Donor-Acceptor π-Conjugated Enamines: Functional Group-Compatible Synthesis from Amides and Their Photoabsorption and Photoluminescence Properties

Article abstract of DOI:10.1021/acs.joc.9b02267

High functional group compatibility of iridium-catalyzed synthesis of enamines from amides and 1,1,3,3-tetramethyldisiloxane (TMDS) realized facile access of a series of donor (D)-π-acceptor (A)-conjugated enamines, in which enamine behaves as a donor functional group. The amide precursors containing reducible functional groups, such as halogen, carbonyl, and nitro groups, underwent reaction with TMDS to give the corresponding enamines in high yields. In most cases, chemoselective hydrosilane reduction of the amide group occurred while other reducible groups remained intact. Absorption and emission properties including solvatochromic behavior for the resulting D-π-A-conjugated enamines were determined using UV-visible and fluorescent spectra, which provided an understanding of the donor properties of the CH=CHNPh₂ group and photofunctional properties of the D-π-A conjugated enamines as a fluorescent dye. Maximum absorption wavelength (λ_abs) of p-ZC₆H₄CH=CHNPh₂ was predictable from λ_abs of p-ZC₆H₄NPh₂, which was supported by density functional theory calculations. Some of the D-π-A-conjugated enamines showed fluorescence with moderate fluorescence quantum yields (φ_fl). Of interest are unusually emissive π-conjugated enamines containing a nitro group, which generally behaves as strong quenchers of fluorescence. The additive effect of B(C₆F₅)₃ resulted in significant red shifts of λ_abs and λ_fl. In some cases, high φ_fl was observed in the solution state.

Full text of DOI:10.1021/acs.joc.9b02267

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Article
Donor-Acceptor #-Conjugated Enamines: Functional
Group Compatible Synthesis from Amides and Their
Photoabsorption and Photoluminescence Properties
Atsushi Tahara, Ikumi Kitahara, Daichi Sakata, Yoichiro Kuninobu, and Hideo Nagashima
J. Org. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.joc.9b02267 • Publication Date (Web): 28 Oct 2019
Downloaded from pubs.acs.org on October 28, 2019
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(
1) Title
Donor-Acceptor -Conjugated Enamines: Functional
Group Compatible Synthesis from Amides and Their
Photoabsorption and Photoluminescence Properties
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(2) Authors’ names and addresses
Atsushi Tahara,† Ikumi Kitahara,‡ Daichi Sakata,‡ Yoichiro Kuninobu,†,‡ Hideo
†
,‡,
Nagashima. *
†
‡
Institute for Materials Chemistry and Engineering, and Interdisciplinary Graduate School
of Engineering Sciences, Kyushu University, Kasuga, Fukuoka 816-8580, Japan.
(3) Corresponding author’s e-mail address
*
E-mail: nagasima@cm.kyushu-u.ac.jp
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(
4) Table of Contents/Abstract Graphic
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(5) Abstract
High functional group compatibility of iridium-catalyzed synthesis of enamines from
amides and 1,1,3,3-tetramethyldisiloxane (TMDS) realized facile access of a series of Donor
D)--Acceptor (A) conjugated enamines, in which enamine behaves as a donor functional
(
group. The amide precursors containing reducible functional groups, such as halogen,
carbonyl, and nitro groups, underwent reaction with TMDS to give the corresponding
enamines in high yields. In most cases, chemoselective hydrosilane reduction of the amide
group occurred while other reducible groups remained intact. Absorption and emission
properties including solvatochromic behavior for the resulting D--A conjugated enamines
were determined using UV-visible and fluorescent spectra, which provided an understanding
of the donor properties of the CH=CHNPh group and photofunctional properties of the D-
2

-A conjugated enamines as a fluorescent dye. Maximum absorption wavelength ( ) of p-
abs
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ZC H CH=CHNPh was predictable from  of p-ZC H NPh , which was supported by
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abs
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DFT calculations. Some of the D--A conjugated enamines showed fluorescence with
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moderate fluorescence quantum yields ( ). Of interest are unusually emissive -conjugated
fl
enamines containing a nitro group, which generally behaves as strong quenchers of
fluorescence. Additive effect of B(C F ) resulted in significant red-shifts of  and  . In
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some cases, high  was observed in the solution state.
fl
(6) Introduction
Enamines have been recognized as a unique synthetic equivalent to enolates since the
1
first report by Stork and coworkers. While condensation of ketones or aldehydes with
secondary amines has been used to produce enamines,1a,1e recent progress in transition metal-
2
catalyzed reactions has provided several other alternative synthetic routes. Among them,
iridium-catalyzed reaction of tertiary amides with 1,1,3,3-tetramethyldisiloxane (TMDS) is
a unique method for preparation of aldenamines with extremely high catalytic efficiency and
3
excellent functional group compatibility. The reaction involves two elementary steps,
hydrosilylation of the C=O group in amides and subsequent elimination of a silanol from the
resulting silylhemiaminal. In most cases, these two elementary reactions occur spontaneously.
