Cross coupling reactions direct oxidative

Abstract This chapter highlights the use of iV-heterocyclic carbenes as supporting ligands in arylation reactions different than the more common cross-coupling reactions, including C-F bond activation, catalytic arylation, homocoupling, direct arylation and oxidative Heck reactions. 

In summary, these results demonstrate that air-stable POPd, POPdl and POPd2 complexes can be directly employed to mediate the rate-limiting oxidative addition of unactivated aryl chlorides in the presence of bases, and that such processes can be incorporated into efficient catalytic cycles for a variety of cross-coupling reactions. Noteworthy are the efficiency for unactivated aryl chlorides simplicity of use, low cost, air- and moisture-stability, and ready accessibility of these complexes. Additional applications of these air-stable palladium complexes for catalysis are currently under investigation. 

The direct electrosynthesis of S—N bonds from disulfides and amines has been shown to occur through a reaction of the amine with the oxidized disulfide being a strong electrophile. In contrast to the results, the cross-coupling of phthahmide (16) with disulfide does not proceed in a direct electrolysis. However, the electrosynthesis of sulfenimides (17) can be achieved by a [Br]" "-mediated cross-coupling reaction of imides with disulfides (Scheme 7). The electrolysis of a mixture of (16) and dicyclohexyl disulfide in an MeCN-NaBr-(Pt) system affords... 

Much work has been directed towards the synthesis of thiophene oligomers and polymers. This is due to the current interest in research on conducting polymers and molecular electronics (92CRV711). Two main approaches have been used for making such polymers (i) chemical (e.g. FeCl3) or electrochemical oxidation of monomeric thiophenes and (ii) transition metal-catalyzed cross-coupling reactions. 

Further investigations on reductive elimination processes showed that this reductive elimination could be the side reaction leading to degradation of active species in C-C cross-coupling reactions. As illustrated in Scheme 31, palladium-based catalyst (93) underwent oxidative addition in the presence of iodobenzene, providing the reaction intermediate (183), which could be involved in the catalytic cycle but also affords the imidazolium salt (184) by direct reductive elimination. Since then, a few other examples of... 

The synthesis of hippadine (44) and ungeremine (48) was performed by a combination of the directed ortho metallation and the modified Suzuki cross coupling reactions 134) (Scheme 15). (7-Bromo-5-mesyloxy)indoline reacted with (6-formyl-3,4-methylenedioxy)benzeneboronic acid under the modified Suzuki reaction conditions to give the pyrrolophenanthridone ring system 128, which was reduced with sodium bis(2-methoxyethoxy)alumi-num hydride (SMEAH) in boiling toluene to produce ungeremine (48) in 54% yield. Also, starting with 7-iodoindoline, a similar reaction sequence afforded anhydrolycorin-7-one (119), which was transformed to hippadine (44) in 90% yield by oxidation with DDQ. 

In the cross-coupling reaction, starting from the simple arene (with directing group), palladation by a Pd(II) salt would lead to the formation of the palladacyclic complex (Ar1Pd(II)L) (Scheme 3). After the transmetallation and reductive elimination processes, the biaryl product is obtained together with Pd(0). If the Pd(0) can be further oxidized to Pd(II) catalyst, a catalytic cycle will be formed. By accomplishing this, arenes (C-H) are used to replace the aryl halides (C-X). Similarly, arenes (C-H) can be used to replace the aryl metals (C-M). 

Simultaneously, Shi and coworkers used an amide as a directing group in the oxidative cross-coupling reaction. Amide was used as a directing group to control... 

The use of aryl tosylates as electrophiles is attractive, since they can be synthesized from readily available phenols with less expensive reagents than those required for the preparation of the corresponding triflates. More importantly, tosylates are more stable towards hydrolysis than are triflates. However, this greater stability renders tosylates less reactive in transition metal-catalyzed coupling reactions. As a result, protocols for traditional cross-coupling reactions of these electrophiles were only recently developed [1], In contrast, catalytic direct arylations with aryl tosylates were not reported previously. However, a ruthenium complex derived from heteroatom substituted secondary phosphine oxide (HASPO) preligand 72 [81] allowed for direct arylations with both electron-deficient, as well... 

---