Visible-Light-Mediated Regioselective Allylation, Benzylation, and Silylation of Methylene-Malononitriles via Photoredox-Induced Radical Cation Fragmentation

Article abstract of DOI:10.1002/ejoc.201900902

Visible-light-mediated regioselective allylation, benzylation, and silylation of methylene-malononitriles using allylic/benzylic silane and disilane reagents have been developed, delivering the corresponding allylated, benzylated, and silylated products i

Full text of DOI:10.1002/ejoc.201900902

A Journal of
Accepted Article
Title: Visible-Light-Mediated Regioselective Allylation, Benzylation, and
Silylation of Methylene-Malononitriles via Photoredox-Induced
Radical Cation Fragmentation
Authors: Rongfang Liu, Shane Pui Mun Chia, Yi Yiing Goh, Han Wen
Cheo, Binbin Fan, Ruifeng Li, Rong Zhou, and Jie Wu
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To be cited as: Eur. J. Org. Chem. 10.1002/ejoc.201900902
Link to VoR: http://dx.doi.org/10.1002/ejoc.201900902
Supported by

10.1002/ejoc.201900902
European Journal of Organic Chemistry
FULL PAPER
Visible-Light-Mediated Regioselective Allylation, Benzylation, and
Silylation of Methylene-Malononitriles via Photoredox-Induced
Radical Cation Fragmentation
Rongfang Liu,[a] Shane Pui Mun Chia,[b] Yi Yiing Goh,[b] Han Wen Cheo,[b] Binbin Fan,[a] Ruifeng Li,[a]
Rong Zhou*[a] and Jie Wu*[b]
Abstract: Visible-light-mediated regioselective allylation, benzylation, radicals for further transformations.[5]
Recently, we have
and silylation of methylene-malononitriles using allylic/benzylic silane
and disilane reagents have been developed, delivering the
corresponding allylated, benzylated, and silylated products in
moderate to excellent yields. These reactions proceed smoothly with
exclusive regioselectivity by employing only an organo-photoredox
catalyst under mild conditions. A photoredox-induced radical cation
fragmentation is proposed for the generation of the key allylic,
benzylic, and silyl radicals.
developed a metal-free direct alkylation of allylic/benzylic sp3
CH bonds via photoredox-catalyzed -CH+· type radical cation
deprotonation.[6] By employing 9-mesityl-10-methylacridinium
-
perchlorate (Mes-Acr+ClO4 ), an organo-photoredox catalyst, a
broad array of substrates including tetra-, tri-, and disubstituted
alkenes and toluenes could serve as effective radical precursors
to smoothly couple with electron deficient alkenes. However,
efforts to extend this methodology to terminal alkenes and
electron-poor toluenes were not successful, mainly due to the
higher oxidative potentials associated with these substrates (e.g.
ox
E1/2 = +2.50 V vs SCE in MeCN for 2-methylpent-1-ene).[7] We
Introduction
envisioned that introducing an electroauxiliary group such as a
trimethylsilyl (TMS) group to the allylic/benzylic substrates may
significantly lower their oxidative potentials (e.g. E1/2ox = +1.58 V
vs Ag/Ag+ in MeCN for allyltrimethylsilane).[8g] This will thus
enable an effective photoredox-induced single electron transfer
(SET) to generate the radical cation, and the subsequent
fragmentation will deliver a reactive carbon radical and a silyl
cation species.
The rapidly growing field of visible-light photocatalysis offers a
valuable platform for the design and discovery of new synthetic
transformations, enabling numerous new bond formations via
open-shell radical species.[1] In this context, the direct addition of
photo-generated radicals to CC bonds represents an attractive
approach for the construction of CC or Cheteroatom bonds.[2]
In general, the photo-induced generation of radicals for
conjugate addition proceeds in two reaction modes: a) via
hydrogen atom transfer (HAT); b) via photoredox process. The
HAT process affords the radical species directly from CH
substrates through hydrogen atom abstraction, offering
enormous opportunities for CH activations. The efficiency of
HAT is controlled by the relative bond dissociation energies
(BDEs) and the polarity of the HAT catalyst and the substrates
involved (Scheme 1, a).[3] On the other hand, photoredox
catalysis can generate radicals from both CH and
Cheteroatom substrates via either single electron oxidation or
reduction, which is effectively controlled by the corresponding
redox potentials of the photocatalyst and the substrates
(Scheme 1, b).[1,2] Over the past decade, significant
developments have been made in both HAT and photoredox
catalysis.
Scheme 1. Two modes of photo-induced generation of radicals for conjugate
addition
Radical cation species generated from photoredox catalysis
have been frequently used in many important transformations.[4]
Radical cation can undergo fragmentation to directly afford
Notably, the use of organosilicon compounds in photo-
induced reactions such as allylation, benzylation, and silylation
have been developed since the late 1980s.[8] However, the
reported examples required high catalyst loading (20-50 mol%),
use of equipment-demanding UV light, or suffered from poor
selectivity.[8] Recently, Melchiorre and co-workers reported an
elegant enantioselective catalytic -alkylation of enals with
alkyltrimethylsilanes under visible-light irradiation, through
carbon radicals generation from organosilicon compounds under
mild visible-light conditions.[9] Moreover, Molander and
coworkers have developed novel dual nickel/photoredox cross-
couplings by use of a new class of hypercoordinate silicates,
which acts as an effective radical precursor under visible-light
photoredox catalysis.[10] Inspired by these pioneering work and
[a]
R. Liu, Dr. R. Zhou, Dr. B. Fan, Dr. R. Li
College of Chemistry and Chemical Engineering
Taiyuan University of Technology
Taiyuan 00024 (P.R. China)
E-mail: zhourong@tyut.edu.cn
[b]
C. P. M. Shane, Y. Y. Goh, H.W. Cheo, Dr. J. Wu
Department of Chemistry
National University of Singapore
3 Science Drive 3, Singapore 117543 (Republic of Singapore)
E-mail: chmjie@nus.edu.sg
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/ejoc.201900902
European Journal of Organic Chemistry
FULL PAPER
also as our continuous interest on visible-light photocatalysis,[11]
we herein report visible-light-mediated allylation, benzylation,
and silylation of methylene-malononitriles with allylsilanes,
benzylsilanes, and disilanes by utilizing only an organo-
photocatalyst via photoredox-induced σ-CSi+· or σ-SiSi+· type
radical cation fragmentation.
