Structure, formation and catalytic studies of a meso-palladioporphyrin intermediate in a Heck reaction

Article abstract of DOI:10.1039/b801163j

A meso-palladioporphyrin intermediate was isolated from a Heck reaction between an iodoporphyrin and a non-activated olefin using a Pd(PPh ₃)₂Cl₂/Et₃N system; its structure was characterized by NMR, MS and X-ray

Full text of DOI:10.1039/b801163j

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www.rsc.org/dalton | Dalton Transactions
Structure, formation and catalytic studies of a meso-palladioporphyrin
intermediate in a Heck reaction†
Michael Y. Jiang, Ji-Young Shin, Brian O. Patrick and David Dolphin*
Received 22nd January 2008, Accepted 21st February 2008
First published as an Advance Article on the web 26th March 2008
DOI: 10.1039/b801163j
A meso-palladioporphyrin intermediate was isolated from a Heck reaction between an iodoporphyrin
and a non-activated olefin using a Pd(PPh3)2Cl2/Et3N system; its structure was characterized by NMR,
MS and X-ray crystallography. Studies on its formation indicate that the Pd(II) catalyst was reduced in
situ by Et3N with the assistance of water. The catalytic activity of the intermediate was confirmed by
stoichiometric and catalytic reactions using a more reactive olefin, ethyl acrylate.
highly electron-rich moiety, 1-ethoxyvinyl benzoate, to the meso-
position of 5,15-diphenylporphyrin (DPP) using a Heck reaction.
Although electronically unfavored, the b-arylation of the 1,1-
disubstituted olefin should be feasible considering the lack of
Introduction
Pd-catalyzed cross-coupling reactions have proven to be mild yet
powerful methodologies in modern organic synthesis.1 The general
catalytic cycles consist of oxidative addition, transmetallation (or
eliminable a-hydrogens and the steric repulsion. Unfortunately,
olefin insertion for Heck reactions) and reductive eliminations.
the common Heck coupling conditions (Pd(OAc)2/PPh3/bases)
Both Pd(0) and Pd(II) catalysts are frequently used and for
failed and we turned to a number of different catalyst formulations.
the latter, a pre-reduction step is generally assumed prior to
The Pd(PPh3)2Cl2/Et3N combination, which is more frequently
the oxidative addition. Despite their clear significance, only
limited information is available regarding the mechanisms of
this reduction process and the “real” in situ reductant has yet
employed in Sonogashira and Suzuki coupling reactions, was
examined. Refluxing a THF solution of 1-ethoxyvinyl benzoate, 1,
Et3N, and Pd(PPh3)2Cl2 with a stoichiometric ratio of 1 : 1 : 2 : 0.5
to be unambiguously determined.2 Phosphines,3 organometallic
for 16 h gave, instead of the expected coupling product, a
nucleophiles,4 amines,5–7 and olefins2 have all been suggested as
the Pd(II) to Pd(0) reductant in the catalytic systems. In the field
of porphyrin chemistry, Pd-catalyzed cross-coupling reactions
have been widely used in peripheral functionalization and multi-
deiodination product 2 and a meso-palladioporphyrin 3 in yields
of 28% and 5%, respectively. Most of the starting olefin and about
two-thirds of 1 were recovered.
The X-ray crystal structure of 3 is shown in Fig. 1. There are two
porphyrin array construction.8 Peripheral palladioporphyrins, the
molecules, A and B, in the unit cell and both show slightly distorted
oxidative addition intermediate9 of these transformations, were
porphyrin skeletons and square planar coordination geometry
first reported by Arnold et al. in 1998.10 They can be readily pre-
pared from the stoichiometric reactions between bromoporphyrins
and Pd(0) species, such as Pd(PPh3)4 or Pd2(dba)3.10–12 However,
around palladium. A and B differ mostly in the orientations of two
meso-phenyl substituents, which are staggered in A and parallel in
B. The torsional angles between the phenyl rings and the porphyrin
mean plane are 52.8◦ and 79.6◦ in A and 64.8◦ and 64.9◦ in B. One
no peripheral palladioporphyrin has been generated from a Pd(II)
precursor. In fact, to the best of our knowledge, the isolation of
phenyl group from each of the PPh3 ligands in both molecules
an oxidative addition intermediate from a catalytic system using
overhangs the porphyrin plane in a sandwich-like manner, due to
the p–p stacking interactions,13 which can be inferred through: (1)
the torsional angles between the top and the bottom phenyl planes
and the porphyrin mean plane are 5.7◦ and 11.5◦ in A and 5.6◦
and 11.9◦ in B; and (2) the mean distances from the top and the
a Pd(II) catalyst has never been reported. Moreover, the catalytic
reactivity of peripheral palladioporphyrins has never been studied.
