Visible-light-induced cyanation of aza-Baylis-Hillman adducts: A Michael type addition

Article abstract of DOI:10.1016/j.tetlet.2014.01.121

Visible-light-induced aerobic oxidative cyanation of aza-Baylis-Hillman (aza-BH) adducts providing valuable allylic cyanides in good to excellent yields has been developed. The protocol involves in situ formation of 4π conjugated iminium ion intermediates, which undergo cyanation at the γ-position to afford Michael type adducts. This is the first example of visible-light-induced catalytic functionalization of aza-BH adducts using air (O₂) as an economical and ecosustainable oxidant and TMSCN as a convenient and readily available cyanide source.

Full text of DOI:10.1016/j.tetlet.2014.01.121

Tetrahedron Letters 55 (2014) 1788–1792
Contents lists available at ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Visible-light-induced cyanation of aza-Baylis–Hillman adducts:
a Michael type addition
⇑
Vishnu P. Srivastava, Arvind K. Yadav, Lal Dhar S. Yadav
Green Synthesis Lab, Department of Chemistry, University of Allahabad, Allahabad 211002, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Visible-light-induced aerobic oxidative cyanation of aza-Baylis–Hillman (aza-BH) adducts providing
valuable allylic cyanides in good to excellent yields has been developed. The protocol involves in situ for-
mation of 4p conjugated iminium ion intermediates, which undergo cyanation at the c-position to afford
Michael type adducts. This is the first example of visible-light-induced catalytic functionalization of aza-
BH adducts using air (O2) as an economical and ecosustainable oxidant and TMSCN as a convenient and
readily available cyanide source.
Received 31 December 2013
Revised 24 January 2014
Accepted 25 January 2014
Available online 5 February 2014
Keywords:
Air oxidation
Ó 2014 Elsevier Ltd. All rights reserved.
Baylis–Hillman adducts
Cyanation
Michael adducts
Photoredox catalysis
Visible light
Recently, visible-light-induced catalytic oxidation of sp3 C–H
bonds adjacent to nitrogen providing reactive iminium ions has be-
come a powerful toolkit for amine functionalization.1 Amines often
act as single electron donors in visible-light-induced photoredox
processes and are oxidized to nitrogen radical cations to execute
iminium ion formation under aerobic conditions. This has led to
the development of several useful synthetic transformations of rel-
evance to material and pharmaceutical sciences.2
However, the benzylic amine substrates acting as an electron
donor are very limited and mostly realized on N-aryltetrahydroiso-
quinoline derivatives.1 Thus, investigations on novel benzylic
amine precursors for their exploitation in photosensitized SET (sin-
gle electron transfer) processes are highly desirable. Baylis–Hill-
man adducts are useful substrates for diverse and highly
functionalized molecules which are required for the synthesis of
various interesting natural and unnatural products of medicinal
importance.3 Our continued efforts for implementation of new
substrates in visible-light-photoredox reactions4 and functionali-
zation of BH adducts,5 prompted us to choose aza-BH adducts for
the present study as depicted in Scheme 1.
