Heck with olefins

Palladium-catalyzed coupling reactions of organic halides with olefins or dienes (R. F. Heck, 1979) are broad in scope and simple to carry out. Anhydrous conditions or any special technique are not required and most functional groups are tolerated. 

Heck reaction, palladium-catalyzed cross-coupling reactions between organohalides or triflates with olefins (72JOC2320), can take place inter- or intra-molecularly. It is a powerful carbon-carbon bond forming reaction for the preparation of alkenyl- and aryl-substituted alkenes in which only a catalytic amount of a palladium(O) complex is required. 

Nanometer size Pd colloids in block copolymer micelles of polystyrene polyvinylpyridine as catalysts have been used is a novel way by Klingelhofer for Heck reaction of C-C coupling of aryl halides with olefins. 

The Heck reaction, a palladium-catalyzed vinylic substitution, is conducted with olefins and organohalides or pseudohalides are frequently used as reactants [15, 16], One of the strengths of the method is that it enables the direct monofunctionalization of a vinylic carbon, which is difficult to achieve by other means. Numerous elegant transformations based on Heck chemistry have been developed in natural and non-natural product synthesis. Intermolecular reactions with cyclic and acyclic al-kenes, and intramolecular cyclization procedures, have led to the assembly of a variety of complex and sterically congested molecules. 

The Heck reaction, first disclosed by the Mori and Heck groups in the early 1970s [65, 66], is the Pd-catalyzed coupling reaction of organohalides (or triflates) with olefins. Nowadays, it has become an indispensable tool for organic chemists. Inevitably, many applications to heterocyclic chemistry have been pursued and successfully executed. In one case, Ohta et al. reacted 2-chloro-3,6-dimethylpyrazine (49) with styrene to furnish ( )-2,5-dimethyl-3-styrylpyrazine (50) [67]. Here, only the E isomer was observed. The outcome will become apparent during the ensuing discussions on the mechanism. 

Reactions of aryllead triacetates with olefins (Heck-type reactions) proceed similarly but do not require Cul as co-catalyst. From the numerous reported reactions, that of phenyllead triacetate with 2,3-dihydrofuran is mentioned as a typical example. This affords the C-C coupling product 27 in 68% yield, together with 10% of the homocoupling product (Equation (ll)).47... 

As shown in the previous sections, a (cr-allenyl)palladium species, which is formed from a propargyl electrophile and a Pd(0) catalyst, reacts with a hard carbon nucleophile in a manner analogous to the Pd-catalyzed cross-coupling reaction to give a substituted allene. The results indicate that the reactivity of the (cj-allenyl)palladium species is similar to that of an alkenylpalladium intermediate. Indeed, it was found that the (cr-allenyl)palladium species reacted with olefins to give vinylallenes, a reaction process that is similar to that of the Heck reaction of alkenyl halides [54]. 

Following our first report on the palladium-catalyzed reaction of vinyl triflates with olefins (Heck-type reaction), oxidative insertion of palladium(O) into the carbon-oxygen bond of easily available vinyl triflates ... 

The intermolecular Heck reactions of oxazoles and thiazoles with olefins are not too common. They are rarely high yielding since in several cases they are biased by dehalogenation. Due to this reason the olefination of these systems is usually achieved through Stille coupling with vinylstannanes. 

Heck reaction [ 17] (organotriflates or halides with olefins),... 

The bisamides and bisesters provide two different families of benzocyclobu-tene monomers and polymers derived from bromobenzocyclobutene 2. Heck and coworkers have demonstrated that aromatic bromides and iodides react with olefins in the presence of a palladium catalyst to afford products where the vinyl group is directly bonded to the aromatic ring [40,42,43], This technology has been used with 4-bromobenzocyclobutene 2 as the starting aromatic halide,in order to prepare more highly functional bis- and monobenzocyclobu-tenes (Fig. 4)... 

Alkenyliodonium salts can be used as highly reactive substrates for Heck-type olefination and similar palladium-catalyzed cross-coupling reactions [63 -65]. In a recent example, a series of dienes 80 were stereo- and regioselectively prepared by a palladium-catalyzed Heck-type reaction of alkenyliodonium salts 79 with a,/3-unsaturated carbonyl compounds (Scheme 37) [64]. 

