Cyclopalladated complex

The cyclopalladated complexes of hydrotiis(pyrazol-l-yl)borate and tetrakis (pyrazol- l-yl)borate have a common feature of bidentacy of the pyrazol- 1-ylborate... 

Another palladium complex, namely, a six-membered cyclopalladate complex of 2-benzoyl pyridine, has also been used for the hydrogenation of polymers [77, 78]. Possible catalytic mechanisms for the hydrogenation of natural rubber [76] and NBR [77] catalyzed by these two complexes were proposed, but unfortunately the authors did not provide sufficient evidence to support their proposed mechanisms. 

Cyclopalladated complexes provide a potential use for regiospecifically controlled organic syntheses.1 Various substituents can be introduced onto the palladated carbon position by reactions of cyclopalladated complexes with various reagents.2-4 Cyclopalladation of aryl-substituted nitrogen or phosphorus bases has been performed mainly by use of [PdCl4]2- or PdCl2(NCPh)2 in the presence or absence of an acetate salt, but this method appears to have limit in applicability.5,6 Here, we describe a preparation of a rare six-membered cyclopalladated complex of 2-benzylpyridine by use of palladium(II) acetate,7 which seems to be a better starting material for syntheses of cyclopalladated species.6,7 Only five reports of six-membered cyclopalladated complexes have appeared.7,8... 

The moderate yield of cyclopalladated products could be increased using ligand exchange reactions starting from another cyclopalladated complex.131 Other thioketones such as thioxanthone, dithioxanthone, and thio-chromone derivatives react in a similar way, affording cyclopalladated compounds.132... 

An intermediate in the reaction of bis(hexafluoroacetylacetonato)-palladium with S = C(C6HdOMe-p)2 to give the cyclopalladated complex 12 could be observed at low temperatures. In the intermediate the thioben-zophenone is 771 coordinated to the metal and one acetylacetonato ligand via the central carbon atom. Subsequent loss of the cr-coordinated acetyl-acetonate afforded 12 (Scheme 4).133... 

Stable, cyclopalladated complexes sometimes may be used in vinyl substitution but these reactions cannot be carried out catalytically. A major limitation is that few cyclopalladated structures are available and only a few types of vinyl substitution products can be made by this procedure. 

The cyclopalladated complexes have limited practical value in organic synthesis because of the high cost of die stoichiometric amount of palladium required. Some compensation for their expense may be obtained, however, if several steps can be saved compared with an alternate method and, of course, the palladium may be recovered relatively easily.18... 

Vinyl substitutions with N,N-dialkylbenzylamine-cyclopalladated complexes have been studied most thoroughly. In the presence of triethylamine the substitution occurs quite selectively with styrene derivatives19 and a,p-unsaturated carbonyl compounds.20 For example, chloride-bridged cyclopalladated NJ4-dimethylbenzylamine dimer and methyl vinyl ketone give 92% of o-dimethylaminomethylbenzalacetone in 1 h at 110 C (equation 6). 

Ryabov, A. D. Cyclopalladated complexes in organic synthesis. Synthesis 1985, 233-252. Grigg, R. MacLachlan, W. S. MacPherson,... 

Camargo M, Dani P, Dupont J, de Souza RF, Pfeffer M, Tkatchenko I (1996) Cationic cyclopalladated complexes new catalyst precursors for the telomerization of butadiene with alcohols. J Mol Catal A Chem 109 127-131... 

Treatment of the cyclopalladate complex (61) with with diphenylacetylene (DPA) gives the new complex (62), which on treatment with AgBF4 and subsequent thermolysis in chlorobenzene yields the dibenzo[M]thiepine salt (63) (Scheme 9). Longer treatment of the salt in chlorobenzene results in conversion to the thiepine (64) <95JOci005>. 

Although introduced separately, transmetalation (catalytic Cycle 2) and cyclopal-ladation (catalytic Cycle 1) were found to be competitive processes under given arylation reaction conditions. Because 6 is ineffective in catalyzing the desired transformation, any condition that favors its formation (fast C-H bond activation before transmetalation) would inhibit the arylation process. Thus, substrates that form stable five membered cyclopalladated complexes will fail to undergo the arylation reaction (see Section 1.4.3). 

Apart from these successful transformations, the method has its limitations. As already mentioned, the success of this arylation and alkenylation reaction depends on a delicate balance of rates of transmetalation and C-H bond activation. For substrate 19, rapid benzylic C-H bond activation is favored over formation of the six-membered cyclopalladated intermediate, which in turn gives rise to complex 20 instead of the arylation reaction (Scheme 6). Furthermore, substrate 21, does not undergo the arylation reaction but follows the competitive pathway to form five membered cyclopalladated complex 22. 

Arylation of a cyclopalladated complex derived from tri-(o-tolyl)-phosphine with phenylstannane J. Louie, J. F. Hartwig, Angew. Chem. Int. Ed. 1996, 35, 2359-2361. 

Coupling of a Schiff base cyclopalladated complex with boronic acids ... 

Bincoletto C, Terariol ILS, Oliveira CR, Dreher S, Fausto DM, Soufen MA, Nascimento FD, Caires ACF (2005) Chiral cyclopalladated complexes derived from N, N-dimethyl-1-phenethylamine with bridging bis(diphenylphosphine)ferrocene ligand as inhibitors of the cathepsin B activity and as antitumoral agents. Bioorg Med Chem 13 3047-3055... 

Scheme 6.16 Homocoupling of iodobenzene with a cyclopalladated complex...

Miscellaneous Reactions of Phosphines. Procedures for the resolution of benzylcyclohexylphenylphosphine have been developed, involving adduct formation with cyclopalladated chiral amine complexes. A similar approach has also been used for the resolution of P-chiral secondary phosphines, e.g. (193). Treatment of t-butyl(di-o-tolyl)phosphine with potassium tetrachloro-palladate(ii) yields a cyclopalladated complex (194), involving chiral phos-... 

Analogous cyclopalladated complexes were also isolated and similar complexes of substituted bipyridines and 2-phenylaniline also exhibit high... 

Cyclopalladated complexes in organic synthesis , Ryabov, A. D., Synthesis, 1985, 233 Mechanisms of intramolecular activation of C-H bonds in transition metal complexes , idem, Chem. Rev., 1990, 90, 403. 

Following some earlier observations [18], the chemistry of cyclopalladated complexes has been greatly developed in the last ten years [31]. Recently, systematic investigations on the luminescence of cyclometallated Pd(II) complexes have also appeared. Most of the available data are gathered in Table 2. 

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