Synthesis of α-silylmethyl-α,β-unsaturated imines by the rhodium-catalyzed silylimination of primary-alkyl-substituted terminal alkynes

Article abstract of DOI:10.1021/jo501431e

In contrast to our previous report on the rhodium-catalyzed reaction of terminal alkynes with equimolar amounts of hydrosilanes and isocyanides leading to (E)- or (Z)-β-silyl-α,β-unsaturated imines A, the addition of an excess molar amount of hydrosilanes relative to isocyanides in the reaction of primary-alkyl-substituted terminal alkynes results in the production of α-silylmethyl-α,β-unsaturated imines B. Various isocyanides bearing tert-butyl and 1-adamantyl groups gave B with good product selectivity. Z isomers were formed stereoselectively in many cases. Regarding the mechanism for this reaction, when the hydrosilane was added to the reaction mixture in two portions, unsaturated imines A were initially formed, which then underwent double-bond isomerization, probably catalyzed by a Rh-H species, to give B.

Full text of DOI:10.1021/jo501431e

Article
pubs.acs.org/joc
Synthesis of α‑Silylmethyl-α,β-Unsaturated Imines by the Rhodium-
Catalyzed Silylimination of Primary-Alkyl-Substituted Terminal
Alkynes
Yoshiya Fukumoto,* Hiroto Shimizu, Aya Tashiro, and Naoto Chatani
Department of Applied Chemistry, Faculty of Engineering, Osaka University, Suita, Osaka 565-0871, Japan
S
* Supporting Information
ABSTRACT: In contrast to our previous report on the rhodium-catalyzed
reaction of terminal alkynes with equimolar amounts of hydrosilanes and
isocyanides leading to (E)- or (Z)-β-silyl-α,β-unsaturated imines A, the
addition of an excess molar amount of hydrosilanes relative to isocyanides
in the reaction of primary-alkyl-substituted terminal alkynes results in the
production of α-silylmethyl-α,β-unsaturated imines B. Various isocyanides
bearing tert-butyl and 1-adamantyl groups gave B with good product
selectivity. Z isomers were formed stereoselectively in many cases.
Regarding the mechanism for this reaction, when the hydrosilane was
added to the reaction mixture in two portions, unsaturated imines A were
initially formed, which then underwent double-bond isomerization,
probably catalyzed by a Rh−H species, to give B.
tertiary-alkyl-substituted isocyanides, such as tert-butyl iso-
cyanide and 1,1,3,3-tetramethylbutyl isocyanide (tert-octyl
isocyanide), and hydrosilanes results in the formation of Z
isomers, while the use of an aryl-substituted isocyanide gave E
isomers. In the related silylformylation reaction, in which
carbon monoxide is used in place of isocyanides, Z isomers
were obtained stereoselectively.3 During the course of our study
on the Z-selective silylimination of primary-alkyl-substituted
alkynes with tert-octyl isocyanide, however, we found that α-
silylmethyl-α,β-unsaturated imines B, which contain an
allylsilane, were also formed when an excess molar amount of
hydrosilane relative to isocyanide was used in the reaction.
Allylsilanes have been utilized extensively for regio- and
stereoselective C−C bond formation with carbon electrophiles
such as aldehydes and acetals in the presence of the Lewis acid,
a reaction known as the Hosomi−Sakurai reaction.4 Alper and a
co-worker reported that α,β-unsaturated aldehydes correspond-
ing to B were produced in the reaction of terminal alkynes with
hydrosilanes under CO/H2 pressure. However, in some cases,
further hydrogenation of the C−C double bond occurred,
resulting in the formation of saturated aldehydes, which
depends on the nature of the substituent in the starting
alkyne.5 Propargyl alcohols and their derivatives6 and
propargylamines7 were also converted into type B aldehydes,
along with the elimination of R3Si−Y (Y = OR′, NR′2). Herein
we report the optimization of the reaction conditions for the
selective formation of B and the scope and limitations of the
substrates. The reason for the product switch caused by the
INTRODUCTION
■
The catalytic addition of silyl functionalities across carbon−
carbon multiple bonds has seen significant development in the
past two decades, and the advances in this area have been
summarized in several reviews.1 The regio- and stereoselective
addition of silicon compounds to CC triple bonds in the
presence of Lewis acids or transition-metal complexes is an
efficient strategy for preparing stereodefined vinylsilanes. We
recently reported the rhodium-catalyzed addition of silyl and
imino groups to alkynes via the use of a combination of
hydrosilanes and isocyanides, leading to the formation of β-
silyl-α,β-unsaturated imines A (Scheme 1).2 The reaction,
which is referred to as silylimination, allowed the stereoselective
production of (E)- and (Z)-vinylsilanes from terminal alkynes,
depending on the specific isocyanide employed in the reaction.
This indicates that silylimination with equimolar amounts of
Scheme 1. Rhodium-Catalyzed Silylimination of Terminal
Alkynes Leading to β-Silyl-α,β-unsaturated Imines A or α-
Silylmethyl-α,β-unsaturated Imines B
Received: June 27, 2014
Published: August 8, 2014
© 2014 American Chemical Society
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Table 1. Product Distribution of Vinylsilanes A, Allylsilanes B, and Imidoylsilanes C in the Rh4(CO)12-Catalyzed Reaction of 1-
a
Octyne (1a) with Dimethylphenylsilane and Isocyanides
isocyanide
amt (mmol)
A
B
entry
amt of HSiPhMe2 (mmol)
R′
yield (%)
E/Z
yield (%)
trace
E/Z
C yield (mmol)
1
2
3
4
5
6
7
8
2
tOct
2
2
2
3
3
2
2
2
2
88 (2aOct
)
30/70
45/55
73/27
30/70
29/71
0.87
0.73
0.67
0.90
1.00
0.70
0.42
0.90
0.14
2.5
3
57
9
32 (3aOct
)
22/78
18/82
78
0
2
90
90
0
3
0
2.5
2.5
2.5
2.5
Ad
tBu
Cy
Xy
93 (3aAd
92 (3aBu
71 (3aCy
0
)
4/96
4/96
18/82
trace
)
)
7 (2aCy
)
99/1
b
9
89 (2aXy
)
86/14
a
Reaction conditions: 1a (1 mmol), Rh4(CO)12 (0.015 mmol), THF (8 mL) at 70 °C for 30 min. Product yields and E/Z ratios were determined by
b
1H NMR spectroscopy. The reaction was run for 4 h.
difference in the molar ratio of the hydrosilane to the
isocyanide was also studied.
allylsilane-type imines could be obtained in pure form.