-
-
-
-
-
-
-
-
-
9
Mes-Acr+ClO4
Mes-Acr+ClO4
Mes-Acr+ClO4
Mes-Acr+ClO4
Mes-Acr+ClO4
Mes-Acr+ClO4
Mes-Acr+ClO4
Mes-Acr+ClO4
Mes-Acr+ClO4
DMF
5
10
11
12
13
14
15
16
17
DMSO
MeOH
0
23
DCE/MeOH (V/V 3/1)
DCE/MeOH (V/V 4/1)
DCE/MeOH (V/V 5/1)
DCE/MeOH (V/V 10/1)
DCE/MeOH (V/V 4/1)
no light or photocatalyst
49
65(60)
55
Results and Discussion
45
We initiated our study with the reaction between
allyltrimethylsilane 1 and benzylidenemalononitrile 2 in the
39[c]
0
-
presence of Mes-Acr+ClO4 (2.5 mol%) in 1,2-dichloroethane
(DCE, 0.1 M) under blue LED irradiation (λmax = 470 nm). To
our delight, the expected allylation product 3 was achieved in
38% yield, with no other regioisomers detected (Table 1,
entry 1). A systematic survey on the reaction conditions was
conducted to further improve the efficiency using 1 and 2 as
the model substrates. Among several common photocatalysts
[a] General conditions: allyltrimethylsilane
1
(63 μL, 0.4 mmol),
benzylidenemalononitrile 2 (31 mg, 0.2 mmol), and photocatalyst (0.005
mmol) in solvent (2.0 mL), irradiated by 18 W blue LED. [b] Yields were
determined by the crude 1H NMR spectra using 1,3,5-trimethoxybenzene
as an internal standard, and data in parentheses are isolated yields. [c] 1.0
mL of solvent was used.
-
investigated, Mes-Acr+ClO4 proved to be the most effective
one (entries 1-5). The organo-photocatalyst 2,4,6-tri(p-
tolyl)pyrylium tetrafluoroborate gave an inferior result, while
Ir(III) and Ru(II) based catalysts were completely ineffective,
probably due to their lower excited-state oxidizing ability
red*
(E1/2
= +1.21, +1.14, +0.77 V vs SCE in MeCN for the
catalysts used in entries 3-5, respectively). Screening of the
reaction medium indicated that a mixed DCE/MeOH was the
most suitable medium for this transformation, probably due to
a nucleophile-assisted silane radical cation fragmentation
(entries 6-15).[12] The volume ratio of the two solvents
influenced the reaction to a certain extent, and the optimal
ratio was found to be DCE/MeOH as 4/1 (entries 12-15). A
declined yield was obtained when the reaction solution was
more concentrated (entry 16). Finally, no reaction occurred in
the absence of either light or photocatalyst, highlighting the
necessity of both components (entry 17).
Table 1. Survey on reaction conditions.[a]
Entry
Catalyst
Solvent
Yield (%)[b]
-
1
2
Mes-Acr+ClO4
DCE
DCE
38
5
2,4,6-tri(p-tolyl)pyrylium
tetrafluoroborate
3
4
5
6
7
8
Ir[dF(CF3)ppy]2(dtbpy)PF6
Ir(dFppy)2(dtbpy)PF6
Ru(bpy)3Cl2·6H2O
DCE
0
DCE
0
DCE
0
-
Mes-Acr+ClO4
DCM
MeCN
Acetone
24
22
20
-
Mes-Acr+ClO4
-
Mes-Acr+ClO4
Scheme 2. Scope of the visible-light-mediated allylation, benzylation, and
silylation reactions. [a] General conditions: silane (0.4 mmol), methylene-
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10.1002/ejoc.201900902
European Journal of Organic Chemistry
FULL PAPER
-
malononitrile (0.2 mmol), and Mes-Acr+ClO4 (2.1 mg, 0.005 mmol) in
DCE/MeOH (2.0 mL, V/V 4/1), irradiated by 18 W blue LED. Isolated
yields. [b] DCE (2.0 mL) was used as the solvent.
to (See Supporting Information Fig. S1 and S2). This supports
the proposed SET between the silane and the light-excited
photocatalyst. Fragmentation of radical cation I yields the
corresponding carbon or silicon radical II.[5,16] Nucleophilic
addition of radical II to electron-deficient methylene-
malononitrile gives radical III. The addition of radical II to the
CC bond occurred exclusively at the β-position to the cyano
group due to the formation of a more stable radical
intermediate III.[17] Single-electron reduction of the radical III
by the reduced form of the acridinium catalyst (E1/2ox = -0.57 V
vs SCE in MeCN) delivers the anion intermediate IV and
regenerates the photocatalyst. Protonation of anion IV by
solvent produces the corresponding allylation, benzylation, or
silylation product. Furthermore, no reaction occurred when
radical scavengers such as TEMPO and hydroquinone were
added, supporting the above radical-based process (See
Supporting Information).
With the optimal conditions in hand (Table 1, entry 13),
we sought to investigate the generality of this protocol
(Scheme 2). Employing allyltrimethylsilane
1
as
a
representative substrate, variety of methylene-
a
malononitriles bearing different electronic and steric
properties were first investigated (Scheme 2a). Allylation
reactions of allyltrimethylsilane 1 with an array of methylene-
malononitriles, bearing either electron-neutral, electron-rich,
or electron-poor aromatic rings, proceeded smoothly to afford
the corresponding products 3-11 in moderate to good yields.
