Towards Visible-Light Photocatalytic Reduction of Hypercoordinated Silicon Species

Article abstract of DOI:10.1002/hlca.201900238

Nowadays, the quest of new radical precursors based on heteroatom complexes occupies an increasingly prominent position in contemporary research. Herein, we investigated the behavior and the limitations of hexa- or pentacoordinated organochlorosilanes and related pentacoordinated silyliums as new families of complexes for the generation of radicals under photocatalytic reductive conditions. Particularly, treatment of chlorophenylbis[N,S-pyridine-2-thiolato(?)]silicon(IV) or the related silylium derivative with the fac-Ir(ppy)₃ (5 mol-%)/NEt₃ (1.5 equiv.) system under blue LEDs irradiation generates a thiopyridyl radical which can participate in the formation of a carbon?sulfur bond by reaction with an allylsulfone. Computational studies supported this experimental finding, and particularly by showing that homolytic fragmentation of C?Ts bond is favored over the fragmentation of thiopyridyl radical.

Full text of DOI:10.1002/hlca.201900238

DOI: 10.1002/hlca.201900238
COMMUNICATION
Towards Visible-Light Photocatalytic Reduction of Hypercoordinated
Silicon Species
Etienne Levernier,a Christophe Lévêque,a Etienne Derat,a Louis Fensterbank,*a and Cyril Ollivier*a
a
Sorbonne Université, CNRS, Institut Parisien de Chimie Moléculaire, 4 Place Jussieu, CC 229, FR-75252 Paris
Cedex 05, France, e-mail: louis.fensterbank@sorbonne-universite.fr; cyril.ollivier@sorbonne-universite.fr
Dedicated to Philippe Renaud in celebration of his 60th birthday and with all our friendship
Nowadays, the quest of new radical precursors based on heteroatom complexes occupies an increasingly
prominent position in contemporary research. Herein, we investigated the behavior and the limitations of hexa-
or pentacoordinated organochlorosilanes and related pentacoordinated silyliums as new families of complexes
for the generation of radicals under photocatalytic reductive conditions. Particularly, treatment of chlorophe-
nylbis[N,S-pyridine-2-thiolato(À )]silicon(IV) or the related silylium derivative with the fac-Ir(ppy)3 (5 mol-%)/NEt3
(1.5 equiv.) system under blue LEDs irradiation generates a thiopyridyl radical which can participate in the
formation of a carbonÀ sulfur bond by reaction with an allylsulfone. Computational studies supported this
experimental finding, and particularly by showing that homolytic fragmentation of CÀ Ts bond is favored over
the fragmentation of thiopyridyl radical.
Keywords: chlorosilane, silylium, hypercoordinated silicon compounds, photoreduction, radicals, cyclic
voltammetry, CÀ S bond, allylation.
tion of the supersilanol (TMS)3SiOH to perform halo-
Introduction
gen abstraction for Nickel/photoredox sp3-sp3 cross-
Over the last few decades, organosilicon compounds coupling reactions.[13] As another interesting property,
have received a great deal of attention in organic organosilicons with an activated CÀ Si bond as well as
synthesis,[1,2] and particularly radical synthetic hypercoordinated – penta- or hexacoordinated –
chemistry, with the generation of transient silicon- silicon derivatives[14–17] have revealed as a potential
centered radicals from silyl hydrides,[3–8] behaving source of carbon-centered radicals under oxidative
both as an effective mediator of radical reactions conditions, which can participate in further synthetic
starting from a range of radical precursors (halides, transformations.
