Transition cross-dehydrogenative couplings

From C-H to C-C Bonds Cross-Dehydrogenative-Coupling 27 Renewable Resources for Biorefineries 28 Transition Metal Catalysis in Aerobic Alcohol Oxidation 29 Green Materials from Plant Oils... 

The cross-dehydrogenative coupling of N-protected acetanilides such as 112 with arenes proceeds with Pd(II) and Cu(II) via paUadacycles 113 (Scheme 11.38) [147]. Presumably, paUadacycles 113 react with the arenes by a proton-abstraction mechanism via transition state 114 to form Pd(II) complexes 115, which evolve by reductive elimination to yield the coupled products 116. A palladacycle was also involved in the related reaction of acetanilides with boronic acids catalyzed by Pd(ll) and Cu(ll) [148]. 

The groundbreaking work of Shue, de Vries and van Leeuwen others on oxidative olefinations and the contributions of Lu, Fagnou, DeBoef and others on direct arylations laid the foundation for the development of an impressive number of transition-metal-catalyzed selective cross-dehydrogenative sp C-H couplings. Notably, the Fujiwara-Moritani reaction is also commonly termed the oxidative Mizoroki-Heck reaction . Progress in this area has been extensively reviewed and will thus only be briefly mentioned here, notwithstanding their great importance. 

Over the last decade, the copper-mediated or copper-catalyzed C-H functionalization has been developed rapidly and greatly by significant efforts of many researchers, and cheap and abundant copper salts now can replace, to some extent, precedented noble transition metal catalysts such as Pd, Rh, and Ru. Moreover, some unique features of copper salts and complexes are observed. The intermolecular dehydrogenative cross-couplings mentioned in this chapter are such good examples, and they are otherwise challenging even under known noble transition metal catalysis. However, there is still a large room for further... 

Various transition metal catalysts of Pt, Pd, and Rh were not effective in this dehydrogenative cross-coupling condensation reaction, even though some of them had been reported as efficient catalysts for linear hydrosilanes and silanols in our previous paper [95,115]. 

Heteroaryl phosphonates are common motifs in biological compounds and have stimulated the development of transition metal-catalyzed methodologies for C-P bond formation [68]. Phosphonated thiophenes 43 are accessible via silver-catalyzed dehydrogenative cross-coupling of thiophene 1 with dialkyl phosphites 42 (Scheme 19) [69]. The reaction is performed in aqueous dichloromethane, proceeds regioselectively at the a-position, and utilizes silver(l) nitrate as catalyst and the oxidant potassium persulfate. 

To complete the above reaction, dehydration, dehydrogenation, and cross-coupling must occur successively. Catalysts active for these reactions are MgO doped with 2 — 15% transition metal ions. 

As mentioned above, successive occurrence of dehydration, dehydrogenation, and cross-coupling is required to yield the products. It seems likely that acid sites and base sites participate in the reaction. The role of base sites is particularly important because the conversion rates correlate well with the acidities of the reactants. Acidities of the reactants are compared to the reaction rates in Table 4.36. The pKa values are associated with the dissociation of the H atom which is abstracted in the cross-coupling step. The easier the dissociation of the H atom as an H, the faster the reaction rate. This suggests that abstraction of the H from the reactant by base sites is the ratedetermining step. Therefore, activity increase is caused by the increase in the basicity of MgO by doping with transition metal ions. The increase in basicity is prominent when the ionic radii of added metal ions are close to magnesium ion radius (see Section 3.2.2)... 

---