Chemoselective methoxycarbonylation of terminal alkynes catalyzed by Pd(ii)-TROPP complexes

DOI: 10.1039/c002976a

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Dalton Transactions p. 6509 - 6517 (2010)

Update date:2022-09-26

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Authors:

Bettucci, Lorenzo

Bianchini, Claudio

Oberhauser, Werner

Vogt, Matthias

Gruetzmacher, Hansjoerg

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Article abstract of DOI:10.1039/c002976a

The mono-phosphanes 5-(diphenyl)-phosphanyl-5H-dibenzo[a,d]cycloheptene (TROPP^Ph) and 5-((2-methoxy)phenyl)-phosphanyl-5H-dibenzo[a,d] cycloheptene (TROPP^(2-MeOPh)) have been employed to coordinate PdCl₂, yielding [PdCl₂(TROPP^Ph)] (1a) and [PdCl₂(TROPP^(2-MeOPh))] (1b), respectively. The corresponding tosylate (OTs) complexes [Pd(OTs)₂(TROPP^Ph)] (2a) and [Pd(OTs)₂(TROPP^(2-MeOPh))] (2b) have been successfully applied in the p-benzoquinone (BQ)-assisted methoxycarbonylation of terminal alkynes to give chemoselectively the corresponding alkynylcarboxylic acid methyl ester with high TOF (up to 980 h^-1). Unlike 2a/b, the Pd^II-(tosylate)-diphosphane complexes [Pd(OTs)(H₂O)(dppp)] (OTs) 2c (dppp = 1,3-bis-di(phenylphosphanyl)propane and [Pd(H ₂O)₂(MeO-dppp)](OTs)₂2d (MeO-dppp = 1,3-bis(di(2-methoxyphenyl)phosphanyl)propane) preferentially catalyzed a double alkyne insertion. The "in situ" spectroscopic observation of the Pd-methoxycarbonyl compound [Pd(COOMe)(TROPP^Ph)](OTs) (3a) in conjunction with the evidence of the fast β-hydride elimination reaction of TROPP-based Pd-catalysts in the dimerization reaction of ethene are indicative for a strongly electrophilic metal centre. The X-ray crystal structures of 1a·0.5C₂H₄Cl₂ and of the neutral and cationic Pd-alkyl complexes [Pd(Me)(Cl)(TROPP^Ph) (7a·0. 5CH₂Cl₂) and [Pd(CH₂CH₂COMe) (TROPP^Ph)](PF₆) (10a·0.5C₆H ₆), respectively, confirm unambiguously the bidentate coordination mode of TROPP-ligands to Pd^II, that persists even in the presence of CO pressure. The Royal Society of Chemistry.

Full text of DOI:10.1039/c002976a

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www.rsc.org/dalton | Dalton Transactions

Chemoselective methoxycarbonylation of terminal alkynes catalyzed by

Pd(^II)-TROPP complexes†

Lorenzo Bettucci,^aClaudio Bianchini,^aWerner Oberhauser,*^aMatthias Vogt^band Hansjo¨rg Gru¨tzmacher^b

Received 12th February 2010, Accepted 7th May 2010

First published as an Advance Article on the web 8th June 2010

DOI: 10.1039/c002976a

The mono-phosphanes 5-(diphenyl)-phosphanyl-5H-dibenzo[a,d]cycloheptene (TROPP^Ph) and

5-((2-methoxy)phenyl)-phosphanyl-5H-dibenzo[a,d]cycloheptene (TROPP^(2-MeOPh)) have been employed

to coordinate PdCl₂, yielding [PdCl₂(TROPP^Ph)] (1a) and [PdCl₂(TROPP^(2-MeOPh))] (1b), respectively. The

corresponding tosylate (OTs) complexes [Pd(OTs)₂(TROPP^Ph)] (2a) and [Pd(OTs)₂(TROPP^(2-MeOPh))] (2b)

have been successfully applied in the p-benzoquinone (BQ)-assisted methoxycarbonylation of terminal

alkynes to give chemoselectively the corresponding alkynylcarboxylic acid methyl ester with high TOF

(up to 980 h^-1). Unlike 2a/b, the Pd^II-(tosylate)-diphosphane complexes [Pd(OTs)(H₂O)(dppp)](OTs) 2c

(dppp = 1,3-bis-di(phenylphosphanyl)propane and [Pd(H₂O)₂(MeO-dppp)](OTs)₂2d (MeO-dppp =

1,3-bis(di(2-methoxyphenyl)phosphanyl)propane) preferentially catalyzed a double alkyne insertion.

The “in situ” spectroscopic observation of the Pd-methoxycarbonyl compound

[Pd(COOMe)(TROPP^Ph)](OTs) (3a) in conjunction with the evidence of the fast b-hydride elimination

reaction of TROPP-based Pd-catalysts in the dimerization reaction of ethene are indicative for a

strongly electrophilic metal centre. The X-ray crystal structures of 1a·0.5C₂H₄Cl₂and of the neutral and

cationic Pd-alkyl complexes [Pd(Me)(Cl)(TROPP^Ph) (7a·0.5CH₂Cl₂) and [Pd(CH₂CH₂COMe)-

(TROPP^Ph)](PF₆) (10a·0.5C₆H₆), respectively, conﬁrm unambiguously the bidentate coordination mode

of TROPP-ligands to Pd^II, that persists even in the presence of CO pressure.

The selectivity towards different carbonylation products largely

depends on the reaction conditions and on the nature of the cata-

The transition metal-catalyzed carbonylation of alkenes and

lyst employed. Signiﬁcant examples for the carbonylation reaction

Introduction

alkynes has been known since the 1930s.¹Alkoxycarbonylation

of terminal alkynes are: (i) The Drent’s methoxycarbonylation of

propene,⁵chemoselectively yielding methyl methacrylate, which

is a large scale chemical intermediate for the production of ho-

mopolymers and copolymers and (ii) the Tsuji’s Pd/Cu-catalyzed

oxidative alkoxycarbonylation of terminal alkynes yielding the

corresponding alkynylcarboxylic acid ester.^3aUnfortunately, the

latter catalytic reaction is chemoselective only when carried out at

a high palladium-substrate ratio, otherwise, 1,4-disubstituted-1,3-

diynes are obtained by a rapid Cu-catalyzed alkyne coupling.^3a,b,e,f,g

Since Pd^IIis reduced to Pd⁰in the course of the catalytic

carbonylation reaction the metal re-oxidation is fundamental for

a successful catalysis and a ligand capable of stabilizing low metal

oxidation states would be important to stabilize Pd⁰single species,

avoiding the formation of palladium black.

reactions of terminal alkynes are of particular interest as they

lead to the formation of useful building blocks, such as a,b-

unsaturated esters (i.e. mono-and di-carbonylated products)²

and alkynylcarboxylic esters.³In particular, the latter esters are

important intermediates in the synthesis of carbapenems, that

are known for their broad spectrum of antibacterial properties.⁴

Several different products obtained from the same terminal alkyne

(R₁C₂H) by oxidative carbonylation reaction carried out in an

alcohol (ROH) are depicted in Scheme 1.

