Oxidative esterification of alkenes via π- and σ-organopalladium complexes: New pathways for the reaction

Article abstract of DOI:10.1016/S0022-328X(01)00825-7

New mechanistic data on the oxidative esterification of alkenes were obtained in the study of the reaction of Pd(II) acetate with hex-1-ene, methylcyclohex-1-ene and racemic α-pinene in a chloroform solution. High yields of unsaturated esters with terminal alcohol group were found in the oxidation of hex-1-ene, while the exocyclic methyl groups in methylcyclohex-1-ene and α-pinene remain untouched.

Full text of DOI:10.1016/S0022-328X(01)00825-7

Journal of Organometallic Chemistry 636 (2001) 69–75
www.elsevier.com/locate/jorganchem
Oxidative esterification of alkenes via p- and s-organopalladium
complexes: new pathways for the reaction
N.Yu. Kozitsyna, A.A. Bukharkina, M.V. Martens, M.N. Vargaftik, I.I. Moiseev *
N.S. Kurnako6 Institute of General and Inorganic Chemistry, Russian Academy of Sciences, 31, Leninski Prospect, GSP-1,
Moscow 117907, Russia
Received 23 February 2001; accepted 1 April 2001
Dedicated to Professor Oleg M. Nefedov in recognition of his outstanding contribution to organometallic chemistry
Abstract
New mechanistic data on the oxidative esterification of alkenes were obtained in the study of the reaction of Pd(II) acetate with
hex-1-ene, methylcyclohex-1-ene and racemic a-pinene in a chloroform solution. High yields of unsaturated esters with terminal
alcohol group were found in the oxidation of hex-1-ene, while the exocyclic methyl groups in methylcyclohex-1-ene and a-pinene
remain untouched. © 2001 Elsevier Science B.V. All rights reserved.
Keywords: Palladium; Hex-1-ene; Methylcyclohex-1-ene; a-Pinene; p-Complex; s-Complex; Oxidative esterification; Regioselectivity; Carboca-
tions; Reaction mechanism
1. Introduction
and RCOO− anion addition to occur asynchronously.
First, the (RCOO)2 Pd(II) group adds to one of C
atoms of the alkene double bond, producing a positive
charge on the neighboring C atom. Then, the palladium
Redox reaction of Pd(II) carboxylates with alkenes
produces a-, b- or g-alkenyl esters and Pd0 (Eq. (1))
s-carbenium complex
that formed under-
goes successive deprotonation and reductive elimination
of the Pd0(OCOR)− group producing an allylic ester.
Due to the asynchronism of the Pd–X across CꢀC
bond addition, a carbenium moiety of the hydrocarbon
ligand has a chance to isomerize, for instance via
1,2-hydride shift, before the deprotonation and elimina-
tion stages, thus resulting in a homoallylic ester.
In our previous study [6] we found that the regio-
and stereoselectivity effects overlap. The aim of this
work is restricted to the regioselectivity of reaction (1).
To diminish the stereoselectivity problems, we chose the
following series of optically inactive substrates: (1) hex-
1-ene (unbranched acyclic alkene without stereo- and
conformational limitations); (2) methylcyclohex-1-ene
(achiral cyclohexene derivative with low ring-conforma-
tional barrier); and (3) racemic a-pinene (chiral cyclo-
hexene derivative with rigidly fixed ring conformation).
In this work we studied the regioselectivity of the
reaction of Pd(II) acetate with the above alkenes in a
chloroform solution.
(1)
via successive formation of the intermediate palladium
p-(
) and s-bound (
) com-
plexes [1–4]. The reaction Eq. (1) is assumed to proceed
through synchronous insertion of the p-coordinated
alkene into the Pd–OCOR bond [1–3] or outer-sphere
attack of the RCOO− anion on the p-coordinated
alkene molecule [5] with simultaneous formation of the
s-Pd–C and C–OCOR bonds. Recently new data were
obtained [6] that suggest the Pd–C s-bond formation
* Corresponding author. Tel.: +7-095-9521203; fax: +7-095-
9541279.
