Allylation of aldehydes by dual photoredox and nickel catalysis

Article abstract of DOI:10.1039/c9cc03344k

Here we report the application of dual nickel/photoredox catalysis to the allylation of aliphatic, aromatic and heteroaromatic aldehydes by using commercially available reagents. The process utilizes the combination of a Ni(ii) complex, [Ru(bpy)₃]²⁺ as a photoredox catalyst, and allylacetate under blue LED irradiation, and allows the synthesis of a large variety of homoallylic alcohols.

Full text of DOI:10.1039/c9cc03344k

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DOI: 10.1039/C9CC03344K
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Allylation of Aldehydes by Dual Photoredox and Nickel Catalysis
Andrea Gualandi,*a Giacomo Rodeghiero,a,b Adriana Faraone,a Filippo Patuzzo,a Marianna
Marchini,a Francesco Calogero,a Rossana Perciaccante,b Thomas Paul Jansen,b Paola Ceroni,*a and
Pier Giorgio Cozzi*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Here we report the application of dual nickel/photoredox catalysis catalytic generation of an active organometallic intermediate
to the allylation of aliphatic, aromatic and heteroaromatic by photoredox processes is restricted to cross-coupling
aldehydes by using commercially available reagents. The process reactions.9 Although interesting allylation reactions of
utilizes the combination of a Ni(II) complex, [Ru(bpy)3]2+ as carbonyls were recently described by applying photoredox
photoredox catalyst, allylacetate under blu LED irradiation, and methodologies,10 these methods lack generality and are
allows the synthesis of a large variety of homoallylic alcohols.
limited in scope. Quite recently, a remarkable example of
diastereoselective allylation of aldehydes (Figure 1) through a
combination of photoredox and chromium(II) catalysis was
reported by Glorius,11a and Kanai.11b Kanai was also able to find
an highly enantioselective version of the reaction. In both
cases, the photocatalyst is acting as oxidant in its excited state,
forming allylic radicals from alkenes that are intercepted by
Cr(II) complexes. As the high oxidation potential, propene was
not reactive in the conditions, differently from other
substituted alkenes. Herein, we disclose a nickel based dual
photoredox approach for allylation reaction.
The allylation of carbonyl compounds is crucial
a
transformation in organic synthesis for the preparation of
homoallylic alcohols.1 Organometallic allylating compounds2
are remarkable reagents for synthesis and they are easily
accessible by Barbier methodologies.3 However, in many
organometallic procedures a stoichiometric amount of metal is
crucial. In the catalytic version of the Nozaki-Hiyama-Kishi
reaction, a chemoselective and well-established allylation
methodology based on chromium,4 the use of an excess of
manganese as terminal reductant is also mandatory, the
reaction is heterogeneous, and the final work-up of the
reaction results in the production of waste (Mn, Mn salts, etc.).
To avoid the employment of stoichiometric amount of metal
as reductant, allyl species formed under electrochemical
conditions were able to react with carbonyl compounds.5
In recent years photoredox catalysis has attracted
considerable interest and by the use of metal complexes, dyes,
or semiconductors, can give interesting methodologies to form
radical species by electron transfer (ET) or energy transfer.6
Metallaphotoredox catalysis, i.e. metal catalysis merged with
photoredox catalysis, is a new and rapidly growing research
subject.7 The key seminal papers, published by Sanford,8a
Molander,8b Doyle and MacMillan,8c were followed by many
other publications, dealing with new concepts, and with the
applications of different metals.7 In these methodologies, the
Glorius (2018)
OH
O
Ir
H
R
Cr
Ni
+
R
R = electron-rich aryl,heteroaryl, amino d.r. >19:1
to 11:1; up to 95% yield
Kanai (2019)
R2
Ni
O
OH
*
R3 R4
H
R1
PC
Cr
*
R4
+
R1
R2
L*
Mg
R3
PC = organophotocatalyst, L* = BOX, R1, R2, R3, R4 = alkyl
d.r. 20:1, ee 85-99%, up to 97% yield
This work: simple and general nickel/photoredox
allylation of aldehydes
O
OH
Ni
Ni
Ru
+
X
R
R
R = H, Ph, alkyl
d.r. 1:1-6:1, up to 89%
a. Dipartimento di Chimica “G. Ciamician”, ALMA MATER STUDIORUM Università di
Bologna, Via Selmi 2, 40126 Bologna, Italy; E-mail: piergiorgio.cozzi@unibo.it;
paola.ceroni@unibo.it; andrea.gualandi10@unibo.it
X = OAc, OC(O)OR', OC(O)tBu
Figure 1. Dual photoredox allylation methodologies.