Yoon,7e and Stephenson1h,7a–c has demonstrated that ruthenium
and iridium based polypyridyl complexes play a pivotal role in
SET processes, as they possess well known photocatalytic proper-
ties and can readily be tuned by the choice of right ligands.7 The
photocatalytic aerobic oxidation of amino benzylic C–H bonds to
generate reactive iminium ion intermediates has been frequently
reported in the recent years.1,6 Among different diverging ap-
proaches from this structural motif, the cyanation of sp3 C–H bonds
adjacent to a nitrogen atom represents one of the most straightfor-
ward and convenient methods for the synthesis of
a-amino
nitriles.1d,g Besides this, such highly reactive iminium ions have
been subsequently trapped with diverse nucleophiles such as enol
silane,6a TMSCF3,6b phosphonates,6c indole,6d terminal alkynes,6c,d
etc.6
The use of atmospheric oxygen as an oxidant in organic synthe-
ses remains very limited although it is readily available, economi-
cal and ecosustainable oxidant.8 The field of visible-light-
photoredox catalysis has opened up a new route to utilize atmo-
spheric oxygen, where it is mostly employed as an oxidant to
regenerate Ru+2 from Ru+1 to complete the catalytic cycle and the
superoxide radical (OÅ2ꢀ) thus formed plays a key role in the syn-
thetic process.1,6
In view of the above valid facts and widening the substrate
scope for the visible-light-photoredox reactions, amino benzylic
carbon in conjugation with a double bond was considered as a no-
vel system for the present study. In the current photoredox cata-
The development of new methods in synthetic organic chemis-
try using photoredox catalysis has attracted significant interest, as
visible light is a ubiquitous, sustainable and clean source of
energy.1,2,6 The pioneering work from the groups of MacMillan,7d
⇑
Corresponding author. Tel.: +91 532 2500652; fax: +91 532 2460533.
E-mail address: ldsyadav@hotmail.com (L.D.S. Yadav).
lytic process a 4
p conjugated iminium ion is generated and
http://dx.doi.org/10.1016/j.tetlet.2014.01.121
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.

V. P. Srivastava et al. / Tetrahedron Letters 55 (2014) 1788–1792
1789
Table 2
Ru(II) or Ir(III), visible light
Optimization of reaction conditionsa
1,6
Previous work: Attack at the α
-position.
Ru(II) or Ir(II), visible light
blue LEDs
air (O2)
N
N
Ru(bpy)3Cl2 (3)
(mol%)
N
N
Cl2
COOMe
COOMe
air,
Nu
Nu
Ar
Ar
N
TMSCN, solvent 18-30 h, rt
N
N
N
N
II
CN
Present work:
Attack at the γ-position.
Ru
1a
2a
Ru(bpy)3Cl2, visible light
N
N
Entry
3 (mol %)
Solvent
Time (h)
Yieldb (%)
3
N
.6H2O
air,
CN
Ar
1
2
3
4
5
6
7
8
1
1
1
1
1
1
2
0.5
CH3CN
DMF
DMSO
THF
EtOH
DCE
CH3CN
CH3CN
18
18
18
18
18
18
18
30
85
80
65
43
47
55
85
67
NC
Ar
3
EWG
EWG
1
2
Scheme 1. Visible-light-induced functionalization of benzylic amines.
subsequently attacked by the cyanide ion at the
afford an allylic cyanide, whereas in the previous cases the attack
takes place at the
-position (Scheme 1).1,6 The known methods
available for the cyanation of amines involve the use of hyperva-
lent-iodine9/I2,10 tropolium ion11 and copper12 catalysis and
electrolysis,13 which are of limited scope and selectivity.
c
-position to
a
Reaction conditions: 1 (1 mmol), 3 (0.5-2 mol %), solvent (3 mL), TMSCN (1.5
equiv), open to air (without bubbling air, irradiation through the flask’s bottom side
using Luxeon Rebel high power blue LEDs.
a
b
Isolated yield of pure product 2a.
In order to verify the practicability of the envisaged protocol for
the target molecule, a model reaction was carried out with
aza-Baylis–Hillman adduct 1a, in CH3CN containing 1 mol % of
Ru(bpy)3Cl2 (3) as a photocatalyst and TMSCN (1.5 equiv) as a
cyanide source. The reaction mixture was open to an air atmo-
sphere (but no air bubbling) and irradiated with visible light (blue
light emitting diodes (blue LEDs),14 kmax = 447.5 nm) at rt. It was
highly satisfying that the reaction delivered the desired allylic
cyanide 2a in 85% isolated yield after 18 h (Table 1, entry 1).