E-mail address: iimois@ionchran.rinet.ru (I.I. Moiseev).
0022-328X/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(01)00825-7

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N.Y. Kozitsyna et al. / Journal of Organometallic Chemistry 636 (2001) 69–75
2. Results and discussion
much less than 1% of its initial concentration. There-
fore, the participation of ‘free’ AcO− anions in the
reactions under study can be neglected. The equili-
brated reaction solutions contained the Pd3(OAc)6 and
[Pd2(OAc)6]2− as the Pd(II) species.
2.1. Reacti6e palladium species
Alkene oxidative esterification shown in Eq. (1) is a
redox reaction that converts a soluble Pd(II) complex
into colloidal Pd clusters or aggregates of small Pd
metal species, so-called palladium black. The latter
species are efficient catalysts for alkene dehydrogena-
tion, isomerization and redox disproportionation.
These undesired side reactions are usually precluded by
the addition of p-benzoquinone (BQ), which re-oxidizes
the Pd0 formed by reaction Eq. (2)
2.2. Reaction products
The products of the oxidative esterification of hex-1-
ene, 1-methylcyclohexene and a-pinene are presented in
Table 1. Even traces of vinylic esters were not found in
all experiments. As can be seen in Table 1, allylic esters
are the main reaction products in the oxidation of all
alkenes under study.1
R%CHꢀCHR%%+Pd(OCOR)2
=Pd0+alkenyl ester+RCOOH
(2)
2.2.1. Hex-1-ene oxidation
The most plentiful allylic esters, cis- and trans-hex-2-
en-1-ol acetates (Table 1), are expected to form via the
‘carbenium’ mechanism [6], in which (1) Pd(II) atom
adds to the terminal C atom of the hex-1-ene molecule
and (2) s-carbenium complex that formed successively
splits off H+ and eliminates the Pd0(OAc)− group (see
Scheme 1).
The formation of the homoallylic ester, hex-3-en-1-ol
acetate, also follows reasonably from this mechanistic
scheme. Meanwhile, the origin of hex-4-en-3-ol acetate
is not so obvious and needs additional comments.
The appearance of the OAc group at the third posi-
tion of the hexenyl moiety can be due to the reversible
isomerization of s-allyl complex 1 to complex 3 via
p-allyl intermediate 2 as depicted in Scheme 2.
Hex-1-en-3ol acetate, the expected product of
Pd(OAc)− reductive elimination from complex 3, was
not found. We suggest that hex-4-en-3-ol acetate is
formed via a series of the ‘protonation–H-shift–depro-
tonation’ transformations of complex 3.
and recover Pd(II) as shown by Eq. (3) [1].
Pd0+Q+2RCOOHꢀPd(OCOR)2+QH2
(3)
In an aprotic solvent and in the absence of free
carboxylic acid RCOOH, re-oxidation of Pd0 by Eq. (2)
does not occur. Instead of this, p-benzoquinone (BQ)
and the alkene molecule (Un) bind the Pd0 species in
statu nascendi to the p-complex BQ·Pd·Un (see Eq. (4))
Pd0+BQ+Un=BQ·Pd·Un
(4)
which is stable in aprotic media [8,9], thus preventing
the formation of the dispersed palladium phase and
alkene side reactions.
Palladium(II) was introduced into reaction solutions
as the complex Pd3(OAc)6. The reaction of the latter
with alkenes is very slow at room temperature but is
noticeably accelerated when 0.1–0.5 mol of additional
AcO− anions per Pd(II) atom is added. The excess
AcO− anions were introduced in the form of the
The allylic esters found could have also been consid-
ered as the products of anti-Markovnikov addition of
the Pd–OAc group to the 1-alkene molecule followed
by Pd–C bond heterolytic cleavage or H–Pd elimina-
tion. There is no reason, however, to expect the anti-
Markovnikov addition.
