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Organometallics 2009, 28, 824–829

Heteroaromatic Analogues of Dibenzylideneacetone (dba) and Pd02(het-dba)3 Complexes: Effect of a Thienyl Moiety on the Reactivity of Pd0(η2-thn-dba)(PPh3)2/Pd0(PPh3)2 (n ) 1 or 2) and Pd0(η2-th2-dba)(dppe)/Pd0(dppe) in Oxidative Addition Reactions with Iodobenzene Petr Sehnal,† Hanaa Taghzouti,‡ Ian J. S. Fairlamb,*,† Anny Jutand,*,‡ Adam F. Lee,† and Adrian C. Whitwood† Department of Chemistry, UniVersity of York, Heslington, York YO10 5DD, U.K., and Ecole Normale Supe´rieure, De´partement de Chimie, UMR CNRS-ENS-UPMC 8640, 24, Rue Lhomond, F-75231 Paris Cedex 5, France ReceiVed October 9, 2008

Palladium(0) complexes containing thienyl analogues of dibenzylidene acetone (dba) have been synthesized and characterized. Their reactivity toward PPh3 and dppe, to generate Pd0(η2-alkene)(PPh3)2 and Pd0(η2-alkene)(dppe) complexes, and their subsequent oxidative addition reactions with PhI, have been investigated. Introduction The application of π-acidic alkenyl ligands in Pd-catalyzed cross-coupling processes1 attracts interest from the organometallic chemistry and synthesis community. The Pd0 precursor source, Pd02(dba-H)3 (dba-H ) E,E-dibenzylidene acetone, e.g., 1, Figure 1), is used in combination with phosphine, amine and N-heterocyclic carbene ligands (L), to generate “Pd0Ln” complexes in situ.2 In these reactions, dba-H, an enone ligand,3 is usually noninnocent.4 The dba-H ligand coordinates in an η2mode to Pd0, as “Pd0(η2-dba-H)Ln”,4b,5 affecting the concentration of “Pd0Ln” complexes in oxidative addition with organohalides, the first committed step in cross-coupling processes. While this appears a hindrance, dba-H coordination to Pd0 reduces the rate of agglomeration, to form larger Pd0 clusters and colloids, ultimately Pd precipitation.6 By subtle electronic * Joint corresponding authors. Fax: 44 1904 434091 (I.J.S.F.); 33 1 4432 2402 (A.J.). E-mail: [email protected] (I.J.S.F.); [email protected] (A.J.). † Department of Chemistry, University of York. ‡ Ecole Normale Supe´rieure, De´partement de Chimie, UMR CNRS-ENSUPMC 8640. (1) (a) Johnson, J. B.; Rovis, T. Angew. Chem., Int. Ed. 2008, 47, 840– 871. (b) Glorius, F. Angew. Chem., Int. Ed. 2004, 43, 3364–3366. (c) Fairlamb, I. J. S. Org. Biomol. Chem. 2008, 6, 3645–3656. (2) Littke, A. F.; Fu, G. C. Angew. Chem., Int. Ed. 2002, 41, 4176– 4211. (3) Moulton, B. E.; Duhme-Klair, A.-K.; Fairlamb, I. J. S.; Lynam, J. M.; Whitwood, A. C Organometallics 2007, 26, 6354–6365, and references cited therein. (4) (a) Amatore, C.; Jutand, A. Coord. Chem. ReV 1998, 178-180, 511– 528. (b) Amatore, C.; Jutand, A.; Meyer, G. Inorg. Chim. Acta 1998, 273, 76–84. (5) Amatore, C.; Jutand, A.; Khalil, F.; M’Barki, M. A.; Mottier, L. Organometallics 1993, 12, 3168–3178. (6) The π-acidic alkene can increase catalyst stability, see: (a) Scrivanti, A.; Beghetto, V.; Matteoli, U.; Antonaroli, S.; Marini, A.; Mandoj, F.; Paolesse, R.; Crociani, B. Tetrahedron Lett. 2004, 45, 5861–5864. (b) Scrivanti, A.; Beghetto, V.; Matteoli, U.; Antonaroli, S.; Marini, A.; Crociani, B. Tetrahedron 2005, 61, 9752–9758. (c) Fluorous dba-type ligands stabilize Pd nanoparticles, see: Moreno-Man˜as, M.; Pleixats, R.; Villarroya, S. Organometallics 2001, 20, 4524–4528. (d) Moreno-Man˜as, M.; Pleixats, R.; Villarroya, S. Chem. Commun. 2002, 60, 61. (e) Tristany, M.; Courmarcel, J.; Dieudonne´, P.; Moreno-Man˜as, M.; Pleixats, R.; Rimola, A.; Sodupe, M.; Villarroya, S. Chem. Mater. 2006, 18, 716–722.

