Engineering Catalysts for Selective Ester Hydrogenation

The development of efficient catalysts
and processes for synthesizing
functionalized (olefinic and/or chiral) primary alcohols and fluoral
hemiacetals is currently needed. These are valuable building blocks
for pharmaceuticals, agrochemicals, perfumes, and so forth. From an
economic standpoint, bench-stable Takasago Int. Corp.’s Ru-PNP,
more commonly known as Ru-MACHO, and Gusev’s Ru-SNS complexes
are arguably the most appealing molecular catalysts to access primary
alcohols from esters and H2 (Waser, M. et al. Org.
Proc. Res. Dev. 2018, 22, 862).
This work introduces economically competitive Ru-SNP­(O)z complexes (z = 0, 1), which combine
key structural elements of both of these catalysts. In particular,
the incorporation of SNP heteroatoms into the ligand skeleton was
found to be crucial for the design of a more product-selective catalyst
in the synthesis of fluoral hemiacetals under kinetically controlled
conditions. Based on experimental observations and computational analysis,
this paper further extends the current state-of-the-art understanding
of the accelerative role of KO-t-C4H9 in ester hydrogenation. It attempts to explain why a maximum
turnover is seen to occur starting at ∼25 mol % base, in contrast
to only ∼10 mol % with ketones as substrates.