Visible Light Photocatalytic Reduction of O-Thiocarbamates: Development of a Tin-Free Barton-McCombie Deoxygenation Reaction

Article abstract of DOI:10.1002/adsc.201400729

The Barton-McCombie deoxygenation is one of the most important transformations in the toolbox of organic chemists which has been the subject of a number of methodological developments. In this study, we report a photocatalyzed redox deoxygenation of secondary and tertiary alcohols from thiocarbamate precursors under visible light activation. The iridium complex Ir(ppy)₃ proved to be the most efficient catalyst in the presence of Hünig's base as sacrifial electron donor. A mechanistic investigation is presented based on fluorescence quenching experiments and cyclic voltammetry.

Full text of DOI:10.1002/adsc.201400729

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DOI: 10.1002/adsc.201400729
Visible Light Photocatalytic Reduction of O-Thiocarbamates:
Development of a Tin-Free Barton–McCombie Deoxygenation
Reaction
Ludwig Chenneberg,a,b Alexandre Baralle,a,b Marion Daniel,a,b
Louis Fensterbank,a,b,* Jean-Philippe Goddard,a,b,c,* and Cyril Olliviera,b,*
a
Sorbonne Universitꢀs UPMC Univ Paris 06, UMR 8232, Institut Parisien de Chimie Molꢀculaire, 4 place Jussieu, C.229,
F-75005 Paris, France
Fax : (+33)-1-4427-7360; e-mail: louis.fensterbank@upmc.fr or cyril.ollivier@upmc.fr
CNRS, UMR 8232, Institut Parisien de Chimie Molꢀculaire, 4 place Jussieu, C. 229, F-75005 Paris, France
Laboratoire de Chimie Organique et Bioorganique EA 4566, Universitꢀ de Haute-Alsace, Ecole Nationale Supꢀrieure de
b
c
Chimie de Mulhouse, 3 rue Alfred Werner, F-68093 Mulhouse Cedex, France
E-mail: jean-philippe.goddard@uha.fr
Received: July 25, 2014; Published online: September 11, 2014
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201400729.
Abstract: The Barton–McCombie deoxygenation is
one of the most important transformations in the
toolbox of organic chemists which has been the sub-
ject of a number of methodological developments.
In this study, we report a photocatalyzed redox de-
oxygenation of secondary and tertiary alcohols
from thiocarbamate precursors under visible light
tions since it allows the reduction of an alcohol func-
tion to the corresponding alkane.[5] The reaction usu-
ally proceeds in two steps through the preliminary
formation of an intermediate thiocarbonyl precursor
such as xanthate, thiocarbonate, thiocarbamate
(Scheme 1). In the original version, the radical chain
reaction was initiated by AIBN under thermal condi-
tions and (n-Bu)3SnH was used as chain mediator.[5a]
Related protocols have been proposed by changing
the initiation process and substituting the toxic tin hy-
dride mediator by silanes,[6] phosphorus derivatives,[7]
boranes[8,9] or cyclohexadienyl compounds.[10] Deoxy-
genation reactions have also been investigated
through the reduction of benzoyl derivatives by or-
ganometallic redox processes[11] as well as photoin-
activation. The iridium complex IrACTHNUTRGENUG(N ppy)3 proved to
be the most efficient catalyst in the presence of
Hꢁnigꢂs base as sacrifial electron donor. A mecha-
nistic investigation is presented based on fluores-
cence quenching experiments and cyclic voltamme-
try.
Keywords: deoxygenation; photocatalysis; photore-
duction; radical reactions; thiocarbamates
Since the development of modern radical chemistry,
a number of related transformations have entered
mainstream chemistry and proved to be highly effi-
cient tools for chemists in molecular and material sci-
ences.[1] Indeed, applications in surface functionaliza-
tion,[2] polymer preparation[3] or organic synthesis[4]
have highlighted the strong impact of this field and
demonstrated its ability to act at the interface of dif-
ferent scientific domains. Among the large number of
known radical reactions, those dedicated to functional
group transformations have certainly been the most
intensively studied for methodological development
and total synthesis. Notably, the Barton–McCombie
Scheme 1. Photoreduction of O-thiocarbamate: A photoca-
talytic alternative to the classical Barton–McCombie deoxy-
deoxygenation is one of the most used radical reac- genation reaction
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Visible Light Photocatalytic Reduction of O-Thiocarbamates
Table 1. Optimization of the reaction conditions for the pho-
tocatalytic reduction of O-thiocarbamate 1a into 2a.