Figure 1. Various alkenyl ligands.

tuning of the aryl groups of dba (e.g., dba-n,n′-Z,Z′; n/n′ ) 3, 4, or 5; Z/Z′ ) OMe, t-Bu, Br, H, F, CF3, NO2) this noninnocent behavior has been exploited experimentally in cross-coupling processes,7 and supported by theoretical studies.8 Dba-n,n′-Z,Z′ ligands also influence transmetalation and reductive elimination steps,9 and reduce the rate of β-hydrogen elimination in “PdIIσ-alkyl” complexes.10 This latter finding facilitates selective intramolecular Heck reactions of alkenyl tethered alkyl bromides to give exo-cyclopentene products (proViding that the dba-4,4′OMe ligand is used). Extending our interests to variants of the “dba” structure we herein report our findings concerning the synthesis of heteroaromatic analogues (het-dba) and the binuclear Pd0 complexes derived from 2a (th2-dba) and 3a (th1-dba), namely Pd02(th2(7) (a) Fairlamb, I. J. S.; Kapdi, A. R.; Lee, A. F. Org. Lett. 2004, 6, 4435–4438. (b) Mace´, Y.; Kapdi, A. R.; Fairlamb, I. J. S.; Jutand, A. Organometallics 2006, 25, 1795–1800. (c) Fairlamb, I. J. S.; Kapdi, A. R.; Lee, A. F.; McGlacken, G. P.; Weissburger, F.; de Vries, A. H. M.; Schmieder-van de Vondervoort, L. Chem. Eur. J. 2006, 12, 8750–8761. (8) Fairlamb, I. J. S.; Lee, A. F. Organometallics 2007, 26, 4087–4089. (9) (a) Zhao, Y.; Wang, H.; Hou, X.; Hu, Y.; Lei, A.; Zhang, H.; Zhu, L. J. Am. Chem. Soc. 2006, 128, 15048–15049. (b) For related ligand effects, see: Liu, Q.; Duan, H.; Luo, X.; Tang, Y.; Li, G.; Huang, R.; Lei, A. AdV. Synth. Catal. 2008, 350, 1349–1354. (c) Luo, X.; Zhang, H.; Duan, H.; Liu, Q.; Zhu, L.; Zhang, T.; Lei, A. Org. Lett. 2007, 9, 4571–4574. (d) Shi, W.; Luo, Y.; Luo, X.; Chao, L.; Zhang, H.; Wang, J.; Lei, A. J. Am. Chem. Soc. 2008, 130, 14713–14720. (10) Firmansjah, L.; Fu, G. C. J. Am. Chem. Soc. 2007, 129, 11340– 11341.

10.1021/om800975w CCC: $40.75  2009 American Chemical Society Publication on Web 01/09/2009

Heteroaromatic Analogues of Dibenzylideneacetone

Organometallics, Vol. 28, No. 3, 2009 825

Scheme 1. Synthesis of Heteroaromatic Variants of dba

dba)3 · th2-dba and Pd02(th1-dba)3 · th1-dba, respectively. Ligands, 2b, 2c and 3b, do not form binuclear Pd0 complexes of this type. The reaction kinetics of in situ generated Pd0(η2-thndba)(PPh3)2 (n ) 1 or 2) and Pd0(η2-th2-dba)(dppe) complexes with iodobenzene have been determined, and the results compared with Pd0 complexes possessing the parent dba-H ligand.

Results and Discussion Symmetrical and unsymmetrical “dba” structures may be accessed synthetically using a number of methods.11 Arguably the most efficient means is by Claisen-Schmidt condensation11a of an appropriate aldehyde and ketone in the presence of NaOH in aqueous EtOH (Scheme 1). The reaction of 2 equiv of thiophene-2-carbaldehyde or furan-2-carbaldehyde with acetone in the presence of NaOH in EtOH affords the symmetrical hetdba analogues 2a and 2b in 93% and 90%, respectively (eq 1).12 Unsymmetrical het-dba analogues were prepared by a similar reaction of one equiv of thiophene-2-carbaldehyde or furan-2-carbaldehyde with (E)-4-phenylbut-3-en-2-one, affording 3a and 3b in 93% and 99%, respectively (eq 2). It has been reported that picolinaldehyde fails to react with acetone to give 2c under these and related reaction conditions; H2O elimination is hindered by the neighboring nitrogen atom through an inductive effect which results in electron withdrawal from the carbon atom bearing the hydroxyl group.13 However, we found that2ccanbepreparedin83%yieldbyHorner-Wadsworth-Emmons reaction of two equivalents of picolinaldehyde with bis(phosphonate) compound 414 with a K2CO3 base (eq 3). Crystals of 2c and 3a, produced by slow evaporation from a CH2Cl2 solution, were found suitable for X-ray diffraction analysis (Figure 2), which demonstrate the expected atom connectivity. Palladium(0) dimer complexes, Pd0x(het-dba)y (het-dba ) 2a or 3a), were prepared using the method described by Ishii and co-workers {as for Pd02(dba-H)3 · dba-H}.15 A methanolic solution of the het-dba ligand (3.3 equiv) and NaOAc (∼8 equiv) were heated to 40-50 °C for 5 min, then PdCl2 (1 equiv) was added (Scheme 2). (11) (a) Conard, C. R.; Dolliver, M. A. Org. Synth. 1943, 2, 167. (b) Mahrwald, R.; Schick, H. Synthesis 1990, 592. (c) Sinisterra, J. V.; GarciaRaso, A.; Cabello, J. A.; Marinas, J. M. Synthesis 1984, 502. (d) Deng, G.; Ren, T. Synth. Commun. 2003, 33, 2995. (12) Rule, N. G.; Detty, M. R.; Kaeding, J. E.; Sinicropi, J. A. J. Org. Chem. 1995, 60, 1665–1673. (13) Marvel, C. S.; Stille, J. K. J. Org. Chem. 1957, 22, 1451–1457. (14) Corbel, B.; Medinger, L.; Haelters, J. P.; Sturtz, G. Synthesis 1985, 1048–1051. (15) Ukai, T.; Kawazura, H.; Ishii, Y.; Bonnet, J. J.; Ibers, J. A. J. Organomet. Chem. 1974, 65, 253–266.

