A highly efficient Ti-catalyst for the deoxygenative reduction of esters under ambient conditions: experimental and mechanistic insights from DFT studies

In this paper, we report the synthesis of dianionic amidophosphineborane-supported titanium chloride [{Ph2P(BH3)N}(2)C6H4TiCl2] (1) and Ti-IV alkyl complex [{Ph2P(BH3)N}(2)C6H4Ti(CH2SiMe3)(2)] (2) using a salt metathesis reaction. Ti-IV complex 1 was obtained by the reaction of the bis-borane ligand [{Ph2P(BH3)N}(2)C6H4] 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 LiCH2SiMe3 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.