Palladacycles reaction modes

This conversion of an acyclic bisdiene to a cychzed enediene constitutes a new Pd-mediated reaction mode, which can be rationalized as outlined in Scheme 7. Com-plexation of bisdiene 17 following oxidative cyclization (i.e., oxidative addition with intramolecular coupling of the diene moieties) affords palladacycle 20. Protonation in an... 

Phenylhydrazones exhibit another mode of reaction (Scheme 65). The reaction of butadiene with the phenylhydrazone of acetaldehyde 204 (1 mol % (Ph3P)4Pd, THF, 110 °C, 24 h) affords a 2 1 mixture of 205 and 206 (86%). A small amount of the protic (H—Y) trapping prodnct was also observed. The formation of 205 can be rationalized by addition of the phenylhydrazone in the fashion of a R (R )C=Y-type electrophile to a palladacycle in an Sgs fashion to 208 followed by hydride transfer to 209. Rednctive elimination conld acconnt for the formation of 205. Thns, 206 could be formed via a similar pathway, by addition of the phenylhydrazone in an rather than fashion. The proposed hydride transfer invoked to rationalize the formation of the observed products offers interesting possibilities and appears worthy of further investigation. Phenylhydrazones derived from propanal, acetone, and methyl ethyl ketone behave similarly (60-95% yield), although the ratio of 205/206 and the proportion of the protic (H—Y-type) trapping product vary. 

An entirely different mode of interaction of electron-rich cyclopalladated Pd(II) complexes with water has been described, involving the oxidation of Pd(IV) species through oxidative addition of water to Pd(ll) with subsequent elimination of dihydrogen.It is not clear whether this process, proved for Pd complex with a very specific coordination sphere, may have a relation to a hypothetical involvement of Pd(ll)/Pd(IV) catalytic cycles in the reactions generally considered as Pd(0)/Pd(n) processes (Heck reaction, cross-coupling, etc.), particularly in those involving palladacycles as catalysts. ... 

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