Figure 2. (A) X-ray structure for compound 2c. Selected bond distances (Å): C(1A)-N(1A) 1.36(2), C(8)-O(1A) 1.157(9), C(6A)-C(7A) 1.329(4), C(7A)-C(8) 1.469(3), C(8)-C(9A) 1.548(4), C(9A)-C(10A) 1.339(4). Selected bond angles (deg): C(6A)-C(7A)-C(8) 117.4(2), O(1A)-C(8)-C(7A) 127.5(3), O(1A)-C(8)-C(9A) 117.1(3), C(9A)-C(10A)-C(11A) 124.7(3). (B) X-ray structure for compound 3a. Selected bond distances (Å): C(4)-S(1) 1.657(4), C(7)-O(1) 1.203(4), C(5)-C(6) 1.334(6), C(6)-C(7) 1.647(5), C(7)-C(8) 1.361(4), C(8)-C(9) 1.338(6). Selected bond angles (deg): C(5)-C(6)-C(7) 124.1(4), O(1)-C(7)C(6) 105.4(2), O(1)-C(7)-C(8) 138.9(2), C(8)-C(9)-C(10) 125.7(5). Scheme 2. Synthesis of Palladium(0) Complexes Containing thn-dba Ligands (n ) 1 and 2)

The reactions were run for 4 h which leads to the precipitation of dark purple solids, which can be filtered following cooling to 25 °C. For th2-dba (2a) the Pd:L stoichiometry was confirmed by elemental analysis as Pd02(th2-dba)3 · th2-dba. An identical Pd:L stoichiometry was observed for Pd02(th1-dba)3 · th1-dba. The former complex is only partially soluble in common organic solvents, whereas the latter complex mirrors the solubility of Pd02(dba-H)3 · dba-H, e.g., soluble in aromatic and chlorinated solvents and insoluble in hydrocarbons or alcohols. An NMR spectroscopic study (at 500 MHz in CDCl3) of the Pd0 complexes has proven to some extent informative (Figure 3). The major isomeric complex of “Pd02(dba-H)3” exhibits six different alkene environments (12 protons; ∼δ 4.8-6.9). The minor isomeric complex is just visible in this spectrum (previously unreported). The broad peaks at ∼δ 6.5-6.6 represent the ortho-C-H protons from the dba-H ligand in the Pd0 complex. For Pd02(th1-dba)3 · th1-dba, similar coordinated alkene protons are observed, which is more intricate, as th1dba is an unsymmetrical bidentate 1,4-diene ligand leading to further positional isomeric complications. The 1H NMR spectrum shows that there are many different Pd-alkene interactions, for which a full analysis has not been attempted. A 1H NMR spectrum of Pd02(th2-dba)3 · th2-dba in CDCl3 was obtained at low concentration, and while some alkene protons, albeit broad

