Aluminum(III) Salen Complexes as Active Photoredox Catalysts

Article abstract of DOI:10.1002/ejoc.201901086

Metallosalen are privileged complexes that have found important applications in catalysis. In addition, their luminescent properties have also been studied and used for sensing and biological applications. Salen metal complexes can be efficient photosensitizers, but they can also participate to electron transfer processes. Indeed, we have found that commercially available [Al(Salen)Cl] is an efficient photoredox catalyst for the synergistic stereoselective reaction of alkyl aldehydes with different bromo ketones and malonates to give the corresponding enantioenriched α-alkylated derivatives. The reaction was performed in the presence of a MacMillan catalyst. [Al(Salen)Cl] is able to replace ruthenium complexes, showing that also aluminum complexes can be used in promoting photoredox catalytic reactions.

Full text of DOI:10.1002/ejoc.201901086

DOI: 10.1002/ejoc.201901086
Full Paper
Earth-Abundant Catalysts | Very Important Paper |
Aluminum(III) Salen Complexes as Active Photoredox Catalysts
Andrea Gualandi,*[a] Marianna Marchini,[a] Luca Mengozzi,[a] Hagos Tesfay Kidanu,[a]
Antoine Franc,[a,b] Paola Ceroni,*[a] and Pier Giorgio Cozzi*[a]
Abstract: Metallosalen are privileged complexes that have synergistic stereoselective reaction of alkyl aldehydes with dif-
found important applications in catalysis. In addition, their lumi- ferent bromo ketones and malonates to give the corresponding
nescent properties have also been studied and used for sensing enantioenriched α-alkylated derivatives. The reaction was per-
and biological applications. Salen metal complexes can be effi- formed in the presence of a MacMillan catalyst. [Al(Salen)Cl] is
cient photosensitizers, but they can also participate to electron able to replace ruthenium complexes, showing that also alumi-
transfer processes. Indeed, we have found that commercially num complexes can be used in promoting photoredox catalytic
available [Al(Salen)Cl] is an efficient photoredox catalyst for the reactions.
Introduction
diamino-N,N′-bis(3,5-di-tert-butylsalicylidene)] 1 (Figure 1) is
bound to Al(III), Zn(II), Sn(II), Si(IV), and Mg(II) an unstructured
fluorescence band appears in the 400–700 nm region with a
lifetime in the nanosecond range.[15] The (1R,2R)-[Al(Salen)Cl]
complex 2[16] also exhibits electrogenerated chemilumines-
cence in acetonitrile solutions, although less efficiently than
[Ru(bpy)3]2+ in the same conditions.[17] Furthermore, Garcia
has reported an interesting photochemical investigation of
[Al(Salen)Cl] complex 2,[18] in conventional and nonconven-
tional solvents. Given these premises, we decided to investigate
the efficiency of Al(III) Schiff bases complexes as photoredox
catalysts[19] in a benchmark reaction, reported by MacMillan in
2008 using [Ru(bpy)3]2+ photocatalyst.[20] This paper practically
restarted the field of photoredox catalysis in organic synthesis,
merging organocatalysis and a photoredox process mediated
Application of new and effective green catalysts for the prepa-
ration of high-value molecules is a modern goal in catalysis and
synthesis.[1] In this perspective, the development of photoredox
mediated methodologies have found an impressive accelera-
tion in the past few years.[2] Photoredox catalysis based on pho-
toinduced electron transfer (PET) or energy transfer processes
were described.[3] Transition metal complexes such as ruth-
enium,[4] iridium,[5] and copper[6] have been used as photocata-
lysts to promote organic reactions, alone, or in synergistic com-
bination with nickel,[7] palladium,[8] and cobalt.[9] Organic
dyes,[10] quantum dots,[11] and semiconductors[12] were also ex-
ploited in many reactions and were found able to replace transi-
tion metal-based photocatalysts in many applications. Lastly,
abundant metals such as iron, chromium, cerium have been
also applied in photoredox catalysis.[13]
by [Ru(bpy)3]2+ [20]
The proposed photochemical mechanism
.
