Organic photoredox catalysis for the oxidation of silicates: Applications in radical synthesis and dual catalysis

Article abstract of DOI:10.1039/c6cc04636c

Metal free photooxidation of alkyl bis(catecholato)silicates with the organic dye 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyano-benzene (4CzIPN) allows the smooth formation of alkyl radicals. The latter can be efficiently engaged either with radical acceptors to provide homolytic addition products or in photoredox/nickel dual catalysis reactions to obtain cross-coupling products.

Full text of DOI:10.1039/c6cc04636c

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Organic Photoredox Catalysis for the Oxidation of Silicates:
Applications in Radical Synthesis and Dual Catalysis
Christophe Lévêque,a Ludwig Chenneberg,a Vincent Corcé,a Cyril Ollivier*a and Louis Fensterbank*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Metal free photooxidation of alkyl bis(catecholato)silicates with (E1/2*(4CzIPN/[4CzIPN]-)=+1.35 V vs SCE) augured well for a
the organic dye, 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyano- possible photooxidation of alkyl bis(catecholato)silicates
benzene (4CzIPN), allows the smooth formation of alkyl radicals. (E1/2(Ox/red)=+0.3 to ̴
+0.9 V vs SCE).3a We thus decided to
The latter can be efficiently engaged either with radical acceptors assess this reactivity with a series of alkylsilicates and the
to provide homolytic addition products or in photoredox/nickel cheapest photocatalyst 4CzIPN ($0.06/mmol for carbazole,
$2.2/mmol for tetrafluoroisophthalonitrile9) in both radical
synthesis and dual catalysis.
dual catalysis reactions to obtain cross-coupling products.
Visible-light photoredox catalysis1 has emerged as
a
The ability of 4CzIPN to oxidize benzylsilicate 1a in spin
trapping experiments with TEMPO, acting both as radical trap
and presumably also as sacrificial electron acceptor, was first
explored (Table 1).3,5 The reaction was performed in a 0.1 M
powerful strategy to generate radical species replacing more
and more tin-mediated or stoichiometric redox
methodologies.2 Many opportunities are now available to
access all kinds of C-centered radicals, based whether on
photooxidative or photoreductive processes.1 In this context,
we and subsequently the group of Molander showed recently
that the photooxidation of alkyl bis(catecholato)silicates3 is a
very convenient source of alkyl radicals. This process
constitutes a significant advance with advantageous features
compared to the photooxidation of carboxylates4 and
trifluoroborates,5 which is limited to stabilized alkyl radicals4,5
Table1 Spin-trapping experiments with TEMPO: screening of
organic photocatalysts with benzylsilicate 1a
(allylic, benzylic and
α
given their lower
-heterosubstituted radicals). In contrast,
oxidation potential, alkyl
Entry
Photocatalyst (mol%)
Eosin Y (10 mol%)
Fluorescein (10 mol%)
Acr+-Mes (10 mol%)
4CzIPN (10 mol%)
4CzIPN (1 mol%)
Yield (%)
0
1
2
3
4
5
bis(catecholato)silicates offer more scope. So far, their
effective oxidation has only been described with
photocatalysts based on expensive metals3 (Ir, Ru).
Alternatively, we have sought to explore more sustainable
0
66
92
94
processes using easily accessible organic dyes.6 In
a
preliminary study,3b it was found that only the highly oxidizing
Fukuzumi acridinium photocatalyst6a showed some activity,
limited to allylic and benzylic silicates. In 2012, Adachi et al.
described a family of carbazoyl dicyanobenzenes as light-
harvesters for organic light-emitting diodes.7 Among them,
1,2,3,5-tetrakis-(carbazol-yl)-4,6-dicyanobenzene
(4CzIPN)
displayed promising features for photoredox catalysis: a high
photoluminescence quantum yield (94.6%) and a long life-time
at the excited state (5.1 µs). Recently, Zhang et al. reported
the redox properties of this photosensitizer8 and the given
value for the oxidative potential of the photoexcited 4CzIPN
DMF solution under blue LEDS irradiation (477 nm) with 1a
(0.3 mmol), TEMPO (0.9 mmol) and the organic dye (10 mol%).
Only Fukuzumi acridinium (Acr+-Mes) and 4CzIPN gave the
expected TEMPO adduct 2a in 66% and 92% yields
respectively.3b Furthermore, the catalytic loading in 4CzIPN
could be diminished down to 1 mol% without any erosion of
the yield (entry 5). These preliminary results excited our
interested in further uses of 4CzIPN.
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Therefore, we engaged various alkylsilicates
1 in a series of
Table 3 Screening of reaction conditions for photoredox/nickel
diagnostic radical addition experiments (Table 2). Using first
allylsulfone 3a, we surmised the following catalytic cycle.3a
Photoexcited 4CzIPN* would oxidize the alkylsilicate derivative
via a SET mechanism leading to the [4CzIPN]- anion and a
hypervalent 5-coordinate silicon radical. Upon homolytic
fragmentation of the C-Si bond, the generated C-centered
dual-catalyzed cross-coupling
radical would add to allylsulfone 3a, providing a
β-sulfonyl
radical which after fragmentation liberates the allylation
adduct and a tosyl radical. A SET step would regenerate the
photocatalyst and give a sulfinate anion.