To this purpose, our attention was attracted by the TROPP

ligands.⁶These ligands are known for their particular shape

which consists of a boat conformation providing a concave rigid

bidentate binding site composed of a s-donor (i.e. phosphorus)

and a p-acceptor moiety (i.e. C–C double bond). It has been

established that the electronic property of TROPP ligands are

remarkable to stabilize unusual oxidation states, as reported for

rhodium^7a,band iridium.^7b,c

Herein, we describe the synthesis and characterization of neutral

and cationic Pd^II-complexes with TROPP ligands (Scheme 2).

The corresponding palladium-tosylate complexes have been

successfully employed as precursors for the oxidative methoxy-

carbonylation of terminal alkynes.

Scheme 1

^aIstituto di Chimica dei Composti Organometallici (ICCOM-CNR), Area di

Ricerca CNR di Firenze, via Madonna del Piano 10, 50019, Sesto Fiorentino,

Italy. E-mail: werner.oberhauser@iccom.cn r .it; Fax: +39-055-5225203

^bETH Zu¨rich, Laboratorium fu¨r Anorganische Chemie, HCI-H131,

Wolfgang-Pauli-Str. 10, CH-8093, Zu¨rich, Switzerland

† CCDC reference numbers 696684, 696685 and 739871. For crystallo-

graphic data in CIF or other electronic format see DOI: 10.1039/c002976a

This journal is

The Royal Society of Chemistry 2010

Dalton Trans., 2010, 39, 6509–6517 | 6509

View Article Online

◦

Table 1 Selected bond lengths (A) and angles ( ) for 1a·0.5C₂H₄Cl₂,

7a·0.5CH₂Cl₂and 10a·0.5C₆H₆

1a·0.5C₂H₄Cl₂

7a·0.5CH₂Cl₂

10a·0.5C₆H₆

Pd(1)–P(1)

Pd(1)–Cl(1)

Pd(1)–Cl(2)

2.233(1)

2.353(1)

2.319(1)

2.208(2)

2.383(3)

2.1981(8)

Scheme 2

Pd(1)–O(1)

Pd(1)–C(8)

Pd(1)–C(9)

Pd(1)–C(28)

Pd(1)–C(ct)

C(8)–C(9)

C(30)–O(1)

Cl(1)–Pd(1)–Cl(2)

Cl(1)–Pd(1)–C(28)

P(1)–Pd(1)–C(8)

P(1)–Pd(1)–C(9)

P(1)–Pd(1)–C(28)

P(1)–Pd(1)–C(ct)

O(1)–Pd(1)–C(28)

2.127(2)

2.318(4)

2.333(3)

2.042(3)

2.213(9)

1.363(5)

1.239(4)

2.249(4)

2.234(4)

2.373(10)

2.346(9)

2.102(9)

2.314(27)

1.373(13)

Results and discussion

2.136(7)

1.391(6)

Synthesis of palladium complexes

The reaction of [PdCl₂(h -COD)] with (TROPP^Ph)⁶and

92.62(4)

(TROPP^(2-MeOPh)) in dichloromethane gave the neutral complexes

[PdCl₂(TROPP^Ph)] (1a) and [PdCl₂(TROPP^(2-MeOPh))] (1b) as yellow

micro-crystalline compounds in 93 and 90% yield, respectively

(Scheme 3(a)).

91.80(30)

92.60(20)

92.30(20)

86.20(30)

92.78(41)

90.89(11)

92.23(12)

94.95(9)

91.58(9)

89.77(10)

93.55(22)

82.14(13)

91.69(20)

ct (centroid of the C(8) C(9) bond).

adventitious water) (Scheme 3(c)). The ³¹P{ H} NMR spectra

of 2a¢/b¢ in CD₃OD showed a broad singlet centred at 129.00

and 130.80 ppm, that became narrower at low temperature. The

replacement of coordinating anions such as tosylate or triﬂate to

palladium by MeOH is rather common for Pd(II) complexes.^8,9

Suitable crystals of 1a·0.5C₂H₄Cl₂for a single crystal X-ray

structure analysis were obtained by slow diffusion of toluene into a

solution of 1a in dichloroethane at room temperature. An ORTEP

diagram of the latter compound is shown in Fig. 1, while selected

bond distances and angles are reported in Table 1.

In the crystal structure of 1a·0.5C₂H₄Cl₂the 1,2-dichloroethane

molecule lies about an inversion centre so that only one half of the

molecule is in the asymmetric unit. The palladium atom exhibits

a planar coordination geometry, built up by two cis-coordinating

chloride atoms, a phosphorus atom and the centroid (ct) of the

Scheme 3

The treatment of either complex with silver tosylate

in dichloromethane gave the corresponding Pd-bis(tosylate)

complexes of the formulae [Pd(OTs)₂(TROPP^Ph)] (2a) and

[Pd(OTs)₂(TROPP^(2-MeOPh))] (2b) (Scheme 3(b)).

The neutral palladium complexes have been characterized in

solution by multinuclear-NMR spectroscopy and in the solid

state by elemental analysis. Additionally, 1a·0.5C₂H₄Cl₂has been

identiﬁed by a single crystal structure analysis (vide infra). The

³¹P{ H} NMR spectra of 1a/b in CD₂Cl₂exhibited a singlet

centred at 107.40 and 112.80 ppm, while the corresponding

¹³C{ H} NMR spectra showed, for the metal-coordinating sp²-

coordinating C C_tropbond (i.e. ct of C(8) C(9) bond). The palla-

dium atom is located in the best coordination plane deﬁned by the

atoms P(1), Cl(1), Cl(2), and ct. Both oleﬁnic carbon atoms, C(8)

and C(9) deviate symmetrically from the latter plane by 0.6926(29)

carbon atoms, a singlet centred at 100.00 ppm, that is signiﬁcantly

high-ﬁeld shifted compared to the corresponding free ligand (i.e.

132.70 ppm).^6,7

The ³¹P{ H} NMR spectra of CD₂Cl₂solutions of 2a/b showed

and -0.6965(31) A, respectively. The coordinating C C_tropbond

a singlet centred at 126.00 and 130.80 ppm, respectively, while

(1.391(6) A) is signiﬁcantly elongated when compared to that

of the free ligand (1.328(4) A) being almost identical to that

the ¹³C{ H} NMR spectra acquired in the same solvent showed

a singlet at 97.30 ppm for the cycloheptene C C_tropdouble

found for the related palladium dichloride complex bearing the

related dibenzo[a,d]cycloheptenyl dibenzophosphole¹⁰as ligand.

bond. The high-frequency shift of the ³¹P{ H} NMR signal and

the low-frequency shift of the ¹³C{ H} NMR signal of 2a/b,

The coordinating C C_tropbond axis is almost perpendicular

as compared to 1a/b reﬂect the stronger trans inﬂuence of the

tosylate vs. the chloride ligand.⁸Importantly, only one ¹H NMR

singlet for the chemically inequivalent tosylate methyl groups

in 2a/b was observed, even at low temperature (i.e. -60^◦C).