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Figure 3. 1H NMR spectra {recorded in CDCl3 (0.7 mL) at 500 MHz}: (A) ) Pd02(dba-H)3 · dba-H (12 mg); (B) ) Pd02(th1dba)3 · th1-dba (10 mg); (C) ) Pd02(th2-dba)3 · th2-dba (∼1 mg; only partially soluble). (/) Shielded ortho-aryl proton environments in Pd02(dba-H)3 · dba-H (revealed from deuterium labeling studies by Ishii and co-workers).15,17

signals, are visible, a detailed analysis of this Pd0 complex is not possible (VT experiments are precluded by the low concentration of the Pd0 complex and dynamic exchange effects16 in solution and further complex precipitation at lower temperatures). The solid-state 13C NMR spectra of Pd02(th1dba)3 · th1-dba and Pd02(th2-dba)3 · th2-dba reveal shielded but broad carbon environments characteristic of Pd-C bonds (e.g., 90-100 ppm) (Figure 4). This are easily identified by a comparison with the solid-state 13C NMR spectra of both th1dba (3a) and th2-dba (2a). The broadness of the 13C signals likely stems from multiple signals for each carbon environment. For example, 12 are possible in the major isomeric complex of Pd02(th2-dba)3 · th2-dba), which are too close to resolve. For Pd02(th1-dba)3 · th1-dba, two broad 13C signals, characteristic of Pd-C bonds, are observed which is expected for the unsymmetrical th1-dba ligand Vide supra. The 13C signals for the CdO groups in the palladium complexes are broadened and also slightly shifted relative to the free ligands {Pd02(th1-dba)3 · th1-dba ) δ 185; th1-dba (3a) ) δ 189; Pd02(th2-dba)3 · th2-dba ) δ 185; th2-dba (2a) ) δ 186}. While Pd02(th2-dba)3 · th2-dba does dissolve in both DMF and DMSO, we note that after ca. 30 min the complex has degraded to give palladium particles.6c There is no evidence by 1H NMR spectroscopy that the th2-dba ligand is stabilizing these particles (only free ligand is observed). The stabilities of both Pd02(th2dba)3 · th2-dba and Pd02(th1-dba)3 · th1-dba have been evaluated by thermogravimetric analysis (TGA) coupled with differential temperature analysis. The TG curves of these complexes display a two-stage decomposition pattern which reveals a high thermal (16) In unpublished studies we have prepared various 13C-labeled Pd02(dba-H)3 · dba-H complexes. 13C EXSY experiments show that dba-H freely exchanges with the “Pd02(dba-H)3” complex. The rate of exchange in these types of Pd0 dimer complexes is dependent on the type of dba ligand present. (17) Kawazura, H.; Tanaka, H.; Yamada, K.; Takahashi, T.; Ishii, Y. Bull. Chem. Soc. Jpn. 1978, 51, 3466–3470.

Figure 4. 13C Solid state NMR spectra: (A) th1-dba (3a); (B) Pd02(th1-dba)3 · th1-dba; (C) th2-dba (2a); (D) Pd02(th2-dba)3 · th2dba.

stability (see Supporting Information). The Pd02(dba-H)3 · dba-H complex releases three dba-H ligands to generate “Pd02(dbaH)1”. Both Pd02(th2-dba)3 · th2-dba and Pd02(th1-dba)3.th1-dba appear to degrade to give sulfur adducts of the type Pd02(thndba)2S2 (n ) 1 or 2), although the intermediate species degrade at much higher temperatures - the Pd02(thn-dba)3 · thn-dba complexes are more thermally stable/robust compared to Pd02(dbaH)3 · dba-H. Het-dba ligands 2b, 2c or 3b do not form Pd0x(het-dba)y complexes. For 2b and 3b, the ligands are fully recovered from the reaction mixtures; 2c slowly degrades under the reaction conditions. Analysis of the solution and solid-state structures of Pd02(dba-H)3 · dba-H reveals that switching of the enone conformation (s-cis T s-trans) involves delocalization of electron density from Pd0 to the ortho-positions (which are shielded (/), see Figure 3) of the aromatic rings, and whereas a neighboring sulfur atom can accommodate this requirement, oxygen and nitrogen could be destabilizing (Figure 5). In the case of 2c, N-coordination could compete with alkene coordination to Pd, although we are unable to rule out an inefficient reduction of the initial PdII complex by NaOAc in methanol. Comparative Reactivity of PhI with the Pd(0) Complexes Generated from Pd02(dba-H)3dba-H, Pd02(th1-dba)3th1-dba or Pd02(th2-dba)3 · th2-dba Associated with 2 or 4 PPh3 in DMF. In previous work, we established that the rate of the oxidative addition of PhI to the Pd(0) complexes generated by addition of PPh3 (PPh3/Pd ) 2) to either Pd02(dba-H)3 or Pd0(dba-H)2 in DMF was affected by the electron-donating or accepting properties of groups Z-substituted on the phenyl groups of dba-H (see 1 in Figure 1): the oxidative addition being faster when Z

Heteroaromatic Analogues of Dibenzylideneacetone

Organometallics, Vol. 28, No. 3, 2009 827 Table 1. Oxidation Peak Potentials of Pd0 Complexes Generated from Pd0(thn-dba)2 or Pd0(dba-H)2 Associated with 2 equiv PPh3 or 1 equiv of dppe in DMF at 20 °C EpO2 (V vs SCE)a 0

Figure 5. Delocalization of electron density onto the ortho-position of the aromatic/heteroaromatic ring systems: successful alkenecoordination to Pd0.

