Metal-free desilylative C-C bond formation by visible-light photoredox catalysis

Article abstract of DOI:10.1039/c8cc10239b

A newly developed methodology for the use of organosilanes as radical precursors under metal-free and visible-light conditions is presented. The strong oxidant character of the 9-mesityl-10-methylacridinium salt in its excited state enables the transforma

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Metal-Free Desilylative C-C Bond Formation by Visible Light
Photoredox Catalysis
Mustafa Uygur,a Tobias Danelzika and Olga García Mancheño*a
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
counterion remain as waste, which points out a suboptimal atom
A newly developed methodology for the use of organosilanes as
radical precursors under metal-free and visible light
conditions is presented. The strong oxidant character of the 9-
mesityl-10-methylacridinium salt in its excited state enables
10 the transformation of simple silanes to the corresponding
carbon-centered radicals, which were trapped by various
acceptor molecules.
50 economy of this methodology. Moreover, expensive Ir(III) or
Ru(II) complexes are used as photocatalysts, which leads to
another drawback as far as up-scaling of this process is considered.
Alternatively, Melchiorre and coworkers reported a photocatalyst-
free approach, in which an in situ formed chiral iminium excited
55 species was used to cleave the C-Si bond of organosilanes and
allow enantioselective -alkylation reactions of aldehydes
(Scheme 1, middle).11 In order to address some of the current
limitations and expand the use of simple organosilanes as alkyl-
radical precursors, we aimed at developing a visible light mediated
60 C-C bond forming methodology in the presence of an
organophotocatalyst, without the necessity of a derivatization step
at the Si-atom and further additives (Scheme 1, bottom).
Visible light photoredox catalysis has gained significant
importance in synthetic organic chemistry in the past few years.1
15 While a great diversity of photocatalysts for photoreductions and
oxidations already exists, the number of substrates acting as alkyl
C-radical precursors is still limited.2 In this regard, halide-
containing precursors provide the corresponding radical upon
reductive conditions, but they often lead to the formation of
20 halogenated side-products (e.g. via atom transfer radical addition
(ATRA) process) and require sacrificial electron donors like
amines in excess to close the catalytic cycle.3 Changing the
reactivity to the photooxidation pathway enables the access to
alkylic C-radicals by using carboxylic acids (Eox = ~ +1.3 – 2.0 V
25 vs. SCE)4 and organotrifluoroborates (Eox ~ +1.4 V vs. SCE)5 in
the presence of strong photooxidants, such as Ir(dF-CF3ppy)2
(dtbpy)3PF6 (E*1/2 (Ir(III)*/ Ir(II)) = +1.21 V vs. SCE)6 or 9-
mesityl-10-methyl-acridinium salt (E*1/2 (Mes-Acr+*/Mes-Acr°) =
+2.06 V vs. SCE).7 However, photocatalytic decarboxylation
30 reactions require a base as additive in superstoichiometric
amounts, as well as sacrificial electron acceptors such as
diphenyldisulfide, which can form further undesired thiol-
containing waste during the photoredox process. The problematic
production of toxic and corrosive side- compounds, such as BF3,
35 might also appear if the substrate is switched to
organotrifluoroborates, which additionally show poor solubility in
organic solvents.
In 2015 and 2016, the groups of Fensterbank8 and Molander9
published new methods for the use of organosilicon compounds
40 under visible light conditions (Scheme 1, top).10 These approaches
require an additional synthetic step, which implies the
transformation of the organosilanes to the corresponding activated
bis(catecholato)silicates. Besides the addition to Michael
acceptors,8a these intermediates allowed the performance of a
45 broad variety of C-Csp3 couplings in the presence of additional
nickel catalyst.8-9 Unfortunately, after the photocatalytic transfer of
the corresponding alkyl-group, the resulting catecholatosilane/
silicate and the stabilizing potassium 18-crown-6 or ammonium
Previous work:
O
O
O
Ir(III)/Ru(II) catalyst
Fensterbank et al.
Molander et al.
Si
R'
R' R''
O
visible light
R''-X
X
*
F
F
Ar
Ar
O
visible
light
R2
SiMe3
N
Melchiore et al.