Here, we report the isolation of a meso-palladioporphyrin as
a reaction intermediate from a Heck coupling reaction using a
Pd(II) catalyst. The subsequent study of its formation mechanism
and catalytic activity contributes to the better understanding of
the mechanism of Pd(PPh3)2Cl2/Et3N catalyzed cross-coupling
reactions.
˚
bottom phenyl carbons to the porphyrin mean plane are 3.33 A
1
˚
˚
˚
and 3.94 A in A and 3.27 A and 4.00 A in B. As a result, H NMR
Result and discussion
To develop new-generation porphyrin-based photosensitizers for
photodynamic therapy (PDT), we attempted to introduce a
Department of Chemistry, University of British Columbia, Rm W300-6174
University Blvd., Vancouver, BC, V6T 1Z3, Canada. E-mail: david.dolphin@
ubc.ca; Fax: +1-604-822-9278; Tel: +1-604-822-4571
† CCDC reference number 637318. For crystallographic data in CIF or
other electronic formats see DOI: 10.1039/b801163j
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Table 1 Controlled experiments to identify the reductant for
Pd(PPh3)2Cl2
Entrya
Pd(II)b
1
TEAc
Olefin
THFd
2e
3f
1
2
3
4
0.5
1
1
1
1
1
1
2(A)
1(A)
No
1
B
B
A
B
28
50
No
No
5
8
No
No
No
No
No
1
5(B)
a All experiments were run at the same scale (0.1 mmol of 1) and refluxed
in THF for 16 h. Equivalents of reagents are shown. In some cases, reagent
grade is indicated. b i.e. Pd(PPh3)2Cl2. c TEA = triethylamine. A: Aldrich,
99.5%; B: rigorously dried over sodium. d A: Aldrich, 99.5%; B: rigorously
dried over sodium. e Yields (%) were determined by NMR. f Isolated yields
(%) relative to Pd(PPh3)2Cl2.
Pd(PPh3)2Cl2, as a result of the shielding effect of the porphyrin
macrocycle.
Complex 3 results from the sequential steps of in situ reduction
of Pd(PPh3)2Cl2 to Pd(0), oxidative addition of 2 to Pd(0) to
give 4, and the conversion of 4 to 3 through I–Cl exchange. The
low isolated yield of 3 is attributed to the instability of meso-
palladioporphyrins in solution and upon chromatography,11,14 as
they readily decompose to give the hydrodepalladation products
(2 in this case). Even though the possibility that 2 came from the
hydrodepalladation of 4 cannot be ruled out, the combined yields
of 2 and 3 indicate that a substantial amount of Pd(PPh3)2Cl2 had
been reduced in situ to Pd(0).
We were prompted to study the in situ reduction of Pd(PPh3)2Cl2
by carrying out a series of control experiments (Table 1), taking
advantage of the fact that the three porphyrin derivatives of
interest, 1, 2 and 3, can be readily distinguished on TLC.15 The
original reaction is listed as Entry 1 for comparison. Refluxing
equimolar amounts of Pd(PPh3)2Cl2, 1 and Et3N in THF without
the olefin substrate gave 2 and 3 in slightly improved yields,
due to the increased Pd(PPh3)2Cl2 loading (Entry 2). This clearly
eliminates the possibility of olefin substrate as the major reductant.
In the absence of Et3N, 1 failed to react even when using the
residual water-containing commercial THF solvent (Entry 3).
