The_Catalyst_Review_August_2023 - 17

Efforts toward Synthetic Photosynthesis: Visible Light-Driven CO2
Urethanes are commonly synthesized from
amines by one of two methods, depending on
whether the carbonyl C1
unit is derived from
carbon monoxide or carbon dioxide (Figure 1).
Both processes are highly energy intensive and
frequently need poisonous or bulky chemicals to
make the process exergonic. Herein, the authors
offer a moisture-tolerant approach for driving
CO2
amino alkylation with visible light energy
utilizing sensitive arylcyclohexenes, which
transfer most of the photon's energy to strain
during olefin isomerization. This strain energy
considerably improves alkene basicity, allowing
for successive protonation through ammonium
carbamate interception.
The authors demonstrated this concept by
irradiating benzylammonium benzylcarbamate
and 4-fluorophenylcy-clohexene with blue
light in the presence of catalytic quantities of
Ir(Fppy)3
Valorization
Figure 1. Conversion of primary and secondary amines to urethanes. (A) Urethanes
using CO as the C1
synthon. (B) Urethanes using CO as the C1
isocyanation (dehydrative) conditions or reactive electrophiles. (C) Urethane using
activated olefin as a proelectrophile. (D) Urethanes from photon-strained olefin.
synthon along with either
, resulting in a gradual conversion to the
corresponding urethane. Several modifications
incorporating solvent, photosensitizer,
moisture tolerance, and concentration yielded
a mitigated 2+2 photodimerization process
generating cyclobutene.
With a set of general reaction conditions in
hand, a series of different cyclohexenes were investigated to probe the significance of the basicity of strained cyclohexenes on
reaction rate (Figure 2).
Figure 2. Reactions performed with optimized urethane synthesis procedures in hand. (A) Investigation of amine scope. Values
reported are isolated percent yields with the isolated mass in parentheses. (B) Urethane from exhalation. (C) Base-mediated
transcarbamoylation as a urethane generalization strategy.
Based on kinetic evidence, the authors posit that upon blue light irradiation in the presence of catalytic Ir(Fppy)3, the arylcyclohexenes
undergo excitation to their triplet excited state, which must twist about the former double bond before they may undergo intersystem
crossing and cross back onto the ground-state surface near the transition state for rotation about the double bond. A bifurcation
occurs, with some returning unchanged to the relaxed isomer and some moving to the highly strained isomer. Meanwhile, the amine
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August 2023
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