Iminium
OTDS
excited
Im*

R2
no catalyst
R1
R1
This work:
Organo-
photocatalyst
- metal-free
- visible light
- mild conditions
R2
R2
visible light
R1
SiMe3
R1
EWG - no pre-Si-derivatization
EWG
Scheme 1. Organosilicon compounds as alkyl-radical precursors.
65
We started our investigations with the optimization of the
oxidative C-Si bond cleavage of the commercially available, cheap
and bench-stable benzyltrimethylsilane (1a) (Table 1). The
redoxpotential of this compound is reported as Eox = +1.68 V vs.
SCE in the literature,12 which induced us to use
70 organophotocatalysts with a highly oxidizing excited state.
Therefore, the 9-mesityl-10-methylacridinium perchlorate salt I
(E*1/2 = +2.06 vs. SCE),7 the imido-acridinium perchlorate salt II
(E*1/2 = +2.40 vs. SCE),13 which was previously developed in our
group, and the triphenylpyrylium tetrafluoroborate salt III (E*1/2
=
75 +1.9 vs. SCE)14 were explored. In the presence of organo-
photocatalyst I-III (10 mol%) and the Michael acceptor N-
phenylmaleimide (3a) (3 equiv.) as trapping reagent, the reaction
mixture was stirred for 18 h in CHCl3 under N2 atmosphere by
irradiation with a stripe of blue LEDs in the photoreactor. To our
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Table 1. Optimization of the photocatalytic C-Si bond cleavage.[a]
(3b) and the unprotected maleimide (3c) gave the corresponding
products 2 in good yields (71-77%). Switching to the closely
related maleic anhydride (3d), as well as to the open form dimethyl
35 fumarate (3e), gave us the dicarboxylic and diacid monoester17
coupling products in moderate to good yields. Furthermore, the
functional group tolerance was tested by the use of the acceptor
molecules phenylsulfonylmethylacrylate (3f) and sulfonylethylene
(3g). In both cases, the desired compounds 2f and 2g were obtained
40 in high yields (71 and 83%, respectively) without the occurrence
of any side-products from the interaction of either the silane 1a or
the radical intermediate with the functional groups.
O
O
3a (x eq.)
N-Ph
TMS
N
O
photocatalyst (x mol%)
CHCl3, r.t., photoreactor
O
1a
2a
O
Ph
Cy
O
N
O
N
N
Ph
-
-
-
BF4
ClO4
ClO4
II
Table 2. Scope of the Michael trapping reagents 3.[a],[b]
photocatalyst I (10 mol%)
I
III
Entry
Cat. (x mol%)
I (10)
3a (x eq.)
Atm.
Yield (%)[b]
EWG 3 (1.5 eq.)
EWG
TMS
CHCl3, air, r.t., 18 h
photoreactor
1
2
3
3
N2
N2
N2
N2
N2
air
O2
air
air
air
30
36
16
--[c]
--[c]
49
17
46
35
78
1a
2
II (10)
Acceptor 3
2, Yield
Acceptor 3
2, Yield
O
O
O
O
3
III (10)
3
N
NH
N
NH
4
Na2-Eosin Y (10)
3
O
O
O
O
5
Ru(bpy)3(PF6)2 (10)
3
3b
2b: 77%
3c
2c: 71%
O
O
CO2Me
6
I (10)
I (10)
I (5)
3
CO2Me
CO2Et
CO2H
7
3
MeO2C
CO2Me
2e: 45%
SO2Ph
SO2Ph
O
3d
2d: 64%[c]
CO2Me
3e
8
3
CO2Me
9
I (2.5)
I (10)
3
SO2Ph
SO2Ph
3g
10
1.5
SO2Ph
3f
2f: 71%
2g: 83%
[a] Reactions in 0.2 mmol scale. [b] Isolated yields. [c] Determined by GC-MS.
45 [a] All reactions in a 0.2 mmol scale. [b] Isolated yields. [c] From ring-opening and
esterification of 3d (Note: the used p.a. CHCl3 contains 0.5-1% EtOH).
delight, the desired product could be obtained in all three cases in
moderate yields (Table 1, entries 1-3). Both acridinium salts I and
II led to similar results (30 and 36% yield, respectively), while the
pyrylium catalyst III showed the lowest catalytic activity (16%).