However, when one equivalent of Et3N was added to the system,
2 and 3 immediately appeared on TLC. Interestingly, when both
rigorously dried Et3N and THF were employed, the reduction
of Pd(PPh3)2Cl2 did not occur (Entry 4). It is obvious that Et3N
together with trace water play important roles in the reduction
of Pd(PPh3)2Cl2 in our system. Grushin and Alper studying the in
situ reduction of a Pd(II) catalyst,16 suggested that the PPh3 ligands
are the in situ reductant for the Pd(PPh3)2Cl2/Et3N system and the
Et3N is merely responsible for the deprotonation of the residual
water to generate OH−, which is involved stoichiometrically in
the reduction. However, in our experiments where substantial
reduction occurred (Entries 1 and 2), the only possible water source
is the residual water in the commercial 99.5% Et3N, which is far
from sufficient for reduction. Furthermore, the fact that neither
2 nor 3 was generated in Entry 3 where PPh3 and residual water
were both present also disfavours Grushin’s mechanism. In fact,
amines have been reported to reduce Pd(II) precursors such as
Fig. 1 ORTEP drawings of 3 (A: top view; B: side view) showing thermal
ellipsoids at 50% probability level. The CH2Cl2 and hydrogen atoms are
◦
˚
omitted for clarity. Selected bond lengths (A) and bond angles ( ) for
molecule A: Pd(1)–P(1) = 2.324(2), Pd(1)–P(2) = 2.329(2), Pd(1)–Cl(1) =
2.384(2), Pd(1)–C(10)
=
2.039(9); C(10)–Pd(1)–P(1)
=
88.9(3),
C(10)–Pd(1)–P(2) = 87.8(3), P(1)–Pd(1)–Cl(1) = 92.36(9), P(2)–Pd(1)–
Cl(1) = 90.87(9). For B: Pd(2)–P(4) = 2.327(2), Pd(2)–P(3) = 2.337(2),
Pd(2)–Cl(2) = 2.377(2), Pd(2)–C(82) = 2.008(10), C(82)–Pd(2)–P(4) =
89.7(3), C(82)–Pd(2)–P(3)
P(3)–Pd(2)–Cl(2) = 90.59(9).
=
88.2(3), P(4)–Pd(2)–Cl(2)
=
91.45(9),
6
PdCl2·2PhCN5 and Pd(acac)2 even without phosphine ligands.
Therefore, we believe that Et3N is the major reductant for the
Pd(PPh3)2Cl2/Et3N system and the reduction is greatly accelerated
by trace water, although the mechanism awaits further exploration.
signals of the PPh3 ligands shift 0.5 ppm and 0.95 ppm upfield for
ortho-Hs and meta-/para-Hs, respectively, compared to those of
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An authentic sample of 4 was independently synthesized by the
oxidative addition of 1 to Pd(PPh3)4. The 1H and 31P NMR spectra
of 3 and 4 allow unambiguous discrimination between them.17
The crude reaction mixture of Entry 2 was immediately subject
to NMR characterization and no 4 was observed. This clearly
indicates that I–Cl exchange was completed before the purification
process and the chloride of 3 came from Pd(PPh3)2Cl2 after its
reduction.18 The exclusive isolation of 3 in the absence of 4 suggests
a thermodynamically favoured I–Cl exchange process. Indeed, 4
was completely converted to 3 within minutes after vigorously
mixing with aqueous KCl solution.19 Amatore and coworkers
studied the oxidative addition mechanism using PhI and elec-
trochemically generated Pd(0) from Pd(PPh3)2Cl2 and concluded
that the I–Cl exchange for the analogous trans-PhPdI(PPh3)2 in
the presence of a large excess of chloride did not occur,20 which
is in direct contrast to our results. However, such a conclusion
is questionable considering only 31P NMR was employed for
structural characterization in their study, which is incapable of
distinguishing trans-PhPdI(PPh3)2 and its chloride counterpart.21
On the other hand, the ready discrimination between closely
analogous 3 and 4 by 1H and 31P NMR spectroscopy showed the
advantages of using porphyrin-based substrates to perform the
mechanistic studies, owing to their unique spectroscopic (NMR,
UV-vis, fluorescence) and spectrometric (MS) characteristics in
contrast to other organic compounds.
reactions. When employed on a catalytic scale (0.1 equivalent), 3
showed similar catalytic efficiency as Pd(PPh3)2Cl2 in catalyzing
the coupling reaction and furnished 5 in 92% yield (reactions C
and D).
On the basis of the above discussion, a mechanism diagram
focussing on the early stages of Heck coupling between 1 and
olefin substrates using the Pd(PPh3)2Cl2/Et3N system is depicted
in Scheme 2. Pd(PPh3)2Cl2 is reduced in situ to Pd(0) by Et3N with
the assistance of trace water, accompanied by the extrusion of Cl−.
Oxidative addition of 1 to Pd(0) gives 4, which is readily converted
to 3 in the presence of Cl−. When non-activated olefin substrates
are used, 3 and 4 slowly decay to 2 via hydrodepalladation.
However, addition of an activated olefin such as ethyl acrylate to
the system resumes the catalytic cycle to produce 5 and regenerates
the Pd(0), assuring continuous turnovers. Such a mechanistic
scheme may be applicable to other cross-coupling reaction systems
where common organic substrates and the Pd(PPh3)2Cl2/Et3N
conditions are used.
