Thiophene electrophilic palladation

Heteroaromatics have high reactivity toward electrophilic palladation and show good regioselectivity. Reactions with pyrrole,thiophene, furan, and indole have been reported (equation 3). The use of stoichiometric copper(II) ion gives a process catalytic in Pd. 

The electron-rich nature of heterocycles such as indoles, furans and thiophenes allows a different type of Heck reaction to be carried out. In this oxidative modification, the aryl palladium derivative is generated by electrophilic palladation with a palladium(II) reagent. 

The electron-rich nature of heterocycles such as indoles, furans, and thiophenes allows a different type of Heck reaction to be carried out. " In this oxidative modification the aryl palladium derivative is generated by electrophilic palladation with a palladium(II) reagent. This process is not catalytic in the standard way, but can be made so by the addition of a reoxidant selective for Pd(0) note, that the catalytic Pd(0) could not effect the first (electrophilic) ring palladation. " ... 

The order of reactivity for the reaction sites of azole compounds in electrophilic reactions is known to be 5 >4> 2 [18]. Thus arylation at the relatively electron-rich 5-position may be regarded as similar to that of pyrroles, furans, and thiophenes (Scheme 1). In contrast, the reaction at the 2-position may be regarded as proceeding differently [3], Although the precise mechanism is still unclear, it may involve base-assisted deprotonative palladation with the ArPd(II) species (path d in Scheme 2). Insertion of the C=N double bond into the Ar-Pd bond is also a possi-... 

The Pd(II)-mediated reaction of benzene with alkenes affords styrene derivatives 164. The reaction can be vmderstood by palladation, insertion of olefin to give 163, and y3-H elimination [67,68]. In addition to benzene and naphthalene derivatives, electron-rich heteroaromatic compounds such as ferrocene, furan and thiophene react with alkenes to give vinyl heterocycles. The effect of substituents in this reaction is similar to that observed in the electrophilic aromatic substitution [69]. 

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