2

Pd (η -dba-H)(PPh3)2 Pd0(η2-th1-dba)(PPh3)2 Pd0(η2-th2-dba)(PPh3)2 Pd0(PPh3)2 Pd0(η2-dba-H)(dppe) Pd0(η2-th2-dba)(dppe) Pd0(dppe)

EpO1 (V vs SCE)a

+0.575 +0.542 +0.506 +0.098;b +0.118c +0.385 +0.281 -0.510;d -0.485e

At a steady gold disk electrode (d ) 2 mm) with a scan rate of 0.5 V s-1 in DMF containing nBu4NBF4 (0.3 M). b Generated from Pd0(th1-dba)2 + 2 PPh3. c Generated from Pd0(th2-dba)2 + 2 PPh3. d Generated from Pd0(dba-H)2 + 1 dppe. e Generated from Pd0(th2-dba)2 + 1 dppe. a

Scheme 4

Figure 6. Cyclic voltammetry at a steady gold disk electrode (d ) 2 mm) performed in DMF (containing nBu4NBF4 0.3 M) with a scan rate of 0.5 Vs-1, at 20 °C. a) Pd0(th1-dba)2 (2 mM) + PPh3 (4 mM); b) Pd0(th1-dba)2 (2 mM) + PPh3 (8 mM). Scheme 3

was an electron-donating group.7b The dba-H ligand plays a crucial role in the kinetics of oxidative additions by controlling the concentration of the reactive Pd0(PPh3)2 in its equilibrium with the major but unreactive complex Pd0(η2-dba-H)(PPh3)2 (equilibrium constant KZ in Scheme 3). The introduction of electron-donating Z-groups results in a lower affinity of 1 for the electron rich Pd0(PPh3)2, resulting in higher KZ leading to the overall oxidative addition being faster (Scheme 3). In this context, the influence of ligand 2a (th2-dba) or 3a (th1dba) on the rate of oxidative additions has been compared to the parent ligand dba-H.5 PPh3 (2 equiv) was added to either Pd02(th1-dba)3.th1-dba, Pd02(th2-dba)3.th2-dba or Pd02(dbaH)3.dba-H (named for convenience in the following section: Pd0(th1-dba)2, Pd0(th2-dba)2 and Pd0(dba-H)2, respectively) at a concentration of 2 mM in DMF. Cyclic voltammetry was performed to identify the Pd0 complexes generated in situ. In both cases, two successive oxidation peaks were observed as was the case for dba-H5 (Figure 6 for th1-dba). The oxidation

peak located at O2 characterizes Pd0(η2-th1-dba)(PPh3)2 (Figure 6a, Table 1). The oxidation peak at O1 characterizes Pd0(PPh3)2 in equilibrium with Pd0(η2-th1-dba)(PPh3)2 (left equilibrium in Scheme 4). In the presence of 4 equiv of PPh3 the first oxidation peak current at O1 increased at the expenses of that of Pd0(η2-th1dba)(PPh3)2 due to the formation of Pd0(PPh3)3 involved in a fast equilibrium with Pd0(PPh3)2, so that they exhibit a common oxidation peak at O14,5 (Figure 6b, Scheme 4). Similarly, the addition of PPh3 (2 equiv) to Pd0(th2-dba)2 (2 mM) also resulted in the formation of Pd0(η2-th2-dba)(PPh3)2 in equilibrium with Pd0(PPh3)2 and Pd0(PPh3)3 in the presence of excess PPh3 (4 equiv) (Scheme 4, Table 1). The kinetics of the oxidative addition of PhI to the Pd(0) complexes generated from Pd0(thn-dba)2 (n ) 1 of 2) and PPh3 (2 or 4 equiv) has been investigated in DMF by means of electrochemical techniques, as reported for Pd0(dba-H)2.4,5 From the kinetic curves shown in Figure 7, it emerges that substitution of one or two phenyl groups in dba by a thienyl group did not affect the rate of the oxidative addition at equal PPh3 loading. The rate was only affected by the concentration of PPh3 added to the Pd(0) precursor (whatever the precursor): t1/2 ) 115 ((5) s for 2 equiv of PPh3 and t1/2 ) 180 ((5) s for 4 equiv of PPh3 due to the partial formation of the unreactive Pd0(PPh3)3, as usually observed.5 From these unexpected results, one concludes that at equal PPh3 loading, the concentration of the common reactive Pd0(PPh3)2 was similar in its equilibrium with Pd0(thndba)(PPh3)2 (n ) 1 or 2) or with Pd0(η2-dba-H)(PPh3)2. Consequently, the equilibrium constant Kthn is not significantly affected by the substitution of the phenyl groups in dba-H by either one or two thienyl groups. Comparative Reactivity of PhI with the Pd(0) Complexes Generated from Pd(dba-H)2 or Pd(th2-dba)2 Associated with 1 equiv of dppe in DMF. As observed for Pd0(dba-H)2,18 the addition of dppe (1 equiv) to Pd0(th2-dba)2 (2 mM) in DMF, led at short times to the formation of Pd0(dppe)2 characterized (18) (a) Amatore, C.; Broeker, G.; Jutand, A.; Khalil, F. J. Am. Chem. Soc. 1997, 119, 5176–5185. (b) Paladino, G.; Madec, D.; Prestat, G.; Maitro, G.; Poli, G.; Jutand, A. Organometallics 2007, 26, 455–458.

