2±CuCl2 combination in the oxidation of camphene, a-pinene, and b-pinene by dioxygen in acetic

acid solutions has been studied. The reactions of a-pinene and b-pinene yield a mixture of carveyl acetate (up to 25% on

reacted ole®n), a-terpenyl acetate, bornyl chloride, and fenchyl chloride. Camphene undergoes a skeletal rearrangement and

an acetic acid/water addition resulting in bornyl acetate as a major product, along with borneol and a-pinene. No oxidation

products are detected. In an attempt to develop a CuCl2-free catalytic system for the selective oxidation of bicyclic

monoterpenes, the oxidation of b-pinene and camphene by hydrogen peroxide catalyzed by Pd(OAc)2 in acetic acid solutions

has been studied. b-Pinene gave the allylic oxidation products, i.e., pinocarveol, pinocarveyl acetate and myrtenyl acetate,

with selectivity up to 75% at virtually complete conversion, and camphene gave camphene glycol monoacetate with a 90%

selectivity at 80% conversion. The oxidation reaction competes with the skeletal rearrangement of monoterpenes accompanied

by a nucleophilic addition of hydroxy and acetoxy groups. The introduction of benzoquinone (BQ) in catalytic amounts exerts

a bene®cial effect on the catalyst stability and selectivity for glycol monoacetate formation. For the Pd(OAc)2±BQ±H2O2

system, more than 200 turnover numbers could be achieved in the acetoxylation of camphene. # 1998 Elsevier Science B.V.

All rights reserved.

Keywords: Oxidation; Palladium catalysts; b-Pinene; Camphene; Hydrogen peroxide

1. Introduction

Selective oxyfunctionalization of available mono-

terpenes represents an interesting route to extend the

utilization of these cheap natural products. Some of

their oxygenated derivatives are commercially impor-

tant materials for pharmaceutical, ¯avor, and perfum-

ery industry as well as useful synthetic intermediates

and chiral building blocks [1,2]. We have previously

reported that allylic acetates, aldehydes, alcohols, and

carboxylic acid derivatives can be obtained in good

yields by the oxidation [3], hydroformylation [4,5],

and alkoxycarbonylation [6] of some naturally occur-

ring monoterpenes. Although the reactions of ole®n

oxidation catalyzed by palladium complexes have

been developed as important synthetic methods, there

is very little information in the literature concerning

their application to natural product synthesis. We have

described in [3] the procedure for the allylic oxidation

of limonene by dioxygen in the presence of the PdCl2/

CuCl2 catalytic combination, which results in trans

Applied Catalysis A: General 174 (1998) 177±186

*Corresponding author. Tel.: 00 55 31 499 5755; fax: 00 55 31

499 5700; e-mail: [email protected]

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PII: S0926-860X(98)00191-4

carveyl acetate in excellent yield. We tried later to

extend this oxidation methodology to the bicyclic

monoterpenes, such as a-pinene, b-pinene, and

camphene. It was found that CuCl2, acting as a Lewis

acid, promotes the undesirable skeletal rearrangement

of these monoterpenes accompanied by a nucleo-

philic addition of chloride and acetate groups resulting

in the formation of bornyl chloride, bornyl acetate,

fenchyl chloride, a-terpenyl acetate, etc., as main

products.

Our efforts are now being made to develop a CuCl2-

free catalytic system for the selective oxidation of

bicyclic monoterpenes. The aim of the present study

was to investigate the possibility of using hydrogen

peroxide as the oxygen source for the oxidation of

limonene and bicyclic monoterpenes, such as a-

pinene, b-pinene, and camphene. Hydrogen peroxide

is a cheap and strong oxidant, with water being formed

as the only by-product, which could lead to the

development of environment-friendly processes. It

has previously been used as the reoxidant in Wacker

type oxidation of ethylene [7] and other terminal

ole®ns to methyl ketones [8]. Recently, a palla-

dium-catalyzed allylic acetoxylation of internal and

simple cyclic ole®ns using hydrogen peroxide as the

terminal oxidant has been described [9,10]. The related

catalytic system, PdCl2ÿAgOAcÿTeO2ÿt BuOOH, has

earlier been developed for the allylic acetoxylation of

cyclic ole®ns and the application to b-pinene oxidation

has been reported [11]. However, this method suffers

fromseriousdisadvantages,suchasacomplicatedwork-

up procedure for product separation, and requires three

days to reach a 26% yield of allylic acetates (based on b-

pinene charged), corresponding to approximately ®ve

turnovers.