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Amides are less likely than other carbonyl groups, such as ketones and esters, to undergo
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hydride reduction. As a consequence, selective reduction of amide groups is difficult when
these reducible functional groups exist in the same molecule.4,5e,6 The iridium-catalyzed
reaction of tertiary amides with TMDS is of particular interest because the reaction is tolerant
of ketone and ester groups to prepare easily the corresponding functionalized aldenamines
from ketoamides and amide esters.3a,3c
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A recent topic in enamine chemistry, which originated from materials science, is the
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excellent donor properties of -conjugated enamines. Since the report by Borsenberger and
coworkers in 1997, hole-transport properties of several aldenamines conjugated to
delocalized aromatic rings have been investigated.7a A recent contribution is facile
preparation of -conjugated aldenamines from N,N-diaryl--arylacetamide derivatives,
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b
which was achieved by the judicious choice of iridium catalysts. Compared with N,N-
dialkylamides, N,N-diarylamides possess lower reactivity toward reaction with TMDS, and
are more difficult to convert into the corresponding enamines when the catalyst
IrCl(CO)(PPh ) (I) was used. Improved catalysts, such as IrCl(CO)[P(OC F ) ] (II),
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6 5 3 2
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IrCl(PPh ) , and a species generated in situ from [IrCl( -1,5-cyclooctadiene)] and
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phosphorous ligands, actually provided a method for the synthesis of a series of -conjugated
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enamines.3b,3c
These results prompted further research. Donor-acceptor -conjugated dyes (D--A
molecules), in which electron-donating (D) and electron-accepting (A) functional groups are
linked by -conjugated systems, have attracted considerable attention, especially in relation
to biocompatible fluorescent dye labels, sensitizers of organic solar cells, components of
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organic light-emitting diodes, and other applications. Since -conjugated enamines act as an
excellent electron donor in organic electronics, they were of interest because of their ability
to act as donor functional groups in D--A molecules, which has not yet been explored. The
present approach described here involves the preparation of a series of -conjugated
enamines containing various acceptor functional groups, along with studies on their UV-
visible absorption and emission. A problem to overcome during synthesis was the selective
hydrosilylation/silanol elimination sequence during the iridium-catalyzed reaction of N,N-
diaryl--arylacetamides with TMDS, which produces the -conjugated enamine donor
portion with the acceptor functional groups remaining intact. Enamine formation from
amides essentially involves hydride reduction of a C=O group, while many acceptor
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-6
functional groups are susceptible to hydride reduction. However, the selective reaction was
accomplished by a judicious choice of the iridium catalyst. The donor properties of -
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conjugated enamines were determined by examination of the UV-visible spectra of the D--
A conjugated enamines associated with TD-DFT calculations, and by evaluating the D--A
conjugated enamines as a fluorescent dye in the fluorescence spectra.
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(7) Results and Discussion
Preparation of D--A conjugated enamines. The representative D--A conjugated
enamines were p-ZC H CH=CHNPh (4), and a series of enamine products with different
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electron-accepting Z groups were synthesized by iridium-catalyzed reaction of the
corresponding tertiary amides, p-ZC H CH CONPh (2), with TMDS. A general reaction
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procedure involved use of IrCl(CO)(PPh ) (1X, 0.05–0.25 mol%) and TMDS (2 equiv. to 2)
3
2
in toluene at room temperature for 2 to 24 h. Reaction proceeded through formation of
silylhemiaminal intermediates (3). In some cases, 3 was a major product of this reaction, and
conversion of 3 to 4 was accomplished with ease by heating at 60 °C. As described previously,
the reaction of N,N-dialkylacetamides with TMDS was complete within 1 h with catalyst
3
a
loadings as low as 0.001 mol%. In contrast, reaction of N,N-diaryl--arylacetamides was
slower, even when using active iridium catalysts such as IrCl(CO)[P(OC F ) ] (1Y) with
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3
b
higher catalyst loadings at higher temperature for longer reaction times. In both cases, E-
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enamine was obtained selectively. Halogens, cyano, carbonyl, and nitro groups were selected
as the acceptor Z groups. These functional groups have a lone pair of electrons for
coordination to the iridium center, which possibly lowers the catalytic activity. Carbonyl and
nitro groups are typical reducible functional groups for hydride reduction, and hydrosilane
reduction of these functional groups is potentially competitive with the enamine syntheses.3-6
In our previous papers, we reported selective reduction of amide groups with ester and ketone
groups remaining unreacted.3a,3c However, nitro, cyano and formyl groups have not been
examined yet.
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Halogen atoms as acceptor A. Table 1 shows the results for conversion of amides [Z = F
(2b), Cl (2c), Br (2d), and I (2e)] to the corresponding enamines 4b–4e. Treatment of the
amide 2b containing F with TMDS at room temperature for 2 h using IrCl(CO)(PPh ) (1X,
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2
0
.05 mol%) resulted in >99% conversion of 2b to give a mixture of the silylhemiaminal 3b
(85%) and the enamine 4b (9%) (entry 1). Heating the reaction mixture at 60 °C for 1 h led
to complete conversion of 3b to 4b, and 4b was obtained in 91% isolated yield. The Cl
analogue 2c reacted with TMDS in a similar manner (entry 2), although the reaction was
somewhat slower and the silylhemiaminal 3c was obtained as a single primary product.
Isolated yield of 4c after heat treatment was 71%. Although reaction of the Br derivative 2d
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was slower than that of 2c, as shown in entry 3, the situation was improved when the more
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reactive catalyst IrCl(CO)[P(OC F ) ] (1Y, 0.03 mol%) was used. Reaction was complete
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in 2 h, and the desired compound 4d was obtained in 82% isolated yield (entry 4). The iodide-
substituted enamine 4e was the most difficult to synthesize among 4b–4e. After optimizing
the reaction conditions, 2e was converted to 4e using 0.3 mol% of 1Y (entry 6, 72% isolated
yield after 6 h). No silylhemiaminal was detected.