The protocol was found to tolerate functional groups such as
halogens, trifluoromethyl groups, acids, nitriles, and nitro
groups. A sterically demanding naphthyl group was also
accommodated, albeit the yield of product 12 was relatively
lower. However, alkene with a hetero-aryl moiety such as 2-
furyl substituted methylene-malononitrile was ineffective for
this allylation. Furthermore, this radical cation fragmentation
protocol could be extended to benzylsilanes, leading to the
benzylation of methylene-malononitriles (Scheme 2b). Under
the optimal conditions, the benzylation of arylidene-
malononitriles with the representative benzylsilanes,
benzyltrimethylsilane and (4-bromobenzyl)trimethylsilane,
occurred smoothly to afford their corresponding products 13-
16 in moderate to excellent yields. Notably, disilanes were
also a competent class of compounds for this protocol
(Scheme 2c). In all the examined cases, disilanes such as
hexamethyldisilane and 1,2-bis(2-methoxyphenyl)-1,1,2,2-
tetramethyldisilane participated in the reaction smoothly to
give the corresponding silylated products 17-26 in moderate
to good yields, with methylene-malononitriles bearing either
electron-rich, electron-poor aryl, hetereoaryl, or sterically
hindered aryl group. It is also worth noting that although
transition-metal-catalyzed hydrosilylation of electron-rich
alkenes has been extensively studied,[13] the hydrosilylation of
electron-deficient alkenes is scarce due to their low reactivity
and poor selectivity toward most of the existing catalysts.[14]
This study thus represents an example of metal-free
hydrosilylation of electron-deficient alkenes under mild
conditions.[11e,15] Remarkably, only one regioisomer was
obtained for all the substrates examined, highlighting the
regioselective process.
Scheme 3. Plausible mechanism for the allylation, benzylation, and
silylation reactions
Importantly, the malononitirle moiety in the products can
be transformed to acid,[18] ester,[19] amide,[20] and other
functionalities in a convenient manner. As exemplified in
Scheme 4, the benzylation product 13 could be easily
converted to the bioactive compound 27 in one-step, a
representative compound serving as M1 antagonist.[21]
A plausible mechanism for the allylation, benzylation, and
silylation reactions was proposed based on all experimental
results and our previous related studies (Scheme 3).[6] Initial
visible-light excitation of the acridinium photoredox catalyst
red*
generates its excited-state [Mes-Acr+]* (E1/2
= +2.06 V vs
Scheme 4. Synthetic transformation of the benzylation product 13
SCE in MeCN),[6] which is sufficiently oxidizing to oxidize the
ox
silane (E1/2
=
+1.58
V
vs Ag/Ag+ in MeCN for
via SET.
allyltrimethylsilane)[8g] to its radical cation
I
Luminescence quenching experiments indicate that the
silanes such as allyltrimethylsilane 1 can quench the excited
state of acridinium ([Mes-Acr+]*), whereas methylene-
malononitrile such as benzylidenemalononitrile 2 are unable
Conclusions
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10.1002/ejoc.201900902
European Journal of Organic Chemistry
FULL PAPER
Prepared according to the typical procedure, allyltrimethylsilane (63 μL,
0.4 mmol) and 2-(2-bromobenzylidene)malononitrile (46.6 mg, 0.2 mmol)
were employed to give 5 (32.5 mg, 59% yield) as a pale yellow liquid. 1H
NMR (400 MHz, CDCl3) δ 7.70 – 7.25 (m, 5H), 5.77 – 5.63 (m, 1H), 5.30
– 5.18 (m, 2H), 4.13 – 4.04 (m, 2H), 2.88 – 2.83 (m, 2H) ppm; 13C NMR
(75 MHz, CDCl3) 135.3, 133.7, 132.3, 130.2, 128.2, 125.1, 119.8, 111.6,
111.1, 43.6, 35.0, 27.7 ppm; HRMSESI [M-H]- calculated for C13H10BrN2
273.1412, found 273.0033.
In conclusion, we have developed visible-light-mediated
allylation, benzylation, and silylation of methylene-malononitriles
using organosilanes as the radical precursors by employing an
organo-photoredox catalyst, affording the corresponding
products in moderated to excellent yields with exclusive
regioselectivity.
A
photoredox-catalyzed radical
cation
fragmentation process was proposed to generate the key allyl,
benzyl, and silyl radicals to couple with methylene-malononitriles.
The reactions benefit from their redox economy, mild conditions,
and good selectivity. The synthetic utility of the methodology
was demonstrated by facile synthesis of a bioactive compound.
Future efforts in our laboratory will be directed towards further
application of the visible-light-induced radical cation
fragmentation protocol for synthesis of other value-added
molecules.
2-(1-(4-Bromo-2-fluorophenyl)but-3-en-1-yl)malononitrile (6)
Prepared according to the typical procedure, allyltrimethylsilane (63 μL,
0.4 mmol) and 2-(4-bromo-2-fluorobenzylidene)malononitrile (50.2 mg,
0.2 mmol) were employed to give 6 (37.5 mg, 64% yield) as a pale yellow
liquid. 1H NMR (400 MHz, CDCl3) δ 7.34 (ddd, J = 11.6, 9.1, 1.6 Hz, 2H),
7.25 – 7.21 (m, 1H), 5.67 – 5.55 (m, 1H), 5.20 (dd, J = 17.1, 1.2 Hz, 1H),
5.15 (dd, J = 10.2, 0.7 Hz, 1H), 4.03 (d, J = 6.5 Hz, 1H), 3.66 (dd, J =
15.1, 6.8 Hz, 1H), 2.71 (ddd, J = 31.1, 14.3, 7.1 Hz, 2H) ppm; 13C NMR
(101 MHz, CDCl3) δ160.4 (d, J = 250.8 Hz), 132.2, 129.9 (d, J = 4.3 Hz),
128.4 (d, J = 3.6 Hz), 123.4 (d, J = 10.1 Hz), 122.7 (d, J = 13.7 Hz),
120.3, 119.9 (d, J = 25.7 Hz), 111.3, 111.2, 38.9, 35.4, 27.6 ppm;
HRMSESI [M-H]- calculated for C13H9BrFN2 290.9933, found 290.9939.