We report herein the results of the oxidation of some

monoterpenes by hydrogen peroxide using palladium

acetate as a catalyst in aqueous acetic acid solutions in

the absence of halogens and co-metals. We have found

that the ole®n structure greatly in¯uences the activity

of the system and product nature. b-Pinene gave the

allylic oxidation products, i.e., pinocarveol, pinocar-

veyl acetate and myrtenyl acetate, and camphene gave

camphene glycol monoacetate in high yields. No

selective oxidation of a-pinene and limonene was

observed under the conditions when a rapid and

selective oxidation of b-pinene and camphene

occurred.

2. Experimental

All chemicals were purchased from commercial

sources and used as received, unless otherwise indi-

cated. (ÿ)-Camphene (1), (1S)-(ÿ)-b-pinene (2),

(1S)-(ÿ)-a-pinene (3), and R-(�)-limonene (4) were

distilled before use.

The reactions were carried out in a stirred glass

reactor equipped with a sampling system and con-

nected to a gas burette to monitor the gas uptake when

dioxygen was used as a ®nal oxidant. Hydrogen

peroxide (30 wt%) was injected to the solution of

ole®n, palladium salt and cooxidant, if any, in acetic

acid (5±10 ml), and the mixture was stirred for

the reported time at the reported temperature. The

reactions were followed by gas chromatography (GC)

using a Shimadzu 14B instrument ®tted with a

Carbowax 20 M capillary column and a ¯ame ioniza-

tion detector. After separation either by column chro-

matography (silica) or by extraction with a pentane±

ether (1:1) mixture, the products were identi®ed by 1H

and 13C NMR on a Brucker CXP-400 spectrometer

with tetramethylsilane as an internal standard and

CDCl3 as a solvent and by GC±MS on a Hewlett-

Packard MSD 5890/Series II instrument operating at

70 eV. Spectral simulations performed with the ADC/

CNMR program were in agreement with the spectra

observed.

Pinocarveol (14) MS (m/z/rel.int.): 134/17; 119/21;

109/20; 95/21; 92/67; 91/45; 83/70; 81/30; 79/25; 77/

16; 70/55; 69/35; 67/23; 55/100; 53/30. 1H NMR: d0.62 (s, 3H, CH3); 1.30 (s, 3H, CH3); 4.36 (m, 1H,

CHOH); 4.75 (br.d, 1H, �CH); 4.93 (br.d, 1H, �CH);13C NMR: d 68.48 (C3); 114.01 (C10); 150.19 (C2).

Pinocarveyl acetate (15) MS (m/z/rel.int.): 134/13;

119/25; 108/13; 93/11; 92/61; 91/100; 79/11; 69/20;

55/13; 53/11. 1H NMR: d 0.57 (s, 3H, CH3); 1.20 (s,

3H, CH3); 1.98 (s, 3H, OAc); 4.94 (d, 1H, �CH,

J�1.6 Hz); 4.98 (m, 1H, CHOAc); 5.90 (d, 1H, �CH,

J�1.6 Hz). 13C NMR: d 66.81 (C3), 111.32 (C10);

155.83 (C2); 170.58 (OCOCH3).

Myrtenyl acetate (16) MS (m/z/rel.int.): 134/10;

119/28; 108/13; 93/12; 92/36; 91/100; 79/14. 1H

NMR: d 0.75 (s, 3H, CH3); 1.22 (s, 3H, CH3); 1.99

(s, 3H, OAc); 4.36 (d, 1H, CHHOAc, J�1.5 Hz); 4.39

(d, 1H, CHHOAc, J�1.5 Hz); 5.49 (m, 1H,�CH). 13C

NMR: d 66.80 (C10); 121.31 (C3); 142.79 (C2); 170.97

(OCOCH3).

