Asymmetric radical alkylation of: N -sulfinimines under visible light photocatalytic conditions

Article abstract of DOI:10.1039/c7cc03724d

In this communication, a new photocatalytic strategy for the addition of alkyl-radical derivatives to N-sulfinimines with complete diastereoselectivity and moderate to good yields is presented. This is the first asymmetric photocatalytic addition to N-sulfinimines under visible light irradiation with smooth conditions and functional group tolerance.

Full text of DOI:10.1039/c7cc03724d

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Asymmetric Radical Alkylation of N-Sulfinimines under Visible
Light Photocatalytic Conditions†‡
Alberto F. Garrido-Castro,a Houcine Choubane,a,b Mortada Daaou,b M. Carmen Maestro*a and
José Alemán*a,c
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
In this communication, a new photocatalytic strategy for the received significant attention over the past decade due to its
addition of alkyl-radical derivatives to N-sulfinimines with ability to achieve bond constructions that are not possible
complete diastereoselectivity and moderate to good yields is using established procedures.5 At the same time, photoredox
presented. This is the first asymmetric photocatalytic addition to catalysts can also be employed to generate radicals to be used
N-sulfinimines under visible light conditions with smooth in a diverse range of proven radical strategies. This approach is
conditions and functional group tolerance.
based on the adeptness transition metal complexes6 and
organic dyes7 display by engaging in SET processes following
photoexcitation with visible light. Numerous photocatalytic
examples of racemic radical transformations performed with
imines have been published.8 However, only one asymmetric
approach has been disclosed by Ooi and co-workers (Scheme
1a).9 In Ooi’s work, the addition of α-alkylamine derivatives to
N-sulfonylimines to afford diamine derivatives was described.
Nonetheless, no examples of visible light photocatalytic
reactions with N-sulfinimines exist to the best of our
knowledge.
Enantiopure sulfinimines, or N-sulfinyl imines, have been
used in the asymmetric synthesis of diverse chiral amino
derivatives, e.g. α−branched amines, α− and β−amino acids,
and α− and β−amino phosphonic acids.1 The N-sulfinyl group
is one of the best chiral auxiliaries for imines because the C=N
bond is activated for nucleophilic addition, diastereofacial
selectivity is provided, and it is easily removable under mild
acidic conditions. However, most reactions with N-sulfinimines
employ organometallic compounds such as organolithium or
Grignard reagents under very restrictive conditions (low
temperature and minimal functional group tolerance).
Therefore, our goal is to deliver a novel radical addition to
N-sulfinimines, using photocatalysis and visible light irradiation
as a viable radical-generating process, and the N-sulfinyl group
as a chiral auxiliary to achieve complete stereoselectivity in the
synthesis of amine derivatives (Scheme 1b).
In this regard, radical reactions offer a better functional
group tolerance under more suitable reaction conditions.
However, only a few examples involving radical-mediated
intermolecular additions to N-sulfinimines have been
reported,2-4 which are impractical because of the large amount
of radical precursors and the use of toxic reagents (Bu3SnH).
These issues are impeding a simple gateway to valuable chiral
amines, and therefore a new straightforward methodology
capable of overcoming these limitations is required.
Previous work:
Ar’’
NH HN
P
NH HN
Ar’’
O
O
(4 mol%)
O
S
O
R2
N
S
[Ir(ppy)2(Me2phen)]BArF (1 mol%)
N
NH R2
R1
+
(a)
Ar’
visible light
toluene, 8 h, r.t.
- R1
N
Ar
On the other hand, visible light photoredox catalysis has
Ar
*
Ar’
R1 = H, TMS
Photocatalytic
Diamine Synthesis
This work:
R1
R1
S
S
*
+
R2-X
HN
*
O
N
O
*
(b)
Ar
photocatalyst
b
L.S.P.B.E, Département de Chimie Organique Industrielle, Faculté de Chimie,
Ar
R2
Université des Sciences et de la Technologie d'Oran- Mohamed Boudiaf- B.P 1505 El
Mnaouer, Oran, 31000, Algérie.
Photocatalytic
Amine Synthesis
C
Institute for Advanced Research in Chemical Sciences (IAdChem), Universidad
Autónoma de Madrid, 28049 Madrid, Spain.
† Authors dedicate this work to Prof. Jose Luis García Ruano on the occasion of his 70th
Scheme 1. Previous reaction and present work of this manuscript.
In order to achieve this goal, we performed an initial study
in which the reactivity of the enantiopure N-sulfinimine 1a (R1
birthday.
‡ Electronic Supplementary Information (ESI) available: See DOI: 10.1039/x0xx00000x.