Experimental Section
Typical procedure for the allylation and benzylation reactions
2-(1-(4-Chlorophenyl)but-3-en-1-yl)malononitrile (7)
To a 25 mL Schlenk tube equipped with a magnetic stir bar was added
-
the photocatalyst Mes-Acr+ClO4 (2.1 mg, 0.005 mmol), and methylene-
malononitrile (0.2 mmol). The Schlenk tube was sealed and degassed via
vacuum evacuation and subsequently backfilled with argon for three
times. After that, anhydrous DCE and methanol (2.0 mL, V/V 4/1), and
silane (0.4 mmol) were added sequentially using a syringe. The reaction
setup was placed under blue LED (2 meter strips, 18 W) with an argon
balloon and irradiated for 12-36 hrs. After completion as monitored by
TLC, the solvent was removed under reduced pressure and the crude
product was purified by column chromatography on silica gel via gradient
elution with hexane / ethyl acetate (15:1-3:1) give the allylation or
benzylation product.
Prepared according to the typical procedure, allyltrimethylsilane (63 μL,
0.4 mmol) and 2-(4-chlorobenzylidene)malononitrile (37.7 mg, 0.2 mmol)
were employed to give 7 (26.3 mg, 57% yield) as a pale yellow liquid. 1H
NMR (300 MHz, CDCl3) δ 7.42 – 7.27 (m, 5H), 5.71 – 5.58 (m, 1H), 5.27
– 5.18 (m, 2H), 4.02 – 4.00 (m, 1H), 3.31 – 3.24 (m, 1H), 2.75 – 2.69
(m,2H) ppm; 13C NMR (75 MHz, CDCl3) 134.9, 134.7, 132.5, 129.4,
129.2, 120.1, 111.6, 111.1, 45.2, 36.3, 28.7; HRMSESI [M-H]-
calculated for C13H10ClN2 229.0533, found 229.0538.
2-(1-(4-(Trifluoromethyl)phenyl)but-3-en-1-yl)malononitrile (8)
2-(1-Phenylbut-3-en-1-yl)malononitrile (3)
Prepared according to the typical procedure, allyltrimethylsilane (63 μL,
0.4 mmol) and 2-(4-(trifluoromethyl)benzylidene)malononitrile (44.4 mg,
0.2 mmol) were employed to give 8 (39.6 mg, 75% yield) as a pale yellow
liquid. 1H NMR (300 MHz, CDCl3) δ 7.70 (d, J = 8.3 Hz, 2H), 7.49 (d, J =
8.2 Hz, 2H), 5.74 – 5.58 (m, 1H), 5.32 – 5.17 (m, 2H), 4.06 (d, J = 5.5 Hz,
1H), 3.42 – 3.31 (m, 1H), 2.85 – 2.68 (m, 2H) ppm; 13C NMR (75 MHz,
CDCl3) 140.1, 132.3, 131.3 (q, J = 33.0 Hz), 128.5, 126.2 (q, J = 3.7 Hz),
123.7 (q, J = 272.3 Hz), 120.5, 111.5, 110.1, 45.6, 36.3, 28.5 ppm;
HRMSESI [M-H]-calculated for C14H10F3N2 263.0796, found 263.0802.
Prepared according to the typical procedure, allyltrimethylsilane (63 μL,
0.4 mmol) and benzylidenemalononitrile (31 mg, 0.2 mmol) were
employed to give 3 (23.5 mg, 60% yield) as a pale yellow liquid. 1H NMR
(300 MHz, CDCl3) δ 7.45 – 7.32 (m, 5H), 5.76 – 5.59 (m, 1H), 5.31 – 5.11
(m, 2H), 4.02 (d, J = 5.6 Hz, 1H), 3.29 (td, J = 7.6, 5.7 Hz, 1H), 2.79 –
2.69 (m, 2H) ppm; 13C NMR (101 MHz, CDCl3) δ 136.3, 132.9, 129.2,
128.9, 127.9, 119.9, 111.8, 111.5, 45.9, 36.4, 28.9 ppm; HRMSESI [M-
H]- calculated for C13H11N2 195.0922, found 195.0928.
4-(1,1-Dicyanopent-4-en-2-yl)benzoic acid (9)
2-(1-(m-Tolyl)but-3-en-1-yl)malononitrile (4)
Prepared according to the typical procedure, allyltrimethylsilane (63 μL,
0.4 mmol) and 4-(2,2-dicyanovinyl)benzoic acid (39.6 mg, 0.2 mmol)
were employed to give 9 (28.8 mg, 60% yield) as an orange liquid. 1H
NMR (400 MHz, CDCl3) δ 8.17 (d, J = 8.4 Hz, 2H), 7.48 (d, J = 8.3 Hz,
2H), 5.72 – 5.57 (m, 1H), 5.24 (dd, J = 21.7, 13.6 Hz, 2H), 4.06 (d, J =
5.7 Hz, 1H), 3.42 – 3.35 (m, 1H), 2.85 – 2.70 (m, 2H) ppm; 13C NMR (101
MHz, CDCl3) 170.9, 142.1, 132.3, 131.1, 128.2, 120.4, 111.6, 111.1, 45.8,
36.3, 28.5 ppm; HRMSESI [M-H]- calculated for C14H11N2O2 239.0821,
found 239.0826.
