Dearomatization and Functionalization of Terpyridine Ligands Leading to Unprecedented Zwitterionic Meisenheimer Aluminum Complexes and Their Use in Catalytic Hydroboration

This paper reports the first example
of dearomatization of ubiquitous
terpyridine (tpy) ligands via 2′/6′-, 3′/5′-,
or 4′-selective alkylation of the central pyridine ring. The
reaction is mediated by the most abundant metal in the Earth’s
crust, aluminum (Al), and depending on the conditions employed, exhibits
ionic or radical character as suggested by experimental and computational
analysis. In the latter case, intermediate formation of an AlIII complex supported by π-radical monoanionic ligand
(tpy•)1– is apparent. The 3′/5′-alkylation
leads to unprecedented zwitterionic Meisenheimer AlIII complexes,
which were identified as efficient precatalysts for the selective
hydroboration of CO and CC functionalities. Turnover
numbers (TONs) up to ∼1000 place the corresponding complexes
in the category of the most efficient Al catalysts reported to date
for the title reaction. The acquired data suggest that aluminum monohydrides,
or more likely dihydrides, could be relevant catalytic species. Alternatively,
one can also imagine a mechanistic scenario in which the dearomatized
“chemically noninnocent” ligand acts as hydride donor,
and a detailed investigation of this is warranted in the future.