duced electron transfer sensitized by N-alkylcarba-
zoles under Hg-lamp irradiation.[12] As an efficient al-
ternative to stoichiometric redox reactions or harsh
UV conditions, visible light photoredox catalysis has
re-emerged in the last decade.[13] A lot of groups in-
cluding ours have taken an active part in the develop-
ment of new catalytic processes based on visible light
to activate organic or organometallic photocatalysts
able to promote electron transfers and radical reac-
tions.[14,15] Very few of them have been dedicated to
À
the challenging reductive cleavage of the C O bond
because of its high bond strength.
Entry Cat.
(mol%)
t
G
Solvent
conc.[f]
Yield
[%][a]
[h] [equiv.]
Recently, Falvey reported a photorelease of carbox-
ylic acids based on a formal deoxygenation reac-
tion.[16] Stephenson combined the iodination of pri-
mary and secondary alcohols with the photoreduction
of the so-formed aliphatic iodides to the correspond-
ing alkanes in a one-pot two-step sequence.[17] Finally,
Overman developed a photoredox-catalyzed fragmen-
1
A (5)
B (5)
C (5)
C (5)
C (5)
C (5)
C (5)
C (5)
C (5)
C (5)
C (5)
C (2)
C (2)
C (1)
C (5)
C (1)
–
24 10
24 10
24 10
18 10
24 10
48 10
48 10
48 10
48 10
24 10
24 10
24 10
CH3CN C1 NR
CH3CN C1 20
CH3CN C1 47
DMPU C1 51
2[b]
3
4
5[c]
6[c]
7
NMP C1
DMF C1
27
42
DMSO C1 47
acetone C1 ND
8[c,e]
9[c]
10
11[c]
12
13
14
15
16
17
18
tation of tertiary alkyl N-phthalimidoyl oxalates
[18]
DCM C1
29
À
aiming at C C bond formation. Based on all these
CH3CN C2 53
CH3CN C2 53
CH3CN C2 54
CH3CN C2 53
CH3CN C2 53
DMPU C1 50
considerations, we surmised that the development of
an efficient tin-free deoxygenation reaction that
would use visible light activation of a photoredox cat-
alyst is of primary importance for the chemical com-
munity.
24
24
24
24
24
24
5
5
5
–
5
5
Thus, we decided to investigate the photoreduction
of thiocarbonyl derivatives and to focus our attention
on the very easy to access imidazolyl-l-thiocarbonyl
(ITC) substrates (Scheme 1). This functional group
has been introduced by Barton and McCombie[5a] and
gave comparable yields with methyldithiocarbonyl
(MDC), and thiobenzoyl (TB) counterparts. ITC sub-
strates have been, for instance, used by Rasmussen
for the deoxygenation of carbohydrates.[19] O-Thiocar-
bamates 1a–j (structures are reported in Table 2)
were obtained in high yields (63–84%) from the cor-
CH3CN C2
3
CH3CN C2 NR
CH3CN C2 NR
C (1)[d]
[a]
Isolated yield.
NMR yield.
Fluorescent bulb, 14 W.
Reaction performed in the dark.
Not determined, only traces of 2a were observed.
C1 =0.1M, C2 =0.067M.