Moreover, as expected, the reaction with Na2-Eosin Y (E*1/2
(EY°*/EY°-) = +0.83 V vs. SCE).15 or Ru(bpy)3(PF6)2 (E*1/2
5
Under the optimized conditions,
a library of alkyl-
trimethylsilanes with various substitution patterns and electronic
properties were next applied in the desilylative C-C bond
50 formation reaction with 3a (Table 3). We were pleased to see that
the coupling products could be obtained in moderate to good yields
for neutral and electron-rich benzylsilanes 1h-r. Different methyl
substituted derivatives were initially investigated, showing that
mainly all the positions at the aromatic system (2h-j, 40-76%) as
55 well as at the benzylic site (2k, 50%) were well tolerated. This
might be not only caused by the higher production of side-
products, but also by the steric hindrance of the formed radical by
the neighbouring methyl group. The reaction with other alkyl (4-
tBuC6H4, 1l) and aryl (2-naphtyl, 1m or 4-PhC6H4, 1n) substituted
60 benzylsilanes also proceeded smoothly, providing the
corresponding products 2l-n. Furthermore, the reaction of
benzylsilanes with strong electron-donating cyclic ketal (1o),
methoxy (1p) and methylsulfide (1q) substituents, which are
known to lower the redoxpotential of the aromatic system and lead
65 to further side-products in the presence of strongly oxidizing
conditions, led to the desired products in up to 76% yield.
Additionally, excellent chemo-selectivity was observed in the case
of 1q, which occurred without formation of the corresponding
sulfoxide by oxidation of the functional group. Halogen-
70 substituents, such as fluoride, bromide and iodine, led to the
coupling products in good yields (2r-t, 89%, 84% and 50%,
respectively), while the more demanding electron-poor substrates,
10 (Ru(III)*/Ru(II)) = +0.77 V vs. SCE)6 gave no conversion (entries
4-5). Due to the availability as well as its better solubility in
chloroform, catalyst I was used for further optimization. The
atmosphere was then changed from nitrogen to air and oxygen
(entries 6-7), which resulted in an increase of the yield to 49%
15 when air was used. However, a significant drop to 17% was
obtained when the reaction was performed under oxygen. We
assume that the regeneration efficiency of the reduced
photocatalyst is enhanced by the reaction with oxygen that leads to
the in situ formation of the super-oxide radical anion (O2•-),16
20 which might be generated in too high concentrations under pure
oxygen atmosphere, leading to a large number of undesired side-
products by radical chain processes. Next, the catalyst loading was
evaluated (entries 8-9). A small decrease of the yield was obtained
when 5 mol% of catalyst I was used, while 2.5 mol% gave 2a in
25 only 35%. Pleasantly, a decrease to 1.5 equivalents of 3a gave the
best results of 78% yield (entry 10). Furthermore, a short solvent
screening showed that in comparison to CHCl3 all the other tested
solvents gave the desired product in notably lower yields (see S.I.
for details).
30
With these results in hand, the scope of applicable Michael
acceptors was explored (Table 2). Thus, the N-methyl protected
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such as the 4-CF3 derivative 1u provided 2u in a moderate 35%
yield. Finally, other organosilanes bearing an amine, thioether or
ether functionality were also efficiently enrolled (50-94% overall
yields, 4a-e). Interestingly, in the reactions between 3a and this
photocatalyst and only traces of product could be obtained in the
presence of I.20 Moreover, we determined the fluorescence
quantum yield for the standard reaction as 0.92 (see S.I.), which
confirms our presumption that a radical chain propagation is not
≤
1). We then focused on the
5
type of substrates with electron rich aromatic substituents, a 30 the predominant pathway (Φ
subsequent ring-closing process was observed, leading to a
tricyclic species 4a’, 4d’ and 4e’ as the major or sole product.
detection of the radical intermediate formed by the oxidation of the
C-Si bond. Therefore, the standard reaction of 1a with 3a in the
presence of the photocatalyst I under visible light irradiation was
conducted with 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO) (1.1
35 equiv.) of as additive (Scheme 2). We were pleased to observe the
TEMPO-adduct 6 of the radical-intermediate 5, as well as the
homo-coupling 7, by GC-MS analysis. However, we were not able
to detect the TEMPO adduct of intermediate 8, since the reactivity
of this species is much higher than that of the radical 5.
