Enantioselective protonation of radical anion intermediates in photoallylation and photoreduction reactions of 3,3-diaryl-1,1-dicyano-2-methylprop-1-ene with allyltrimethylsilane

Article abstract of DOI:10.3390/molecules24142677

Photoreactions of acetonitrile solutions of 3,3-diaryl-1,1-dicyano-2-methylprop-1-enes (1a–c) with allyltrimethylsilane (2) in the presence of phenanthrene as a photoredox catalyst and acetic acid as a proton source formed photoallylation (3) and photored

Full text of DOI:10.3390/molecules24142677

molecules
Article
Enantioselective Protonation of Radical Anion
Intermediates in Photoallylation and Photoreduction
Reactions of 3,3-Diaryl-1,1-dicyano-2-methylprop-1-ene
with Allyltrimethylsilane
Hajime Maeda *,† , Masayuki Iida, Daisuke Ogawa and Kazuhiko Mizuno *
Department of Applied Chemisty, College of Engineering, Osaka Prefecture University, 1-1 Gakuen-cho,
Naka-ku, Sakai, Osaka 599-8531, Japan
* Correspondence: maeda-h@se.kanazawa-u.ac.jp (H.M.); kmizunophotochem@gmail.com (K.M.);
Tel./Fax: +81-76-264-6290 (H.M.)
† Present address: Division of Material Chemistry, Graduate School of Natural Science and Technology,
Kanazawa University, Kanazawa, Ishikawa 920-1192, Japan.
ꢀꢁꢂꢀꢃꢄꢅꢆꢇ
Academic Editor: Yasuharu Yoshimi
Received: 25 June 2019; Accepted: 22 July 2019; Published: 23 July 2019
ꢀꢁꢂꢃꢄꢅꢆ
Abstract: Photoreactions of acetonitrile solutions of 3,3-diaryl-1,1-dicyano-2-methylprop-1-enes (1a
with allyltrimethylsilane ( ) in the presence of phenanthrene as a photoredox catalyst and acetic acid
as a proton source formed photoallylation ( ) and photoreduction ( ) products via photoinduced
electron transfer pathways. When (S)-mandelic acid was used as the proton source, the reactions
proceeded with 3.4 and 4.8 %ee for formation of and , respectively. The results of studies of the effect
–c)
2
3
4
3
4
of aryl ring substituents and several chiral carboxylic acids suggested that the enantioselectivities of
the reactions are governed by steric controlled proton transfer in intermediate complexes formed by
π-π and OH-π interactions of anion radicals derived from 1a–c and chiral carboxylic acids.
Keywords: photoreaction; photoinduced electron transfer; photoredox catalyst; Felkin-Anh model;
radical anion; electron deficient alkene; allylsilane; mandelic acid; enantioselectivity; enantiomer
1. Introduction
Coupling reactions proceeding through photoinduced electron transfer (PET) pathways have
been extensively studied from both a synthetic as well as a mechanistic viewpoint [
radical ions that serve as intermediates in these processes are short-lived and highly reactive, control
of the stereochemistry of these reactions is often difficult [15 21]. We have previously developed
photoallylation and photoreduction reactions of electron deficient alkenes with allyltrimethylsilane
that occur via PET pathways [22 24]. In addition, we also demonstrated that diastereoselectivity of
this process can be achieved by steric control of allyl radical or proton addition to radical anions that
are generated from electron deficient alkenes (Scheme 1) [25 27]. The current study was aimed at the
1–14]. Because
–
–
–
development of enantioselective PET promoted coupling reactions, and specifically, at assessing the
effect of chiral carboxylic acids on the stereochemical outcomes of photoallylation and photoreduction
reactions of prochiral electron deficient alkenes. The results showed that these processes took place
with maximum 3.4–4.8 %ee when (S)-mandelic acid was used as the chiral proton source.
Molecules 2019, 24, 2677; doi:10.3390/molecules24142677
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Scheme 1. Our previous works.
