Methanolysis of acyl-Pd(II) complexes relevant to CO/ethene coupling reactions

Article abstract of DOI:10.1039/b402275k

Alcoholysis of acyl-Pd(II) complexes relevant to palladium catalysed CO/ethene coupling reactions such as polyketone synthesis/alkoxycarbonylation reactions is shown, in a highly active catalyst system, to proceed via coordination of methanol to the Pd centre prior to nucleophilic attack at the acyl carbon.

Full text of DOI:10.1039/b402275k

Methanolysis of acyl–Pd(II) complexes relevant to CO/ethene coupling
reactions†
Jianke Liu, Brian T. Heaton, Jonathan A. Iggo* and Robin Whyman
Department of Chemistry, Donnan and Robert Robinson Laboratories, University of Liverpool, P. O. Box
147, Liverpool, UK L69 7ZD. E-mail: iggo@liv.ac.uk; Fax: 44 151 794 3588; Tel: 44 151 794 3538
Received (in Cambridge, Uk) 10th February 2004, Accepted 2nd April 2004
First published as an Advance Article on the web 28th April 2004
Alcoholysis of acyl–Pd(II) complexes relevant to palladium
catalysed CO/ethene coupling reactions such as polyketone
synthesis/alkoxycarbonylation reactions is shown, in a highly
active catalyst system, to proceed via coordination of methanol
to the Pd centre prior to nucleophilic attack at the acyl
carbon.
[Pd(dppomf)(C(O)CH3)]OTs is fast whereas the cation is un-
reactive toward ethene,3 methanolysis thus appears not to require a
vacant coordination site at Pd. Cole-Hamilton has drawn attention
to a third possibility in which decoordination of one arm of the
phosphine occurs, eqn. (3).13,14 Methanol can then enter the vacant
site and attack the acyl carbon directly, eqn. (3a), or protonate the
free phosphine centre to generate a methoxy ligand on Pd, eqn.
(3b). Elimination of ester and transfer of the proton from
phosphorus to Pd with recoordination of the phosphine completes
the methanolysis process. As part of our continuing studies in this
area8,9 we have been investigating the mechanistic pathways
available to the prototypical catalyst system Pd/dibpp (dibpp =
(Bui2P)2C3H6).15
In this paper, we report the effects of solvent, counter-ion and
occupancy of the fourth site on (i) the species present in solutions
of the acyl intermediate [Pd(dibpp)(C(O)CH3)L]n+ (n = 0 or 1; L
= counter-ion, solvent, or ligand) and (ii) on the methanolysis
reaction, which suggest that coordination of CH3OH to the Pd
centre is an essential prerequisite to methanolysis of the Pd–acyl
bond.
The 31P{1H} NMR spectrum of [Pd(dibpp)(CH3)(OTf)] (OTf =
CF3SO32) 1a‡ displays two broad singlets at 293 K which resolve
into a pair of doublets at 193 K. The exchange process is attributed
to coordination–decoordination of the triflate anion, rather than to
phosphine dissociation, since the temperature dependence of the
spectrum is suppressed by any group capable of binding more
strongly than OTf to the Pd centre, e.g. 1b–e,h, Scheme 2, the
31P{1H} NMR spectra of which show well resolved pairs of
doublets at 293 K.
The acyl complex [Pd(dibpp)(C(O)CH3)(CO)]+ 2c is formed
quantitatively when excess CO is bubbled through a CH2Cl2
solution 1a, whereas a mixture of [Pd(dibpp)(C(O)CH3)L]n+ (n =
0, L = OTs; n = 1, L = CO) 2h,k is formed from the tosylate
complex 1h. Finally, bubbling CO through a CH2Cl2 solution of 1e
gives 2e exclusively, the acyl carbonyl species 2g is not seen.
Bubbling CO through a CH2Cl2/CH3CN (9 : 1) solution of 1b gives
2b and through a CH2Cl2/CH3CN (9 : 1) solution of the mixture 1e
and 1f gives a mixture containing both [Pd(dibpp)(C(O)CH3)TFA]
2e and [Pd(dibpp)(C(O)CH3)(CH3CN)]TFA 2f. Addition of 1
equivalent of CH3CN to a CH2Cl2 solution containing 2h,k results
in complete displacement of the tosylate anion or CO by CH3CN.