With the aim to develop economically and environmentally
sustainable synthetic methodologies, our group was involved
in developing catalytic processes using abundant metal Salen
complexes.[14] As Salen is an effective ligand, easily prepared
from available precursor, able to bind many metals, we won-
dered if metal Salen complexes could represent a new class of
effective photoredox catalysts. Some years ago, we have re-
ported the photophysical properties of Salen compounds,[15]
discovering that when ligand (1R,2R)-(–)-[1,2-cyclohexane-
[a] Dipartimento di Chimica “G. Ciamician”,
Alma Mater Studiorum – Università di Bologna,
Via Selmi 2, 40126, Bologna, Italy
E-mail: andrea.gualandi10@unibo.it
piergiorgio.cozzi@unibo.it
paola.ceroni@unibo.it
https://site.unibo.it/stereoselective-metal-photoredox-catalysis-lab
https://site.unibo.it/photoactive-system/en
[b] École nationale supérieure de chimie de Paris,
11, rue Pierre et Marie Curie, 75231, Paris, Cedex 05, France
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejoc.201901086.
Figure 1. Chemical structures of Salen 1 ligand and Al(III) complexes used in
this study.
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was a reductive photoinduced electron transfer of the Ru(II)
complex with the chiral enamine, formed in situ by the reaction
of an imidazolidinone catalyst and an aldehyde.
Table 1. Screening of Al(III) complex in photocatalytic stereoselective alkyl-
ation of aldehyde 7a with 8a.[a]
From this seminal paper, many efficient processes based on
Ru(II) or Ir(III) complexes were described.[21] Recently, other ex-
perimental evidences obtained by Yoon[22] has shown that the
reaction proceeds through a radical chain mechanism. This re-
action was successfully performed also with different photocat-
alysts as organic dyes,[23] semiconductors,[24] iron complexes,[25]
under visible light irradiation. Even chiral enamines can behave
as organic photocatalysts: upon photoexcitation, they become
strong reductants able to reduce the electron poor bromo de-
rivatives, thus initiating the catalytic process.[26]
We have chosen this reaction to evaluate the properties of
[Al(Salen)Cl] as photocatalysts for multiple reasons. First, alumi-
num is quite abundant and replacement of low abundant met-
als such as ruthenium and iridium is actually one of forefront
research in photoredox catalysis. Second, [Al(Salen)Cl] can have
a dual role, as a photocatalyst as well as a Lewis acid catalyst.[16]
Third, [Al(Salen)Cl] has been widely used as chiral catalyst,[16]
and these properties could be combined with its photophysical
properties.
Entry
photosensitiser
Conversion [%][b]
ee [%][c]
1
2
3
4
5
6
7
1
2
3
4
5
6
–
37
n.d.
89
88
89
89
99 (83)[d]
86
50
68
72
12
88
n.d.
[a] Reaction was performed on 0.2 mmol of 8a in DMF (1 mL). [b] Conversion
determined by 1H NMR analysis of reaction crude using internal standard
method. [c] Determined by HPLC analysis on crude reaction mixture using
chiral stationary phase. [d] Yield determined after chromatographic purifica-
tion. n.d. = not determined.
and TMSCN addition to aldehydes.[29] In addition, 2 was de-
scribed to promote the enantioselective addition of HCN to
imines.[30] Moreover, recent strategies have been developed in
which, a chiral metal complex can serve as a sensitizer for
photoredox catalysis and at the same time provide very effec-
tive asymmetric induction for the enantioselective alkylation re-
actions.[31] Thus, taking into account the fact that chiral Salen
metal complexes constitute an established class of catalysts for
asymmetric transformations, we have performed the model re-
action using 20 mol-% of morpholine as the organic catalyst
Results and Discussion
We tested several M(III) Schiff bases complexes as photocata-
lysts, using the standard reaction conditions developed by Mac-
Millan (Table 1 and Table S1). Al(III) complexes were found the
most effective catalysts. Although Al(III) complexes with Salo-
phen ligand, complexes 5 and 6, were also conceivable catalysts
(Table 1 entry 5 and 6), better conversion and yield were ob-
tained using (R,R)-[Al(Salen)Cl] (2).
