Transition metal catalysts Heck reaction

Ionic liquids serve as very useful reaction media, which can facilitate easy separation of product(s) and a catalyst after the reaction. These substitutes of organic solvents as reaction media have already been successfully applied to a variety of transition metal-catalyzed organic reactions such as Heck reaction, Suzuki cross-coupling, hydroformyla-tion, and alkoxycarbonylation. ... 

The counter-ions of some of the quaternary onium groups were exchanged with an anionic phosphine compound, which was then used to complex palladium. Thus, a polymer material containing phase transfer catalyst and transition-metal catalyst groups was obtained (Fig. 20). The Heck-type vinyla-tion reaction [137] was used to examine the catalytic activity of the heterogeneous system. The polymer-supported catalyst was found to compare favourably with the homogeneous system (Fig. 21). 

A wide variety of synthetic processes have been rendered asymmetric through the use of a chiral catalyst. In addition to the types of reaction described above, chiral transition metal catalysts have been used to influence the stereochemical course of isomerization, cyclization, and coupling reactions. As an example, an approach towards the natural product (-)-epibatidine (158) was recently reported by Namyslo and Kaufmann (166). Epibatidine is a potent analgesic and a nicotinic receptor agonist. The synthesis involves an asymmetric Heck-type hydroarylation between the bicyclic alkene (155) and pyridyl iodide (156). A number of bidentate chiral li-... 

Fig. 14.5 Generation of addressable libraries on chips (a) Scheme of Heck reaction under employment of an transition metal catalyst (Reprinted with permission from Tian et al.33 Copyright (2005) American Chemical Society) (b) Scheme of reductive amination reactions (Reprinted with permission from Tesfu et al.34 Copyright (2006) American Chemical Society)...

As ionic liquids are stiU expensive in comparison with conventional molecular solvents, their efficient recycling is an important issue that addresses the economics of their use, especially in large-scale applications. The most studied examples are the 1,3-dialkylimidazolium-based ionic liquids where the ionic solvent is usually recycled through several cycles of the reaction. Numerous examples describe the immobilization of a transition metal catalyst in the ionic liquid phase of a biphasic system. A variety of palladium coupling reactions, such as the Heck [27], Suzuki... 

Transition metal catalysts encapsulated within the ligand-template nanoreactor G, P(Py)s [ZnJs, have been applied to catalyze industrially relevant processes such as hydroformylation and Heck reaction. Nanoreactor [G 3 Rh(CO)(acac)] encapsulates a Rh-species that contains only one tris(meta-pyridyl)phosphine ligand, P(m-Py)3, surrounded by three Zn-porphyrins or Zn-salphens. Under syngas pressure (H2/CO), rhodium species like Rh(CO)(acac)P(Py)3 transform into a complex of type HRh(CO)3P(Py)3, which is the active species for the hydroformylation reaction, hi this reaction terminal alkenes are converted into linear and/or branched aldehydes, and the ratio of these products strongly depends on the specific catalyst applied. Hydroformylation of 1-octene by encapsulated rhodium, [G 3 HRh(CO)3], resulted in a 10-fold rate enhancanent compared... 

Just like all other transition metal catalysts discussed herein, Chang and coworkers s ruthenium catalyst provided good to excellent stereoselectivities in Mizoroki-Heck-type functionahzation reactions of alkenes, yielding predominantly or exclusively the corresponding (ii)-isomer. A stereochemistry that is not only complementary to these catalysts, but also to the conunonly employed palladium complexes (Scheme 10.38, ((>)), was achieved recently [64, 65]. Thus, a ruthenium catalyst allowed for regio- and diastereo-selective arylations of 2-pyridyl-substituted alkenes via a C-H bond functionalization process (Scheme 10.38, (a)). 

Transition-metal catalysts play an ever-increasing and important role in modem chemistry [3]. Numerous transition-metal-catalyzed coupling reactions have been developed and applied in the total synthesis of natural products, such as the Suzuki reaction, the Negishi reaction, the Heck reaction, and many others [4]. Interestingly, the power of transition-metal catalysts is even more visible in the area of domino reactions, where terms such as palladium walking show the value of transition metals in bond formations. 

Phenanthridinone derivatives have been reported to be found in a number of natural alkaloids and exhibit a wide range of biological activities. In the case of starting from bi-functionalized arenes with transition metal catalysts, the intramolecular cyclization of 2-bromo-iV-arylbenzamides via C-H activation is the most direct pathway, which has been applied in the synthesis of anti-hepatitis C virus agents and materials. Remarkably, Yao, Xu and their co-workers developed a one-pot procedure for the synthesis of a pyrrolophenanthridone skeleton via an intramolecular Heck reaction and oxidation of N-(2-bromobenzyl) substituted indoles. Moderate to good yields of the desired products were isolated in one step (Scheme 3.67). From the point view of academic interest, the... 

In the last quarter of the 20th century a new family of C-C forming reactions based on transition metal catalysts evolved as powerful tools in synthesis. Palladium-catalyzed cross-coupling reactions (Heck, Stile, Suzuki) and metathesis of olefins, enynes, and alkynes have had a profound impact on the art of total synthesis. 

One remarkable reaction catalyzed by Rn complexes is the Murai conpling, in which alkyl arenes 44 are formed by hydroarylations of olefins. Interestingly, P-hydride elimination is not a problem in these reactions, as would be expected with other transition metal catalysts." " Murai-type protocols are especially nseful as they can employ electron-rich olefins, which are typically low-yielding snbstrates in Pd-catalyzed, oxidative Heck (Fujiwara-Moritani) reactions." " ... 

Kim JG, Jang DO (2009) Indium-catalyzed reaction for the synthesis of caibamates and carbonates selective protection of amino groups. Tetrahedron Lett 50(22) 2688-2692 Paul F (2000) Catalytic synthesis of isocyanates or carbamates from nitroaromatics using group viii transition metal catalysts. Coord Chem Rev 203(l) 269-323 Dieck HA, Laine RM, Heck RE (1975) Low-pressure, palladium-catalyzed iV4V -diarylurea synthesis from nitro compounds, amines, and carbon monoxide. J Org Chem 40(19) 2819-2822... 

The separation of reaction products from catalysts is a recurrent problem in homogeneous catalysis. The major drawback of the common separation techniques applied in homogeneous catalysis is the extensive (and usually destmctive) postreaction workup required. OSN membranes, being selective between high MW catalysts (>450 Da) and reaction products, are able to perform this separation. Nair et al. (2002) presented a membrane-based (STARMEM 122) process for the separation of a phase transfer catalyst (PTC) and a Heck reaction transition metal catalyst from the reaction media. For the PTC catalyst the process was so efficient that rejections superior to 99% were observed for both pre- and postreaction mixtures and no reaction rate decline was observed for two consecutive catalyst recycles. 

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