The propagation steps in the copolymerisation of CO with alkenes
and in alkoxycarbonylation of alkenes catalysed by cationic Pd
diphosphine complexes1 have been extensively studied and are now
well understood.2–9 The termination steps of the reaction, however,
have been little studied with most information derived from an
analysis of the end groups of the polymer chain.10 Notable
exceptions to this are the recent studies by van Leeuwen and
Zuideveld,11 Bianchini,3 and ourselves9 that have succeeded in
identifying some or all of the intermediates involved in chain
transfer by alcoholysis of the acyl and alkyl intermediates. As a
consequence several conflicting reaction pathways have been
proposed for the alcoholysis of the acyl intermediate (Scheme 1,
eqn. (1)–(3)). Thus, van Leeuwen observed that methanolysis is
effectively suppressed by diphosphine ligands that adopt a trans
geometry at Pd and concluded that methanolysis of the acyl must be
by intra-molecular attack of cis coordinated CH3OH on the acyl
carbon, eqn. (1).11 Similarly, Bianchini notes that methanolysis of
the acetyl intermediate [Pd(dppomf)(C(O)CH3)]OTs, (dppomf =
(C5Me4PPh2)2Fe) requires a change in hapticity of the ligand from
h3-P,P,Fe (trans Ps) to h2-P,P (cis Ps) to provide a free
coordination site for CH3OH cis to the acyl and may therefore be
rate limiting in that reaction.12 In a later paper however, Bianchini
proposes direct inter-molecular attack of CH3OH at the acyl
carbon, eqn. (2), based on the observation that methanolysis of
Scheme 1 Proposed pathways for the methanolysis of Pd–acyl com-
plexes.
† Electronic supplementary information (ESI) available: experimental
details and representative 31P{1H} NMR spectra. See http://www.rsc.org/
suppdata/cc/b4/b402275k/
Scheme 2 Methanolysis of Pd(dibpp)acyl complexes. Methanolysis does
not occur in the presence of coordinating ligands such as TFA or MeCN.
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C h e m . C o m m u n . , 2 0 0 4 , 1 3 2 6 – 1 3 2 7
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

methanolysis reaction to proceed. Thus, addition of 10% CH3OH to a
CH2Cl2 solution of 2c and CO at 243 K, results in progressive formation of
the Pd–acyl–(CH3OH) complex 2d and formation of methyl acetate
(detected by 13C{1H} NMR).
To summarize, only trifluoacetate anion competes effectively with
MeCN for the fourth coordination site and does so even in the
presence of a large excess of CH3CN. The CH3OH containing
cations are only obtained in the absence of CH3CN. These results
indicate that the affinity for the Pd centre of the various anions is
31P{1H} and 13C{1H} NMR spectroscopic data for 1a–k and 2a–k: P
trans to CH3 or C(O)CH3 given first; coupling to Ptrans is given first; J in Hz.
a Recorded in CH2Cl2 b Recorded in CH2Cl2/CH3CN (9 : 1) c Recorded in
CH2Cl2/CH3OH (9 : 1): 1a [Pd(dibpp)CH3(CF3SO3)] a dP 18.8 d, 216.6 d
(JPP = 41); 1b [Pd(dibpp)CH3(CH3CN)][CF3SO3] b dP 11.0 d, 215.6 d (JPP
= 41); 1c [Pd(dibpp)CH3(CO)][CF3SO3] a dP 20.5 d, 212.3 d (JPP = 47),
dC 181.6 dd (JPC = 114, 16); 1d [Pd(dibpp)CH3(CH3OH)][CF3SO3)] c dP
18.8 d, 214.2 d (JPP = 41); 1e [Pd(dibpp)CH3(CF3CO2)] a dP 12.7 d, 211.4
TFA
> OTs > OTf, as might be expected, and, perhaps
surprisingly, that CH3CN has a higher affinity for the Pd centre than
CO which has a slightly greater affinity for Pd than OTs, i.e. the
affinities of these ligands for the Pd(dibpp) centre is in the order,
TFA > CH3CN > CO > OTs > CH3OH > OTf. The equilibrium
position will, of course, be influenced by the relative concentrations
of the species competing for the fourth site. The IR stretching
vibration of the carbonyl ligand in 2c occurs at 2123 cm21 in accord
with the report of Drent.16 However, the relatively weak affinity of
CO for Pd in these complexes has not previously been recognized,
and has implications for related Pd catalysed carbonylation
reactions.
