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The +IV Oxidation State in Organopalladium Chemistry RECENT ADVANCES and POTENTIAL INTERMEDIATES in ORGANIC SYNTHESIS and CATALYSIS by Allan J

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The +IV Oxidation State in Organopalladium Chemistry RECENT ADVANCES and POTENTIAL INTERMEDIATES in ORGANIC SYNTHESIS and CATALYSIS by Allan J The +IV Oxidation State in Organopalladium Chemistry RECENT ADVANCES AND POTENTIAL INTERMEDIATES IN ORGANIC SYNTHESIS AND CATALYSIS By Allan J. Canty Department of Chemistry, University of Tasmania, Hobart, Australia The organometallic chemistry of palladium is dominated by the +II oxida- tion state, and the chemistry of complexes containing simple organic groups bonded to palladium in the +I V oxidation state has developed only recently Organic synthesis and catalytic reactions that may involve undetected palla- dium( I V)intermediates have been suggested frequentlx and the new oxidation state +I V chemistry provides some support for these proposals, and gives encouragement for the development of new systems involving palladium( I V). The chemistry of organopalladium( I V) is reviewed here, and possible cat- alytic roles forpalladium( I V)are discussed. The synthesis and decomposition reactions of palladium( I V) complexes provide "models" for catalytic pro- posals. The palladium( I V) complexes are formed by oxidative addition of organohalides to palladium( II) complexes, and most complexes decompose under mild conditions by carbon-carbon bond formation in reductive elimina- tion reactions, for example,for methyl(pheny1) (2,2'-bipyridyl)palladium(I I) as a substrate, oxidative addition of benzyl bromide gives Pd" BrMePh(CH,Ph) (bpy), which reductively eliminates toluene to form the complex Pd"Br(CH,Ph) (bpy). The organometallic chemistry of palladium is and has been reviewed recently (8). The new dominated by the oxidation state +II, and in the chemistry includes reaction systems that are catalytic applications of palladium complexes the ideal for studies of mechanisms in organometal- most common oxidation states are +II and 0 (1). lic chemistry, and that model some of the pro- On many occasions, possible roles for palladi- posed roles for palladium(IV) in catalysis. um(IV) in organic synthesis and catalysis have Synthesis is based on the oxidative addition been proposed (2, 3) but, apart from several reaction of organohalides with palladium(I1) pentafluorophenylpalladium(IV) complexes iso- complexes, as illustrated in Equation (i). The lated in the mid-1970s (4), organopalladium0V) bromine atom in PdBrMe,(CH,Ph) (bpy) may complexes were not characterised until the report also be replaced on reaction with silver salts and of the preparation and crystal structure of a 2,2'- anions to give a range of complexes bipyridyl complex in 1986 (5): PdXMe,(CH,Ph)(bpy), where X may be F, N, or 0,CPh (9). Typical complexes are shown in Figure 1, and X-ray structural studies of sev- eral complexes show the expected octahedral co-ordination characteristic of organoplat- inum(IV) chemistry (8). The organometallic chemistry of palladi- Kinetic studies of the oxidative addition of um(IV) has developed rapidly since 1986 (6-8) Me1 and PhCH,Br to PdMe,(bpy) and Platinum Metals Rev., 1993, 37, (l), 2-7 L Fig. 1 Examples of organopalladium(1V) com- plexes, including complexes with ally1 groups in (a) and (b): (a) R = CHSh, CH,- CH=CHz, CH,-CH=CHPh (8, I- 10); (b) R = Me, CHSh, CH,- Br CH=CH, (6); (c) isomers of PdIMe,Ph(bpy) (11); and complexes of (d) poly(pyra- zol-1-yl)borates (R = H, pz) (8), and (e) 1,4,7-trithiacy- clononane, IpdMe,(BSB)] + X- (X = I; and NO, formed on reaction of the iodide salt with I &NO,) (8312) PdMez(phen) are consistent with the occur- these systems in the solid state has allowed the rence of the classical SN2mechanism, involv- first estimate of palladium-carbon bond ener- ing Pd(I1) as the nucleophile (13). 'H NMR gies, for example -130 kJ/mol for Pd-Me (13a). spectra at low temperature allow detection There is a high selectivity for methyl elimina- of cationic intermediates, see Equation (ii) tion, see Equations (iii), (iv) and (v). (14), and, in support of this mechanism, PdMez(NM&H,CH&IMez) reacts with methyl PdnMePh(bpy)+PhCH,Br+ triflate in CD,CN to form [PdMe,(NMe,- Pd"BrMePh(CH,Ph) (bpy) - CHzCHzNMez)(NCCD,)]'OSOzCF3- (6a) Pd"Br(CH,Ph)(bpy) + MePh (iii) PdIMe,Ph(bpy)- 0.8[PdIPh(bpy) + Me-Me] + 0.2[PdIMe(bpy) + Me-Ph] (iv) Kinetic studies of reductive elimination from In contrast to platinum chemistry, the cations PdIMe,(bpy) indicate that a pre-equilibrium are fluxional, and another illustration of the occurs to form a cation, and that reductive elim- greater lability of palladium(IV) is the occur- ination occurs from a five-co-ordinate cation rence of an equilibrium between PdIMe,(bpy) (or octahedral complex involving solvent co- and [PdMe,(bpy) (NCMe)]T in acetonitrile ordination) (1 3a), for example, for (8). PdBrMe,(CH,Ph) (bpy) in acetone: Most palladium(n3 complexes undergo facile PdBrMe,(CHzPh)(bpy).