Therefore, the compounds were isolated as aldehydes after
hydrolysis (see the Experimental Section).
With the optimized reaction conditions in hand, various
terminal alkynes were examined in terms of producing the
corresponding allylsilanes (Table 2). Functional groups such as
phenyl (entry 2), ester (entry 3), cyano (entry 4), amide (entry
5), THP (entry 6), and siloxy groups (entry 7) did not interfere
with the reaction. The reaction of prop-2-yn-1-ylcyclohexane
(1h) under the standard reaction conditions afforded a mixture
of 3h in 79% yield and the byproduct vinylsilane 2h in 5% yield.
2h disappeared completely when the added HSiPhMe2 was
increased to 4 mmol, and 3h was obtained in 83% yield with a
93% Z selectivity (entry 8). Similar reactivities were observed in
the reaction of 1i (entry 9). Whereas benzylacetylene (1j) was
also transformed under the standard reaction conditions into
the phenyl-substituted product 3j (entry 10), the vinylsilanes
2k,l were formed as the sole products in the reactions of
adamantylmethylacetylene (1k, entry 11) and cyclohexylacety-
lene (1l, entry 12).
We envisioned several pathways that could explain the
formation of allylsilanes, especially the origin of the alkene
isomerization, as shown in Scheme 2. Alper proposed the
formation of the silylallene dihydridorhodium complex X as a
key intermediate in the aforementioned reaction of terminal
alkynes with hydrosilanes under CO/H2 pressure.5 Considering
the similarity of the products in both reactions, it appears that
the catalytic reaction also involves the formation of
intermediate X, in which a hydrogen and an imino group are
added to the carbon−carbon double bond of the allene ligand
(path a). In an analogous fashion, the isomerization of an
alkyne to an allene Y and the subsequent silylimination of the
carbon−carbon double bond is also a possibility (path b).
However, the formation of vinylsilanes followed by the
isomerization of the carbon−carbon double bond to afford
allylsilanes is the most likely explanation (path c), because of
the fact the product ratio of 3aOct/2aOct increased with
increasing amounts of hydrosilane (entries 1−3 in Table 1).
To confirm that the present reaction proceeds via path c, an
excess molar amount of hydrosilane relative to isocyanide was
added to the reaction mixture in two portions (Scheme 3).
RESULTS AND DISCUSSION
■
As reported in our previous study, when 1-octyne (1 mmol; 1a)
was reacted with dimethylphenylsilane (2 mmol) and tert-octyl
isocyanide (2 mmol) in the presence of Rh4(CO)12 (0.015
mmol) in THF under reflux for 30 min, the vinylsilane 2aOct
was produced in 88% yield with an E/Z ratio of 30/70. N-tert-
Octylformimidoylsilane8,9 was also produced as a byproduct
(entry 1, Table 1). Whereas 2aOct was isolated by column
chromatography on Al2O3 followed by bulb-to-bulb distillation
under reduced pressure in 48% yield, the imidoylsilane
disappeared during the chromatographic operation. Attempts
to isolate the imidoylsilane by bulb-to-bulb distillation from the
reaction mixture were also unsuccessful. The fact that moderate
Z selectivity was observed in the reaction of primary-alkyl-
substituted alkynes prompted us to reinvestigate the reaction
conditions in order to improve the stereoselectivity of this
reaction. To our surprise, we found that the allylsilane 3aOct was
formed in 32% yield along with 2aOct in 57% yield when the
amount of HSiPhMe2 was increased to 2.5 mmol (entry 2).
The addition of 3 mmol of HSiPhMe2 resulted in a further
increase in the yield of 3aOct to 78% (entry 3). On the other
hand, when an excess or an equimolar amount of HSiPhMe2
relative to tert-octyl isocyanide was added to the reaction
system, 2aOct was obtained in 90% yield (entries 4 and 5, and
also entry 1). A series of isocyanides (2 mmol) were next
examined in the reaction of 1-octyne (1 mmol) and HSiPhMe2
(2.5 mmol). Other tertiary-alkyl-substituted isocyanides such as
tert-butyl isocyanide and 1-adamantyl isocyanide (AdNC)
reacted to give the corresponding allylsilanes 3aAd and 3aBu in
93 and 92% yields, respectively, with good Z selectivity (entries
6 and 7). Despite the fact that the reaction with cyclohexyl
isocyanide also afforded mainly the desired 3aCy (entry 8), the
use of 2,6-dimethylphenyl isocyanide (XyNC) resulted in the
production of the vinylsilane 2axy with no detectable formation
of the allylsilane (entry 9). HSiEt2Me was also applicable to the
reaction with AdNC, giving the allylsilane 3a′Ad in 80% yield
with an 85% Z selectivity. Although we isolated 2axy by bulb-to-
bulb distillation under reduced pressure in 62% yield, no
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a
Table 2. Silylimination of Some Terminal Alkynes
Scheme 2. Possible Pathways for the Formation of
Allylsilanes
Scheme 3. Sequential Addition of HSiPhMe2 to the Reaction
of 1a with CNAd
mura and Suginome reported on the migration of the double-
bond of 2-borylvinylsilanes to the corresponding allylsilanes
catalyzed by a palladium complex.11
A proposed mechanism for the catalytic reaction is presented
in Scheme 4, which involves three types of catalytic cycles.