2. Results and Discussion
Irradiation of an acetonitrile solution containing 1,1-dicyano-2-methyl-3,3-diphenylprop-1-ene
1a), 3 equiv of allyltrimethylsilane ( ), a catalytic amount of phenanthrene (Phen) as a photoredox
catalyst in a Pyrex vessel using a 300 W high-pressure mercury lamp was found to produce reduction
product 4a in 53% yield (Scheme 2, Table 1, entry 1, supplementary). Photoreaction of 1a with in
the presence of acetic acid produced the allylated product 3a in addition to 4a in 34 and 31% yields,
respectively (entry 2). The corresponding products 3b and 4b were produced in photoreactions
(
2
2
–
c
–c
of bis(p-methoxyphenyl) (1b) and bis(p-chlorophenyl) (1c) derivatives conducted under the same
conditions (entries 3–6). The irradiation times used for these processes are those required for complete
consumption of 1a
–c. The observed efficiencies of the reactions, based on the required irradiation
times, decreased in the order 1c > 1a > 1b.
Scheme 2. Photoallylation and photoreduction of 1a–c by using allyltrimethylsilane (2).
Table 1. Photoallylation and photoreduction of 1a–c by using allyltrimethylsilane (2) a.
Yields/%
Irradiation
Time/h
Entry
Substrate
Additive
3
4
0 b
34 b
0 d
53 b
31 b
44 d
46 d
36 d
47 d
1
2
3
4
5
6
1a (Ar = Ph)
1a (Ar = Ph)
none
acetic acid c
none
4
4
24
24
2
1b (Ar = p-MeOC6H4)
1b (Ar = p-MeOC6H4) acetic acid c
20 d
0 d
1c (p-ClC6H4)
1c (p-ClC6H4)
none
acetic acid c
2
33 d
a
Conditions: 1a
–c (0.14 mmol), 2 (0.42 mmol), phenanthrene (0.07 mmol), CH3CN (8 mL), 300 W high-pressure
mercury lamp, Pyrex, r.t. Determined by using GC. c 1 mL. d Determined by using 1H-NMR.
b
Structures of photoproducts 3a–c and 4a–c were determined by using spectroscopic methods.
In 1H-NMR spectra of CDCl3 solutions of 3a and 4a (Figure 1), the chemical shifts of resonances for
protons that are bonded to the asymmetric carbons, i.e., Hb in 3a and Hh in 4a, were 2.97 (qd) and
3.02 (qt) ppm, respectively. Authentic samples of the photoproducts were prepared by hydrogenation
of 1a using Pd/C to form 4a and ensuing allylation of 4a using allyl chloride to form 3a (Scheme 3).
The spectral data for the synthesized compounds were identical to those of photoproduced 3a and 4a
.

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Figure 1. 300 MHz 1H-NMR spectra of 3a and 4a in CDCl3.
Scheme 3. Synthesis of authentic samples of 3a and 4a.
In order to explore the enantioselectivities of these photoreactions, samples of 3a and 4a were
subjected to HPLC using a chiral stationary phase with the effluents being monitored by using UV and
CD detectors (Figure 2a–d). The results showed that two peaks in the HPLC trace for the enantiomers of
3a and 4a were completely resolved. Unfortunately, HPLC conditions could not be found for resolution
of the enantiomers of 3b and 4b. Moreover, the enantiomers of 3c and 4c can be separated by using GC
with a chiral capillary column (Figure 2e).