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= p-Tol) was tested with different radical donors (see. S.I. for Table 2. N-Sulfinimine screening with different chiral auxiliaries.a
10
O
O
R
R
Ir(ppy)3 3c (1 mol%)
DIPEA / HE
more details). Reactions performed with isobutyric acid (2a
11
)
iPr
S
S
and isopropyl oxalate (2b
)
were unsuccessful; probably
+
O
N
HN
O
N
O
420 nm blue LEDs (15 W)
MeCN, 18 h, r.t.
- CO2, phthalimide
caused by the effect the required additives (K2HPO4 and water)
inflicted upon the N-sulfinimine, observing its hydrolysis in the
crude mixture. However, under fairly anhydrous conditions,
the reaction of N-(isobutyryloxy)phthalimide 2c12 with N-
Ph
Ph
iPr
O
1a-e
4
2c
sulfinimine 1a catalyzed by [Ru(bpy)3]2+
(3a) gave rise to our
tBu
O
desired product in moderate conversion after 14 hours (entry
1, Table 1).
S
S
S
S
S
HN
O
HN
O
HN
O
HN
O
HN
O
Ph
iPr
4a/4a´
conv.b = 100%
drb = 66:34
Ph
iPr
Ph
iPr
Ph
iPr
Ph
iPr
4e
conv. = 100%
dr > 98:2
Following this positive result, we tried to identify the
optimal photocatalyst that would allow us to increase the final
conversion. As revealed in Table 1, the use of some metal
photocatalysts (entries 2, 4 and 5) occasionally led to N-
sulfinimine hydrolysis, whereas reactions performed with
4c/4c´
conv. = 100%
dr = 90:10
4b
4d
complex
mixture
conv. = 43%
dr > 98:2
a
Reaction conditions: 0.1 mmol scale using 1.0 equiv. of N-sulfinimine 1, 1.5
equiv. of phthalimide 2c, 2.0 equiv. of DIPEA, 1.5 equiv. of Hantzsch ester (HE)
and 1 mL of MeCN. b Determined by 1H NMR.
organic dyes (entries 6-8) yielded 4a without hydrolysis of 1a
.
Nicely, Ir(ppy)3 3c displayed the highest conversion (entry 3),
and it was consequently used as photocatalyst during the
solvent screening (entries 9-12). Gratifyingly, reactions
performed with Ir(ppy)3 in highly polar solvents (entries 10-12)
desired product, obtaining a complex mixture under the
present conditions (Table 2). These results prompted us to
conduct an evaluation of three different N-sulfinimines bearing
led to complete conversion to sulfinamide 4a/4a’.
bulkier aryl groups (1c-e), anticipating that the stereoinduction
Despite reaching optimal conversion, dr values obtained
during screening studies confirmed that the reaction displayed
low diastereoselectivity under the optimized conditions.
Variation of different reaction parameters such as
temperature (0, -30 °C instead of r.t.) and concentration (0.03
M instead of 0.10 M) seemed to have no effect upon
diastereoselectivity. We performed several tests with N-tert-
butanesulfinimine 1b, aiming to improve this feature by taking
would improve upon increase of the steric size of the N-sulfinyl
moiety (Table 2). To our delight, all three of them delivered
excellent diastereoselectivity, especially the 2-methoxy-1-
naphthyl- and mesityl-substituted N-sulfinimines (1d and 1e,
respectively). Since 1d was less reactive than 1e after 18 hours,
we selected the latter as the optimal substrate to complete
the scope with different imines (Table 3), and different radical
donors (Table 4), using DMSO as solvent because higher yields
were achieved after isolation by flash chromatography (see
S.I.).
t
advantage of the bulkier Bu group. However, none of these
trials provided the
The reaction was found to be compatible with a large
variety of aromatic rings (Table 3).13 Therefore, electron
donating and electron withdrawing groups underwent the
Table 1. Catalyst and solvent screening of the visible light-mediated
alkylation of N-sulfinimines.a
alkylation with excellent diastereoselectivity (4f, 4g and 4h),
and good chemoselectivity in the case of 1h, where no
addition to the nitrile group was detected. Pleasantly, this
photocatalytic protocol tolerated the use of halogen
substituents at para and ortho positions without reduction or
loss of the bromide and iodide atoms (4i and 4j). Different
electron rich and poor heterocycles were used, yielding 4k and
4l with an excellent diastereoselectivity.
