Pyridine‐Directed Palladium‐Catalyzed Phosphonation of C(sp²)H Bonds

Redox-Induced Aromatic C–H Bond Functionalization in Metal Complex Catalysis from the Electrochemical Point of View

“…For example, as phosphonate-bridged palladacycles are significantly more difficult to oxidize than acetate-bridged ones [25], the reactivity of both complexes (key intermediates) should be considered in C-H phosphorylation reactions, such as the phosphorylation of 2-phenylpyridine with H-phosphonates catalyzed by palladium acetate [14,15,25,30]. Selective C-H phosphonation requires the correct choice of an oxidant (or electrolysis potential) to promote the oxidation of the phosphonate-bridged dipalladacycle.…”

“…In the case of a "weak" oxidant (or a low electrolysis potential), the oxidation of an acetate-bridged dipalladacycle is the dominant process that results in the formation of an acetoxylated byproduct. The problem of finding an appropriate chemical oxidant likely explains the low yields and very few aromatic C-H phosphonation reactions in the literature, which are usually carried out at high temperature, with special additives (NMMI, Scheme 6 [30]). …”

Phosphorylation of C–H bonds of aromatic compounds using metals and metal complexes

“…This approach was further developed in Pd-catalyzed ortho-C(sp 2 )7H functionalization reactions promoted by ligands and weakly coordinated directing groups. 205 The results of an independent study of a similar reaction of 2-arylpyridine phosphonation by Murakami's research group 206 were published in 2013, almost at the same time as the study of Yu and co-workers. They found that hydroxyalkylphosphonate generates an H-phosphonate acting as a masked phosphonylating reagent.…”

“…at 120 8C for 48 h, the product of phosphorylation of ortho-C7H-bond 107g was obtained in the yield of only 12% (Scheme 41). 206 However, when phosphite 120a was replaced by a-hydroxyalkylphosphonate 120b, readily obtainable from phosphite 120a and acetone in a single step, the product 107g was surprisingly obtained in 70% yield along with a small amount (6%) of diphosphonated product. Thus, a-hydroxyalkylphosphonate 120b proved to O be more effective phosphonation agent, probably, due to the fact that it gradually generates phosphite 120a.…”

“…Other reaction conditions were investigated using phosphite 120b as a latent phosphonation agent; however, the use of such oxidants as Cu(OAc) 2 and Ag 2 CO 3 , stronger bases (K 3 PO 4 ) and other solvents gave worse results. 206 …”