Computationally Probing the Mechanism of the Blue-Light-Driven O–H Functionalization of Alcohols by Aryldiazoacetates: Photobasicity or Carbene Chemistry

Photochemistry provides green alternatives
to traditional reaction
conditions and opens up routes toward products that are otherwise
difficult to make. Recent work by Koenigs and co-workers demonstrated
the blue-light-driven O–H functionalization of alcohols by
aryldiazoacetates. Based on spectroscopic and computational analyses,
Koenigs and co-workers demonstrated that the alcohols form a hydrogen-bonding
complex with aryldiazoacetates prior to the light absorption, with
the strength of hydrogen bonding correlated with the product yield.
Because methyl phenyldiazoacetate (MPDA) was observed to preferentially
react with alcohols over cyclopropanation with styrene, the reaction
was speculated to occur via excited-state proton transfer, with MPDA
acting as a photobase. In this paper, we use time-dependent density
functional theory to show that the electronic excited state of aryldiazoacetates
is inconsistent with photobasicity. Instead, we argue that the reaction
proceeds via a carbene intermediate generated through the photolysis
of the aryldiazoacetate. Using density functional theory, we demonstrate
that the reaction between the singlet state of the carbene intermediate
and the alcohol is thermodynamically favorable and very fast. Moreover,
we provide a rationalization for the experimentally observed preference
for O–H functionalization with alcohols over cyclopropanation
with alkenes. Overall, this work provides a refined mechanistic understanding
of an interesting photochemical transformation.