Figure 2. Resolution of the enantiomers of (
both at 270 nm, ( 4a by chiral HPLC with UV and CD detectors both at 270 nm, and (
chiral GC with a MS detector.
a
,
b
)
3a by using chiral HPLC with UV and CD detectors
c,
d)
e)
3c and 4c by

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In order to prove that these separation techniques led to the individual enantiomers, the effluents
of peaks A–D in Figure 2a–d were collected, concentrated in vacuo and the residues in ethanol were
subjected to UV-vis absorption and CD spectroscopic analysis (Figures 3 and 4). The UV-vis absorption
spectra of substances in effluents corresponding to peaks A and C were identical to those from peaks
B and D, respectively. In addition, 1H-NMR and mass spectra of the respective substances in peaks
A and C were also identical to those in peaks B and D, respectively. Moreover, CD spectral traces of
substances comprising peaks A and B, and peaks C and D, respectively, were mirror images relative to
the horizontal base line. The combined results indicated that the enantiomers of these substances can
be resolved by using chromatographic methods.
Figure 3. UV-vis absorption spectra of ethanol solutions of substances from (a) peak A, (b) peak B, (c)
peak C, and (d) peak D.
Figure 4. Circular dichromism (CD) spectra of ethanol solutions of substances from (
(b) peaks C and D.
a) peaks A and B,
To assess the potential of introducing enantioselectivity into the photoreactions described above,
irradiations were carried out on solutions of 1a and allyltrimethylsilane ( ) containing chiral
–
c
2
carboxylic acids. The yields and percent enantiomeric excesses (%ee) of products formed in these
processes are listed in Table 2. The %ee in each case was calculated using the ratio of areas under
the chiral HPLC or GC peaks corresponding to the enantiomers as %ee when acetic acid was used
becoming zero. A positive %ee value corresponds to a situation in which the major isomer is the second
peak, while a negative value shows that the major isomer is the first peak. The absolute structures
could not be decided. The data arising from photoreactions in the absence or presence of achiral acetic
acid are also included in Table 2 for comparison purposes.
Use of 1 equiv of (R)-mandelic acid in photoreaction of 1a with
4a with respective +1.5 and +4.1 %ee values (entry 3). A reversal in major enantiomers of the
products arose from the reaction of 1a with conducted in the presence of (S)-mandelic acid (entry
2 led to formation of 3a and
2
4), which supports the reaction proceeding in an enantioselective manner. Also, when l-lactic acid
was used in this photoreaction, the major enantiomers were the reverse of those formed in reactions
in the presence of (S)-mandelic acid (entry 5). The use of C2 symmetric dibenzoyl l-tartaric acid
did not promote an increase of %ee of either product (entry 6). The photoreaction of 1b with 2 also

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occurred when (R)- and (S)-mandelic acids were used, however the %ee of either product could
not be determined (entries 9 and 10). Like in the case of 1a, photoreaction of 1c produced products
3c and 4c in which the major enantiomers were reversed when (R)- and (S)-mandelic acids were
utilized (entries 13 and 14). Moreover, the results showed that the %ee improved up to 3.5 when
(S)-2-(6-methoxy-2-naphthyl)propionic acid was used as a chiral acid (entry 15).
Table 2. Enantioselective photoallylations and photoreductions of 1a–c a.