The best conditions for performing the reaction were: 5 mol- (see SI, Table S2). Also in this case the desired product was
% of the photocatalyst 2, 14 mol-% of the imidazolidinone cata- isolated in good yield, but in a racemic form. Moreover, the
lyst 9 in DMF with 2,6-lutidine as the organic base and compact use of the commercially available (S,S)-[Al(Salen)Cl] ent-2 gave
fluorescent lamp (CFL, 23 W) as light source. Chiral (Salen)- practically identical results (see Table S2). Both experiments are
metal complexes catalyze an array of asymmetric nucleophile- ruling out a possible role played by the chiral (R,R)-Cl[Al(Salen]
electrophile reactions[16] including TMSN3 and carboxylic acid metal complexes in transmitting chiral information in these re-
additions to meso epoxides,[27] hetero Diels-Alder reaction,[28] actions. These optimized reaction conditions were explored
Scheme 1. Enantioselective [Al(Salen)Cl] promoted α-alkylation of different aldehydes with bromoderivatives 8a–e.
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with a series of aldehydes (7a–e, Scheme 1). In general, good a dynamic quenching process according to the Stern-Volmer
yields and enantiomeric excesses were collected for the se- Equation (1):
lected aldehydes. In addition, the reaction proceeded also with
bromo ketones (8c–e).
τ0/τ = 1 + KSV[Q] = 1 + kq τ0 [Q]
(1)
where τ0 and τ are the lifetimes in the absence and in the
presence of the quencher Q (i.e. diethyl bromomalonate 8a),
respectively, KSV is the Stern-Volmer constant and kq is the
In all the tested reactions, the use of [Al(Salen)Cl] photocata-
lyst yielded conversion values comparable to those obtained in
the case of [Ru(bpy)3]2+ [20]
although the photocatalyst loading
,
was 10 times higher in the present case compared to the origi-
nal paper.
quenching constant. The analysis of the plot in Figure 3 yielded
–1
the following quenching constant: kq = 9.5 × 108
M
s–1.
The reaction is completely inhibited when carried out in the
presence of TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy) or oxy-
gen, confirming the radical nature of the process (Scheme 2).
Scheme 2. Control test to support a radical reaction pathway.
A photophysical analysis of photocatalyst 2 and of the reac-
tion mechanism was carried out.
Compound 2 absorbs mainly in the UV region with a tail in
the visible up to 420 nm and emits with a maximum at 475 nm
and a lifetime of 1.08 ns in DMF (Figure 2). [Al(Salen)Cl] (2) is
stable under the tested reaction conditions, as confirmed by
the absorption spectrum of the reaction mixture after 6 hours
of irradiation (Figure S3). A further proof of the stability is the
fact that the lifetime of the fluorescent excited state of 2 in the
reaction mixture does not change at the end of the reaction.
Figure 3. Stern-Volmer plot of [Al(Salen)Cl] 2 in DMF solution in the presence
of increasing amounts of diethyl bromomalonate 8a.
The observed quenching process is consistent with a photo-
induced oxidative electron transfer in which compound 2 is
oxidized and 8a is reduced. Indeed, the estimated potential for
oxidation of the fluorescent excited state *2 is –1.47 V vs. SCE,
sufficiently negative to drive 8a reduction (–0.62 vs. SCE).[32]
The estimation is based on the reported potential for the first
oxidation (+1.44 V vs. SCE in acetonitrile[33]) and the energy of
the fluorescent excited state of 2 (E00 = 2.91 eV).