chalcogens, xanthates, etc…) and as substrates for
A SET oxidation-desilylation of benzyltrimethysi-
radical functionalization such as the hydrosilylation of lanes involving visible-light excited chiral iminium
unsaturated systems.[3–8] The homolytic SiÀ H cleavage ions,[18] 9-mesityl-10-methylacridinium perchlorate
can be obtained for instance with the (TMS)3SiH salt[19] or graphitic carbon nitride[20] as a catalytic and
Chatgilialoglu’s reagent[3–6,9,10] or the Et3SiH/RSH sys- strong photooxidant, allowed the formation of ben-
tem for polarity reversal catalysis introduced by zylic radicals which can be trapped by ground-state
Roberts.[11] More anecdotal, silyl radicals can be iminium ions or Michael-type acceptors respectively. In
generated by photolysis of a SiÀ B bond for the the same vein, α-alkoxymethyl radicals were success-
silylboration of olefins[12] or by photocatalytic oxida- fully generated from the corresponding α-silyl
ethers[21–23] by photocatalyzed oxidation, as well as
acyl radicals from acyl silanes.[24] But interestingly,
what about the reactivity of hypercoordinated silicon
derivatives under oxidative conditions? This question
Supporting information for this article is available on the
WWW under https://doi.org/10.1002/hlca.201900238
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Helv. Chim. Acta 2020, 103, e1900238
was first answered by Kumada who reported the
oxidation of organopentafluorosilicates to provide
organic radicals which can react with a second
equivalent of copper chloride and liberate the chlori-
nated product. However, this transformation required
stoichiometric amount of copper salts and the low
solubility of these substrates limits their use for
synthesis.[25,26] In that context, Nishigaishi reported the
photoallylation[27,28] of benzyl-type derivatives and
dicyanobenzene with the more soluble allyl biscate-
cholato silicate,[29] but these studies were limited to
the generation of allylic radical. Following our initial
work on the oxidation of alkyltrifluoroborates in
2010,[30] we prepared a series of bench stable alkyl
biscatecholato silicates with a low oxidation potential
(<1 V vs. SCE) and tested them under visible-light
photooxidative condition[31] in order to generate
functionalized alkyl radicals and particularly primary
ones.[32,33–49] The scope of substrates for simple radical
trapping and also for photoredox/nickel dual catalysis
proved to be quite large from primary to tertiary
radicals.[31,32,50–54]
Scheme 1. Hypercoordinated silicon compounds as a source of
radicals under photooxidative or photoreductive conditions.
To date, the use of hypercoordinated silicon
derivatives in radical synthesis has been limited to the
generation of radicals under oxidative regimes and no
attempt has been made in a reductive manner. Herein,
we reported the first results about the behavior of
hypercoordinated chlorosilanes and the related cati-
onic silicon derivatives under visible-light photoreduc-
tive conditions to generate radical species. We
presumed that the presence of hypercoordinated
ligands or the formation of silylium salts would favor
the labilization of the SiÀ Cl bond during the reduction
step to provide the formation of hypercoordinated
silyl radicals. The latter may subsequently expel a
carbon- or heteroatom-centered radical (R1 or X,
respectively) (Scheme 1).
Scheme 2. Synthesis of hexacoordinated silicon compound 1a
with 8-oxyquinolinato ligands and reactivity in the presence of
allylsulfone 2 under visible-light photoreductive conditions.
THF, it could not be determined. Nevertheless, we
explored the photoreduction of 1a using the highly
reductant photocatalyst fac-Ir(ppy)3 (5 mol-%, E Ir (IV)/
Ir (III)*=À 1.7 V vs. SCE) under blue LEDs irradiation in
Results and Discussion
We thus tested the photoreduction of a set of hyper- the presence 1.5 equiv. of allyl sulfone 2 as a radical
coordinated chlorosilanes whose synthesis has already acceptor and 1.5 equiv. of N,N-diisopropylethylamine
been described in the literature. We started to (DIPEA) as sacrificial electron donor for the regener-
investigate the reactivity of hexacoordinated silicon ation of the photocatalyst (Scheme 2). Whatever the
compounds with bidentate N,O ligands such as 8- nature of the solvent (0.1 mol/L), acetonitrile (CH3CN)
oxyquinolinato ligands. We first prepared compound or dimethylformamide (DMF), no reaction occurred
1a from phenyltrichlorosilane and 8-hydroxyquinoline leaving the allylsulfone 1a unchanged. Butyl and
using the procedure developed by Wägler in 2014 benzyl chlorosilanes were also efficiently synthetized
(Scheme 2).[55] We tried to evaluate the reduction (86% and 84% yield, respectively) and investigated
potential of 1a by cyclic voltammetry (CV) but, due to but no reaction was observed.
