Heck reaction

The Heck reaction is one of the best-studied organometallic reactions and is widely used in organic synthesis. For her it is a palladium -catalyzed cross-coupling. It allows for the direct olefination of aryl halides, wherein the halide is replaced by the corresponding alkenyl. It can be simple alkenes, aryl-substituted alkenes or electrophilic alkenes such as acrylic esters. The Heck reaction is related to the Suzuki coupling, and accordingly mechanistically similar.

This reaction was developed in 1972 by the American chemist Richard F. Heck, after which it was named, as well as John Paul Nolley Jr. of the University of Delaware. For his research in the field of palladium -catalyzed cross couplings in organic synthesis Heck, in 2010, together with the two Japanese chemists Ei-ichi Negishi and Akira Suzuki the Nobel Prize for Chemistry.

Mechanism

The catalytic cycle begins with the oxidative addition step (A) of the halide to the palladium ( 0) species ( 1), formally, a palladium (II ) complex ( 2) is formed, then a π - complex ( 3) alkene component inserted (B). Of Pd (II) σ intermediate ( 4) is initially cleaved, the alkene ( 5) by β - hydride elimination, (C ), then by a reductive elimination of (D) of HX from 6 the Pd (0) species ( 1) regressed. The liberated HX is bound by the base.

Wherein the Heck reaction is selectively obtain the trans-substituted double bond. The reason for this is that first runs a syn -addition, the final β - hydride elimination, (C), however, in turn, extends syn.

For this reason, run around the CC single bond and ultimately yields the trans product meantime rotation.

Depending on whether the double bond of the olefin used acceptor (EEC, electron Withdrawing group) or donor- substituted (EDG, electron donating group) is, different isomers arising from the Heck reaction. While acceptor -substituted olefins rather provide the trans product, more products are obtained with a terminal double bond of donor -substituted olefins. While this selectivity for the acceptor - substituted olefins is usually very high, strong mixtures of terminal double bond and trans product are obtained in the case of donor-substituted olefins. This can be explained on the electrophilicity of the cationic Pd ( II) species, which are added in the insertion step (B ) itself to the electron-deficient position. Pd (II ) species acts as a Lewis acid which is analogous to the addition of a proton to a double bond. The addition of a Lewis acid, the adjacent C atom is strongly positive law. The addition takes place so that the positively charged carbon atom is better stabilized, ie in the alpha- position to an EDG, but at a terminal EEC. At this position then to the Pd -bound rest is transmitted.