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dba-H)(dppe) than with Pd0(η2-th2-dba)(dppe) (KH > Kth2, Scheme 5) or/and that Pd0(η2-dba-H)(dppe) is intrinsically more reactive than Pd0(η2-th2-dba)(dppe) (kH > kth2). From these kinetic studies the rate of oxidative addition with PhI could be modulated (slowed) by substitution of one or two phenyl groups of dba-H by a thienyl group only in the case of a bidentate ligand such as dppe. This could be potentially an advantage in reactions where oxidative addition is fast (as is the case for activated organohalides, or electrophilic substrates for nucleophilic allylic substitution processes), leading to the accumulation of higher concentrations of PdII intermediates. Figure 7. Kinetics of the oxidative addition of PhI (2 mM) to Pd0(PPh3)2 generated from: (b) Pd0(dba-H)2 (2 mM) + PPh3 (4 mM); (9) Pd0(th1-dba)2 (2 mM) + PPh3 (4 mM); ([) Pd0(th2-dba)2 (2 mM) + PPh3 (4 mM) in DMF at 20 °C. Plot of the molar fraction x of Pd0(PPh3)2 (x ) [Pd0]t/[Pd0]0 ) i/i0; i: oxidation current of Pd0(PPh3)2 at t, i0: initial oxidation current of Pd0(PPh3)2). Kinetics of the oxidative addition of PhI (2 mM) to the Pd0(PPh3)2 complex generated from (O) Pd0(dba-H)2 (2 mM) + PPh3 (8 mM); (0) Pd0(th1-dba)2 (2 mM) + PPh3 (8 mM); (]) Pd0(th2-dba)2 (2 mM) + PPh3 (8 mM) in DMF at 20 °C.

Figure 8. Kinetics of the oxidative addition of PhI (200 mM) to the Pd0 complexes generated from: (b) Pd0(dba-H)2 (2 mM) + dppe (2 mM); (() Pd0(th2-dba)2 (2 mM) + dppe (2 mM) in DMF at 20 °C. Scheme 5

by an oxidation peak in cyclic voltammetry (at -0.41 V vs SCE). After 10 min, the cyclic voltammogram exhibited the oxidation peak for Pd0(η2-th2-dba)(dppe) (major one) and that of Pd0(dppe) in equilibrium (Table 1, Scheme 5). The oxidative addition of PhI was slower than that involving PPh3 as ligand. The kinetics of the oxidative addition was investigated at 20 °C in the presence of a large excess of PhI (100 equiv). From the kinetic curves shown in Figure 8, it emerges that the substitution of the two phenyl groups in dba by two thienyl groups affected the rate of the oxidative addition. The oxidative addition was indeed slower when using Pd0(th2dba)2 as precursor instead of Pd0(dba-H)2: t1/2 ) 6000 ((50) s and t1/2 ) 1800 ((50) s, respectively. From previous work it is known that Pd0(η2-dba-H)(dppe) and Pd0(dppe) react in parallel with PhI.18b The fact that {Pd0(dba-H)2 + dppe} was more reactive than {Pd0(th2-dba)2 + dppe} suggests that either the Pd0(dppe) concentration is higher in its equilibrium with Pd0(η2-

{Pd0(dba-H)2 + 2PPh3} ) {Pd0(th1-dba)2 + 2PPh3} ) {Pd0(th2-dba)2 + 2PPh3} (1) {Pd0(dba-H)2 + dppe} > {Pd0(th2-dba)2 + dppe}

(2)

{Pd0(th2-dba)2 + 2PPh3} > {Pd0(th2-dba)2 + 2dppe} (3) In summary, we have prepared heteroaromatic analogues of dibenzylidene acetone. Only the thienyl containing analogues were able to form new binuclear Pd0 complexes, similar to Pd02(dba-H)3 · dba-H. The reactivity of both Pd02(th1-dba)3 · th1dba and Pd02(th2-dba)3 · th2-dba toward PPh3 and dppe and the kinetics of the subsequent oxidative addition reactions of in situ generated Pd0L(PPh3)2 and Pd0L(dppe) complexes with PhI have been determined. In the case of PPh3 it was shown that there is essentially no difference in the rate of oxidative addition with PhI. However, the th2-dba ligand was able to slow oxidative addition when the bidentate ligand, dppe, was used. More broadly, the het-dba analogues reported herein could be useful diene ligands for many other transition metal complexes, where dba-H coordination is reported, e.g., [Rh(η5C5Me5)(η2,η2-dba-H)],19 [Fe(CO)3[η4-{(CO)(CH)CHPh)2}],20 and other Fe21 and Ru22 complexes. We are currently exploring the catalytic applications of Pd02(th1-dba)3 · th1-dba and Pd02(th2dba)3 · th2-dba complexes.