Initially, Rh4(CO)12 reacts with a hydrosilane to generate the
Rh-SiR′3 species I and the Rh−H species II or a complex
containing both Rh−Si and Rh−H bonds, although the details
of the actual catalyst species are unclear at this time.
Vinylsilanes A are then formed according to the catalytic
cycle i. The addition of I to the alkyne C−C triple bond gives
the β-silylvinylrhodium complex III. The subsequent insertion
of an isocyanide into the Rh−C bond in III then affords the
iminorhodium complex IV, which reacts with a hydrosilane to
give A, with the regeneration of I. I also functions as a catalyst
for the formation of imidoylsilanes C via the insertion of an
isocyanide into the Rh−Si bond in I to generate V (cycle ii). In
the case where the molar amount of hydrosilane in the reaction
mixture is equal to or lower than that of the isocyanide, the
hydrosilane would be completely consumed, with A and C
being produced, and II would also disappear from the reaction
mixture. As a consequence, A would remain without reacting
further. On the other hand, II would continue to be present in
the reaction mixture when an excess molar amount of
hydrosilane relative to isocyanide is added. Therefore, the
addition of II to the vinylsilane A would give VI, as depicted in
cycle iii.12 The Rh species is eliminated with the less sterically
hindered β-hydrogen close to the substituent R to afford the
allylsilane B. A β-hydride elimination of the 1-silylalkan-2-
a
Reaction conditions: alkyne (1 mmol), HSiPhMe2 (2.5 mmol), 1-
adamantyl isocyanide (2 mmol), in THF (8 mL), at 70 °C for 30 min.
Product yields and E/Z ratios were determined by 1H NMR
b
c
spectroscopy. HSiPhMe2 (4 mmol) was used. When the reaction
was run under the standard reaction conditions, 3h was obtained in
79% yield (E/Z = 12/88) along with the vinylsilane 2h in 5% yield (E/
Z = 29/71). When the reaction was run under the standard reaction
conditions, 3i was obtained in 75% yield (E/Z = 21/79) along with the
d
vinylsilane 2i in 7% yield (E/Z = 22/78).
First, 1a was treated with 1 mmol of HSiPhMe2 and 2 mmol of
AdNC at 70 °C for 30 min. As expected, the vinylsilane 2aAd
1
was formed, as confirmed by an H NMR spectrum of an
aliquot of the reaction mixture. At this point, an additional 1.5
mmol of HSiPhMe2 was added to the reaction mixture, with the
result that the total molar amount of HSiPhMe2 exceeded that
of AdNC. After the resulting reaction mixture was stirred for
30 min at 70 °C, 2aAd was quantitatively transformed into 3aAd.
This provides evidence consistent with the silylimination of an
alkyne and the subsequent isomerization of the carbon−carbon
double bond of the initial product vinylsilanes. The rearrange-
ment of vinylsilanes to allylsilanes has also been observed in
some rhodium-catalyzed hydrosilylations of 1-hexyne.10 Oh-
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Scheme 4. Proposed Reaction Mechanism
1e,g,k were purchased. 1e,k were prepared as described below. 1g was
prepared following a procedure described in the literature.15 All
alkynes were purified by distillation over CaH2 prior to use. Among
the isocyanides shown in Table 1, 1-adamantyl isocyanide16 and 2,6-
dimethylphenyl isocyanide17 were prepared following procedures
described in the literature. 1,1,3,3-Tetramethylbutyl isocyanide (tert-
octyl isocyanide), tert-butyl isocyanide, and cyclohexyl isocyanide were
commercially available and were used as received without any further
purifications. HSiPhMe2 and HSiEt2Me were purchased and were
distilled over CaH2 before use. Rh4(CO)12 was purchased from Strem
and was used without further purification.
ylmetal species leading to allylsilanes was proposed in the
transition-metal-catalyzed silyl-Heck reactions of primary-alkyl-
substituted alkenes.13 The fact that the reaction of 1k resulted
in the production of not the allylsilane but, rather, the
vinylsilane can be attributed to the shorter bond length of the
carbon−carbon bond (ca. 154 pm) in comparison to that of the
carbon−silicon bond (ca. 188 nm),14 thereby resulting in an
increase in the steric influence of the adamantyl group.
CONCLUSION
■
Preparation of N,N-Diethylhex-5-ynamide (1e).18 The proce-
dure reported by Merlic et al.19 was modified by using diethylamine in
place of dibutylamine to produce 1e from hex-5-ynoic acid (2.5 mL,
22.5 mmol) in 76% yield (2.86 g) as a colorless oil. 1H NMR
(CDCl3): δ 1.11 (t, J = 7.1 Hz, 3H), 1.19 (t, J = 7.1 Hz, 3H), 1.88
(quint, J = 7.1 Hz, 2H), 1.98 (t, J = 2.7 Hz, 1H), 2.28 (td, J = 6.7, 2.4
Hz, 2H), 2.45 (t, J = 7.1 Hz, 2H), 3.33−3.38 (m, J = 7.2 Hz, 4H). 13C
NMR (CDCl3): δ 13.0, 14.2, 17.8, 23.8, 31.2, 39.9, 41.7, 68.7, 83.8,
171.1. IR (ATR): 3297 w, 2972 w, 2935 w, 2874 w, 2151 w, 1632 s
cm−1. MS: m/z (relative intensity) 167 (5), 115 (67), 100 (34), 86
(11), 72 (40), 67 (21), 58 (100), 55 (20). HRMS: calcd for
C10H17NO (M+): 167.1310. Found 167.1309.