Table 1. Reaction of haloamides 2b-2e with TMDS catalyzed by 1X or 1Y.a
toluene, 60 °C, 1h,
OC
PPh3
Cl
1X =
1Y =
Ir
Ir
cat. 1X or 1Y
2 equiv. TMDS
Ph3P
Z
Z
Z
O
OSi
NPh2
H
+
OC
P(OC6F5)3
Cl
Toluene
temp, time
NPh2
NPh2
(C F O) P
6
5
3
H
2b - 2e
Z = F (2b), Cl (2c), Br (2d), I (2e)
3b - 3e
4b - 4e
TMDS =
Si
Si
H
H
O
( -Si = -SiMe OSiMe H )
2
2
NMR Yield (%)b
Substrate
Cat.
mol%)
Temp.
(°C)
Time
(h)
Conv. of
Isolated yield of 4 after
Entry
2 (%)b
the heat treatment,
(
(Z = )
3
86
95
49
16
0
4
9
1
2
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6
2b (Z = F)
2c (Z = Cl)
2d (Z = Br)
2d (Z = Br)
2e (Z = I)
1X (0.05)
1X (0.05)
1X (0.05)
1Y (0.03)
1X (0.05)
1Y (0.30)
25
25
25
30
25
30
2
4
97
95
91
71
--
0
4
48
0
2
>99
12
83
9
82
--
24
6
2e (Z = I)
>99
0
92
72
aReactions of the amide (1.00 mmol for entries 1-3 and 5, 0.50 mmol for entries 4 and 6) with TMDS (2.00 mmol for entries
1
-3 and 5, 1.00 mmol for entries 4 and 6) were carried out in toluene (10 mL for entries 1-3 and 5, 5 mL for entries 4 and
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) in the presence of the catalyst 1X or 1Y (0.05 mol% ~0.3 mol%) under inert gas atmosphere. Products existing in the
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reaction mixture. The yields were determined by H NMR in the presence of anisole as an internal standard.
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Palladium complexes often promote oxidative addition of aryl halides, i.e., replacement
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of a C-X bond by a C-H bond in the presence of a hydrosilane. Although the lower reactivity
of 2d and 2e compared to 2b and 2c may be ascribed to oxidative addition of a C-X bond to
a Vaska-type iridium complex, this possibility was excluded because oxidative addition of
3
2
the C(sp )-X bond to 1X is known, but that of a C(sp )-X bond has not been reported in the
1
0
literature. No C-X bond reduction by TMDS was actually observed in the reactions shown
in Table 1. The lower reactivity of 2d and 2e instead was considered to be due to coordination
of a lone pair of bromide or iodide to the catalytically active iridium center.
Benzothiadiazole as acceptor A. Benzothiadiazole (BTD) is a typical electron acceptor
1
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utilized for organic semiconductors, having lone pairs on either nitrogen or sulfur atoms
which potentially disturb the iridium-catalyzed reactions of the amide with TMDS. Thus, it
was possible to assume that enamine formation could occur from 2f, the amide containing
BTD as an acceptor, though the reaction was disturbed by coordination of sulfur or nitrogen
atoms in the BTD moiety. Reaction of 2f to 4f did not occur when 1X was used as the catalyst.
The use of more reactive catalyst 1Y improved the situation, and treatment of 2f with TMDS
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in the presence of 0.5 mol% 1Y gave 4f as a single product with >99% NMR yield and 42%
isolated yield (Scheme 1).
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9
0
1
2
3
4
5
6
7
8
9
0
Scheme 1. Reaction of amide 2f bearing benzothiadiazole moiety with TMDS catalyzed
by 1X or 1Y.
0.5 mol% IrCl(CO)(PPh3)2 (1X) or
0
.5 mol% IrCl(CO){P(OC F ) } (1Y)
6 5 3 2
O
H
N
S
N
S
N
2
equiv. TMDS
N
NPh2
Toluene, 30 °C, 2 h
NPh2
2
f (0.5 mmol)
4f
0%a (-)b
with 1X :
with 1Y : >99%a (42%)b
a1H NMR yield (anisole was used as an internal standard) bIsolated yield
Cyano and alkoxycarbonyl groups as acceptor A. Cyano and alkoxycarbonyl groups undergo
hydride reduction upon treatment with a strong reducing reagent such as LiAlH4.5,6 Their
lone pairs potentially coordinate to the iridium center, slowing the reaction. While potential
reduction of the Z groups in 2g (Z = CN) and 2h (Z = CO Me) was possible, these reactions
2
occurred selectively only for the amide as shown in Table 2. No reaction occurred with 2g in
the presence of 0.05 mol% 1X (entry 1). Use of 1Y in higher catalyst loadings (0.5 mol%) at
3
0 °C led to complete conversion of 2g after 1 h to form the corresponding silylhemiaminal
quantitatively. After heat treatment, the desired enamine was isolated in 58% yield. Reaction
of 2h catalyzed by 0.1 mol% 1X resulted in low conversion of 2h, due to the low solubility
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of amide 2h in toluene (entry 3). Use of CH Cl instead of toluene improved the conversion
2
2
of 2h, as shown in entry 4. Improvement also was made by using 1Y as the catalyst in toluene,
leading to >99% conversion of 2h after 2 h, and a mixture of 3h (32%) and 4h (66%) was
obtained.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Table 2. Reaction of amides bearing cyano (2g) or methoxycarbonyl (2h) groups with
TMDS catalyzed by 1X or 1Y.a
toluene, 60 °C, 1h
cat. IrCl(CO)(PPh3)2 (1X) or
cat. IrCl(CO){P(OC F ) } (1Y)
Z
6
5 3 2
Z
Z
O
OSi
H
2
equiv. TMDS
+
Toluene, temp, time
NPh2
NPh2
NPh2
H
2g, 2h
3g, 3h
4g, 4h
Z = CN (2g), CO Me (2h)
( -Si = -SiMe OSiMe H )
2
2
2
NMR Yield
%)b
Substrate
Entry
Cat.