The preparation and characterization of the series of complexes
2b–k allows us to probe directly, for a highly active catalytic
system, the effect of blocking of the fourth coordination site on the
methanolysis reaction, Scheme 2.§11,12 Rapid reaction is seen on
addition of excess (10% v/v) CH3OH at 243 K to CH2Cl2 solutions
of the acyl complexes, however no reaction is seen in CH2Cl2/
CH3CN (9 : 1) solutions in which CH3CN occupies the fourth site,
2b, 2f or 2i.¶ The reaction of 2e with CH3OH in both CH2Cl2 and
CH2Cl2/CH3CN mixtures requires further comment. No reaction is
observed at 243 K on addition of 1–10 equivalents of CH3OH to a
CH2Cl2 solution of 2e even on standing overnight. However,
addition of (10% v/v) MeOH to a CH2Cl2 solution of 2e in the
presence of 13CO∑ results in immediate formation of the Pd acyl
carbonyl complex 2g, observed in situ by 31P{1H} NMR, followed
by methanolysis to give methyl acetate. These observations can be
explained as follows: in the presence of near stoichiometric
amounts of CH3OH, TFA is not dissociated and effectively blocks
the fourth coordination site on Pd to incoming CH3OH, however, in
the presence of a large excess of CH3OH, dissociation of the anion
is aided by its solvation by methanol. The first formed solvento
cation (which cannot be directly observed) is trapped by the excess
CO present in the solution to give 2g. Methanolysis of 2g then
occurs. Support for this interpretation comes from the observation
that addition of (10% v/v) CH3OH to a mixture of 2e and 2f in
CH2Cl2/CH3CN (9 : 1) (vide supra) results in complete conversion
of 2e to 2f (which is resistant to methanolysis).
b
d (JPP = 41); 1f [Pd(dibpp)CH3(CH3CN)][CF3CO2] dP 10.8 d, 215.6 d
(JPP = 41); 1g [Pd(dibpp)CH3(CO)][CF3CO2 ] a dP 20.8 d, 213.5 d (JPP
=
48), dC 181.6 dd (JPC = 114, 16); 1h [Pd(dibpp)CH3(CH3C6H4SO3)] a dP
17.2 d, 212.2 d (JPP = 42); 1i [Pd(dibpp)CH3(CH3CN)][CH3C6H4SO3] dP
11.1 d, 215.7 d (JPP = 42); 1j [Pd(dibpp)CH3(CH3OH)][CH3C6H4SO3] c
dP 18.9 d, 214.3 d (JPP = 42); 1k [Pd(dibpp)CH3(CO)][CH3C6H4SO3] a dP
20.6 d, 213.4 d (JPP = 47), dC 181.7 dd (JPC = 114,16); 2b [Pd(dibpp)–
(C(O)CH3)(CH3CN)][ CF3SO3] b dP 5.4 d, 2 19.6 (JPP = 70), dC 242.6 dd
a
(JPC = 112, 10); 2c [Pd(dibpp)(C(O)CH3)(CO)][CF3SO3
]
dP 26.7 d,
219.2 d (JPP = 73), dC 235.2 dd (JPC = 88, 5); 176.9 dd (JPC = 80, 20);
c
2d [Pd(dibpp)(C(O)CH3)(CH3OH)][CF3SO3] dP 13.4 d, 219.1 d (JPP
=
a
66), dC 243 dd (JPC = 116, 12); 2e [Pd(dibpp)(C(O)CH3)(CF3CO2)] dP
10.0 d, 215.8 d (JPP
= 67), dC 247.8 dd (JPC = 125, 10); 2f
[Pd(dibpp)(C(O)CH3)(CH3CN)][CF3CO2] b dP 4.9 d, 219.7 d (JPP = 70),
dC 242.8 dd (JPC = 112, 10); 2g [Pd(dibpp)(C(O)CH3)(CO)][CF3CO2 ] c dP
26.1 d, 218.5 d (JPP = 73), dC 234.7 dd (JPC = 88,6); 176.9 dd (JPC
=
79,20); 2h [Pd(dibpp)(C(O)CH3)(CH3C6H4SO3)] a dP 12.5 d, 216.6 d (JPP
= 70), dC 244.6 dd (JPC = 122,12); 2i [Pd(dibpp)(C(O)CH3)(CH3CN)]
[CH3C6H4SO3] b dP 4.9 d, 219.7 d (JPP = 70), dC 242.6 dd (JPC = 113,10);
2j [Pd(dibpp)(C(O)CH3)(CH3OH)][CH3C6H4SO3] c dP 13.4 d, 219.2 d (JPP
= 66), dC 245.5 dd (JPC = 117, 12); 2k [Pd(dibpp)(C(O)CH3)(CO)]
[CH3C6H4SO3] a dP 26.8 d, 218.6 d (JPP = 73), dC 235.5 dd (JPC = 88,
5); 176.9 dd(JPC = 80, 20).
