Diverting Ni‐Catalyzed Direct Benzylic C−H Hydroxylation towards Trifluoroethoxylation

Abstract

Nickel(II) complexes with the simple N4 donor aminopyridine ligands BPMEN (N,N′‐dimethyl‐N,N′‐bis(2‐pyridylmethyl)‐1,2‐diaminoethane) and TPA (tris(2‐pyridylmethyl)amine) have been demonstrated to perform as efficient catalysts for the regioselective oxygenation of benzylic C−H groups with m‐chloroperoxybenzoic acid under mild conditions (at 20 °C), typically ensuring >90 % substrate conversions within 1 h at 1 mol % catalyst loadings. Unprecedentedly, replacing the commonly used mixed reaction solvent CH3CN/CH2Cl2 with 2,2,2‐trifluoroethanol diverts the selectivity of the above Ni‐based catalyst systems from hydroxylation/ketonization to trifluoroalkoxylation, with the formation of 1‐phenylalkyl 2,2,2‐trifluoroethyl ethers. The use of 2,2,2‐trifluoroethanol ensures high substrate conversion values (up to 97 %), as well as high selectivity for the ether formation (up to 92 %). Preliminary discussion of the reaction mechanism is given based on the catalytic and literature data.

Catalyst and Medium Control over Rebound Pathways in Manganese-Catalyzed Methylenic C–H Bond Oxidation

“…In particular, the oxidation of S1 catalyzed by 8 delivered the alcohol and ketone products ( P1u-OH and P1-O ) in 40% combined yield, accompanied by 10% of P1u-OCH 2 CF 3 and 5% of P1r-OCH 2 CF 3 , confirming that the formation of the cationic intermediate is disfavored by the use of such catalyst. Most importantly, the observed ether products are formed as single diastereoisomers, suggesting common mechanistic features for TFE and carboxylate rebound, indicating that also TFE can bind to the metal center acting by all means as a co-ligand. , Accordingly, we propose that the 2,2,2-trifluoroethoxy group in the rearranged alcohol product observed in the oxidation of ent -trachyloban-19-oate in TFE (Figure ) also derives from the transfer of the OCH 2 CF 3 to a cationic intermediate.…”

A synthetically useful catalytic system for aliphatic C–H oxidation with a nonheme cobalt complex and m-CPBA

“…In stark contrast, selective C(sp 3 )–H oxidation by late transition metal (groups 9–11) complexes under substrate-limiting conditions has rarely been reported, 25–50 and only recently have the corresponding oxo complexes of tetragonal symmetry been explored, 51–64 because of the widely known ‘oxo wall’. 19,65–68 Herein, we report a synthetically usefully catalytic system for aliphatic C(sp 3 )–H oxidation with a biologically inspired nonheme cobalt complex and m -CPBA via a metal-based oxidant ( e.g.…”