In conclusion, we report on a switch in end products, caused by
the molar ratio of hydrosilanes and isocyanides in the rhodium-
catalyzed reaction of primary-alkyl-substituted terminal alkynes
with hydrosilanes and isocyanides. The addition of an excess
molar amount of hydrosilane relative to the isocyanide in the
reaction resulted in the production of α-silylmethyl-α,β-
unsaturated imines. The choice of isocyanides bearing tert-
butyl and 1-adamantyl groups gave the desired allylsilane-type
compounds with good product selectivity. A variety of
functional groups are tolerated in the reaction. Mechanistic
studies revealed that tandem rhodium-catalyzed reactions
involving silylimination, leading to the production of β-silyl-
α,β-unsaturated imines and the subsequent carbon−carbon
double bond isomerization, are involved in the formation of the
final product.
Preparation of 1-(Prop-2-yn-1-yl)adamantane (1k). This
compound was produced from 2-(adamantan-1-yl)acetaldehyde20
(3.89 g, 22.0 mmol) following a method for the synthesis of 3-
ethylhept-1-yne described in the patent21 in 41% yield (1.59 g) as a
1
colorless oil. H NMR (CDCl3): δ 1.57 (bs, 6H), 1.62−1.70 (c, 6H),
1.94 (m, 2H), 1.98 (c, 4H). 13C NMR (CDCl3): δ 28.5, 32.4, 33.6,
36.8, 41.8, 70.0, 81.8. IR (ATR): 3308 w, 2899 s, 2847 m, 2116 w
cm−1. MS: m/z (relative intensity) 136 (11), 135 (100), 107 (15), 93
(31), 91 (12), 79 (34), 77 (12), 67 (13). Anal. Calcd for C13H18: C,
89.59; H, 10.41. Found: C, 89.30; H, 10.57.
EXPERIMENTAL SECTION
■
General Methods. 1H NMR and 13C NMR spectra were recorded
on 400 and 100 MHz spectrometers (all compounds except 2aOct and
2aXy) or 270 and 68 MHz spectrometers (2aOct and 2aXy) using CDCl3
and C6D6 as solvents. Data are reported as follows: chemical shifts in
ppm (δ), multiplicity (s = singlet, d = doublet, t = triplet, q = quartet,
m = multiplet, c = complex), coupling constant (Hz), integration, and
interpretation. Infrared spectra (IR) were recorded using ATR (all
compounds except 2aOct and 2aXy) or in KRS-5 cells (2aOct and 2aXy).
Absorption data are reported in reciprocal centimeters with the
following relative intensities: s (strong), m (medium), or w (weak).
Mass spectra were obtained using a spectrometer with a quadrupole
mass analyzer at 70 eV. High-resolution mass spectra (HRMS) were
obtained using a spectrometer with a double-focusing mass analyzer.
Analytical gas chromatography (GC) was carried out on a chromato-
graph equipped with a flame ionization detector. High-performance
liquid chromatography (HPLC) was performed on a chromatograph
equipped with a UV detector. THF was purified by passage through
activated alumina under a positive pressure of N2. All alkynes except
General Procedure for the Rh4(CO)12-Catalyzed Reaction of
1-Octyne with HSiPhMe2 and Isocyanides Leading to β-Silyl-
α,β-unsaturated Imines (2aOct and 2aXy). A 20 mL reaction flask,
equipped with a reflux condenser, was dried for 1 h in an oven at 110
°C and then purged with N2. After the flask was cooled to room
temperature, Rh4(CO)12 (11.2 mg, 0.015 mmol), THF (8 mL), and
the isocyanide (2 mmol) were placed in the flask. The mixture was
stirred at room temperature for 5 min. HSiPhMe2 (0.31 mL, 2 mmol)
and alkyne (1 mmol) were added to the resulting yellow solution. The
reaction mixture was refluxed in an oil bath at 75 °C until the starting
alkyne disappeared. After the mixture was cooled to room temper-
ature, the volatiles were removed in vacuo. The product was isolated
by bulb-to-bulb distillation under reduced pressure or by column
chromatography on Al2O3 (0.063−0.200 mm, Activity Stage I, neutral)
followed by bulb-to-bulb distillation under reduced pressure.
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(E)- and (Z)-N-2-(Dimethylphenylsilylmethylene)oct-1-ylidene-
2,4,4-trimethylpentan-2-amine (2aOct). The compound was isolated
by alumina column chromatography (1.8 cm i.d. × 8 cm height, Rf =
0.11 in hexane/EtOAc 200/1) followed by bulb-to-bulb distillation
(170 °C/0.7 mmHg) in 48% yield (184 mg) as a colorless oil (E/Z =
23/77). 1H NMR (C6D6): δ 0.43 (s, 6H), 0.46 (s, 6H), 0.89−0.94 (c,
3H), 1.00 (s, 9H), 1.05 (s, 9H), 1.07 (s, 6H), 1.22 (s, 6H), 1.28−1.50
(c, 6H), 1.58 (s, 2H), 1.63−1.73 (c, 2H), 1.65 (s, 2H), 2.65 (t, J = 8.1
Hz, 2H), 2.74 (t, J = 7.9 Hz, 2H), 6.06 (s, 1H), 6.29 (s, 1H), 7.17−
7.27 (c, 3H), 7.54−7.59 (c, 2H), 7.82 (s, 1H), 8.18 (s, 1H). 13C NMR
(C6D6): δ −0.7, 0.1, 14.6, 23.2, 23.3, 29.6, 29.9, 30.07, 30.12, 30.5,
30.7, 31.7, 32.2, 32.3, 32.36, 32.38, 32.40, 34.9, 56.7, 56.8, 60.7, 61.2,
128.25, 128.33, 129.3, 129.4, 134.1, 134.2, 134.6, 136.3, 139.1, 139.9,
156.3, 158.6, 159.2, 159.4. IR (neat): 3064 w, 2960 s, 2904 m, 1630 m
cm−1. MS: m/z (relative intensity) 385 (4), 314 (14), 272 (23), 250
(36), 188 (21), 140 (71), 138 (24), 136 (11), 135 (55), 121 (11), 84
(10), 74 (16), 73 (11), 59 (27), 58 (33), 57 (100), 55 (16). Anal.
