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 [{Ph 2 P(BH 3 )N} 2 C 6 H 4 TiCl 2 ] (1) and Ti IV alkyl complex [{Ph 2 P(BH 3 )N} 2 C 6 H 4 Ti(CH 2 SiMe 3 ) 2 ] (2) using a salt metathesis reaction. Ti IV complex 1 was obtained by the reaction of the bis-borane ligand [{Ph 2 P(BH 3 )NH} 2 C 6 H 4 ] and TiCl 4 in toluene followed by the addition of 2 equivalents of [LiN(SiMe 3 ) 2 ] at ambient temperature. Ti IV bis-alkyl complex 2 was isolated from the reaction of complex 1 with 2.5 equivalents of LiCH 2 SiMe 3 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.