Temp. Time Conv. of
Isolated yield of 4 after
(
(
mol%)
(°C)
(h)
2 (%)b
the heat treatment
(Z =)
3
0
4
0
1
2
2g (Z = CN)
2g (Z = CN)
1X (0.05)
1Y (0.5)
1X (0.1)
1X (0.1)
1Y (0.1)
25
30
30
30
30
24
1
0
--
58
--
>99
10
>99
10
0
3
2h (Z = CO
2h (Z = CO
2h (Z = CO
2
2
2
Me)
Me)
Me)
19
19
2
0
4c
5
>99
>99
>99
32
0
61
--
66
a
Reactions of the amide (0.50 mmol) with TMDS (1.0 mmol) were carried out in the presence of and the catalyst 1X or 1Y
(
0.05 mol% ~0.5 mol%) under inert gas atmosphere. Toluene was used as the solvent unless otherwise noted. b Products
1
existing in the reaction mixture. The yields were determined by H NMR in the presence of anisole as an internal standard.
cCH
Cl was used as solvent.
2
2
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1
1
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1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Formyl and acetyl groups as acceptor A. Aldehydes and ketones are more susceptible than
carboxylic acid derivatives to hydride reduction.5,6 As a consequence, iridium-catalyzed
enamine formation from amides and TMDS is accompanied by reduction of the C=O bond
in formyl or acetyl groups as a side reaction. The lone pairs on the oxygen atom of the
aldehydes and ketones have the potential to suppress the catalytic activity. This effect can be
seen in Table 3, entries 1 and 3. The ketoamide 2i (Z = COMe) did not react with TMDS
with 0.01 mol% 1X, whereas both amide and acetyl groups underwent reaction with TMDS
to form enamines with a siloxy group 4j, when 1Y (0.05 mol%) was used as the catalyst. The
situation was improved when the amount of 1X was increased to 0.25 mol% (entry 2);
selective preparation of ketoenamine 4i was achieved after 2 h (>99% conversion of 2i, >99%
selectivity of the reaction, 68% isolated yield of 4i). Aldehydes are more reactive than
ketones toward hydride reduction. Reaction conditions were optimized based on the
experimental results shown in entries 4–11. Similar to the reactions of 2i, no reaction of 2k
occurred with a low catalyst loading (0.01 mol%, entry 4), whereas use of catalyst 1Y
resulted in hydrosilylation of both amide and formyl groups to form 3l (entry 11). When the
catalyst loading was increased to 0.1 mol% and the amount of TMDS was reduced from 2
equiv. per 2k to 1 equiv., conversion of 2i was moderate (entries 6-10). The optimized result
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
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was use of 0.1 mol% of 1X with 2 equiv. of TMDS to 2i at room temperature for 2 h, which
gave three products, 3k, 4k, and 3l. After heat treatment of the mixture, all 3k and 3l were
converted to 4k and 4l, respectively. Chromatographic separation from 4l resulted in isolation
of 4k in 44% yield (entry 5).
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Table 3. Reaction of amides bearing acetyl (2i) and formyl (2k) group with TMDS
catalyzed by 1X or 1Y.a
O
O
OSi
OSi
cat. 1X or 1Y
R
O
R
OSi
NPh2
R
OSi
NPh2
R
O
TMDS
H
H
+
+
NPh2
2i (R = Me), 2k (R = H)
Toluene
temp, time
NPh2
2j (R = Me), 2l (R = H)
H
H
3i (R = Me), 3k (R = H)
3j (R = Me), 3l (R = H)
6
[
0 °C, 1 h
R = H ]
O
OSi
R
H
R
H
1
X = IrCl(CO)(PPh3)
2
+
+
H
1Y = IrCl(CO){P(OC F ) }
Si = SiMe OSiMe H
6
5 3 2
NPh2
NPh2
2
2
4
i (R = Me), 4k (R = H)
4j (R = Me), 4l (R = H)
NMR yields (%)b
Equiv.
of
Conv.
Isolated
Yields
of 4
En
try
Cat.
Temp.
(°C)
Time
(h)
-R
of 2
[ Desired product ]
[ Undesired product ]
(mol%)
TMDS
(%)b
3
i or 3k
4i or 4k
3j or 3l
4j or 4l
2j or 2l
1
2
3
4
5
6
1X (0.01)
1X (0.25)
1Y (0.05)
1X (0.01)
1X (0.1)
1X (0.1)
2.0
2.0
2.0
2.0
2.0
1.3
30
30
30
25
25
25
19
2
0
>99
>99
3
-
0
-
>99
0
-
0
-
0
-
-
-
Me
68c
0
0
0
1
6
(
2i)
0.5
4
0
0
>99
0
-
-
3
0
0
-H
2
80
67
37
0
12
7
0
44d
-
(2k)
4
57
0
0
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1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
7
8
9
1X (0.1)
1X (0.1)
1X (0.2)
1X (0.1)
1Y (0.1)
1.1
1.0
1.0
1.0
2.0
25
25
25
30
25
4
4
4
4
4
56
63
45
41
48
34
0
0
0
4
8
0
0
3
5
-
-
-
-
-
79
16
10
0
10
6
0
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
0
1
53
0
3
1
>99
0
68
29
aReactions of 2i or 2k (0.50 mmol) with TMDS (1.0 mmol) were carried out in toluene (5.0 mL) in the presence of the
b
1
catalyst (0.01 mol% ~0.25 mol%) under an inert gas atmosphere. Determined by H NMR in the presence of anisole as an
c
d
internal standard. Isolated yield after short column chromatography. A reaction mixture was subjected to heat treatment
to convert 3k to 4k. The resulting product was purified by column chromatography to separate 4k from byproducts.
