Further Mechanistic Investigations into the Blue Light-Driven O–H Functionalization of Alcohols by Aryldiazoacetates

The blue light-driven
O–H functionalization of alcohols
by aryldiazoacetates represents a modern synthetic application of
diazo photochemistry. Previous work by us and the Koenigs group demonstrated
that this chemistry occurs through a free singlet carbene intermediate
generated from the photolysis of the aryldiazoacetate. However, there
is significant controversy in the literature concerning the exact
photochemical pathway through which the photolysis of aryldiazoacetates
occurs, with at least three proposed mechanisms in the literature.
One group argued that photolysis requires protonation of the carbonyl
to occur, another group described the C–N bond-breaking occurring
on T1 after intersystem crossing from S1, and
a third group argued that the singlet carbene is generated on S0 after internal conversion from S1. Additionally,
no mechanistic explanation exists in the literature for the dependence
of the O–H functionalized product yield on the chemical structure
of the aryldiazoacetate. In this paper, we apply high-level quantum
chemical methods to develop a refined picture of aryldiazoacetate
photolysis and to build a mechanistic explanation for what controls
the O–H functionalized product yield. Overall, our work provides
new insights into an important chemical transformation initiated by
the absorption of visible light.