Simple and general procedure for the synthesis of α,β-alkynyl ketones from nitriles using alkynyldimethylaluminum reagents

Article abstract of DOI:10.1055/s-0033-1339482

A simple and efficient approach for the synthesis of α,β-alkynyl ketones from nitriles and alkynyldimethylaluminum reagents, derived from trimethylaluminum and alkynes, is described. This methodology provides access to a wide range of α,β-alkynyl ketones with aliphatic, aromatic, and heteroaromatic substituents in moderate to high yield (53-90%). In the cases of aryl-substituted nitriles, the product can also be obtained as α,β-alkynyl N-H ket-imines in high yield (88-93%). Georg Thieme Verlag Stuttgart, New York.

Full text of DOI:10.1055/s-0033-1339482

LETTER
▌1953
lSetitemr ple and General Procedure for the Synthesis of α,β-Alkynyl Ketones from
Nitriles Using Alkynyldimethylaluminum Reagents
α,β-Alkynyl Ketones from Nitriles and Alkynylaluminum Reagents
Balaji L. Korbad, Sang-Hyeup Lee*
Department of Life Chemistry, Catholic University of Daegu, Gyeongsan-si, Gyeongbuk 712-702, Korea
Fax +82(53)8503728; E-mail: leeshh@cu.ac.kr
Received: 29.05.2013; Accepted after revision: 01.07.2013
terminal alkynes to nitriles using Ln[N(TMS)2]3 catalyst
Abstract: A simple and efficient approach for the synthesis of α,β-
(Ln = lanthanide metals) and primary amine additive,
alkynyl ketones from nitriles and alkynyldimethylaluminum re-
however, the reported yields are not very satisfactory till
agents, derived from trimethylaluminum and alkynes, is described.
This methodology provides access to a wide range of α,β-alkynyl
now.8
ketones with aliphatic, aromatic, and heteroaromatic substituents in
moderate to high yield (53–90%). In the cases of aryl-substituted ni-
oped and utilized in organic synthesis due to their inherent
triles, the product can also be obtained as α,β-alkynyl N-H ket-
So far, many organoaluminum reagents have been devel-
reactivity, stability, low cost, and commercial availabili-
imines in high yield (88–93%).
ty.9 Since organoaluminum reagents are highly Lewis
Key words: α,β-alkynyl ketones, α,β-alkynyl N-H ketimines,
alkynes, trimethylaluminum, nitriles
acidic, they can interact with heteroatoms, such as oxygen
and nitrogen, and even with carbon to form a strong coor-
dinate complex (1:1).10,11 As a latter example, Micouin et
al. developed the alkynyldimethylaluminum reagent from
α,β-Alkynyl ketones are one of the very important motifs
the reaction of alkynes with trimethylaluminum using a
that have a wide range of synthetic applications due to
catalytic amount of Et3N.11b This reagent has been utilized
their occurrence in biologically active molecules, natural
in a wide range of organic transformations, such as al-
products, and pharmaceutical materials.1–3 Conventional-
kynylative ring opening of activated epoxides12 or oxa-
ly, α,β-alkynyl ketones are prepared from the reaction of
zolidines,13 Pd-catalyzed cross-coupling reaction,14
aldehyde with alkynyl lithium or magnesium reagents fol-
addition reaction with acid chloride to form α,β-alkynyl
lowed by the oxidation.4 One of the direct methods to ac-
ketones,15 and in the synthesis of leustroducsin B,16 alumi-
cess α,β-alkynyl ketones involves the coupling reaction of
noisoxazoles, and pyrazoles,17 etc.
alkynyl organometallic reagent with acid chloride.5,6 Un-
As a part of ongoing research on solid-phase synthesis of
small organic molecules, we have extensively utilized alu-
minum-amide reagents which were prepared from amines
and trimethylaluminum. This reagent has been utilized in
an amidation reaction, as we dubbed as ‘smart cleavage’,
to introduce further diversity onto the core scaffold during
release of product from the solid support.18 In addition, we
recently reported the addition of aluminium amide re-
agents to alkyl- or aryl-substituted nitriles affording ami-
dines.19 These prompted us to investigate the addition
reactions of alkynyldimethylaluminums onto nitriles and
here our findings concerning the synthesis of α,β-alkynyl
ketones are described (Scheme 1).
fortunately, these methodologies are usually accompanied
by competitive homocoupling and also limited with re-
spect to functional-group tolerance and substrate stability.
An alternative approach is the palladium-catalyzed car-
bonylative Sonogashira coupling reaction of terminal al-
kynes or the metalated derivatives with aryl halides in the
presence of carbon monoxide.7 However, they also suffer
from disadvantages like the use of metal salt and noncar-
bonylative Sonogashira product as a side product. In addi-
tion, aforementioned methodologies are necessary to use
expensive metal catalysts, ligands, and they require high
CO pressure, longer reaction time, and tedious purifica-
tion. Recently, Shen et al. reported the direct addition of
i) toluene
60 °C, 6–24 h
ii) SiO2-CHCl3
NH
O
r.t., 0–12 h
R1
C
N
Me2Al
R2
+
R1
R1
R2
R2
1
2
3
4
R1 = alkyl, aryl, heteroaryl
R2 = alkyl, aryl
Scheme 1 Synthesis of α,β-alkynyl ketones 4 and N-H ketimines 3
SYNLETT 21709013, 2415, 1953–1958
Advanced online publication: 14.08.2013
0
9
3
6
-
5
2
1
4
1
4
3
7
-
2
0
9
6
DOI: 10.1055/s-0033-1339482; Art ID: ST-2013-U0494-L
© Georg Thieme Verlag Stuttgart · New York

