Heck reaction
The Heck reaction is one of the best-studied organometallic reactions and is widely used in organic synthesis. It is a palladium - catalyzed coupling. It enables the direct olefination of aryl halides , the halide being replaced by the corresponding alkenyl group. Simple alkenes, aryl-substituted alkenes or electrophilic alkenes such as acrylic esters can be used. 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 whom it was also named, and John Paul Nolley jr. from the University of Delaware . For his research in the field of palladium-catalyzed cross-coupling in organic synthesis, Heck received the Nobel Prize in Chemistry in 2010 together with the two Japanese chemists Ei-ichi Negishi and Akira Suzuki .
mechanism
The catalytic cycle begins with the oxidative addition ( A ) of the halide to the palladium (0) species ( 1 ), formally forming a palladium (II) complex ( 2 ), which is then transformed into a π complex ( 3 ) the alkene component is inserted ( B ). The alkene ( 5 ) is first split off from the Pd (II) σ intermediate ( 4 ) with β-hydride elimination ( C ), then the Pd (0) species is formed by reductive elimination ( D ) of HX from 6 ( 1 ) regressed. The released HX is bound by the base.
In the Heck reaction, the trans -substituted double bond is selectively obtained. The reason for this is that a syn addition initially takes place, but the final β-hydride elimination ( C ) is again syn .
For this reason, a rotation around the CC single bond has to take place in the meantime and the trans product is ultimately obtained .
Depending on whether the double bond of the olefin used is acceptor (EWG, electron withdrawing group) or donor- substituted (EDG, electron donating group), different stereoisomers result from the Heck reaction. While acceptor-substituted olefins tend to provide the trans product, products with a terminal double bond are more likely to be obtained from donor-substituted olefins. While this selectivity for acceptor-substituted olefins is usually very high, in the case of donor-substituted olefins strong mixtures of terminal double bond and trans product are obtained. This can be explained by the electrophilicity of the cationic Pd (II) species, which itself adds to the more electron-rich position in the insertion step ( B ). The Pd (II) species acts as a Lewis acid that is analogous to the addition of a proton to a double bond. By adding a Lewis acid, the neighboring carbon atom is strongly positive. The addition takes place in such a way that the positively charged carbon atom is better stabilized, i.e. H. in α-position to an EDG, but terminally to an EWG. The residue bound to the Pd is then transferred to this position.
See also
literature
- Richard Fred Heck, John Paul Nolley Jr. (University of Delaware): Palladium-catalyzed vinylic hydrogen substitution reactions with aryl, benzyl, and styryl halides . In: J. Org. Chem. Band 37 , no. 14 , 1972, p. 2320-2322 , doi : 10.1021 / jo00979a024 (received January 13, 1972).