Entry
1
Photocatalyst
Solvent
DCM
DCM
DCM
DCM
DCM
DCM
DCM
DCM
Tol
Conv. (%)b
39
drb
[Ru(bpy)3]Cl2·6H2O (3a)
[Ir(ppy)2(bpy)]PF6 (3b)
Ir(ppy)3 (3c)
73:27
-
c
2
-
3
67
72:28
-
c
4
Ir(dFppy)3 (3d)
-
The simple one-step preparation of the phthalimide
reagent (see S.I.) led us to conduct a study of the addition of
c
-
5
[Ir{dF(CF3)ppy}2(dtbpy)]PF6 (3e)
Eosin Y (3f)
-
6
58
68:32
75:25
67:33
71:29
69:31
66:34
68:32
different alkyl radicals derived from 2 to the imine 1e (Table 4).
7
Rose Bengal (3g)
Rhodamine 6G (3h)
Ir(ppy)3 (3c)
46
As a result, secondary alkyl groups were easily added with
excellent yields (74-79%) and diastereoselectivity, as well as
the tert-butyl group (4n). Primary alkyl radicals have been
traditionally problematic in terms of their generation, and
other methods have failed to successfully complete the
addition to the N-sulfinimine.4 Remarkably, primary alkyl
8
29
9
30
10
11
12
Ir(ppy)3 (3c)
DMF
MeCN
DMSO
100
100
100
Ir(ppy)3 (3c)
Ir(ppy)3 (3c)
a Reaction conditions: 0.1 mmol scale using 1.5 equiv. of N-sulfinimine 1a, 1.0 equiv. of
phthalimide 2c, 2.0 equiv. of DIPEA, 1.5 equiv. of Hantzsch ester (HE) and 1 mL of radical addition was achieved with an excellent dr, albeit in
solvent. b Conversion and diastereomeric ratio were determined by 1H NMR analysis of
moderate yield due to its difficult isolation (4o).
c
the crude mixture. Reliable data could not be obtained due to partial N-sulfinimine
hydrolysis.
The absolute configuration of compound 4e was
determined by comparison of the spectroscopic data of a
2 | J. Name., 2012, 00, 1-3
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a
Table 3. N-Sulfinimine scope with (isobutyryloxy)phthalimide 2c
.
A plausible mechanism of the radical addition to N-
sulfinimines is depicted in Scheme 2. The radical formation of
the reaction pathway is based on mechanistic studies
performed on our system along with fundamental concepts
related to photoredox catalytic mechanisms.5,6 Photoredox
catalysts can reach an excited state upon irradiation with
visible light. Therefore, Ir(ppy)3 absorbs irradiation with blue
light, populating the highly reducing excited *Ir(ppy)3 species
O
O
O
Ir(ppy)3 3c (1 mol%)
DIPEA / HE
iPr
+
N
S
S
420 nm blue LEDs (15 W)
DMSO, 36 h, r.t.
N
O
HN
O
O
- CO2, phthalimide
iPr
2c
R
R
1e-l
(SS,S)-4e-l
(
ꢀꢁ/ꢂ IrIV/*IrIII = -1.73 V vs SCE).6 The photoexcited catalyst
ꢃꢄꢅ
engages in a SET process with phthalimide 2c
(
ꢀ
= -1.14 V
ꢁ/ꢂ
vs SCE, see cyclic voltammetry in S. I.), generating anion radical
S
S
S
S
HN
O
HN
O
HN
O
HN
O
iPr
iPr
iPr
I
through an oxidative quenching pathway (see Scheme 2). The
iPr
photocatalytic cycle is completed with the reduction of the IrIV
species (ꢀꢁ/ꢂ IrIV/IrIII = +0.77 V vs SCE)6 by an electron donor
such as DIPEA (ꢀꢁꢆ/ꢇꢂ = +0.94 V vs SCE, see cyclic voltammetry in
S. I.), regenerating the photocatalyst. The resultant reductive
elimination of the phthalimide group leads to decarboxylation
Me
MeO
NC
4e
4f
4g
yield = 61%
> 98:2
4h
yieldb = 79%
yield = 59%
dr > 98:2
yield = 63%
dr > 98:2
dr
drc > 98:2
S
S
S
S
and subsequent formation of radical II. Next, N-sulfinimine
undergoes radical addition (II) to afford N-centered radical III
Finally, Hantzsch ester donates a hydrogen atom to III, giving
rise to sulfinamide via hydrogen atom transfer (HAT). The
1
HN
O
Br HN
O
HN
O
HN
O
N
iPr
.
iPr
iPr
S
iPr
I
4i
4k
4l
4j
yield = 70%
dr > 98:2
yield = 60%
dr > 98:2
yield = 56%
dr > 98:2
yield = 64%
dr > 98:2
4
reaction also proceeds adequately in absence of Hantzsch
ester, although requiring longer reaction times, because the
and 1 mL of DMSO. b Isolated yields after flash chromatography. c Determined by oxidized species of DIPEA IV can transfer a hydrogen atom.
a
Reaction conditions: 0.1 mmol scale using 1.0 equiv. of N-sulfinimine 1, 1.5
equiv. of phthalimide 2c, 3.0 equiv. of DIPEA, 0.5 equiv. of Hantzsch ester (HE)
1H NMR.
This step is part of the widely-known DIPEA decomposition
pathway in photocatalytic procedures following electron
donation.5,6
known compound in the literature (see S.I.),14 assuming the
same stereochemical behaviour for all compounds . We have
carried out the desulfinylation under acidic conditions of 4e
giving rise to amine which has been correlated with a known
4
In order to validate this proposal, different mechanistic
,
studies were performed. Stern-Volmer quenching evaluation
allowed us to establish the first step of the mechanism (see
S.I.). The excited state of our photocatalyst 3c can decay via
phosphorescence (λem = 518 nm).15 However, we observed
that phthalimide 2c interfered with this process, quenching the
excited state of the photocatalyst.
5
(S)-amine in the literature (see S.I.). The stereoselectivity of
these reactions can be rationalized from the more stable s-cis
arrangement conformation of the sulfinylimine (enforced by
the intramolecular hydrogen bond interaction).4 The approach
of the radical to the less hindered si-face of imines explains the
S configuration assigned to the obtained compounds 4e-o
(bottom-left, Scheme 2).
a
Table 4. Phthalimide screening with N-sulfinimine 1e
.
a
Reaction conditions: 0.1 mmol scale using 1.0 equiv. of N-sulfinimine 1, 1.5
equiv. of phthalimide 2c, 3.0 equiv. of DIPEA, 0.5 equiv. of Hantzsch ester (HE)
and 1 mL of DMSO. b Isolated yields after flash chromatography. c Determined by
1H NMR.
Scheme 2. Proposed mechanism for the decarboxylative alkylation.
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1
2
(a) P. Zhou, B.-C. Chen and F. A. Davis, Tetrahedron, 2004,
60, 8003; (b) F. A. Davis, J. Org. Chem., 2006, 71, 8993; (c) F.
Ferreira, C. Botuha, F. Chemla and A. Pérez-Luna, Chem. Soc.
Rev., 2009, 38, 1162; d) M. T. Robak, M. A. Herbage and J. A.
Ellman, Chem. Rev., 2010, 110, 3600.
Although Lin’s reductive cross-coupling with aldehydes is not
a radical alkyl addition, it can be considered a radical
mediated process, see: Y.-W. Zhong, Y.-Z. Dong, K. Fang, K.
On the other hand, DIPEA and N-sulfinimine
1 did not display
this quenching ability. Therefore, the mechanism should follow
an oxidative quenching pathway, starting with reduction of
phthalimide
Finally, to ensure whether this reaction is governed by a
radical chain propagation process or visible light-
2.
a
photocatalytic one, we carried out light/dark experiments,
(Figure 1).16 In addition, the radical chain mechanism seems to
be incoherent since the N-centered radical III and phthalimide
2c should both reduced.17 Predictably, we observed that
product formation (4e) only occurs during visible light
irradiation periods, indicating that radical formation is taking
Izumi, M.-H. Xu and G.-Q. Lin, J. Am. Chem. Soc., 2005, 127
11956.
,
3
4
T. Akindele, Y. Yamamoto, M. Maekawa, H. Umeki, K.-I.
Yamada and K. Tomioka, Org. Lett., 2006, , 5729.
8
a) J. A. Fernández-Salas, M. C. Maestro, M. M. Rodríguez-
Fernández, J. L. García-Ruano and I. Alonso, Org. Lett., 2013,
15, 1658. For pioneering works on intramolecular radical
additions to N-sulfinimines, see: b) E. M. Rochette, W. Lewis,
A. G. Dossetter and R. A. Stockman, Chem. Commun, 2013,
49, 9395; c) E. Lacôte and M. Malacria, C.R. Acad. Sci., Ser.
place. Moreover, the quantum yield (
measured (see S.I.) to provide further information in this
regard. Since (420 nm) = 0.077, the photocatalyst must
φ) of this reaction was
φ
IIc: Chim., 1998, 1, 191.
absorb 13 photons to afford one equivalent of product,
indicating that a chain propagation-type mechanism should
not be taking place.