Yields/% (ee/%)
Irradiation
Time/h
Entry
Substrate
Additive
3
4
0 b
34 b
53 b
31 b
1
2
1a (Ar = Ph)
1a (Ar = Ph)
none
4
4
acetic acidc
34 b (+1.5 d,e
)
)
27 b (+4.1 d,e
26 b (−4.8 d,f
39 b (+3.2 d,e
16 b (−3.6 d,f
)
)
)
)
3
4
5
6
1a (Ar = Ph)
1a (Ar = Ph)
1a (Ar = Ph)
1a (Ar = Ph)
(R)-mandelic acid
(S)-mandelic acid
l-lactic acid
4
4
4
4
22 b (−3.4 d,f
28b (+0.6 d,e
)
)
31 b (−2.0 d,f
dibenzoyl l-tartaric acid
7
8
1b (Ar = p-MeOC6H4)
1b (Ar = p-MeOC6H4)
none
24
24
0 g
20 g
44 g
46 g
acetic acid c
54 g (nd h
)
)
28g (nd h
)
)
9
1b (Ar = p-MeOC6H4)
1b (Ar = p-MeOC6H4)
(R)-mandelic acid
(S)-mandelic acid
24
24
13 g (nd h
48g (nd h
10
11
12
1c (p-ClC6H4)
1c (p-ClC6H4)
none
2
2
0 g
33 g
36 g
47 g
acetic acid c
27 g (+2.0 i,j
24 g (−2.6 i,k
0 g
)
55 g (+2.0 i,j
)
13
14
1c (p-ClC6H4)
1c (p-ClC6H4)
1c (p-ClC6H4)
(R)-mandelic acid
(S)-mandelic acid
2
2
2
)
61 g (−0.6 i,k
)
(S)-2-(6-methoxy-2-
naphthyl)propionic acid
68g (−3.5 i,k
)
15
a
Conditions: 1a–c (0.14 mmol),
2
(0.42 mmol), phenanthrene (0.07 mmol), CH3CN (8 mL), additive (0.14 mmol), 300
W high-pressure mercury lamp, Pyrex, r.t. Determined by using GC. c 1 mL. d Determined by using chiral HPLC.
b
e
f
Major isomer corresponds to the second peak in the HPLC chart. Major isomer corresponds to the first peak in
the HPLC chart. Determined by using 1H-NMR. h Ee could not be determined. Determined by using chiral GC.
g
i
j
k
Major isomer corresponds to the second peak in the GC chart. Major isomer corresponds to the first peak in the
GC chart.
Each photoreaction described above takes place through a process termed photoredox sensitization
by phenanthrene (Phen) (Scheme 4) [22 27]. In the pathway, the excited singlet state of Phen, generated
–
by light absorption, transfers one electron (SET (single electron transfer)) to the electron-deficient alkene
1 to form the phenanthrene radical cation (Phen•+) and the alkene radical anion 1•−. The subsequent
SET from allyltrimethylsilane (
which undergoes nucleophile-assisted Si-C bond cleavage [28
anion 1•− is protonated by the carboxylic acid to produce radical
radical generates the allylation product . In a competitive pathway, radical
abstraction or one-electron reduction followed by protonation or disproportionation to form reduction
product 23]. The inefficiency of the photoreaction of the MeO-substituted substrate 1b and high
2 ,
) to Phen•+ generates recovered Phen and the radical cation 2•+
–
30] to form the allyl radical. Also, radical
, which upon coupling with the allyl
undergoes hydrogen
5
3
5
4
[
efficiency of the reaction of Cl-substituted reactant 1c are likely consequences of the stabilities of the
corresponding radical anions 1b•– and 1c•– which governs their rates of formation by SET from relative
to unproductive decay of the excited singlet state of Phen.

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Based on the results of molecular orbital calculations with related compounds, it is estimated
that the radical contribution to radical anion 1•– is large at the dicyano substituted carbon (
) and
that negative charge density is large at the dialkyl substituted carbon ( ) [23 24 26 27]. In accord with
this conclusion, the photoreaction of 1a with using CH3COOD as the additive produced mainly
α
β
,
,
,
2
mono-deuteriated forms of 3a and 4a in which deuterium is present at the stereogenic carbons marked
with * in Scheme 4. Therefore, enantioselectivity is governed at the step where protonation of the
radical anion takes place.
Scheme 4. Mechanism for photoallylation and photoreduction of 1.
The stereochemistry of protonation of the radical anion 1•– can be discussed using a Felkin-Anh
model (Scheme 5) [31
–
33]. Specifically, in reaction of 1a in the presence of (S)-mandelic acid, proton
transfer to the Re face of 1a•– should be preferred in a complex in which a
π-π
stabilizing interaction
occurs between the phenyl groups and the OH group of the acid is located in a sterically less hindered
position. Proton transfer to the Re face of 1a leads to the eventual formation of (S)-3a and (S)-4a. On the
other hand, in the reaction of 1a in the presence of l-lactic acid, an OH-π interaction between the
OH group of the acid and the phenyl group of 1a•– takes place to form a complex in which proton
transfer from the carboxylic acid group occurs preferentially to the Si face to minimize steric repulsion
of methyl group. This process then gives rise to formation of (R)-3a and (R)-4a. In photoreaction of 1c
in the presence of (S)-2-(6-methoxy-2-naphthyl)propionic acid, the main enantiomers produced were
the same as those generated in reaction of 1c in the presence of (S)-mandelic acid, and %ee increased.
