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Recent Advances of Modern Protocol for C-C Bonds—The Suzuki Cross-Coupling

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Recent Advances of Modern Protocol for C-C Bonds—The Suzuki Cross-Coupling Advances in Chemical Engineering and Science, 2013, 3, 19-32 http://dx.doi.org/10.4236/aces.2013.33A1003 Published Online July 2013 (http://www.scirp.org/journal/aces) Recent Advances of Modern Protocol for C-C Bonds—The Suzuki Cross-Coupling Jadwiga Sołoducho*, Kamila Olech, Agnieszka Świst, Dorota Zając, Joanna Cabaj Faculty of Chemistry, Wrocław University of Technology, Wrocław, Poland Email: *[email protected] Received April 30, 2013; revised May 30, 2013; accepted June 30, 2013 Copyright © 2013 Jadwiga Sołoducho et al. This is an open access article distributed under the Creative Commons Attribution Li- cense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. ABSTRACT Over the past 20 years, small molecule solid phase synthesis has become a powerful tool in the discovery of novel mol- ecular materials. In the development of organic chemistry, the carbon-carbon bond formation has always been one of the most useful and fundamental reaction. The current review summarizes recent developments in metal-catalyzed cou- pling reactions. The following method is discussed in detail—the cross-coupling of aryl halides with aryl boronic acids (the Suzuki coupling), and the others C-C bond formation reactions as the palladium-catalyzed reaction between an aryl and (or) alkyl halide and a vinyl functionality (the Heck reaction); and the palladium-catalyzed cross-coupling reaction of organostannyl reagents with a variety of organic electrophiles (the Stille reaction)—are mentioned. Keywords: C-C Bond Formation; Coupling Reactions; Palladium Catalysts; Suzuki Coupling; Heterocyclic Copolymers 1. Introduction [4] reported that cross-coupling reactions between al- kenylboranes and organic halides were efficiently cata- Recently, small-molecule solid-phase synthesis has beco- lyzed by a catalytic amount of tetrakis (triphenylphoshine) me a powerful tool in the discovery of new drugs or mo- palladium (Pd(PPh ) ) in the presence of a suitable base. lecular materials. The methods are investigated by the 3 4 In this context, using the conventional bridged deri- time in details: the cross-coupling of aryl halides with vatives of phenoxazine [5], phenothiazine, fluorene, car- aryl boronic acids (the Suzuki coupling); the palladium- catalyzed reaction between an aryl and (or) alkyl halide bazole as a functional segments, the palladium-catalyzed and a vinyl functionality (the Heck reaction); and the condensation halide arylenes by Suzuki-, and other cou- palladium-catalyzed cross-coupling reaction of organ- pling reactions were used as well-established procedure ostannyl reagents with a variety of organic electrophiles in modern organic synthesis [6]. The coupling products (the Stille reaction) [1]. find good applications as intermediates in the preparation Organic transformations in aqueous media have re- of materials (Figure 1). However, the Suzuki-type reac- ceived also much attention because water is mainly tions catalyzed by heterogeneous palladium involving harmless to the environment. The cross-coupling reaction bromoarenes are relatively unexplored. In this context, of alkenyl and aryl halides with organoborane derivatives polymer-supported catalysts have become valuable tools in the presence of a palladium catalyst and a base (Su- offering many advantages such as simplification of zuki reaction) has often been carried out in an organic- product synthesis. aqueous mixed solvent. Although Davidson and Triggs Nowadays, palladium-catalyzed Suzuki-Miyaura co- [2] discovered in 1968 that arylboronic acids reacted upling reaction [7,8] of aryl halides is one of the most with palladium(II) acetate to give corresponding biaryls, effective methods for the construction of biaryls or sub- and Garves [3] in 1970 that arylsulfinic acids could be stituted aromatic moieties in organic synthesis. Although coupled to biaryls using Pd(II) in aqueous solvents, it was homogeneous palladium catalysts have been extensively not until 1979 when biaryls could efficiently be prepared investigated [9-14], their industrial applications remained by a palladium-catalyzed reaction. Miyaura and Suzuki limited due to the difficulty in the separation process from the products for recycling [9-15]. Heterogenization * Corresponding author. of the existing homogeneous palladium catalysts offers Copyright © 2013 SciRes. ACES 20 J. SOŁODUCHO ET AL. Figure 1. Applications of molecular materials synthesized by Suzuki coupling. an attractive solution to this problem [16-19]. From a In general, transition metals, and in particular pall- practical point of view, the use of aryl chlorides is highly adium, have been of importance for the development of desirable compared with the more expensive aryl iodides, reactions for the formation of carbon-carbon bonds. In aryl bromides and aryl triflates. 