the insolubility of this organosilicon in acetonitrile and
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In a second phase, we looked at more soluble osilane to be reduced. Compound 3 was then
silicon derivatives. In 2012, Tacke’s group reported the subjected to the same reaction conditions used for 1a,
formation of pentacoordinate chlorosilicon(IV) com- but again no reaction was observed by TLC and
plexes with tridentate dianionic O,N,O ligands such as confirmed by 1H-NMR (Scheme 3). Only starting materi-
the
tridentate
2-{(E)-[(3Z)-4-hydroxypent-3-en-2- als were recovered. Even after addition of additives
ylidene]amino}phenol ligand.[56] We then synthesized such as TMSI or TMSOTf to generate in situ the
the corresponding complex 3 as reported in Scheme 3 corresponding electron deficient silylium derivatives
and measured its reduction potential by CV. To our 4a and 4b (not characterized),[57] no product was
delight, we were pleased to see that the reduction obtained, and only degradation was seen. Switching
potential of the hypercoordinated species 3 is higher from fac-Ir(ppy)3 to a highly reducing organic photo-
than the tetravalent phenyl trichlorosilane (PhSiCl3), catalyst such as N-phenylphenothiazine (Ph-PTZ) (E Ph-
that is, À 1.60 V vs. SCE compared to À 1.95 V vs. SCE PTZ+·/Ph-PTZ*=À 2.5 V vs. SCE),[58] did not afford any
(see cyclic voltammogram of 3 (Figure 1) and Support- product either.
ing Information). This suggests that this tridentate
Thereafter, we were particularly interested in the
ligand significantly increases the ability of the chlor- photoreduction of hexacoordinated silicon complexes
with two bidentate N,S-pyridine-2-thiolato ligands.[57]
In this case, we may benefit from the possible
stabilization of the generated silyl radical by the sulfur
atom as already observed in the disulfide bond.[59]
Chlorophenylbis[N,S-pyridine-2-thiolato(À )]silicon(IV)
(5a) was then synthesized following the protocol
reported by Tacke in 2013 as outlined in Scheme 4. Its
reduction potential can be estimated by CV and
showed a value of about À 1.49 V vs. SCE (Figure 2),
higher than the reduction potentials previously deter-
mined for the hypercoordinated silicon species 3 (vide
supra). This increase of potential should facilitate the
reduction of compound 5a by the excited state of the
photocatalyst fac-Ir(ppy)3 (Scheme 4). This substrate
was then treated under the same conditions as 1a and
3. Allylsulfide 6, resulting from the addition of the
ligand 2-thiopyridine to the allylsulfone 2, was
obtained in a 24% yield. We wondered if the yield in 6
Scheme 3. Synthesis of pentacoordinated silicon compound 3,
using 2-{(E)-[(3Z)-4-hydroxypent-3-en-2-ylidene]amino}phenol li-
gand, and the corresponding silylium derivatives 4a and 4b as
well as their reactivity in the presence of allylsulfone 2 under can be improved and if this transformation involves a
visible-light photoreductive conditions.
radical or an ionic pathway.
Figure 1. Electrochemical studies: Cyclic voltammogram of 3
°
(10 mM) performed at 22 C in dried and degassed THF
containing Bu4NPF6 (100 mM) as the supporting electrolyte at a
scan rate of 0.1 VsÀ 1. Glassy carbon, platinum plate and
saturated calomel were used as working, counter and reference
electrodes, respectively.
Scheme 4. Synthesis of hexacoordinated silicon compound 5a,
using N,S-pyridine-2-thiolato ligand followed by their reactivity
in the presence of allylsulfone 2 under visible-light photo-
reductive conditions. Formation of allylsulfide 6.