+ reductive elimination in the solid state and at [PdMe2(CHzPh)(bpy)l'Br---j moderate temperatures in solution, for exam- PdBr(CH,Ph)@py) + Me-Me (v) ple, Reaction (iii) occurs at 0-C in acetone. Differential scanning calorimetry of some of Related work suggests that cation formation Platinum Metals Rev., 1993, 37, (1) 3 CH,Ph Me\ ‘>N> + PhlPdLN Me Br Br Fig. 2 Examples of transfer of alkyl halides from Pd(IV) to Pt(I1) (13b) and Pd(n) (ll),where N-N = 2,2’-bipyridyl, N’-N’= 2,2’-bipyrimidine is also a key step in new redox reaction systems as in Equation (vi) for transfer from Pd(IV) to for alkyl halide transfer from Pd(IV) to Pt(II) or Pd(II), cation formation is expected to enhance Pd(II), as illustrated in Figure 2. In a mecha- nucleophilic attack at an axial benzyl group to nism directly related to oxidative addition, such give PdMePh(bpy) as the leaving group and a I I I I ; +I Me Me A0 &ye II + Pd ? Fig. 3 (a) Possible schemes for coupling of acetanilide with iodomethane, adapted from the presentation in Ref. 2e; (b) adapted from Ref. 16, illustrating that benzenonium species may form via oxidative addition Platinum Merals Rev., 1993, 37, (1) 4 PdOcation which reacts rapidly with bromide. tive addition-reductive elimination sequences resulting in carbon-carbon bond formation (2), PdNBrMePh(CH,Ph)(bpy) + reactions now established as characteristic of PdnMel(bpy) , [Pd"MePh(CH,Ph) (bpy)]' Pd(1V) chemistry (8), and those that require Br- Pd"MePh(bpy)+[PdNMez(CH,Ph)(bpy)]+---' other types of reaction (3), in particular C-H activation via oxidative addition. PdNBrMe,(CH2Ph)(bpy) (4 Coupling reactions to form C-C bonds that Proposals for the involvement of PdOinter- are catalysed by palladium complexes general- mediates in organic synthesis and catalysis fall ly proceed via a Pd(0)-Pd(I1) oxidative addi- into two categories: those that require oxida- tion-reductive elimination cycle (15), but the Pd4Br LxB: RX = Me1 CHo=CHCHzBr CHz=CHCHzCI Fig 4 (a) Scheme for the syntheais of hexahydromethanoh.ipheoylen~,adapted from the pre- sentation of mechanism in Refs. 2g and 2h; (a) model reactions for oxidative addition to Pd(II) and reductive elimination from Pd(IV) (2g, 7) Platinum Metals Rev., 1993, 31, (1) 5 development of an extensive chemistry of PdO oped, using a phenanthroline complex (2g, 7) indicates that Pd(II)-Pd(IV) cycles are feasible. (Figure 4b). For catalyses that may involve Pd(I1)-Pd(IV) However, the model reactions to date employ sequences, intermediates may be either octa- alkyl or ally1 halides for oxidative addition, hedral, with PdIh4e3(bpy) and related complexes whereas the catalysis proposal requires aryl as models, or five-co-ordinate with cationic halide oxidative addition. The interaction of species such as those shown in Equations (ii) aryl halides with palladium(I1) has yet to be and (v) as models. explored, although precedents do exist for Examples of proposed catalyses are illustrat- oxidative addition of aryl halides to platinum(lI) ed in Figures 3a and 4a. For the coupling of (17). acetanilide with iodomethane (Figure 3a) the Organopalladium(IV) chemistry is providing Pd(I1) cyclometallated complex is known to be new comparisons of structure, solution dynam- an intermediate, and formation of the product ics, and reactivity among the nickel triad ele- could occur via either a benzonium Pd(TI) inter- ments, and its development is commencing mediate or an oxidative addition Pd(IV) inter- some 80 years after the synthesis of plat- mediate (2e). Choosing between these possi- inumo complexes by Pope and Peachey (18). bilities is not straightforward, although it is The new chemistry is providing a firmer basis interesting to note that theoretical calculations for proposals involving palladium(IV) in catal- (1 6a) suggest that related Pt(I1) benzonium ysis, in particular the occurrence of oxidative species (16b) are formed via oxidative addition addition-reductive elimination sequences for (Figure 3b). The benzenonium species may also carbon-carbon bond formation. be regarded as possible intermediates in reduc- tive elimination from aryl(alky1)palladiumOV) Acknowledgements complexes, such as in Reactions (iii) and (iv). This work was hded by grants hrnthe Ausaalian The formation of hexahydromethanotriph- Research Council, and travel grants for collaborative studies hrnthe Ian Potter Foundation (Australia), the enylenes and related molecules using palladi- Department of Industry, Technology, and Commerce um(0) catalysts is believed to involve both PdO (Australia), the Australian National University, the and Pd(IV) intermediates (2g, 2h, 2j), as illus- Natural Sciences and Engineering Research Council (Canada), and the Netherlands Organisation for trated in Figure 4a. Model reactions for the Scientific Research. Johnson Matthey provided gen- Pd(II)-PdO-Pd(IL) sequence have been devel- erous loans of palladium salts. References 1 R. F. Heck, “Palladium Reagents in Organic 3 (a) A. D. Ketley, L. P. Fisher, A. J. Berlin, C. R. Synthesis”, Academic, New York, 1985 Morgan, E.
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