11 (19)
SOMO activation using organocatalysis: Enamines are nucleophiles that are characterized by
having a relatively high energy HOMO and that react with electrophiles. MacMillan and co-
workers hypothesized that a one-electron oxidation of an enamine should generate the
corresponding radical cation with a singly occupied molecular orbital (SOMO) that is activated
toward enantioselective coupling with π-rich nucleophiles (Scheme 4).
56
For such a strategy to
be successful, the enamine must undergo selective oxidation (step 2) in the presence of a
secondary amine and an aldehyde, and the catalyst must induce high enantiomeric selectivity in
the coupling step (step 3).
This has indeed proven to be possible and this chemistry has been applied for the α-allylation,
α-arylation and intramolecular cyclisation of aldehydes, furnishing the products in high yield
and er. As an example, this chemistry has been applied in an efficient synthesis of the naturally
occurring indolizidine alkaloid (–)-tashiromine (Scheme 5).
57
In this synthesis, the
organocatalytic SOMO activation is used to construct the fused bicyclic ring system in
compound 33 by allowing the cation radical, which is formed by oxidizing the enamine that is
obtained from the aldehyde and catalyst 32 to add to the pyrrole moiety, and simultaneously
install one new stereocentre in high er.
Scheme 4. Catalytic cycle for the allylation of aldehydes using SOMO catalysis. Aldehyde 26
condenses with the organocatalyst to form enamine 27 (step 1, 27 is in equilibrium with the
corresponding iminium ion, which is not shown). A one-electron oxidation of 27 furnishes
cation radical 28 (step 2), which can couple with
π
-rich nucleophiles (step 3) such as
allyltrimethyl silane (28) to furnish intermediate 29. Fragmentation of this species will give
iminium ion 30 (step 4), which is hydrolysed to the allylated aldehyde 31 and regenerates the
organocatalyst (step 5).
12 (19)
Scheme 5. Synthesis of (–)-tashiromine using organocatalyst 32.
Merging organocatalysis with photoredox catalysis: The possibility to convert solar energy into
chemical energy is of great importance for developing a sustainable society. The inspiration for
this research stems from photosynthesis, where plants use solar energy to convert a simple
feedstock into chemical energy in the form of carbohydrates. One possible way to mimic this
chemistry is to use transition metal catalysts (photoredox catalysts, P) to harvest light, which
can then activate stable organic molecules by single-electron oxidation or reduction. This
furnishes open-shell intermediates that are not readily accessible and opens the possibility to
trigger otherwise difficult two-electron reaction pathways by using two one-electron transfer
steps mediated by the photocatalyst.
In 2008, Nicewicz and MacMillan merged this chemistry with organocatalysis, resulting in an
efficient α-alkylation of aldehydes (eq. 13).
58
The role of the photocatalyst P in this reaction is to
reduce the alkyl halide to an alkyl radical and a halide ion (Scheme 6, step 2). The alkyl radical
then adds to an enamine, forming a carbon-carbon bond and a new alkyl radical (step 3). This
species is then oxidized by the photocatalyst to yield an iminium ion (step 4), which is
hydrolysed to the product and returns the organocatalyst (step 5). Two catalytic cycles are
involved, one with the organocatalyst and another with the photoredox catalyst, with two points
of contact.
Nicewicz and MacMillan’s investigation, together with those led by Yoon
59
and Stephenson,
60
spurred considerable interest in the chemistry community, and much effort has been invested in
developing photoredox-catalysed reactions. The power of this chemistry is that by using
sustainable reaction conditions, it allows access to intermediates not attainable by traditional
thermal activation. New chemistry has been developed, and photoredox catalysis has now been
applied in most areas of organic chemistry, both in academia and industry.
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