178 E. Gusevskaya et al. / Applied Catalysis A: General 174 (1998) 177±186

Camphene glycol monoacetate (18) (new com-

pound as far as we know) MS (m/z/ int. rel.): 71/

100; 59/30; 112/25; 95/15. 1H-RMN d 1.19 (s, 3H,

CH3); 1.24 (s, 3H, CH3); 2.18 (s, 3H, OCOCH3); 4.32

(s, 2H, CH2OAc). 13C-RMN d 26.12 (C5); 26.26 (C6);

28,00 (C9 ou C10); 28.25 (C9 ou C10); 30.36 (C7);

38.96 (C2); 47.27 (C1); 52.02 (C4); 67.20 (C8); 71,7

(C3).

3. Results and discussion

We have previously reported that limonene can be

oxidized by dioxygen at 60±808C and oxygen pressure

of 0.1 MPa in glacial acetic acid containing LiCl, in

the presence of the PdCl2±CuCl2 catalytic combina-

tion, giving rise to the formation of carveyl acetate (7)

(85% trans) with up to 90% selectivity [3]. Table 1

shows the product distributions at different reaction

times during the reactions of some bicyclic mono-

terpenes, such as camphene (1), b-pinene (2) and a-

pinene (3), under the conditions similar to those used

for limonene oxidation.

It has been found that 1 undergoes a skeletal

rearrangement and an acetic acid/water addition

resulting in bornyl acetate (5) as a major product,

along with borneol (6) and 3 (Table 1, run 1; Scheme

1). No oxidation products are detected. Varying the

catalyst component concentrations and temperature

results in some changes in the product distribution but

not in the product nature. In the absence of PdCl2, the

same products are formed (run 2).

A virtually complete conversion of 2 and 3 was

observed after 3 h of reaction at 808C. The main

products formed are carveyl acetate (7) (ca. 85%

trans) (up to 25%), a-terpenyl acetate (8), a-terpineol

(9), bornyl chloride (10), and fenchyl chloride (11)

(Scheme 2). Besides, the products of the skeletal

rearrangement of pinenes, i.e., camphene (1), limo-

nene (4), a-terpinolene (12) and g-terpinene (13) are

detected. Very similar product distributions are

obtained regardless of whether the starting material

is a-pinene or b-pinene. A GC analysis of the reaction

mixture shows that at short reaction times the rates of

the isomerization, resulting mainly in limonene, and

the nucleophilic addition reactions, resulting in 8, 9,

10, and 11, exceed signi®cantly the rate of oxidation

Table 1

Reactions of bicyclic monoterpenes in acetic acid solutions containing PdCl2, CuCl2 and LiCl at a dioxygen pressure of 0.1 MPa

Run Olefin Time (h) Conversiona (%) Product distributiona (%)

1 3 4 5 6 7 8 9 10 11 12�13

1b Camphene 1 10 18 55 27 tr.e tr.e

3 20 14 63 23 tr.e tr.e

2b,c Camphene 2 34 24 68 8 tr.e tr.e

3d a-Pinene 0.4 83 16 6 27 4 25 9 13

3 99 2 4 26 21 6 29 10 2

4c,d a-Pinene 0.4 80 7 19 29 tr.e 24 8 13

3 99 7 18 tr.e 25 4 27 10 9

5d b-Pinene 0.4 99 5 10 16 tr.e 23 tr.e 23 13 10

3 99 5 3 25 19 3 28 15 2

6c,d b-Pinene 0.4 99 5 6 17 25 tr.e 13 22 12

3 99 6 19 tr.e 23 tr.e 27 15 10

aDetermined by gas chromatography.bReaction conditions: [olefin]�1.00 mol lÿ1, [PdCl2]�10ÿ2 mol lÿ1, [CuCl2�2H2O]�0.2 mol lÿ1, [LiCl]�0.5 mol lÿ1, O2 (0.1 MPa), 808C.c[PdCl2]�0.dReaction conditions: [olefin]�1.00 mol lÿ1, [PdCl2]�10ÿ2 mol lÿ1, [CuCl2�2H2O]�0.1 mol lÿ1, [LiCl]�0.7 mol lÿ1, O2 (0.1 MPa), 808C.eTrace amounts.

Scheme 1.

