Intermediates of cobalt-catalysed PTC carbonylation of benzyl halides

Article abstract of DOI:10.1016/S1381-1169(03)00303-0

Intermediates of the Co carbonyl-catalyzed carbonylation of ortho-substituted benzyl halides were identified by IR, ¹H, and ¹³C-NMR spectra. The molecular structures of the acyl-type derivatives 2-MeC₆H₄CH₂

Full text of DOI:10.1016/S1381-1169(03)00303-0

Journal of Molecular Catalysis A: Chemical 204–205 (2003) 227–233
Intermediates of cobalt-catalysed PTC carbonylation
of benzyl halides
Howard Alpera, Lajos Benczeb, Roland Boesec, Luciano Cagliotid, Robert Kurdib,
Gyula Pályie,∗, Stefania Tiddiae, Davide Turrinie, Claudia Zucchie
a
Department of Chemistry, University of Ottawa, 10 Marie Curie, Ottawa, Ont., Canada K1N 6N5
b
Müller Laboratory, Department of Organic Chemistry, University of Veszprém, Egyetem-u.6, H-8200 Veszprém, Hungary
c
Institute of Inorganic Chemistry, University of Essen, Universitätsstrasse 5-7, D-45117 Essen, Germany
d
Department of Chemistry and Technology of Biologically Active Compounds,
University “La Sapienza” Roma, P.zale Moro 5, I-00185 Roma, Italy
Department of Chemistry, University of Modena and Reggio Emilia, Via Campi 183; I-41100 Modena, Italy
e
Received 4 October 2002; received in revised form 13 March 2003; accepted 25 March 2003
Dedicated to Professor Renato Ugo on the occasion of his 65th birthday
Abstract
Intermediates of the cobalt carbonyl-catalysed carbonylation of ortho-substituted benzyl halides were identified by IR,
1H- and 13C-NMR spectra. The molecular structures of the acyl-type derivatives 2-MeC6H4CH2C(O)Co(CO)3PPh3 and
2-PhC6H4CH2C(O)Co(CO)3PPh3 were determined by X-ray crystallography. Structural features of these complexes are
discussed.
© 2003 Elsevier Science B.V. All rights reserved.
Keywords: Phase transfer catalysis; Hydroxycarbonylation; Carbonylation; Acylcobalt carbonyls; Alkylcobalt carbonyls; ␩3-benzylcobalt
carbonyls
1. Introduction
The highly complicated multiple interactions oper-
ative in these techniques, however, make sometimes
difficult the chemical and physical fine tuning of these
reactions. It is, therefore, of primary interest to gain
insight into the mechanisms. These can be substan-
tially different from those of conventional only-TM,
or even of the (related) biphasic-TM variants [9–15].
A few years ago we published an easy, cheap and
efficient TM–PTC variant of the cobalt-catalysed
hydroxycarbonylation of benzyl halides, using poly-
(ethylene-glycol)s (PEG) as phase transfer catalysts
[16]. The mechanistic studies in course of this work
provided a good example of the above arguments,
disclosing new (catalytic) loops of the TM–PTC
Phase transfer catalysis (PTC) became a well-estab-
lished, environmentally friendly (green) method in
preparative organic chemistry [1]. The combination
of PTC with transition metal molecular catalysts
(TM) is one of the most promising potentialities of
transition metal chemistry. TM–PTC is characterised
by dramatic increase in reaction rates, consequently
enables mild conditions, favourable operational costs
and high selectivities [2–8].
∗
Corresponding author. Fax: +39-059-373543.
E-mail address: palyi@unimo.it (G. Pa´lyi).
1381-1169/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S1381-1169(03)00303-0