Calcd for C25H43NSi: C, 77.85; H, 11.24; N, 3.63. Found: C, 77.88; H,
11.51; N, 3.63.
107 (12), 75 (21). Anal. Calcd for C17H26OSi: C, 74.39; H, 9.55.
Found: C, 74.25; H, 9.62.
(E)- and (Z)-2-Diethylmethylsilylmethyloct-2-enal (4a′). This
compound was isolated in 75% yield (178 mg) as a pale yellow oil
(Rf = 0.10 in hexane/EtOAc 20/1) with an E/Z ratio of 3/97. Z-4a′
was obtained by column chromatography on Al2O3 (Rf = 0.23 in
hexane/EtOAc 20/1). 1H NMR (CDCl3): δ −0.12 (s, 3H), 0.41−0.52
(c, 4H), 0.88−0.92 (c, 9H), 1.30−1.36 (c, 4H), 1.46−1.50 (m, 2H),
1.69 (s, 2H), 2.26 (q, J = 7.3 Hz, 2H), 6.27 (t, J = 7.3 Hz, 1H), 9.30 (s,
1H). 13C NMR (CDCl3): δ −5.7, 5.4, 7.2, 11.4, 14.0, 22.5, 28.3, 29.3,
31.6, 141.9, 151.7, 195.1. IR (ATR): 2954 m, 2930 w, 2874 w, 2813 w,
2707 w, 1685 s cm−1. MS: m/z (relative intensity) 225 (6), 211 (42),
141 (43), 113 (22), 101 (64), 89 (67), 75 (10), 73 (100), 61 (33).
HRMS: calcd for C14H28OSi (M+) 240.1909, found 240.1910.
(E)- and (Z)-2-Dimethylphenylsilylmethyl-5-phenylpent-2-enal
(4b). This compound was isolated in 83% yield (257 mg) as a
colorless oil (Rf = 0.14 in hexane/EtOAc 20/1) with an E/Z ratio of 3/
1
97. Z-4b was obtained by flash column chromatography on SiO2. H
NMR (CDCl3): δ 0.28 (s, 6H), 1.94 (s, 2H), 2.34 (q, J = 7.5 Hz, 2H),
2.63 (t, J = 7.5 Hz, 2H), 6.30 (t, J = 7.2 Hz, 1H), 7.09 (d, J = 6.8 Hz,
2H), 7.20−7.40 (c, 6H), 7.52 (m, 2H), 9.32 (s, 1H). 13C NMR
(CDCl3): δ −2.7, 14.5, 30.8, 34.4, 126.2, 127.8, 128.2, 128.4, 129.2,
133.6, 138.3, 140.7, 141.6, 150.8, 194.8. IR (ATR): 3066 w, 3026 w,
2955 w, 2898 w, 2863 w, 2719 w, 1681 s cm−1. MS: m/z (relative
intensity) 293 (6), 136 (13), 135 (100), 107 (10), 91 (32), 75 (19).
Anal. Calcd for C20H24OSi: C, 77.87; H, 7.84. Found: C, 77.62; H,
8.11.
(E)- and (Z)-N-2-(Dimethylphenylsilylmethylene)oct-1-ylidene-
2,6-dimethylaniline (2aXy). The compound was isolated by bulb-to-
bulb distillation (192 °C/1 mmHg) in 62% yield (232 mg) as a pale
1
yellow oil (E/Z = 89/11). H NMR (C6D6): δ 0.27 (s, 6H), 0.42 (s,
6H), 0.85−0.95 (c, 3H), 1.26−1.51 (c, 6H), 1.60−1.71 (c, 2H), 2.03
(s, 6H), 2.11 (s, 6H), 2.74 (t, J = 8.1 Hz, 2H), 2.82 (t, J = 8.0 Hz, 2H),
6.07 (s, 1H), 6.48 (s, 1H), 6.85−7.03 (c, 3H), 7.20−7.23 (c, 3H),
7.39−7.49 (c, 2H), 7.50−7.54 (c, 2 H), 7.53 (s, 1H), 8.18 (s, 1H). 13C
NMR (C6D6): δ −0.9, 0.1, 14.6, 18.7, 18.8, 23.26, 23.31, 29.9, 30.5,
30.7, 31.5, 32.4, 35.4, 123.9, 124.0, 126.6, 126.8, 128.4, 129.5, 129.6,
134.0, 134.2, 138.6, 139.0, 139.5, 140.7, 151.8, 151.9, 157.4, 158.0,
163.0, 167.0. IR (neat): 3068 w, 3020 w, 2960 s, 2932 s, 2860 m, 1626
s, 1594 m cm−1. MS: m/z (relative intensity) 377 (5), 243 (18), 242
(100), 135 (61), 132 (21), 120 (30), 105 (20), 79 (14), 77 (13), 59
(19). Anal. Calcd for C25H35NSi: C, 79.51; H, 9.34; N, 3.71. Found: C,
79.53; H, 9.59; N, 3.93.