On the basis of the photophysical evidences, we propose
the reaction mechanism illustrated in Scheme 3. Contrary to
[Ru(bpy)3]2+, [Al(Salen)Cl] acts as a reductant for initiating the
chain mechanism. The ability of the amidoalkyl radical IV to
behave as strong reducing agent induces the reduction of
bromoderivative, regenerating radical III. The addition of radical
III to enamine II is the enantiodiscriminating step. The same
photocatalytic mechanism was previously proposed by some of
us in the case of [Fe(bpy)3]2+ photocatalyst.[25]
To explore the possibility offered by the new photocatalyst
2, application towards the synthesis of Pregabalin[34] was inves-
tigated. Pregabalin is a drug used for the treatment of epilepsy,
neuropathic pain or other anxiety disorders. The photoredox
access to racemic pregabalin was reported by MacMillan, by a
Giese type reaction.[35] For accessing Pregabalin we have con-
sidered the retrosynthetic analysis depicted in Scheme 4. Key
intermediate of the synthesis is the chiral oxo-malonate deriva-
tive accessible by our photocatalytic methodology from alde-
hyde 7f.
Figure 2. Absorption (solid line) and emission spectra (dashed line) of
[Al(Salen)Cl] 2 in DMF. λex = 340 nm.
We investigated the first step of the photocatalytic cycle by
quenching experiments. Upon addition of each of the reaction
components (at the same concentration used in the optimized
reaction conditions) to a DMF solution of complex 2, we discov-
ered that diethyl bromomalonate 8a was the only one able to
quench the fluorescence of the photocatalyst. In particular, the
Stern-Volmer plot (Figure 3) shows a linear correlation between
4-Methyl-pentanal 7f is not commercially available and was
the ratio τ0/τ and the quencher concentration, as expected for prepared by oxidation, after a careful study of different method-
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tion with NaBH4 in MeOH gave the intermediate 12 in 71 % of
yield as mixture of diastereoisomer. After hydrolysis of the ester
followed by decarboxylation, the key intermediate 13[38] for the
synthesis of Pregabalin was obtained in 83 % yield.
Scheme 6. Formal synthesis for Pregabalin.
Conclusions
We have shown that [Al(Salen)Cl] is an effective photocatalyst
based on an earth abundant element. The photophysical inves-
tigations suggested that [Al(Salen)Cl] in its fluorescent excited
state is able to directly reduce the bromo derivatives, forming
the corresponding radical species after debromination. Applica-
tion of the new photocatalyst towards the synthesis of pre-
gabalin was reported. As the Lewis acidic properties of
[Al(Salen)Cl] and its photoredox properties, we are currently ex-
ploring the use of [Al(Salen)Cl] in synergistic type of catalysis.
Scheme 3. Proposed mechanism for the Al(salen) mediated alkylation of alde-
hydes.
Acknowledgments
We thank the University of Bologna for financial support.
Scheme 4. Retrosynthetic approach to Pregabalin.
Keywords: Aluminium · Photoredox catalysis · MacMillan
catalyst · Stereoselective alkylation · Pregabalin
ologies. For all the tested methods the major problem encoun-
tered was connected with the extreme volatility of the aldehyde
7f. In addition, traces of CH2Cl2, used as reaction solvent for the
synthesis of 7f, were always present after a careful and difficult
fractional distillation and were found to inhibit the photoredox
reaction. In order to obtain the aldehyde 7f avoiding the use
of chlorinated solvents, we select the Stahl[36] procedure and 7f
was obtained after distillation in 64 % yield as 77 % mixture
with CH3CN. The solution was used directly in photocatalytic
reaction.
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Received: July 25, 2019
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
Earth-Abundant Catalysts
A benchmark photoredox alkylation of
aldehydes is effectively promoted by
using 5 mol-% of the commercially
available [Al(Salen)Cl], in the presence
of a MacMillan organocatalyst (14 mol-
%) under irradiation with a CFL light
bulb.
A. Gualandi,* M. Marchini,
L. Mengozzi, H. T. Kidanu,
A. Franc, P. Ceroni,* P. G. Cozzi* .... 1–6
Aluminum(III) Salen Complexes as
Active Photoredox Catalysts
DOI: 10.1002/ejoc.201901086
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim