Catalyst Design of Vaska-Type Iridium Complexes for Highly Efficient Synthesis of π-Conjugated Enamines

Article abstract of DOI:10.1021/acs.organomet.5b00636

The appropriate design of a ligand (L) in IrCl(CO)(L)₂ (4) realized the efficient synthesis of π-conjugated enamines possessing hole-transport properties. The iridium complex with electron-withdrawing phosphorus ligands catalyzed the hydrosilylation of amides to the corresponding silylhemiaminals, which were transformed to the enamines by heat or by treatment with acids. High catalytic efficiency (TON > 10,000) was achieved, which made it possible for the residual iridium in the enamine product to be below 20 ppb.

Full text of DOI:10.1021/acs.organomet.5b00636

Article
pubs.acs.org/Organometallics
Catalyst Design of Vaska-Type Iridium Complexes for Highly Efficient
Synthesis of π‑Conjugated Enamines
Atsushi Tahara,† Yasumitsu Miyamoto,‡ Ryuta Aoto,‡ Keisuke Shigeta,‡ Yuta Une,‡ Yusuke Sunada,†
Yukihiro Motoyama,† and Hideo Nagashima*,†,§
‡
†Institute for Materials Chemistry and Engineering and Graduate School of Engineering Sciences, Kyushu University, Fukuoka
819-0395, Japan
§CREST, Japan Science and Technology Agency (JST), Kasuga, Fukuoka 816-8580, Japan
S
* Supporting Information
ABSTRACT: The appropriate design of a ligand (L) in IrCl(CO)(L)2
(4) realized the efficient synthesis of π-conjugated enamines
possessing hole-transport properties. The iridium complex with
electron-withdrawing phosphorus ligands catalyzed the hydrosilylation
of amides to the corresponding silylhemiaminals, which were
transformed to the enamines by heat or by treatment with acids.
High catalytic efficiency (TON > 10,000) was achieved, which made it
possible for the residual iridium in the enamine product to be below 20
ppb.
Chart 1. π-Conjugated Enamines 2-i−vii Possessing Hole-
Transport Properties and Their Amide Precursors 1-i−vii
INTRODUCTION
■
Enamines have been useful synthetic intermediates as “enolate
equivalents” in organic synthesis since their discovery by Stork
and co-workers.1 Except for enamines conjugated to carbonyl
groups such as β-ketoenamines,2 enamines are generally
unstable to moisture and easily undergo hydrolysis to afford
aldehydes and amines. Thus, most enamines have been either
isolated by careful experiments under anhydrous conditions to
avoid hydrolysis or prepared in situ and used for further organic
reactions without isolation.3 Although good reactivity of
enamines toward electrophiles suggests the potential of
enamines as excellent electron donors, their application to
materials did not receive much attention until a report by
Borsenberger and co-workers in 1997.4 They investigated hole-
transport properties of several aldenamines conjugated to
delocalized aromatic rings, and found that these weakly polar
aldenamines were stable enough to prepare vapor-deposited
enamine glasses and enamine-doped polymers,4a which showed
good room-temperature mobilities.2 These interesting proper-
ties of enamines were later supported by further studies by
Jankauskas and co-workers on the hole-transport properties of
certain π-conjugated enamines4b−f and actual applications of π-
conjugated enamines to electrophotographic photoreceptors.5,6
The π-conjugated enamines, typical examples of which are
shown in Chart 1, were synthesized by conventional preparative
methods using the condensation of aldehydes or ketones with
secondary amines.1,4b−f However, the reaction was essentially
reversible, and effective removal of water was necessary for
isolation of the product at high temperatures. Yields of the
products were low to moderate.1,7
attracted attention and includes cross-coupling of vinyl halides
with secondary amines,8 dehydrogenation of amines,9 hydro-
amination of alkynes,10 and cross-coupling of ynamides and
boronic acids.11
Among these methods, catalytic hydrosilane reduction of
tertiary amides provides unique access to aldenamines from
easily available amides.12−15 Although reduction of amides to
amines has recently attracted attention from organic and
We are interested in the development of new and efficient
synthetic methods for π-conjugated aldenamines. The synthesis
of enamines by virtue of transition-metal catalysts has recently
Received: April 30, 2015
© XXXX American Chemical Society
A
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organometallic chemists,16−18 relatively little has been inves-
tigated for the preparation of enamines from amides. The first
report of this transformation was from Corriu and co-workers
in 1984, in which RhCl(PPh3)3 catalyzed the reaction of
PhCH2CONEt2 (1-viii) with Me2HSi(CH2)2SiHMe2
(BDMSE) or 1,2-(Me2HSi)2C6H4 (BDSB) to give PhCH
CHNEt2 (2-viii) in moderate yields.12a Our discovery in 2009
of a highly efficient reaction of a series of aliphatic tertiary
amides to aldenamines was achieved by using a catalytic
amount of IrCl(CO)(PPh3)2 (Vaska’s complex 4f, shown in
Chart 2) and certain hydrosilanes, Me2HSiOSiHMe2 (TMDS)
the hydrosilane reduction of PhCH2CONEt2, which was
achieved at room temperature in 30 min. The use of extremely
low catalyst loadings is important for the preparation of
chemicals for applications to materials whose properties are
affected by contamination by metals. Highly efficient catalytic
reactions achievable by a very small amount of metal
compounds lead to facile removal of the metal residues from
the catalyst component.16h
These excellent features of IrCl(CO)(PPh3)2-catalyzed
aldenamine synthesis suggested that it should be useful for
the preparation of the π-conjugated enamines shown in Chart
1. However, we were disappointed to see that Vaska’s complex
4f showed no catalytic activity for the reaction of π-conjugated
enamine precursors with TMDS even at high temperature. We
pursued a solution to this problem by the appropriate ligand
design of IrCl(CO)(L)2 (4) on the basis of Tolman’s χ values19
and succeeded in discovering that IrCl(CO)(L)2 with electron-
withdrawing phosphorus ligands provided highly efficient
selective preparation of π-conjugated enamines.
Chart 2. Vaska-Type Complexes 4a−j
RESULTS AND DISCUSSION
■
and Me3Si[OSiHMe]n-OSiMe3 (PMHS).13 This IrCl(CO)-
(PPh3)2-catalyzed reaction has several potential advantages for
the synthesis of π-conjugated enamines for applications as
materials. The first advantage is the selectivity of the reaction to
form aldenamines. As shown in Scheme 1, transition-metal-
Synthesis of IrCl(CO)(L)2 (4). To improve the catalytic
activity toward the reaction of π-conjugated aldenamine
precursors, we prepared the series of Vaska-type complexes
shown in Chart 2 by changing the phosphorus ligands L. For
the complexes bearing electron-donating phosphorus ligands,
six phosphine and one phosphite complexes 4a−g were
synthesized according to the procedure in the literature.20
Among the ligands in these complexes, P(OPh)3 was the most
electron withdrawing. To investigate the effect of more electron
withdrawing ligands, we prepared three new Vaska-type
complexes, where L = P(NC4H4)3, P(OC6F5)3, P{OCH-
(CF3)2}3. The unique electron-withdrawing effects of P-
(NC4H4)3 were investigated by several rhodium complexes,21a
for which rhodium compounds bearing P{OCH(CF3)2}3
showed significant reactivity in several catalytic reactions.21b,c
A platinum complex of P(OC6F5)3 was prepared, but no
detailed studies in reactivity were undertaken.21d
The synthesis of three new complexes, 4h−j, was performed
by the treatment of {Ir(COD)Cl}2 with the corresponding
phosphorus ligand under a CO atmosphere.20 All of the
synthesized complexes 4h−j were characterized by NMR and
IR spectroscopy and elemental analyses, and their structures
were determined by X-ray diffraction studies. The molecular
structures of 4h−j are similar to that of the Vaska complex 4f,
which has a square-planar geometry with the iridium atom
bound to two mutually trans PPh3 ligands, a CO group, and a
chlorine atom. A representative ORTEP drawing of 4i is shown
in Figure 1. The Ir−P(1), Ir−P(2), Ir−C(1), and Ir−Cl(1)
bond distances indicated in the figure caption are comparable
to those found in 4f. The molecular structures of 4h,j are
summarized in the Supporting Information. NMR and IR
spectra of 4h−j were consistent with those expected from the
molecular structure of each complex, showing that the structure
in the solid state is maintained in solution. A single 31P
resonance was observed at δ 79.5 (4h), 108.0 (4i), and 113.2
(4j), indicating that the two PR3 ligands are in the trans
configuration. A 13C signal due to the CO ligand was observed
at δ 166.4 (4h), 165.1 (4i), and 162.6 (4j). IR absorption bands
(νCO) were visible at 2011 cm−1 (4h), 2052 cm−1 (4i), and
2059 cm−1 (4j). 31P signals of 4h−j showed significant upfield
shifts in comparison with that of Vaska’s complex 4f (δP 24.7).
Scheme 1. Hydrosilane Reduction of Tertiary Amides
catalyzed reactions of tertiary amides with hydrosilanes
generally afford the corresponding tertiary amines. The reaction
proceeds through a silylhemiaminal as an intermediate, which is
formed by addition of a Si−H bond of the hydrosilane across a
CO bond in the tertiary amide (hydrosilylation of the amide,
eq 1). Further reduction of the silylhemiaminal by hydrosilanes
results in the formation of tertiary amines (deoxygenative
reduction, eq 2), whereas formal elimination of silanol gives the
enamine (eq 3). In fact, we have investigated hydrosilane
reduction reactions of tertiary amides catalyzed by ruthenium,
platinum, and iron complexes, which generally afford tertiary
amines as a single product, though aldenamines are sometimes
detected as a byproduct under special reaction conditions such
as when the catalyst loadings are low.16 Recently, Brookhart
and co-workers reported the reduction of tertiary amides with
hydrosilanes catalyzed by pincer-type iridium complexes, in
which a mixture of the corresponding aldenamine and the
tertiary amine was obtained.14 It is a notable feature of the
IrCl(CO)(PPh3)2-catalyzed hydrosilane reduction that alden-
amines were obtained as a single product without the formation
of tertiary amines.13 The second advantage is that amides,
which are easily available from the corresponding carboxylic
acids and amines, can be used as the starting materials for π-
conjugated aldenamines. The third advantage is the high
catalytic efficiency of TOF > 104 (TON > 5000) observed in
B
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Scheme 2. Conversion of 1-viii to 2-viii
a
Table 2. Catalyst Screening for the Reaction of 1-viii
b
b
entry complex yield of 2-viii (%) entry complex yield of 2-viii (%)
1
2
3
4
5
4a
4b
4c
4d
4e
11
0
6
4f
4g
4h
4i
>99
>99
>99
>99
45
7
23
45
3
8
9
Figure 1. Molecular structure of 4i (thermal ellipsoids with 50%
probability). One THF molecule exists in the unit cell as a
crystallization solvent, which is omitted for clarity. Selected bond
lengths (Å) and angles (deg): Ir−P(1) = 2.249(2), Ir−P(2) =
2.257(2), Ir−Cl(1) = 2.343(2), Ir−C(1) = 1.838(10), C(1)−O(1) =
1.149(12); P(1)−Ir−P(2) = 174.98(9), P(1)−Ir−Cl(1) = 87.85(8),
P(2)−Ir−Cl(1) = 90.97(8), Cl(1)−Ir−C(1) = 178.3(3), P(1)−Ir−
C(1) = 90.5(3), P(2)−Ir−C(1) = 90.6(3).
c
10
4j
a
All reactions were carried out with amide 1-viii (1.0 mmol) in the
presence of 0.5 mol % of 4 and 1,1,3,3-tetramethyldisiloxane (2.0
mmol) in C6D6 (0.5 mL) at room temperature for 30 min. Yields
were determined by H NMR with hexamethylbenzene (0.05 mmol)
as an internal standard. A mixture of the enamine 2-viii and the
corresponding amine PhCH2CH2NEt2 was obtained in a ratio of
45:55.
b
1
c
In contrast, νCO of 4f was observed at 1967 cm−1, which was
markedly lower than those of 4h−j.
It is important for appropriate catalyst design to understand
the steric as well as electronic effects of ligands. Although
detailed computational studies22 have recently been performed
for possible prediction of phosphorus ligand properties,
Tolman’s cone angles and χ values are still convenient to
consider the ligand effect.19 Table 1 shows the CO stretching
bearing electron-donating ligands in the left column were less
reactive than those having electron-withdrawing ligands in the
right column. In addition to Vaska’s complex 4f reported
previously, the analogous Vaska-type complexes 4g−j bearing
electron-withdrawing phosphorus ligands (χ value of L >12.8,
νCO of 4 >1967 cm−1) showed high catalytic activity and
brought the reaction to completion within 30 min. The
complexes shown in entries 3−5 have χ and νCO values similar
to those of 4.
The low catalytic activity of 4e may indicate that less
sterically bulky ligands are more appropriate for efficient
conversion of 1-viii to 2-viii, when the χ values of L and νCO
values of 4 are similar.
Table 1. νCO Value of 4 vs χ Value and Cone Angle of L
νCO of
χ value cone angle of
entry complex
L
4 (cm−1
)
of L19
L (deg)19
1
2
3
4
5
6
7
8
9
10
4a
4b
4c
4d
4e
4f
PCy3
PEt3
1932
1942
1955
1959
1961
1967
2003
2011
2052
2059
0.3
5.6
170
132
122
136
194
145
128
137
137
130
The reactions of π-conjugated aldenamine precursors 1-i,ii
were different from those of 1-viii and did not directly afford
the enamines 2-i,ii, respectively. NMR spectra indicated
formation of a single products, which were assignable as the
silylheminals 3-i,ii, as shown in Scheme 3. Quantitative
PMe2Ph
9.2
PMePh2
10.9
10.5
12.8
29.2
P(o-Tol)3
PPh3
4g
4h
4i
P(OPh)3
P(Pyrrolyl)3
P(OC6F5)3
P{OCH(CF3)2}3
Scheme 3. Conversion of 1-i−vii to 2-i−vii
4j
51.3
frequency for the iridium complexes (νCO(Ir)) with the cone
angle and χ values of the corresponding phosphines and
phosphites. These indicate the electronic properties of the
iridium center bonded with the phosphorus ligands and
contribute to achieving a better understanding of the ligand
effect, as described in Catalyst Screening.
Catalyst Screening. We selected three tertiary amides for
screening of the iridium catalysts 4. One was N,N-dialkylamide
1-viii for comparison with our previous results, and the other
two were the amide precursors of π-conjugated enamines 1-i
and 1-ii. Scheme 2 and Table 2 show the reaction of 1-viii with
TMDS (2 equiv), which gave 2-viii by catalysis of 0.5 mol % of
the iridium complex at room temperature. Although the
reaction proceeded through the silylhemiaminal intermediate
3-viii, only 2-viii was visible in the NMR spectrum in all cases
shown in the table, even in the initial stage of the reaction
(Scheme 2). It is clear from Table 2 that the four complexes
formation of 3-i,ii from 1-i,ii was suggested by comparison of
the integral value of the signals of 3-i,ii with that from the
added internal standard. Since 3-i,ii were sensitive to moisture,
attempted chromatographic purification gave a mixture of
products, including the corresponding aldehyde. However, the
crude silylheminals were quantitatively converted to the
corresponding π-conjugated enamines 2-i,ii by thermal treat-
ment (method A) or reaction with HCl in Et2O (method B).
Table 3 shows the reactions of 1-i,ii with TMDS catalyzed by
five selected Vaska-type complexes, 4f−j, which showed
excellent catalytic activity in the conversion of 1-viii to 2-viii.
For the conversion of 1-i to 3-i, the reaction was performed at
room temperature for 0.5 h in the presence of 0.5 mol % of the
C
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a
Table 3. Catalyst Screening for the Hydrosilylation of 1-i,ii by the Iridium Catalysts 4f−j
b
b
entry
amide
catalyst
temp (°C)
time (h)
yield of 3 (%)
method
yield of 2 (%)
1
2
3
4
5
6
7
8
9
10
1-i
4f
4g
4h
4i
room temp
60
0.5
21
0.5
0.5
0.5
2
0
>99
>99
>99
>99
0
1-i
A
A
A
A
90
92
94
93
1-i
room temp
room temp
room temp
room temp
room temp
room temp
room temp
room temp
1-i
1-i
4j
1-ii
1-ii
1-ii
1-ii
1-ii
4f
4g
4h
4i
2
0
21
2
>99
>99
>99
B
B
B
> 99
> 99
> 99
4j
2
a
All reactions were carried out with amide 1-i (1.0 mmol) or 1-ii (1.0 mmol) in the presence of 0.5 mol % of 4f ∼ 4j and 1,1,3,3-
b
1
tetramethyldisiloxane (2.0 mmol) in toluene (4.0 mL). Yields were determined by H NMR with anisole (1.0 mmol) as an internal standard.
Method A: heating at 100 °C for 6 h. Method B: addition of HCl in Et2O and stirring at room temperature for 0.5 h.
a
Table 4. Isolation of 2-i−vii via the Silylhemiaminal Intermediates 3-i−vii by the Iridium Catalyst 4i
b
c
entry
amide
time (h)
NMR yield of 3 (%)
diastereomer ratio of 3
method
isolated yield of 2 (%)
E:Z ratio of 2
1
2
3
4
5
6
7
1-i
0.5
2
>99
>99
98
96:4
A
B
A
B
A
A
C
91
86
90
96
93
92
77
90:10
1-ii
1-iii
1-iv
1-v
2
95:5
98:2
16
2
>99
95
87:13
21:79
100:0
1-vi
1-vii
30
2
95
>99
a
All reactions were carried out with amides 1-i−vii (1.0 mmol) in the presence of 0.5 mol % of 4i and 1,1,3,3-tetramethyldisiloxane (2.0 mmol) in
b
c
1
toluene (4.0 mL) at room temperature. Yields were determined by H NMR with anisole (1.0 mmol) as an internal standard. Isolated yield.
Method A: heating at 80−100 °C for 6 h. Method B: addition of HCl in Et2O and stirring at room temperature for 0.5 h. Method C: addition of
P2O5 and stirring at room temperature for 0.5 h.
catalyst (entries 1−5). No reaction took place when 4f,g were
used as the catalysts. The reaction did not occur with 4f even at
elevated temperatures and prolonged reaction times, while 4g
catalyzed the reaction at 60 °C, and the silylhemiaminal 3-i was
formed quantitatively after 21 h. In sharp contrast to these two
catalysts, the reactions catalyzed by 4h−j were complete at
room temperature in 0.5 h to give 3-i quantitatively. In these
reactions, the best yield of 3-i was obtained when 2 equiv of
TMDS was used with respect to 1-i. For instance, the yield of
3-i was decreased in the reaction catalyzed by 4i with lower
amounts of TMDS (>99% (2 equiv of TMDS) > 83% (1 equiv)
> 41% (0.5 equiv)).
Since 1-ii was less reactive than 1-i, we set the standard
conditions at room temperature for 2 h in the presence of 0.5
mol % of 4. As shown in entries 6−10, no reaction took place
with 4f,g, whereas the reaction was slow but complete after 21
h with 4h. It is notable that 4i,j both resulted in complete
conversion of 1-ii to 3-ii within 0.5 h. The results shown in
Table 3 clearly demonstrate that Vaska-type complexes bearing
electron-withdrawing fluorinated phosphites are active catalysts
for the conversion of π-conjugated enamine precursors to the
corresponding silylhemiaminals, which can be transformed to
the desired enamines by further treatment as in methods A and
B.
aldenamine. Three procedures were used for the reaction of 3
to 2,:the aforementioned methods A and B and treatment with
P2O5 (method C) reported previously. The seven π-conjugated
enamines 2-i−vii were obtained in good to high yields. In four
cases, shown in entries 1, 3, 5, and 7, the enamine formed had
geometrical isomers. Only one isomer of 2-vii was detected in
the reaction of 1-vii, and the coupling constant between the
two olefinic protons of 2-vii in the 1H NMR spectrum indicated
the isomer to be exclusively trans (3JH−H = 14.4 Hz). A mixture
of E and Z isomers was obtained in the reactions of 1-i,iii,v, in
ratios of 90:10, 98:2, and 21:79, respectively. The stereo-
chemistry of the isomers was determined by NOESY spectra, as
summarized in the Supporting Information. The major isomer
of 2-i,iii was E, whereas that of 2-v was Z. It was curious that
the major isomer was different between 2-i (and 2-iii) and 2-v,
even though the structures of these enamines were similar. To
investigate the stereospecificity of the elimination of silanol
from the silylhemiaminal, stereochemical analysis of 3-i,iii,v by
1H NMR was undertaken, and the results are summarized in
Preparation of Enamines with Low Catalyst Loadings.
Preparation of Enamines with Low Catalyst Loadings.
As noted in the Introduction, our previous paper demonstrated
the high catalytic activity of 4f, which was proved by the
reaction of 1-viii with TMDS with low catalyst loadings. In the
extreme case, the reaction was complete within 0.5 h at room
temperature, and the TOF exceeded 104. We attempted further
improvement of the catalyst efficiency in this reaction by using
the new catalyst 4i and found that selective synthesis of 2-viii
was achieved by catalysis of 0.001 mol % of 4i at room
temperature within 30 min (TOF = 2 × 105 h−1, TON = 105).
Similarly, the preparation of 2-iii in gram quantities was
Isolation of π-Conjugated Enamines 2-i−vii. Experi-
ments aimed at the isolation of 2-i−vii were performed using 4i
as the catalyst. The results are summarized in Table 4. All
reactions were carried out at room temperature in the presence
of 0.5 mol % of 4i. The reactions were complete after 0.5−30 h.
The complete conversion of the amide to the silylhemiaminal
was confirmed by 1H NMR, and the crude product was
subjected to a reaction to eliminate a silanol to form the desired
D
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examined with a minimum concentration of the catalyst 4i.
Hydrosilylation of 1-iii to 3-iii proceeded smoothly with 0.01
mol % of catalyst loading (40 °C, 24 h, 98% yield), and the
TON reached 10000. The crude 3-iii was subjected to
elimination of silanol by method A to give 2-iii in 93% yield.
In this case, iridium catalyst and silane wastes were easily
separable from the products by a short silica gel column, and
there was no detectable Ir species in the enamine product by
ICP-MS analysis (<20 ppb; see the Supporting Information).
NMR Analysis of Silylhemiaminal Intermediates 3-i−
vii. The silylhemiaminals 3-i−vii are intermediates in the
synthesis of π-conjugated enamines as described above. Most of
them are unstable to moisture or the reagent that cleaves the
O−Si bond. For example, signals due to the silylhemiaminal
reaction (67:33 (after 30 min, conversion of 3-i 20%), 84:16,
(after 1 h, conversion of 3-i 54%)) and came close to 90:10
after 4 h (conversion >99%). Although the mechanism is
unclear at present, E/Z isomerization took place under the
reaction conditions.
Mechanistic Considerations. As described above, marked
differences in reactivity were present between N,N-dialkyla-
mides such as 1-viii and N,N-diarylamides 1-i−vii. The iridium
complexes 4a−j all showed some catalytic activity for the
reaction of the former with TMDS, but only a limited number
of iridium complexes exhibited activity for the reaction of the
latter. It is also interesting that the products of the reactions of
N,N-diarylamides 1-i−vii were the corresponding silylhemia-
minals 3-i−vii, whereas the reaction of 1-viii gave the enamine
2-viii and the silylheminal intermediate 3-viii was never
observed. A primary clue for understanding these results is
the electronic nature of both the iridium catalysts and the
amide precursors.
The electronic nature of the iridium centers of 4a−j was
estimated from νCO(Ir). Since 4a−j have similar square-planar
geometries and two phosphorus ligands are in the trans
orientation, νCO(Ir) is strongly correlated with Tolman’s value
(Table 1). The complexes 4a−e, where L is electron donating
(χ = 0.3−10.9; νCO(Ir) 1932−1961 cm−1) showed catalytic
activity lower than that of Vaska’s complex 4f (χ = 12.8; νCO of
4f 1967 cm−1) in the reaction of N,N-diethylamide 1-viii with
TMDS to give the corresponding enamine 2-viii. In sharp
contrast, the iridium complexes bearing electron-withdrawing
phosphorus ligands (χ = 29.2−51.3; νCO(Ir) 2003−2059 cm−1)
exhibit high catalytic activity toward the hydrosilylation of 1-i−
vii to 3-i−vii, whereas no reaction occurred with 4f. The results
shown in Table 3 indicate the order of increasing catalytic
activity as 4j ≈ 4i > 4h > 4g, which is consistent with the order
of νCO(Ir): 2059 ≈ 2052 > 2011 > 2003 cm−1. The results
clearly showed that an iridium complex having an electron-
deficient iridium center is a better catalyst for the hydro-
silylation of the amides 1-i−vii.
1
were clearly observed by H NMR from C6D6, whereas peaks
due to the corresponding aldehyde were observed rather than
those from the hemiaminal in CDCl3. Among the seven
hemiaminals, 3-vii was relatively stable and isolable. Treatment
of 1-vii with 2 equiv of TMDS in the presence of 4i (0.5 mol
%) was followed by removal of TMDS under vacuum. The
yield was determined by quantitative 1H NMR spectroscopy in
the presence of an internal standard (hexamethylbenzene). The
1H NMR spectrum of crude 3-vii showed well-resolved signals
without any peaks derived from impurities. The spectrum
measured at room temperature consisted of four signals due to
the diastereotopic SiMe2 groups (O−SiMe2−O−SiHMe2,
3
δ−0.26 (s, 3H), − 0.11 (d, JH−H = 2.8 Hz, 3H), − 0.10 (d,
3JH−H = 2.8 Hz, 3H), − 0.09 (s, 3H)), those showing an ABX
3
pattern due to CH2CH(OSi)(NR2) (δ 3.28 (dd, JH−H = 6.9
2
3
2
Hz, JH−H = 13.7 Hz, 1H), 3.50 (dd, JH−H = 6.9 Hz, JH−H
=
3
13.7 Hz, 1H) and δ 6.41 (dd, JH−H = 6.9, 6.9 Hz, 1H)), and
signals due to a Si−H group (4.72 (sept, 3JH−H = 2.8 Hz, 1H))
and the aromatic moieties. Sixteen resonances were visible in
the 13C NMR spectrum, whereas two resonances appeared in
the 29Si NMR spectrum. The high-resolution mass spectrum
(EI) showed the [M]+ signal. These spectral data are consistent
with 3-vii.
The IR spectra of amides (νCO(amide)) 1-i−viii also
correlated with the reactivity. The νCO(amide) value was in
the range 1632−1697 cm−1 and increased in the order 1-viii
(1632 cm−1) < 1-i (1659 cm−1) < 1-ii−iv (1667−1672 cm−1) <
1-v,vi (1680−1684 cm−1) < 1-vii (1697 cm−1). Amides have
the two resonance forms A and B, as shown in Chart 3, and the
Isolation of the other six silylhemiaminals was unsuccessful
due to contamination by a small amount of silicon products
derived from TMDS, which were not removed under vacuum.
Attempted chromatographic purification led to decomposition
of the silylhemiaminal. Nevertheless, these species provided
1
characteristic H signals due to one methyl and two vicinal
methine protons in Ph(Me)CHCH(OSi)(NR2) of 3-i,iii,v and
those due to two vicinal methine protons in Ph2CHCH(OSi)-
(NR2) of 3-ii,iv,vi (see the Experimental Section and the
Supporting Information). The α-methyl-α-phenyl derivatives 3-
i,iii,v exist as a mixture of diastereomers in ratios of 96:4, 95:5,
and 85:15, respectively. The signal due to the α-methyl group
of the major isomer for 3-i,iii,v appeared as a doublet at δ 1.22,
which was a chemical shift higher than that for the minor
isomer. Similarly, the signal due to −SiOCH− was always
observed in the downfield region (δ 5.55, 5.74, and 5.31,
respectively). These results suggest that all of the three major
isomers possess the same relative stereochemistry. As described
above, the major isomer of 2-i,iii was E; this is different from
the major isomer of 2-v (Z). They were formed from the major
diastereomer of 3-i,iii,v having the same configuration. A
reasonable explanation for this is elimination of silanol, which
did not proceed stereospecifically. It is also important that the
E:Z ratio changed in the progress of the reaction, suggesting
that it was controlled thermodynamically. For example, the
ratio of (E)-2-i to (Z)-2-i was lower at the initial stage of the
Chart 3. Resonance Forms of Amides and Amide Rotation
lower νCO stretching is due to a higher contribution of the
dipole resonance form B. Since B has two rotamers, the energy
barrier of interconversion between the two rotamers can
predict the reactivity of the amides. A higher energy barrier
suggests a higher contribution of B in comparison to that of A.
Solution dynamics allow qualitative comparison of the energy
barrier. NMR analysis of 1-i−viii clearly showed that the
amides were divided into three groups. The first group is 1-viii,
1
which gave well-resolved H and 13C NMR spectra assignable
to the dipole resonance form B at room temperature. In
E
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contrast, 1-v−vii are in a second group, which afforded 1H and
13C resonances corresponding to the neutral resonance form A
above room temperature. The remaining four amides, 1-i−iv,
belong to a third group that exhibited 1H and 13C NMR spectra
with extensive broadening of signals due to the NAr2 group at
room temperature. The spectra changed by cooling or heating,
as shown in Figure S-1(a) in the Supporting Information as a
took place for the α,α-diphenyl analogue under the same
conditions.
It is necessary to gain additional mechanistic insight into the
catalytic cycles proposed for the hydrosilylation of carbonyl
compounds.23 The Chalk−Harrod mechanism was proposed
for the platinum-catalyzed hydrosilylation of alkenes in the
earlier stages of the hydrosilylation studies, and later a modified
Chalk−Harrod cycle was discussed both experimentally and
theoretically.24 The hydrosilylation of carbonyl compounds has
been considered to proceed through the modified Chalk−
Harrod cycle as proposed by Ojima and Kogure in 1975.25 In a
recent review on the rhodium-catalyzed hydrosilylation of
ketones, several other possible mechanisms were described for
the reaction with di- or trihydrosilanes;23a however, these are
not suitable for the present hydrosilylation with TMDS, in
which the silicon center is trisubstituted. When Ojima’s
mechanism shown in Scheme 4a is adapted to the iridium-
catalyzed hydrosilylation of amides to silylhemiaminals, the
initial step is oxidative addition of a Si−H bond to a transition-
metal center to form the H−Ir−Si moiety X, as shown in
Scheme 4b. A CO bond in the amide is inserted in the
resulting M−Si bond to give the intermediate Y. Reductive
elimination of a silyl ether from Y gives the silylhemiaminal. It
is important that the Si−Ir bond of X is polarized as Irδ− and
Siδ+ due to the difference in electronegativity between Si and Ir.
In an extreme case, the R3Si−Ir−H species is dissociated to
form R3Si+ and [Ir−H]−. The negative charge of the [Ir−H]−
species is mitigated by electron-withdrawing ligands such as
P(NC4H4)3, P(OC6F5)3, and P{O(CHCF3)2}3, which are
bound to the iridium phosphorus center. This increases the
thermodynamic stability of [Ir−H]− and enhances the
electrophilicity of R3Si+.
1
typical example. Variable-temperature H NMR of 1-i in THF-
d8 showed two sets of signals at −40 °C due to the NPh2 group,
which correspond to the dipole resonance form B. The peaks
began to broaden at −20 °C and coalesced at 25 °C. Broad
signals assignable to the neutral resonance form A were
observed at 40 °C and became sharp at 65 °C. Similar
temperature-dependent phenomena were seen in the 13C NMR
spectra of 1-i (Figure S-1(b)). These clearly demonstrate that
the rotational barrier of the C(O)−N bond decreased in the
order 1-viii > 1-i−iv > 1-v−vii. This is consistent with the order
of νCO described above, suggesting that the contribution of the
two rotamers was dependent on the substituents on the amide
nitrogen atom.
The relationship of the reactivity of the amides to the
catalytic activity of the iridium complexes is primarily correlated
to that of the νCO(Ir) and νCO(amide) stretches, as shown in
Figure 2. A higher νCO(Ir) value corresponds to a lower
On the other hand, in the reaction of the two resonance
forms A and B of amides with the intermediate X, the dipole
resonance form B, having a negative charge on the oxygen
atom, should react with the highly electrophilic silicon atom in
the R3Si−Ir−H species X with ease. This explains the higher
reactivity of N,N-dialkylamides such as 1-viii in the present
iridium-catalyzed reactions; the reaction was achieved even with
Vaska’s complex 4f. In contrast, the contribution of B is smaller
in the amides 1-i−vii; this made the reaction catalyzed by 4f
difficult. In sharp contrast, when the electrophilicity of the
silicon atom in X is enhanced by the electron-drawing
phosphorus ligands, the less reactive neutral resonance form
A of 1-i−vii is able to react with R3Si−Ir−H.