General Procedure for the Rh4(CO)12-Catalyzed Reaction of
Alkynes with Hydrosilanes and Isocyanides Leading to α-
Silylmethyl-α,β-unsaturated Imines (3aAd-3j) and Subsequent
Hydrolysis of the Product Imines to Aldehydes (4a−j). A 20 mL
reaction flask, equipped with a reflux condenser, was dried for 1 h in an
oven at 110 °C and then purged with N2. After the flask was cooled to
room temperature, Rh4(CO)12 (11.2 mg, 0.015 mmol), THF (8 mL),
and the isocyanide (2 mmol) were placed in the flask. The mixture was
stirred at room temperature for 5 min. HSiPhMe2 (0.38 mL, 2.5
mmol) and alkyne (1 mmol) were added to the resulting yellow
solution. The reaction mixture was refluxed in an oil bath at 75 °C for
30 min. After the mixture was cooled to room temperature, the
volatiles were removed in vacuo. The yield and E/Z ratio of the
resulting imine were determined by 1H NMR spectroscopy with
phthalan as an internal standard. The imine was hydrolyzed to give an
aldehyde by passage through a silica gel column containing 10 wt %
water (3.3 cm i.d. × 15 cm height, 0.075−0.150 mm) in hexane/
AcOEt (20/1) as an eluent. Analytically pure Z isomer was obtained
by column chromatography on Al2O3 (1.7 cm i.d. × 15 cm height,
0.063−0.200 mm, Activity Stage I, neutral; all aldehydes except
4b,d,g−i), flash column Chromatography on SiO2 (1.7 cm i.d. × 15
cm height, 0.040−0.063 mm; 4b,d,g,h), or by HPLC (Nacalai
COSMOSIL Cholester (2.0 cm i.d. × 15 cm height; 4i).
(E)- and (Z)-Methyl 6-Dimethylphenylsilyl-5-formylhex-4-enoate
(4c). This compound was isolated in 80% yield (230 mg) as a colorless
oil (Rf = 0.26 in hexane/EtOAc 5/1) with an E/Z ratio of 3/97. Z-4c
was obtained by column chromatography on Al2O3 (Rf = 0.19 in
1
hexane/EtOAc 10/1). H NMR (CDCl3): δ 0.28 (s, 6H), 1.97 (s,
1H), 2.24−2.30 (c, 4H), 3.67 (s, 3H), 6.21 (t, J = 6.6 Hz, 1H), 7.31−
7.35 (c, 3H), 7.50 (m, 2H), 9.32 (s, 1H). 13C NMR (CDCl3): δ −2.8,
14.5, 24.4, 32.4, 51.7, 127.7, 129.1, 133.5, 138.1, 142.0, 149.0, 172.6,
194.6. IR (ATR): 3070 w, 3048 w, 3000 w, 2954 w, 2901 w, 2822 w,
2712 w, 1738 m, 1681 s cm−1. MS: m/z (relative intensity) 290 (1),
151 (11), 136 (13), 135 (100), 107 (13), 105 (11), 89 (11), 75 (14).
HRMS: calcd for C16H22O3Si (M+) 290.1338, found 290.1337.
(E)- and (Z)-5-Cyano-2-dimethylphenylsilylmethylpent-2-enal
(4d). This compound was isolated in 80% yield (203 mg) as a
colorless oil (Rf = 0.11 in hexane/EtOAc 5/1) with an E/Z ratio of 3/
1
97. Z-4d was obtained by flash column chromatography on SiO2. H
NMR (CDCl3): δ 0.30 (s, 6H), 1.95 (s, 2H), 2.06 (t, J = 7.2 Hz, 2H),
2.17 (q, J = 7.2 Hz, 2H), 6.16 (t, J = 7.2 Hz, 1H), 7.35−7.38 (c, 3H),
7.47 (m, 2H), 9.36 (s, 1H). 13C NMR (CDCl3): δ −3.0, 15.1, 16.0,
24.7, 118.5, 127.9, 129.4, 133.6, 137.7, 143.2, 144.9, 194.1. IR (ATR):
3069 w, 3048 w, 2957 w, 2900 w, 2827 w, 2713, 2247 w, 1682 s cm−1.
MS: m/z (relative intensity) 243 (1), 136 (14), 135 (100), 105 (11).
Anal. Calcd for C15H19NOSi: C, 69.99; H, 7.44; N, 5.44. Found: C,
69.85; H, 7.55; N, 5.44.
(E)- and (Z)-6-Dimethylphenylsilyl-5-formyl-N,N-diethylhex-4-en-
amide (4e). This compound was isolated in 89% yield (292 mg) as a
colorless oil (Rf = 0.40 in EtOAc) with an E/Z ratio of 1/99. Z-4e was
obtained by column chromatography on Al2O3 (Rf = 0.38 in EtOAc).