This outcome might be a consequence of a strong
methoxynaphthyl groups.
π-π interaction between the chlorophenyl and the

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Scheme 5. Plausible explanation for the enantioselectivity.
3. Experimental Section
3.1. Materials and Equipments
THF was distilled from CaH2 and then from Na/benzophenone. CH3CN was distilled from
P2O5 then from Ca(OH)2. Hexane and 2-propanol were distilled without using a drying agent.
Allyltrimethylsilane (2) was prepared using a reported procedure [27]. Activated alumina was dried at
200 ◦C for 2 h before use. Most other chemical substances were used after purification by distillation
or recrystallization.
Column chromatography was conducted by using Wakogel C-70~230 (Fujifilm Wako Pure
Chemical Corporation, Osaka, Japan). Thin-layer chromatography was performed by using Merck
Kiesel gel 60 F254 plates (Merck KGaA, Darmstadt, Germany). HPLC separations (achiral) were
performed on a recycling preparative HPLC equipped with Jasco PU-987 pump, UV-970 UV detector,
and a Chemcosorb I-5Si column (Chemco Plus Scientific Co., Ltd., Osaka, Japan) using hexane-AcOEt
or hexane-2-propanol as an eluent, or a recycling preparative HPLC equipped with Jasco PU-2086
pump, RI-2031 differential refractometer (Jasco Corporation, Tokyo, Japan), and Megapak GEL 201F
columns (GPC) using CHCl3 as an eluent (Jasco Corporation, Tokyo, Japan).
1H and 13C-NMR spectra were recorded using a Varian MERCURY-300 (300 MHz and 75 MHz,
respectively, (Varian Inc., Palo Alto, CA, USA) spectrometer with Me4Si as an internal standard. Mass
spectra (EI, achiral) were recorded on a SHIMADZU GCMS-QP5050 (Shimadzu Corporation, Kyoto,
Japan) operating in the electron impact mode (70 eV) equipped with GC-17A and DB-5MS column
(J&W Scientific Inc., Serial: 8696181, Folsom, CA, USA). UV-vis spectra were recorded using a Jasco
V-530 spectrophotometer (Jasco Corporation, Tokyo, Japan).
3.2. Preparation of 1a
A mixture of 1,1-diphenylacetone (1.051 g, 5.0 mmol), malononitrile (0.330 g, 5.0 mmol) and
◦
activated alumina (1.5 g) was stirred at 60 C for 1 h [34]. The solids were removed by filtration.
Concentration of the filtrate gave a residue that was subjected to silica gel column chromatography
followed by recrystallization from hexane to give 2-(1,1-diphenylpropan-2-ylidene)malononitrile (1a,
white solid, 0.531 g, 2.06 mmol, 41% yield). Lit [35].

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3.3. Preparation of 1b
A THF (50 mL) solution of 4-bromoanisole (17.53 mL, 140.0 mmol) was added dropwise to stirred
Mg turnings (3.889 g, 160.0 mmol). A small amount of I2 was added to facilitate the reaction. A THF
(20 mL) solution of ethyl l-lactate (4.587 mL, 40.0 mmol) was added dropwise to the solution, and the
resulting mixture was stirred at reflux, cooled, and extracted with Et2O and NH4Cl aq [36]. The organic
layer was dried over Na2SO4, filtered, and concentrated in vacuo, giving a residue that was subjected to
silica gel column chromatography to give 1,1-bis(4-methoxyphenyl)propane-1,2-diol (5.76 g, 20.0 mmol,
50% yield, including inpurity).