Entry
Silicate
(nb eq.)
4CzIPN
loading
Nickel
source/loading
NiCl2.dme
5 mol%
NiCl2.dme
3 mol%
NiCl2.dme
2 mol%
Ni(COD)2
3 mol%
Yield (%)
1
2
3
4
5
1.5
1.5
1.5
1.5
1.2
2 mol %
2 mol %
1 mol %
2 mol %
1 mol %
81%a
77%a
Based on this catalytic cycle, allylation adducts were
obtained in excellent yields for stabilized radicals (4ab
and secondary radical (4ad), to moderate yields for primary
radicals (4ae 4af, 4ag) (Table 2). Interestingly, silicate 1f
, 4ac)
79%a
(83 %)b
,
0%
furnished the carbon allylation adduct, without fragmentation
of the Ph2PO radical.10 Encouraged by these results, we
engaged cyclohexylsilicate 1d in various radical reactions like
alkynylation,11 vinylation11 and Giese-type reaction leading
NiCl2.dme
2 mol%
57%a
a1H NMR yield with butadiene sulfone as internal standard , 0.3 mmol
scale. bIsolated yield.
efficiently to products 4bd, 4cd and 4dd. It should be noted
that similar additions can be performed by photocatalytic
reduction of alkyl halides or related precursors but they when a Ni(0) (Ni(COD)2) precatalyst was tested (entry 4). This
generally rely on activated substrates or very special finding suggests a catalytic cycle starting from a Ni(II) species
functions.12
as previously described by Johannes et al.15
The scope of this reaction was investigated focusing
initially on the influence of aryl ring substitution (Table 4). A
better yield was observed with an electron neutral (6cg) or an
Table 2 Reaction of silicates with various radical acceptors
Table 4 Reaction of silicates with various aryl bromides
Zhang et al. also reported8 the first examples of the use of
4CzIPN as photocatalyst in photoredox/nickel dual-catalyzed13
cross-coupling reactions of aminocarboxylates and benzyl-
trifluoroborates with arylbromides. In the same vein, we
planned to extend this mixed photoorganic/metallic dual-
catalysis to alkylsilicates.14 Thus, a mixture of 4’-bromoaceto-
phenone 5a with acetoxypropylsilicate 1g in the presence of
4CzIPN (2 mol%), NiCl2.dme (5 mol %) and 4,4’-di-tert-butyl-
LED) for 24 hours at rt. In these conditions, an excellent 81%
yield of coupling product 6ag was observed (Table 3).
Furthermore, we were able to reduce the catalytic loading in
photocatalyst and nickel to 1 mol% and 2 mol% respectively,
giving 6ag in 83% isolated yield (entry 3). However, decreasing
the amount of alkylsilicate drastically affected the yield of the
reaction (entry 5). Interestingly, no product was observed
aExperiment realized with 4’-chloroacetophenone. bYield determined in
a mixture with ethyldiphenylphosphine oxide (15% of byproduct).
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electron withdrawing group (6ag) than with a methoxy
substituted substrate (6bg). Polyaromatic substrates also gave
Table 6 Reaction of silicates with alkenyl halides
the expected products (6dg-6fg) in good to moderate yields.
Moreover, when 4’-chloroacetophenone was engaged the
yield of the reaction dropped dramatically (from 83% to 14% of
6ag). This highlights the importance of the oxidative addition
step, in accordance with the postulated mechanism.13,14
Heteroaryl bromides can also act as electrophilic partners.
Pyridines
(6gg and 6hg), quinoline (6ig) or benzofuran
derivatives (6jg) provided coupling products in moderate to
good yields. In addition, the compatibility of several types of
alkylsilicates was explored with 4’-bromoacetophenone. An
assortment of key functions such as amine (6ab), ester (6ah
and 6ag) or phosphine oxide (6af) was well tolerated in the
reaction conditions. In all cases, products were obtained in
good to fair yields regardless of the stability of the generated C
centered radical. Concerning silicate 1f as before, no product
derived the generation of
a P-centered radical was
observed.10,16 Not surprisingly, chloromethylsilicate 1h did not
give the desired product (6ha). Finally, the reactivity of hex-1-
enylsilicate 1j was examined with 4’-bromoacetophenone and
aAlkenyl bromide used as substrate. bStarting from a mixture of
diastereoisomer Z/E: 15/85. cAlkenylchloride used as substrate.
dReaction performed during 48 hours.
resulted in a mixture of products 6ja, 6j’a and 6j’’a in a
10/13/77 ratio and 82% overall yield. Here, the intermediate 5-
interesting to compare this finding with the metallic
photocatalysts Ru(bpy)3(PF6)2 and [Ir(dF(CF3)ppy)2(bpy)](PF6)
(Table 7). Previously reported conditions3c,14b led to the
hexenyl radical would directly react to provide product 6ja and
6j’a resulting from post-isomerization of 6ja 17
Interestingly,
.