Prepared according to the typical procedure, allyltrimethylsilane (63 μL,
0.4 mmol) and 2-(3-methylbenzylidene)malononitrile (33.6 mg, 0.2 mmol)
were employed to give 4 (16.4 mg, 39% yield) as a pale yellow liquid. 1H
NMR (400 MHz, CDCl3) δ 7.30 (t, J = 7.9 Hz, 1H), 7.16 (dd, J = 20.6, 6.5
Hz, 3H), 5.72 – 5.61 (m, 1H), 5.25 (dd, J = 17.1, 1.2 Hz, 1H), 5.17 (d, J =
10.1 Hz, 1H), 4.00 (d, J = 5.7 Hz, 1H), 3.25 (dd, J = 13.4, 7.5 Hz, 1H),
2.80 – 2.68 (m, 2H), 2.38 (s, 3H) ppm; 13C NMR (101 MHz,CDCl3) 138.9,
136.3, 133.1, 129.7, 129.1, 128.6, 124.8, 119.7, 111.9, 111.5,45.9, 36.5,
28.9, 21.5 ppm; HRMSESI [M-H]- calculated for C14H13N2 209.1079,
found 209.1084.
2-(1-(4-Cyanophenyl)but-3-en-1-yl)malononitrile (10)
2-(1-(2-Bromophenyl)but-3-en-1-yl)malononitrile (5)
Prepared according to the typical procedure, allyltrimethylsilane (63 μL,
0.4 mmol) and 2-(4-cyanobenzylidene)malononitrile (35.8 mg, 0.2 mmol)
were employed to give 10 (23 mg, 52% yield) as a yellow liquid. 1H NMR
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10.1002/ejoc.201900902
European Journal of Organic Chemistry
FULL PAPER
(300 MHz, CDCl3) δ 7.74 (d, J = 8.0 Hz, 2H), 7.49 (d, J = 7.7 Hz, 2H),
5.72 – 5.56 (m, 1H), 5.32 – 5.19 (m, 2H), 4.08 (dd, J = 5.4, 1.7 Hz, 1H),
3.36 (dd, J = 13.3, 7.3 Hz, 1H), 2.85 – 2.66 (m, 2H) ppm; 13C NMR (75
MHz, CDCl3) 141.3, 132.9, 131.9, 128.9, 120.7, 117.9, 113.2, 111.3,
110.8, 45.6, 36.1, 28.3 ppm; HRMSESI [M-H]- calculated for C14H10N3
220.0875, found 220.0880.
8.2 Hz, 2H), 7.39 – 7.27 (m, 3H), 7.20 (d, J = 6.5 Hz, 2H), 3.89 (d, J = 5.0
Hz, 1H), 3.58 – 3.49 (m, 1H), 3.28 (dd, J = 7.8, 3.6 Hz, 2H) ppm; 13C
NMR (75 MHz, CDCl3) 140.3, 135.9, 131.4 (q, J = 32.8 Hz), 129.3, 128.8,
128.6, 127.8, 126.2 (q, J = 3.7 Hz), 123.7 (q, J = 272.1 Hz), 111.7, 111.0,
48.0, 38.3, 28.1 ppm; HRMSESI [M-H]- calculated for C18H12F3N2
313.0953, found 313.0958.
2-(1-(4-Nitrophenyl)but-3-en-1-yl)malononitrile (11)
2-(2-(3-Bromophenyl)-1-(4-(trifluoromethyl)phenyl)ethyl)malononitrile (16)
Prepared according to the typical procedure, allyltrimethylsilane (63 μL,
0.4 mmol) and 2-(4-nitrobenzylidene)malononitrile (39.8 mg, 0.2 mmol)
were employed to give 11 (19.3 mg, 40% yield) as an orange liquid. 1H
NMR (400 MHz, CDCl3) δ 8.30 (d, J = 8.7 Hz, 2H), 7.56 (d, J = 8.7 Hz,
2H), 5.72 – 5.59 (m, 1H), 5.32 – 5.22 (m, 2H), 4.09 (d, J = 5.5 Hz, 1H),
3.46 – 3.40 (m, 1H), 2.85 – 2.71 (m, 2H) ppm; 13C NMR (101 MHz,
CDCl3) 143.1, 131.8, 129.1, 124.3, 120.9, 111.2, 110.7, 45.4, 36.2, 28.3
ppm; HRMSESI [M-H]- calculated for C13H10N3O2 240.0773, found
240.0779.
Prepared according to the typical procedure, benzyltrimethylsilane (97.2
mg, 0.4 mmol) and 2-(4-(trifluoromethyl)benzylidene)malononitrile (44.4
mg, 0.2 mmol) were employed to give 16 (37.7 mg, 48% yield) as a
yellow liquid. 1H NMR (300 MHz, CDCl3) δ 7.69 (d, J = 8.3 Hz, 2H), 7.47
(dd, J = 11.4, 8.3 Hz, 4H), 7.04 (d, J = 8.3 Hz, 2H), 3.89 (d, J = 5.3 Hz,
1H), 3.56 – 3.46 (m, 1H), 3.33 – 3.16 (m, 2H) ppm; 13C NMR (75 MHz,
CDCl3) 139.8, 134.8, 132.4, 131.6 (q, J = 33.0 Hz), 130.5, 128.5, 126.3
(q, J = 3.8 Hz), 124.7 (d, J = 277.1 Hz), 121.8, 111.4, 110.9, 47.8, 37.8,
28.4 ppm; HRMSESI [M-H]- calculated for C18H11BrF3N2 391.0058,
found 391.0063.
2-(1-(Naphthalen-1-yl)but-3-en-1-yl)malononitrile (12)
Typical procedure for the silylation reactions
Prepared according to the typical procedure, allyltrimethylsilane (63 μL,
0.4 mmol) and 2-(naphthalen-1-ylmethylene)malononitrile (40.8 mg, 0.2
mmol) were employed to give 12 (17.2 mg, 35% yield) as a brown liquid.