This experimental evidence is in accordance with a OTs-exchange

process at palladium, that is too fast to be observed on the NMR

time scale.

(87.9(2)^◦) oriented with respect to the best coordination plane.

The overall conformation of the coordinating TROPP^Phligand

is deﬁned by the parameters a (52.40^◦) and b (24.7^◦) as shown

in Scheme 4 (a = 47.80^◦and b = 24.40^◦for the free TROPP^Ph

ligand).

Catalytic methoxycarbonylation of terminal alkynes

Conductivity measurements of 2a/b carried out in CH₂Cl₂and

MeOH (i.e. reaction medium for the carbonylation reactions),

showed that the latter compounds behave in CH₂Cl₂as non-

electrolyte, whereas in MeOH they behave as 1 : 2 electrolyte, which

is consistent with the chemical formulae [Pd(TROPP^Ph)(S)₂](OTs)₂

(2a¢) and [Pd(TROPP^(2-MeOPh))(S)₂](OTs)₂(2b¢) (S = MeOH or

Compounds 2a/b were tested as catalyst precursors for the

methoxycarbonylation of terminal alkynes (i.e. phenylacetylene,

4-tolylacetylene and 4-bromo-phenylacetylene) in the presence

of p-benzoquinone (BQ). For

comparative purpose,

analogous reactions, catalyzed by the Pd-diphosphane

6510 | Dalton Trans., 2010, 39, 6509–6517

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Table 2 Methoxycarbonylation of terminal alkynes (R–C₂H) catalyzed by the precursors 2a–d^a

Entry

Precatalyst/mmol

Substrate (R)

t/h

Conv. (%)

TOF^b

Sel. A (%)

Sel. B (%)

Sel. C (%)

1^c

2a (0.02)

2a (0.001)

2a (0.02)

2a (0.001)

2b (0.02)

2b (0.001)

2b (0.02)

2b (0.001)

2c (0.02)

2d (0.02)

PdCl₂/CuCl₂

4-Tolyl

4-BrPh

0.5

—

68.0

—

58.0

—

32.0

—

42.0

—

2^d

trace

100

—

4^e

5^f

540

510

180

960

920

420

12^c

13^d

19^g

21^c

25^c

29^h

—

100

—

nd.

—

520

186

920

880

400

170

—

136

—

4-Tolyl

4-BrPh

4-Tolyl

4-BrPh

4-Tolyl

4-BrPh

nd.

^aCatalytic conditions: MeOH, 25 mL; substrate, 2.00 mmol; BQ, 2.40 mmol; p(CO), 7 bar; temperature, 70 ^◦C; stirring rate, 800 rpm. ^bTurn over

frequency expressed as mmol (alkyne) converted ¥ (mmol (Pd) ¥ h)^-1. ^cWithout BQ. ^dTsOH, 0.60 mmol. ^eBQ, 1.20 mmol. ^fp(CO), 28 bar. ^gReaction in

2-propanol. ^hPdCl₂, 0.02 mmol, CuCl₂, 2.40 mmol, NaOAc, 2.40 mmol.

Scheme 4

dppp)](OTs)₂(2d)^8a(MeO-dppp = 1,3-bis(di(2-methoxyphenyl)-

phosphanyl)propane) that share with 2a/b a similar ligand bite

angle of ca. 90^◦, were carried out. The result of this comparative

catalytic study is shown in Table 2.

Regardless of the precatalyst employed, catalytic methoxycar-

bonylation reactions of terminal alkynes carried out in the absence

of BQ yielded the unsaturated monoesters A and B (Scheme 5(a))

with the branched ester A as the major product.

Fig. 1 ORTEP diagram of 1a·0.5C₂H₄Cl₂with 30% probability ellipsoids.

Hydrogen atoms and the solvent molecule are omitted for clarity.

complexes [Pd(OTs)(H₂0)(dppp)](OTs) (2c)¹¹(dppp

bis(diphenylphosphanyl)propane) and [Pd(H₂O)₂(MeO-

1,3-

Scheme 5

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Under the latter catalytic conditions, 2a/b were almost inactive

(entries 1 and 12, Table 2), whereas upon addition of HOTs, an

increased catalytic conversion was observed with the branched

ester A as the major product (entries 2/13 vs. 1/12, Table 2). The

diphosphane complexes 2c/d showed a good catalytic activity also

in the absence of a Brønsted acid, with a slight chemoselectivity

in favour of the branched ester of 68.0 and 58.0%, respectively

(entries 21 and 25, Table 2).

The catalytic cycle comprises the transformation of the precat-

alyst 2a/b (Scheme 6) into the Pd-alkoxycarbonyl species 3a/b,

that undergoes a cis-2,1-insertion in the presence of the terminal

alkyne, yielding the Pd-alkenyl intermediate 4a/b.^3dThe latter

compound is proposed to undergo an isomerization reaction

into 5a/b enabling a b-hydride elimination reaction.^3dA fast b-

hydride elimination reaction generates the Pd-hydride 6a/b with

the concomitant release of the corresponding alkynylcarboxylic

acid ester. Finally, the BQ-assisted conversion of 6a/b into 3a/b

closes the catalytic cycle.¹²The following experimental evidence

corroborates the proposed catalytic cycle:

Catalytic methoxycarbonylation reactions of terminal alkynes

promoted with 2a/b in the presence of BQ exclusively yielded the

corresponding alkynylcarboxylic acid methyl ester C in high yield

(i.e. TOF up to 960 h^-1). In contrast, 2c/d gave under identical

catalytic conditions a mixture of C and the 2,4-di-substituted

2,4-pentadienylcarboxylic acid methyl ester D^2d(Scheme 5(b))

as the major compound (Table 2). This ﬁnding clearly indicates

that, unlike 2a/b, 2c/d preferentially undergo a double 1,2-alkyne

insertion. The highest substrate conversion into C was found to be

obtained in the CO pressure range from 7 to 21 bar. Importantly,

even at CO pressures higher than 21 bar, the diesters formed upon

a double methoxycarbonylation reaction were not obtained. A

strong dependence of the catalytic activity on the amount of

BQ present in the reaction mixture was observed (entry 3 vs. 4,

Table 2), indicating a BQ-promoted oxidation of Pd⁰to Pd^IIin the

course of the catalytic cycle. Furthermore, decreasing the amount

of precatalyst from 0.02 to 0.001 mmol while keeping the amount

of substrate constant at 2 mmol, remarkably increased the TOF

of the catalytic reactions. We interpret this result due to a reduced

Pd black formation at lower Pd concentrations.

The substitution of BQ by CuCl₂in the 2a/b-catalyzed methoxy-

carbonylation reactions chemoselectively gave the corresponding

1,4-di-substituted 1,3-butadiyne, which is typically obtained as

a major side-product in the Pd/Cu-based carbonylation reac-

tions of terminal alkynes.^3aAs a matter of fact, the catalytic

system comprising PdCl₂/CuCl₂/NaOAc gave under the present

catalytic conditions C in 3% yield, while the corresponding 1,4-

di-substituted-1,3-butadiyne was the major product (entry 29,

Table 2).