1954
B. L. Korbad, S.-H. Lee
LETTER
Table 1 Optimization of the Reaction Conditions for the Addition of Alkynyldimethylaluminum Reagent 2a to Nitrile 1a
O
NH
Ph
Me2Al
Pent
Ph
C
N
+
Ph
Pent
Pent
1a
2a
3aa
4aa
Entry
Temp (°C)
Ratio of 1a/2b
Solvent
Yield (%)a
1
2
3
4
5
6
7
8
r.t.
40
60
110
60
60
60
60
1:2
1:2
1:2
1:2
1:1
1:4
1:2
1:2
toluene
toluene
toluene
toluene
toluene
toluene
DCE
n.r.b
40
85
75
60
84
80
THF
trace
a Isolated yield based on 1a.
b No reaction.
Our investigation began by treating benzonitrile (1a, 1 gave slightly better yield compared to unsubstituted aryl
equiv) with an alkynyldimethylaluminum reagent (2a, 2 nitriles (Table 2, entries 1 and 2) and those with electron-
equiv) in toluene at room temperature. However, reaction donating substituents (Table 2, entries 3–5). A heteroaryl
did not proceed at all at room temperature (Table 1, entry nitrile (2-thiophenecarbonitrile) smoothly reacted to af-
1). The reaction was still sluggish at 40 °C, and the reac- ford the desired product (Table 2, entry 8), although it re-
tion was not complete after 12 hours (Table 1, entry 2). By quired longer reaction time (10 h) compared to
increasing the reaction temperature to 60 °C, all the start- aforementioned aryl nitriles (6 h). Alkyl nitriles also re-
ing material was consumed within six hours to give N-H acted well with different terminal alkynes to give the de-
ketimine 3aa as monitored by TLC (vide infra). Subse- sired product in moderate yield (Table 2, entries 9–12).
quent hydrolysis of 3aa using silica for 12 hours provided
the desired ketone 4aa in satisfactory yield (Table 1, entry
It should be noted that the stability of the initial addition
product, α,β-alkynyl N-H ketimines 3, were quite differ-
3). Employment of higher reaction temperature (110 °C)
ent depending on the nature of R1 substituents. When R1
resulted in the formation of multispots on TLC and gave
is alkyl, N-H ketimines 3 were quite unstable and even
low yield (Table 1, entry 4). Then the molar ratios of the
cannot be observed by TLC. On the contrary, 3 (R1 = aryl)
substrate and reagent as well as effect of the solvent were
were stable enough to be observed by TLC, and hydroly-
investigated. The use of twofold excess of alkynyldimeth-
sis of those by silica gel treatment took about 12 hours at
ylaluminum reagent 2a gave better yield than the employ-
room temperature (note: hydrolysis of ketimines using
stronger acid, such as aqueous HCl or AcOH, lead to mul-
ment of stoichiometric amount of 2a (Table 1, entries 3
and 5). However, excessive use of reagent 2a (fourfold)
did not further improve the reaction remarkably in terms
of yield and reaction rate (Table 1, entry 6). Of the solvent
tiple spots on TLC and gave poor yield of the product).
Even in these cases of R1 = aryl, 3 were successfully iso-
lated in high yield when the reaction was quenched by the
tested, toluene and 1,2-dichloroethane (Table 1, entry 3
treatment of moist THF (Table 3, entries 1–4).20
and 7) gave similarly good results; however, THF gave
In general, only the diaryl N-H ketimines are known to be
only a trace amount of product (Table 1, entry 8).
sufficiently stable to be isolated.21 Till now, these diaryl
With optimized reaction conditions (nitrile/alkynyldi-
variants are utilized in a range of reactions, such as reduc-
methylaluminum reagent = 1:2 in toluene at 60 °C) in
tion to amine,22 transimination followed by further trans-
hand, the scope of this reaction was examined using com-
formation,23 catalyzed [3+2] annulation with alkynes,24
bination of series of nitriles and terminal alkynes (Table
oxidative alkynylation with terminal alkynes,25 and dia-
2).20 In brief, the reaction was tolerant for variety of ni-
stereoselective allylation.26 Considering this, aryl alkynyl
triles and terminal alkynes to afford the corresponding
N-H ketimines 3 may be utilized as promising intermedi-
ates in many organic transformations.
products in good to high yields. Aryl nitriles having elec-
tron-withdrawing substituents (Table 2, entries 6 and 7)
Synlett 2013, 24, 1953–1958
© Georg Thieme Verlag Stuttgart · New York