Following this experiment, a series of control experiments were
performed, which demonstrated that either there was no product
formation or it was formed in traces in the absence of any one of
the reagents/catalyst (Table 1, entries 2–4). Only a trace of product
was detected under an N2 atmosphere (Table 1, entry 5). Moreover,
the use of another catalyst Ir(bpy)3PF6 (1 mol %) or eosin Y
(2 mol %) was not as effective as Ru(bpy)3Cl2 (1 mol %) (Table 1, en-
try 1 vs 6 and 7). Irradiation with a household light (18 W CFL) pro-
duced considerably lower yield in a longer time as compared to
blue LEDs irradiation (Table 1, entry 1 vs 8). The presence of O2
(air) was found to be essential, because only a trace of product for-
mation was detected under the degassed condition or in an N2
atmosphere (Table 1, entry 1 vs 4 and 5). Moreover, the reaction
was quenched with TEMPO (1 mol %) (Table 1, entry 9) showing
that it follows a radical reaction pathway.
Then, the reaction was optimized for an effective solvent sys-
tem and catalyst loading (Table 2). CH3CN was found to be the best
among the tested solvents DMF, DMSO, DCE, EtOH, and THF, hence
it was used throughout the present study (Table 2, entry 1 vs 2–6).
The optimum catalyst loading delivering the highest yield in min-
imum reaction time was 1 mol %, because lowering the amount of
the catalyst to 0.5 mol % decreases the yield significantly and takes
a longer reaction time (Table 2, entry 1 vs 8), while there was no
effect on the yield and reaction time on increasing the catalytic
loading from 1 to 2 mol % (Table 2, entry 7).
Table 1
Screening and control experimentsa
To investigate the scope of the substrate, we tried various types
of aza-BH adducts 1, which successfully underwent the reaction to
produce the corresponding allylic nitriles in good to excellent
yields (Table 3). Many functionalities like MeO, NO2, Cl, Br, CN,
and COOMe are well tolerated in the present reaction conditions.
The aza-BH adducts 1 bearing an electron-withdrawing group on
the aromatic ring undergo slower reaction and give slightly lower
yields as compared to those containing an electron-donating group
(Table 3, entries 2 and 3 vs 4–6). The requisite aza-BH adducts 1
were prepared by the method reported in the literature.12b,16
Based on the above observations and the literature prece-
dents,1,7,10,12 a plausible mechanistic pathway is depicted in
Scheme 2. The benzylic tertiary amine 1 undergoes SET to generate
the radical cation A and Ru(I) with activated Ru(II)⁄ in the presence
of blue LEDs. Ru(I) is oxidized to Ru(II) with atmospheric oxygen,
which is reduced to superoxide radical anion (OÅ2ꢀ) to complete
the photoredox cycle. The radical cation A is converted into imin-
ium ion B by H-abstraction from the benzylic position by the
superoxide radical anion (O2Åꢀ). The conjugated iminium ion thus
generated in situ undergoes nucleophilic addition with the cyanide
ion to afford the desired allylic cyanides 2.
air
Ru(bpy)3Cl2 (3) (1 mol%),
N
N
(O2)
TMSCN (1.5 equiv)
COOMe
COOMe
CN
CH3CN, visible light, 18-72 h, rt
2a
1a
Entry
Visible light
Photocatalyst
Air
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
8
9
+
ꢀ
+
+
+
+
+
+
+
+
+
ꢀ
+
+
+
+
+
+
+
+
+
ꢀ
N2
+
+
+
18
24
24
24
24
18
18
24
72
85
n.r.c
n.r.
Traced
Trace
72e
57f
55g
+
Traceh
a
Reaction conditions: 1 (1 mmol), 3 (1 mol %), CH3CN (3 mL), TMSCN (1.5 equiv),
open to air (without bubbling air), irradiation through the flask’s bottom side using
Luxeon Rebel high power blue LEDs.
b
Isolated yield of pure product 2a.
c
Reaction was carried out in dark; n.r. = no reaction.
Reaction was performed under degassed condition.