NO group as acceptor A. Nitro groups are not reduced with many hydride reagents, although
2
they are easily converted to an NH or NHOH group by catalytic reduction with hydrogen12
2
1
3
14
or hydrosilanes, or by reduction with Fe powder in acetic acid. Nitro groups have lone
pairs on the oxygen, which potentially coordinate to transition metals and reduce catalytic
activity. The reaction of 2m (Z = NO ) with TMDS in the presence of 0.5 mol% 1Y at room
2
temperature resulted in quantitative conversion of 2m after 2 h, while no reaction proceeded
when 1X was used as the catalyst. After heat treatment, 4m was obtained as a single product
in 61% isolated yield (Table 4).
a
Table 4. Synthesis of -conjugated enamines containing a NO group.
2
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O
H
1
) 0.5 mol% IrCl(CO){P(OC6F5)3}2 (1Y),
equiv. TMDS, Toluene, 30°C, 6 h
O2N
Ar'
O2N
Ar'
2
NAr2
NAr2
H
2) Toluene, 60°C, 1 h
R
R
Amide 2, 5-8
Enamine 4, 9-12
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
yield
%)b,c
entry
amide
O
enamine
(
O2N
O2N
N
N
>99%
(61%)
1
2
2m
4m
O
O
O
97%
73%)
O2N
N
O2N
N
(
2
m'
4m'
98%
75%)
N
N
N
N
3
4
(
NO2
m"
NO2
4m"
2
O2N
OMe O2N
OMe
98%
(
79%)
5
9
OMe
N
OMe
N
O2N
O2N
O2N
O
O
96%
40%)
5
6
(
O
O
6
7
10
11
O2N
95%
58%)
N
N
(
O2N
7
O2N
O
95%
N
N
(
59%)
8
12
aReactions of amides (0.50 mmol) with TMDS (1.0 mmol) were conducted in toluene (5.0 mL) in the presence of 1Y (0.5
b
1
c
mol%) under an inert gas atmosphere. Determined by H NMR in the presence of anisole as an internal standard. Isolated
yield in parentheses. Primary products obtained by the reactions of the amides are a mixture of the silylhemiaminal and the
enamine, which were converted to the enamine by thermal treatment.
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
The ease of synthesizing enamine 4m bearing a strongly electron-accepting NO group
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
prompted the preparation of a series of nitro enamines other than p-O NC H CH=CHNPh .
2
6
4
2
The derivatization improved understanding of the donor properties of enamines in D--A
molecules. The first point of the derivatization was the position change of the NO group
2
from the para- to the meta- (4m’) or ortho-position (4m”). The second point is alteration of
the NPh group to more electron-donating N(p-anisyl) (9) or phenoxazinyl moiety (10). The
2
2
third point is introduction of a methyl group on the C=C moiety at the -position of the NPh2
group (11). The fourth is elongation of the -conjugate bridge by introduction of a C H
6
4
moiety (12). As summarized in Table 4, all of the derivatives were synthesized with the same
procedure used for preparation of 4m. Conversion of the starting materials and NMR yield
of the product were >99%, but the isolated yields were lower because of losses during the
purification process.
Absorption and emission spectra of 4. Properties of D--A enamines synthesized as
described above were investigated by UV-visible spectroscopy with the aid of TD-DFT
calculations. Table 5 shows experimentally obtained absorption wavelengths in toluene, in
the solid state, and absorption wavelengths calculated by TD-DFT of compounds 4b-4i, 4k,
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4
m, 4m’, 4m”, and 9–12. The calculated energy gap between the HOMO and LUMO of each
compound also is provided. Solvent effects were not included in the TD-DFT data described
in this table, indicating that the calculated values are those in gas phase. The calculated gas-
phase absorption could be compared to those in non-polar solvents such as toluene.
Experimental and calculated details are summarized in SI.
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Table 5. Summary of experimentally obtained absorption and emission wavelengths (in
toluene/solid state), calculated wavelengths (in gas phase) and HOMO-LUMO gap
energies for enamines 4b–4m” and 9–12.