1H NMR (400 MHz, CDCl3) δ 8.00 – 7.87 (m, 3H), 7.65 – 7.60 (m, 1H),
7.59 – 7.51 (m, 3H), 5.76 – 5.65 (m, 1H), 5.32 – 5.24 (m, 1H), 5.16 (dd, J
= 10.2, 1.3 Hz, 1H), 4.32 (dd, J = 13.1, 7.1 Hz, 1H), 4.16 (d, J = 5.6 Hz,
1H), 2.97 (t, J = 7.1 Hz, 2H) ppm; 13C NMR (101 MHz, CDCl3) 134.4,
132.9, 132.2, 131.2, 129.9, 129.7, 127.5, 126.4, 125.6, 124.8, 121.6,
120.1, 112.3, 111.8, 38.2, 36.2, 28.7 ppm; HRMSESI [M-H]- calculated
for C17H13N2 245.1079, found 245.1084.
To a 25 mL Schlenk tube equipped with a magnetic stir bar was added
-
the photocatalyst Mes-Acr+ClO4 (2.1 mg, 0.005 mmol), and methylene-
malononitrile (0.2 mmol). The Schlenk tube was sealed and degassed via
vacuum evacuation and subsequently backfilled with argon for three
times. After that, anhydrous DCE (2.0 mL), and silane (0.4 mmol) were
added sequentially using a syringe. Then the reaction was placed under
blue LED (2 meter strips, 18 W) with an argon balloon and irradiated for
12-36 hrs. After completion as monitored by TLC, the solvent was
removed under reduced pressure and the crude product was purified by
column chromatography on silica gel via gradient elution with hexane /
ethyl acetate (15:1-3:1) give the allylation or benzylation product.
2-(1,2-Diphenylethyl)malononitrile (13)
2-(Phenyl(trimethylsilyl)methyl)malononitrile (17)
Prepared according to the typical procedure, benzyltrimethylsilane (76 μL,
0.4 mmol) and benzylidenemalononitrile (31 mg, 0.2 mmol) were
employed to give 13 (44.3 mg, 90% yield) as a yellow liquid. 1H NMR
(300 MHz, CDCl3) δ 7.47 – 7.27 (m, 8H), 7.22 – 7.17 (m, 2H), 3.85 (d, J =
5.1 Hz, 1H), 3.47 (td, J = 7.8, 5.2 Hz, 1H), 3.31 – 3.23 (m, 2H) ppm; 13C
NMR (75 MHz, CDCl3) 136.5, 136.4, 129.1, 129.0, 128.9, 128.8, 127.9,
127.5, 111.9, 111.3, 48.3, 38.5, 28.4 ppm; HRMSESI [M-H]- calculated
for C17H13N2 245.1079, found 245.1084.
Prepared according to the typical procedure, hexamethyldisilane (81.7 μL,
0.4 mmol) and benzylidenemalononitrile (31 mg, 0.2 mmol) were
employed to give 17 (29.2 mg, 64% yield) as a yellow liquid. 1H NMR
(300 MHz, CDCl3) δ 7.41 – 7.32 (m, 2H), 7.32 – 7.24 (m, 1H), 7.22 – 7.14
(m, 2H), 4.08 (d, J = 7.8 Hz, 1H), 2.64 (d, J = 7.8 Hz, 1H), 0.16 (s, 9H)
ppm; 13C NMR (75 MHz, CDCl3) δ 137.2, 129.2, 127.8, 127.4, 113.2,
113.1, 37.8, 25.2, -2.1 ppm; HRMSESI [M-H]- calculated for C18H15N2Si
227.1010, found 227.1010.
2-(1-(4-Bromo-2-fluorophenyl)-2-phenylethyl)malononitrile (14)
2-(m-Tolyl(trimethylsilyl)methyl)malononitrile (18)
Prepared according to the typical procedure, benzyltrimethylsilane (76 μL,
0.4 mmol) and 2-(4-bromo-2-fluorobenzylidene)malononitrile (50.2 mg,
0.2 mmol) were employed to give 14 (63.8 mg, 93% yield) as a yellow
liquid. 1H NMR (300 MHz, CDCl3) δ 7.38 – 7.24 (m, 6H), 7.15 (d, J = 6.2
Hz, 2H), 3.95 (d, J = 6.3 Hz, 1H), 3.87 (dd, J = 14.4, 7.3 Hz, 1H), 3.30
(dd, J = 14.0, 7.8 Hz, 1H), 3.18 (dd, J = 14.0, 7.6 Hz, 1H) ppm; 13C NMR
(75 MHz, CDCl3) 160.3 (d, J = 251.1 Hz), 135.8, 129.9 (d, J = 4.0 Hz),
129.1, 128.8, 128.3 (d, J = 3.6 Hz), 127.7, 123.3 (d, J = 10.1 Hz), 122.7
(d, J = 13.4 Hz), 119.9 (d, J = 25.7 Hz), 111.4, 111.2, 41.1, 37.6, 27.4
ppm; HRMSESI [M-H]- calculated for C17H11BrFN2 341.0090, found
341.0095.
Prepared according to the typical procedure, hexamethyldisilane (81.7 μL,
0.4 mmol) and 2-(3-methylbenzylidene)malononitrile (33.6 mg, 0.2 mmol)
were employed to give 18 (29 mg, 60% yield) as a colorless liquid. 1H
NMR (400 MHz, CDCl3) δ 7.12 – 7.04 (m, 1H), 6.92 (d, J = 7.6 Hz, 1H),
6.81 (d, J = 7.6 Hz, 2H), 3.90 (d, J = 8.0 Hz, 1H), 2.44 (d, J = 8.0 Hz, 1H),
2.19 (s, 3H), 0.00 (s, 9H) ppm; 13C NMR (101 MHz, CDCl3) δ 138.8,
137.1, 128.9, 128.6, 128.1, 124.7, 113.3, 113.2, 37.7, 25.2, 21.5, -2.1
ppm; HRMSESI [M-H]- calculated for C14H17N2Si 241.1166, found
241.1161.