LETTER
α,β-Alkynyl Ketones from Nitriles and Alkynylaluminum Reagents
1955
Table 2 Synthesis of α,β-Alkynyl Ketones 4 from Nitriles 1 and Alkynyldimethylaluminum Reagents 2 via Scheme 1a
Entry
R1
R2
Product
Reaction time (h)
Yield (%)b
O
O
1
6
85
2a
1a
4aa
2
3
4
6
6
6
78
75
79
1a
2b
2a
4ab
O
O
MeO
MeO
1b
4ba
MeO
MeO
1b
2b
2c
2b
2c
4bb
O
MeO
MeO
5
6
7
6
6
6
76
85
90
1b
4bc
O
Cl
Cl
1c
4cb
O
Cl
Cl
1c
4cc
O
S
S
8
9
10
10
74
72
1d
2b
2a
4db
4ea
O
1e
© Georg Thieme Verlag Stuttgart · New York
Synlett 2013, 24, 1953–1958

1956
B. L. Korbad, S.-H. Lee
LETTER
Table 2 Synthesis of α,β-Alkynyl Ketones 4 from Nitriles 1 and Alkynyldimethylaluminum Reagents 2 via Scheme 1a (continued)
Entry
10
R1
R2
Product
Reaction time (h)
Yield (%)b
O
10
75
1e
2b
4eb
O
11
12
10
24
64
53
1e
1f
2c
2c
4ec
4fc
O
a Reaction conditions: nitrile (1 mmol), alkynyldimethylaluminum (2 mmol), toluene (4 mL), 60 °C, 6–24 h, and then the reaction was quenched
with the treatment of SiO2 and CHCl3 (10 g, 100 mL).
b Isolated yield based on nitrile.
On the basis of the results described above, a possible re- N-H ketimines 3 were then subsequently hydrolyzed by
action pathway is shown in Scheme 2. Terminal alkyne 5 the treatment of SiO2–CHCl3 to provide the correspond-
reacts with trimethylaluminum with the evolution of ing α,β-alkynyl ketones 4 in moderate to high yield. In the
methane to afford alkynyldimethylaluminum 2. Then this cases of R1 = aryl, the product can also be obtainable as
reagent 2 coordinates and thus activates the nitrile 1. Via N-H ketimines 3 in high yield.
a four-membered transition state, alkyne attack to the ac-
tivated CN bond to afford the aluminated imine interme-
Acknowledgment
diate. Then the product can be obtained as N-H ketimine
3 (where R1 = aryl) after quenching with THF–water mix-
This article was supported by research grand from Catholic Univer-
sity of Daegu.
ture. On the other hand, the product can be obtained as ke-
tone 4 by SiO2-mediated decomposition of imine to
carbonyl.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synlett. Included are
In conclusion, we developed an addition reaction of al-
kynyldimethylaluminum reagents 2 on to a range of alkyl,
aryl, and heteroaryl nitriles 1. The resultant α,β-alkynyl
general information, experimental details, characterization data for
all compounds and copies of 1H and 13C NMR spectra.SnoIufproi
m
tgioSrantnugIifoop
r
imaotrtn
References and Notes
R1
C
N+
N
C
R1
Me
Me
Al
AlMe3
– CH4
1
–
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hydrolysis
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R1
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3
4
Scheme 2 A plausible reaction mechanism
Synlett 2013, 24, 1953–1958
© Georg Thieme Verlag Stuttgart · New York