[b]
[c]
[d]
[e]
[f]
responding secondary and tertiary alcohols and N,N’- usual Ru
thiocarbonyldiimidazole (1.2 equiv.) with a catalytic Pr)2NEt in acetonitrile without success (Table 1,
amount of DMAP (0.4 equiv.) (Scheme 2). entry 1). Under the same conditions, the use of the
We selected O-thiocarbamate 1a for the optimiza- slightly more reductive [Ir(dtbppy)(ppy)2]PF6 estab-
ACHTUNGTERN(NUNG bpy)3Cl2 (5 mol%) with 10 equivalents of (i-
A
ACHTUNGTRENNUNG
tion of the reaction parameters. We investigated the lished the viability of our photoreductive approach af-
photoreductive conditions catalyzed by polypyridine fording 2a in 20% yield (Table 1, entry 2). The most
ruthenium or iridium complexes in the presence of reductive – IrACTHNURTGNENG(U ppy)3 – of the catalyst set proved to be
Hꢁnigꢂs base [(i-Pr)2NEt] as sacrificial electron donor superior and yielded the reduced product 2a in 47%
under blue LED light (Table 1).[20] We first tested the (Table 1, entry 3). Then, we settled on this catalyst for
the rest of the study. Changing acetonitrile to DMPU
(N,N’-dimethylpropyleneurea) gave comparable yield
(Table 1, entry 4) but any other solvent modification
(NMP, DMF, DMSO, acetone and DCM; Table 1, en-
tries 5–9) did not improve the photoreduction process.
Fluorescent bulb lamp (14 W) and blue LED gave
similar results (Table 1, entries 10 and 11) which is
consistent with a visible light activation of the cata-
lyst. The next step was to decrease the catalyst con-
centration and the stoichiometry of the (i-Pr)2NEt
(Table 1, entries 12–14). Thus, the Ir(ppy)3 loading
Scheme 2. One-step synthesis of O-thiocarbamates from al-
cohols and commercially available reagents.
ACHTUNGTRENNUNG
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Ludwig Chenneberg et al.
Table 2. Scope of the reaction.
was fixed to only 1 mol% in the presence of 5 equiva-
lents of Hꢁnigꢂs base. The reaction was conducted in
acetonitrile during 24 h under blue LED irradiation
and 2a was obtained in 53% isolated yield (Table 1,
entry 14, conditions A). We decided to set alternative
conditions involving DMPU that could be a helpful
solvent for the deoxygenation of very lipophilic sub-
strates. A higher catalyst loading (5 mol%) was neces-
sary to keep the same efficiency of the reduction
(Table 1, entry 15, conditions B).
Control reactions have been pursued in order to
secure the role of each reagent. Only 3% of 2a was
obtained when (i-Pr)2NEt was omitted (Table 1,
entry 16). No reaction occurred in the absence of Ir-
ACHTUNGTRENNUNG(ppy)3 and without visible light activation (Table 1,
entries 17 and 18).
Our photocatalytic conditions were compared to
the conventional Barton–McCombie reaction. The de-
oxygenation of 1a was run in refluxing toluene (c=
0.02 mM) with a slow addition of AIBN (10 mol%)
and (n-Bu)3SnH (2 equiv.) solution over 4 h. Com-
plete conversion was observed after two additional
hours and 2a was obtained in 46% (NMR yield)
(Scheme 3). This finding was highly suggestive that
a photocatalytic Barton–McCombie deoxygenation
could be competitive with the original method and
constitute a versatile tool for applications in organic
synthesis.
Then, we investigated the scope of the reaction by
applying the optimized conditions to a series of O-thi-
ocarbamate derivatives 1a–j (Table 2). The model
substrate was modified by changing the N-sulfonyl
group to a Boc carbamate (1b) that was also compati-
ble with the reaction conditions and 2b was obtained
in a comparable yield of 53%. The isolation of 2c
(41%) was a bit tedious due to its relative volatility
but the benzylidene acetal was tolerated under such
mild reduction conditions. Due to its poor solubility
in acetonitrile, lithocholic acid derivative 1d was re-
duced under conditions B. Indeed, DMPU was more
suitable for this reaction and the corresponding deoxy
compound was obtained in 65% yield. Other very in-
teresting chemoselectivities were highlighted in the
reduction of 1e and 1f. Indeed, the diacetonide-d-glu-
cofuranose derivative 1e was deoxygenated at the 3-
position to afford the corresponding 3-deoxy com-
[a]
Conditions A.
Isolated yield.
Conditions B.
[b]
[c]
Scheme 3. Tin-mediated radical reduction of O-thiocarba-
mate 1a to 2a.
[d]
Conditions A with 2 equiv. of (i-Pr)2NEt.
Ratio meso/dl=1/1.