2-(2-Phenyl-1-(4-(trifluoromethyl)phenyl)ethyl)malononitrile (15)
2-((4-Methoxyphenyl)(trimethylsilyl)methyl)malononitrile (19)
Prepared according to the typical procedure, benzyltrimethylsilane (76 μL,
0.4 mmol) and 2-(4-(trifluoromethyl)benzylidene)malononitrile (44.4 mg,
0.2 mmol) were employed to give 15 (52.8 mg, 84% yield) as a yellow
liquid. 1H NMR (300 MHz, CDCl3) δ 7.70 (d, J = 8.2 Hz, 2H), 7.54 (d, J =
Prepared according to the typical procedure, hexamethyldisilane (81.7 μL,
0.4 mmol) and 2-(4-methoxybenzylidene)malononitrile (36.8 mg, 0.2
mmol) were employed to give 19 (26.3 mg, 51% yield) as a yellow semi-
1
solid. H NMR (300 MHz, CDCl3) δ 7.12 (d, J = 8.6 Hz, 2H), 6.89 (d, J =
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10.1002/ejoc.201900902
European Journal of Organic Chemistry
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8.6 Hz, 2H), 4.03 (d, J = 7.5 Hz, 1H), 3.81 (s, 3H), 2.58 (d, J = 7.6 Hz,
1H), 0.16 (s, 9H) ppm; 13C NMR (75 MHz, CDCl3) δ 158.8, 129.1, 128.9,
114.6, 113.3, 113.1, 55.2, 36.9, 25.5, -2.1 ppm; HRMSESI [M-H]-
calculated for C14H17N2OSi 257.1116, found 257.1116.
Prepared according to the typical procedure, hexamethyldisilane (81.7 μL,
0.4 mmol) and 2-(naphthalen-1-ylmethylene)malononitrile (40.8 mg, 0.2
mmol) were employed to give 25 (36.7 mg, 66% yield) as a yellow liquid.
1H NMR (400 MHz, CDCl3) δ 7.97 (d, J = 8.3 Hz, 1H), 7.90 (d, J = 7.5 Hz,
1H), 7.79 (d, J = 8.2 Hz, 1H), 7.62 – 7.45 (m, 3H), 7.38 (d, J = 7.2 Hz,
1H), 4.24 (d, J = 9.0 Hz, 1H), 3.73 (d, J = 9.0 Hz, 1H), 0.15 (s, 9H) ppm;
13C NMR (101 MHz, CDCl3) δ 134.2, 134.0, 131.5, 129.3, 127.8, 126.6,
126.1, 125.3, 123.9, 122.5, 113.4, 113.3, 30.8, 25.4, -1.9 ppm;
HRMSESI [M-H]- calculated for C17H17N2Si 277.1166, found 277.1166.
2-((4-Chlorophenyl)(trimethylsilyl)methyl)malononitrile (20)
Prepared according to the typical procedure, hexamethyldisilane (81.7 μL,
0.4 mmol) and 2-(4-chlorobenzylidene)malononitrile (37.7 mg, 0.2 mmol)
were employed to give 20 (37.2 mg, 71% yield) as a yellow liquid. 1H
NMR (500 MHz, CDCl3) δ 7.35 (d, J = 8.4 Hz, 2H), 7.14 (d, J = 8.4 Hz,
2H), 4.06 (d, J = 7.5 Hz, 1H), 2.63 (d, J = 7.5 Hz, 1H), 0.17 (s, 9H) ppm;
13C NMR (126 MHz, CDCl3) δ 135.6, 133.3, 129.4, 129.2, 113.0, 112.8,
37.3, 25.1, -2.2 ppm; HRMSESI [M-H]- calculated for C13H14ClN2Si
261.0620, found 261.0620..
2-(((2-Methoxyphenyl)dimethylsilyl)(phenyl)methyl)malononitrile (26)
Prepared according to the typical procedure, 1,2-bis(2-methoxyphenyl)-
1,1,2,2-tetramethyldisilane
(132
mg,
0.4
mmol)
and
benzylidenemalononitrile (31 mg, 0.2 mmol) were employed to give 26
(55 mg, 69% yield) as a yellow solid. 1H NMR (300 MHz, CDCl3) δ 7.55 –
7.45 (m, 1H), 7.40 – 7.31 (m, 4H), 7.25 – 7.17 (m, 2H), 7.05 (t, J = 7.3 Hz,
1H), 6.98 (d, J = 8.3 Hz, 1H), 4.18 (d, J = 7.1 Hz, 1H), 3.97 (s, 3H), 3.03
(d, J = 7.1 Hz, 1H), 0.45 (s, 3H), 0.37 (s, 3H) ppm; 13C NMR (75 MHz,
CDCl3) δ 163.8, 136.9, 136.2, 132.4, 128.9, 128.4, 127.2, 122.2, 121.4,
113.6, 113.5, 110.0, 55.2, 37.2, 25.5, -2.7, -3.9 ppm; HRMSESI [M-H]-
calculated for C19H19N2OSi 319.1272, found 319.1278.
2-((4-Cyanophenyl)(trimethylsilyl)methyl)malononitrile (21)
Prepared according to the typical procedure, hexamethyldisilane (81.7 μL,
0.4 mmol) and 2-(4-cyanobenzylidene)malononitrile (35.8 mg, 0.2 mmol)
were employed to give 21 (42 mg, 83% yield) as a yellow liquid. 1H NMR
(400 MHz, CDCl3) δ 7.68 (d, J = 8.4 Hz, 2H), 7.33 (d, J = 8.3 Hz, 2H),
4.13 (d, J = 7.5 Hz, 1H), 2.75 (d, J = 7.5 Hz, 1H), 0.18 (s, 9H) ppm; 13C
NMR (101 MHz, CDCl3) δ 142.8, 132.9, 128.5, 118.2, 112.8, 112.5,
111.4, 38.2, 24.5, -2.3 ppm; HRMSESI [M-H]- calculated for C14H14N3Si
252.0962, found 252.0967.