bis(triphenylphosphoranilidene)ammonium
acetate,
(PNP)+OAc−. The corresponding anionic Pd(II) com-
plexes formed according to Eqs. (5) and (6)
Pd3(OAc)6+3(PNP)+OAc−
X 3/2(PNP)+2 [Pd2(OAc)6]2−
(5)
Pd3(OAc)6+6(PNP)+OAc− X 3(PNP)2+[Pd(OAc)4]2−
2.2.2. Methylcyclohex-1-ene oxidation
As seen in Table 1, allylic esters are also the main
products of methylcyclohex-1-ene oxidative esterifica-
tion. In the frameworks of the known ‘allylic’ mecha-
nism [4], the allylic esters 4 and 5 are expected to form
according to Scheme 3. However, neither 4 nor 5 ester
was found among the reaction products. It is notewor-
thy that the methyl group of the initial cycloalkene
remains untouched in all of the esters formed in the
reaction.
(6)
are well soluble in chloroform, so that the reaction
between Pd(II) and alkene proceeds in a homogeneous
system smoothly.
Under the experimental conditions (0.05 mol l−1 of
the total Pd(II) and 0.01–0.02 mol l−1 of the (PNP)+
OAc− added, preliminary equilibrated during 12–20 h
at 20°C), practically all the additional AcO− anions
were involved into the composition of the anionic
Pd(II) complexes. Estimation by 1H-NMR spectra
showed that a fraction of ‘free’ (unbound with Pd(II))
AcO− anions in the equilibrated CDCl3 solution was
1 Nearly the same results were obtained in experiments with the use
of tetraethylammonium acetate, (NEt4)+OAc−, instead of (PNP)+
OAc−
.

N.Y. Kozitsyna et al. / Journal of Organometallic Chemistry 636 (2001) 69–75
71
Table 1
Oxidative acetoxylation of alkenes by Pd(II) acetato complexes in chloroform at 20°C
Scheme 1.
2.2.3. h-Pinene oxidation
Unlike the ‘allylic’ mechanism [4], the ‘carbenium’
mechanism [6] reasonably explains the formation of the
most abundant reaction product, methylcyclohex-1-en-
6-ol acetate, as shown in Scheme 4.
Our experiments showed that the 2-methyl group of
the a-pinene molecule undergoes no transformations by
the action of Pd(II) acetates, similarly to that found for
The ways of formation of the other esters are not so
obvious. The main problem is why the location of the
OAc group in their molecules relatively to the Me
group is different from that expected for the three
known mechanisms, viz., synchronous ‘classical’ [1–3],
‘allylic’ [4] and asynchronous ‘carbenium’ [6] mecha-
nisms. The resolution of the issue was found by the
results for a-pinene oxidative esterification.
Scheme 2.

72
N.Y. Kozitsyna et al. / Journal of Organometallic Chemistry 636 (2001) 69–75
charge is partly distributed over the C atoms of the
hydrocarbon cyclic system. The highest positive charge
is expected to be in position six, thus inducing the
cleavage of the strained C–C bond of the four-mem-
bered cycle and producing carvyl acetate as shown in
Scheme 5.
The vicinity of the Pd atom to the H–C(7) bond
opens up a new avenue for transformation of complex
8 including its rearrangement via migration of the Pd
atom from positions three to seven, affording finally
a-pinen-7-ol acetate, as shown in Scheme 6. The ten-
dency of the Pd atom to capture an H atom seems to
facilitate the insertion of the Pd atom, while it is
splitted from the C(3) atom, into the C(7)–H bond.
The driving force for this rearrangement seems to be
a rather strong Pd–H interaction in transition state 9,
resulting in the elimination of HPdOAc, similarly to
that proposed by Heck [11] for phenylacetylene car-
bonylation with Pd complexes. A similar rearrangement
occurring in the reaction of cyclohex-1-ene (in a boat
conformation) with Pd(II) acetate can result in the
found products of oxidative esterification, methylcyclo-
hex-1-en-3-ol, methylcyclohex-6-en-3-ol and methylcy-
clohex-1-en-4-ol (Table 1).
