Pd-Catalyzed Cross-Couplings: On the Importance of the Catalyst Quantity Descriptors, mol % and ppm

This Review examines parts per million (ppm) palladium
concentrations
in catalytic cross-coupling reactions and their relationship with
mole percentage (mol %). Most studies in catalytic cross-coupling
chemistry have historically focused on the concentration ratio between
(pre)­catalyst and the limiting reagent (substrate), expressed as mol
%. Several recent papers have outlined the use of “ppm level”
palladium as an alternative means of describing catalytic cross-coupling
reaction systems. This led us to delve deeper into the literature
to assess whether “ppm level” palladium is a practically
useful descriptor of catalyst quantities in palladium-catalyzed cross-coupling
reactions. Indeed, we conjectured that many reactions could, unknowingly,
have employed low “ppm levels” of palladium (pre)­catalyst,
and generally, what would the spread of ppm palladium look like across
a selection of studies reported across the vast array of the cross-coupling
chemistry literature. In a few selected examples, we have examined
other metal catalyst systems for comparison with palladium.