b. Cyanagen Srl, Via Stradelli Guelfi 40/C, 40138, Bologna, Italy
† Electronic Supplementary Information (ESI) available: Screening tests, further
substrates studied. Photophysical measurements, electrochemical and quenching
studies. Detailed procedures, description and copies of NMR spectra for all
compounds. See DOI: 10.1039/x0xx00000x
The catalytic use of Ni(II) with bipyridine (bpy) ligands in
allylation and Reformatsky reactions was described by
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Durandetti and Périchon,12 in the presence of a stoichiometric moderate as
Journal Name
a
function of the presence of electron-
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amount of manganese or zinc metal as terminal reductants.13 withdrawing groups. In the case of aromatic aldehydes, the
In addition, an enantioselective allylation reaction of reaction is clean, and no reduction of pinacol coupling by-
DOI: 10.1039/C9CC03344K
1
aldehydes described with catalytic amount of Ni(II) complexes products was observed. In the H-NMR spectra of the crude
in the presence of BOX ligands and zinc as the stoichiometric reaction mixtures just signals of the un reacted aldehydes are
reductant, was reported.14 Therefore, we began our present. Sensitive pyrrole or electron rich substrates are quite
investigations by studying the allylation of 2-naphthaldehyde compelling reactants. Linear aliphatic and unsaturated
(1a), as model substrate, in the presence of different allylating substituted aldehydes are suitable substrates as illustrated in
reagents, nickel sources, photocatalysts, reaction solvents, and Scheme 2. More encumbered alkyl aldehydes showed a
additives (see ESI for full details). The systematic evaluation of reduced reactivity while pivalaldehyde was found not reactive
all reaction parameters enabled us to optimize the reaction (see Scheme S1), probably due to sterical reasons. In the case
conditions, obtaining 3a in 75% isolated yield after 48 h of of aliphatic aldehydes, byproducts derived from aldol
irradiation employing [NiBr2(glyme)] (10 mol%), allylacetate 2a condensation are revealed.
(3 equiv.),15 diisopropylethylamine (DIPEA,
3
equiv.),
[NiBr2(glyme)], 10 mol%
[Ru(bpy)3]Cl2, 1 mol%
OH
[Ru(bpy)3]Cl2 photocatalyst (1 mol%), o-phenanthroline (15
mol%) as ligand for nickel(II), under blue LEDs irradiation at
450 nm (Table 1). It is important to remark that we observed a
decisive role of the ligand of Ni(II) ions and its sterical
encumbrance (see ESI for all ligands screened).
O
OAc
+
Ar
Ar
1b-q
o-phenanthroline, 15 mol%
DIPEA, 3 equiv.
2a (3 equiv.)
3b-q
CH3CN, Blue LEDs, rt, 48h
(0.2 mmol)
OH
OH
OH
Table 1. Allylation reaction and variation of some reactions parameters
3d, 56%
3b, 59%
3c, 80%
tBu
Ph
O
O
OH
[NiBr2(glyme)], 10 mol%
[Ru(bpy)3]Cl2, 1 mol%
OH
OH
OH
OAc
+
O
o-phenanthroline, 15 mol%
2a (3 equiv.)
1a
DIPEA, 3 equiv.
3a
O
3g, 89%
3e, 75%
3f, 97%
MeO
CH3CN, Blue LEDs, rt, 16h
OH
OH
OH
Entry
Deviation from standard
conditions
Conversion (Yields)b
MeO
Me2N
3j, 26%
3i, 29%
3h, 32%
1
2
none
68
65
NC
[Ir(ppy)2(dtbbpy)](PF6) instead
of [Ru(bpy)3]Cl2
O
O
OH
OH
Cl
OH
3
{Ir[dF(CF3)ppy]2(dtbbpy)}(PF6)
instead of [Ru(bpy)3]Cl2
No DIPEA
Allyl bromide instead of
allylacetate
31
3k, 66%
3m, 58%
3l, 70%
CF3
OH
Cl
4
5
0
6
OH
OH
OH
Boc
S
N
Ph
S
N
3n, 63%
3o, 52%
6
7
No nickel
No light
0
0
3p, 67%
Boc
3q, 45%
8
9
10
11
No degassed solvent
30 mol% o-phenanthroline
[Ni(II)(o-phen)3](BF4)2
48 h of irradiation
0
51
45
Scheme 1. Dual nickel photoredox allylation of aromatic aldehydes.
