DPPF ligand formation

Alkyl- and arylmercury(II) halides are used for the ketone formation[402]. When active methylene compounds. such as /f-keto esters or malonates are used instead of alcohols, acylated / -keto esters and malonates 546 are produced, For this reaction, dppf is a good ligand[403]. The intramolecular version of the reaction proceeds by trapping the acylpalladium intermediate with eno-late to give five- and six-membered rings smoothly. Formation of 547 by intramolecular trapping with malonate is an example[404]. 

An interesting parallel was found while the microwave-enhanced Heck reaction was explored on the C-3 position of the pyrazinone system [29]. The additional problem here was caused by the capability of the alkene to undergo Diels-Alder reaction with the 2-azadiene system of the pyrazinone. An interesting competition between the Heck reaction and the Diels-Alder reaction has been noticed, while the outcome solely depended on the substrates and the catalyst system. Microwave irradiation of a mixture of pyrazinone (Re = H), ethyl acrylate (Y = COOEt) and Pd(dppf)Cl2 resulted in the formation of a mixture of the starting material together with the cycloaddition product in a 3 1 ratio (Scheme 15). On the contrary, when Pd(OAc)2 was used in combination with the bulky phosphine ligand 2-(di-t-butylphosphino)biphenyl [41-44], the Heck reaction product was obtained as the sole product. When a mixture of the pyrazinone (Re = Ar) with ethyl acrylate or styrene and Pd(dppf)Cl2 was irradiated at 150 °C for 15 min, both catalytic systems favored the Heck reaction product with no trace of Diels-Alder adduct. 

Due to the potential problems associated with f3-H elimination, the first examples that were reported involved the intramolecular formation of G-O bonds between tertiary alcohols and aryl bromides using Pd(OAc)2 with 2,2 -bis(di-/>-tolylphosphino)-l,l -binapthyl (tol-BINAP) or bis(diphenylphosphino)ferrocene (dppf) as the ligands (Equation (12)).91 Although the coupling with primary and secondary alcohols was troublesome with this system, the more recent introduction of ligands 23-28 (Figure 3) has ameliorated many of these difficulties (Equation (13)).92... 

The Pd-catalysed intermolecular reaction of aryl bromides containing electron-withdrawing substituents with a wide variety of alcohols including MeOH, 2-propanol, benzyl alcohol and i-butyl alcohol gives the aryl ethers 416 under milder conditions than uncatalysed reactions. Bidentate ligands such as BINAP and DPPF (XLIX) are effective [206,207]. The aryl Pd alkoxide 417 was isolated as an intermediate, and the formation of the aryl ethers 418 by reductive elimination of 417 was confirmed. 

The formation of alkoxo intermediates may be occurring when monophosphines are used, but the stability of the amine complexes favors the deprotonation of coordinated amine. Instead, the alkoxo complexes may be important in catalytic systems involving chelating ligands [51]. Indeed, the DPPF complex [Pd(DPPF)(p-Bu C6H4)(0-f-Bu) reacted with diphenyl amine, aniline, or piperidine, as shown in Eq. (48), to give the product of amine arylation in high yields in each case [51]. Since, no alkali metal is present in this stoichiometric reaction, the palladium amide is formed by a mechanism that cannot involve external deprotonation by alkali metal base. 

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