(i) The Pd-methoxycarbonyl compound [Pd(COOMe)-

(CD₃OD)(TROPP^Ph)](OTs) (3a¢) (Scheme 6) was observed spec-

troscopically by bubbling ¹³CO through a solution of 2a in

CD₃OD.¹³The latter compound could not be isolated due to its

rapid CO-de-insertion reaction in the absence of CO but it was

unambiguously identiﬁed. The ¹³C{ H} NMR spectrum of 3a¢

revealed a doublet centred at 175.00 ppm with a ²J_PCof 16.3 Hz for

the carboxyl C-atom, which is consistent with a cis-coordination

of the methoxycarbonyl moiety with respect to the phosphorus

atom of the ligand.^13aThe related Pd-acyl model compound of

the formula [Pd(COMe)(H₂O)(TROPP^Ph)](PF₆) (9a) (Scheme 7),

shows also a cis-stereochemistry of the palladium coordinating

acyl moiety with respect of the phosphorus donor atom. As

a consequence, its ¹H NMR spectrum, acquired in CD₂Cl₂,

exhibited a doublet at 1.89 ppm (⁴J_PHof 1.5 Hz) for COCH₃, while

the corresponding ¹³C{ H} NMR spectrum showed a singlet at

118.70 ppm for the palladium coordinating sp²-hybridized carbon

atoms. It is important to stress, that CO neither coordinates to

3a/a¢ and 9a nor displaces the coordinating C C_tropbond from

palladium, even at high CO pressure (i.e. 20 bar). This ﬁnding is

in agreement with a strong electrophilic palladium centre.¹⁴

The substitution of methanol by 2-propanol as carbonylation

reaction medium had no effect neither on the chemoselectivity nor

on the catalytic activity (entry 19 vs. 17, Table 2).

For the chemoselective 2a/b-catalyzed oxidative methoxycar-

bonylation reaction of terminal alkynes we propose the catalytic

cycle shown in Scheme 6.

Scheme 7

(ii) The catalytic activity has been found to be independent

of the type of alcohol employed as reaction medium (entry

19 vs. 17, Table 2). This evidence rules out a methanolysis

reaction path (i.e. reaction of methanol with a palladium acyl

species) as termination reaction.¹⁵Instead, the palladium b-

hydride elimination, which is fostered by an electrophilic metal

centre, is favoured.¹⁶In order to experimentally prove the rapid b-

hydride elimination activity of the TROPP-based Pd-catalyst, the

model compound [PdMe(H₂O)(TROPP^Ph)](PF₆) (8a) (Scheme 7)

was used to catalyze the oligomerization reaction of ethene in

toluene at 50^◦C. As a result, a stoichiometric amount of propene

(i.e. with respect to the amount of the precursor) and a 1 : 18

mixture of 1- and cis/trans 2-butenes with a TOF of 160^-hwas

obtained.

Scheme 6

6512 | Dalton Trans., 2010, 39, 6509–6517

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Table 3 Crystallographic data for 1a·0.5C₂H₄Cl₂, 7a·0.5CH₂Cl₂and 10a·0.5C₆H₆

1a·0.5C₂H₄Cl₂

7a·0.5CH₂Cl₂

10a·0.5C₆H₆

Empirical formula

Formula weight

T/K

C₂₈H₂₃Cl₃PPd

603.18

170.0(2)

Monoclinic

P2₁/n

8.6468(6)

14.0728(8)

20.6612(12)

95.573(6)

2502.3(3)

1.601

1.141

1212

0.20 ¥ 0.20 ¥ 0.10

MULTI-SCAN

6900

C_28.5H₂₅Cl₂PPd

575.76

293.2(2)

Monoclinic

P2₁/c

8.907(5)

19.718(5)

15.161(5)

92.175(5)

2660.8(2)

1.437

0.973

1164

0.30 ¥ 0.20 ¥ 0.20

PSI-SCAN

4654

C₃₄H₃₁F₆OP₂Pd

737.93

180.2(2)

Monoclinic

P2₁/c

17.9962(5)

8.9054(2)

21.4435(5)

110.763(3)

3213.42(14)

1.525

0.738

1492

0.40 ¥ 0.30 ¥ 0.20

MULTI-SCAN

10 409

Crystal system

Space group

a/A

b/A

c/A

b/^◦

V/A

D_c/mg m^-3

m/mm^-1

F(000)

Crystal size/mm

Absorption correction

Reﬂections collected

q range for data collection/^◦

Reﬁnement method

Data/restraints/parameter

Goodness-of-ﬁt on F²

Final R indices [I > 2s(I)]

3.81–30.67

2.51–24.98

3.72–32.48

Full-matrix least-squares on F²

3929/0/298

0.923

2463/3/293

1.029

6560/2/383

1.039

R₁= 0.0572

wR₂= 0.0919

R₁= 0.1153

wR₂= 0.1008

1.257, -0.934

R₁= 0.0686

wR₂= 0.1659

R₁= 0.1759

wR₂= 0.2076

0.881, -0.480

R₁= 0.0509

wR₂= 0.1371

R₁= 0.0890

wR₂= 0.1615

1.57, -0.846

R indices (all data)

-3

Largest diff. peak, hole/e A

(TROPP^Ph)](PF₆) (10a) were synthesized (Scheme 7). The ¹H and

¹³C{ H} NMR spectra of the latter Pd-alkyl compounds are

consistent with a trans-coordination of the alkyl moiety with

respect to meta coordinating C C_tropbond of the ligand. Due

to the higher trans-inﬂuence of the alkyl moieties vs. chloride or

tosylate, the ¹³C resonances of the C C_tropdouble bond were shif-

ted to higher frequencies by 12–19 ppm compared to 1a and 2a.

Single crystal X-ray structure analyses of 7a·0.5CH₂Cl₂and

10a·0.5C₆H₆conﬁrmed the relative stereochemistry observed in

solution. Suitable crystals for an X-ray diffraction experiment of

7a·0.5CH₂Cl₂and 10a·0.5C₆H₆were obtained by a slow diffusion

of toluene into a CH₂Cl₂solution of the corresponding compound.

Benzene has been found to be present as an impurity (<0.5%) in

toluene used for crystallization. ORTEP diagrams of 7a·0.5CH₂Cl₂

and 10a·0.5C₆H₆are shown in Fig. 2 and 3, respectively, while

selected bond lengths and angles as well as crystallographic data

are reported in Tables 1 and 3, respectively.

Scheme 8

(iii) The 2a/b-catalyzed carbonylation reactions of terminal

alkynes carried out in the absence of BQ led to the selective

formation of the monoesters A and B (Scheme 5(a)). The proposed

catalytic cycle for their formation (Scheme 8) comprises a 1,2- and

2,1-insertion of the terminal alkyne into the Pd–H bond of the

intermediates 6a/b (Scheme 8), yielding the Pd-alkenyl isomers

11a/b. Subsequent insertion of CO, followed by methanolysis

releases the esters A and B and regenerates 6a/b (Scheme 5(a)).