To study the catalytic activities of 3 and 4, a highly reactive
olefin substrate ethyl acrylate rather than 1-ethoxyvinyl benzoate
was employed (Scheme 1). Stoichiometric reaction of 3 (or 4),
ethyl acrylate and Et3N gave 5 in quantitative yield within 1 h
(reactions A and B). It is noteworthy that the hydrodepalladation
of 3 (or 4) was completely suppressed as no 2 was found in these
Scheme 2 Catalytic cycle of the Heck reaction using the Pd(PPh3)2Cl2/
Et3N system.
Conclusions
In summary, a meso-palladioporphyrin 3 has been isolated as a
reaction intermediate from a Heck reaction using Pd(PPh3)2Cl2/
Et3N. Its mechanism of formation as well as its catalytic activity
have been studied. Our work has shown that porphyrins can be an
advantageous platform for mechanistic studies.
Experimental
General directions
All reactions were performed under an atmosphere of argon
in flame-dried glassware, unless otherwise indicated. Anhydrous
THF was distilled from sodium/benzophenone under argon
immediately prior to use. Anhydrous triethylamine was distilled
from sodium immediately prior to use. All other reagents were
used as commercially supplied. Flash chromatography (FC) was
performed on silica gel (Silicycle, 230–400 mesh). Thin layer
chromatography (TLC) was performed on Merck aluminium
backed plates pre-coated with silica (0.2 mm, 60 F254). The
Scheme 1 Catalytic study of 3 and 4. Reaction conditions: a THF, reflux,
1 h, 3 : olefin : Et3N = 1 : 2 : 2; b THF, reflux, 1 h, 4 : olefin : Et3N = 1 : 2 : 2; c
THF, reflux, 16 h, 1 : olefin : 3 : Et3N = 1 : 2 : 0.1 : 2, yield based on 63%
d
conversion of 1; THF, reflux, 16 h, 1 : olefin : Pd(PPh3)2Cl2 : Et3N =
1 : 1.2 : 0.1 : 2, yield based on 72% conversion of 1.
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NMR spectra were recorded on a Bruker AV-400 instrument.
Chemical shifts are quoted in parts per million (ppm) and
referenced to the appropriate residual solvent peak. Low and
high resolution electron spray ionization (ESI) mass spectra were
obtained on a Micromass LCT spectrometer, with only molecular
ions (M+, MH+, MNa+) being reported. High resolution ESI mass
spectrometry (HRESIMS) measurements are valid to 5 ppm.
Elemental analyses were performed on a Carlo Erba Elemental
Analyzer 1108. UV-Vis spectra were recorded on a Varian Cary-
50 spectrophotometer.
Typical procedure for catalytic studies (stoichiometric scale,
reaction A, Scheme 1)
To a dry THF solution (5 mL) of 3 (6.6 mg, 0.0056 mmol)
was added ethyl acrylate (1.2 lL, 0.011 mmol) and triethylamine
(1.6 lL, 0.011 mmol). After refluxing for 1 h, the crude mixture
was concentrated in vacuo and purified by silica chromatography
(CH2Cl2) to give 5 (3.3 mg, quantitative yield). Rf (THF : hexane =
◦
1
1 : 1): 0.48; (CH2Cl2): 0.41; H NMR (400 MHz, CDCl3, 25 C,
TMS): d = 10.25 (d, 3J(H,H) = 15.7 Hz, 1H), 10.17 (s, 1H), 9.55
(d, 3J(H,H) = 4.7 Hz, 2H), 9.32 (d, 3J(H,H) = 4.5 Hz, 2H), 9.03 (d,
3J(H,H) = 4.7 Hz, 2H), 9.01 (d, 3J(H,H) = 4.5 Hz, 2H), 8.21–8.19
(m, 4H), 7.80–7.75 (m, 6H), 6.70 (d, 3J(H,H) = 15.7 Hz, 1H), 4.43
(q, 3J(H,H) = 7.1 Hz, 2H), 1.48 (t, 3J(H,H) = 7.1 Hz, 3H); ESIMS
(m/z): 622.0 (M+); HRESIMS (m/z): calcd for C37H26N4O2Zn
(M+) 622.1347, found 622.1344; UV-Vis (CH2Cl2) kmax/nm (log
e): 422 (5.49), 551 (4.27), 592 (3.99). Anal. calcd (found) (%) for
C37H26N4O2Zn·0.5H2O: C, 70.20 (70.10); H, 4.30 (4.41); N, 8.85
(8.56).