The higher contribution of neutral resonance form A in
comparison to that of dipole resonance form B in 1-i−vii is due
to delocalization of lone-pair electrons of the nitrogen atom
over conjugated π systems in the adjacent N-aryl substituents.
This also explains why the reaction of 1-viii directly afforded 2-
viii but those of 1-i−vii were terminated after the
silylhemiaminals 3-i−vii were formed. As shown in Scheme 4,
elimination of a silanol anion assisted by heat or acid from the
silylhemiaminal 3 forms iminium cation Z. Deprotonation of Z
takes place with ease to form the enamine 2. In other words,
the E1 elimination of silanol is preferable to the E2 process.
Elimination of silanol by the E1 process does not occur in a
stereospecific manner; this explains the formation of both E and
Z isomers of 2 from the same diastereomeric configuration of 3.
Figure 2. Catalytic activity of 4: correlation with νCO(amide) and
barrier of amide rotation vs νCO(Ir).
electron density of the iridium center. In contrast, a higher
νCO(amide) value results from a lower contribution of dipole
resonance form B. The results revealed that the amides with a
lower contribution of dipole resonance form B (νCO(amide)
>1650 cm−1) were more difficult to hydrosilylate, but the
reaction was achieved by a catalyst a more electron deficient
iridium center (νCO(Ir) >2000 cm−1), which showed higher
activity for the amide having a lower contribution of the
resonance form B (νCO(amide) >1650 cm−1). A typical
example is the reaction of 1-vii (νCO(amide) 1697 cm−1)
catalyzed by 4i (νCO(Ir) 2052 cm−1), which is in sharp contrast
to the facile hydrosilylation of 1-viii (νCO(amide) 1632 cm−1)
by 4f (νCO(Ir) 1967 cm−1). It should also be noted that steric
hindrance around the carbonyl group of the amides is an
additional factor in the reactivity of the amides. For example,
longer reaction times were needed to perform the hydro-
silylation of α,α-diphenylacetamides 1-ii,iv,vi in comparison
with that of α-methyl-α-phenylacetamides 1-i,iii,v, respectively,
having the same N-aryl group. The reaction of 1-vii having one
α-phenyl substituent was achieved by 4i. However, no reaction
CONCLUSION
As reported previously, transformation of N,N-dialkylamides to
aldenamines by treatment with TMDS is efficiently catalyzed by
■
F
DOI: 10.1021/acs.organomet.5b00636
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Article
Scheme 4. Modified Chalk−Harrod Cycle25 Proposed by Ojima and a Possible Mechanism for the Hydrosilylation of Amides
and Conversion of Silylhemiaminals to Enamines
Vaska’s complex 4f. However, N,N-diarylamides 1-i−vii, which
are precursors of the π-conjugated enamines 2-i−vii, hardly
reacted with TMDS under the same conditions. A new
discovery presented in this paper is that IrCl(CO)L2 complexes
bearing electron-withdrawing fluorinated phosphites as L such
as 4i,j are effective catalysts for the preparation of 2-i−vii. In
contrast to N,N-dialkylamides such as 1-viii that directly afford
the corresponding aldenamines, the reaction of N,N-diary-
lamides with TMDS gives silylhemiaminals 3 as the primary
products, which are converted to π-conjugated enamines by
subsequent treatment with heat or acid. The new catalyst 4i
showed extremely high catalytic activity for both the one-step
conversion of N,N-dialkylamides 1-viii to 2-viii and the two-
step procedure to form 2-iii from N,N-diarylamide 1-iii. The
TON of the formation of 2-viii exceeded 105, which was
accomplished at room temperature within 30 min. A TON of
104 was achieved at 40 °C when 1-iii was the substrate. In both
cases, the level of residual iridium in the product was below the
detection limit (20 ppb) of ICP-MS analysis. The remarkable
catalytic activity of 4i,j is ascribed to the generation of an
electron-deficient iridium center in the catalytic cycle, which
was achieved by the ligand effect of electron-withdrawing
fluorinated phosphites. The electron-deficient iridium species
activates a Si−H bond to form the oxidative adduct including
an electrophilic silicon atom, which is capable of reacting with
the neutral resonance form of the N,N-diarylamides. Since the
residual metal has to be minimized when transition-metal-
catalyzed reactions are applied to the preparation of electronic
materials, the present method will be useful for synthesis of π-
conjugated enamines as hole-transport materials. Good donor
properties of metal-free π-conjugated enamines could open
further applications of these compounds to control photo and
electron functionality.
recent account.16h It is also important to investigate further
insights into the catalytic intermediates and transition states
involved in the catalytic cycle. We referred to discussion of the
Chalk−Harrod and modified Chalk−Harrod mechanisms in a
previous section. Other than these, an alternative outer-sphere
route where the amide directly attacks the silicon atom of the
oxidative adduct, H−M−Si, was proposed in the rhodium-
catalyzed hydrosilylation of ketones with Ph2SiH2.23a The
recent development of hydrosilylation catalyzed by cationic
iridium or ruthenium complexes suggested ionic outer-sphere
mechanisms,26 which were also summarized as a recent
review.23b The iridium complexes used in this study are neutral
and not cationic; however, the possible involvement of these
outer-sphere mechanisms should not be excluded in further
detailed studies. We are currently investigating these mecha-
nistic aspects by isolation of the catalytic intermediates and
DFT calculations.
EXPERIMENTAL SECTION
■
General Procedure. All reactions were carried out under an argon
atmosphere using standard Schlenk techniques or were performed in a
N2-filled glovebox. 1H, 13C, 19F, 29Si, and 31P NMR spectra were
measured on JEOL ECA 400 (396 MHz) and ECA600 (600 MHz)
1
spectrometers. Chemical shifts for H and 13C were given in parts per
million relative to the solvent signal. Chemical shifts for 19F, 31P, and
29Si are given in parts per million downfield from hexafluorobenzene
(δF −163.6), phosphoric acid (δP 0), and tetramethylsilane (δSi 0) as
external standards, respectively. IR spectra were measured on a JASCO
FT/IR 4200 spectrometer. Elemental analyses were measured on a
PerkinElmer 2400II/CHN analyzer. HR-MS analyses, ICP-MS
analyses, and X-ray diffraction analyses were performed at the
Analytical Center of the Institute for Materials Chemistry and
Engineering, Kyushu University. Dehydrated solvents (toluene, 1,4-
dioxane, THF, Et2O, n-pentane) were purchased from Kanto Chemical
Co. Ltd. and used as received. N,N-Diethyl-2-phenylacetylamide (1-
viii), 1,1,3,3- tetramethyldisiloxane (TMDS), and P2O5 were
purchased from Wako Pure Chemical Industries, AZmax Co. Ltd.,
and Merck, respectively. The phosphorus ligands PCy3, PEt3, PMe2Ph,
PMePh2, P(o-tolyl)3, PPh3, and P(OPh)3 were purchased from Kanto
Chemical Co. Ltd. The syntheses of the three phosphorus compounds
tris(N-pyrrolyl)phosphine (P(NC4H4)3),21a tris(1,1,1,3,3,3-hexafluoro-
2-propyl) phosphite (P{OCH(CF3)2}3),21b,c and tris(2,3,4,5,6-penta-
fluorophenyl) phosphite (P(OC6F5)3)21d have been reported in the
Elucidation of the mechanism awaits further investigation.
One of the questions that has remained unsolved is why only
TMDS is useful for the conversion from an amide to a
silylhemiaminal, while no reaction took place with other
hydrosilanes such as Me3SiOSiHMe2, PhMe2SiH, Ph2SiH2, and
PhSiH3. Similar results where use of hydrosilanes bearing two
closely located Si−H groups is crucially important are seen in
several metal-catalyzed hydrosilylation, as summarized in our
G
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Article
27
literature. Ir(COD)Cl}2 and 4a−g were prepared according to
tion from ethanol afforded the desired amide. The preparation of 1-vii
was performed according to the literature method.29b From carbazole
(2.17 g, 13.0 mmol) and phenylacetyl chloride (1.54 g, 10 mmol), 1-vii
was obtained as a white solid (2.35 g, 8.24 mmol, 82%).
Although amides 1-i,vii are known compounds, NMR data reported
in the literature were ambiguous due to the amide rotation on the
NMR time scale or contamination by impurities.30,31 We performed
variable-temperature NMR studies for 1-i−vi in several different
solvents. Assignments of NMR data were performed with the aid of
1H−1H COSY experiments (protons are labeled as shown in Chart 4).
known methods.20,28
Preparation of Ir{P(NC4H4)3}2(CO)Cl (4h), Ir{P(OC6F5)3}2(CO)Cl
(4i), and Ir[P{CH(CF3)2}3]2(CO)Cl (4j). These iridium complexes were
synthesized from {Ir(COD)Cl}2 and the corresponding phosphorus
ligands. In a typical example, {Ir(COD)Cl}2 (133.5 mg, 0.20 mmol),
tris(N-pyrrolyl)phosphine (192.0 mg, 0.64 mmol), and THF (5 mL)
were placed in a 50 mL Schlenk flask at −78 °C, the solution was
degassed by three freeze−pump−thaw cycles, and the atmosphere was
replaced by carbon monoxide. The solution was stirred for 1 h at −78
°C and for an additional 12 h at ambient temperature. The solution
turned from orange to light yellow. The solution was filtered through a
short pad of Celite, and the solvent was then removed under reduced
pressure. The desired complex 4h was obtained as a yellow solid (307
mg, 0.39 mmol, 96%). 1H NMR (600 MHz, CDCl3, room
temperature): δ 6.42 (br, 12H, NC4H4), 7.05 (br, 12H, NC4H4).
13C{1H} NMR (151 MHz, CDCl3, room temperature): δ 114.1
Chart 4. Labels for Identification of Amide 1
(NC4H4), 124.5 (NC4H4), 166.4 (CO). 31P{1H} NMR (243 MHz,
CDCl3, room temperature): δ 79.5 (P(NC4H4)3). IR (KBr, cm−1):
2011 (νCO). Dec pt: 152 °C. Anal. Calcd for C25H24N6OP2ClIr: C,
42.05, H, 3.39, N, 11.77. Found: C, 42.15, H, 3.41, N, 11.58.
In a similar fashion, 4i was synthesized from {Ir(COD)Cl}2 (100.0
mg, 0.15 mmol) and tris(2,3,4,5,6-pentafluorophenyl) phosphite
(345.0 mg, 0.60 mmol). The crude compound was recrystallized
from THF and pentane to afford 4i as a yellow solid (64.0 mg, 0.05
mmol, 30%). 13C{19F} NMR (151 MHz, CDCl3, room temperature):
δ 125.0 (ipso-C6F5), 138.1 (m-C6F5), 140.0 (p-C6F5), 141.0 (o-C6F5),
165.1 (CO). 19F NMR (565 MHz, CDCl3, room temperature): δ −
3
3
161.7 (dd, JF−F = 20.7 Hz, m-C6F5), − 156.7 (t, JF−F = 20.7 Hz, p-
C6F5), − 151.8 (d, JF−F = 20.7 Hz, o-C6F5). 31P{19F} NMR (243
MHz, CDCl3, room temperature): δ 108.0 (P(OC6F5)3). IR (KBr,
cm−1): 2052 (νCO). Dec pt: 176 °C. Anal. Calcd for C37O7F30P2ClIr:
C, 31.39, H, 0.00. Found: C, 31.56, H, 0.12.
