Palladium complexes, with cyclic cationic

Beyond palladium, it has recently been shown that isoelectronic metal complexes based on nickel and platinum are active catalysts for diyne reductive cyclization. While the stoichiometric reaction of nickel(O) complexes with non-conjugated diynes represents a robust area of research,8 only one example of nickel-catalyzed diyne reductive cyclization, which involves the hydrosilylative cyclization of 1,7-diynes to afford 1,2-dialkylidenecyclohexanes appears in the literature.7 The reductive cyclization of unsubstituted 1,7-diyne 53a illustrates the ability of this catalyst system to deliver cyclic Z-vinylsilanes in good yield with excellent control of alkene geometry. Cationic platinum catalysts, generated in situ from (phen)Pt(Me)2 and B(C6F5)3, are also excellent catalysts for highly Z-selective reductive cyclization of 1,6-diynes, as demonstrated by the cyclization of 1,6-diyne 54a.72 The related platinum bis(imine) complex [PhN=C(Me)C(Me)N=Ph]2Pt(Me)2 also catalyzes diyne hydrosilylation-cyclization (Scheme 35).72a... 

Furthermore, the decarboxylative allylic alkylation was extended to cyclic dienol carbonates 132 by Cossy and coworkers. Upon treatment with the palladium complex of Trost s ligand (U,U)-14, butenolides 133 with a quaternary stereogenic center were obtained in fair chemical and optical yields (Scheme 5.44) [64]. The preferred Si-face approach of the allylpalladium cation reaction is rationalized by an outer-sphere mechanism, adapting a transition state model related to that shown in Scheme 5.42. 

Cationic BINAP-palladium and platinum complexes 30a,b can catalyze highly enantioselective cycloaddition reactions of arylglyoxals with acyclic and cyclic... 

We mentioned that by mixing vinyl epoxides and zerovalent palladium, the alcoholate formed was usually sufficiently basic to deprotonate the pronucleophile entity. In some cases, especially with ketones, low reactivity and yields were reported (Table To overcome the problem of the weak basicity of the alcoholate, silyl enol ethers, keto adds, or preformed lithium enolates have successfully been employed.f f" f f /3-Keto acids are masked enolates via the decarboxylation of the intermediary Tr-allylpalladium ]3-ketocarboxylate complexes. The main limitation of the use of keto adds as pronucleophiles seems to be their low reactivity toward the hindered cyclic vinyl epoxides. In these cases, the cationic n-allylpalladium complex undergoes ]S-elimination. Indeed, the reaction between benzoyl acetic acid and cyclobutadiene monoxide in the presence of Pd(PPh3>4 gives only the corresponding cyclopentanone and acetophenone as the... 

---