Some light has been thrown on this unusual reaction by a study of the reaction of cobalt hydrocarbonyl with olefins under nitrogen (14). It has also be discussed recently by Heck (59). 

Glorius, F. Palladium-catalyzed Heck-type reaction of 2-chloro acetamides with olefins. Tetrahedron Lett. 2003, 44, 5751—5754. 

Ruthenium(O) complexes such as Ru(COD)(COT) catalyze the dehydrohalo-genative coupling of vinyl halides with olefins to give substituted conjugated dienes in a Heck-type reaction [11]. Thus, alkenyl halides readily react with activated olefins to produce dienes 16 (Eq. 7). Oxidative addition of vinyl halide, followed by regioselective insertion of an electron-deficient olefin and by -hydrogen elimination leads to the diene. 

Heck RF (1979) Palladium-catalyzed reactions of organic halides with olefins. Acc Chem Res 12 146-151... 

Microwave-assisted Heck coupling reactions have been reported in several instances.[75,81 88] Using microwave dielectric heating, reaction times as short as only a few minutes can be realised and some data from such experiments are listed in Table 6.3. A wide selection of arylhalides were reacted with olefins with Pd/C immobilised in [C8Ciim][BF4] under microwave irradiation.[88]... 

More recently, cationic intermediates have been observed in the Heck reactions of arene diazonium salts catalyzed by triolefinic macrocycle Pd(0) complexes [17,59], o-iodophenols and enoates to form new lactones [60], and o-iodophenols with olefins (the oxa-Heck reaction) [61 ]. In the first case ions were formed by oxidation of the analyte at the capillary, or by association of [NH4] or Na". In the two other cases ionization occurred through the more typical loss of a halide ligand. The oxa-Heck reaction provides a good example of how these experiments are typically performed and the type of information that can be obtained. The oxyarylations of olefins were performed in acetone, catalyzed by palladium, and required the presence of sodium carbonate as base. Samples from the reaction mixtures were diluted with acetonitrile and analyzed by ESI(+)-MS. Loss of iodide after oxidative addition of o-iodophenol to palladium afforded positively-charged intermediates. Species consistent with oxidative addition, such as [Pd(PPh3)2(C6H50)], and the formation of palladacycles of the type seen in Scheme 8 were observed. Based on this, a mechanism for the reaction was proposed (Scheme 8). 

In this context, a functionalized ionic liquid, 1-(2-hydroxyethyl)-3-methyl imidazolium tetrafluoroborate [hemim][BF4], is reported as an efficient and recyclable reaction medium for the palladium catalyzed Heck reaction. The olefination of iodoarenes and bromoarenes with olefins generates the corresponding products in good to excellent yields under phosphine-ffee reaction conditions. After separation of the product, fresh starting materials are charged into the recovered ionic liquid which entraps the palladium catalyst. The reactions still proceed quantitatively for six cycles, without significant loss of catalytic activity. " The effect of both the cation and the anion on the chemical yield is shown in Figure 28. 

The 1-naphthylmethyl to 8-naphthyl palladium migration was discovered as a side reaction in a Heck reaction of l-(chloromethyl)naphthalene with olefins... 

The palladium-catalysed Heck reaction of aryl or vinyl halides with olefins has been widely used in synthetic chemistry. The reaction works impressively well with a wide range of electron-deficient and neutral olefins, generally affording (1-arylatcd products. In the case of electron-rich olefins, however, a mixture of regioisomers is usually obtained under standard Heck reaction conditions (Figure 5.1).[1] This... 

Discussion and Conclusions. In the present work a large variety of Pd/C catalysts with different properties was studied as catalysts in Heck reactions of aryl bromides with olefins. The activity of the catalysts strongly depends on the Pd dispersion, the Pd oxidation state in the fresh catalyst, the water content (wet or dry catalysts) and the catalyst preparation conditions (in regnation method, pretreatment conditions). The effects are significant, i.e. Pd on the same activated carbon support is eitiier found to be a nearly inactive catalyst or, in the other extreme, a catalyst with the highest activity ever reported up to now for heterogeneous systems for the conversion of aryl bromides (Table 6.)... 

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