Organopalladium intermolecular reaction

The reaction is thought to proceed by co-ordination of the alkene with the organopalladium(II) species, followed by carbopalladation. Subsequent p-hydride elimination regenerates an alkene and releases palladium(II). This is reduced (reductive elimination) to palladium(O) in the presence of a base, to allow further oxidative addition and continuation of the cycle (1.211). The carbopalladation and p-hydride elimination steps occur syn selectively. Excellent regioselectivity, even for intermolecular reactions, is often observed, with the palladium normally adding to the internal position of terminal alkenes (except when the alkene substituent is electron-rich as in enamines or enol derivatives), thereby leading to linear substitution products. 

The intennolecular acylpalladation corresponds to the addition of an acyl-palladium bond onto a rr-bond system of another molecule this elementary step can also be referred to as an insertion (Scheme I). This produces another organopalladium complex, which can in principle participate in subsequent propagation or termination reactions. This excludes processes that involve alkoxycarbonylation (R— = R O—) and hydrocarbonyla-tion (R— = H—). This section will focus on nonpolymeric intermolecular reactions of acylpalladium complexes with different 7r-bond systems (alkenes, imines, dienes, and alkynes). 

Carbopalladation occurs with soft carbon nucleophiles. The PdCl2 complex of COD (100) is difficult to dissolve in organic solvents. However, when a heterogeneous mixture of the complex, malonate and Na2C03 in ether is stirred at room temperature, the new complex 101 is formed. This reaction is the first example of C—C bond formation and carbopalladation in the history of organopalladium chemistry. The double bond becomes electron deficient by the coordination of Pd(II), and attack of the carbon nucleophile becomes possible. The Pd-carbon n-bond in complex 101 is stabilized by coordination of the remaining alkene. The carbanion is generated by treatment of 101 with a base, and the cyclopropane 102 is formed by intramolecular nucleophilic attack. Overall, the cyclopropanation occurs by attack of the carbanion twice on the alkenic bond activated by Pd(II). The bicyclo[3.3.0]octane 103 was obtained by intermolecular attack of malonate on the complex 101 [11]. 

Hosokawa, T., Murahashi, S.-l. Other intermolecular oxypalladation-dehydropalladation reactions. Handbook of Organopalladium... 

Larhed, M. and Hallberg, A. (2002) Scope, mechanism, and other fundamental asf>ects of the intermolecular Heck reaction, in Elandbook of Organopalladium Chemistry for Organic Synthesis, Vol 1 (ed. E.-i. Negishi), John Wiley Sons, Inc., New York, pp. 1133-78. 

An interesting sequential reaction, consisting of an intermolecular alkene carbometallation and subsequent intermolecular alkyne cross-coupling, has been reported (Scheme 8.87) [601]. Starting from an immobihzed tropane framework 459, stoichiometric carbopalladation yields a stable organopalladium intermediate, which in the presence of copper(I) iodide undergoes coupling with an added terminal acetylene. 

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