5
a) M. Fagnoni, D. Dondi, D. Ravelli and A. Albini, Chem. Rev.,
2007, 107, 2725; b) J. M. R. Narayanam and C. R. J.
Stephenson, Chem. Soc. Rev., 2011, 40, 102; c) L. Shi and W.
Xia, Chem. Soc. Rev., 2012, 41, 7687.
In conclusion, the first highly diastereoselective
photocatalytic addition to N-sulfinimines is presented. This is
an efficient transformation with full conversion under mild
conditions in absence of toxic reagents. The reaction presents
a wide scope, affording different sulfinamides with high
diastereomeric control. We propose a mechanism based on
the reductive decarboxylation of the phthalimide to generate
the alkyl radical which is added to the N-sulfinimine via
oxidative quenching of the photocatalyst, as it is confirmed by
Stern-Volmer studies. Furthermore, the quantum yield
indicates that the reaction does not involve a radical chain
mechanism.
6
7
C. K. Prier, D. A. Rankic and D. W. C. MacMillan, Chem. Rev.,
2013, 113, 5322.
a) D. Ravelli, M. Fagnoni and A. Albini, Chem. Soc. Rev., 2013,
42, 97; b) S. Fukuzumi and K. Ohkubo, Chem. Sci., 2013,
561; c) D. P. Hari and B. König, Chem. Commun., 2014, 50
4
,
,
6688; d) N. A. Romero and D. A. Nicewicz, Chem. Rev., 2016,
116, 10075.
For racemic photocatalytic addition to sulfonylimines, see: a)
J. K. Matsui, S. B. Lang, D. R. Heitz and G. A. Molander, ACS
Catal., 2017, 7, 2563. For racemic photocatalytic additions to
N-arylimines, see: b) M. Nakajima, E. Fava, S. Loescher, Z.
Jiang and M. Rueping, Angew. Chem. Int. Ed., 2015, 54, 8828;
c) L. Qi and Y. Chen, Angew. Chem. Int. Ed., 2016, 55, 13312.
For racemic photocatalytic addition to ketimines, see: d) J. L.
Jeffrey, F. R. Petronijević and D. W. C. MacMillan, J. Am.
Chem. Soc., 2015, 137, 8404; e) For an intramolecular
photocatalytic racemic example, see: S.-Y. Hsieh and J. W.
Bode, Org. Lett., 2016, 18, 2098.
8
Spanish Government (CTQ2015-64561-R) and the
European Research Council (ERC-CG, contract number:
647550) are acknowledged.
9
For asymmetric additions, see: a) D. Uraguchi, N. Kinoshita,
T. Kizu and T. Ooi, J. Am. Chem. Soc., 2015, 137, 13768; b) T.
Kizu, D. Uraguchi and T. Ooi, J. Org. Chem., 2016, 81, 6953.
10 L. Chu, C. Ohta, Z. Zuo and D. W. C. MacMillan, J. Am. Chem.
Soc., 2014, 136, 10886.
11 C. C. Nawrat, C. R. Jamison, Y. Slutskyy, D. W. C. MacMillan
and L. E. Overman, J. Am. Chem. Soc., 2015, 137, 11270.
12 J. Yang, J. Zhang, L. Qi, C. Hu and Y. Chen, Chem. Commun.,
2015, 51, 5275.
13 Alkyl N-sulfinimines were found to be unreactive under the
present conditions. Alkenyl N-sulfinimines yielded a complex
mixture in which 1,2- and 1,4-additions were detected in the
crude NMR.
14 a) C. Roe, T. M. Solá, L. Sasraku-Neequaye, H. Hobbs, I.
Churcher, D. MacPherson and R. A. Stockman, Chem.
Commun., 2011, 47, 7491. b) C. Roe, H. Hobbs and R. A.
Stockman, J. Org. Chem., 2011, 76, 9452.
15 J.-H. Seo, N.-S. Han, H.-S. Shim, J.-H. Kwon and J.-K. Song,
Bull. Korean Chem. Soc., 2011, 32, 1457.
16 a) M. A. Cismesia and T. P. Yoon, Chem. Sci., 2015, 6, 5426. b)
For fundamental review, see: a) D. M. Arias-Rotondo and J.
K. McCusker, Chem. Soc. Rev., 2016, 45, 5803.
17 Intermediate III (see Scheme 2) would be reduced to the
Figure 1. Light/dark experiment for the decarboxylative alkylation of N-
sulfinimines. Conversion determined by 1H NMR. Areas shaded in blue indicate
irradiation and those shaded in black indicate darkness.
corresponding anion followed by protonation to yield 4.
Notes and references
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