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9 Generation and cycloaddition of azomethine ylides via demetallation of
imminium salts have been reported. For examples see: (a) W. H. Pearson,
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electron-deficient alkyne, was used, pyrrole derivative 5a was
obtained (entry 4).
Analogously, the reactions of N-methyl-N-(stannylmethyl)thio-
amide 1d with dipolarophiles, electron-deficient alkenes and an
alkyne, gave the corresponding cycloadducts in good yields under
very mild conditions (entries 5–8). Among these, it is noteworthy
that cycloadduct 4d in the reaction with methyl acrylate (2f),
an unsymmetrical dipolarophile, was produced with complete
regioselectivity.
Surprisingly, the cycloaddition of azomethine ylides having
destabilizing substituents or no substituents could also be carried out
(Table 3). The introduction of an alkyl group to the 1- or
3-position of 1,3-dipoles causes extensive destabilization of the 1,3-
dipoles. However, when N-(stannylmethyl)thioisobutyramide 1e
was employed in the reactions with dimethyl fumarate (2a) and
fumaronitrile (2c), the corresponding cycloadducts 3c and 4e were
readily obtained (entries 1 and 2). The reactions of N-(stannyl-
methyl)thioformamide 1f and N-methyl-N-(stannylmethyl)thiofor-
mamide 1g with these dipolarophiles afforded 2,5-unsubstituted
pyrrolines 3d, 4f, 4g, and 4h in good yields respectively (entries 3–
6). It is noteworthy that the azomethine ylide generated from
thioamide 1f is an ultimately non-substituted nitrile ylide
equivalent, and thus, it is a very useful reactive species for the
synthesis of less-substituted N-containing 5-membered hetero-
cycles. When DMAD was employed, the reaction proceeded at
ambient temperature to give pyrrole derivative 5c in high yield
(entry 7). The 2,5-unsubstituted pyrrolines and pyrroles are
known, for example, to be precursors of polypyrroles which are
mainly useful for conducting polymers.
In summary, we demonstrated an unprecedented method of
effective generation of azomethine ylides on the basis of a new
strategy using 1,4-stannatropy. Cycloaddition of the azomethine
ylides, or nitrile ylide equivalents, proceeded under mild conditions
to give pyrroline and pyrrole derivatives in good to excellent yields.
The present strategy is also expected to be applicable to generation
of other 1,3-dipoles and extension to 1,6-stannatropy for
generation of 1,5-dipoles.
10 References on nitrile ylides or their equivalents: (a) H.-J. Hansen and
H. Heimgartner, 1,3-Dipolar Cycloaddition Chemistry, Vol. 1, ed.
A. Padwa, John Wiley & Son, New York, 1984, 177; (b) T. Ibata and
K. Fukushima, Nitrogen, Oxygen and Sulfur Ylide Chemistry, ed. S. J.
Clark, Oxford University Press, Oxford, 2002, 279.
Notes and references
1 R. Huisgen, 1,3-Dipolar Cycloaddition Chemistry, Vol. 1, ed. A. Padwa,
John Wiley & Son, New York, 1984, 1.
528 | Chem. Commun., 2006, 526–528
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