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H. Alper et al. / Journal of Molecular Catalysis A: Chemical 204–205 (2003) 227–233
system, catalysed by one of the “intermediates”. This
prompted us to perform a systematic, structurally
controlled study on the intermediates of this reac-
tion. In a recent paper we described the isolation and
X-ray structure of some para-substituted benzylcobalt
derivatives [17]. Here we report on organocobalt
complexes obtained from ortho-substituted benzyl
halides.
with MP solvent: 64% (TON 19.2); 2-PhC6H4CH2Br:
3.1% (TON 0.45), with MP solvent: 5.8% (TON 0.7);
o,oꢀ-BrCH2C6H4C6H4CH2Br: 7.8% (TON 0.9), with
MP solvent: 30.5% (TON 3.7). [TON: turnover num-
bers, calculated per Co atom introduced.] For com-
parison: PhCH2Br with t-AmOH solvent 65% (TON
24.4) [16]; with MP solvent 10.2% (TON 3.8). Similar
procedure using benzene as organic solvent yielded,
with 2-MeC6H4CH2Br exclusively the coupling prod-
uct o,oꢀ-dimethyl-1,2-diphenylethane (6a) in 48.7%
yield. (Table SM7).
2. Experimental
The organic phases of the carbonylation reaction
mixtures were analysed time-to-time by infrared
spectroscopy and the intermediate cobaltorganic com-
plexes (2, 3, 4) were identified by model preparations
as described in the following part of this paper.
Starting materials were of commercial origin, with
the exception of dicobalt octacarbonyl, which was
prepared according to a published procedure [18].
All operations were performed by standard Schlenk
techniques [19]. Instruments used were as follows:
IR: Bruker FT-IR IFS 113 V; 1H- and 13C-NMR:
Bruker AMX 400; mass spectra: HP 5890 GC-HP
5890 AMS instrument; X-ray diffraction: Nicolet
R 3 m/V four circle diffractometer, Mo K␣ radia-
tion (at 298 K); diffractometer measurement device:
Siemens SMART CCD area detector system; diffrac-
tometer control software, Bruker AXS SMART Vers.
5.054 1997/1998; computing data reduction: Bruker
AXS SAINT program Vers. 6.02A; structure solu-
tion and refinement: Bruker AXS SHELXTL Vers.
5.10 DOS/WIN95/NT. Atomic scattering factors were
taken from SHELXTL and [20,21]. Corrections for
anomalous dispersion were made according to [22].
The procedures followed in the preparation of
the benzyl- and phenylacetylcobalt complexes were
variants of the procedures described in [16,23–25].
Catalytic and model experiments are summarised in
Scheme 1.
2.2. Preparation of
2-PhC6H4CH2C(O)Co(CO)3PPh3
Na[Co(CO)4] (2 mmol) was prepared (with Na/Hg
from Co2(CO)8) in 20 ml of diethyl ether (Et2O) and
cooled to −30 ◦C. To this solution, under an Ar atmo-
sphere, while being stirred, 2-phenylbenzylbromide
(1b) (0.48 g [0.36 ml], 2 mmol) was added at once.
The reaction mixture, almost immediately became
turbid, because of the formation of a white precip-
itate (NaBr). The reaction mixture was stirred for
additional 30 min at −30 ◦C and then it was left to
warm gradually to r.t. and stirring was continued for
1–2 h.
At this point, a sample (0.5 ml) was taken and
analysed by IR spectroscopy (2200–1600 cm−1
range), which indicated the complete disappear-
ance of the strong, broad band of [Co(CO)4]−
at ∼1900 cm−1, as well as the evolution of a
new band system, which showed the presence of
␩3-benzylCo(CO)3 (4) (main product) [9,16,24] and
␩1-phenylacetylCo(CO)4 (2) [16,17,23–26] as well as
low amount of ␩1-benzylCo(CO)4 (3) [16,17,23–26]
type complexes together with minor amounts of
Co2(CO)8 and Co4(CO)12 as the only cobalt carbonyl
complexes in the solution. The NaBr precipitate was
filtered off and the resulting clean brown solution was
used for additional experiments.
2.1. Catalytic carbonylations
The procedure described in [16] was followed, with
the following parameters: r.t., 1 bar CO, 12 mmol sub-
strate (benzyl halide) (1), 1 mmol PEG 400, 12.5 ml
tert-amyl alcohol (t-AmOH), 12.5 ml 15% aqueous
NaOH, 0.5 mmol Co2(CO)8. Carbonylations using
4-methylpentane-2-one (MP) as organic phase were
performed similarly, substituting only t-AmOH with
the ketone solvent.
Treatment of the reaction mixture with CO
gas resulted in the increase of the amount of the
␩1-phenylacetylCo(CO)4 type complex as well as
Carbonylation yields were (corresponding pheny-
lacetic acids (5)): 2-MeC6H4CH2Br: 22% (TON 6.6),