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a comparable yield while no reaction was observed
with Hantzsch ester (Table 1, Entries 2, 3 and 4). An
extended irradiation time from 24 h to 48 h did not
improve the photoreductive process (Table 1, Entries 3
and 5). No more than 45% yield was reached with an
excess of allylsulfone 2 (4 equiv., Table 1, Entry 6). By
contrast, if we consider allylsulfone 2 as the limiting
substrate, addition of 2 or 3 equiv. of organochlorosi-
lane 5a led to 66 or 78% of 6, respectively (Table 1,
Entries 7 and 8). An organic dye such as fluorescein (F)
(E (F+/F*)=À 1.61 V vs. SCE) was investigated as an
alternative to fac-Ir(ppy)3, but only 4% of 6 was
obtained after 24 h of irradiation (Table 1, Entry 10).
Control experiments have been performed to
secure the role of the photocatalyst and a radical
process (Table 1, Entries 11–13). Without any photo-
catalyst and amine, 26% of product was obtained due
to the degradation of the silicon complex (Table 1,
Entry 14). In the absence of photocatalyst, we ran the
reaction under blue LEDs irradiation and allylsulfide 6
was obtained in 33% yield (Table 1, Entry 11). This
Figure 2. Electrochemical studies: Cyclic voltammogram of 5a
°
(10 mM) performed at 22 C in dried and degassed THF having
Bu4NPF6 (100 mM) as the supporting electrolyte at a scan rate of
0.1 VsÀ 1. Glassy carbon, platinum plate and saturated calomel
were used as working, counter and reference electrodes,
respectively.
Encouraged by these preliminary results, we first slight increase of yield in comparison to entry 16 may
optimized the reaction conditions by screening the arise from a photoinduced electron-transfer (PET)
nature of the photocatalyst or the electron donor, by between NEt3 and the organosilane 5a. No reaction
using different solvents and by modulating the occurred without blue LEDs activation. Only 26% for 6
reaction time or the ratio radical trap/hypercoordi- was obtained when NEt3 was omitted (due to the
nated silicon (2/5a). Both DMF and CH3CN have degradation of the complex). Even if we showed that
proved to be beneficial solvents for the reaction, while the presence of the photocatalyst fac-Ir(ppy)3 is
the reaction was less effective in THF (Table 1, required to reach a good yield, we tested the reactivity
Entries 1–3 and 9). With respect to the sacrificial between pyridine-2-thiol and acceptor allylsulfone 2
electron donor, changing DIPEA to triethylamine gave under basic conditions. As expected, no trace of 6 was
formed showing that the formation of the allylation
adduct does not involve an ionic pathway and a
radical process is likely to occur (Scheme 5). Of note,
addition of fac-Ir(ppy)3 (5 mol-%) to this mixture led to
only 9% yield of 6. From these last studies, we can
deduce that most of thiopyridyl radicals were gen-
erated by reduction of silane 5a leading to adduct 6,
and not from the decomposition of 5a. We also can
mention that energy transfer instead of electron
Table 1. Optimization of the reaction conditions and control
reactions
Entry Solvent PCat
Equiv. Time Donor
Yield[a]
(2/5a) [h]
1
2
3
4
THF
fac-Ir(ppy)3 1.5/1
fac-Ir(ppy)3 1.5/1
fac-Ir(ppy)3 1.5/1
fac-Ir(ppy)3 1.5/1
24
24
24
24
DIPEA
DIPEA
Et3N
Hantzsch
ester
Et3N
DIPEA
Et3N
Et3N
Et3N
DIPEA
Et3N
–
24%
35%
36%
0%
DMF
DMF
DMF
5
6
7
8
DMF
DMF
DMF
DMF
MeCN
MeCN
DMF
DMF
DMF
DMF
fac-Ir(ppy)3 1.5/1
fac-Ir(ppy)3 4/1
fac-Ir(ppy)3 1/2
fac-Ir(ppy)3 1/3
fac-Ir(ppy)3 1.5/1
Fluorescein 1.5/1
48
24
24
24
24
24
24
24
24
24
37%
45%
66%
78%
32%
4%
33%
26%
0%[b]
26%
9
10
11
12
13
14
–
1/3
fac-Ir(ppy)3 1/3
fac-Ir(ppy)3 1/3
Et3N
–
–
1/3
Scheme 5. Control experiment: reactivity between pyridine-2-
thiol and acceptor allylsulfone 2 under basic conditions.