Ir(bpy)3PF6ꢁ6H2O (1 mol %) was used as the catalyst.
d
In conclusion, we have disclosed aza-BH adducts as a novel sub-
e
f
strate for visible-light-mediated functionalization at the
with respect to the amino group. The protocol involves an aerobic
oxidative photocatalytic -cyanation of aza-BH adducts to provide
c-position
Eosin Y (2 mol %) was used as the catalyst.
g
Reaction was performed using 18 W CFL (compact fluorescent lamp, Philips).
h
Reaction was quenched with TEMPO (1 mol %).
c

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V. P. Srivastava et al. / Tetrahedron Letters 55 (2014) 1788–1792
Table 3
Substrates scope
blue
LEDs
Ru(bpy)3Cl2 (3) (1 mol%),
TMSCN (1.5 equiv)
N
air
N
(O2)
EWG
CN
EWG
Ar
CH3CN, 14-24 h, rt
Ar
2
1
Entrya
Substrate 1
Product 2
Time (h)
18
Yieldb (%)
N
N
1
85
COOMe
COOMe
CN
2a
1a
N
N
2
3
4
5
6
14
14
20
20
24
89
92
80
82
73
COOMe
CN
COOMe
COOMe
2b
1b
H3C
H3C
N
N
COOMe
CN
2c
1c
H3CO
H3CO
N
N
COOMe
COOMe
COOMe
COOMe
1d
2d
CN
Cl
Br
Cl
N
N
COOMe
CN
1e
2e
Br
N
N
COOMe
CN
2f
1f
O2N
O
O2N
O
N
N
7
8
18
18
80
84
COOMe
COOMe
COOMe
CN
1g
N
2g
N
COOMe
CN
2h
1h
N
N
9
18
14
79
80
CN
CN
CN
2i
1i
N
N
10
Ph
CN
Ph
2j
1j
NC
CN

V. P. Srivastava et al. / Tetrahedron Letters 55 (2014) 1788–1792
1791
Table 3 (continued)
Entrya
Substrate 1
Product 2
Time (h)
14
Yieldb (%)
N
N
11
85
Ph
COOMe
Ph
2k
NC
1k
COOMe
a
Reaction conditions: 1 (1.0 mmol), 3 (1.0 mol %), TMSCN (1.5 equiv) in CH3CN (3 mL) open to air, irradiation with blue LEDs irradiation (for the detailed procedure, see:
Ref. 15).
b
Isolated yields of pure products 2 after column chromatography.
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N
N
SET
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EWG
EWG
Ar
Ar
1
A
*
Ru(I)
Ru(II)
blue
Photocatalytic
cycle of Ru(II)
HO2
Ru(II),
O2
LEDs
air
(O2)
CN
Ru(
II)
O2
(air)
N
N
CN source
EWG
CN
EWG
Ar
Ar
B
2
Scheme 2. Plausible mechanistic pathway for cyanation of aza-BH adducts.
6. (a) Zhao, G.; Yang, C.; Guo, L.; Sun, H.; Chen, C.; Xia, W. Chem. Commun. 2012,
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Chem. Eur. J. 2012, 18, 5170; (d) Zhu, S.; Rueping, M. Chem. Commun. 2012,
11960; (e) Rueping, M.; Zhu, S.; Koenigs, R. M. Chem. Commun. 2011, 8679.
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4509; (b) Dai, C.; Narayanam, J. M. R.; Stephenson, C. R. J. Nat. Chem. 2011, 3,
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12886.
allylic cyanides in good to excellent yields. Mild reaction
conditions, low catalyst loading, and use of atmospheric oxygen
and visible light as cost effective, readily available and ecosustain-
able reagents to afford Michael type addition products are the
attractive features of the methodology.
