C O M M U N I C A T I O N S
azanickelacycle (run 5).6e 1,2-Dialkyl-substituted alkynes such as
3-hexyne did not react efficiently because of the rapid formation
of the undesired seven-membered azanickelacycle even with slow
addition of a mixture of the alkyne and AlMe3 (run 6).6e For several
cases cited in runs 4, 5, and 6, formation of 8 was observed in the
1H NMR spectra of the crude products. These results are consistent
with the mechanism proposed in Scheme 3.
Table 1. Nickel-Mediated Cyclocondensation of Alkynes, N-BBSA,
a
and AlMe3
In summary, we have demonstrated the nickel-catalyzed three-
component cyclocondensation of imines, alkynes, and AlMe3 to
yield unique azaaluminacyclopentenes. Nickelacycle 1, generated
by the oxidative cyclization of an alkyne and an imine, is a key
intermediate in the cyclocondensation reaction as well as in the
three-component coupling reaction with ZnMe2. Further study will
focus on the related formation of oxaaluminacyclopentenes.
alkyne
run
R1
R2
product
yield (%)b
yield of 8 (%)c
1
2
Ph
Ph
4a
4b
4c
4d
4ef
4fg
86 (71)
85 (82)
90 (99)
85 (73)
65 (44)
27
-
-
-
7
13
59
p-MeC6H4
p-MeC6H4
p-CF3C6H4
TMS
3
p-CF3C6H4
4d
5e
6
Ph
Ph
Et
Me
Et
Acknowledgment. The authors are grateful for financial support
through Grants-in-Aid for Scientific Research (21245028 and 21750102)
and a Grant-in-Aid for Scientific Research on Priority Areas (19028038,
Chemistry of Concerto Catalysis) from the Ministry of Education,
Culture, Sports, Science and Technology, Japan.
a General conditions: alkyne and N-BBSA (0.3 mmol each), solvent
(10 mL). After dropwise addition of AlMe3, the reaction mixture was
stirred until the color derived from 1 (typically purple) disappeared
(1.0-6.0 h). b Isolated yields as allylamines 5 after protolysis. Cited
yields in parentheses are of isolated 4. c The cited yields, determined by
1H NMR, were of the corresponding protonated products. d The reaction
was carried out using 1.5 mmol of TMSCt CPh. e The reactions were
carried out with concomitant addition of AlMe3 and the alkyne. f The
Supporting Information Available: Detailed experimental proce-
dures, analytical and spectral data for all new compounds, and
crystallographic data (CIF) for 2a and 4a. This material is available
minor regioisomer (11%) was also obtained. g Formation of
1,2-dihydropyridine derivative was also observed (also see ref 6e).
a
References
and was found to accelerate with the addition of 10 mol % Ni(cod)2
and PCy3 (Scheme 3, right circle).10 Therefore, the slow addition
of AlMe3 improved the yield of 4a as a result of suppression of
the competitive reaction. Finally, the three-component cyclocon-
densation of N-BBSA, diphenylacetylene, and AlMe3 (via slow
addition over 30 min) in the presence of 10 mol % Ni(cod)2 and
PCy3 afforded 4a in 71% isolated yield (Table 1, run 1).11 While
the isolated yield of 4a was somewhat decreased because of losses
during the purification process, NMR analysis indicated that this
catalytic reaction proceeded quantitatively. In fact, protolysis of
the crude product gave the corresponding allylamine 5a in 86%
isolated yield (run 1). The same reaction conditions were applied
successfully to diphenylacetylene derivatives, which led to clean
formation of 4b and 4c (runs 2 and 3, respectively).
Nonsymmetric alkynes were used as coupling components in
the cyclocondensation with N-BBSA and AlMe3; 4d was formed
in 85% yield as a single regioisomer, but use of 5 equiv of 1-phenyl-
2-trimethylsilylacetylene was essential for the promotion of a
smooth reaction, probably as a result of its diminished ability to
coordinate to Ni(0) (run 4). In contrast, the reaction with 1-phenyl-
1-propyne gave 4e with 86:14 regioselectivity only when slow
addition of both AlMe3 and the alkyne was conducted in order to
circumvent the insertion of the second alkyne into a five-membered
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Scheme 3. Plausible Mechanism for Catalytic Formation of 4
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(11) In contrast, slow addition of ZnMe2 was unnecessary because the
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reaction did not occur.
JA903175G
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J. AM. CHEM. SOC. VOL. 131, NO. 26, 2009 9161