Table 3. Scope of the benzyltrimethylsilane derivatives 1.[a],[b]
3 (1.5 eq.)
photocatalyst I (10 mol%)
R2
R2
R1
R1
TMS
EWG
O
CHCl3, r.t., air, photoreactor
O
2 or 4
1
O
N
N
O
Ph
N
Ph
Ph
Ph
Ph
O
O
O
2a: 78%
2j: 76%
2h: 54%
2i: 40%
O
O
N
N
Ph
N
tBu
O
O
O
O
O
2k: 50%[c]
2l: 62%
O
O
N
Ph
N
Ph
N
Ph
O
40
Ph
O
O
O
2m: 37%
2n: 58%
2o: 40%
Figure 2. Fluorescence quenching of I by different concentrations of 1a (Q).
O
O
O
Our control experiments (see S.I., Table S7) indicated the
importance of the presence of small amounts of HCl, formed
presumably by the decomposition of CHCl3 over the time under
45 light and moisture exposure. Thus, when freshly distilled CHCl3
was used, low conversions and the formation of 8% of the radical
homocoupling product 7 were observed. Moreover, extra-added
H2O generally hampered the reaction (see S.I. TS6-7).21 We were
then interested in identifying the source of the necessary proton for
50 the formation of the products 2 and 4a-c. Therefore, deuterium-
labelling experiments were carried out. While the formation of the
deuterated product 2aD-d or 2a-d was not observed in the reaction
with benzylTMS-d7 (Scheme 2, eq. 2) or with dry CDCl3 as
solvent (Scheme 2, eq. 3), the use of CHCl3 in the presence of DCl
55 in D2O resulted in the formation of 2a-d as the major compound in
a 2a:2a-d 22:78 ratio (Scheme 2, eq. 3).21 Moreover, the blank
reaction using HCl as potential promoter without the presence of
the catalyst and light irradiation did not proceed (see S.I., TS7).
With these results in hand, a postulated reaction mechanism is
60 outlined in Scheme 2 (bottom). The excitation of the catalyst in its
ground state Mes-Acr+ I by absorption of visible light generates
the photoexcited state I+*. In this state, I+* is able to oxidize the C-
Si bond of 1 by a single electron transfer (SET) to form the radical
intermediate 5 and the reduced catalyst I• after desilylation
65 promoted by a weak nucleophilic solvent (e.g. 0.5-1% of EtOH
present in CHCl3 or water).22 Radical 5 adds to the double bond of
the acceptor 3 to form the radical 7. Then, a back-electron transfer
(BET) from I• to 8, forming the anionic intermediate 9 and
regenerating the photocatalyst I in its ground state, closes the
70 catalytic cycle.16 Finally, the product 2 is formed after the
protonation of 9 by traces of HCl present in the solvent.
N
Ph
N
Ph
N
Ph
X
S
O
O
O
MeO
2p: 76%
2q: 72%
2r: X = F, 89%
2s: X = Br, 84%
2t: X = I,
2u: X = CF3, 35%
50%
SO2Ph
SO2Ph
O
Ph
O
N
N
N
Ph
O
4b: 72%
N
O
N
SO2Ph
SO2Ph
4a:4a': 39:61
N
60%
Boc
4c: 50%
O
N
O
O
O
O
Ph
N
Ph
N
Ph
4d:4d': 45:55
94%
S
O
O
O
42%
52%
4e': 50%
10 [a] Reactions in a 0.2 mmol scale. [b] Isolated yields. [c] Obtained in a 79:21 d.r.
Motivated by these results, we decided to analyse the
mechanism of the photocatalytic process (Scheme 2). Initially,
fluorescence experiments were carried out by excitation of catalyst
Mes-Acr
I at 420 nm and measuring the corresponding
15 fluorescence in the presence of different amounts of silane 1a as
quencher (Q) (Figure 2).18 As already predicted, a decay in the
fluorescence was observed upon addition of increasing
concentrations of the quencher, which also evidences that the
singlet excited state of the photocatalyst is responsible for the
20 reactivity. To prove that the reaction is photocatalyzed and not
initiated by radical chain processes caused by the homolytic
cleavage of the C-Cl-bond of chloroform to the corresponding
chlorine and trichloromethane radical, a test reaction in BrCCl3 as
a radical starter was next performed (Scheme 2, eq. 1).19 No
25 conversion of 1a could be achieved in the absence of the
Conclusions
In summary, we were able to develop a novel application of simple
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35 2 a) L. Fensterbank, J.-P. Goddard and C. Ollivier, Visible-Light-
Mediated Free Radical Synthesis, In Visible Light Photocatalysis in
Organic Chemistry, Wiley-Blackwell, 2018, p. 25-71; b) J.-P.