1H NMR (CDCl3): δ 0.28 (s, 6H), 1.11 (t, J = 7.3 Hz, 3H), 1.13 (t, J =
7.3 Hz, 3H), 1.99 (s, 2H), 2.17 (t, J = 7.3 Hz, 2H), 2.37 (q, J = 7.3 Hz,
2H), 3.21 (q, J = 7.3 Hz, 2H), 3.36 (q, J = 7.3 Hz, 2H), 6.30 (t, J = 7.3
Hz, 1H), 7.33−7.34 (c, 3H), 7.50 (m, 2H), 9.32 (s, 1H). 13C NMR
(CDCl3): δ −2.9, 12.9, 14.0, 14.2, 24.7, 31.0, 39.9, 41.5, 127.5, 128.8,
133.4, 138.0, 141.2, 150.8, 170.0. 195.7. IR (ATR): 3068 w, 3048 w,
2971 w, 2933 w, 2902 w, 2818 w, 2710, 1680 m cm−1. MS: m/z
(relative intensity) 316 (4), 231 (18), 196 (19), 137 (12), 136 (14),
135 (100), 115 (19), 107 (14), 105 (14), 100 (69), 75 (40), 74 (17),
72 (51), 58 (12). Anal. Calcd for C19H29NO2Si: C, 68.83; H, 8.82; N,
4.22. Found: C, 68.49; H, 8.98; N, 4.23.
(E)- and (Z)-2-Dimethylphenylsilylmethyloct-2-enal (4a). This
compound was isolated in 91% yield (250 mg) as a pale yellow oil (Rf
= 0.14 in hexane/EtOAc 20/1) with an E/Z ratio of 3/97. Z-4a was
obtained by column chromatography on Al2O3 (Rf = 0.24 in hexane/
EtOAc 20/1). The stereochemistry of the compound was determined
by an NOE experiment. 1H NMR (CDCl3): δ 0.27 (s, 6H), 0.88 (t, J =
7.1 Hz, 3H), 1.22−1.32 (c, 6H), 1.95 (s, 2H), 2.01 (q, J = 7.1 Hz, 2H),
6.28 (t, J = 7.1 Hz, 1H), 7.34−7.36 (c, 3H), 7.51 (m, 2H), 9.32 (s,
1H). 13C NMR (CDCl3): δ −2.8, 13.9, 14.3, 22.4, 28.1, 29.2, 31.5,
127.7, 129.0, 133.5, 138.4, 141.0, 152.6, 194.9. IR (ATR): 3069 w,
3049 w, 2956 w, 2927 w, 2858 w, 2708 w, 1683 s cm−1. MS: m/z
(relative intensity) 274 (8), 137 (14), 136 (13), 135 (100), 127 (11),
(E)- and (Z)-2-Dimethylphenylsilylmethyl-5-(tetrahydro-2H-
pyranyl)oxypent-2-enal (4f). This compound was isolated in 90%
yield (300 mg) as a colorless oil (Rf = 0.06 in hexane/EtOAc 20/1)
with an E/Z ratio of 3/97. Z-4f was obtained by column
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Article
1
chromatography on Al2O3 (Rf = 0.13 in hexane/EtOAc 10/1). H
NMR (CDCl3): δ 0.27 (s, 6H), 1.50−1.79 (c, 6H), 1.97 (s, 2H), 2.32
(q, J = 6.7 Hz, 2H), 3.34 (dt, J = 9.6, 6.7 Hz, 1H), 3.49 (m, 1H), 3.71
(dt, J = 9.6, 6.7 Hz, 1H), 3.81 (m, 1H), 4.53 (m, 1H), 6.37 (t, J = 6.7
Hz, 1H), 7.33−7.36 (c, 3H), 7.50 (m, 2H), 9.35 (s, 1H). 13C NMR
(CDCl3): δ −2.8, −2.7, 14.4, 19.4, 25.3, 29.8, 30.5, 62.3, 65.5, 98.8,
127.7, 129.1, 133.5, 138.3, 142.3, 148.3, 194.7. IR (ATR): 3068 w,
3049 w, 2971 w, 2947 m, 2902 w, 2870 w, 2709 w, 1680 s cm−1. MS:
m/z (relative intensity) 247 (1), 135 (48), 95 (100), 75 (13), 67 (17),
57 (14). HRMS: calcd for C19H28O3Si (M+ + H) 333.1886, found
333.1881.
1684 m cm−1. MS: m/z (relative intensity) 325 (1), 323 (25), 261
(10), 232 (15), 137 (26), 136 (12), 135 (100), 107 (19), 93 (29), 91
(14), 79 (31), 75 (15), 67 (14). HRMS: calcd for C22H30OSi (M+)
338.2066, found 338.2067.
(E)- and (Z)-2-Cyclohexyl-3-dimethylphenylsilylpropenal (5l).2
This compound was isolated in 92% yield (250 mg) after hydrolysis
of 2l as a colorless oil (Rf = 0.28 in hexane/EtOAc 20/1) with an E/Z
1
ratio of 11/89. H NMR (CDCl3): δ 0.50 (s, 6H), 0.51 (s, 6H), 0.97
(m, 2H), 1.10−1.24 (c, 3H), 1.32−1.43 (c, 2H), 1.65−1.85 (c, 5H),
2.30 (m, 1H), 2.61 (m, 1H), 6.70 (s, 1H), 6.89 (s, 1H), 7.34−7.40 (c,
3H), 7.50−7.57 (c, 2H), 9.44 (s, 1H), 9.78 (s, 1H). 13C NMR
(CDCl3): δ −1.7, 0.1, 25.6, 26.2, 26.5, 29.6, 32.5, 38.1, 43.0, 128.0,
128.1, 129.36, 129.41, 133.5, 133.6, 137.4, 138.1, 146.4, 151.7, 161.9,
162.3, 193.2, 196.1.
(E)- and (Z)-5-tert-Butyldimethylsiloxy-2-dimethylphenylsilylme-
thyloct-2-enal (4g). This compound was isolated in 93% yield (338
mg) as a colorless oil (Rf = 0.08 in hexane/EtOAc 20/1) with an E/Z
ratio of 3/97. Z-4g was obtained by flash column chromatography on
1
SiO2. H NMR (CDCl3): δ 0.03 (s, 6H), 0.27 (s, 6H), 0.88 (s, 9H),
ASSOCIATED CONTENT
■
1.96 (s, 2H), 2.22 (q, J = 7.0 Hz, 2H), 3.56 (t, J = 7.0 Hz, 2H), 6.37 (t,
J = 7.0 Hz, 1H), 7.34−7.36 (c, 3H), 7.50 (m, 2H), 9.34 (s, 1H). 13C
NMR (CDCl3): δ −5.3, −2.7, 14.5, 18.3, 25.9, 32.8, 61.4, 127.7, 129.1,
133.6, 138.4, 142.2, 148.8, 194.9. IR (ATR): 3069 w, 3049 w, 2954 w,
2929 w, 2897 w, 2816 w, 2710 w, 1685 m cm−1. MS: m/z (relative
intensity) 347 (1), 210 (12), 209 (18), 136 (13), 135 (100), 129 (19),
91 (11), 89 (27), 75 (41), 73 (54). Anal. Calcd for C20H34O2Si2: C,
66.24; H, 9.45. Found: C, 66.03; H, 9.45.