H. Alper et al. / Journal of Molecular Catalysis A: Chemical 204–205 (2003) 227–233
229
Scheme 1. Preparative experiments: (i) and (iii) PTC (t-AmOH or 4-methylpentane-2-one as organic phase); (ii) model experiment with
Na[Co(CO)4]/Et2O; (iv) PTC (benzene as organic phase); (v) thermal decomposition, 0 ◦C; (vi) PPh3/Et2O.
treatment with Ar atmosphere (bubbling through the
solution) resulted considerable enrichment of the
␩3-derivative. These operations could be repeated,
however, no enrichment of the ␩1-benzylCo(CO)4
(alkyl) (3) derivative could be achieved, this complex
was present always in low concentrations, just on the
limit of being observed.
Then, the enrichment of the reaction mixture in the
␩1-acyl derivative was achieved by treatment with CO
gas. To this solution, while being stirred, under Ar at-
mosphere at r.t., triphenylphosphine (524 mg, 2 mmol)
was added in one portion. The resulting mixture was
stirred for 3 h, in course of which a gradual colour
change from brown to light yellow was observed. A
sample was taken for IR analysis, which indicated
the presence of ␩1-phenylacetylCo(CO)3PPh3 type (8)
complex [16,17,23–26] as practically the only one
cobalt carbonyl species in solution. This product was
then obtained in pure form by evaporating the Et2O
solvent and recrystallising the crude product from (lay-
ered) Et2O/n-hexane.
The yellow crystalline product (8b) (471.7 mg, 41.2
%) was characterised by spectra, elemental analyses
(Tables SM4–SM6 and SM9) and X-ray diffraction
(Tables SM10–SM17).
An attempt at thermal decarbonylation [26] of
2-PhC6H4CH2C(O)Co(CO)3PPh3 (8b) (Et2O 32 ◦C,
8 h, Ar atmosphere), showed a particular stability of
this complex, only minor amounts of the alkyl-type
derivative 2-PhC6H4CH2Co(CO)3PPh3 (9b) were
formed, just enabling spectroscopic detection of this
species (Tables SM4–SM6).

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H. Alper et al. / Journal of Molecular Catalysis A: Chemical 204–205 (2003) 227–233
Preparation of 2-MeC6H4CH2C(O)Co(CO)3PPh3
ence can, most probably, be attributed to changes in
the relative stabilities, and therefore the distribution of
the intermediate cobalt complexes (which in fact was
detected, vide infra).
An attempt at using the “classical” organic solvent
of TM–PTC carbonylations (benzene) led to only ben-
zyl/benzyl coupling product (6a) indicating again the
sensitivity of these systems towards all details of the
experimental set-up.
(8a) was performed similarly. Spectra, analyses and
results of the X-ray structure determination are shown
in Tables SM4–SM6, SM9 and SM10–SM17, respec-
tively.
Similar treatment of o,oꢀ-BrCH2C6H4C6H4CH2Br
(1c) resulted very instable ␩3-benzyl-, ␩1-alkyl and
␩1-acyl-type complexes, which decomposed dur-
ing spectroscopic characterisation as well as dur-
ing attempts at isolation or substitution by PPh3.
The decomposition yielded a monocarbonylated
ring-closure derivative in high yields (81.6%): 3,4;5,6-
dibenzocycloheptanone (7c) [27], which was charac-
The reactions of the in situ generated [Co(CO)4]−
catalyst precursor were modelled by reactions of
Na[Co(CO)4] (prepared more “cleanly” by Na/Hg
in Et2O). These model reactions yielded a mixture
of ␩1-acylCo(CO)4 (2), ␩1-alkylCo(CO)3 (3) and
␩3-benzylCo(CO)3 (4) complexes, as expected on the
basis of earlier experience [16,17,23–26], but with
some important differences:
1
terised by mass as well as H and 13C NMR spectra
(Table SM8).
“Model” experiments for the study of the differ-
ences in the distribution of intermediates 2, 3 and 4
under PTC conditions were performed as described
above for the preparation of 2b. The solvent from
the final sample was evaporated and the residue was
dissolved in t-AmOH or MP. These solutions were
flushed by Ar or CO gas to achieve decarbonyla-
tion/carbonylation reactions. These reactions were fol-
lowed by IR spectroscopy.
(a) only low amounts of alkyl-type complexes 3 were
formed (with respect to unsubstituted benzyl
derivatives);
(b) the ortho-substituted derivatives showed an ele-
vated tendency to form ␩3-benzylCo(CO)3 type
complexes (4), which enabled also the isolation of
4a (see [4]);
(c) the cobalt complexes obtained from the disubsti-
tuted biphenyl substrate 1c were unusually insta-
ble, starting to decompose already a 0 ◦C, yielding
the interesting C7-ring derivative 7c.
3. Results and discussion
3.1. Preparative results
The all-carbonyl complexes were oily substances,
which could not be isolated except the ␩3-derivative
(4a) [16].
The preparative results are summarised in Scheme 1.
The TM–PTC hydroxycarbonylation of 1a–c pro-
ceeded with low to medium yields (and TON-s). It
is interesting to compare the carbonylation yields ob-
tained with the two solvents (t-AmOH versus MP)
with the yields observed for unsubstituted PhCH2Br
([16] and this work) in these solvents, used as organic
phases. While for unsubstituted PhCH2Br t-AmOH
provides significantly higher yield, than the ketone,
substrates 1a, 1b and 1c show higher yields using MP.
This “inverted solvent effect” might be attributed to
the different distributions of intermediates 2, 3 and 4.
Model experiments, carbonylation/decarbonylation
of mixtures of complexes 2, 3 and 4 in these sol-
vents indicate that there are significant differences in
these distributions between unsubstituted benzyl- ver-
sus ortho-substituted benzyl derivatives, as well as for
the same derivative in different solvents. This differ-
3.2. Structural results
The ␩1-acyl-type complexes 8a and 8b could be ob-
tained in crystalline form, the X-ray diffraction struc-
ture determination resulted the structures shown in
Fig. 1. Experimental details and some structural fea-
tures are shown in Tables SM10–SM17.
The crystal systems and point groups of the com-
plexes 8a and 8b are different, 8a: monoclinic P21/n,
¯
while 8b: triclinic P1 with Z = 4 and 2, respectively.
Both are centrosymmetric. In spite of these differences
the main structural features of the complexes are fairly
similar.
The overall geometries are approximately trigonal
bipyramidal, with the non-carbonyl ligands in the