3
The preparation of 4j was performed by treatment of {Ir(COD)-
Cl}2 (100.0 mg, 0.15 mmol) and tris(1,1,1,3,3,3-hexafluoro-2-propyl)
phosphite (317.0 mg, 0.60 mmol) in a manner similar to that above.
The crude product was recrystallized from THF and pentane to afford
4j as a yellow solid (31.0 mg, 0.02 mmol, 16%). 1H NMR (600 MHz,
CDCl3, room temperature): δ 5.64 (br, 6H, −CH(CF3)2). 13C{19F}
1
NMR (151 MHz, CDCl3, room temperature): δ 71.7 (d, JC−H = 151
In this Experimental Section, the data measured at the temperature
whiched afford the best-resolved resonances are described, and other
data are summarized in the Supporting Information.
Hz, −CH(CF3)2), 119.6 (s, −CH(CF3)2), 162.6 (s, CO). 19F NMR
(565 MHz, CDCl3, room temperature): δ − 73.38 (s, −CH(CF3)2).
31P{1H} NMR (243 MHz, CDCl3, room temperature): δ 113.20
(P{OCH(CF3)2}3). IR (KBr, cm−1): 2059 (νCO). Dec pt: 184 °C.
Anal. Calcd for C19H6O7F36P2ClIr: C, 17.29, H, 0.46. Found: C, 17.55
H, 0.39.
Preparation and Characterization of Carboxamides. The
following procedure, reported for the preparation of 1-viii,13 was
adopted to synthesize the five carboxamides 1-i,ii,iv−vi. In a 500 mL of
three-necked flask, amine (13 mmol) and pyridine (26 mmol) were
dissolved in CH2Cl2 (10 mL). A solution of acyl chloride (10 mmol)
in CH2Cl2 (10 mL) was then added dropwise at −78 °C. The solution
was then stirred at ambient temperature for 8 h. The reaction was
quenched with 1 M aqueous HCl, and the organic layer was separated,
washed with saturated aqueous NaHCO3, and dried over MgSO4.
After filtration, the solution was concentrated in vacuo. The residue
was purified by chromatography on silica gel (hexane/EtOAc 10/1).
Further purification by recrystallization from ethanol afforded the
desired amide.
The amide 1-iii was prepared by a procedure similar to that
reported for the preparation of PhCH2(CO)NPh2.29a To a solution
of 1-naphthylamine (1.75 g, 8.0 mmol) in 1,4-dioxane (20 mL) was
added a solution of 2-phenylpropyonyl chloride (1.68 g, 10.0 mmol) in
1,4-dioxane (10 mL) dropwise at ambient temperature in a 300 mL
three-necked flask. The solution was then heated under reflux for 8 h.
The reaction mixture was cooled and poured into cool water. The
white precipitates that formed were collected, washed with hexane (5
mL × 5), and dried under vacuum to afford 1-iii (1.56 g, 4.43 mmol,
44%) as a white solid. The residue was purified by chromatography on
silica gel (hexane/EtOAc 10/1). Further purification by recrystalliza-
N,N-Diphenyl-2-phenylpropionamide (1-i).30 From diphenyl-
amine (2.20 g, 13.0 mmol) and 2-phenylpropyonyl chloride (1.68 g,
10.0 mmol), 1-i was obtained as a white solid (2.02 g, 6.72 mmol,
67%). Measurement at −40 °C provided spectra due to the dipole
resonance form B. 1H NMR (600 MHz, THF-d8, −40 °C): δ 1.37 (d,
3
3JH−H = 6.8 Hz, 3H, −CHMePh), 3.83 (q, JH−H = 6.8 Hz, 1H,
−CHMePh), 7.02 (d, 3JH−3H = 7.2 Hz, 2H, o-Ph1), 7.08 (t, 3JH−H = 7.2
3
Hz, 1H, p-PhN), 7.16 (d, JH−H = 7.6 Hz, 2H, o-PhN), 7.17 (t, JH−H
=
7.2 Hz, 1H, p-Ph1), 7.21 (dd, JH−H = 7.6, 7.6 Hz, 2H, m-Ph1), 7.23
3
(dd, JH−H = 7.2 Hz, 2H, m-PhN), 7.33 (br-dd, 1H, p-PhN′); 7.35 (br,
3
2H, m-PhN′); 6.7−7.8 (br, 2H, o-PhN′). 13C{1H} NMR (151 MHz,
THF-d8, −40 °C): δ 20.9, 44.8, 126.0, 127.2, 127.3, 128.2, 128.4,
129.0, 129.1, 130.2, 130.3, 142.9, 143.7, 144.3, 173.2. Measurement at
1
+65 °C provided spectra due to the neutral resonance form A. H
3
NMR (600 MHz, THF-d8, 65 °C): δ 1.38 (d, JH−H = 6.8 Hz, 3H,
−CHMePh), 3.81 (q, 3JH−H = 6.8 Hz, 1H, −CHMePh), 7.04 (d, 3JH−H
= 7.2 Hz, 2H, o-Ph1), 7.11 (d, JH−H = 7.2 Hz, 4H, o-NPh2), 7.14 (t,
3
3JH−H = 7.2 Hz, 1H, p-Ph1), 7.27 (dd, 3JH−H = 7.2 Hz, 2H, m-Ph1), 7.28
3
3
(t, JH−H = 7.2 Hz, 2H, p-NPh2), 7.35 (dd, JH−H = 7.2 Hz, 4H, m-
NPh2). 13C{1H} NMR (151 MHz, THF-d8, 65 °C): δ 20.7, 45.1, 127.0
(br), 127.1, 128.3, 128.7 (br), 128.9, 129.5 (br), 143.2, 144.4, 173.5. IR
(ATR, cm−1): 1659 (νCO). Mp: 118−119 °C. HRMS-EI(+) (m/z):
[M]+ calcd for C21H19NO, 301.1467; found, 301.1468. Anal. Calcd for
C21H19NO: C, 83.69, H, 6.35, N, 4.65. Found: C, 83.51, H, 6.38, N,
4.46.
N,N-Diphenyl-2,2-diphenylacetamide (1-ii). From diphenylamine
(2.20 g, 13.0 mmol) and diphenylacetyl chloride (2.31 g, 10.0 mmol),
1
1-ii was obtained as a white solid (2.28 g, 6.28 mmol, 63%). The H
H
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NMR spectrum at 70 °C gave well-resolved signals due to the neutral
C21H17NO2, 315.1259; found, 315.1273. Anal. Calcd for C21H17NO2:
C, 79.98, H, 5.43, N, 4.44. Found: C, 79.72, H, 5.54, N, 4.52.
N-Phenoxazinyl-2,2-diphenylacetamide (1-vi). From phenoxazine
(2.38 g, 13.0 mmol) and diphenylacetyl chloride (2.31 g, 10.0 mmol),
1-vi was obtained as a dark green solid (3.01 g, 7.98 mmol, 80%).
Measurement from room temperature to +70 °C provided spectra due
1
resonance form A. H NMR (600 MHz, CD3CN, 70 °C): δ 5.20 (s,
3
3
1H, −CHPh2), 7.23 (d, JH−H = 7.2 Hz, 4H, o-Ph1), 7.24 (d, JH−H
=
3
7.2 Hz, 4H, o-NPh2), 7.27 (t, JH−H = 7.2 Hz, 2H, p-NPh2), 7.31 (t,
3JH−H = 7.2 Hz, 2H, p-Ph1), 7.31 (dd, 3JH−H = 7.2, 7.2 Hz, 4H, m-Ph1),
7.37 (dd, JH−H = 7.2, 7.2 Hz, 4H, m-NPh2). 13C{1H} NMR (151
3
1
to the neutral resonance form A. H NMR (600 MHz, CD3CN, 70
MHz, THF-d8, −40 °C): 55.7, 126.3, 127.3, 127.5, 128.8, 128.9, 129.1,
129.7, 130.1, 130.5, 140.9, 143.96, 144.02, 171.2. IR (ATR, cm−1):
1672 (νCO). Mp: 83−84 °C. HRMS-EI(+) (m/z): [M]+ calcd for
C26H21NO, 363.1623; found, 363.1620. Anal. Calcd for C26H21NO: C,
85.92, H, 5.82, N, 3.85. Found: C, 86.10, H, 5.93, N, 3.86.
°C): δ 5.80 (s, 1H, −CHPh2), 7.07 (d, 3JH−H = 8.2 Hz, 2H, Hd), 7.17
(d, 3JH−H = 7.2 Hz, 2H, o-Ph), 7.19 (dd, 3JH−H = 7.2, 7.2 Hz, 2H, Hb),
7.24 (dd, 3JH−H = 8.2, 7.2 Hz, 2H, Hc), 7.25 (t, 3JH−H = 6.8 Hz, 2H, p-
Ph), 7.28 (dd, 3JH−H = 6.8 Hz, 4H, m-Ph), 7.59 (d, 3JH−H = 8.2 Hz, 2H,
Ha). 13C{1H} NMR (151 MHz, CD3CN, 70 °C): δ 55.5, 118.0, 124.9,
126.9, 128.4, 128.7, 129.7, 130.2, 131.1, 140.6, 152.9, 172.9. IR (ATR,
cm−1): 1684 (νCO). Mp: 125−127 °C. HRMS-EI(+) (m/z): [M]+
calcd for C26H19NO2, 377.1416; found, 377.1409. Anal. Calcd for
C26H19NO2: C, 82.74, H, 5.07, N, 3.71. Found: C, 82.86, H, 4.98, N,
3.63.
N-(1-Naphthyl)-N-phenyl-2-phenylpropionamide (1-iii). Measure-
ment at −20 °C provided spectra due to the dipole resonance form B.
1
A mixture of four rotamers was observed in a ratio of 54:26:13:7. H
NMR (600 MHz, CDCl3, −20 °C): δ 1.38, 1.47, 1.55, and 1.59
3
(54:26:13:7, d each, JH−H = 6.8 Hz, 3H in total, −CHMePh), 3.38,
3
3.72, 4.21, and 4.08 (54:26:13:7, q each, JH−H = 6.8 Hz, 1H in total,
N-Carbazolyl-2-phenylacetamide (1-vii).31 Measurement at room
temperature provided spectra due to the neutral resonance form A. 1H
NMR (600 MHz, CDCl3, room temperature): δ 4.51 (s, 2H,
−CHMePh), 6.74−7.54 (a mixture of signals, 15H in total, Ph1, PhN
and Naphthyl), 7.98, 7.84, 7.75, and 7.78 (54:26:13:7, d each, 3JH−H
=
7.6 Hz, 1H in total, Hd), 8.14, 7.95, 7.82, and 8.10 (54:26:13:7, d each,
3JH−H = 7.6 Hz, 1H in total, Hg). 13C{1H} NMR (151 MHz, CDCl3,
−20 °C): 20.9, 20.2, 19.4, and 21.7 (54:26:13:7, −CHMePh), 45.0,
44.4, 43.6, and 43.7 (54:26:13:7, -CHMePh), 125.3−143.2 (13C
resonances due to Ph1, PhN and Naphthyl), 174.7, 175.1, 174.6, and
174.9 (54:26:13:7, CO). IR (ATR, cm−1): 1667 (νCO). Mp: 95−96
°C. HRMS−EI(+) (m/z): [M]+ calcd for C25H21NO, 351.1623;
found, 351.1618.