[a]
[b]
NMR yield using TMOP as internal standard. In the dark.
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transfer from the iridium complex fac-Ir(ppy)3 is
negligible.1
In parallel, we examined the behavior of the
commercially available AldrithiolTM under the same
photocatalytic conditions, as a potential source of
thiopyridyl radical. Starting from three equivalents of
this compound, only a 18% yield of 6 was obtained
compared to 78% from 5a. This study showed all the
interest of 5a as an efficient precursor of thiopyridyl
radical for synthetic transformations (Scheme 6).
Scheme 6. Photocatalytic reaction using AldrithiolTM as a radical
precursor.
Computational studies have been realized, namely
at the B3LYP-D3/def2-SV(P) level with the TURBOMOLE
suite of programs. Upon reduction, it was showed that
the SET process is occurring on the π system of the
pyridine moiety of 5a (Figure 3, lower part), in a similar
manner to what have been observed with our
biscatecholato silicates except that in this case this is
an electron capture by the photocatalyst.[31] If we
compare the various BDEs of the substrates before
and after oxidation, this reduction strongly weakens all
SiÀ X (X=C, S, Cl) bonds. Particularly, the energy of the
SiÀ S bond (35 kcal/mol) is lower than the one of the
SiÀ C bond (46 kcal/mol) and the SiÀ Cl bond (63 kcal/
mol) and should favor the formation of the thiopyridyl
radical (Figure 3). We also investigated the non-hyper-
coordinated tetravalent species with two phenylthio
ligands, to directly compare the various BDE values
with 5a. This species is plotted on the right side of
Figure 3. The reduction process is taking place on the
Figure 3. Computational studies on 5a and on phenylbis
(phenylthiol)chlorosilane. Upper part: bond dissociation ener-
gies before and after reduction, in kcal/mol. Lower part: spin
density isosurfaces.
phenylthio moiety, but the energy of the SiÀ S bond 27 kcal/mol after reduction. The other bonds are
before reduction (76 kcal/mol) is higher than for the similarly affected by the reduction with the SiÀ Cl bond
hypervalent silane 5a (66 kcal/mol). This comparison having a BDE of 64.5 kcal/mol and the SiÀ Ph being at
shows also that while 5a has a positive electron 40 kcal/mol.
affinity (10.35 kcal/mol), this is not the case for (PhS)2Si
Based on these experimental studies, DFT calcu-
(Cl)Ph (À 1.96 kcal/mol), highlighting the ease of reduc- lations and in accordance with the literature,[33–49] we
tion of 5a. As for 5a, we also observed a small BDE of can propose the following mechanism where the
photoactivated catalyst under blue LEDs irradiation
reduces hypercoordinated phenylchlorosilane 5a at
the pyridine moiety and promotes preferentially the
1 In response to a referee that we thank for her/his
suggestions, we first measured the UV/Visible spectrum
which showed a characteristic band centered at 290 nm
homolytic cleavage of the SiÀ S bond between the
thiopyridine ligand and silicon atom. The expelled
and a large one between 320 nm and 440 nm. This might
thiopyridyl radical can be trapped by allylsulfone 2.
explain the formation of thiopyridyl radical both under
The photocatalyst is regenerated by action of NEt3
UV (350 nm) or blue LEDs irradiation. Thus, we performed
(Scheme 7). It should be mentioned that formation of 6
a reaction with benzophenone as photosensitizer (with a
involved sequential addition of the thiopyridine radi-
cal, generation of the transient radical intermediate A
triplet energy (69.1 kcal/mol) higher than that of fac-Ir
(ppy)3) (58.1 kcal/mol) under UV irradiation (350 nm). The
yield did not increase significantly which suggests that
the energy transfer from the iridium complex fac-Ir(ppy)3
would be negligible compared to the electron transfer
process. This was also supported by the photoinduced
electron-transfer (PET) reaction observed between NEt3
and the organosilane 5a in Table 1, Entry 11.