Absorption
Emission
Exp. in toluene Exp. in solid state
Exp.a
Calcd. (gas phase)b
Com
pou
nd
En
try
EHO
abs [nm]

abs [nm] in toluene
ex
fl
ex
fl
MO -
in solid
calc [nm]
d
d

fl

fl
-1
-1
[nm] [nm]
[nm]
[nm]
( max [cm ·M ] )
LUMO
state
[eV]
<0.01e
–f
–f
1
2
3
4
4b
4c
4d
4e
309 (25,700), 332 (19,200)
319 (22,800), 345 (24,600)
319 (19,100), 347 (21,900)
309 (29,500), 349 (34,200)
316 (28,300), 345 (18,500),
325, 351
344, 363
337, 364
350, 392
329, 355
331, 357
331, 359
331, 360c
3.99
3.92
3.90
3.90c
310
345
350
350
386
325
405 <0.01e 363
405 <0.01e 364
408 <0.01e 392
419
422
437
0.05
<0.01
<0.01
326, 366,
470
338, 371,
547
5
4f
2.57
425
552
0.61
470
624
0.07
4
26 (12,100)
6
7
8
4g
4h
4i
292 (33,700), 370 (31,700)
315 (11,100), 370 (34,300)
306 (10,600), 380 (30,200)
314, 390
307, 399
315, 394
256, 373
257, 378
285, 389
3.64
3.61
3.48
370
370
380
426 <0.01e 390
432 <0.01e 399
440 <0.01e 394
457
474
489
0.01
0.06
0.04
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1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
9
4k
4m
4m’
290 (15,000), 392 (26,500)
291 (14,500), 424 (21,700)
317 (20,500), 341 (27,000)
307, 408
304, 487
354, 548
284, 392
299, 440
346, 524
3.41
3.03
2.83
390
425
340
450 <0.01e 408
525 0.54 487
511
–f
0.05
–f
1
0
1
1
1
585 <0.01e 345
–f
–f
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
87 (13,500), 334 (10,300),
12 (3,400)
340, 362,
551
322, 346,
501
2
4m”
2.94
335
460 <0.01e 300
494
0.01
4
–f
–f
–f
–f
1
1
3
4
9
291 (20,200), 445 (19,500)
288 (11,900), 431 (17,000)
290 (20,800), 425 (12,800)
309 (23,800), 413 (26,000)
311, 466
308, 421
310, 474
318, 447
299, 464
280, 451
306, 478
361, 524
2.87
2.98
2.89
2.62
445
430
425
415
575
494
562
550
0.22
466
446
474
450
<0.01e
10
11
12
15
16
0.13
0.64
605
–f
0.01
–f
a
-5
Maximum absorption wavelengths of enamines in the solid state were described. Those for toluene solution in 10 M were
also described with max. bCalculation was performed under the condition described in experimental section unless otherwise
c
d
noted. 12 excited states were calculated for TD-DFT. SDD basis set was selected for I atom. Absolute fluorescence
e
quantum yield was described unless otherwise noted. Determined by intensity relative to quinine sulfite (fl 0.55, ex. 350
f
nm) in 0.5 M sulfuric acid. Not detected.
Two absorption peaks were observed at 300–500 nm for the compounds listed in the table,
but the assignment of the peaks for the halogen derivatives 4b–4e was different than those
for the other compounds. In 4b (Z = F), 4c (Z = Cl), 4d (Z = Br), and 4e (Z = I), TD-DFT
predicted two signals at ca. 330 and ca. 360 nm, which agree with those observed in UV-vis
spectra in toluene. As shown in Fig. 1, the HOMO and LUMO of these compounds were 
and *, respectively, in which orbitals are extensively delocalized in the molecules. The
LUMO+1 of 4c and 4d, and LUMO+2 of 4b and 4e, consisted of * of the NPh moiety
2
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(
2*), which has an energy level close to the LUMO. The two absorption maxima observed
in the UV-vis spectra of 4b–4e were ascribed to -* and - * transitions. In contrast, other
2
1
1
1
1
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0
compounds except 4f provided a small and a large absorption peak around 260–340 nm and
3
70–500 nm. The former was a mixture of local transitions including  to * of the NPh2
moiety, whereas the latter was due to the -* transition. Although the reason is not clear,
the TD-DFT calculations of 4f did not reproduce the experimentally observed absorption
spectra.
Z
Z
Z
N
N
N
 (HOMO)
* (LUMO)
 2
*
Z = Z for 4a - 4e
Z = Z for 4g - 4m
Z = Z for 4a - 4e
Z = Z for 4g - 4m
LUMO+1 for 4c, 4d
LUMO+2 for 4b, 4e
Figure 1. Molecular orbitals for enamines 4.
Figure 2 shows plots of HOMO and LUMO energy levels of 4a–4m. The HOMO and
LUMO levels of each compound were lower than those calculated for 4a having no
substituent on the benzene ring. While the energy level change was relatively small in the
halogen derivatives 4b–4e, fewer of the compounds showed a lowering in the energy level
of the HOMO (0.2–0.5 eV) and, more significantly, that of LUMO (0.6–1.5 eV). These
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significant reductions in the energy level in the LUMO were due to the excellent -acceptor
nature of C≡N, C=O(OMe), C=O(Me), C=O(H), and NO , to which -electrons in the
2
0
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enamines extensively delocalized. The effect of the NO group was the most significant,
2
which lowered the HOMO and LUMO levels of 4m by 0.53 and 1.49 eV compared with
those of 2a. Similar effects of the NO group were generally seen in the HOMO and LUMO
2
levels of a series of nitro derivatives, 4m, 4m’, 4m’’, and 9–12 (see 2-1 in SI). The HOMO-
LUMO gap of 4b–4e was similar to that of 4a, whereas that of the other compounds was
smaller. This is in accord with the red-shift of the absorption of 4f–12 compared to that of
4
a–4e.
Figure 2. Calculated HOMO, LUMO and HOMO-LUMO gap (EHOMO-LUMO) for enamines
a~4m [eV].
