C Coupling Reactions

Cationic phosphine ligands containing guanidiniumphenyl moieties were originally developed in order to make use of their pronounced solubility in water [72, 73]. They were shown to form active catalytic systems in Pd-mediated C-C coupling reactions between aryl iodides and alkynes (Castro-Stephens-Sonogashira reaction) [72, 74] and Rh-catalyzed hydroformylation of olefins in aqueous two-phase systems [75]. 

The ease of formation of the carbene depends on the nucleophilicity of the anion associated with the imidazolium. For example, when Pd(OAc)2 is heated in the presence of [BMIM][Br], the formation of a mixture of Pd imidazolylidene complexes occurs. Palladium complexes have been shown to be active and stable catalysts for Heck and other C-C coupling reactions [34]. The highest activity and stability of palladium is observed in the ionic liquid [BMIM][Brj. Carbene complexes can be formed not only by deprotonation of the imidazolium cation but also by direct oxidative addition to metal(O) (Scheme 5.3-3). These heterocyclic carbene ligands can be functionalized with polar groups in order to increase their affinity for ionic liquids. While their donor properties can be compared to those of donor phosphines, they have the advantage over phosphines of being stable toward oxidation. 

Those processes that can be described as intramolecular coupling belong to the same type of reaction as discussed thus far in this chapter. Intramolecular N-, O-, S-, and C-coupling reactions are known. The older literature on such ring closure reactions is particularly extensive. We will discuss only a few representative examples published in the last two decades. The older literature was reviewed briefly by Wulfman (1978, p. 277) and, in more detail, by Saunders and Allen (1985, Chap. 8). 

The most important intramolecular C-coupling reaction is the synthesis of 4,5-benzo-pyrazoles, originally called indazoles (6.70), as shown in Scheme 6-47. The indazole synthesis was discovered by Witt et al. (1890). Nolting (1904) investigated which substituted 2-methylbenzenediazonium salts yield indazoles (review Behr, 1967). 

Stimulated by extensive research activities on donor/acceptor substituted stilbenes, Mullen and Klarner have reported a donor/acceptor substituted poly(4,4 -biphenyl-diylvinylene) derivative (85) in which the NR2 donor and CN acceptor substituents are located on the vinylene unit [111]. The synthesis is based on a C-C-coupling reaction of in situ generated carbanion functions with a (pseudo)cation function, followed by a subsequent elimination of MeSH with formation of the olefinic double bond. 

Also, the Fe methyl contact distance of 3.005(2) A in 34 shortens to 2.085(5) A when the Fe—C-l bond is formed. The most dramatic feature of this rearrangement is that the two acyl carbon donor atoms of 34 have undergone a transannular, interligand C—C coupling reaction. 

Scheme 2. A representative description for the interligand C—C coupling reaction.

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