Article reviewPalladium catalyzed Heck ReactionAbstract:The Heck reaction is one of the most studied coupling reactions and was awarded the Nobel Prize in Chemistry. It has been a famous topic for all with many studies been done on it. There are various ways of describing Heck reaction and its mechanism. The use of palladium catalyst and also the use of ethylene containing compounds like Et3N has always been the center of studies for most of the studies. Different kind of catalyst could be used for Heck reaction however the brief look at the palladium was studied in this review.Keywords: Heck reaction; catalysis; mechanism; application, carbon-carbon bond forming1. IntroductionHeck is one of the coupling reactions like Heck, Suzuki, Sonogoshira, Negishi, Kumada , Stille , Tsuji-Trost , etc. To shape the chemical synthesis and manipulate it for the product, this type of coupling reactions could be really decisive and important ones. The general Heck reaction could be given asFig1 generalization form of Hecks reaction Heck reaction is used greatly in many syntheses, including drug synthesis, agrochemicals etc. The idea of Heck reaction was proposed by Mizoroki and Heck independently in 1970s. Because of its high efficiency and simplicity, there has been a lot of attention drifted towards these reactions. Synthetic aspect of Heck reaction is due to its high chemo selectivity and mild reaction conditions. The toxicity of the reaction is also low including the low cost of the reagent in the cases of catalytic regeneration through recycling.The Heck reaction generally includes vinylation or arylation of olefins where a large variety of olefins or derivatives of acrylates and styrenes could be used. The reagent (aryl halides) could be any aryl bromides, iodides, aromatic triflates, aroyl chlorides, aryl sulfonyl chlorides, aromatic diazonium salts, aroyl anhydrides, aryl chlorides and aryl silanols. The importance of catalysis in this reaction mechanism could not be under stated where a variety of metals along with a huge range of ligands can be studied. Use of catalysis could really help the overall reaction to be carried out in fewer steps and Palladium is usually the preferred metal as it can be applied with a wide variety of functional groups and it has a remarkable ability to assemble C-C bonds between appropriately functionalized substrates. Most palladium based methodologies proceed with stereo- and region selectivity and with excellent yields and also the less crowded structure is preferred during Heck reaction, and often favours a trans product. Sometimes compounds such as TBAB (Tetra butyl ammonium bromide) are added to the reaction mixture along with organic or inorganic bases needed for the sequestration of acid generated. Mostly used solvents for the Heck reaction are solvents like DMF (dimethyl formamide) and NMP (N-methyl-2-pyrrolidone) but the reaction can also be performed in many other different. Also the recycling of catalyst for its reuse in recent studies has been critical for further improvement of its application field which also include aqueous medium which implies the potential for greener approach for organic reactions.This review has concentrated its discussion on various catalysis techniques with certain aspects and mainly focuses on the use of platinum as a catalyst in the Heck reaction.2. Reviews on Platinum as Heck catalystMostly the studies on the heck reaction depend upon the reaction mechanism and the use of different catalyst type will redirect the mechanism towards different direction finally giving the similar type of product. So use of catalyst which can really shorten the reaction time and mechanism is the must and also the majority of the work on Heck reaction has been focused on the catalyst development.For any catalyst to be suitable for the Heck reaction the properties it should possess is its stability and its ability to make the reaction quick. This review discusses the simple N-heterocyclic carbine palladium based complexes as effective catalyst and it can be used in variety of cross coupling reactions and have been proved to a suitable choice phosphine ligands in homogeneous catalysis in different catalytic processes. The efficiency of this catalyst depends upon the binding ability for transition metal and their ability to stabilize and activate metal centers in different steps of organic synthesis. Also N/heterocyclic carbenes as ligands where ligands and metal complexes are selectively obtained and active towards Heck reaction giving TONs upto 1.7*106.these ligands don’t only bind with the transition metal but also the main group elements.it has been explained as that bulky NHC ligands for successful reactions with catalysts made of these ligands. Palladium complexes are electron donating ligands such as carbenes, phospines. Nitrogen donor ligands also increase the activity, stability and the selectivity properties of the catalysts. However, palladium nanoparticles can also act as pre catalyst by putting thecatalyst on a polymer support. Palladacycles are found to be active against a broad spectrum of reactions and its high activity at low palladium ligands ratio does increases its thermal capabilities as well. However, there are also some problems like scope, purity, and the time for developing high turnover for the cross coupling and Heck reactions. As compared to the most of the traditional catalyst , carbine ligands along with palladium nanoparticles do have high/TON and thus have more robust than most of the other phosphine ligands. For heterogeneous catalysis, palladium catalysed coupling reactions are efficient and reliable for introducing of new carbon-carbon bonds onto molecules attached to a solid support. The main advantage of using palladium metal catalyst to heck reaction is the simplification of the procedure and the possibility of the recovery of the precious metal. For the industrial approach, there has been many developments on the mechanism of the palladium catalysed C-C bond formation and change in methodologies during its innovation and evolution.fig2: schematic representation of C-H activation and transmetallationfig 3: schematic representation of heck reaction mechanism via Pd (II)/Pd(IV) cycle3. ConclusionHeck reaction has been a robust and efficient method for carbon-carbon bond formation and it could promise for new discoveries resulting in better and efficient catalyst formation and the great functional properties of palladium makes Heck reaction possible even on most sensitive substrates and it could also reduce the number of reaction steps by use of the palladium catalyst by concentrating on protection and deprotection procedures. hence , the palladium catalysed heck reaction has been a integral part of pharmaceutical and industrial research and production.4. References1.https://chem.libretexts.org/Core/Inorganic_Chemistry/Catalysis/Catalyst_Examples/Heck_Reaction accessed on 10.12.2017 2. https://heterocyclist.com/tag/heck-reaction/ accessed on 10.12.2017 3. http://www.organic-chemistry.org/abstracts/lit2/591.shtm accessed on 10.12.2017 4.