1 E. Drent and P. H. M. Budzelaar, Chem. Rev., 1996, 96, 663.
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4 Z. Freixa and P. van Leeuwen, Dalton Trans., 2003, 1890.
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7 C. Bianchini, A. Meli, G. Muller, W. Oberhauser and E. Passaglia,
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We conclude that, in this system, methanolysis proceeds via
coordination of CH3OH to Pd, followed by intra-molecular
nucleophilic attack on the acyl carbon and the pathway is not
affected by the acid used, i.e. methanolysis occurs via the
mechanism shown in eqn. (1) or (3).** We cannot conclusively
distinguish between mechanisms (1) and (3), however we note that
there is no evidence in our 31P{1H} NMR spectra9 to suggest that
decoordination/recoordination of the diphosphine ligand occurs on
the NMR timescale. The situation with regard to [Pd(dppomf-
)(C(O)Me)]OTs is complicated by the presence of an intra-
molecular Fe?Pd bond which reasonably accounts for the
observed reactivity of that system.
8 J. Liu, B. T. Heaton, J. A. Iggo and R. Whyman, Angew. Chem. Int. Ed,
2004, 43, 90.
9 W. Clegg, G. R. Eastham, M. R. J. Elsegood, B. T. Heaton, J. A. Iggo,
R. P. Tooze, R. Whyman and S. Zacchini, Organometallics, 2002, 21,
1832; W. Clegg, G. R. Eastham, M. R. J. Elsegood, B. T. Heaton, J. A.
Iggo, R. P. Tooze, R. Whyman and S. Zacchini, J. Chem. Soc., Dalton
Trans., 2002, 3300; G. R. Eastham, B. T. Heaton, J. A. Iggo, R. P.
Tooze, R. Whyman and S. Zacchini, Chem. Commun., 2000, 609.
10 W. P. Mul, E. Drent, P. J. Jansens, A. H. Kramer and M. H. W.
Sonnemans, J. Am. Chem. Soc., 2001, 123, 5350.
11 P. van Leeuwen, M. A. Zuideveld, B. H. G. Swennenhuis, Z. Freixa, P.
C. J. Kamer, K. Goubitz, J. Fraanje, M. Lutz and A. L. Spek, J. Am.
Chem. Soc., 2003, 125, 5523; M. A. Zuideveld, P. C. J. Kamer, P. W. M.
N. van Leeuwen, P. A. A. Klusener, H. A. Stil and C. F. Roobeek, J. Am.
Chem. Soc., 1998, 120, 7977.
12 C. Bianchini, A. Meli, W. Oberhauser, P. van Leeuwen, M. A.
Zuideveld, Z. Freixa, P. C. J. Kamer, A. L. Spek, O. V. Gusev and A. M.
Kal’sin, Organometallics, 2003, 22, 2409.
13 D. J. Cole-Hamilton and R. A. M. Robertson, in Catalytic Synthesis of
Alkene-Carbon Monoxide Copolymers and Cooligomers, ed. A. Sen,
Kluwer Academic Publishers, Dordrecht, 2003, Chap. 3.
14 G. R. Eastham, PhD, University of Durham, 1996.
The financial support of EPSRC, grant GR/R37685, is gratefully
acknowledged.
Notes and references
‡ See ref 8 for a general procedure for the preparation of solutions of the
complexes 1a–k and 2a–k and details of the NMR instrumentation.†
§ Effects due to variation of e.g. the diphosphine ligand are excluded.
¶ Decarbonylation reactions dominate on warming to 293 K.
∑ Excess13CO was added to drive the reaction 1e ? 2e to completion.
** Under catalytic conditions, where methanol is the solvent and excess
CO is present, mass action will ensure that methanol competes effectively
with the anion or CO for the fourth coordination site allowing the
15 R. I. Pugh and E. Drent, in Catalytic Synthesis of Alkene-Carbon
Monoxide Copolymers and Cooligomers, ed. A. Sen, Kluwer Academic
Publishers, Dordrecht, 2003, Chap. 2.
16 W. P. Mul, H. Oosterbeek, G. A. Beitel, G. J. Kramer and E. Drent,
Angew. Chem. Int. Ed., 2000, 39, 1848.
C h e m . C o m m u n . , 2 0 0 4 , 1 3 2 6 – 1 3 2 7
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