Experimental Section General experimental details are given in the Supporting Information. The following compounds were prepared according to literature procedures12 (characterization data can be found in the Supporting Information): (1E,4E)-1,5-di(2-thienyl)penta-1,4dien-3-one (2a), (1E,4E)-1-phenyl-5-(2-thienyl)penta-1,4-dien-3one (3a), (1E,4E)-1,5-di(2-furyl)penta-1,4-dien-3-one (2b) and (1E,4E)-1-phenyl-5-(2-furyl)penta-1,4-dien-3-one (3b). (1E,4E)-1,5-Di(2-pyridinyl)penta-1,4-dien-3-one (2c). A 25 mL flask was charged with 1,3-bis(diethylphosphonato)acetone (0.50 g, 1.51 mmol) and 2-pyridinecarboxaldehyde (0.324 g, 3.02 mmol). A solution of potassium carbonate (2.80 g, 20.3 mmol) in water (2.5 mL) and ethanol (1.5 mL) were added and the biphasic mixture was stirred rapidly at room temperature for 1 h. The reaction mixture was extracted with ethyl acetate (10 mL) and the extract was (19) (a) Lee, H. B.; Maitlis, P. M. J. Organomet. Chem. 1973, 57, C87– C89. (b) Ibers, J. A. J. Organomet. Chem. 1974, 73, 389–400. (20) Berne`s, S.; Toscano, R. A.; Cano, A. C.; Mellado, O. G.; AlvarezToledano, C.; Rudler, H.; Daran, J. -C. J. Organomet. Chem. 1995, 498, 15–24. (21) (a) Alvarez-Toledano, C.; Delgado, E.; Donnadieu, B.; Hernandez, E.; Martin, G.; Zamora, F. Inorg. Chim. Acta 2003, 351, 119–122. (b) Ortega-Jimenez, F.; Ortega-Alfaro, M. C.; Lopez-Cortes, J. G.; GutierrezPerez, R.; Toscano, R. A.; Velasco-Ibarra, L.; Pena-Cabrera, E.; AlvarezToledano, C. Organometallics 2000, 19, 4127–4133. (c) Rivomanana, S.; Mongin, C.; Lavigne, G. Organometallics 1996, 15, 1195–1207. (22) Osintseva, S. V.; Dolgushin, F. M.; Shtel´tser, N. A.; Petrovskii, P. V.; Kreindlin, A. Z.; Rybin, L. V.; Antipin, M. Y. Organometallics 2005, 24, 2279–2288.