Acknowledgments
8. For selected reviews on oxidative reactions with molecular oxygen, see: (a)
Roduner, E.; Kaim, W.; Sarkar, B.; Urlacher, V. B.; Pleiss, J.; Glser, R.; Einicke, W.-
D.; Sprenger, G. A.; Beifuß, U.; Klemm, E.; Liebner, C.; Hieronymus, H.; Hsu, S.-
F.; Plietker, B.; Laschat, S. ChemCatChem 2013, 5, 82; (b) Wu, W.; Jiang, H.;
Adimurthy, S. Acc. Chem. Res. 2012, 45, 1736; (c) Shi, Z.; Zhang, C.; Tang, C.; Jiao,
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Angew. Chem., Int. Ed. 2011, 50, 11062; (e) Wendlandt, A. E.; Suess, A. M.; Stahl,
S. S. Angew. Chem. 2011, 123, 11256; (f) Punniyamurthy, T.; Velusamy, S.; Iqbal,
J. Chem. Rev. 2005, 105, 2329; (g) Stahl, S. S. Angew. Chem., Int. Ed. 2004, 43,
3400.
We sincerely thank the SAIF, Punjab University, Chandigarh, for
providing spectra. V.P.S. is grateful to the Department of Science
and Technology (DST) Govt. of India, for the award of a DST-Inspire
Faculty position (Ref. IFA-11CH-08) and financial support and
A.K.Y. is grateful to the CSIR, New Delhi, for the award of a Senior
Research Fellowship.
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15. General procedure for the visible-light-induced cyanation of aza-Baylis–Hillman
adducts to afford allylic cyanides 2 (Table 3): To a solution of aza-Baylis–Hillman
adduct 1 in CH3CN (3 mL) was added Ru(bpy)3Cl2 (1 mol %) and TMSCN (1.5
equiv). The reaction mixture was irradiated with blue LEDs with stirring at rt
for 14–24 h (Table 3). After completion of the reaction (as monitored by TLC),
water (5 mL) was added and the mixture was extracted with EtOAc (3 ꢂ 5 mL).
The combined organic phase was dried over MgSO4, filtered, and evaporated
9
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V. P. Srivastava et al. / Tetrahedron Letters 55 (2014) 1788–1792
under reduced pressure. The resulting product was purified by silica gel
column chromatography using a gradient mixture of hexane/ethyl acetate as
117.8, 114.2, 62.3, 43.5, 28.2; HRMS (EI): calcd for C18H16N2 260.1314, found
260.1315.
eluent to afford an analytically pure sample of 2. The characterization data of
representative compounds 1 and 2 are given below:
Compound 2a: (Yellow oily liquid); IR (neat liquid): mmax = 3440, 3060, 3024,
2217, 2952, 2923, 2850, 2717, 1962, 1729, 1614, 1576, 1492, 1436, 1377, 1261,
Compound 1a: (Yellow oily liquid); IR (neat liquid):
mmax = 3440, 3062, 3023,
1003, 767, 700 cmꢀ1 1H NMR (400 MHz, CDCl3): d = 7.37–7.31(m, 5H), 3.59 (s,
;
2950, 2924, 2856, 2719, 1960, 1728, 1619, 1572, 1490, 1438, 1379, 1263, 1009,
3H), 3.09 (s, 2H), 1.32–1.25 (t, 4H), 1.07–0.97 (m, 6H); 13C NMR (100 MHz,
CDCl3): d = 166.7, 156.4, 133.9, 128.7, 127.3, 126.8, 116.0, 95.9, 54.2, 52.3, 26.8,
24.0, 11.9; HRMS (EI): calcd for C17H20N2O2 284.1525, found 284.1520.
765, 712 cmꢀ1 1H NMR (400 MHz, CDCl3): d = 7.30–7.01 (m, 5H), 6.27 (s, 1H),
;
5.60 (s, 1H), 4.12 (s, 1H), 3.76 (s, 3H), 1.54–1.43 (t, 4H), 1.02–0.92 (br0 s, 6H);
13C NMR (100 MHz, CDCl3): d = 166.0 139.4, 128.3, 126.7, 124.9, 122.1, 93.7,
83.1, 51.5, 51.1, 25.6, 24.3; HRMS (EI): calcd for C16H21NO2 259.1572, found
259.1570.
