Communications
doi.org/10.1002/ejoc.202001640
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Photoredox Allylation Reactions Mediated by Bismuth in
Aqueous Conditions
Simone Potenti,[a, b] Andrea Gualandi,*[a] Alessio Puggioli,[a] Andrea Fermi,[a]
This paper is dedicated to Prof. Franco Cozzi on the occasion of his 70th birthday.
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e.g. an amine (DIPEA or TEA) or Hantzsch’s ester. In the case of
nickel, cobalt, and titanium, the catalytic cycle is feasible with
the use of an organic reducing agent. In both cases, the
allylation reaction is performed in the presence of a photo-
catalyst (iridium complex or an organic dye), although recently
Gansäuer pointed out[16] that titanium complexes themselves
can act as photocatalysts under precise conditions (green light
irradiation). The described methodologies are certainly innova-
tive and can be further expanded towards other C–C bond-
forming transformations. Barbier reactions with certain types of
metals (zinc, indium, bismuth, gallium) were developed in
aqueous solvents.[17] Tofurther extend the advantage of photo-
redox allylation reactions, and explore the use of green and
sustainable conditions, we wondered if the mentioned metals
could be employed in photoredox allylation reactions under
aqueous Barbier conditions, and herein we report the successful
endeavor of our investigations.
Bismuth is an inexpensive, safe, and environmentally-benign
metal, that is commonly used in cosmetics, and as a component
of oral gastrointestinal drugs.[18] Bismuth has been used in
allylation reactions under various conditions,[19] and in the
presence of stoichiometric metals as reductants, such as Al, Mg,
Fe, and Zn.[20] Interestingly, also sodium borohydride was a
suitable reductant for Barbier-typeallylation reaction with
bismuth.[21] Based on these reports, we selected bismuth salts as
metal catalysts for the catalytic photoredox allylation of
aldehydes. Starting by employing 4-chlorobenzaldehyde as the
model substrate, we have optimized the allylation reaction
using allyl bromide and Bi(OTf)3 (Table 1). We have avoided the
employment of metal photocatalysts based on iridium and
ruthenium, focusing our investigation on the class of thermally
activated delayed fluorescence (TADF) organic dyes based on
carbazoyl and diphenylamine substituted dicyanoarenes.[22]
Among all the TADF dyes, 3CzClIPN[23] was the dye of choice.
We also varied – in the model reaction – the solvent, finding
that a 1:1 mixture of EtOH/H2O was convenient to satisfyingly
perform the reaction (Table 1, entry 6). Furthermore, the
reaction does not require strictly de-oxygenated solvents and
can be conveniently settled without a freezing-pump thaw
procedure to eliminate traces of oxygen (Table 1, entry 7). The
reaction is not sensitive to the presence of radical scavengers
like (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO, Table 1, en-
try 8). Different bismuth salts were also tested in the model
reactions (see Table S3) and we found that BiBr3 was also a
compelling catalyst for the reaction (Table 1 entry 14). Due to
Organometallic allylic reagents are widely used in the con-
struction of CÀ C bonds by Barbier-type reactions. In this
communication, we have described a photoredox Barbier
allylation of aldehydes mediated by bismuth, in absence of
other metals as co-reductants. Mild reaction conditions, toler-
ance of oxygen, and use of aqueous solvent make this photo-
redox methodology attractive for green and sustainable syn-
thesis of homoallylic alcohols.
In recent years, photoredox catalysis has been developed
towards effective and practical new synthetic methodologies
for CÀ C and CÀ X (X=O, N, S, P) bond-forming reactions.[1]
Metalla photoredox catalysis, developed from the seminal work
of Sanford,[2] Molander,[3] and MacMillan-Doyle,[4] has consider-
ably expanded the repertoire of reactivity and scope, allowing
the
development
of
new,
mild,
and
interesting
transformations.[5] From the application of metalla photoredox
catalysis in the context of cross-coupling reactions[6] the
methodology has evolved to consider radical to polar crossover
mechanism,[7] in which carbanion[8] or carbenium ion[9] are
formed by the reaction of a metal with a radical generated
under photoredox conditions. Recently, allylation reactions
were described in Barbier conditions, with the involvement of
nickel,[10] chromium,[11] titanium,[12] and cobalt.[13] Stereoselective
allylation reactions with chromium[14] and nickel[15] were also
developed, showing the possibilities offered by the use of
photoredox conditions, in the presence of metals, for asymmet-
ric reactions. Essentially, photoredox Barbier conditions avoid
the need of a metal co-reductant (often Mn or Zn). In the
examples reported with chromium, the photoredox cycle does
not need a sacrificial reducing agent such as organic molecules,
[a] S. Potenti, Dr. A. Gualandi, A. Puggioli, Dr. A. Fermi, Prof. G. Bergamini,
Prof. P. G. Cozzi
Dipartimento di Chimica “G. Ciamician”
ALMA MATER STUDIORUM, Università di Bologna
Via Selmi 2, 40126, Bologna, Italy
E-mail: andrea.gualandi10@unibo.it
[b] S. Potenti
Laboratorio SMART, Scuola Normale Superiore,
Piazza dei Cavalieri 7, 56126, Pisa, Italy
Supporting information for this article is available on the WWW under
Eur. J. Org. Chem. 2021, 1624–1627
1624
© 2021 Wiley-VCH GmbH