Notably, the ﬁrst Pd-hydride that enters the catalytic cycle

stems from the methanolysis of intermediates 3a/b), releasing a

stoichiometric amount of dimethyl carbonate that was actually

detected by a GC-MS analysis of the catalytic reaction mixture.

In order to prove the trans-coordination of the alkenyl moiety

in the intermediates 4a/b and 5a/b (Scheme 6) with respect

The crystal structure of 7a·0.5CH₂Cl₂shows the expected planar

coordination sphere for the palladium atom. The metal centre

deviates from the best coordination plane, deﬁned by the atoms

P(1), Cl(1), ct (i.e. centroid of the C(8) C(9) bond) and C(28) by

0.020(3) A in the direction of C(8). The methyl group (C(28)) is

located trans to the C C_tropbond of the cycloheptene moiety,

showing a Pd–C bond length of 2.102(9) A, which is in the

range of previously reported Pd-methyl bond length.¹⁷Apparently,

the higher trans-inﬂuence of the methyl group vs. chloride is

responsible for a shortening of the coordinating cycloheptene C–

C double bond to 1.373(13) A, as compared to 1a·0.5C₂H₄Cl₂

(1.391(6) A). Nevertheless, a signiﬁcant elongation of the latter

double bond with respect to the free TROPP^Phligand (1.328(4) A)

is observed.⁶The bite angle of the ligand in 7a·0.5CH₂Cl₂may

be described by the P(1)–Pd(1)–ct angle of 92.78(41)^◦. The two

inter-planar angles a and b, deﬁned as shown in Scheme 4,^5ashow

to the C C_tropunit of the ligand, neutral and mono-cationic

Pd-alkyl compounds of the formulae [PdMeCl(TROPP^Ph)] (7a),

[PdMe(H₂O)(TROPP^Ph)](PF₆) (8a) and [Pd(CH₂CH₂COMe)-

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The palladium atom deviates by 0.047(3) A from the least-

square coordination plane, deﬁned by the atoms O(1), C(28),

P(1), and ct. The carbon atom C(28) coordinates trans to the

C–C double bond of the ligand and the C(8) C(9) bonding

distance of 1.363(5) A is almost identical to that found for 7a·0.5

CH₂Cl₂. The Pd(1)–O(1) bonding distance of 2.127(2) A is in the

range of the bond lengths reported for related palladium-b-keto-

chelate complexes.^13b,18aImportantly, this latter bond persists also

in solution. In fact, an IR spectrum of 10a acquired in CH₂Cl₂

exhibited a n(CO) of 1632 cm^-1. The corresponding ¹³C{ H} NMR

spectrum acquired in CD₂Cl₂, showed for the carbonyl carbon

resonances a singlet centred at 235.81 ppm. The overall concave

conformation of TROPP^Phin 10a·0.5C₆H₆(i.e. a = 50.3(2) and b =

24.1(2)^◦) is comparable to that of 1a·0.5C₂H₄Cl₂and 7a·0.5CH₂Cl₂

(vide supra).

Conclusions

Neutral and cationic TROPP-modiﬁed Pd^IIcomplexes have been

synthesized and characterized. NMR spectroscopic analysis and

crystal structure data conﬁrm the bidentate coordination mode of

TROPP ligands to a palladium(II) centre. Even in CO-pressurized

Fig. 2 ORTEP diagram of 7a·0.5CH₂Cl₂with 30% probability ellipsoids.

The solvent molecule and hydrogen atoms are omitted for clarity.

solutions a de-coordination of the C C_tropdouble bond from the

palladium centre was not observed.

The cationic Pd-tosylate complexes 2a/b have been successfully

applied in the BQ-assisted methoxycarbonylation of terminal

alkynes to give chemoselectively the corresponding alkynylcar-

boxylic acid methyl ester in high yield (TOF up to 980 h^-1).

This catalytic activity strongly contrasts with the one obtained

from analogous reactions promoted by Pd/Cu-based or Pd-

diphosphane precatalysts. These systems lead either to alkyne-

coupling products or double alkyne insertion products as major

by-products.

The “in situ” spectroscopic detection of the Pd-

methoxycarbonyl species 3a¢, combined with the experimental

evidence of the exclusive dimerization reaction of ethene, catalyzed

by the cationic Pd–Me model compound 8a, corroborates the

presence of a strongly electrophilic metal centre in TROPP-

modiﬁed Pd^II-catalysts.

Experimental

Materials and physical measurements

All reactions and manipulations were carried out under a ni-

trogen atmosphere by using the standard Schlenk-type tech-

niques. The solvents were dried by passing them through

columns of suitable molecular sieves under nitrogen. The reagents

were used as purchased from Aldrich or Fluka, unless stated

otherwise. [PdCl₂(h -COD)],¹⁹[PdClMe(h -COD)],²⁰2c,¹¹2d,^8a

TROPP^{Ph 6}, 5-chloro-5H-dibenzo[a,d]cycloheptene²¹and di(2-

methoxyphenyl)phosphane²²wereprepared accordingto literature

methods. Carbonylation reactions were performed in a 320 mL

stainless steel autoclave, constructed at ICCOM-CNR (Florence,

Italy), equipped with a magnetic drive stirrer and a Parr 4842

temperature and pressure controller. GC analyses were performed

on a Shimadzu GC-14 A gas chromatograph equipped with a

ﬂame ionization detector and a 30 m (0.25 mm i.d., 0.25 mm

ﬁlm thickness) SPB-1 Supelco fused silica capillary column.

Fig. 3 ORTEP diagram of 10a·0.5C₆H₆with 30% probability ellipsoids.

Hydrogen atoms, the counter-anion and the solvent molecule are omitted

for clarity.

a value of 51.8(5)^◦and 25.1(6)^◦, respectively, that is comparable

to the value found for 1a·0.5C₂H₄Cl₂(vide supra).

The crystal structure of 10a·0.5C₆H₆(Fig. 3) reveals one

molecule of 10a and a molecule of benzene in the asymmetric unit

with an occupancy factor of 0.5.

The planar coordination sphere around the palladium centre

consists of one phosphorus donor atom, the centroid ct of the

C(8) C(9) bond of the cycloheptene moiety, the carbonyl oxygen

atom (O(1)) and the sp³-hybridized carbon atom (C(28)). The

two latter atoms belong to a palladium-b-keto-chelate ring.^13b,18

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GC-MS analyses were performed with a Shimadzu QP2100S

apparatus equipped with a column identical with that used for

GC analysis. Deuterated solvents for routine NMR measurements

NMR (CDCl₃, 75.49 MHz, ppm): d 55.19 (d, J_PC= 21.0 Hz,

PCH), 100.01 (s, CH CH), 128.47–135.50 (Ar–C). P{ H} NMR

(CDCl₃, 121.49 MHz, ppm): d 107.38 (s).

were dried over activated molecular sieves. ¹H, ¹³C{ H}, ³¹P{ H}

NMR spectra were obtained on either a Bruker Avance II DRX

300 spectrometer at 300.13, 75.49, 121.49 MHz, respectively, or

on a Bruker Avance DRX-400 spectrometer at 400.13, 100.62

and 161.98 MHz, respectively. Chemical shifts (d) are reported

in ppm relative to TMS, referenced to the chemical shifts of

residual solvents resonances (¹H and ¹³C NMR) or 85% H₃PO₄.