X-Ray structure determination of 3
A red needle crystal of 3 was obtained by slow diffusion of hexane
vapour into a CH2Cl2 solution of 3. The crystal is formed as a
racemic twin so the domains of both enantiomorphs are present
in the crystal. Crystallographic data: 2[PdCl(ZnDPP)(PPh3)2]·
0.84CH2Cl2, C136.84H99.68Cl3.68N8P4Pd2Zn2, M = 2453.88, 0.50 ×
0.10 × 0.10 mm, monoclinic, P21 (no. 4), a = 13.304(1), b =
◦
3
˚
˚
29.414(3), c = 14.427(1) A, b = 90.818(3) , V = 5644.8(8) A , Z =
2, qcalcd = 1.452 g cm−3, l(Mo-Ka) = 9.42 cm−1, T = 173 K, 44251
measured reflections, 22575 unique reflections, R1 = 0.079 (I >
3r(I)), wR2 = 0.221 (all data), GOF = 1.05. CCDC 637318.
Typical procedure for catalytic studies (catalytic scale, reaction C,
Scheme 1)
To a dry THF solution (5 mL) of 1 (20 mg, 0.03 mmol) and 3
(3.5 mg, 0.003 mmol) was added ethyl acrylate (6.4 lL, 0.06 mmol),
and triethylamine (8.3 lL, 0.06 mmol). After refluxing for 16 h,
the crude mixture was concentrated in vacuo and purified by silica
chromatography (CH2Cl2 : hexane = 1 : 1) to give 5 (10.8 mg, 92%
based on converted 1), and 7.4 mg (37%) of the starting porphyrin
was recovered.
Typical procedure of control experiments (Entry 2, Table 1)
To a dry THF solution (5 mL) of 10-iodo-5,15-diphenylporphy-
rinatozinc(II)22 1 (65 mg, 0.1 mmol) was added bis(triphenylpho-
sphine)palladium(II) dichloride (70 mg, 0.1 mmol) and triethy-
lamine (14 lL, 0.1 mmol). After refluxing for 16 h under argon, the
mixture was concentrated in vacuo and pre-purified by recrystal-
lization with CH2Cl2/hexane. Most of the unconverted 1 and deio-
dination product 2 precipitated and the filtrate was concentrated
and subject to silica gel chromatography (CH2Cl2 : hexane = 1 : 1)
to give meso-palladioporphyrin 3 (10 mg, 8%). Rf (THF : hexane =
Acknowledgements
We thank Prof. Craig M. Jensen of University of Hawaii at Manoa
for providing a palladium PCP complex for catalyst screening and
theNatural Sciences and Engineering Research Councilof Canada
(NSERC) for financial support.
◦
1
1 : 1): 0.67; (CH2Cl2): 0.41; H NMR (400 MHz, CD2Cl2, 25 C,
TMS): d = 9.95 (s, 1H), 9.83 (d, 3J(H,H) = 4.5 Hz, 2H), 9.23 (d,
3J(H,H) = 4.6 Hz, 2H), 8.92 (d, 3J(H,H) = 4.6 Hz, 2H), 8.50 (d,
3J(H,H) = 4.4 Hz, 2H,), 8.13–8.11 (m, 4H), 7.75–7.72 (m, 6H),
7.23–7.18 (m, 12H), 6.48–6.42 (m, 18H); 31P NMR (161.8 MHz,
CD2Cl2, 25 ◦C): d = 24.5 (s); ESIMS (m/z): 1190.1 (M+); UV-Vis
(CH2Cl2) kmax/nm (log e): 429 (5.25), 552 (3.86), 596 (3.51).
Notes and references
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2 I. P. Beletskaya and A. V. Cheprakov, Chem. Rev., 2000, 100, 3009–
3066.
Synthesis of 4
3 C. Amatore, A. Jutand and M. A. Mbarki, Organometallics, 1992, 11,
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4 K. Sonogashira, Y. Tohda and N. Hagihara, Tetrahedron Lett., 1975,
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8 W. M. Sharman and J. E. Van Lier, J. Porphyrins Phthalocyanines,
2000, 4, 441–453; J. Setsune, J. Porphyrins Phthalocyanines, 2004, 8,
93–102.
To a DMF solution (5 mL) of 1 (65 mg, 0.1 mmol) was added
tetrakis(triphenylphosphine)palladium(0) (116 mg, 0.1 mmol).