2005 the Nobel Prize in chemistry was awarded to This short review summarizes the advantages of the metal-catalyzed reactions for the formation of car- Suzuki protocol over other similar cross-coupling reac- bon-carbon double bonds. The 2010 year the Nobel Prize tions, several structurally different ligands that promote in chemistry is awarded to the formation of car- the reaction in water [20-23], recent results from ligan- bon-carbon single bonds through palladium-catalyzed dless Suzuki procedure, and target molecules mainly cross-coupling reactions. produced by the method. 3. Suzuki Coupling 2. Transition Metals in Synthetic Organic The Scheme 1 shows the first published Suzuki coupling, Chemistry which is the palladium-catalyzed cross-coupling between During the second half of the 20th century, transition organoboronic acid and halides. Recent catalyst and metals have come to play an important role in organic methods developments have broadened the possible ap- chemistry and this has led to the development of a large plications enormously, so that the scope of the reaction number of transition metal-catalyzed reactions for creat- partners is not restricted to aryls, but includes alkyls, ing organic molecules. Transition metals have a unique alkenyls and alkynyls. Potassium trifluoroborates and ability to activate various organic compounds and th- organoboranes or boronate esters may be used in place of rough this activation they can catalyze the formation of boronic acids. Some pseudohalides (for example triflates) new bonds. One metal that was used early on for cata- may also be used as coupling partners. lytic organic transformations was palladium. One event that stimulated research into the use of palladium in or- 4. Mechanism of the Suzuki Coupling ganic chemistry was the discovery that ethylene is oxi- In the Suzuki (Scheme 2) as well as Stille mechanism is dized to acetaldehyde by air in a palladium-catalyzed reaction and this became the industrially important Wacker process. Subsequent research on palladium-ca- talyzed carbonylation led to new reactions for the forma- tion of carbon-carbon bonds. Scheme 1. Typical Suzuki procedure. Copyright © 2013 SciRes. ACES J. SOŁODUCHO ET AL. 21 need of activation, and in the Suzuki coupling the bo- tion of R1Pd-OR from R1Pd-X (Scheme 4). Reaction ronic acid must be activated, for example with base. The does not occur in the absence of base. Exact mechanism activation of the boron atom features the polarization of is unclear. the organic ligand, and facilitates transmetalation. If starting materials are substituted with base labile groups 4.3. Reductive Elimination (i.e., esters), powdered KF effects this activation while This final step of catalytic cycle gives the desired product leaving base labile groups unaffected. It is worth to men- and it also regenerates the palladium catalyst so that it tion, that F- is also advantageous in Stille reaction. can participate again in the catalytic cycle (Scheme 5). Due to the stability, ease of preparation and low toxi- Require the complex to revert back to the cis conforma- city of the boronic acid compounds, there is currently tion before reductive elimination can occur. widespread interest in applications of the Suzuki cou- pling, with new developments and refinements being 5. Suzuki Cross-Coupling as Synthetic reported constantly. Protocol for Branched Heterocyclic Units 4.1. Oxidative Addition Suzuki reactions are very useful tool for the formation of Csp2-Csp2 bonds. This procedure is, however, convenient Attaching of the palladium catalyst to the alkyl halide in the synthesis of conducting polymers. Suzuki reaction gives rise to the organopalladium complex (Scheme 3). is also a powerful method for the formation of Csp2-Csp3 The complex is at the beginning in the cis conformation bond. This coupled protocols was also used in our previ- but isomerizes to the trans conformation. Stereochemis- ous work for obtaining branched or hyper-branched try with vinyl halides are retained but inversion of sterically crowded heterocyclic structures [24,25] (Scheme stereochemistry occurs with allylic or benzylic halides. 6). Phenothiazine, as well as phenoxazine, is a well- 4.2. Transmetalation known very strong electron donor and has high HOMO energy level because of sulfur atom. Therefore, it was The role of base in the Suzuki coupling is to activate the expected phenothiazine ring to be an excellent building boron-containing reagent, and also facilitate the forma- block for lowering the ionization potential of conjugated polymers. However, efficient electrogenerated chemilu- minescence was observed in co-oligomers of methyl-
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