4
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Fluorescence spectra of the compounds listed in Table 5 indicated that some of them were
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emissive. Relatively high quantum yields ( ) were available for 4f (Z = BTD) and nitro
fl
derivatives, 4m, 9, 11, and 12. Of particular interest, is the remarkably high  of the nitro
fl
derivatives. For example, 4m and 12 produced strong fluorescence at 525 and 550 nm, with
quantum yields reaching 54% and 64%, respectively. Nitroarenes are well known to be
1
5
quenchers of fluorescence, and only a few examples have been reported for highly emissive
nitro compounds.16,17 Almost all compounds in Table 5 did not produce fluorescence in the
solid state. Although  was lower than 10%, 4c (Z = Cl), 4f–4k (Z = BTD, CN, CO Me,
fl
2
COMe, and CHO), and nitro derivatives, 4m”, and 11, were emissive. The solvatochromic
behavior of these compounds is discussed later.
Absorption spectra of p-ZC H CH=CHNPh , 4b–4m, are predictable from those of p-
6
4
2
ZC H NPh having the same Z group. As shown in Fig. 3, maximum absorption wavelengths
6
4
2
of 4a–4e and 4g–4m calculated by TD-DFT ( ) linearly correlated with those of p-
calc
ZC H NPh having the same Z group. Calculated absorption wavelength of p-ZC H NPh
2
6
4
2
6
4
1
8
was consistent with that obtained experimentally. These are in accord with the additive rule
of absorption spectra; calc of p-ZC H CH=CHNPh approximately corresponds to  of p-
6
4
2
calc
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ZC H NPh plus 40 nm. In contrast, emission behavior was complicated. Diphenylamino
6
4
2
compounds, p-ZC H NPh (Z = CN and CHO18a,b) reportedly showed fluorescence in THF
18a
6
4
2
0
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0
at  of 421 and 465 nm with moderate quantum yields (0.29 and 0.36). In contrast, the
fl
corresponding enamine p-ZC H CH=CHNPh [4g (Z = CN), 4k (Z = CHO)] showed weak
6
4
2
emission at  of 443 and 472 nm in THF. While p-O NC H NPh is not fluorescent in
fl
2
6
4
2
MeCN,18c 4m showed weak emission at 671 nm (vide infra).
Figure 3. Plots of calculated absorption wavelengths for enamines 4 vs. those for the
corresponding amines.
Solvatochromism in absorption and emission. Solvatochromisms typically observed in D-
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
-A molecules, which are useful for understanding the ground as well as excited state
2
0
properties of the molecules, have received considerable attention recently. Figure 4 shows
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0
plots of  corresponding to -* transitions measured in six solvents: hexane, toluene, THF,
abs
CHCl , CH Cl , and DMF. From 4b to 4g, where Z = F, Cl, Br, I, BTD, and CN, a red shift
3
2
2
of 5–8 nm was observed upon an increase in solvent polarity, indicating the appearance of
positive solvatochromism. A relatively larger red shift of abs (13–19 nm) was observed for
carbonyl molecules, ester (4h), ketone (4i), and aldehyde (4k). The positive solvatochromism
was most significant for the nitro derivative 4m, which red-shifted more than 40 nm. In all
2
0a
cases, values of the red-shift had a linear relation with solvent polarity parameter E (30)
T
(see 1-3 in SI).
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9
0
-
5
Figure 4. Plots of abs for enamines 4b–4m, 4m”, and 9 measured in six solvents (10 M
solutions in hexane, toluene, THF, CHCl , CH Cl , DMF).
3
2
2
In general, positive solvatochromism is observed when the molecule in an excited state
2
0
is better stabilized by polar solvents than that in the ground state. Intramolecular charge
transfer (ICT) of D--A compounds (Scheme 2) explains the present results. For compound
4, an ionic resonance form B resulting from ICT (Scheme 2) contributed to stabilization of
the excited states of 4. Thus, contribution of B tends to be NO > carbonyls > CN, BTD,
2
halogen.
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Scheme 2. Resonance structures of enamine 4.
Z
Z
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Ar
Ar
N
N
Ar
Ar
A
B
As described above, 4m shows fluorescence in remarkably high quantum yield in toluene
compared with those reported for other nitro compounds,16b,17 making 4m suitable for
studying solvatochromic behavior in both absorption and emission spectra. The UV-vis and
fluorescent spectra of 4m were obtained in 12 aprotic solvents, of which E (30) increased in
T
the order: hexane < CCl < toluene < 1,4-dioxane < THF < o-dimethoxybenzene (o-DMB) <
4
CHCl < CH Cl < acetone < DMF < DMSO < MeCN. In addition, protic solvents EtOH and
3
2
2
MeOH also were used for the measurements. Spectra in H O were not available due to lack
2
of solubility of 4m. Results are summarized in Table 6 and Figures 5 and 6. Relatively strong
fluorescence ( > 0.27) was observed in CCl , toluene, 1,4-dioxane, THF and o-DMB with
fl
4
the fluorescence wavelength in the range of 508–595 nm. The quantum yield reached 0.69 in
2
1
1,4-dioxane. Although the quantum yield was low, the longest emission wavelength of 671
nm was observed in MeCN. The spectrum in hexane gave two peaks and the quantum yield
was lower than that in toluene. This may be explained by aggregation of 4m in hexane like
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2
2
prodan. However, we do not have clear answer to this problem, because UV-vis absorption
-5
-6
and fluorescence measurement of 4m in hexane in two concentrations, 10 and 10 M,
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
showed no difference in the peak ratio.
Table 6. Summary of experimentally obtained absorption and fluorescence wavelengths
of 4m in various solvents.