H. Alper et al. / Journal of Molecular Catalysis A: Chemical 204–205 (2003) 227–233
231
Fig. 1. ORTEP drawing of the molecular structures of complexes 8a and 8b (one enantiomer each). Some characteristic parameters
are: distances: Co–C(1), Co–C(2), Co–C(3) average 1.792, 1.785; Co–C(4) 2.012(3), 2.031(3); Co–P 2.2642(8), 2.2498(8); C(4)–O(4)
1.202(4), 1.193(4) Å; angles: C(2)–Co–C(3), C(2)–Co–C(1), C(3)–Co–C(1) 117.10–124.32, 114.91–125.80; C(4)–Co–P 177.17(9), 178.12(9);
C(5)–C(4)–O(4) 118.7(3), 120.7(3)◦, respectively.
two axial positions, as usual in similar complexes
[17,24,28–32]. The equatorial carbonyl ligands are
deformed because of the steric interactions with the
other ligands, as reflected by the non-equivalence of
the OC–Co–CO angles. The geometry around the
acyl group is “regular”.
The organic ligands show a unique orientation:
these are turned “side-on” towards the cobalt. This
situation appears to be of autosolvation-type interac-
tion [33–37], which is also reflected by the “bending”
of the C5, C8 axes (3–5◦ in both complexes). The
two ortho-C-s in the aromatic rings are (within 1.7%)
equidistant from the Co atom.
(in 8a) is quantitatively correlated to P helicity of
the phosphine, while the re/P (in 8b) correlation
in the other complex is only apparent, because
of the change in the priority in the corresponding
CIP rule;
(ii) in the elementary cell of complex 8a two pairs
of conformational racemates si/P versus re/M are
present, while in the more restricted cell of 8b
only one pair (re/P and si/M).
The particular structural features of the phenylacetyl
ligand in these complexes give hints at the reasons
for the unusual preference of the ␩1-acyl complexes
2a, 8a, 2b and 8b but still more structural work is
necessary to achieve a more detailed interpretation.
The concertedness of the orientations of the enan-
tiofaces of the aromatic ring in the phenylacetyl lig-
and with the enantiomeric conformations of the PPh3
ligand provides an important new element to our re-
cent findings with alkylcobalt carbonyl complexes
The conformations of both complexes show peculiar
features:
(i) both ortho-substituents show the same orienta-
tion with respect to the acyl-O and with respect
to the PPh3 conformation. The ring si (enantio-
face turned towards the Co atom) conformation

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H. Alper et al. / Journal of Molecular Catalysis A: Chemical 204–205 (2003) 227–233
[38–42]. This correlation clearly shows, how sensi-
tively can “react” an apparently achiral ligand (here
the ortho-substituted aromatic ring) to chiral informa-
tions from an another ligand (PPh3) mediated by the
transition metal.
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4. Supporting material
Additional supporting material (SM) is available
upon request from the authors. This comprises Ta-
bles of IR (SM1, SM4), 1H- and 13C-NMR (SM2,
SM3, SM5, SM6) of compounds 2, 3, 4, 8 and 9 as well
1
as H-, 13C-NMR and MS of compounds 6a (SM7)
and 7c (SM8), elemental analyses of compounds 8a
and 8b (SM9) as well as X-ray diffraction crystal
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Acknowledgements
Financial support is acknowledged to the Italian
Ministry of University and Research (MURST) the
[Italian] National Research Council (CNR, Rome)
and the [Hungarian] OTKA program (contract no.
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