the formation of the major product 6j’’a implies a 5-exo-trig
cyclization step prior to the coupling process.
expected products 8dg,8d’g in 75% (Z/E: 15/85) yield with
ruthenium and 90% (Z/E: 33/67) with iridium (Table 7, entries
2 and 3). We also verified that the reaction does not follow a
In order to illustrate the versatility of 4CzIPN as
photocatalyst for this type of dual catalysis, a scale-up
experiment was accomplished using acetoxypropylsilicate 1g
and 4’-bromoacetophenone. The reaction conditions were the
same as previously except the reaction time. Indeed, after 24 h
of reaction, only 63% yield of 6ag was obtained, which could
be further improved to 76% yield by increasing the reaction
time to 48 h (Table 5).
pure radical vinylation mechanism involving an addition−β
-
elimination tandem. However, only traces of products (8fd and
8fd’) were obtained when running the reaction in the absence
Table 7 Comparison of experimental conditions for
photoredox/nickel dual-catalysis with β-bromostyrene
Table 5 Scale-up reaction
Photocatalyst
(mol %)
4CzIPN
(1 mol%)
Ru(bpy)3(PF6)2
(2 mol%)
Nickel source
(mol %)
Yield
(E/Z)
Entry
NiCl2.dme
(2 mol%)a
NiCl2.dme
(3 mol%)a
Ni(COD)2
73%b
1
2
3
4
5
6
7
(90/10)
75%b
(85/15)
93%b
(2/1)
a7% of the starting material recovered. b63% obtained after 24h of
reaction
[Ir(dF(CF3)ppy)2(bpy)](PF6)
(2 mol%)
4CzIPN
(3 mol %)a
Finally, a series of alkenyl bromides was treated under the
dual catalysis conditions (Table 6). Unactivated alkenyl
bromides were converted to their corresponding products in
No nickel
No nickel
No nickel
No nickel
<5%a
(1 mol%)
4CzIPN
(1 mol%)
S.M.b ,c
(65/35)
S.M.b ,c
(85/15)
S.M.b,c
moderate yields
bromides such as styryl bromide, styryl chloride, and
dichlorostyrene gave fair to good yields of coupling products,
(
8ag
-
8cg). However, activated alkenyl
β
-gem-
Ru(bpy)3(PF6)2
(2 mol%)
[Ir(dF(CF3)ppy)2(bpy)](PF6)
(2 mol%)
respectively 8dg and 8d’g
,
8eg and 8fg
.
(40/60)
Concerning β-bromostyrene 6d
/
6d’, complete retention of the
a
Nickel catalyst and 4,4'-di-tert-butyl-2,2'-bipyridine engaged in a 1/1
ratio. bStarting from a commercial mixture of diastereoisomer E/Z:
85/15. cExperiment performed without silicate.
double bond geometry was observed when using each pure
isomer. Same result was observed starting from the
commercial mixture of both isomers (ratio Z/E: 15/85). It was
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Chem. Commun., 2013, 49, 7249; (c) Y. Li, K. Miyazawa, T.
Koike and M. Akita, Org. Chem. Front., 2015, , 319.
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of nickel salt and dtbbpy ligand (entry 4) which precluded the
2
exclusive radical pathway. Isomerization of β-bromostyrene in
6
7
the presence of a photosensitizer has been already reported.18
Control experiments were realized in the absence of nickel
catalyst and silicate and showed that the bromide slightly
isomerized in the presence of 4CzIPN (E/Z 65:35) compared to
an E/Z ratio of 40:60 with the iridium complex (entries 5 and
7). In this case, a heavy atom effect, brought by the bromide
atom would easily give access to the triplet state19 of the styryl
derivatives which promotes the isomerization. In contrast, no
isomerization was observed with the ruthenium complex
(entry 6) probably due to the lower energy state of the
photoexcited [Ru(bpy)3]2+*. Thus, even if this organic-dye can
4
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In comparison: $1.3k/mmol for [Ir(dF(CF3)ppy)2(bpy)](PF6),
$110/mmol for Ru(bpy)3(PF6)2. Prices available at
http://www.Sigmaaldrich.com.
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In conclusion, all these results demonstrate that 4CzIPN is
as efficient as metal based photocatalysts for the
photooxidation of alkyl bis(catecholato)silicates. The
generated radicals can be engaged efficiently with radical
acceptors and also in organic photoredox/nickel dual-catalysed
processes with (hetero)aryl halides and alkenyl halides
affording in that case alkene derivatives with high
stereoselectivity. Requiring only a low catalytic loading (1
mol%), 4CzIPN opens very exciting perspectives for more
sustainable photoredox catalysis.
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We thank CNRS, UPMC, IUF, MSER (ASN PhD grant to CL),
LABEX MiChem (ANR-11-IDEX-0004-02), La Région Martinique
(PhD grant to LC), ANR NHCX (11-BS07-008, postdoc grant to
VC) and COST Action CM1201. Omar Khaled is also
acknowledged for HRMS.
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