[e]
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Visible Light Photocatalytic Reduction of O-Thiocarbamates
Scheme 4. Reaction in absence of the (i-Pr)2NEt: O-Thiocar-
bamate transfert to S-thiocarbamate.
pound in 56% yield. Even more interestingly, the ben-
zoyl group at the 5-position of xylofuranose derivative
1f stayed untouched and the reduction proceeded
well in 70% yield.[11] Secondary aliphatic O-thiocarba-
mate 1g was smoothly deoxygenated in 51% yield.
Secondary benzylic and allylic alcohols were engaged
in this photocatalyzed deoxygenation sequence. Benz-
Figure 1. Fluorescence quenching studies of Ir
O-thiocarbamate 1a.
ACHTUNGRTENN(UNG ppy)3* with
ACHTUNGTRENNUNGhydrol derivative 1h was reduced under conditions A
to afford 1,1-diphenylmethane 2ha (21%) and dimer
1,1,2,2-tetraphenylethane 2hb (37%), characteristic of
Fluorescence quenching has been done by addition
benzylic radical reactivity. Interestingly, 1i was effi- of thiocarbamate 1a to a solution of IrACTHUNTRGNE(UNG ppy)3 in
ciently reduced to afford the corresponding dimeriza- DMF.[23] The fluorescence signal was recorded under
tion compound 2i in quantitative yield as a 1:1 mix- excitation at 385 nm for a maximum of emission at
ture of dl and meso diastereoisomers.[21] The reduc- 528 nm (Figure 1). To a 1 mM solution of Ir
ACHTUNGTRENNUNG(ppy)3
tion of the tertiary O-thiocarbamate 1j coming from maintained in the dark, 1a was added to obtain 0.6,
the quinic acid exhibited the potentiality of the 0.7 and 0.99 mM as final concentrations. The corre-
method to deoxygenate functionalized tertiary sub- sponding fluorescence spectra showed a significant
strates in good yield (2j, 60%).
decrease of the intensity while the concentration of
In order to gain insights into the mechanism of this 1a increased. The fluorescence intensity ratio (I0/I) at
reaction and understand the role of each constituent, 528 nm was plotted against 1a concentration. A linear
we engaged O-thiocarbamate 1a without (i-Pr)2NEt variation was observed and followed a Stern–Volmer
as sacrificial donor (Scheme 4). Very interestingly, this law that supports the energy or electron transfer pro-
reaction allowed us to obtain the deoxygenated prod- cess between Ir
ACHTUNGERTN(NUNG ppy)3* and 1a with a quenching rate
uct 2a (3%) and the S-thiocarbamate 3 in 15% yield constant kq=6.5ꢄ105 MÀ1 sÀ1.[24]
while 1a was recovered in 65% yield, as determined
by H NMR with 3-sulfolene as internal standard.
To balance these two processes, we did cyclic vol-
tammetry that allowed us to determine the reduction
1
The formation of 3 presumably originates from potentials of a representative set of thiocarbamates
a thiocarbamate group radical transfer. Thus, valuable (Table 3).[25] For substrates 1a, 1b, 1f and 1j, these po-
information can be deduced: without (i-Pr)2NEt, the tentials are in the range of À1.56 to À1.73 V (SCE).
carbon-centered radical is formed and the (i-Pr)2NEt 1i appeared to be more favorably reduced at À1.11 V
plays an important role in the terminal hydrogen (SCE). These measurements support that IrACHTUNGTRENNUNG
transfer to afford the resulting reduced product.[22] {E1/2 ([Ir(ppy)3]+/Ir
ACTHNUGTRENNUGN ACHTUNGTRENNNUG
Indeed, in the absence of the Hꢁnigꢂs base, the to reduce the O-thiocarbamates to the corresponding
carbon-centered radical would add on the sulfur atom radical anions. Moreover, no reduction wave is ob-
of 1a to propagate the radical chain reaction. This served for the S-thiocarbamate 3 up to À2 V. This is
chain reaction seems to be moderately efficient be- consistent with the observed radical thiocarbamate
cause 1 mol% of photocatalyst resulted in 15% of the group transfer (Scheme 4) when the reaction is con-
group transfer compound 3 but is quite interesting in ducted without Hꢁnigꢂs base and confirms that Ir-
terms of mechanistic implications and perspectives.