3
3
−CH2Ph), 7.34 (t, JH−H = 6.8 Hz, 1H, p-Ph), 7.36 (d, JH−H = 7.6
3
Hz, 2H, o-Ph), 7.41 (dd, JH−H = 6.8,7.6 Hz, 2H, m-Ph), 7.42 (dd,
3JH−H = 8.2, 7.6 Hz, 2H, Hc), 7.48 (dd, JH−H = 8.9, 7.6 Hz, 2H, Hb),
3
8.02 (d, JH−H = 8.2 Hz, 2H, Hd), 8.26 (d, JH−H = 8.9 Hz, 2H, Ha).
3
3
13C{1H} NMR (151 MHz, CDCl3, room temperature): δ 45.5, 116.7,
120.0, 124.0, 126.7, 127.5, 127.6, 129.0, 129.7, 133.7, 138.7, 171.3. IR
(ATR, cm−1): 1697 (νCO); Mp: 113−114 °C (lit. mp 123−124 °C).
HRMS-EI(+) (m/z): [M]+ calcd for C20H15NO, 285.1154; found
285.1150. Anal. Calcd for C20H15NO: C, 84.19, H, 5.30, N, 4.91.
Found: C, 84.21, H, 5.31, N, 4.80.
N-(1-Naphthyl)-N-phenyl-2,2-diphenylacetamide (1-iv). From 1-
naphthylphenylamine (2.85 g, 13.0 mmol) and diphenylacetyl chloride
(2.31 g, 10.0 mmol), 1-iv was obtained as a white solid (2.88 g, 6.96
mmol, 70%). Measurement at −20 °C provided spectra due to the
dipole resonance form B. A mixture of two rotamers was observed in a
ratio of 86:14 at −20 °C. Data for the major rotamer are as follows. 1H
NMR (600 MHz, CDCl3, −20 °C): δ 4.76 (s, 1H, −CHPh2), 7.03 (t,
Screening of Iridium Catalysts for the Reduction of 1-viii
with TMDS (Table 2). In an NMR tube equipped with a J. Young
valve were placed N,N-diethyl-2-phenylacetamide (1-viii; 190 μL, 1.00
mmol) and an iridium catalyst selected from 4a−j (0.005 mmol, 0.5
mol %). Hexamethylbenzene was also added (0.05 mmol) as an
internal standard. Then 1,1,3,3-tetramethyldisiloxane (2.00 mmol) and
3
3JH−H = 7.2 Hz, 2H, Ph1 or PhN), 7.08 (d, JH−H = 7.6 Hz, 1H, Ha),
3
7.12 (t, JH−H = 7.2 Hz, 1H, Ph1 or PhN), 7.14−7.22 (a mixture of
1
C6D6 (0.2 mL) were added, and the reaction was monitored by H
signals, 5H in total, Ph1 or PhN), 7.24−7.42 (a mixture of signals, 10H
NMR spectroscopy. During the reaction, generation of H2 gas was
observed. The conversion of 1-viii and yield of 2-viii were calculated
by integral ratios on the basis of the internal standard. The results are
summarized in Table 2. Since no byproduct was formed, the
conversion of 1-viii matched the yield of 2-viii. The exception was
the reaction with 4j, giving a mixture of 2-viii (45%) and an amine,
PhCH2CH2NEt2 (55%). Only the trans isomer was formed for 2-viii.
Screening of Iridium Catalysts for the Reduction of 1-i and
1-ii with TMDS Catalyzed by Selected Iridium Complexes 4f−j
(Table 3). In a 50 mL two-necked flask were placed N,N-diphenyl-2-
phenylpropionamide (1-i; 301 mg, 1.00 mmol) and one of the iridium
catalysts (0.005 mmol, 0.5 mol %) selected from 4f−j. Then 1,1,3,3-
tetramethyldisiloxane (2.00 mmol) and toluene (4.0 mL) were added,
and the mixture was stirred at ambient temperature for 30 min. After
the reaction, anisole (1.00 mmol) was added as an internal standard.
The sample was subjected to 1H NMR analysis, and the conversion of
1-i and yield of silylhemiaminal 3-i were determined by comparison of
the integral ratios relative to that of the internal standard, as shown in
Table 3. The silylhemiaminal 3-i was obtained as a mixture of two
diastereomers (vide infra). The solvent and unreacted TMDS in the
solution were removed under reduced pressure, and the residue was
dissolved in toluene (15 mL). Conversion of 3-i to 2-i was performed
by method A. The resulting solution of 3-i was heated to 100 °C for 6
h. The solvent was then removed under reduced pressure, and anisole
(1.00 mmol) was again added as an internal standard. The sample was
3
in total, Ph1 or PhN), 7.39 (dd, JH−H = 7.6 Hz, 1H, Hb), 7.53 (dd,
3
3JH−H = 7.6, 7.6 Hz, 1H, Hf), 7.57 (d, JH−H = 7.6, 7.6 Hz, 1H, He),
3
3
7.93 (d, JH−H = 7.6 Hz, 1H, Hc), 7.96 (d, JH−H = 7.6 Hz, 1H, Hd),
3
8.02 (d, JH−H = 7.6 Hz, 1H, Hg). 13C{1H} NMR (151 MHz, CDCl3,
−20 °C): 56.2, 122.9, 125.2, 125.6, 125.9, 126.9, 127.0, 127.2, 127.8,
128.2, 128.3, 128.6, 128.6, 128.7, 128.7, 129.0, 129.4, 130.9, 134.7,
137.7, 139.0, 139.5, 142.0, 172.6. 1H and 13C signals due to the minor
1
rotamer were visible in the same spectra. H NMR of minor rotamer
(600 MHz, CDCl3, −20 °C): δ 5.42 (s, 1H, −CHPh2), 7.00−7.60 (a
mixture of signals, 19H in total, Ph1, PhN and Naphthyl), 7.71 (d, 3JH−H
= 7.6 Hz, 1H, Hc), 7.78 (d, 3JH−H = 7.6 Hz, 1H, Hd), 7.85 (d, 3JH−H
=
7.6 Hz, 1H, Hg). 13C{1H} NMR (151 MHz, CDCl3, −20 °C): 54.6,
122.7, 125.8, 126.0, 126.3, 127.1, 127.2, 127.4, 127.7, 128.1, 128.6,
128.8, 128.9, 129.1, 129.8, 129.9, 134.4, 139.4, 139.5, 139.6, 143.2,
172.4. The other two peaks were unable to be observed due to overlap
with the major rotamer. IR (ATR, cm−1): 1667 (νCO). Mp: 124−125
°C. HRMS-EI(+) (m/z): [M]+ calcd for C30H23NO, 413.1780; found,
413.1778.
N-Phenoxazinyl-2-phenylpropionamide (1-v). From phenoxazine
(2.38 g, 13.0 mmol) and 2-phenylpropyonyl chloride (1.68 g, 10.0
mmol), 1-v was obtained as a dark green solid (1.74 g, 5.51 mmol,
55%). Measurement at 50 °C provided spectra due to the neutral
1
resonance form A. H NMR (600 MHz, CD3CN, 50 °C): δ 1.42 (d,
3
3JH−H = 6.8 Hz, 3H, −CHMePh), 4.48 (q, JH−H = 6.8 Hz, 1H,
1
subjected to H NMR analysis, and the yield of enamine 2-i were
−CHMePh), 6.97 (d, 3JH−H = 7.2 Hz, 2H, o-Ph), 7.02 (d, 3JH−H = 7.6
determined by comparison of the integral ratios relative to that of the
internal standard, as shown in Table 3.
Hz, 2H, Hd), 7.17 (dd, 3JH−H = 7.6 Hz, 2H, Hb), 7.18 (dd, 3JH−H = 7.2
3
3
Hz, 2H, m-Ph), 7.19 (t, JH−H = 6.8 Hz, 1H, p-Ph), 7.21 (dd, JH−H
=
In a similar fashion, screening of the iridium catalyst was performed
for the reaction of 1-ii with TMDS. After the conversion of 1-ii and
the yield of silylhemiaminal 3-ii were determined by 1H NMR,
volatiles were removed under reduced pressure. Conversion of 3-ii to
2-ii was performed by method B; toluene (10 mL) and 1 M HCl in
8.2, 7.6 Hz, 2H, Hc), 7.54 (d, 3JH−H = 8.2 Hz, 2H, Ha). 13C{1H} NMR
(151 MHz, CD3CN, 50 °C): δ 20.6, 43.8, 117.7, 124.7, 126.8, 127.9,
128.3, 128.3, 129.7, 130.9, 142.3, 152.6, 174.7. IR (ATR, cm−1): 1680
(νCO). Mp: 139−140 °C. HRMS-EI(+) (m/z): [M]+ calcd for
I
DOI: 10.1021/acs.organomet.5b00636
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Et2O (5 mL) were placed in the flask. The solution was stirred for 30
min at ambient temperature. The solvent was then removed under
reduced pressure, and anisole (1.00 mmol) was again added as an
internal standard. The sample was subjected to 1H NMR analysis, and
the yield of enamine 2-ii was determined by comparison of the integral
ratio relative to that of the internal standard, as shown in Table 3.
Isolation of 2-i−vii by Reactions Catalyzed by 4i. The general
procedure is as follows: in a 20 mL two-necked found flask were
placed carboxamides 1-i−vii (1.00 mmol) and the iridium catalyst 4i
(7.1 mg, 0.005 mmol, 0.5 mol %). Then 1,1,3,3-tetramethyldisiloxane
(2.00 mmol) and toluene (4.0 mL) were added, and the mixture was
stirred at ambient temperature for 2 h. The reaction of 1-iii required
16 h to complete, whereas that of 1-v needed 21 h. After the reaction
was complete, the 1H NMR spectrum of the crude sample was
measured in the presence of 1.00 mmol of anisole as an internal
standard. The conversions of 1-i−vii and the yields of 3-i−vii were
estimated as shown in Table 4. Some silylhemiaminals, 3-i,iii,v, were
obtained as mixtures of two diastereomers (3-i, 96:4; 3-iii, 95:5; 3-v,
87:13). The resulting silylhemiaminals 3-i−vii were converted to the
enamines 2-i−vii by the method indicated in Table 4. The solvent was
removed under a reduced pressure, and the resulting residue was
purified by chromatography on silica gel (eluents hexane and toluene)
to afford the corresponding enamines 2-i−vii. Assignments of NMR
data were performed with the aid of 1H−1H COSY experiments
(protons are labeled as shown in Chart 5). Although 2-i−vii have all
2-i (600 MHz, CDCl3, rt): δ 2.14 (s, 3H, −CHCMePh), 6.30 (s,
3
1H, −CHCMePh), 6.87 (t, JH−H = 7.6 Hz, 2H, p-PhN), 7.01 (d,
3JH−H = 7.6 Hz, 4H, o-PhN), 7.11 (t, JH−H = 7.6 Hz, 1H, p-Ph1), 7.13
3
3
(dd, JH−H = 7.6 Hz, 4H, m-PhN). The other signals overlapped with
those of the major E isomer. IR (ATR, cm−1): 1627, 1583 (νCCN).