E. Gusevskaya et al. / Applied Catalysis A: General 174 (1998) 177±186 179

resulting in 7. The latter seems to be a product of the

allylic oxidation of the intermediary formed limonene.

In the absence of PdCl2, no oxidation product is

detected (runs 4 and 6).

Therefore, the results obtained show that the appli-

cation of the catalytic system PdCl2/CuCl2 for the

oxidation of 1, 2, and 3 with dioxygen is limited, since

CuCl2, acting as a Lewis acid, promotes the undesir-

able skeletal rearrangements of these bicyclic mono-

terpenes, accompanied by a nucleophilic addition of

chloride and acetate groups. Selectivity for the oxida-

tion product does not exceed 25%. We concentrated

our efforts on developing a CuCl2-free catalytic sys-

tem for the selective oxidation of bicyclic monoter-

penes.

The activity of the Pd(OAc)2±LiNO3 combination

in the oxidation of limonene, a-pinene, b-pinene and

camphene with dioxygen has been examined. The

results concerning limonene, a-pinene, b-pinene have

been published in our previous work [3]. Although

nitrate ions readily oxidize the reduced Pd species in

acetic acid solutions and are reoxidized back by

dioxygen, neither the oxygen consumption nor the

formation of the oxidation products in signi®cant

amounts are observed for all examined monoterpenes.

Avery low conversion of camphene (<5%) is observed

for 2 h at 608C and oxygen pressure of 0.1 MPa in

acetic acid containing Pd(OAc)2 (0.02 equiv.) and

LiNO3 (0.8 equiv.). The small amounts of the uni-

denti®ed products are detected in the reaction mixture

after the run.

In an attempt to develop a CuCl2-free catalytic

system for the oxidation of bicyclic monoterpenes

we have studied the possibility of using hydrogen

peroxide as a ®nal oxidant. It has been found that

b-pinene undergoes an allylic oxidation (Table 2,

Scheme 3) with hydrogen peroxide, in the presence

of palladium acetate, giving rise to the formation of

the commercially valuable pinocarveol (14), pinocar-

veyl acetate (15), and myrtenyl acetate (16) with

selectivity up to 75% at virtually complete conversion

of 2. A skeletal isomerization and nucleophilic addi-

tion of acetate group or water occur in the reaction

solutions and compete with the allylic oxidation

Scheme 2.

Scheme 3.


resulting in a wide variety of by-products, the major of

them being a-terpenyl acetate (8), along with bornyl

acetate (5), a-terpineol (9), and fenchyl acetate (17).

In the absence of palladium acetate, the products 8and 9 are mainly formed and only trace amounts of the

allylic products are detected (run 2). Without the

oxidizable substrates hydrogen peroxide decomposes

with palladium acetate to molecular oxygen, but this

decomposition is fairly slow with respect to palla-

dium-catalyzed oxidation of b-pinene. In an attempt to

decrease the excess of hydrogen peroxide we studied

the effect of its amounts on the product distribution

and reaction rate (Table 2, runs 1, 4±6). As it can be

seen, lowering the hydrogen peroxide concentration

exerts no effect on the selectivity for the allylic

oxidation products, but decreases signi®cantly the

reaction rate. For example, a complete conversion

of 2 is obtained after 40 min at 1.8 mol lÿ1 of hydro-

gen peroxide (H2O2 /ole®n �3:1), while with

0.9 mol lÿ1 of hydrogen peroxide (H2O2 /ole®n

�1.8:1) the time required for a 86% conversion

increases to 140 min (runs 1 and 5). At 0.7 mol lÿ1

of hydrogen peroxide (H2O2 /ole®n �1.4:1), we

achieved a 66% conversion for 1.5 h, no more allylic

oxidation products being formed at longer reaction

times. To complete the substrate conversion, the injec-

tion of the additional amounts of hydrogen peroxide is

required. Thus, due to a thermal and palladium-cat-

alyzed decomposition, the use of a large excess of

hydrogen peroxide (H2O2:ole®n >2) is necessary to

achieve a complete conversion of b-pinene.