ACHTUNGTRENNUNG
At that stage, questions arose: How can 1a be re-
duced by Ir
the catalytic process? We decided to answer these deoxygenation
questions by carrying out fluorescence quenching (Scheme 5). Under visible light activation, Ir
studies of Ir(ppy)3* (Figure 1). able to reach an excited state that could transfer one
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
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Table 3. Reduction potential of thiocarbamates.[a]
Substrate (Sub)
1a
1b
1f
1i
1j
3
Ered (V)[b] [Sub]/
[Sub]CÀ
À1.68
À1.73
À1.67
À1.11
À1.56
<À2
[a]
For cyclic voltammograms, see the Supporting Information.
[b]
The redox potentials are given in volt versus SCE.
Experimental Section
Conditions A
Into a Schlenk tube were introduced the O-thiocarbamate
(0.5 mmol), Ir(ppy)3 (3.3 mg, 0.005 mmol), then CH3CN
AHCTUNGTRENNUNG
(7.5 mL) and (i-Pr)2NEt (0.435 mL, 2.5 mmol). The mixture
was degassed using the freeze-pump-thaw method (3 cycles)
and irradiated by blue LEDs (470 nm) until completion of
the reaction (monitored by TLC). The mixture was concen-
trated under vacuum and the crude was purified by flash
column chromatography.
Conditions B
Into a Schlenk tube were introduced the O-thiocarbamate
(0.5 mmol), IrACTHNUTRGNE(UNG ppy)3 (16.4 mg, 0.025 mmol) then DMPU
Scheme 5. A proposed mechanism.
(5 mL) and (i-Pr)2NEt (0.435 mL, 2.5 mmol). The mixture
was degassed using the freeze-pump-thaw method (3 cycles)
and irradiated by blue LEDs (470 nm) until completion of
the reaction (monitored by TLC). Water was added and the
mixture was extracted with Et2O. (3ꢄ25 mL). The organic
phases were gathered, dried over MgSO4, filtered and con-
centrated under vacuum. The crude was purified by flash
column chromatography.
electron to the thiocarbamate moiety.[26] This reduc-
tion process leads to the oxidation of Ir
species that would be reduced in turn to IrACHTUNGTRENNNUG
ACHTUNGTRENNUNG
(i-Pr)2NEt and starts a new catalytic cycle. Then, the
thiocarbamate radical anion decomposes by fragmen-
tation to afford the intermediate carbon-centered rad-
ical. The hydrogen transfer occurs either from the
amine radical cation coming from the reduction of iri-
dium catalyst or directly by (i-Pr)2NEt.[22] This mecha-
nism is supported by the fluorescence quenching ex-
periments and the fact that in the absence of “H-
donor”, the thiocarbamate transfer is observed.
In conclusion, we report a novel photocatalyzed de-
oxygenation method based on the activation of Ir-
ACHTUNGTRENNUNG(ppy)3 by visible light under reductive conditions. This
transformation allowed us to convert secondary and
tertiary aliphatic and benzylic alcohols to the corre-
sponding alkanes in good yields. Interesting functional
group tolerances were observed. A preliminary mech-
anistic study based on fluorescence suggested the role
of the thiocarbamate substrates as oxidative quenches
Acknowledgements
We thank UPMC, CNRS, IUF (L.F.), ANR (grant “Credox”)
and Emergence UPMC 2010. L. C. was awarded a graduate
fellowship by the Rꢀgion Martinique, which is gratefully ac-
knowledged. This work was supported by the LabEx
MiChem part of French state funds managed by the ANR
within the Investissements d’Avenir programme under refer-
ence ANR-11-IDEX-0004-02. We warmly thank Dr. Sandrine
Sagan (LBM, UMR CNRS 7203) for the access to the fluor-
ometer. Benjamin Doistau and Dr. Vincent Corcꢀ are thanked
for helpful assistance with cyclic voltammetry, Marine Hatit
and Matej Zabka are acknowledged for preliminary contribu-
tions. The support and sponsorship of the COST Action
CM1201 on Biomimetic Radical Chemistry is gratefully ac-
knowledged
of IrACHTUNGTRENNUNG(ppy)3 excited state. Moreover, the thiocarbamate
transfer reaction would open new perspectives to
highly versatile transformations in the fields of organ-
ic synthesis and polymer sciences.
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