and homolytic fragmentation of the CÀ Ts bond. We
were surprised by the fact that intermediate A is more
prone to give the homolytic fragmentation of the CÀ Ts
bond rather than the reversible extrusion of the
thiopyridine radical moiety. It is indeed known in the
literature that a thiophenyl radical is a good leaving
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Scheme 8. Photocatalytic reaction using phenyl[tris(phenylsul-
fanyl)]silane as a radical precursor.
silane and thiophenol according to the literature
procedure.[62] Its reduction potential was then meas-
ured. A value of À 2.00 V vs. SCE was obtained which
appears to be lower than the reduction potential of 5a
(Figure 6). This difference of potential may explain why
no allylation adduct 6 was formed from 5a under the
same reductive conditions (Scheme 8).
We next evaluated the reactivity of the hyper-
coordinated methylchlorosilane 5b under photocata-
lytic reductive conditions. We first prepared it by using
the same procedure as for 5a starting from meth-
Scheme 7. Proposed mechanism for the formation of 6.
group, presumably better than the phenylsulfonyl yltrichlorosilane, and then we measured its reduction
one.[60,61] To rationalize the observed selectivity, DFT potential, which was estimated by CV at about
calculations of β-fragmentation processes on inter- À 1.68 V (vs. SCE) (Figure 7). This value still suggests
mediate A and on an analogous intermediate (A’) that the photo-excited photocatalyst fac-Ir(ppy)3 (E Ir
where the pyridine moiety is replaced by a phenyl (IV)/Ir(III)*=À 1.7 V vs. SCE) can reduce this substrate.
were thus conducted, at the same level of theory Treatment of 5b (3 equiv.) with 2 (1 equiv.) in the
(namely B3LYP-D3/def2-SV(P)). On A’, calculations presence of fac-Ir(ppy)3 (5 mol-%) and Et3N (1.5 equiv.)
indicate that departure of the tosyl radical is slightly in DMF under blue LEDs irradiation gave 6 in 31%
favored over the thiophenyl radical fragmentation yield (Scheme 9). It should be noted that no addition
(barriers of 8.4 kcal/mol vs. 9.3 kcal/mol). But the of the methyl group was observed. For comparison, its
situation is different in the case of intermediate A: benzyl analog 5c has also been investigated. This
while the tosyl radical departure is still in the same compound was prepared in-situ from benzyltrichlorosi-
energetic range with a barrier of 9 kcal/mol, the
thiopyridyl radical extrusion proves to be unfeasible
with an always increasing potential energy surface
(see Figure 4), reaching value over 18 kcal/mol. It turns
out that the thiopyridyl radical is particularly unstable,
and observation of the geometry highlights that this
radical is trying to establish non-covalent interactions
between the nitrogen of the pyridine moiety and the
homobenzylic CÀ H to stabilize itself (see Figure 5).
Therefore, the mechanism proposed in Scheme 7 is
supported by this computational analysis.
Due to the weakness of the SiÀ S bond and its
ability to generate thiyl radicals under reductive
conditions, we compared the reactivity of our previous
system (5a) with the non-hypercoordinated tetravalent
phenylsilane bearing three phenylthio ligands 7
(Scheme 8). This one was synthesized from phenyl-
Scheme 9. Synthesis of hexacoordinated silicon compounds 5b
and 5c, using N,S-pyridine-2-thiolato ligand followed by their
reactivity in the presence of allylsulfone 2 under visible-light
photoreductive conditions. Formation of allylsulfide 6.
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Figure 4. Calculated β-fragmentation processes for intermediate A and A’, at the B3LYPÀ D3/def2-SV(P) level.
Figure 6. Electrochemical studies: cyclic voltammetry of
7
°
(10 mM) performed at 22 C in dried and degassed THF
containing Bu4NPF6 (100 mM) as the supporting electrolyte at a
scan rate of 0.05 VsÀ 1. Glassy carbon, platinum plate and
saturated calomel were used as working, counter and reference
electrodes, respectively.
