Cobalt‐Catalyzed Direct Arylation and Benzylation by CH/CO Cleavage with Sulfamates, Carbamates, and Phosphates

Cobalt‐Catalyzed C4‐Selective Direct Alkylation of Pyridines

“…Thus, Et 3 B does not seem to act as a simple Lewis acid to increase the electrophilicity of pyridines, but may selectively decelerate the C2 alkylation pathway, resulting in an increase in the C4 selectivity for C4-unsubstituted pyridines. [23] Although there is no direct evidence to exclude a reaction mechanism involving oxidative addition/insertion/reductive elimination promoted by a low valent cobalt species generated in situ, [24,25] a mechanism involving hydrometalation/ nucleophilic addition/rearomatization (Scheme 1) appears to be more plausible for the present reaction, based on the following observations: 1) The low-valent cobalt catalyst reported by Yoshikai et al [25a] for CÀH bond functionalization by oxidative addition was not effective in the present pyridine alkylation reaction. [26] 2) The C3-alkylation adduct was not detected at all under the present conditions.…”

Nickel-Catalyzed Cross-Coupling Reactions of Unreactive Phenolic Electrophiles via C–O Bond Activation

“…Based on the finding of an intramolecular kinetic isotope effect of 2.8, C-H bond cleavage was proposed to proceed via a concerted metalation deprotonation pathway. On a related cobaltcatalyzed reaction that deserves to be noted: aryl carbamates can be coupled with C-H bonds of arenes intermolecularly when the arene contains an ortho-directing group such as a 2-pyridyl group [13].…”

“…Reactions are classified by the substrate involved (i.e., aryl esters, aryl ethers and arenols) and subcategorized by the coupling partner. Cross-coupling reactions of inert phenol derivatives by other transition metal catalysts, such as Ru [6][7][8][9][10], Fe [11,12], Co [13], Rh [14,15] and Cr [16], are beyond the scope of this review. Catalytic transformations via alkenyl [17][18][19][20][21][22][23][24][25][26][27][28] and benzylic C-O bond [29,30] activation are not covered.…”

Mild metal-catalyzed C–H activation: examples and concepts

“…55 Although these transformations generally required a reaction temperature of 60 1C, in some cases reasonable catalytic efficiency was also observed at room temperature. Similarly, a closely related catalytic system was used to perform alkylation reactions using alkyl chlorides as coupling partners.…”

“…55,57 Scheme 24 Proposed mechanism of the Co-catalyzed alkylation. Scheme 25 Iron-catalyzed C-H allylation.…”