LETTER
α,β-Alkynyl Ketones from Nitriles and Alkynylaluminum Reagents
1957
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Table 3 Synthesis of N-H Ketimines 3 from Nitriles 1 and Alkynyl-
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Entry R1 R2 Product
Yield (%)b,c
NH
NH
1
2
3
4
1a 2a
90
3aa
3ab
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1a 2b
88
91
93
NH
NH
1c
1c
2a
2b
Cl
3ca
Cl
3cb
a Reaction conditions: nitrile (1 mmol), alkynyldimethylaluminum
(2 equiv), toluene (4 mL), 60 °C, 6 h, and then the reaction was
quenched with a THF–H2O mixture (5 mL, 8:2).
b Isolated yield based on nitrile.
c Purified by flash chromatography on silica using 3% Et3N in
EtOAc–hexane (1:9) as eluent.
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(20) General Experimental Procedure for the Synthesis of
Ketone 4aa (Table 2, Entry 1) and N-H Ketimine 3aa
(Table 3, Entry 1)
The alane (alkynyldimethylaluminum) solution 2 was
prepared according to the procedure described in ref. 11b.
Alane 2a solution (1.42 mL, 1.4 M in toluene, 2.0 mmol)
was added dropwise to a solution of nitrile 1a (0.103 g, 1.0
mmol) in toluene (4 mL). The resulting reaction mixture was
stirred at 60 °C for 6 h, and the progress of the reaction was
monitored by TLC. After cooling, the reaction was quenched
by pouring of the reaction mixture into a slurry of Merck
silica gel 60 (10 g, 230–400 mesh) in CHCl3 (100 mL). After
being stirred for 12 h, the resultant mixture was filtered and
washed with CHCl3. The combined filtrate was evaporated
in vacuo to provide the crude residue. This was then purified
by column chromatography on silica gel using EtOAc–
hexanes (1:20) as eluent to provide the ketone 4aa (0.170 g,
0.85 mmol, 85%) as colorless oil. To synthesize N-H
ketimine 3aa as product, the reaction was performed under
the same reaction condition as described above. Then, the
reaction mixture was diluted with THF (5 mL), and
quenched with careful addition of a THF–H2O mixture (5
mL, 8:2). Then the resulting mixture was stirred for 20 min,
filtered through Celite, and washed with THF. The
combined filtrate was evaporated to dryness to provide the
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© Georg Thieme Verlag Stuttgart · New York
Synlett 2013, 24, 1953–1958

1958
B. L. Korbad, S.-H. Lee
LETTER
(21) (a) Robertson, G. M. Imines and their N-Substituted
crude product, which then purified by column
chromatography on silica gel using EtOAc–hexanes (1:9)
containing 3% Et3N as eluent to provide the N-H ketimine
3aa (0.180 g, 0.90 mmol, 90%) as colorless oil.
1-Phenyloct-2-yn-1-one (4aa)
Derivatives: NH, NR and N-Haloimines, In Comprehensive
Organic Functional Group Transformations; Katritzky, A.
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1H NMR (400 MHz, CDCl3): δ = 8.18–8.09 (m, 2 H), 7.63–
7.54 (m, 1 H), 7.51–7.41 (m, 2 H), 2.49 (t, J = 7.2 Hz, 2 H),
1.68 (quint, J = 7.2 Hz, 2 H), 1.50–1.32 (m, 4 H), 0.93 (t,
J = 7.2 Hz, 3 H). 13C NMR (100 MHz, CDCl3): δ = 178.5,
137.2, 134.1, 129.8, 128.7, 97.1, 79.9, 31.3, 27.7, 22.3, 19.4,
14.1. HRMS (EI+): m/z [M]+ calcd for C14H16O: 200.1201;
found: 200.1198.
(23) O’Donmell, M. J.; Polt, R. L. J. Org. Chem. 1982, 47, 2663;
and references cited therein.
1-Phenyloct-2-yn-1-imine (3aa)
1H NMR (400 MHz, CDCl3): δ = 10.12 (br s, 1 H), 8.07 (s,
2 H), 7.50–7.30 (m, 3 H), 2.43 (t, J = 7.2 Hz, 2 H), 1.64
(quint, J = 7.2 Hz, 2 H), 1.45–1.27 (m, 4 H), 0.928 (t, J = 7.2
Hz, 3 H). 13C NMR (150 MHz, CDCl3): δ = 160.5, 136.7,
131.2, 128.2, 127.5, 95.1, 78.0, 31.0, 27.8, 22.1, 19.1, 13.9.
MS (EI): m/z (%) calcd for C14H17N 199; found: 199 (10)
[M]+, 198 (84), 156 (100), 143 (77), 115 (38), 77 (25).
(24) Sun, Z.-M.; Chen, S.-P.; Zhao, P. Chem. Eur. J. 2010, 16,
2619.
(25) Laouiti, A.; Rammah, M. M.; Rammah, M. B.; Marrot, J.;
Couty, F.; Evano, G. Org. Lett. 2012, 14, 6.
(26) Dhudshia, B.; Tiburcio, J.; Thadani, A. N. Chem. Commun.
2005, 5551.
Synlett 2013, 24, 1953–1958
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