Figure 5. Structure of the intermediate A.
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efficiently as phenylchlorosilane 5a and participate in
radical reactions (Scheme 10).
Interestingly, as the thiopyridine moiety is encoun-
tered in various bioactive molecules (Figure 8),[63,64]
efficient reactions allowing the synthesis of these
compounds in mild conditions remains interesting.[65]
Conclusions
In conclusion, we have demonstrated that the mod-
ification of the ligands of hexacoordinated chlorosilane
derivatives can efficiently modulate their reactivity
towards photoreductive conditions. By using a thiopyr-
idine ligand, the corresponding thiyl radical has been
liberated and efficiently trapped by an allylsulfone as
an acceptor. Even if these radicals could be formed
through the oxidation of pyridine thiolate like phenyl
thiolate,[66] no methodology has already been reported
by using a reductive pathway. Experiments are
currently ongoing in our laboratory for the formation
Figure 7. Electrochemical studies: Cyclic voltammogram of 5b
°
(10 mM) performed at 22 C in dried and degassed THF
containing Bu4NPF6 (100 mM) as the supporting electrolyte at a
scan rate of 0.1 VsÀ 1. Glassy carbon, platinum plate and
saturated calomel were used as working, counter and reference
electrodes, respectively.
lane but could not be isolated. Nevertheless, the of carbon-centered radicals by photoreduction of
photocatalytic reaction was carried on the crude other types of hypercoordinated silicon species.
mixture and the allylsulfide 6 was obtained in 15%
yield as the only product.
From those experiments, only the thiopyridyl
radical was formed as evidenced by trapping experi-
ments with allylsulfone 2. The phenyl derivative was
revealed to be the most reactive and provided the
best yield in allylsulfide 6. Interestingly, formation of
phenyl, methyl or benzyl radical was never observed
in these studies.
We investigated by DFT calculations the fate of the
silyl anion originating from the reduction of hyper-
coordinated alkylchlorosilanes 5 and subsequent frag-
mentation of the thiopyridyl radical. We found that
Scheme 10. Synthesis of silylium derivative 5d, using N,S-
chloride anion dissociation occurs with an energy
pyridine-2-thiolato ligand followed by their reactivity in the
presence of allylsulfone 2 under visible-light photoreductive
conditions. Formation of allylsulfide 6.
barrier of 8.6 kcal/mol providing the formation of a
transient highly reactive silylene.2
Finally, we looked at the photoreduction of the
highly electron deficient silylium derivative 5d readily
obtained by action of TMSI on the corresponding
phenylchlorosilane 5a.[57] Under the optimized photo-
catalytic conditions determined for 5a, 5d provided 6
as the sole product with the same yield as 5a. This
result showed that silylium 5d can be reduced as
2 We are grateful to the referees for their questions on the
nature of the generated Si-species generated after the
reaction, and particularly the fate of the silyl anion.
Figure 8. Bioactive molecules containing a thiopyridine scaf-
fold.
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[9] C. Chatgilialoglu, J. Lalevée, ‘Recent Applications of the
Experimental Section
(TMS)3SiH Radical-Based Reagent’, Molecules 2012, 17,
527–555.
To a Schlenk flask was added the silicon complex 5a–
5d, fac-Ir(ppy)3 (5 mol-%), and allyl sulfone 2. The
degassed solvent (0.1 mol/L or 0.067 mol/L) was then
introduced followed by the amine (1.5 equiv.) and the
mixture was irradiated with blue LEDs (470 nm) at
room temperature for 24 h under an argon atmos-
phere. The reaction mixture was diluted with diethyl
ether, washed with water (two times), dried over
MgSO4 and evaporated under reduced pressure. The
crude residue was purified by flash chromatography.
[10] C. Chatgilialoglu, C. Ferreri, Y. Landais, V. I. Timokhin, ‘Thirty
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E. L., C. L., L. F. and C. O. conceived and designed the
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We thank Sorbonne University, CNRS, ANR-17-CE07-
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Received October 9, 2019
Accepted November 26, 2019
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