A highly efficient Ti-catalyst for the deoxygenative reduction of esters under ambient conditions: experimental and mechanistic insights from DFT studies. Issue 15 (28th March 2022) - Bhattacharjee, Jayeeta
Abstract : A combined experimental & DFT study of the catalytic deoxygenative reduction of organic esters with HBpin using a Ti IV alkyl complex as a competent catalyst to afford corresponding boryl ether at room temperature under neat conditions is reported. Abstract : In this paper, we report the synthesis of dianionic amidophosphineborane–supported titanium chloride [{Ph2 P(BH3 )N}2 C6 H4 TiCl2 ] (1 ) and Ti IV alkyl complex [{Ph2 P(BH3 )N}2 C6 H4 Ti(CH2 SiMe3 )2 ] (2 ) using a salt metathesis reaction. Ti IV complex 1 was obtained by the reaction of the bis-borane ligand [{Ph2 P(BH3 )NH}2 C6 H4 ] and TiCl4 in toluene followed by the addition of 2 equivalents of [LiN(SiMe3 )2 ] at ambient temperature. Ti IV bis-alkyl complex 2 was isolated from the reaction of complex 1 with 2.5 equivalents of LiCH2 SiMe3 in toluene. The solid-state structure of complex 1 is established by single-crystal X-ray diffraction analysis. Ti IV bis-alkyl complex 2 has proved to be a competent catalyst in the deoxygenative reduction of aliphatic and aromatic esters with pinacolborane (HBpin) to afford corresponding boryl ethers at room temperature under solvent-free conditions. Catalyst 2 exhibits chemoselectivity toward ester functionalities over halides, heteroatoms, olefins, and amino functional groups. DFT studies demonstrate that the active form of catalyst 2 is capable of easily transferring its hydrides to ester substrates at room temperature. The studies further reveal that the rate-limiting step (RLS) in an ester-to-boryl ether conversion is the cleavage of the C–O bond of an ester. In brief, the titanium-catalysed ester-to-boryl ether conversions are found to be downhill processes having small activation barriers along all mechanistic steps.