High-pressure NMR (HPNMR) experiments were carried out

on a Bruker ACP 200 spectrometer operating at 200.13 and

1b. Yield: 90.0% (231.4 mg, 0.377 mmol). C₂₉H₂₅Cl₂O₂PPd

(613.59): calc. C, 56.77; H, 4.07; found: C, 56.69; H, 3.99%. H

NMR (CDCl₃, 400.13 MHz, ppm): d 3.59 (s, 6H, OCH₃), 5.79

(d, ²J_PH= 16.0 Hz, 1H, PCH), 6.32 (s, 2H, CH CH), 6.73–7.75

(m, 16H, Ar–H). ¹³C{ H} NMR (CDCl₃, 100.62 MHz, ppm):

d 51.99 (d, J_PC= 22.1 Hz, PCH), 55.57 (s, OCH₃), 100.01

(s, CH CH), 100.22–160.13 (Ar–C). ³¹P{ H} NMR (CDCl₃,

161.98 MHz, ppm): d 112.84 (s).

81.01 MHz for ¹H and ³¹P{ H}, respectively, using a 10 mm

Synthesis of [Pd(OTs)₂(L)] (L = TROPP^Ph, 2a; TROPP^(2-MeOPh)

sapphire NMR tube (Saphikon, Milford, NH) equipped with

titanium high-pressure charging head constructed at the ICCOM-

CNR (Florence, Italy).²³Conductivity measurement were carried

2b. 1a/b (0.200 mmol) were suspended in deaerated CH₂Cl₂

(10 mL), followed by the addition of silver tosylate (117.1 mg,

0.420 mmol). The obtained suspensions were allowed to stir for 4 h

at room temperature, followed by their ﬁltration through a small

plug of Celite. The resulting yellow solutions were concentrated to

a small volume (2 mL) and on addition of diethyl ether (20 mL)

the products precipitated as yellow microcrystalline powders that

were separated by ﬁltration, washed with diethyl ether (10 mL)

and then dried in a stream of nitrogen.

out with an Orion model 101 conductivity meter with 10^-3

solutions.²⁴Elemental analyses were performed using a Carlo Erba

Model 1106 elemental analyzer. Infrared spectra were recorded on

a FT-IR Perkin-Elmer BX spectrometer.

Preparations

Synthesis of TROPP^(2-MeOPh)

solution of di(2-

2a. Yield: 88% (145.2 mg, 0.176 mmol). C₄₁H₃₅O₆PS₂Pd

(824.92): calc. C, 59.70; H, 4.24; found: C, 59.59; H, 4.19%. H

NMR (CD₂Cl₂, 400.13 MHz, ppm): d 2.35 (s, 6H, CH₃), 5.29 (d,

methoxyphenyl)phosphine²²(246.1 mg, 1.00 mmol) was added to

a deaerated solution of 5-chloro-5H-dibenzo[a,d]cycloheptene²¹

(226.6 mg, 1.00 mmol) in toluene (30 mL) under vigorous

stirring. After a reaction time of 10 min the corresponding

hydrochloride compound started to precipitate. The suspension

was than reﬂuxed for 10 min, followed by its cooling to room

temperature. At the latter temperature a deaerated solution of

sodium carbonate in water (15 mL) (10%, w/w) was added and the

suspension was again reﬂuxed under vigorous stirring, obtaining

two clear phase, that were separated at room temperature. The

water phase was again extracted with toluene (15 mL) and

then the combined toluene phases were dried over Na₂SO₄.

The inorganic salt was then separated by ﬁltration, the solvent

removed and the residue recrystallized from acetonitrile. Yield:

65.0% (283.7 mg, 0.650 mmol). C₂₉H₂₅O₂P (436.49): calc. C,

J_PH= 15.2 Hz, 1H, PCH), 7.09–7.87 (m, 28H, Ar–H + CH CH).

¹³C{ H} NMR (CD₂Cl₂, 100.62 MHz, ppm): d 21.09 (s, CH₃),

52.73 (d, J_PC= 25.3 Hz, PCH), 97.33 (br s, CH CH), 123.36–

141.09 (Ar–C). ³¹P{ H} NMR (CD₂Cl₂, 161.98 MHz, ppm): d

125.99 (br s).

2b. Yield: 80.0% (141.6 mg, 0.160 mmol). C₄₃H₃₉O₈PS₂Pd

(884.92): calc. C, 58.36; H, 4.41; found: C, 58.28; H, 4.35%. H

NMR (CD₂Cl₂, 400.13 MHz, ppm): d 2.36 (s, 6H, CH₃), 3.64 (s,

6H, OCH₃), 5.58 (d, J_PH= 16.0 Hz, 1H, PCH), 6.78–7.78 (m,

26H, Ar–H + CH CH). ¹³C{ H} NMR (CD₂Cl₂, 100.62 MHz,

ppm): d 15.13 (s, CH₃), 51.66 (d, J_PC= 27.2 Hz, PCH), 97.33

(br s, CH CH), 111.11–160.57 (Ar–C). ³¹P{ H} NMR (CD₂Cl₂,

161.98 MHz, ppm): d 130.76 (br s).

79.80; H, 5.72; found: C, 79.74; H, 5.68%. H NMR (CDCl₃,

Dissolution of 2a/b in CD₃OD gave the cationic complexes

300.13 MHz, ppm): d 3.51 (s, 6H, OCH₃), 5.14 (d, ²J_PH= 4.2 Hz,

[Pd(L)(CD₃OD)₂](OTs)₂(L = TROPP^Ph, 2a¢; TROPP^(2-MeOPh), 2b¢).

1H, PCH), 6.58–7.45 (m, 18H, Ar–H + CH CH). ¹³C{ H}

2a¢. ¹H NMR (CD₃OD, 400.13 MHz, ppm): d 2.35 (s, 6H,

NMR (CDCl₃, 75.49 MHz, ppm): d 54.20 (d, J_PC= 21.6 Hz,

PCH), 55.00 (s, OCH₃), 109.80–161.90 (Ar–C + CH CH).

CH₃), 5.89 (d, J_PH= 13.2 Hz, 1H, PCH), 7.20–7.93 (m, 28H,

³¹P{ H} NMR (CDCl₃, 121.49 MHz, ppm): d -36.00 (s).

Ar–H + CH CH). ¹³C{ H} NMR (CD₃OD, 100.62 MHz,

ppm): d 19.97 (s, CH₃), 51.62 (d, J_PC= 26.1 Hz, PCH), 98.87

Synthesis of [PdCl₂(L)] (L = TROPP^Ph, 1a; TROPP^(2-MeOPh), 1b).