Compound 2g: (Yellow oily liquid); IR (neat liquid):
mmax = 3446, 3058, 3022,
2952, 2917, 2853, 2712, 2223, 1960, 1726, 1619, 1578, 1490, 1432, 1375, 1260,
1190, 1143, 1021, 760, 711 cmꢀ1 1H NMR (400 MHz, CDCl3): d = 7.57–7.23 (m,
;
Compound 1g. (Yellow oily liquid); IR (neat liquid):
mmax = 3449, 3065, 3028,
5H), 3.69 (s, 3H), 2.81–2.14 (br0 s, 8H), 1.97 (s, 2H); 13C NMR (100 MHz, CDCl3):
d = 165.7, 154.6, 132.9, 127.8, 126.7, 116.0, 95.7, 66.1, 51.3, 48.6, 24.3, 12.7;
HRMS (EI): calcd for C16H18N2O3 286.1317, found 286.1315.
2946, 2918, 2850, 2713, 2219, 1956, 1727, 1621, 1570, 1490, 1437, 1375, 1262,
1198, 1141, 1018, 767, 709 cmꢀ1 1H NMR (400 MHz, CDCl3): d = 7.28-7.04 (m,
;
5H), 6.17 (s, 1H), 5.59 (s, 1H), 4.32 (s, 1H), 3.67 (s, 3H), 2.54–2.43 (br0 s, 8H); 13C
NMR (100 MHz, CDCl3): d = 166.3, 139.8, 136.0, 128.8, 127.3, 126.1, 82.7, 74.8,
67.5, 52.4, 52.1; HRMS (EI): calcd for C15H19NO3 261.1365, found 261.1361.
Compound 2j: (Yellow oily liquid); IR (neat liquid):
2959, 2921, 2856, 2713, 2219, 2232, 1968, 1723, 1615, 1572, 1497, 1438, 1379,
1264, 1197, 1148, 1013, 760, 713 cmꢀ1 1H NMR (400 MHz, CDCl3): d = 7.30–
mmax = 3107, 3068, 3023,
;
Compound 1j:12b (Yellow oily liquid); IR (neat liquid):
m
max = 3063, 3026, 2913,
2851, 2221, 1623, 1595, 1508, 1473, 1387, 1324, 1300, 1224, 1157, 1115,
1040 cmꢀ1 1H NMR (400 MHz, CDCl3): d = 7.27–7.21 (m, 5H), 7.18–7.16 (m,
7.15 (m, 4H), 7.24 (t, 2H), 7.21 (m, 2H), 7.20 (q, 1H), 6.77 (t, 1H), 3.39 (d, 2H),
3.08 (s, 4H); 13C NMR (100 MHz, CDCl3): d = 148.7, 135.4, 134.1, 132.9, 127.3,
126.1, 124.6, 120.0, 118.3, 117.7, 115.8, 72.9, 50.3, 43.3, 27.6, 24.2, 12.3; HRMS
(EI): calcd for C19H15N3 285.1266, found 285.1263.
;
1H), 6.85 (d, J = 8.4 Hz, 2H), 6.82 (dd, J = 7.6, 7.6 Hz, 1H), 5.95 (s, 1H), 5.85 (d,
J = 1.2 Hz, 1H), 5.27 (s, 1H), 3.74 (ddd, J = 11.2, 5.2, 5.2 Hz, 1H), 3.50 (ddd,
J = 13.2, 8.4, 4.8 Hz, 1H), 3.02–2.91 (m, 2H); 13C NMR (100 MHz, CDCl3):
d = 148.0, 135.7, 132.3, 130.4, 129.0, 128.3, 127.9, 127.6, 126.4, 125.3, 118.4,
16. (a) Kim, S. J.; Lee, H. S.; Kim, J. M.; Kim, J. N. Bull. Korean Chem. Soc. 2008,
29, 265; (b) Lee, K. Y.; Kim, T. H.; Kim, J. N. Bull. Korean Chem. Soc. 2004,
25, 1966.