S
* Supporting Information
1
Copies of NMR and 13C spectra of compounds 1e, 1k, 2aOct
,
2aXy, Z-4a-Z-4j, Z-5k, and 5l, and H−1H COSY, HMBC,
DEPT 135, and NOE spectra of Z-4a. This material is available
free of charge via the Internet at http://pubs.acs.org/.
1
AUTHOR INFORMATION
Corresponding Author
*E-mail for Y.F.: fukumoto@chem.eng.osaka-u.ac.jp.
■
(E)- and (Z)-3-Cyclohexyl-2-dimethylphenylsilylmethylpropenal
(4h). This compound was isolated in 80% yield (230 mg) as a colorless
oil (Rf = 0.24 in hexane/EtOAc 20/1) with an E/Z ratio of 1/99. Z-4h
1
was obtained by flash column chromatography on SiO2. H NMR
Notes
(CDCl3): δ 0.28 (s, 6H), 1.02−1.12 (c, 5H), 1.35−1.38 (c, 2H), 1.60−
1.66 (c, 3H), 1.95 (s, 2H), 2.10 (m, 1H), 6.07 (d, J = 10.1 Hz, 1H),
7.34−7.36 (c, 3H), 7.51 (m, 2H), 9.29 (s, 1H). 13C NMR (CDCl3): δ
−2.8, 14.1, 25.1, 25.7, 31.6, 38.2, 127.7, 129.0, 133.6, 138.5, 139.0,
157.3, 195.5. IR (ATR): 3068 w, 3046 w, 2925 w, 2850 w, 2706 w,
1682 s cm−1. MS: m/z (relative intensity) 286 (3), 137 (21), 136 (13),
135 (100), 119 (24), 107 (17), 106 (12), 105 (20), 92 (11), 91 (18),
75 (32). Anal. Calcd for C18H26OSi: C, 75.46; H, 9.15. Found: C,
75.46; H, 9.23.
(E)- and (Z)-4-Methyl-2-dimethylphenylsilylmethylpropenal (4i).
This compound was isolated in 79% yield (196 mg) as a colorless oil
(Rf = 0.25 in hexane/EtOAc 20/1) with an E/Z ratio of 10/90. Z-4i
was obtained by HPLC (Nacalai COSMOSIL Cholester column) with
iPrOH as an eluent. 1H NMR (CDCl3): δ 0.28 (s, 6H), 0.89 (d, J = 6.8
Hz, 6H), 1.95 (s, 2H), 2.51 (d of sext, J = 6.8 10.1 Hz, 1H), 6.06 (d, J
= 10.1 Hz, 1H), 7.34−7.36 (c, 3H), 7.51 (m, 2H), 9.30 (s, 1H). 13C
NMR (CDCl3): δ −2.7, 14.1, 21.7, 28.4, 127.7, 129.1, 133.6, 138.5,
138.8, 158.7, 195.4. IR (ATR): 3069 w, 3053 w, 2960 w, 2871 w, 1684
s cm−1. MS: m/z (relative intensity) 231 (3), 137 (13), 135 (100), 75
(25). Anal. Calcd for C15H22OSi: C, 73.11; H, 9.00. Found: C, 72.72;
H, 8.87.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We wish to thank the Instrumental Analysis Center, Faculty of
Engineering, Osaka University, for assistance with HRMS and
elemental analyses.
REFERENCES
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1
hexane/EtOAc 20/1). H NMR (CDCl3): δ 0.24 (s, 6H), 2.40 (s,
2H), 7.10 (s, 1H), 7.29−7.40 (c, 8H), 7.49 (m, 2H), 9.50 (s, 1H). 13C
NMR (CDCl3): δ −2.4, 15.0, 127.7, 128.5, 129.0, 129.1, 129.4, 133.7,
135.4, 138.3, 141.4, 146.9, 195.5. IR (ATR): 3068 w, 3048 w, 3023 w,
2957 w, 2898 w, 2717 w, 2708 w, 1679 s cm−1. MS: m/z (relative
intensity) 280 (4), 202 (15), 135 (100), 129 (11), 128 (31), 107 (16),
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(E)- and (Z)-2-(1-Adamantylmethyl)-3-dimethylphenylsilylprope-
nal (5k). This compound was isolated in 82% yield (278 mg) after
hydrolysis of 2k as a colorless oil (Rf = 0.20 in hexane/EtOAc 20/1)
with an E/Z ratio of 1/99. Z-5k was obtained by flash column
1
chromatography on SiO2. H NMR (CDCl3): δ 0.53 (s, 6H), 1.42 (s,
6H), 1.57−1.70 (c, 6H), 1.94 (m, 3H), 2.15 (s, 2H), 6.83 (s, 1H),
7.37−7.38 (c, 3H), 7.55 (m, 2H), 9.75 (s, 1H). 13C NMR (CDCl3): δ
0.0, 28.6, 33.3, 36.9, 42.4, 45.4, 128.1, 129.4, 133.5, 138.1, 152.5, 153.3,
193.3. IR (ATR): 3068 w, 3049 w, 2900 m, 2845 m, 2741 w, 2715 w,
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