Scheme 3.
Scheme 4.
methylcyclohex-1-ene (see above). Only two esters, a-
pinene-7-ol acetate (6) and carvyl acetate (7) were
formed by the oxidative esterification of a-pinene (see
Eq. (7) and Table 1).
3. Experimental
Solvents (chloroform, acetic acid and benzene, all
Reakhim (Russia), reagent grade) were purified by stan-
dard procedures [12]. Hex-1-ene (Aldrich, 99+% pu-
rity), methylcyclohex-1-ene (Aldrich, 97+% purity)
and a-pinene (Aldrich, 98% purity, racemate, [a]22 0°
(neat)) were distilled and stored under argon with
(7)
The origin of these reaction products can be rational-
ized taking into account a carbenium nature of the
hydrocarbon group of the s-complex (8). The carboca-
tion moiety of complex 8 has some similarities with the
‘non-classical’ homoallylic systems [9,10]. The positive
,
molecular sieve 4 A. p-Benzoquinone (Reakhim (Rus-
Scheme 5.
Scheme 6.

N.Y. Kozitsyna et al. / Journal of Organometallic Chemistry 636 (2001) 69–75
73
sia), reagent grade) was sublimated in vacuum before
3.4. bis(Triphenylphosphoranilidene)ammonium acetate
experiments. Palladium(II) chloride and sodium boro-
hydride (both Reakhim (Russia), reagent grade) were
used as received. Bis(triphenylphosphoranilidene)-
ammonium chloride, tetraethylammonium hydroxide
(40% aqueous soln. and (9)-2-methylbutyric acid (all
Fluka, reagent grade) were used as received.
bis(Triphenylphosphoranilidene)ammonium acetate
was prepared by the modified method [15]. To a solu-
tion of (Ph3P)2NCl (1 g, 1.75 mmol) in 40 ml of CHCl3
a solution of AgOAc (0.44 g, 2.62 mmol, 20% excess) in
30 ml of CHCl3 was added. The reaction mixture was
stirred during 1 h and the AgCl precipitated was filtered
off. The solution was evaporated on a rotary evaporator
to a volume of 10 ml, and diethyl ether was added to the
beginning of crystallization. The main amount of PNP
acetate was first precipitated as a colorless oil, which
crystallizes on storing in a fridge for 10–12 h. The sub-
stance was purified by recrystallization from CHCl3–
diethyl ether. Yield 0.94 g (90% based on (Ph3P)2NCl).
Anal. Found: C, 76.43; H, 5.61; N, 2.37. Calc. for
3.1. Analysis methods
Elemental C,H,N-microanalysis of the Pd(II), PNP
and NEt+4 carboxylates were performed on an auto-
matic C,H,N-analysator ‘Carlo Erba Strumentazione’,
Italy. 1H-NMR spectra were recorded on a ‘Bruker
WP-200’ spectrometer. GC/MS analyses of the reaction
products were performed on an ‘Automass 150’ chro-
matomass-spectrometer (‘Delsi Nermag’, France, capil-
lary columns with OV-1). GLC analyses were conducted
on chromatographs ‘Varian 3600’ (USA) and ‘3700’
(Russia) with different capillary columns, stationary
phases OV-1, HP-1, SE-30 and XE-60.
1
C39H33NO2P2: C, 76.37; H, 5.57; N, 2.34%. H-NMR
spectrum (CDCl3, l/ppm): 1.95 (s, 3H, OAc−); 7.3–7.7
(m, 30H, Ph).
3.5. Tetraethylammonium (9)-2-methylbutyrate
Tetraethylammonium (9)-2-methylbutyrate was pre-
pared by the method for the synthesis of tetraethylam-
monium acetate [16].
