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2017 ; 139
(12
): 4559-4567
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Studies on the Enantioselective Iminium Ion Trapping of Radicals Triggered by an
Electron-Relay Mechanism
#MMPMID28281754
Bahamonde A
; Murphy JJ
; Savarese M
; Brémond É
; Cavalli A
; Melchiorre P
J Am Chem Soc
2017[Mar]; 139
(12
): 4559-4567
PMID28281754
show ga
A combination of electrochemical, spectroscopic, computational, and kinetic
studies has been used to elucidate the key mechanistic aspects of the previously
reported enantioselective iminium ion trapping of photochemically generated
carbon-centered radicals. The process, which provides a direct way to forge
quaternary stereocenters with high fidelity, relies on the interplay of two
distinct catalytic cycles: the aminocatalytic electron-relay system, which
triggers the stereoselective radical trap upon iminium ion formation, and the
photoredox cycle, which generates radicals under mild conditions. Critical to
reaction development was the use of a chiral amine catalyst, bearing a
redox-active carbazole unit, which could rapidly reduce the highly reactive and
unstable intermediate generated upon radical interception. The carbazole unit,
however, is also involved in another step of the electron-relay mechanism: the
transiently generated carbazole radical cation acts as an oxidant to return the
photocatalyst into the original state. By means of kinetic and spectroscopic
studies, we have identified the last redox event as being the turnover-limiting
step of the overall process. This mechanistic framework is corroborated by the
linear correlation between the reaction rate and the reduction potential of the
carbazole unit tethered to the aminocatalyst. The redox properties of the
carbazole unit can thus be rationally tuned to improve catalytic activity. This
knowledge may open a path for the mechanistically driven design of the next
generation of electron-relay catalysts.