Heteroaromatic Analogues of Dibenzylideneacetone purified by rapid column chromatography on silica (eluent: ethyl acetate to ethyl acetate/ethanol 90:10, v/v). Collected fractions were evaporated in Vacuo to yield the product (0.296 g, 83%). 1H NMR (400 MHz, CDCl3) δ (ppm): 8.68 (ddd, J ) 4.8, 1.7, 0.8 Hz, 2H), 7.75 (d, J ) 15.6 Hz, 2H), 7.73 (td, J ) 7.7, 1.8 Hz, 2H), 7.61 (d, J ) 15.7 Hz, 2H), 7.49 (dt, J ) 7.8, 1.2 Hz, 2H), 7.29 (ddd, J ) 7.6, 4.8, 1.1 Hz, 2H). 13C NMR (100 MHz, CDCl3) δ (ppm): 189.5, 153.2, 150.2, 142.1, 136.8, 128.7, 124.9, 124.4. HRMS (ESI): calculated for C15H13N2O [M + H+], 237.1022; found, 237.1020. Pd02(th2-dba)3 · th2-dba. Following the reported procedure for Pd02(dba-H)3.dba-H.15 A 100 mL three-necked flask was charged with sodium acetate (0.9278 g, 11.3 mmol) and 2a (1.1496 g, 4.67 mmol) and flushed with nitrogen. Methanol (36 mL) was added and the reaction mixture was heated to 60 °C until a clear solution was obtained. Palladium(II) chloride (0.250 g, 1.41 mmol) was added and the reaction mixture was heated to 40 °C for 4 h. After cooling to room temperature, the dark precipitate was filtered off, washed with water (2 × 10 mL), acetone (5 mL) and dried in Vacuo to yield the title compound as a dark solid (0.6966 g, 82%). Anal. Calcd for C26H20O2PdS4: C, 52.12; H, 3.36. Found: C, 52.27; H, 3.26. This satisfies the composition Pd2L3.L (i.e., PdL2). The complex does not dissolve well in common organic solvents. However, in DMF and DMSO, a pale brown turbid solution is obtained after ca. 30 min, which is likely the result of decomposition (formation of palladium particles). MS (ESI): 739, 593, 515, 397. A 1H NMR spectrum (500 MHz, CDCl3) is illustrated in Figure 3. A 13C solid-state NMR spectrum (100 MHz) is illustrated in Figure 4. Pd02(th1-dba)3 · th1-dba. Following the reported procedure for Pd02(dba-H)3.dba-H.15 A 100 mL three-necked flask was charged with sodium acetate (0.9286 g, 11.3 mmol) and 3a (1.121 g, 4.67 mmol) and flushed with nitrogen. Methanol (36 mL) was added and the reaction mixture was heated to 50 °C for 5 min until a clear solution was obtained. Palladium(II) chloride (0.250 g, 1.41 mmol) was added and the reaction mixture was heated to 40 °C for 4 h. After cooling to room temperature, the dark precipitate was filtered off, washed with water (5 mL), acetone (2 mL) and dried in Vacuo to yield the title compound as a dark purple solid (0.7441 g, 90%). Anal. Calcd for C30H24O2PdS2: C, 61.38; H, 4.12. Found: C, 61.47; H, 4.04. This satisfies the composition Pd2L3.L (i.e., PdL2). The solubility appears to mirror the parent complex, Pd2(dba-H)3 · dba-H, i.e. very well soluble in aromatic and chlorinated solvents and insoluble in hydrocarbons and alcohols. MS (ESI): 627, 581, 503, 479. A 1H NMR spectrum (500 MHz, CDCl3) is illustrated in Figure 3. A 13C solid-state NMR spectrum (100 MHz) is illustrated in Figure 4.

Organometallics, Vol. 28, No. 3, 2009 829 General Procedures for Cyclic Voltammetry and the Kinetics of the Oxidative Addition. Experiments were carried out in a three-electrode thermostated cell (20 °C) connected to a Schlenk argon line. The counter electrode was a platinum wire of ca. 1 cm2 apparent surface area. The working electrode was steady gold disk (d ) 2 mm). The reference was a saturated calomel electrode separated from the solution by a bridge filled with 0.3 M nBu4NBF4 solution in DMF (1.5 mL). DMF (15 mL), containing nBu4NBF4 (0.3 M), were poured into a cell, followed by the addition of PPh3 (15.7 mg, 60 µmol, 4 mM), then Pd0(th1-dba)2 (17.6 mg, 30 µmol, 2 mM). The cyclic voltammetry was performed at a scan rate of 0.5 Vs-1. The kinetics of the oxidative addition of PhI (3.4 µL, 30 µmol, 2 mM) was followed by amperometry at a rotating gold disk electrode (d ) 2 mm, EDI 65109, Radiometer) with an angular velocity ω ) 105 rad.s-1. The rotating electrode was polarized at +0.2 V on the plateau of the oxidation wave of Pd0(PPh3)2. The decrease of the oxidation current was recorded versus time after addition of PhI up to 100% conversion at 20 °C. The same experiments were performed in the presence of PPh3 (31.4 mg, 120 µmol, 8 mM). Similar experiments were performed with Pd0(th2dba)2 (17.9 mg, 30 µmol, 2 mM) in the presence of PPh3 (15.7 mg, 60 µmol, 4 mM; or, 32 mg, 120 µmol, 8 mM). Similar experiments were performed with Pd0(th2-dba)2 (17.9 mg, 30 µmol, 2 mM) in the presence of dppe (12 mg, 30 µmol, 2 mM). The kinetics of the oxidative addition was investigated with PhI (340 µL, 3 mmol, 200 mM).

Acknowledgment. We are grateful to the EPSRC for funding (EP/D078776/1), the Royal Society and AstraZeneca (Dr. D. M. Hollinshead) for an unrestricted research award (to I.J.S.F). A generous loan of palladium salts from Johnson-Matthey (I.J.S.F. and A.J.) is acknowledged. We thank the EPSRC Service at Durham University (U.K.) for running the 13C solid-state NMR experiments (Dr. D. C. Apperley). Supporting Information Available: Experimental details (including characterization data for 2a, 3a, 2b and 3b), figures showing TGA results, and tables of X-ray data (for 2c and 3a), and a cif file containing the crystallgraphic data for 2c and 3a. This material is available free of charge via the Internet at http://pubs.acs.org. OM800975W