abs [nm

max [cm-

fl [nm
Stokes shift
flc
Entry
1
Solvent
Hexane
T
E (30)
eV)]a
1
-1
(eV)]b
[cm-1]
(
·M ]
458 (2.707)
2918
3655
31.0
404 (3.069)
26,000
0.02
4
74 (2.616)
2
3
4
5
6
7
8
9
CCl
4
32.4
33.9
36.0
37.4
38.4
39.1
40.7
42.2
43.2
45.1
45.6
51.9
55.4
62.8
416 (2.981)
424 (2.925)
425 (2.918)
431 (2.877)
441 (2.812)
438 (2.831)
439 (2.825)
433 (2.864)
446 (2.780)
452 (2.743)
434 (2.857)
429 (2.890)
430 (2.883)
–
23,000
21,700
26,300
30,200
11,400
24,200
22,200
15,500
22,400
21,000
34,800
18,000
14,300
–
508 (2.441)
525 (2.361)
546 (2.271)
568 (2.183)
595 (2.084)
637 (1.947)
640 (1.937)
623 (1.990)
655 (1.893)
669 (1.854)
671 (1.848)
634 (1.956)
643 (1.928)
–
4353
4537
5214
5596
5869
7132
7154
7043
7154
7176
8138
7537
7703
0.27
0.54
0.69
0.65
0.38
0.09
0.05
0.08
0.03
0.02
<0.01
<0.01
<0.01
–
Toluene
1,4-Dioxane
THF
o-Dimethoxybenzene
CHCl
3
CH Cl
2
2
Acetone
DMF
1
1
1
1
1
0
1
2
3
4
DMSO
MeCN
EtOH
MeOH
1
5d
2
H O
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a
b
c
-5
Maximum absorption wavelengths of 4m in various solvent at 1.0 × 10 M were described [nm and eV in the parentheses].
-6
Maximum fluorescence wavelengths of 4m in various solvent at 1.0 × 10 M were described [nm and eV in the parentheses].
d
Absolute fluorescence quantum yield was described. The data was not available due to lack of solubility of 4m in H
2
O.
1
1
1
1
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0
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0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Figure 5. Emission spectra of 4m obtained in various solvents. (Those for ROH were omitted
for clarity. See Figure S57(f) in SI for complete spectra).
Figure 6. A series of solutions of 4m in various solvents under UV light (365 nm)
A recent study on solvatochromism by Mennucci and coworkers involved the analysis of
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solvatochromic behavior in solvents with different solvent polarity by solvent-induced shifts
2
0b
of absorption  (abs), fluorescent  (flu), and Stokes shift  (SS). To estimate solute-
pol
pol
pol
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
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8
9
0
1
2
3
4
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7
8
9
0
1
2
3
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9
0
1
2
3
4
5
6
7
8
9
0
solvent hydrogen bonding, solvent-induced shifts of absorption HB (abs), fluorescent HB
flu), and Stokes shift HB (SS) also were proposed as parameters. Table 7 shows
(
experimentally obtained pol (abs), pol (flu), and pol (SS) data when the solvent polarity
increased from dioxane to DMSO. The table also includes HB (abs), HB (flu), and  (SS)
HB
upon changing the solvent from DMSO to MeOH. The data are compared with those obtained
2
3
by common ICT probes, 6-propionyl-2-dimethylamino naphthalene (PRODAN) and 7-
diethylamino-9,9-dimethyl-9H-fluorene-2-carbaldehyde (FR-0).23 The pol (abs) and pol
(
flu) values of 4m are larger than those of PRODAN and FR-0, whereas  (SS) is between
pol
those of PRODAN and FR-0. Different from the negative HB (abs) and HB (flu) values of
PRODAN and FR-0, those of 4m are positive, while  (SS) of 4m is smaller than  (SS)
HB
HB
of PRODAN and FR-0. These indicate that 4m behaves as a typical ICT probe in common
organic solvents, but hydrogen bonding between 4m and aprotic solvents does not provide a
large effect on solvatochromic properties.
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Table 7. Experimentally obtained solvent-induced shifts on absorption and emission
-1
energies (eV) and Stokes shift (cm ) for 4m.

pol (Dioxane-DMSO) (upper)
HB (DMSO-MeOH) (lower)
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
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5
6
0
1
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9
0
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2
3
4
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6
7
8
9
0
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4
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6
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1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
compound

Stokes shift [cm-1]
Absorption [eV] Emission [eV]
-
0.18
-0.42
+0.07
-0.25
-0.22
-0.38
-0.20
1962
527
4m
+0.14
-0.11
1161
1339
2804
1485
Prodan23
FR-023
-0.06
-0.03
-0.02
The solvatochromism of 4m and other nitro derivatives was categorized into three
classes: large shift (> 30 nm; 4m, 4m”, 9), medium (10–20 nm; 10, 11), and small (< 10 nm;
4
m’, 12). Despite the large red-shift, the absorption coefficient of 4m” corresponding to the
-* transition is small, due to the steric repulsion between the ortho-NO group and the

2
enamino group, which disturbs the NO group coplanar with the benzene ring conjugated
2
with the enamino group. The large shift is ascribed to a contribution of resonance form B,
whereas the small red shift is explained by a factor that reduces the contribution of B. Among
the two compounds showing small shifts, the m-nitroenamine 4m’ in the solid state showed
two absorptions at 354 and 548 nm, which were in accord with the spectrum obtained by TD-
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