After stirring at 110 ◦C for 18 h, the mixture was concentrated in
vacuo and purified by silica gel chromatography (THF : hexane =
1 : 1) to give meso-palladioporphyrin 4 (85 mg, 66%). Rf (THF :
hexane = 1 : 1): 0.45; 1H NMR (400 MHz, CD2Cl2, 25 ◦C, TMS):
3
d = 9.96 (s, 1H), 9.83 (d, 2H, J(H,H) = 4.5 Hz), 9.24 (d, 2H,
3
3J(H,H) = 4.5 Hz), 8.93 (d, J(H,H) = 4.4 Hz, 2H), 8.55 (d,
3J(H,H) = 4.4 Hz, 2H), 8.15–8.11 (m, 4H), 7.77–7.69 (m, 6H),
7.12–7.18 (m, 12H), 6.47–6.40 (m, 18H); 31P NMR (161.8 MHz,
CD2Cl2, 25 ◦C): d = 23.3 (s); ESIMS (m/z): 1281.6 (M+); UV-Vis
(THF) kmax/nm (log e): 433 (5.22), 562 (3.99), 606 (3.78). Anal.
calcd (found) (%) for C68H49IN4P2PdZn: C, 63.67 (64.00); H, 3.85
(4.05); N, 4.37 (4.70).
9 Oxidative addition products with a general formula trans-PdRClL2,
where R is the aryl group, have been readily prepared and well studied.
Examples see: A. Casado and P. Espinet, Organometallics, 1998, 17,
954–59; P. Espinet and A. M. Echavarren, Angew. Chem., Int. Ed.,
2004, 43, 4704–34.
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10 D. P. Arnold, Y. Sakata, K. Sugiura and E. I. Worthington, Chem.
Commun., 1998, 2331–2332.
11 D. P. Arnold, P. C. Healy, M. J. Hodgson and M. L. Williams,
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12 R. D. Hartnell and D. P. Arnold, Organometallics, 2004, 23, 391–399;
M. J. Hodgson, V. V. Borovkov, Y. Inoue and D. Arnold, J. Organomet.
Chem., 2006, 691, 2162–2170.
13 C. A. Hunter and J. K. M. Sanders, J. Am. Chem. Soc., 1990, 112,
5525–5534.
14 A. Kato, R. D. Hartnell, M. Yamashita, H. Miyasaka, K. Sugiura
and D. P. Arnold, J. Porphyrins Phthalocyanines, 2004, 8, 1222–
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15 The Rf value of 3 is much smaller than those of 1 and 2, which are close
to each other. 2 shows a characteristic fluoresence on TLC upon UV
irradiation while 1 does not.
16 V. V. Grushin and H. Alper, Organometallics, 1993, 12, 1890–1901.
17 The 1H NMR signals of the b-hydrogen on carbon 2 (and 8) are
8.50 ppm for 3 and 8.55 ppm for 4 (CD2Cl2). 31P NMR signals are
24.5 ppm for 3 and 23.3 ppm for 4.
18 Br–Cl exchange was observed between meso-palladioporphyrin bro-
mide and the chlorinated solvent such as CH2Cl2. For examples, see ref.
11, 12 and 14. However, we found only 15% I–Cl exchange occurred
when a CH2Cl2 solution of 4 was stirred for one week.
19 The relative affinities of X− for the Pd centre in [(Ph3P)2Pd(Ph)]+ was
reported to decrease in the sequence: F− > Cl− > Br− > I−, see: J. P.
Flemming, M. C. Pilon, O. Y. Borbulevitch, M. Y. Antipin and V. V.
Grushin, Inorg. Chim. Acta, 1998, 280, 87–98.
20 C. Amatore, A. Jutand and A. Suarez, J. Am. Chem. Soc., 1993, 115,
9531–9541.
21 The 31P NMR signal of 24.17 ppm (in THF) was assigned to trans-
PhPdI(PPh3)2 in ref. 20. However, in another report from the same
group, the 31P NMR signal of trans-PhPdI(PPh3)2 was reported to
be 23.42 ppm (in THF) and 24.44 ppm (in DMF) while that of
trans-PhPdCl(PPh3)2 was reported to be 24.40 ppm (in DMF), see:
C. Amatore, E. Carre, A. Jutand, M. A. Mbarki and G. Meyer,
Organometallics, 1995, 14, 5605–5614.
22 R. W. Boyle, C. K. Johnson and D. Dolphin, J. Chem. Soc., Chem.
Commun., 1995, 527–528.
2602 | Dalton Trans., 2008, 2598–2602
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