25% H2SO4 aq (15 mL) was added to stirred 1,1-bis(4-methoxyphenyl)propane-1,2-diol (5.76 g,
20.0 mmol, including inpurity), and the resulting solution was stirred at reflux for 3.5 h, cooled,
neutralized with Na2CO3 and extracted with Et2O [36]. The organic layer was dried over Na2SO4,
filtered, and concentrated in vacuo, giving a residue that was subjected to silica gel column
chromatography to give 1,1-bis(4-methoxyphenyl)propan-2-one (1.047 g, 3.87 mmol, 19% yield).
A mixture of 1,1-bis(4-methoxyphenyl)propan-2-one (1.047 g, 3.87 mmol), malononitrile (0.384 g,
◦
5.82 mmol) and activated alumina (3.0 g) was stirred at 90 C for 1.5 h [34]. The solids were
removed by filtration. Concentration of the filtrate gave a residue that was subjected to HPLC to
give 2-[1,1-bis(4-methoxyphenyl)propan-2-ylidene]malononitrile (1b, 0.728 g, 2.29 mmol, 59% yield).
1H-NMR (300 MHz, CDCl3)
δ 2.21 (s, 3H), 3.82 (s, 6H), 5.59 (s, 1H), 6.89 (d, J = 8.6 Hz, 4H), 7.05 (d,
J = 8.6 Hz, 4H) ppm.
3.4. Preparation of 1c
A THF (12 mL) solution of 4-bromochlorobenzene (6.647 g, 34.7 mmol) was added dropwise to
stirred Mg turnings (0.729 g, 30.0 mmol). A small amount of I2 was added to facilitate the reaction.
A THF (5 mL) solution of ethyl l-lactate (1.247 mL, 10.9 mmol) was added dropwise to the solution,
and the resulting solution was stirred at reflux, cooled, and extracted with Et2O and NH4Cl aq [36].
The organic layer was dried over Na2SO4, filtered, and concentrated in vacuo, giving a residue that
was subjected to silica gel column chromatography to give 1,1-bis(4-chlorophenyl)propane-1,2-diol
(4.023 g, including inpurity).
To stirred 1,1-bis(4-chlorophenyl)propane-1,2-diol (4.023 g, including inpurity) was added 25%
H2SO4 aq (12 mL), and the resulting solution was stirred at reflux for 3.5 h, cooled, neutralized
with Na2CO3 and extracted with Et2O [36]. The organic layer was dried over Na2SO4, filtered,
and concentrated in vacuo, giving a residue that was subjected to silica gel column chromatography
1
to give 1,1-bis(4-chlorophenyl)propan-2-one (0.934 g, 3.36 mmol, 31% yield (two steps)). H-NMR
(300 MHz, CDCl3) δ 2.24 (s, 3H), 5.05 (s, 1H), 7.11-7.32 (m, 8 H) ppm.
A mixture of 1,1-bis(4-chlorophenyl)propan-2-one (0.934 g, 3.36 mmol), malononitrile (0.444 g,
◦
6.72 mmol) and activated alumina (3.0 g) was stirred at 90 C for 1.5 h [34]. The solids were
removed by filtration. Concentration of the filtrate gave a residue that was subjected to HPLC to
give 2-[1,1-bis(4-chlorophenyl)propan-2-ylidene]malononitrile (1c, 0.602 g, 1.84 mmol, 55% yield).
1H-NMR (300 MHz, CDCl3)
δ 2.21 (s, 3H), 5.62 (s, 1H), 7.05 (d, J = 8.4 Hz, 4H), 7.37 (d, J = 8.4 Hz, 4H)
ppm; 13C-NMR (75 MHz, CDCl3)
δ 21.33, 55.87, 88.80, 111.37, 111.49, 129.47, 129.94, 134.47, 135.75,
179.56 ppm; MS (EI) m/z (relative intensity, %) = 114 (52), 139 (89), 165 (59), 291 (100), 326 (62, M+).