These are due to the formation of secondary amine from the
decomposition of DIPEA, capable of forming enamines. The
amount of these byproducts was reduced by operating in the
presence of water, that avoids the formation of reactive
enamines, and self-condensation of aliphatic aldehydes.
Therefore, the experimental conditions were slightly modified
in order to improve the yields (see ESI for details about
optimization). In general, for both aromatic and aliphatic
aldehydes functional groups were well tolerated, including
halides, CF3, esters, nitriles, ethers, as well as acidic NH groups.
Reaction with chiral aldehydes gave quite moderate
diastereoisomeric ratio and allyl acetates bearing substituents
in three position can be employed (See ESI for details). The
photochemical mechanism of the reaction was investigated by
analysis of the quenching of the photocatalyst luminescence
by each of the components of the reaction. In particular, no
change of the decay of the emission intensity of [Ru(bpy)3]Cl2
94(75%)c
a Reactions were performed on 0.1 mmol scale. b Determined by 1H-NMR analysis.
Isolated yield after chromatographic purification, reaction performed on 0.2
mmol scale.
c
For convenience, we have used the in-situ protocol adding 15 mol%
of o-phenanthroline as the better yield obtained (see entry 9 and 10
in Table 1,16 and S8). From a practical point of view, the reaction
was sensitive to traces of oxygen, as expected by the use of Ir(III)
and Ru(II) polypyridine complexes, characterized by long lifetime of
the lowest excited state, but degassing the reaction mixture by
freeze-pump-thaw cycles was sufficient for
outcome. It is worth noting that the photocatalyst does not
decompose during the photoreaction (see Figure S2).
a good reaction
The generality of our initial finding was studied with a variety
of aromatic and aliphatic aldehydes, and Schemes 1 and 2
illustrate the salient results obtained. Yields are good to
2 | J. Name., 2012, 00, 1-3
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was observed upon addition of allyl acetate 0.6 M, o- stoichiometric reductants, Weix in 2012 also suggested a
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phenanthroline 0.03 M, or hydrocinnamaldehyde 4e 0.2 M similar mechanistic picture.21 However,
a
careful
DOI: 10.1039/C9CC03344K
(same concentrations used to perform the reaction, Figure S3). electrochemical investigation, performed in CH3CN, the same
solvent used in the reactions, suggests a different scenario.
The cyclic voltammogram of [NiII(phen)3]2+in CH3CN21 shows a
[NiBr2(glyme)], 10 mol%
[Ru(bpy)3]Cl2, 1 mol%
OH
O
OAc
reversible one-electron reduction process at -1.31 V (vs SCE,
see Table S11 and Figure S8). Upon addition of allyl acetate,
the cyclic voltammogram displays a chemically irreversible
reduction process with a cathodic peak at almost the same
potential value and higher cathodic current, compared to
[NiII(phen)3]2+. These experimental results can be accounted
for by a chemical reaction of [NiI(phen)3]+ with allyl acetate,
immediately followed by its reduction at less negative
potential compared to the pristine [NiII(phen)3]2+ complex.39
This mechanism is confirmed by digital simulation of cyclic
voltammograms (see ESI for more details, Figure S9).22
+
R
R
o-phenanthroline, 15 mol%
DIPEA, 3 equiv.
4a-m
(0.2 mmol)
2a (3 equiv.)
5a-m
CH3CN:H2O, 9:1
Blue LEDs, rt, 48h
OH
OH
OH
6
4
5c, 54%
5b, 42%
5a, 68%
OH
OH
OH
O
5e, 60%
5f, 48%
OH
5d, 25%
O
OH
OH
BocHN
CbzHN
BnOOC
TosHN
NC
- e-, - H+
N
N
5h, 42%
OH
5i, 54%
5g, 59%
N
OH
OH
BnO
SET
4
4
3
5k, 69%
5j, 46%
OH
5l, 38%
*[Ru(bpy)3]2+
oxidant
[Ru(bpy)3]+
reductant
_
(+)
OH
5m, 43%
5n, 30% anti:syn 1:3
Scheme 2. Dual nickel photoredox allylation of aliphatic aldehydes.