Mp: 92−93 °C. HRMS-EI(+) (m/z): [M]+ calcd for C21H19N,
285.1517; found, 285.1511.
N-(2,2-Diphenylvinyl)-N,N-diphenylamine (2-ii).33 From 1-ii
(363.2 mg, 1.00 mmol), 2-ii was obtained as a white solid (298.6
mg, 0.86 mmol, 86%), though 2-ii was reported as a yellow oil in the
literature.32 Conversion of 3-ii to 2-ii was achieved by treatment of a
toluene solution of 3-ii with 1 M HCl in Et2O (5 mL) at ambient
1
temperature for 30 min (method B). H NMR (600 MHz, CDCl3,
room temperature): δ 6.72 (s, 1H, −CHCPh2), 6.87 (t, 3JH−H = 7.2
Hz, 2H, p-PhN), 6.92−6.96 (a mixture of signals, 6H, Ph1 or Ph1′,
overlapped with o-PhN), 6.97 (d, 3JH−H = 7.2 Hz, 4H, o-PhN), 7.08 (dd,
3JH−H = 7.2, 7.2 Hz, 4H, m-PhN), 7.20−7.28 (a mixture of signals, 4H,
Ph1 or Ph1′, overlapped with a residual proton of CDCl3). The signals
1
were assigned by H−1H COSY. 13C{1H} NMR (151 MHz, CDCl3,
room temperature): δ 122.6, 122.8, 126.4, 126.8, 127.6, 127.7, 128.3,
128.9, 130.2, 130.5, 131.4, 139.1, 142.2, 145.9. IR (ATR, cm−1): 1611,
1584, 1566 (νCCN). Mp: 153 °C. HRMS-FAB(+) (m/z): [M]+ calcd
for C26H21N, 347.1677; found, 347.1672.
N-(2-Phenyl-2-methylvinyl)-N-phenyl-N-(1-naphthyl) amine (2-
iii).4c From 1-iii (351.2 mg, 1.00 mmol), 2-iii was obtained as a 98:2
mixture of E and Z isomers (320.0 mg, 0.90 mmol, 90%). Conversion
of 3-iii to 2-iii was achieved by thermal treatment of 3-iii in toluene at
Chart 5. Labels for Identification of Enamine 2
1
80 °C for 6 h (method A). H NMR of (E)-2-iii (600 MHz, CDCl3,
3
room temperature): δ 1.59 (d, JH−H = 1.4 Hz, 3H, −CHCMePh),
3
3
6.80 (d, JH−H = 7.2 Hz, 2H, o-PhN), 6.89 (q, JH−H = 1.4 Hz, 1H,
−CHCMePh), 6.92 (t, 3JH−H = 7.2 Hz, 1H, p-PhN), 7.20 (dd, 3JH−H
= 7.2 Hz, 2H, m-PhN), 7.24 (t, JH−H = 7.2 Hz, 1H, p-Ph1), 7.34 (dd,
3
3JH−H = 7.2 Hz, 2H, m-Ph1), 7.39 (dd, JH−H = 7.2 Hz, 1H, He), 7.45
3
(d, JH−H = 7.2 Hz, 2H, o-Ph1, overlapped with Ha), 7.45 (d, JH−H
=
=
3
3
7.2 Hz, 1H, Ha), 7.48 (dd, 3JH−H = 7.2 Hz, 1H, Hf), 7.51 (dd, 3JH−H
7.2 Hz, 1H, Hb), 7.76 (d, 3JH−H = 7.2 Hz, 1H, Hc), 7.89 (d, 3JH−H = 7.2
Hz, 1H, Hd), 7.91 (d, 3JH−H = 7.2 Hz, 1H, Hg). 13C{1H} NMR of 2-iii
(151 MHz, CDCl3, room temperature): δ 16.0, 118.3, 120.9, 124.6,
124.7, 125.3, 125.7, 126.19, 126.24, 126.3, 126.4, 126.5, 128.5, 128.7,
1
129.1, 130.4, 132.9, 135.1, 142.3, 142.4, 148.9; H NMR of (Z)-2-iii
(600 MHz, CDCl3, room temperature): δ 2.09 (s, 3H, −CH
CMePh), 6.44 (s, 1H, −CHCMePh). The other signals were
overlapped with those of the major E isomer; IR (ATR, cm−1): 1627,
1589 (νCCN). Mp: 135 °C (lit. mp 135 °C determined by DSC).
HRMS-EI(+) (m/z): [M]+ calcd for C25H21N, 355.1674; found,
355.1675.
N-(2,2-Diphenylvinyl)-N-phenyl-N-(1-naphthyl)amine (2-iv).4c
From 1-iv (413.2 mg, 1.00 mmol), 2-iv was obtained as a white
solid (381.3 mg, 0.96 mmol, 96%). Conversion of 3-iv to 2-iv was
achieved by treatment of a toluene solution of 3-iv with 1 M HCl in
1
Et2O (5 mL) at ambient temperature for 30 min (method B). H
NMR (600 MHz, CDCl3, room temperature): δ 6.64−6.70 (a mixture
of signals, 4H, Ph1 or Ph1′), 6.65 (s, 1H, −CHCPh2, overlapped
with Ph), 6.86 (d, 3JH−H = 7.2 Hz, 2H, o-PhN), 6.92 (t, 3JH−H = 7.2 Hz,
2H, p-PhN), 7.12 (d, 3JH−H = 3.4 Hz, 3H, Ph1 or Ph1′) 7.17 (dd, 3JH−H
= 7.2 Hz, 2H, m-PhN), 6.64−6.70 (a mixture of signals, 5H, Ph1, Ph1′,
been reported in the literature,4c,32−34 some of the spectroscopic data
were not described. In the cases of 2-i,iii,v, analyses of the
stereoisomers were not performed. In this context, 1H and 13C
NMR, IR, and HRMS data are reported below.
3
3
and Naphthyl), 7.27 (dd, JH−H = 7.2 Hz, 1H, Hf), 7.33 (dd, JH−H
=
7.2 Hz, 1H, He), 7.43 (quint, 3JH−H = 3.4 Hz, 1H, Ph1 or Ph1′), 7.65 (d,
3JH−H = 8.3 Hz, 1H, Hd), 7.68 (d, 3JH−H = 8.3 Hz, 1H, Hg). The signals
N-(2-Phenyl-2-methylvinyl)-N,N-diphenylamine (2-i).32 From 1-i
(301.2 mg, 1.00 mmol), 2-i was obtained as a 90:10 mixture of E and Z
isomers (259.5 mg, 0.91 mmol, 91%). Conversion of 3-i to 2-i was
achieved by thermal treatment of 3-i in toluene at 100 °C for 6h
(method A). 1H NMR of (E)-2-i (600 MHz, CDCl3, room
1
were assigned by H−1H COSY. 13C{1H} NMR (151 MHz, CDCl3,
room temperature): δ 118.6, 121.5, 124.6, 125.60, 125.66, 125.7, 125.8,
126.1, 126.4, 126.5, 127.1, 127.2, 127.9, 128.25, 128.27, 129.1, 129.8,
130.1, 132.8, 134.8, 138.7, 140.8, 142.5, 148.5. IR (ATR, cm−1): 1613,
1588, 1572 (νCCN). Mp: 142 °C (lit. mp 131 °C determined by
DSC4c). HRMS-EI(+) (m/z): [M]+ calcd for C30H23N, 397.1830;
found, 397.1830.
3
temperature): δ 1.74 (d, JH−H = 1.4 Hz, 3H, −CHCMePh), 6.59
(q, 3JH−H = 1.4 Hz, 1H, −CHCMePh), 7.04 (t, 3JH−H = 7.6 Hz, 2H,
3
3
p-PhN), 7.17 (d, JH−H = 7.6 Hz, 4H, o-PhN), 7.27 (t, JH−H = 7.6 Hz,
1H, p-Ph1), 7.31 (dd, 3JH−H = 7.6 Hz, 4H, m-PhN), 7.37 (t, 3JH−H = 7.6
Hz, 2H, m-Ph1), 7.49 (d, 3JH−H = 7.6 Hz, 2H, o-Ph1). 13C{1H} NMR of
(E)-2-i (151 MHz, CDCl3, room temperature): δ 16.3, 122.0, 122.5,
125.7, 126.5, 126.7, 128.5, 129.3, 130.8, 142.0, 146.7. 1H NMR of (Z)-
N-(2-Phenyl-2-methylvinyl)phenoxazine (2-v).4c From 1-v (315.1
mg, 1.00 mmol), 2-v was obtained as a 21:79 mixture of E and Z
isomers (245.3 mg, 0.82 mmol, 82%). Conversion of 3-v to 2-v was
achieved by thermal treatment of 3-v in toluene at 100 °C for 6h
J
DOI: 10.1021/acs.organomet.5b00636
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(method A). 1H NMR of (Z)-2-v (600 MHz, CDCl3, room
temperature): δ 2.28 (s, 3H, −CHCMePh), 5.77 (s, 1H, −CH
yield). The amount of the residual metal in the enamine 2-iii isolated
was determined by ICP-MS, which clearly showed that there were no
detectable iridium species in the enamine 2-iii (<20 ppb).
3
3
CMePh), 6.55 (d, JH−H = 8.3 Hz, 2H, Hd), 6.61 (d, JH−H = 8.3 Hz,
2H, Ha), 6.64 (dd, 3JH−H = 8.3 Hz, 2H, Hc), 6.69 (dd, 3JH−H = 8.3 Hz,
2H, Hb), 7.19 (t, 3JH−H = 7.2 Hz, 1H, p-Ph1), 7.25 (dd, 3JH−H = 7.2 Hz,
Preparation of 2-viii. In a 50 mL two-necked found flask equipped
with a Dimroth condenser were placed carboxamide 1-viii (2.0 g, 10.5
mmol) and the iridium complex 4i (0.15 mg, 1.0 × 10−5 mmol). Then
1,1,3,3-tetramethyldisiloxane (2.0 equiv with respect to amide 1-viii)
was added, and the mixture was stirred without any solvent at 100 °C
for 30 min. During the reaction, generation of H2 gas was observed.
3
2H, m-Ph1), 7.39 (d, JH−H = 7.2 Hz, 2H, o-Ph1). The signals were
assigned by 1H−1H COSY. 13C{1H} NMR of (Z)-2-v (151 MHz,
CDCl3, room temperature): δ 21.5, 113.6, 115.5, 120.4, 121.1, 121.7,
123.5, 126.9, 28.1, 128.5, 138.3, 141.1, 144.2. 1H NMR of (E)-2-v (600
MHz, CDCl3, room temperature): δ 2.07 (s, 3H, −CHCMePh),
1
After the reaction was over, the H NMR spectrum was measured in
3
6.13 (s, 1H, −CHCMePh), 6.51 (d, JH−H = 8.3 Hz, 2H, Hd), 6.61
the presence of anisole as an internal standard. The conversion of 1-
viii, the same as the yield of 2-viii, was estimated as >99% from the
integral ratios of the signals of the starting materials to those of the
internal standard. The solvent and unreacted TMDS were removed
under reduced pressure, and the resulting residue was purified by short
column chromatography on silica gel (eluent hexane) to afford the
enamine 2-viii as a colorless oil (1.53 g, 8.71 mmol, 83% in isolated
yield). The amount of the residual metal in the isolated enamine 2-viii
was determined by ICP-MS, which clearly showed that there were no
detectable iridium species in the enamine 2-viii (<20 ppb).
