Transition metal catalysts, crosscoupling reactions

This chapter describes recent advances in transition metal-catalyzed crosscouplings involving the cleavage of C(sp )-0 bonds in non-sulfonylated phenol and enol derivatives. The chapter is divided into three main sections The first details the most intensively studied nickel-based catalyst system. The second deals with C-O bond transformations using metals other than nickel. Finally, the last section describes several examples that exemplify the potential utility of these classes of cross-coupling reactions. 

Transition metal-catalyzed C—P cross-coupling affords powaful tool for the preparation of aryl phosphorus compounds [198]. The scope of the phosphorus-based nucleophiles used in crosscoupling with aryl electrophiles in the presence of transition metal (Pd-, Cu-, and Ni-based) catalysts is very broad. Figure 20.5 summarizes the common phosphoms nucleophiles used for crosscoupling reactions (for mechanism, see Scheme 20.2). 

The utility of these reactions were dramatically improved by the crosscoupling reactions involving transition metals as catalysts. The most prominent among these is the Suzuki coupling (Figure 17) (60-62). These reactions allow for aryl-aryl couplings also (62). Recently Buchwald (65) and Fu (64) developed modified phosphines which will allow the inclusion of aryl chlorides in Suzuki coupling reactions. 

In biatyl synthesis, although transition metal catalyzed aryl-aryl crosscoupling reactions have been widely exemplified using Suzuki-type reactions, CDC reactions between sp C-H bonds leading to sp C-C bonds are scarce. In 2010, Katsuki reported an enantioselective CDC reaction between two naphthol moieties catalyzed by a chiral iron(salan) complex under aerobic oxidative conditions (Scheme 4.23). Using 4 mol% of the iron(salan) complex 23-A as the catalyst in toluene at 60 °C for 48 h, two different 2-naphthols were coupled and the cross-coupled derivatives were isolated with moderate yields (44-70%) and good ee (87-95%). It must be pointed out that the homocoupling derivatives are also obtained in low yields. 

In the following sections, we will review some of the most relevant computational mechanistic studies on the three steps of some Pd-catalyzed C-C crosscoupling reactions and how the phosphine ligands coordinated to the transition metal center affect the catalyst reactivity within these steps. Most of the results reported have been obtained using the Density Functional Theory (DPT) with the hybrid functional B3LYP, although other functionals as B3PW91 have been also employed. 

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