3.5. General Procedure for Photoreactions
CH3CN (8 mL) solutions of a 3,3-diaryl-1,1-dicyano-2-methylprop-2-ene (1a
–c, 0.14 mmol),
allyltrimethylsilane ( , 0.42 mmol), phenanthrene (0.07 mmol), and CH3COOH (1 mL) or a chiral
2
carboxylic acid (0.14 mmol) in Pyrex vessels were degassed by argon bubbling for 5 min and then
the vessels were sealed. The solutions were irradiated by using a 300 W high pressure mercury lamp
(Eikosha, EHB-W-300 or PIH-300) for 2–24 h at room temperature, maintained by using circulated
cooling water. The photolysates were extracted with Et2O. The organic layer was washed with H2O,

Molecules 2019, 24, 2677
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dried over Na2SO4, filtered, and concentrated in vacuo, giving a residue that was subjected to HPLC
to give 3a–c and 4a–c.
2-Allyl-2-(1,1-diphenylpropan-2-yl)malononitrile (3a): 1H-NMR (300 MHz, CDCl3)
δ 1.20 (d,
J = 6.8 Hz, 3H), 2.42–2.59 (m, 2H), 2.91–3.02 (m, 1H), 4.16 (d, J = 9.3 Hz, 1H), 5.28 (d, J = 17.6 Hz, 1H),
5.37 (d, J = 9.6 Hz, 1H), 5.76-5.92 (m, 1H), 7.21–7.40 (m, 10H) ppm; MS (EI) m/z = 41, 65, 77, 91, 102, 115,
128, 151, 165, 167, 193, 300 (M+).
1
2-Allyl-2-[1,1-bis(4-methoxyphenyl)propan-2-yl]malononitrile (3b): H-NMR (300 MHz, CDCl3)
δ
1.18 (d, J = 6.7 Hz, 3H), 2.44-2.60 (m, 2H), 2.80–2.91 (m, 1H), 3.76 (s, 3H), 3.78 (s, 3H), 4.08 (d, J = 9.2 Hz,
1H), 5.29 (d, J = 16.9 Hz, 1H), 5.36 (d, J = 10.0 Hz, 1H), 5.77–5.91 (m, 1H), 6.81–6.87 (m, 4H), 7.19–7.26
(m, 4H) ppm.
1
2-Allyl-2-[1,1-bis(4-chlorophenyl)propan-2-yl]malononitrile (3c): H-NMR (300 MHz, CDCl3)
δ
1.18 (d, J = 6.7 Hz, 3H), 2.52 (dd, J = 13.8, 7.5 Hz, 1H), 2.62 (dd, J = 13.9, 6.7 Hz, 1H), 2.88 (dq, J = 8.9,
6.7 Hz, 1H), 4.15 (d, J = 9.1 Hz, 1H), 5.31 (d, J = 16.9 Hz, 1H), 5.40 (d, J = 10.2 Hz, 1H), 5.76–5.91 (m, 1H),
7.19–7.36 (m, 8H) ppm; 13C-NMR (75 MHz, CDCl3)
δ 15.47, 41.40, 42.40, 42.79, 54.31, 114.18, 123.29,
128.44, 129.24, 129.27, 129.67, 129.93, 133.41, 133.76, 139.09, 139.24 ppm.
1
2-(1,1-Diphenylpropan-2-yl)malononitrile (4a): H-NMR (300 MHz, CDCl3)
δ
1.30 (d, J = 6.6 Hz,
3H), 2.95–3.06 (m, 1H), 3.64 (d, J = 3.3 Hz, 1H), 3.80 (d, J = 11.7 Hz, 1H), 7.21–7.36 (m, 10H) ppm; MS
(EI) m/z = 51, 63, 77, 83, 102, 128, 151, 165, 167, 193, 300 (M+).