OAc
OAc
[LnNiI]+
(II)
On the other hand, DIPEA and [NiBr2(glyme)] quench the
emission of the photocatalyst with quenching constants of 7.7
x 106 M-1s-1and 4.7 x 109M-1s-1, respectively, as determined
from Stern-Volmer plots (Figures S4 and S5). However, it is
worth noting that [NiBr2(glyme)] is a precatalyst and, in the
presence of o-phenanthroline ligand, different Ni(II) complexes
are formed in which bromide and glyme ligands are replaced
by one, two or three o-phenanthrolines (see titration
experiments reported in Figure S7).17 Interestingly, quenching
of the photocatalyst is no longer observed upon concomitant
addition of [NiBr2(glyme)] precatalyst and o-phenanthroline
ligand (ca. 3 equiv. per Ni(II) ion, Figure S6). Therefore, we
suggest the mechanism depicted in Figure 2. The dual catalytic
cycle is initiated by the well-known reductive quenching of the
[Ru(bpy)3]2+ photocatalyst by DIPEA,18 producing the reduced
SET
SET
[Ru(bpy)3]2+
[LnNiII]
L = phen, glyme
(III)
[LnNiII]2+
(I)
HO
OAc
R
[LnNiII]
R
RCHO
H+
O
(IV)
Figure 2. Suggested mechanism for the allylation reaction with the allylacetates.
In conclusion, we developed a novel photoredox nickel
catalyzed allylation of aldehydes, by the in situ formation of
3-allylnickel complex, using inexpensive tertiary amine as
sacrificial reductant, and avoiding the use of metals such as
magnesium, zinc, or manganese. The reaction shows broad
scope and it is quite tolerant to functional groups. Pleasantly,
esters and ketones did not affect the reaction. Further studies
about the preparation of organometallic reagents by
photoredox dual approaches and about the improvement of
the enantioselective variant of this reaction24 are under active
investigations in our laboratory.
[Ru(bpy)3]+ complex (E =-1.33 V vs SCE).19 The photo-
½
generated [Ru(bpy)3]+ complex is able to reduce not only
[LnNiII]2+(I) to [LnNiI]+ (II), but also the nickel complex
intermediate containing the allyl ligand to generate the
catalytically active species [LnNiII(3-allyl)(OAc)]41 (III). The
homoallylic alcohol is presumably formed by protonation of
the homoallylic 1-nickel complexes (IV, Figure 2), or by
reductive elimination of the formed Ni hydride, due to the
presence of proton released by the decomposition of oxidized
DIPEA.23
The catalytic nickel-mediated allylation reactions reported in
literature20,21 suggested the presence of a Ni(II)/Ni(0) cycle. In
cross coupling reaction of aromatic substrates, performed with
allylic acetates in the presence of Zn(0) or Mn(0) as
Conflicts of interest
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 3
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13 Martin has described a nickel-catalytic carboxylation of allylic
There are no conflicts to declare.
View Article Online
alcohols with CO2, in the presenceDOoIf: 1a0.10p3h9e/nCa9CnCth0r3o3l4in4Ke
derivative as ligand, and zinc as stoichiometric reductant;
see: M. van Gemmeren, M. Börjesson, A. Tortajada, S.-Z.
Sun, K. Okura and R. Martín, Angew. Chem. Int. Ed. 2017, 56,
6558-6562.
Acknowledgements
Cyanagen is acknowledged for financial support to G.
Rodeghiero, and for the gift of chemicals and supply. Johnson
Matthey Catalysis is acknowledged for the generous gift of
precious metals. Dr. G. Bergamini is acknowledged for
scientific discussions.
14 Z. Tan, X. Wan, Z. Zang, Q. Qian, W. Deng and H. Gong, Chem
Comm. 2014, 50, 3827-3831.
15 For the important enantioselective allylation protocol
described by Krische, that uses allylacetate in the presence
of available [Ir(COD)Cl]2 and chiral phosphines, see: S. W.
Kim, W. Zhan and M. J. Krische, Acc. Chem. Res. 2017, 50,
2371-2380.
16 [Ni(phen)3](BF4)2 was prepared by modification of reported
procedure (we have used NH4BF4 instead NaClO4), see SI for
further details; W. H. Smith and Y.-M. Kuo, J. Electroanal.
Chem. Interfacial Electrochem. 1985, 188, 189-202.
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3
4
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5
6
Recently, electrochemical methodologies applied to organic 21 (a) Y. G. Budnikova, D. G. Yakhvarov, V. I. Morozov, Y. M.
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the presence of different amount of phenanthroline. As
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stoichiometry exist in equilibrium, and the corresponding
voltammetric curves are rather complex. As the isolated
[Ni(o-phen)3](BF4)2 was active in the reaction, and gave quite
similar results in term of yields, we have performed the
voltammetric studies with this complex.
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7
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% ee.
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