The selectivity for the allylic oxidation products

increases slightly when the reaction temperature is

lowered from 608C to 308C (run 3), however, the

reaction rate decreases markedly. With the decrease

in palladium acetate concentration from 5�10ÿ2 to

5�10ÿ3 mol lÿ1 (runs 5 and 7) the selectivity drops to

56% from the 75% level. The relative amounts of the

addition products (5, 8, 9, and 17) increase at the

expense of the allylic oxidation products (14±16). This

result can be explained by difference in the kinetics of

the competing reactions, i.e., allylic oxidation and

skeletal isomerization/nucleophilic addition. As it was

foundinourpreviouswork[3]therateoftheformationof

the addition products from limonene (8 and 9) hardly

dependedonthepalladiumconcentrationandthekinetic

studyshowedtheorderofnearlyzero,whereastherateof

the allylic oxidation increased with the increase in

palladium concentration.

Reoxidants such as benzoquinone (BQ) and

Fe(NO3)3 were tested as additives to the palladium

catalyst. Fe(NO3)3 quickly decomposes hydrogen per-

Table 2

Oxidation of b-pinene by hydrogen peroxide in acetic acid solutions containing Pd(OAc)2a

Run [H2O2]

(mol lÿ1)

Time

(min)

Conversionb

(%)

Product distributionb (%) Selectivity for allylic

oxidation products (%)Oxidation products Other products

15 14 16 5 8 9 17

1 1.8 20 83 29 20 17 5 8 5 2 72

40 100 32 19 21 5 7 3 4 74

2c 1.8 90 80 5 tr.f tr.f 10 40 40 5 5

3d 1.8 90 50 40 20 20 tr.f 7 3 tr.f 80

4 1.35 120 100 28 19 23 3 11 3 6 70

5 0.9 120 85 30 20 24 3 11 3 3 74

140 86 30 21 24 3 11 3 3 75

6 0.7 90 66 29 20 24 2 13 3 3 73

110 68 28 19 23 2 14 3 3 70

7e 0.9 120 65 26 15 15 6 18 7 6 56

aReaction conditions: [b-pinene]�0.50 mol lÿ1, [Pd(OAc)2]�10ÿ2 mol lÿ1, 608C.bDetermined by gas chromatography. Along with reported products, same unidentified products (5±10%) were observed.cIn the absence of Pd(OAc)2.d308C.e[Pd(OAc)2]�5�10ÿ3 mol lÿ1.fTrace amounts.


oxide. In the presence of a Pd(OAc)2±Fe(NO3)3 com-

bination, the products 8 and 9 are mainly formed from

b-pinene, as a result of its acid catalyzed transforma-

tions: isomerization and addition of acetic acid or

water.

The bene®cial effect of benzoquinone on the palla-

dium-catalyzed allylic oxidation of ole®ns by hydro-

gen peroxide was earlier reported by Akermark et al.

[9] and Mimoun and coworkers [10]. Benzoquinone as

co-catalyst increases the stability of the palladium

catalyst due to the effective reoxidation of Pd(0).

No palladium metal precipitation is observed in the

presence of benzoquinone even at low concentrations

of hydrogen peroxide and long reaction times. The

hydroquinone formed is reoxidized by hydrogen per-

oxide [10]. The addition of benzoquinone is not highly

critical for the b-pinene oxidation. We have investi-

gated the effect of the benzoquinone concentration on

the product distribution and the reaction rate

(Table 3). The molar ratio Pd/BQ which favors allylic

oxidation is 1:4 (run 3). The addition of benzoquinone

does not improve the selectivity for allylic oxidation

products, which is different from the result obtained

for the allylic oxidation of cyclohexene [10].

We have also applied the Pd(OAc)2±H2O2 system to

the oxidation of other monoterpenes. With a-pinene

(3), which contains an endocyclic double bond, a

complex mixture of products has been obtained after

2 h (ca. 50% conversion of 3) with only trace amounts

of the allylic oxidation products (14, 15 and 16) being

detected. Limonene (4), a monocyclic terpene con-

taining both endo- and exocyclic double bonds, does

not undergo any oxidation by hydrogen peroxide in the

presence of Pd(OAc)2 during 1.5 h under the condi-

tions when a rapid oxidation of b-pinene occurs. So,

the extension of the allylic acetoxylation system

Pd(OAc)2±H2O2 to other monoterpenes is strongly

in¯uenced by their structure.

We have observed a dramatic effect of the mono-

terpene structure on the product nature studying the

oxidation of camphene (1). This bicyclic monoterpene

has an exocyclic disubstituted double bond, like b-

pinene, but the only allylic hydrogen is at a bridgehead

position and not easily abstractable. Therefore, we had

no expectations of obtaining the allylic derivatives. It

has been found that at mild conditions (0.1 MPa, 608C,

1 h) in aqueous acetic acid solutions containing hydro-

gen peroxide and catalytic amounts of palladium

acetate camphene undergoes a 80% conversion with

the formation of the only major product 18 (Table 4,

run 1). Using GC±MS, 1H and 13C NMR it has been

identi®ed as camphene glycol monoacetate (18)

(Scheme 4). This product is not detected at all if

palladium acetate is excluded (run 2), therefore, its

formation cannot be explained by a non palladium-

Table 3

Oxidation of b-pinene by hydrogen peroxide in acetic acid solutions containing Pd(OAc)2 and benzoquinone (BQ)a

Run [BQ]

(mol lÿ1)

Coversionb

(%)

Product distributionb (%) Selectivity for allylic

oxidation products (%)Oxidation products Other products

14 15 16 5 8 9 17

1 0 64 19 20 30 2 13 4 6 69

2 10ÿ2 60 18 22 28 tr.c 25 2 tr.c 68

3 2�10ÿ2 82 20 22 30 3 14 4 2 72

4 6�10ÿ2 68 24 21 16 5 22 2 5 61

5 10ÿ1 47 19 24 28 tr.c 15 5 tr.c 71

aReaction conditions: [b-pinene]�0.50 mol lÿ1, [Pd(OAc)2]�5�10ÿ3 mol lÿ1, [H2O2] �1.0 mol lÿ1, 608C, reaction time 1 h.bDetermined by gas chromatography. Along with reported products, some unidentified products (5±10%) were observed.cTrace amounts.

Scheme 4.


centered epoxidation of camphene by the peroxyacetic

acid generated in situ followed by ring opening of

epoxide by acetic acid.

Along with 18, a number of different unidenti®ed

products are formed and in run 1 (Table 4) the selec-

tivity for 18 does not exceed 50% based on reacted

camphene. In an attempt to increase the reaction

selectivity and ®nd the most favorable conditions

for the glycol monoacetate synthesis we study the

effects of the catalyst composition and reaction vari-

ables on the product distribution (Tables 4 and 5).

The selectivity for the formation of 18 increases

markedly with lowering the reaction temperature. A

selectivity of 50% for 18 is observed at 808C (Table 4,

run 3) while 95% at 408C (run 4) at a 50±60%

conversion of 1. However, due to the decrease in

reaction rate at 408C (run 4) the injection of the

additional amounts of hydrogen peroxide is required

to complete the reaction which leads to a dramatic

drop in selectivity (to 60%). The palladium acetate

concentration seems also to in¯uence strongly the

reaction selectivity (runs 1, 5, and 6). With the

decrease in the palladium acetate concentration to

2.5�10ÿ3 from 2.5�10ÿ2 mol lÿ1 the selectivity for

18 increases to 85% from the 55% level (runs 6 and 1)

at ca. 50% conversion. We tried a slow injection of

hydrogen peroxide (for 30 min, run 6, and for

120 min, run 7) in order to avoid its accumulation

in the reaction medium and decrease the excess of

hydrogen peroxide used. However, this resulted in a

reaction deceleration and drastic decrease in selectiv-

ity. It should be mentioned that, in the absence of

benzoquinone, we always observed lower selectivities

at higher conversions of 1.

We have found that using the catalytic amounts of

benzoquinone as reoxidant has the bene®cial effect on

the stability of a palladium catalyst and selectivity for

glycol monoacetate (Table 5). Even at high conver-

sions of 1 (80±90%) the 85±90% selectivities are

achieved (runs 2, 3, 7, and 8). An addition of benzo-

quinone to the reaction is necessary to obtain high

yields of camphene glycol monoacetate. When hydro-

Table 4

Oxidation of camphene by hydrogen peroxide in acetic acid solutions containing Pd(OAc)2a

Run [Pd(OAc)2] �102 (mol lÿ1) Temperature (8C) Time (min) Conversionb (%) Selectivity for 18b (%)

1 1 60 30 50 57

50 82 50

120 97 43

2c 0 60 120 10 0

3 1 80 45 60 50

65 65 47

90 99 45

4d 1 40 45 28 95

100 50 95

220 87 60

5 0.5 60 60 58 75

130 68 70

6 0.25 60 60 47 85

120 66 75

245 76 77

7e 1 60 60 66 37

140 97 38

8f 1 60 60 30 30

140 55 25

aReaction conditions: [camphene]�0.50 mol lÿ1, [H2O2]�1.0 mol lÿ1.bDetermined by gas chromatography.cA complex mixture of unidentified products is observed.dAfter 2 h the additional amounts of H2O2 (1 mol lÿ1) were injected to complete the conversion of 1.eSlow injection (30 min) of H2O2 at stirring.fSlow injection (120 min) of H2O2 at stirring.


gen peroxide was excluded and benzoquinone was

used as a stoichiometric oxidant, in the presence of the

catalytic amounts of palladium acetate (run 6), no

traces of 18 could be detected after 2 h of reaction with

only starting material being recovered (camphene

conversion was lower than 5%).

A molar BQ/Pd ratio strongly affects the reaction

selectivity (Table 5, runs 1±4). The positive effect of

benzoquinone reaches its maximum at a molar ratio BQ/

Pd�10 anda 86%selectivityhasbeen achieved at a 83%

conversion(run3).Thefurtherincreaseinbenzoquinone

concentration leads to a signi®cant decrease in glycol

monoacetate selectivity (to ca. 50%, at BQ/Pd�20, run

4; compare also run 8 with BQ/Pd�10 and run 10 with

BQ/Pd�20). Aslowinjectionofhydrogenperoxidealso

results in a disappointingly low selectivity (ca. 40%, run

5).The Pd(OAc)2±BQ±H2O2 system is very ef®cient for

camphene oxidation even at low concentrations of pal-

ladium acetate. At a molar ratio Pd(OAc)2/

camphene�400 ([Pd(OAc)2]�1.25�10ÿ3, run 9) a

75% conversion is achieved at a 80% selectivity, corre-

sponding to 240 turnovers. In run 8, at a ratio BQ/Pd�10

a 90% selectivity is maintained up to more than 80%

conversion, however, a larger excess of benzoquinone

suppresses the formation of 18 and at BQ/Pd�20 (run

10) the selectivity decreases to the 75% level.

The bene®cial effect of both lowering the palladium

acetate concentration and benzoquinone addition on

the selectivity of the camphene oxidation could be

related with the participation of the soluble and/or

insoluble forms of palladium(0) in the undesirable

transformations of camphene. In the absence of ben-

zoquinone, we have observed the formation of metal-

lic palladium in some runs. The palladium(0) species

formed at relatively high palladium acetate concen-

tration, which contribute to the decrease in selectivity,

are effectively reoxidized by benzoquinone and no

palladium precipitation is observed in the presence of

benzoquinone even at low concentrations of hydrogen

peroxide and long reaction times. In these systems

Table 5

Oxidation of camphene by hydrogen peroxide in acetic acid solutions containing Pd(OAc)2 and benzoquinone (BQ)a

Run [Pd(OAc)2] �102 (mol lÿ1) [BQ]�102 (mol lÿ1) Time (min) Conversionb (%) Selectivity for 18b (%)

1 1 4 55 70 64

80 96 53

2 1 8 55 72 80

120 87 84

3 1 10 55 70 90

120 83 86

4 1 20 55 68 56

130 83 46

5c 1 8 65 73 36

120 93 35

6d 1 25 120 <5 0

7 0.5 5 55 65 90

120 84 88

8 0.25 2.5 60 51 96

120 67 91

215 82 90

9 0.125 2.5 60 33 90

120 52 88

220 75 80

10 0.25 5 60 46 80

120 73 76

220 87 75

aReaction conditions: [camphene]�0.50 mol lÿ1, [H2O2]�1.0 mol lÿ1, 608C.bDetermined by gas chromatography.cSlow injection (60 min) of H2O2 at stirring.dIn the absence of hydrogen peroxide. [camphene]�0.25 mol lÿ1. Trace amounts of bornyl acetate 5 were detected along with a number of

unidentified products.