Goddard, C. Ollivier and L. Fensterbank, Acc. Chem. Res., 2016, 49,
1924-1936; c) J. D. Nguyen, E. M. D’Amato, J. M. R. Narayanam and
organosilanes as radical precursors in visible light photoredox
aerobic catalysis in the presence of an organophotocatalyst for C-
C bond formation reactions, without the necessity of an additional
Si-atom derivatization/activation step or external additives. The
scope of the applied organosilanes and Michael acceptors proved
the excellent functional group tolerance of this process. In addition,
quenching and deuterium experiments were performed to
enlighten the reaction mechanism, in which the singlet excited
state of the catalyst is responsible for the reactivity and traces of
5
40
C. R. J. Stephenson, Nat. Chem., 2012, 4, 854-859.
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4
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6
See e.g.: J. D. Griffin, M. A. Zeller and D. A. Nicewicz, J. Am. Chem.
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45
10 HCl in the solvent might facilitate the final protonation step.
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60
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a) M. Jouffroy, D. N. Primer and G. A. Molander, J. Am. Chem. Soc.,
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10 See also: T. Guo, L. Zhang, X. Liu, Y. Fang, X. Jin, Y. Yang, Y. Li,
B. Chen and M. Ouyang, Adv. Synth. Catal., 2018, 360, 4459-4463.
For a UV-mediated photocatalytic C-Si cleavage/coupling, see: S.
Montanaro, D. Ravelli, D. Merli, M. Fagnoni and A. Albini, Org. Lett.,
2012, 14, 4218-4221.
11 a) M. Silvi, C. Verrier, Y. P. Rey, L. Buzzetti and P. Melchiorre, Nat.
Chem., 2017, 9, 868-873. For an intramolecular organophoto-catalytic
reaction with imines, see: b) M. K. Jackl, L. Legnani, B. Morandi and
J. W. Bode, Org. Lett., 2017, 19, 4696-4699.
Scheme 2. Mechanistic investigations and proposed mechanism. Solv: solvent or H2O
12 T. Maruyama, Y. Mizuno, I. Shimizu, S. Suga and J.-I. Yoshida, J. Am.
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Notes and references
a Münster University, Organic Chemistry Institute, Corrensstraße 40,
15 48149 Münster, Germany. Fax: +49 251 83 36503; E-mail:
olga.garcia@uni-muenster.de
† Electronic Supplementary Information (ESI) available: Full optimization
screeing, experimental procedure and mechanistic studies, analytical data
and NMR collection of new compounds. See DOI: 10.1039/b000000x/
20 WWU Münster is gratefully acknowledged for generous support.
13 A. Gini, M. Uygur, T. Rigotti, J. Alemán and O. García Mancheño,
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90
95
regeneration of the catalysts into its ground state.
17 Exposure of the maleic anhydride to the standard reaction conditions
led to the maleic acid mono-ethylester, as the actual Michael acceptor.
18 It is reported that the excited-state of I is only able to oxidize electron-
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1
a) C. R. J. Stephenson, T. P. Yoon and D. W. C. MacMillan, Visible
Light Photocatalysis in Organic Chemistry, Wiley-VCH, 2018,
Weinheim. Selected reviews: a) K. Zeitler, Angew. Chem. Int. Ed.,
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25
30
20 No conversion of 1a was observed in the presence of H2O2, which
could be produced in the catalytic cycle under aerobic conditions.
100 21 Freshly distilled dry solvents or the presence of H2O led to notable
lower yields (14-23% vs. 78%), whereas the addition of aq. HCl did
not improve the outcome (see TS6 in S.I.), suggesting that as much as
~1-2% vol. of HCl and/or H2O hampers the reaction.
22 K. P. Dockery, J. P. Dinnocenzo, S. Farid, J. L. Goodman, I. R. Gould
105
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