Heck reaction cyclopalladation

Scheme8.22. Cyclopalladation during Pd-mediated homocoupling of aryl iodides [156] and Heck reaction with norbornene [157]. Ar = 2-(tBu)C6H4.

Yang, R, Zhang, Y.M., Zheng, R. et al. (2002) Cyclopalladated complexes of tertiary arylamines as highly efficient catalysts using in the Heck reactions. /. Organomet. Chem., 651, 146-8. 

In this section, only examples of Mizoroki-Heck reactions where a proper addition of the cr -aryl- or a -alkeny Ipalladium(II) complex to a double bond of an alkene or alkyne occurs are considered. As a consequence, an often-met deviation from the classic Mizoroki-Heck mechanism, the so-called cyclopalladation, will not be treated in further detail [12, 18]. However, as it is of some importance, especially in heterocycle formation and mainly because it will be encountered later during polycyclization cases, it shall be mentioned briefly below. Palladacycles are assumed to be intermediates in intramolecular Mizoroki-Heck reactions when j3-elimination of the formed intermediate cannot occur. These are frequently postulated as intermediates during intramolecular aryl-aryl Mizoroki-Heck reactions under dehydrohalogenation (Scheme 6.1). The reactivity of these palladacycles is strongly correlated to their size. Six-membered and larger palladacycles quickly undergo reductive elimination, whereas the five-membered species can, for example, lead to Mizoroki-Heck-type domino or cascade processes [18,19]. 

In 1999, cyclopalladated tolylphosphine compounds were reported to show highly catalytic activities, not only for Heck reactions of which the turnover numbers (TONs) are up to 1,000,000, for example, but also for all other metal-catalyzed cross-coupling reactions [115, 116]. In 2005, Heck reactions with naphthyl phosphines [117] or 77-heterocyclic carbene phosphapalladacycles [118] were also reported to show highly catalytic activities, as evidenced by their TONs of up to 300,000 and 10,800, respectively. 

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. ... 

Scheme 36, A and Turnover numbers up to approximately 500,000 were reported for the reaction of iodobenzene with methyl acrylate and an impressive nnmber (about 100,000) was also achieved with bromobenzene and 4-methoxy-bromobenzene. Characteristic featnres of these tridentate PCP catalysts are thermal stability and air stability. A competitive experiment, including 4-bromoiodobenzene, iodobenzene, 4-methyliodoben-zene, and 4-methoxy-iodobenzene with methyl acrylate provided a linear correlation with Hammet cr-valnes, which, however, exhibited a low p valne (1.39). It was concluded that nucleophilic aromatic substitution is not rate determining, bnt a snbsequent step with different electronic requirements, such as alkene insertion, may acconnt for this observation. Chlorobenzene could not be coupled with these catalyst. Very recently, Milstein s group reported new cyclopalladated, phosphine-free imine complexes as catalysts in the Heck arylation reaction (Scheme 36, The new dimeric imine complexes show... 

This section gives a concise overview on the construction of heterocycles by Heck-type reactions all reactions covered here mechanistically start with an oxidative addition of a C-haUde or another appropriate C-heteroatom bond to a Pd(0) species and go on either with a carbopalladation step (intramolecular in Sect. B and intermolecular in Sect. C) or a cyclopalladation step (Sect. D). The examples presented in the subsections are organized according to types of starting materials, since certain substructures turned out to be typical. 

Ndjera C, Alonso DA (2008) Appheation of cyclopalladated compounds As catalysts for Heck and Sonogashira reactions. In Dupont J, Pfeffer M (eds) Palladacycles. Synthesis, characterization and appheations. Wiley-VCH, Weinheim, p 155... 

R. F. Heck, G. Wu, W. Thao, and A. L. Rheingold, Chem. Ind., 1990, 40, 169-188. Alkene and Alkyne Reactions with Cyclopalladated Organopalladium Complexes. 

---