benzoquinone plays an important role, acting not only

as an oxidant but also as a ligand of palladium [12]. It

seems reasonable to explain the decrease in selectivity

at relatively high excess of benzoquinone (BQ/

Pd�20) by its inhibitive coordination to the palla-

dium. This can result in the retardation of the rate of

palladium-catalyzed camphene oxidation and

decrease in selectivity due to a number of non-palla-

dium catalyzed side reactions. However, the additional

experiments are needed to support this suggestion.

The palladium-catalyzed acetoxylation of b-pinene

and camphene in aqueous acetic acid solutions con-

taining hydrogen peroxide can be seen as a nucleo-

philic attack by three nucleophiles, i.e., H2O2, H2O,

and HOAc on the ole®n coordinated to palladium in a

� or a �-allylic mode [10]. Judging from the product

distribution in the oxidation of b-pinene, water and

hydrogen peroxide seem to be poorer nucleophiles

compared to acetic acid under the reaction conditions,

however, it is dif®cult to compare the reactivities of all

nucleophiles which are present in the solutions in

different concentrations and could be involved in

the oxidation of camphene and b-pinene. The nature

of the reaction products depends strongly on the

coordination mode, which, in turn, is determined by

the ole®n structure. Palladium hydroperoxidic species

(AcOPdOOH) probably obtained by the addition of

hydrogen peroxide to palladium acetate seem to be the

most likely active intermediates in this system [8]. The

allylic derivatives of b-pinene (14, 15 and 16) appear

to be formed as a result of the external and intramo-

lecular nucleophilic attack of acetate on the (�-allyl)-

palladium intermediate via the mechanism similar to

that proposed for the allylic oxidation of cyclic and

internal ole®ns by Mimoun and coworkers [10].

The only allylic hydrogen in camphene molecule is

at the bridgehead position and not easily abstractable.

Furthermore, there are no hydrogens at b-carbon of

camphene double bond. The product 18 is not formed

at all in the absence of palladium acetate, therefore, its

formation cannot be explained by a non palladium-

centered epoxidation of camphene by the peroxyacetic

acid generated in situ. Thus, we suppose two possible

routes for the formation of camphene glycol acetate

18. First, via hydroxypalladation of ole®n in a �-

camphene±palladium complex followed by heteroly-

sis of a carbon±palladium s-bond, since there is no

hydrogen at b-carbon. Second, via peroxypalladation

of ole®n in a �-camphene±palladium complex with

the formation of epoxide followed by ring opening of

epoxide with acetic acid (Scheme 5):

In conclusion, we have developed a new method for

the oxidation of bicyclic monoterpenes, i.e., b-pinene

and camphene, by hydrogen peroxide catalyzed by

palladium acetate in acetic acid solutions. This reac-

tion leads to the formation of allylic products from b-

pinene, and glycol monoacetate from camphene. The

addition of benzoquinone in catalytic amounts exerts a

bene®cial effect on the catalyst stability and selectiv-

ity for glycol monoacetate. For the Pd(OAc)2±BQ±

H2O2 system, more than 200 turnover numbers could

be achieved in the acetoxylation of camphene.

Scheme 5.


Acknowledgements

Financial support from the CNPq (Conselho Nacio-

nal de Desenvolvimento CientõÂ®co e TecnoloÂgico),

FAPEMIG (Fundac,aÄo de Amparo aÁ Pesquisa do

Estado de Minas Gerais) and PRPq, Universidade

Federal de Minas Gerais is gratefully acknowledged.

We thank CNPq and FAPEMIG for the student

(VMSF) and doctoral (PARD) fellowships. The

authors wish to thank Prof. Rodinei Augusti for his

help in GC/MS analysis and Mr. JoseÂ Ailton Gon-

c,alves for the experimental assistance.

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Palladium-catalyzed oxidation of bicyclic monoterpenes by hydrogen peroxide