(br s, CH CH), 125.66–141.20 (Ar–C). ³¹P{ H} NMR (CD₃OD,

An appropriate amount of ligand (0.420 mmol) was added to a

161.98 MHz, ppm): d 129.00 (br s). K_M(MeOH, 21.0 ^◦C): 130

X^-1cm²mol^-1.

deaerated solution of [PdCl₂(h -COD)] (120.0 mg, 0.420 mmol) in

CH₂Cl₂(10 mL). The resulting yellow suspension was allowed to

stir for 20 min, followed by its concentration to a small volume

(5 mL). The addition of diethyl ether to the latter solution caused

the precipitation of the product as microcrystalline yellow powder,

that was successively separated by ﬁltration, washed with diethyl

ether (2 ¥ 5 mL) and dried in a stream of nitrogen.

2b¢. ¹H NMR (CD₃OD, 400.13 MHz, ppm: d 2.34 (s, 6H,

CH₃), 3.64 (s, 6H, OCH₃), 5.88 (d, J_PH= 14.2 Hz, 1H, PCH),

7.02–7.98 (m, 26H, Ar–H + CH CH). ¹³C{ H} NMR (CD₃OD,

100.62 MHz, ppm): d 19.67 (s, CH₃), 51.42 (d, J_PC= 26.2 Hz,

PCH), 98.93 (br s, CH CH), 110.11–161.57 (Ar–C). ³¹P{ H}

NMR (CD₃OD, 161.98 MHz, ppm): d 134.00 (br s). K_M(MeOH,

21.0 ^◦C): 129 X^-1cm²mol^-1.

1a. Yield: 93.0% (216.2 mg, 0.390 mmol). C₂₇H₂₁Cl₂PPd

(553.59): calc. C, 58.58; H, 3.79; found C, 58.42; H, 3.61%. H

NMR (CDCl₃, 300.13 MHz, ppm): d 5.15 (d, J_PH= 21.2 Hz,

“In situ” synthesis of [Pd(COOMe)(CD₃OD)(TROPP^Ph)](OTs)

(3a¢) in CD₃OD. 2a (25.0 mg, 0.030 mmol) was dissolved in

1H, PCH), 7.08–7.70 (m, 20H, Ar–H + CH CH). ¹³C{ H}

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deaerated CD₃OD at room temperature. Then CO was bubbled

through the latter solution for half a minute at room temperature.

During this time the colour of the solution turned red. The red

solution was then transferred into a 5 mm NMR tube in order to

acquire NMR spectra.

IR spectrum. H NMR (CD₂Cl₂, 300.13 MHz, ppm): d 1.89 (d,

⁴J_PH= 1.5 Hz, 3H, COCH₃), 5.25 (d, ²J_PH= 16.2 Hz, 1H, PCH),

6.25 (br s, 2H, H₂O), 7.02–7.48 (m, 20H, Ar–H + CH CH).

¹³C{ H} NMR (CD₂Cl₂, 75.49 MHz, ppm): d 33.60 (d, J_PC

25.9 Hz, COCH₃), 56.81 (d, J_PC= 22.6 Hz, PCH), 118.75 (s,

Integrals of ¹H NMR signals are not reported due to the

concomitant presence of TsOH in solution that rapidly exchanges

the counter-anion with 3a¢. ¹H NMR (CD₃OD, 400.12 MHz,

CH = CH), 126.71–134.53 (Ar–C), 223.24 (s, COCH₃). ³¹P{ H}

NMR (CD₂Cl₂, 121.49 MHz, ppm): d 48.21 (s). IR (CH₂Cl₂,

n(CO)/cm^-1) 1716 (s).

ppm): d 2.38 (s, CH₃), 5.79 (d, J_PH= 16.0 Hz, PCH), 7.08

Synthesis of [Pd(CH₂CH₂COMe)(TROPP^Ph)](PF₆) (10a).

Ethene was bubbled for 5 min through a solution of 9a (132.1 mg,

0.200 mmol) in deaerated CH₂Cl₂(4 mL), that was obtained as

described above, at room temperature. Afterwards the solution

was concentrated to a small volume (2 mL) and diethyl ether was

added, precipitating 10a as yellow microcrystalline compound,

that was separated by ﬁltration, washed with diethyl ether (8 mL)

and dried in a stream of nitrogen. Yield: 78.0% (109.0 mg, 0.156

mmol). C₃₁H₂₈F₆OP₂Pd (698.63): calc. C, 53.29; H, 4.00; found:

(s, CH CH), 7.35–7.77 (m, Ar–H). ¹³C{ H} NMR (CD₃OD,

100.62 MHz, ppm): d 13.9 (s, CH₃), 54.80 (d, J_PC= 24.5 Hz,

PCH), 115.54 (s, CH CH), 124.90–140.46 (Ar–C), 174.9 (d,

²J_PC= 16.3 Hz, COCD₃). ³¹P{ H} NMR (CD₃OD, 161.98 MHz,

ppm): d 68.35 (d, ²J_PC= 16.3 Hz).

Synthesis of [PdClMe(TROPP^Ph)] (7a). [PdClMe(h -COD)]

(106.0 mg, 0.400 mmol) was dissolved in deaerated CH₂Cl₂

(10 mL) followed by the addition of TROPP^Ph(150.9 mg, 0.400

mmol) at room temperature. The solution was allowed to stir for

20 min, followed by its concentration to a small volume (2 mL).

After the addition of diethyl ether the product precipitated as

white microcrystalline powder, that was separated by ﬁltration,

washed with diethyl ether and dried in a stream of nitrogen. Yield:

90.0% (191.9 mg, 0.360 mmol). C₂₈H₂₄ClPPd (533.15): calc. C,

C, 53.19; H, 3.92%. H NMR (CD₂Cl₂, 400.12 MHz, ppm): d

1.86 (td, J_HH= 6.1 Hz, J_PH= 2.1 Hz, 2H, PdCH₂), 2.57 (s,

3H, COCH₃), 3.20 (t, J_HH= 6.1 Hz, 2H, COCH₂), 5.35 (d,

J_PH= 16.4 Hz, 1H, PCH), 7.14–7.63 (m, 20H, Ar–H + CH CH).

¹³C{ H} NMR (CD₂Cl₂, 100.62 MHz, ppm): d 27.98 (s, COCH₃),

32.76 (s, PdCH₂), 51.49 (s, COCH₂), 55.59 (d, J_PC= 18.0 Hz,

PCH), 112.40 (s, CH CH), 112.40–134.38 (Ar–C), 235.81 (s,

63.08; H, 4.50; found: C, 62.98; H, 4.41%. H NMR (CDCl₃,

COCH₃). ³¹P{ H} NMR (CD₂Cl₂, 161.98 MHz, ppm): d 73.74

300.13 MHz, ppm): d 0.92 (d, ³J_PH= 2.9 Hz, 3H, PdCH₃), 5.16 (d,

(s). IR (CH₂Cl₂, n(CO)/cm^-1) 1632 (s).

J_PH= 15.1 Hz, 1H, PCH), 6.93–7.39 (m, 20H, Ar–H + CH CH).