(d, 3JH−H = 8.3 Hz, 2H, Ha), 6.64 (dd, 3JH−H = 8.3 Hz, 2H, Hc), 6.75 (a
mixture of signals, 4H, Hb and Hc), 7.31 (t, 3JH−H = 7.2 Hz, 1H, p-Ph1),
7.43 (dd, 3JH−H = 7.2 Hz, 2H, m-Ph1), 7.57 (d, 3JH−H = 7.2 Hz, 2H, o-
Ph1). The other signals derived from Ha could not be observed due to
the signals for the major Z isomer. 13C{1H} NMR of (E)-2-v (151
MHz, CDCl3, room temperature): δ 16.0, 113.2, 115.8, 121.4, 121.9,
123.7, 126.2, 128.4, 128.8, 132.7, 139.8, 141.9, 144.3. IR (ATR, cm−1):
1622, 1588 (νCCN). Mp: 153 °C (lit. mp 100 °C determined by
DSC). HRMS-EI(+) (m/z): [M]+ calcd for C21H17NO, 299.1310;
found, 299.1304.
1H NMR Analysis of the Silylhemiaminals 3-i−vii. In an NMR
tube equipped with a J. Young valve were placed carboxamides 1-i−vii
(0.1 mmol), the iridium catalyst 4k (0.7 mg, 0.5 μmol, 0.5 mol %), and
C6D6 (0.4 mL). Hexamethylbenzene (C6Me6) was also added as an
internal standard. After the integral ratio between amides 1 and C6Me6
N-(2,2-Diphenylvinyl)phenoxazine (2-vi).4c From 1-vi (377.2 mg,
1.00 mmol), 2-vi was obtained as a yellow solid (332.3 mg, 0.92 mmol,
92%). Conversion of 3-vi to 2-vi was achieved by thermal treatment of
1
3-vi in toluene at 100 °C for 6 h (method A). H NMR (600 MHz,
C6D6, room temperature): δ 5.93 (s, 1H, −CHCPh2), 6.48 (ddd,
1
4
3JH−H = 8.3, 8.3 Hz, JH−H = 1.4 Hz, 2H, Hb), 6.53 (a mixture of
was confirmed by H NMR, 1,1,3,3-tetramethyldisiloxane (0.2 mmol)
was added and the reaction was monitored by 1H NMR. The
conversions of 1-i−vii and the yields of 3-i−vii were estimated from
the integral ratio relative to the internal standard. Since 3-i−vii were
unstable toward moisture, those other than 3-vii were characterized by
3
3
signals, 4H, Ph1′), 6.67 (d, JH−3 H = 7.6 Hz, 2H, Hd), 6.85 (t, JH−H
=
7.6 Hz, 1H, p-Ph1), 6.91 (dd, JH−H = 7.6 Hz, 2H, Hc), 7.13 (1H, p-
Ph1′, overlapped with m-Ph1), 7.13 (d, JH−H = 7.6 Hz, 2H, m-Ph1),
3
7.25 (dd, 3JH−H = 8.3 Hz, 4JH−H = 1.4 Hz, 2H, Ha), 6.67 (d, 3JH−H = 7.6
Hz, 2H, o-Ph1). 13C{1H} NMR (151 MHz, C6D6, room temperature):
δ 113.8, 116.0, 121.8, 122.3, 123.6, 128.4, 128.5, 128.57, 128.60, 128.8,
129.2, 132.1, 138.1, 140.5, 144.7, 144.8. IR (ATR, cm−1): 1592.0
(νCCN). Mp: 189 °C (lit. mp 195 °C determined by DSC). HRMS−
EI(+) (m/z): [M]+ calcd for C26H19NO, 361.1467; found, 361.1460.
trans-N-(2,2-Diphenylvinyl)carbazole (2-vii).34 From 1-vii (285.1
mg, 1.00 mmol), 2-vii was obtained as a white solid (219.5 mg, 0.77
mmol, 77%). Conversion of 3-vii to 2-vii was achieved by treatment of
a toluene solution of 3-vii with P2O5 (30 mg) at ambient temperature
for 30 min (method C). 1H NMR (600 MHz, CDCl3, room
temperature): δ 7.08 (d, 3JH−H = 14.4 Hz, 1H, −CHCHPh), 7.33 (t,
1
specific signals of the H NMR spectrum of the crude mixture which
was obtained by an experiment in the absence of C6Me6. Three
enamines, 3-i,iii,v, were obtained as mixtures of two diastereomers in
ratios of 96:4, 95:5, and 85:15, respectively. Details of these are
described in the Supporting Information.
Spectroscopic Identification of the Silylhemiaminal Inter-
mediate 3-vii. The silylhemiaminal 3-vii was relatively stable enough
to allow removal of excess TMDS to give NMR spectra without
impurities. Treatment of 1-vii (28.5 mg, 0.1 mmol) with TMDS at
ambient temperature for 21 h gave 3-vii. The conversion of 1-vii and
1
1
the yield of 3-vii were estimated by H NMR to be >99%. H NMR
3
3JH−H = 7.2 Hz, 1H, p-Ph), 7.34 (dd, JH−H = 7.2 Hz, 2H, Hb), 7.44
(600 MHz, C6D6, 70 °C): δ −0.23 (s, 3H, -SiMe2), −0.09 (d, 3JH−H
2.8 Hz, 3H, -SiMe2H), −0.09 (d, JH−H = 2.8 Hz, 3H, −SiMe2H),
−0.08 (s, 3H, −SiMe2), 3.31 (dd, 3JH−H = 6.9 Hz, 2JH−H = 13.7 Hz, 1H,
−CH2Ph), 3.50 (dd, 3JH−H = 6.9 Hz, 2JH−H = 13.7 Hz, 1H, −CH2Ph),
=
3
3
(dd, JH−H = 7.2 Hz, 2H, m-Ph), 7.52 (dd, JH−3H = 7.2 Hz, 2H, Hc),
3
3
7.56 (d, JH−H = 7.2 Hz, 2H, o-Ph), 7.72 (d, JH−H = 14.4 Hz, 1H,
−CHCHPh, overlapped with Hd), 7.74 (d, 3JH−H = 7.2 Hz, 2H, Hd),
8.12 (d, 3JH−H = 7.2 Hz, 2H, Ha). The signals were assigned by 1H−1H
COSY. 13C{1H} NMR (151 MHz, CDCl3, room temperature): δ
110.72, 120.0, 120.5, 120.9, 123.5, 124.3, 126.0, 126.4, 127.4, 129.0,
136.5, 139.7. IR (ATR, cm−1): 1651, 1620, 1597 (νCCN). Mp: 104.1
°C (lit. mp 116−120 °C). HRMS−FAB(+) (m/z): [M]+ calcd for
C20H15N, 269.1205; found, 269.1203.
3
3
4.69 (sept, JH−H = 2.8 Hz, 1H, −SiMe2H), 6.43 (dd, JH−H = 6.9 Hz,
1H, −SiOCH−), 6.88−6.98 (a mixture of signals, 5H, Ph), 7.17 (dd,
3JH−H = 8.2 Hz, 2H, Hb), 7.34 (dd, 3JH−H = 8.2 Hz, 2H, Hc), 7.60 (br,
2H, Ha), 7.98 (d, 3JH−H = 8.2 Hz, 2H, Hd). 13C{1H} NMR (151 MHz,
C6D6, room temperature): δ −1.3, −1.2, 0.37, 0.40, 43.0, 81.2, 119.8,
120.6, 124.0 (br), 125.9, 126.9, 128.3, 128.5, 129.7, 137.1, 139.5 (br).
29Si NMR (119 MHz, C6D6, room temperature): δ −8.72 (SiMe),
−4.64 (SiMe). HRMS-EI(+) (m/z): [M]+ calcd for C24H29NO2Si2,
419.1737; found, 419.1736.
Preparation of Enamines with Low Catalyst Loadings.
Preparation of 2-iii. In a 20 mL two-necked found flask equipped
with a Dimroth condenser were placed carboxamide 1-iii (1.08 g, 2.85
mmol) and a 2.83 M solution of iridium complex 4i in toluene (0.1
mL, 2.83 × 10−4 mmol). Then 1,1,3,3-tetramethyldisiloxane (2.0 equiv
with respect to amide 1-iii) and toluene (3 mL) were added, and the
mixture was stirred at 40 °C for 24 h. After the reaction was over, the
1H NMR spectrum was measured in the presence of anisole as an
ASSOCIATED CONTENT
■
S
* Supporting Information
internal standard. The conversion of 1-iii was estimated as >99% from
the integral ratios of the signals of the starting materials to the internal
standard. The solvent and unreacted TMDS in the solution of 3-iii
were removed under reduced pressure, and again toluene (20 mL) was
placed in the flask. Thermolysis of a solution of 3-iii in toluene at 80
°C for 6 h gave the enamine 2-iii in 93% 1H NMR yield, which was a
90:10 mixture of E and Z isomers. The solvent was removed under
reduced pressure, and the resulting residue was purified by short
column chromatography on silica gel (eluent hexane) to afford the
enamine 2-iii as a white powder (0.84 g, 2.51 mmol, 88% in isolated
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.5b00636.
Detailed experimental details, molecular structures of
4h−j, details of crystallographic studies (4h−j), and
spectral data for all new and some reported compounds
(PDF)
Crystallographic data for 4h−j (CIF)
K
DOI: 10.1021/acs.organomet.5b00636
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(12) (a) Corriu, R. J. P.; Moreau, J. J. E.; Patau-Sat, M. J. Organomet.
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(13) Motoyama, Y.; Aoki, M.; Takaoka, N.; Aoto, R.; Nagashima, H.
Chem. Commun. 2009, 1574−1576.
AUTHOR INFORMATION
■
Corresponding Author
* E-mail: nagasima@cm.kyushu-u.ac.jp.
Notes
(14) Park, S.; Brookhart, M. J. Am. Chem. Soc. 2012, 134, 640−653.
(15) The recently reported base-catalyzed hydrosilane reduction of
tertiary amides to enamines may be an alternative solution for the
preparation of metal-free enamines. However, application of the base-
catalyzed process to π-conjugate enamines has not been examined.
Volkov, A.; Tinnis, F.; Adolffson, H. Org. Lett. 2014, 16, 680−683.
(16) (a) Matsubara, K.; Iura, T.; Maki, T.; Nagashima, H. J. Org.
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by the Core Research
Evolutional Science and Technology (CREST) Program of the
Japan Science and Technology Agency (JST).
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