1
2-[1,1-Bis(4-methoxyphenyl)propan-2-yl]malononitrile (4b): H-NMR (300 MHz, CDCl3)
δ
1.28
(d, J = 6.6 Hz, 3H), 2.82–2.95 (m, 1H), 3.66 (d, J = 3.3 Hz, 1H), 3.69 (d, J = 11.7 Hz, 1H), 3.77 (s, 6H),
6.83–6.89 (m, 4H), 7.18–7.24 (m, 4H) ppm.
2-[1,1-Bis(4-chlorophenyl)propan-2-yl]malononitrile (4c): 1H-NMR (300 MHz, CDCl3)
δ 1.29
(d, J = 6.6 Hz, 3H), 2.87-2.98 (m, 1H), 3.60 (d, J = 3.4 Hz, 1H), 3.78 (d, J = 11.8 Hz, 1H), 7.18–7.37
(m, 8H) ppm.
3.6. Resolution of Enantiomers
Resolutions of enantiomers of 3a and 4a were performed on a recycling preparative HPLC equipped
with Jasco PU-980 pump, Jasco UV-970 and CD-2095 detectors (Jasco Corporation, Tokyo, Japan),
Daicel CHIRALCEL OJ (3a) or OJ-H (4a) columns (Daicel Corporation, Osaka, Japan). Eluents were
hexane:2-propanol = 7:3 (3a) or 9:1 (4a). [3a] = 0.068 M, [4a] = 0.078 M. Both 3a and 4a were detected
by UV and CD detectors at 270 nm.
Resolutions of enantiomers of 3c and 4c were performed by using a SHIMADZU GCMS-QP5050
(Shimadzu Corporation, Kyoto, Japan) operating in the electron impact mode (70 eV) equipped with
SUPELCO GAMMA DEXTM 225 colu◦mn (Sigma-Aldrich Co., LLC, St. Louis, MO, USA). Detector
◦
temp = 215 C, injection temp = 220 C, inlet pressure = 93.7 kPa, flow rate = 1.0mL/min, linear
velocity = 28.1 cm/s, split ratio = 50, carrier gas = N2.
4. Conclusions
In summary, we found that photoreactions of prochiral 3,3-diaryl-1,1-dicyano-2-methylprop-1-enes
1a–c with allyltrimethylsilane, carried in the presence of enantiomerically pure chiral carboxylic acids,
generates photoallylation and photoreduction products with low but finite levels of enantioselectivity.
The percent enantiomeric excesses in the products of the process was highest (4.8 %ee) when
(S)-mandelic acid was used. Enantioselectivities in these reactions are a consequence of sterically
governed asymmetric proton transfer in intermediate complexes formed by π-π and OH-π interactions
between radical anions of the prochiral alkenes and the chiral carboxylic acids.
1
Supplementary Materials: The following are available online: H-NMR spectra of 1b
13C-NMR spectra of 1c and 3c.
, 1c, 3a, 3b, 3c, 4a, and 4c,
Author Contributions: Project administration, H.M.; investigation, M.I. and D.O.; supervision, K.M.

Molecules 2019, 24, 2677
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Funding: This study was supported by Grant-in-Aids for Scientific Research on Priority Areas “Reaction Control
of Dynamic Complexes (420)” (16033252), Scientific Research (B) (15350026), Scientific Research (C) (17K05777),
and Young Scientists (B) (16750039), and the Cooperation for Innovative Technology and Advanced Research in
Evolutional Area (CITY AREA) program from the Ministry of Education, Culture, Sports, Science and Technology
(MEXT) of Japan.
Acknowledgments: We thank Masahito Oka at Osaka Prefecture University for permission to use circular
dichromism (CD) spectrometer.
Conflicts of Interest: The authors declare no conflict of interest.
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Sample Availability: Samples of the compounds are not available from the authors.
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