Coupling reactions homocoupling

Coupling reactions Homocoupling of allyl substrates Zinc, 346... 

Novel heteroquaterphenoquinones were synthesized by a stepwise cross-coupling reaction or by a more convenient one-pot oxidative homocoupling reaction of the heterocycle-substituted phenols (Scheme 20, <96JOC4784 see also 95TL8055>). 

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. 

Trost s group examined the possibility of carrying out cross-coupling reactions of alkynes and transformed this into a very powerful synthetic method. Either homocoupling or perhaps, more interesting, heterocoupling procedures were performed using catalytic amounts of a palladium salt (Equation (191)). 

Application in organic synthesis of pentacoordinated triorganodifluorosilicate anions, such as [Bu4N][Ph3SiF2] 825, have been extended to palladium(0)-catalyzed cross coupling reactions (solvents DMF, TFIF, dioxane) with arene halides (Scheme 111).825 This method is tolerant to various palladium(O) catalysts and provides excellent yields of mainly heterocoupled products and only small amounts of homocoupled byproducts. 

A similar cross-coupling reaction between triarylbismuth(V) compounds and aryl-, heteroaryl-, or styryltributylstannanes smoothly proceeded under the same reaction conditions as those for the iodonium salts (Scheme 17) [34]. Homocoupling side products were not mentioned. 

Low-valent niobium and tantalum are also effective for homocoupling of aldehydes leading to 1,2-diols. " Not only commercially available NbClglDME) but also a combination of NbCls and zinc can be used for the pinacol coupling. Reactions in a mixed solvent of 1,4-dioxane and toluene (1 4) give better diastereoselectivity than those in DME or THE (Equation (52)). ... 

In a manner similar to the Sml2-mediated homocoupling of enantiomerically pure (o-methylbenzaldehyde)-Cr(CO)3, the coupling reaction of the [methyl(o-tolylmethylene)amine]Cr(CO)3 complex 17 with SmE gives only ry -diamine 18 along with the reduced amine 19 (Equation (63)). ... 

Most transition metal-catalyzed cross-coupling reactions also yield small quantities of the product of homocoupling of the nucleophilic reactant[16, 114,115]. In particular terminal alkynes [116, 117] or metalated terminal alkynes [118] readily dimerize to the corresponding 1,3-butadiynes (Scheme8.14). 

Numerous examples of unwanted or deliberate Pd-mediated homocoupling of organometallic reagents have been reported (Scheme 8.14) [38, 122, 123]. Suitable oxidants include a-halo ketones [116, 124, 125] (which can, therefore, not be used as electrophilic component in cross-coupling reactions), oxygen [120, 124, 126], 1,2-diiodoethene [127], 2,3-dibromopropionic acid esters [119], and CuCl2 [128]. 

Hennings, D. D. Iwama, T. Rawal, V. H. Palladium-catalyzed (Ullmann-type) homocoupling of aryl halides a convenient and general synthesis of symmetrical biaryls via inter- and intramolecular coupling reactions. Org. Lett. 1999, 1, 1205-1208. 

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