¹³C{ H} NMR (CDCl₃, 75.49 MHz, ppm): d 8.45 (s, PdCH₃),

Methoxycarbonylation of terminal alkynes. Typically, a solu-

tion of the substrate (2.00 mmol) in deaerated MeOH (20 mL)

was introduced by suction into an autoclave (320 mL), previously

evacuated by a vacuum pump, containing the desired amount

of catalyst precursor and BQ or TsOH. The autoclave was then

charged at room temperature with CO (1 bar), followed by its

56.37 (d, J_PC= 21.1 Hz, PCH), 113.49 (s, CH CH), 127.27–

134.63 (Ar–C). ³¹P{ H} NMR (CDCl₃, 121.49 MHz, ppm): d

76.06 (s).

Synthesis

[PdMe(H₂O)(TROPP^Ph)](PF₆)

(8a). 7a

(106.6 mg, 0.20 mmol) was dissolved in deaerated CH₂Cl₂

(8 mL) at room temperature, followed by the addition of AgPF₆

(53.1 mg, 0.21 mmol). The resulting suspension was allowed to

stir for 20 min at room temperature. AgCl was then removed

upon ﬁltration of the suspension through a plug of Celite. The

yellow solution obtained was concentrated to dryness and the

residue suspended in diethyl ether and stirred for 10 min at room

temperature. Afterwards the yellow microcrystalline product was

separated by ﬁltration and dried in a stream of nitrogen. Yield:

80.0% (105.7 mg, 0.160 mmol). C₂₈H₂₆F₆OP₂Pd (660.60): calc. C,

◦

heating to 70 C. Once the latter temperature had been reached

the autoclave was charged with the desired CO pressure and

stirring (800 rpm) was started. After the desired reaction time,

the autoclave was successively cooled to 10 ^◦C by means of an ice-

water bath, unreacted CO released and n-decane (standard, 98.0

mL) added. The catalytic solution was then subjected to a GC and

GC-MS analysis.

Dimerization of ethene. A solution of 8a (13.2 mg, 0.02 mmol)

in toluene (30 mL) was introduced by suction into an autoclave

(320 mL), previously evacuated by a vacuum pump. The autoclave

was then successively charged at room temperature with ethene (1

50.91; H, 3.93; found: C, 50.82; H, 3.79%. H NMR (CD₂Cl₂,

300.13 MHz, ppm): d 0.83 (s, 3H, PdCH₃), 4.11 (s, 2H, H₂O),

◦

5.28 (d, J_PH= 15.3 Hz, 1H, PCH), 7.02 (s, 2 H, CH CH),

bar) and heated to 50 C. Once the latter temperature had been

7.07–7.50 (m, 18H, Ar–H). ¹³C{ H} NMR (CD₂Cl₂, 75.49 MHz,

reached stirring was started and the ethene pressure was adjusted

to 21 bar, maintaining this latter pressure constant by continuously

feeding the autoclave with ethene from a reservoir. After a reaction

time of 2 h, the autoclave was cooled to 0 ^◦C by means of an ice-

acetone bath, the excess ethene was vented off slowly and the clear

yellow reaction solution was subjected to a GC (standard, hexane

(98.0 mL) and GC-MS analysis.

ppm): d 11.73 (s, PdCH₃), 56.18 (d, ¹J_PC= 24.1 Hz, PCH), 113.56

(s, CH H), 126.22–134.15 (Ar–C). ³¹P{ H} NMR (CD₂Cl₂,

121.49 MHz, ppm): d 77.70 (s).

“In situ” synthesis of [Pd(COMe)(H₂O)(TROPP^Ph)](PF₆) (9a).

8a (66.1 mg 0.100 mmol) was dissolved in deaerated CD₂Cl₂(2 mL)

at room temperature. The latter solution was then transferred

into a 10 mm sapphire tube under nitrogen, followed by its

pressurization with CO (75 psi) at room temperature. After a

reaction time of 5 min the sapphire tube was depressurized and the

solution transferred into a 5 mm NMR tube in order to acquire

NMR spectra. An identical synthetic procedure was applied to

obtain a CH₂Cl₂solution of 9a used for the acquisition of an

X-Ray crystallographic data collection and reﬁnement of the

structures

The crystallographic data for 1a·0.5C₂H₄Cl₂, 7a·0.5CH₂Cl₂and

10a·0.5C₆H₆are summarized in Table 3. The X-ray diffraction

intensity data for 7a·0.5CH₂Cl₂were collected on a Enraf

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Nonius CAD4 diffractometer, while those for 1a·0.5C₂H₄Cl₂

and 10a·0.5C₆H₆were collected on an Oxford Diffraction CCD

diffractometer (Xcalibur 3), applying graphite monochromated

7 (a) H. Scho¨nberg, S. Boulmaaˆz, M. Wo¨rle, L. Liesum, A. Schweiger and

H. Gru¨tzmacher, Angew. Chem., Int. Ed., 1998, 37, 1423–1426; (b) F.

Breher, H. Ru¨egger, M. Mlakar, M. Rudolph, S. Deblon, H. Scho¨nberg,

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6220–6228.

Mo-Ka radiation (l = 0.71073 A). Absorption correction was

carried out applying psi-scans (7a·0.5CH₂Cl₂) or SADABS^25a

(1a·0.5C₂H₄Cl₂, 10a·0.5C₆H₆). All structure determination cal-

culations were performed with the WINGX package^25bthat

comprises SIR-97,^25cSHELXL-97^25dand ORTEP-3 programs.^25e

Final reﬁnements based on F²were carried out with anisotropic

thermal parameters for all non-hydrogen atoms, except those

belonging to the solvent molecules (i.e. CH₂Cl₂and C₆H₆). The

occupancy factors for the solvents’ carbon atoms were reﬁned

giving a value close to 0.5. As a consequence, the occupancy factor

of the latter atoms was ﬁxed to 0.5 in the last reﬁnement cycle.

The geometry of the solvent molecules was modeled applying

DFIX restraints. Hydrogen atoms were included in the reﬁnement

using a riding model with isotropic U values depending on

the U_eq. CCDC reference number for 1a·0.5C₂H₄Cl₂: 739871,

7a·0.5CH₂Cl₂: 696685 and 10a·0.5C₆H₆: 96684.

10 C. Thoumazet, L. Ricard, H. Gru¨tzmacher and P. Le Floch, Chem.

Commun., 2005, 1592–1594.

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Acta, 1995, 233, 5–9.

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Meli, W. Oberhauser, P. W. N. M. van Leeuwen, M. A. Zuideveld, Z.

Freixa, P. C. J. Kamer, A. L. Spek, O. V. Gusev and A. M. Kal’sin,

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Meli and W. Oberhauser, Organometallics, 2003, 22, 913–915.

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Acknowledgements

The authors thank the European Commission for ﬁnancing the

project IDECAT (NoE contract no. NMP3-CT-2005-011730).

Additional support by the Swiss National Science Foundation

and the ETH Zu¨rich is gratefully acknowledged.

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Cabinato and L. Toniolo, J. Organomet. Chem., 1990, 398, 187–195;

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