3.2. Palladium(II) acetate
Palladium(II) acetate was obtained by the oxidation
of fresh-prepared Pd black (prepared from PdCl2 and
NaBH4) with concentrated HNO3 in glacial AcOH by
the method [13]. The raw acetate was purified from
traces of nitrato and nitrito complexes by refluxing in
glacial AcOH with fresh-prepared Pd black and recrys-
tallized from hot AcOH.
3.6. General procedure for the oxidation of hex-1-ene,
methylcyclohex-1-ene and h-pinene
Palladium carboxylate (0.111 mmol), PNP or te-
traethylammonium carboxylate (0.042 mmol) and p-
benzoquinone (0.223 mmol) were placed into
a
two-neck flask. The flask was evacuated and filled with
argon. 1.5–2.0 ml of chloroform and 0.223 mmol of
alkene were added in an argon flow. The reaction
mixture was stirred at a room temperature for 10–24 h,
and the reaction products were analyzed by GLC and
GC/MS and identified by mass spectra. Our attempts to
separate the reaction products by microrectification (in
accessible for us scale, 2–3 g) gave samples inadequate
3.3. Palladium(II) (9)-2-methylbutyrate
(9)-2-Methylbutyric acid (1 g 10 mmol) in 5 ml of
benzene was added to a solution of 0.5 g of palladiu-
m(II) acetate (2.23 mmol) in 30 ml of benzene and
refluxed under mixing during 2 h. The dark precipitate
formed after cooling was filtered off, the mother liquid
was evaporated on a rotary evaporator until a viscous
orange oil was formed. The oil was dissolved in pentane,
the excess acid was washed off with water (three to four
times by 200 ml), and the oil was dried over CaCl2 and
then over a molecular sieve. Yield 0.52 g (75% based on
Pd). According to GC/MS data, the substance obtained
as a viscous orange oil contains ꢀ1% of excess (9)-2-
methylbutyric acid. The sample was used in experiments
on hex-1-ene oxidation in the form of solutions in
chloroform without additional purification. IR spec-
1
to H- and 13C-NMR analyses. Therefore, we used the
GC/MS techniques. Our GS/MS findings were prelimi-
narily compared with the Wiley 275 Database and then
the MS and GLC (repeated analyses with a variety of
stationary phases) data were checked by both the mass
spectra and retention indices with the NIST/EPA/NIH
Standard Reference Database (version 1.7).
The results are presented in Table 1.
3.7. Mass spectra of the reaction products (Automass
150, EI, 70 eV)
trum (vaseline oil), wCO/cm−1: 1637, 1425. H-NMR
1
spectrum (CDCl3, l/ppm, J/Hz): 0.62 (t, 3H, 3J=7.36);
3
0.83 (d, 3H, J=7.01); 1.41 (m, 2H); 2.2 (m, 1H). The
Our attempts to separate the reaction products by
microrectification (in accessible for us scale, 2–3 g) gave
samples inadequate to 1H- and 13C-NMR analyses.
Therefore, we used the GC/MS techniques. Our
molecular structure of Pd3(m2-S(+)-MeC*H(Et)COO)6
has been established by X-ray diffraction study of the
single crystal [14].

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N.Y. Kozitsyna et al. / Journal of Organometallic Chemistry 636 (2001) 69–75
The ‘abnormal’ product of propylene oxidation with
Pd(II) acetate, allyl acetate, was recently uncovered to be
a sole reaction product in aprotic solvents (e.g. chloro-
form, dichloromethane, liquid CO2 and THF) [6,7]. An
additional example of the anti-Markovnikov oxidation
was found in this work: hexen-1-ol esters prevail among
the products of hex-1-ene oxidation by Pd(II) carboxy-
lato complexes in a chloroform solution. As seen in Table
1, the total yield of hexen-1-ol acetates is 67% and that
of hexen-1-ol 2-methylbutyrates reaches 90%.
Such a pronounced change in the reaction regioselec-
tivity is barely attributable to a change in the direction
of the Pd(II) electrophilic attack on the alkene CꢀC bond.
