Lewis Acid Induced Toggle from Ir(II) to Ir(IV) Pathways in Photocatalytic Reactions: Synthesis of Thiomorpholines and Thiazepanes from Aldehydes and SLAP Reagents

Redox
neutral photocatalytic transformations often require careful pairing
of the substrates and photoredox catalysts in order to achieve a catalytic
cycle. This can limit the range of viable transformations, as we recently
observed in attempting to extend the scope of the photocatalytic synthesis
of N-heterocycles using silicon amine protocol (SLAP) reagents to
include starting materials that require higher oxidation potentials.
We now report that the inclusion of Lewis acids in photocatalytic
reactions of organosilanes allows access to a distinct reaction pathway
featuring an Ir­(III)*/Ir­(IV) couple instead of the previously employed
Ir­(III)*/Ir­(II) pathway, enabling the transformation of aromatic and
aliphatic aldehydes to thiomorpholines and thiazepanes. The role of
the Lewis acid in accepting an electroneither directly or
via coordination to an iminecan be extended to other classes
of photocatalysts and transformations, including oxidative cyclizations.
The combination of light induced reactions and Lewis acids therefore
promises access to new pathways and transformations that are not viable
using the photocatalysts alone.