Typical procedure for synthesis of bioactive compound 27
To a 25 mL Schlenk tube equipped with a magnetic stir bar was added 1-
methylpiperazine (40 mg, 0.4 mmol), benzylation product 13 (49.2 mg,
0.2 mmol), K2CO3 (55.2 mg, 0.4 mmol), and CH3CN (2.0 mL). The
resulting mixture was stirred at room temperature with an oxygen balloon
until the compound 13 was completely consumed as monitored by TLC.
The solution was filtered, then concentrated and purified through column
chromatography on silica gel by gradient elution with hexane / ethyl
acetate / methanol to give the compound 27 (48 mg) as a colorless oil in
78% yield. A known compound and the data are in accordance with
those reported in the literature.[20] 1H NMR (500 MHz, CDCl3) δ 7.38 –
7.32 (m, 2H), 7.32 – 7.25 (m, 5H), 7.25 – 7.20 (m, 1H), 7.18 – 7.12 (m,
2H), 4.08 – 4.00 (m, 1H), 3.82 – 3.71 (m, 1H), 3.65 – 3.51 (m, 2H), 3.46 –
3.33 (m, 2H), 3.02 (dd, J = 13.5, 6.7 Hz, 1H), 2.39 – 2.30 (m, 1H), 2.31 –
2.23 (m, 1H), 2.21 (s, 3H), 2.10 – 2.02 (m, 1H), 1.88 – 1.80 (m, 1H) ppm.
2-((4-(Trifluoromethyl)phenyl)(trimethylsilyl)methyl)malononitrile (22)
Prepared according to the typical procedure, hexamethyldisilane (81.7 μL,
0.4 mmol) and 2-(4-(trifluoromethyl)benzylidene)malononitrile (44.4 mg,
0.2 mmol) were employed to give 22 (37.9 mg, 64% yield) as a colorless
liquid. 1H NMR (400 MHz, CDCl3) δ 7.64 (d, J = 8.2 Hz, 2H), 7.33 (d, J =
8.1 Hz, 2H), 4.11 (d, J = 7.5 Hz, 1H), 2.74 (d, J = 7.5 Hz, 1H), 0.18 (s,
9H) ppm; 13C NMR (100 MHz, CDCl3) δ 141.3, 129.7 (q, J = 32.7 Hz),
128.1, 126.2 (q, J = 3.7 Hz), 123.9 (q, J = 272.0 Hz), 112.9, 112.7, 37.9,
24.8, 25.5, -2.2 ppm; HRMSESI [M-H]- calculated for C14H14F3N2Si
295.0884, found 295.0889.
4-(2,2-Dicyano-1-(trimethylsilyl)ethyl)benzoic acid (23)
Acknowledgments
Prepared according to the typical procedure, hexamethyldisilane (81.7 μL,
0.4 mmol) and 4-(2,2-dicyanovinyl)benzoic acid (39.6 mg, 0.2 mmol)
1
were employed to give 23 (27 mg, 50% yield) as a white semi-solid. H
We are grateful for the financial support provided by National
Natural Science Foundation of China (Grant No. 21702142,
21871205), and the Scientific and Technological Innovation
Programs of Higher Education Institutions in Shanxi (Grant No.
201802024).
NMR (300 MHz, CDCl3) δ 8.12 (d, J = 8.3 Hz, 2H), 7.31 (d, J = 8.4 Hz,
2H), 4.14 (d, J = 7.8 Hz, 1H), 2.79 (d, J = 7.8 Hz, 1H), 0.19 (s, 9H) ppm;
13C NMR (75 MHz, CDCl3) δ 170.9, 143.5, 131.1, 128.4, 127.8, 112.9,
112.7, 38.2, 24.7, -2.2 ppm; HRMSESI [M-H]- calculated for
C14H15N2O2Si 271.0908, found 271.0919.
Keywords: radical cation • fragmentation • allylation •
benzylation • siylation
2-(Furan-2-yl(trimethylsilyl)methyl)malononitrile (24)
Prepared according to the typical procedure, hexamethyldisilane (81.7 μL,
0.4 mmol) and 2-(furan-2-ylmethylene)malononitrile (28.8 mg, 0.2 mmol)
were employed to give 24 (24.8 mg, 57% yield) as a yellow liquid. 1H
NMR (500 MHz, CDCl3) δ 7.22 (dd, J = 5.0, 0.8 Hz, 1H), 7.04 – 6.99 (m,
2H), 4.03 (d, J = 6.6 Hz, 1H), 2.96 (d, J = 6.6 Hz, 1H), 0.23 (s, 9H) ppm;
13C NMR (126 MHz, CDCl3) δ 138.3, 127.6, 126.1, 124.6, 112.8, 112.7,
32.9, 26.4, -2.2 ppm; HRMSESI [M-H]- calculated for C11H13N2OSi
217.0803, found 217.0808.
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Rongfang Liu, Shane Pui Mun Chia, Yi
Yiing Goh, Han Wen Cheo, Binbin Fan,
Ruifeng Li, Rong Zhou*, Jie Wu*
Page No. – Page No.
Visible-Light-Mediated Regioselective
Allylation, Benzylation, and Silylation
of Methylene-Malononitriles via
Photoredox-Induced Radical Cation
Fragmentation
Visible-light-mediated regioselevtive allylation, benzylation, and silylation of
methylene-malononitriles with silanes have been realized. The reactions proceed
via a photoredox-catalyzed σ-CSi+· or σ-SiSi+· type radical cation fragmentation.
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