The formation of homoallylic esters is much less rational-
ized in the frameworks of the conventional mechanism
of alkene into Pd–OAc bond insertion [1–4]. At the same
time, all facts available can be rationalized by assuming
a scheme involving the Pd(II) atom attack on the
least-branched position of the alkene molecule (C1 in
1-alkenes and C3 in a-pinene) according to the
Markovnikov rule. The following reductive elimination
of the Pd(OCOR)− group, after deprotonation of the
palladium s-carbenium complex, should give rise to the
reaction products found. In this scheme, the AcO group
appears at the C atom of the starting alkene at which the
Pd(II) initial attack was directed.
Scheme 7.
GS/MS findings were preliminarily compared with the
Wiley 275 Database and then the MS and GLC (repeated
analyses with a variety of stationary phases) data were
checked by both the mass spectra and retention indices
with the NIST/EPA/NIH Standard Reference Database
(version 1.7).
Hex-2-en-1-ol acetate (Mw=142); 100 (5.5); 82 (9.1);
67 (16.5); 55 (9.7); 43 (100).
Hex-4-en-3-ol acetate (Mw=142); 142 (0.1); 100
(11.1); 85 (3.1); 71 (50.5); 58 (6.8); 43 (100).
Hex-3-en-1-ol acetate (Mw=142); 82 (6.7); 67 (17.7);
54 (40.4); 43 (100).
Methylcyclohex-1-en-3-ol acetate (Mw=154); 154
(7.4); 112 (16.6); 97 (29.6); 95 (25.9); 94 (33.3); 79 (100);
67 (14.8); 55 (25.9); 43 (40.7).
Methylcyclohex-1-en-6-ol acetate (Mw=154); 154
(1.8); 112 (9.3); 94 (55.5); 79 (100); 67 (11.1); 55 (16.7);
43 (44.4).
Methylcyclohex-1-en-5-ol acetate (Mw=154); 112
(7.4); 97 (100); 84 (37.0); 69 (37.0); 55 (35.2); 41 (29.6).
Methylcyclohex-1-en-4-ol acetate (Mw=154); 110
(36.5); 95 (1.9); 82 (100); 67 (15.4); 54 (55.8); 39 (28.8).
a-Pinene-7-ol acetate (Mw=194); 152 (1.8); 134 (11.1);
119 (29.6); 108 (11.1); 92 (31.5); 91 (100); 79 (12.9); 76
(11.1); 65 (5.5); 55 (3.7); 53 (7.4); 43 (48.1); 40 (14.8); 38
(7.4).
Carvyl acetate (Mw=194); 152 (1.8); 150 (9.2); 134
(51.8); 132 (74.1); 121 (11.1); 119 (18.5); 107 (29.6); 106
(14.8); 91 (18.5); 79 (7.4); 77 (11.1); 67 (3.7); 57 (7.4); 43
(100); 41 (11.1).
Acknowledgements
The work was financially supported by the Russian
Foundation for Basic Research (projects nos. 99-03-
32291 and 00-03-40104), the Program of the Government
of the Russian Federation for support the leading
Russian scientific schools (grant 00-15-97429), the Pro-
gram of the Moscow Government for support studies of
chromatography
and
chromatomass-spectrometry
(grant GB-26/9) and the Program ‘Integration’ of the
Ministry of Science and Technologies of the Russian
Federation (grant S/O 225-2.1). Authors are also grateful
to S.G. Sakharov for recording NMR spectra and A.E.
Gekhman and I.P. Stolarov for help in GC/MS analyses.
4. Conclusions
The oxidation of alkenes higher than ethylene by
Pd(II) halides in aqueous and alcohol solutions normally
produces ketones or ketals [1–3]. These are the expected
products when the lyate ion (OH− in water and RO− in
alcohol) and electrophilic species PdX−3 add to the CꢀC
double bond according to the Markovnikov rule (see
Scheme 7). Both electronic and steric factors are believed
to facilitate this pathway.
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