Umpolung alkynes

Recently, a further unique domino methodology has been reported by Lu and coworkers (Scheme 2.74) [173]. Herein, a triphenyl phosphine-catalyzed umpolung addition/cyclization of allenes and alkynes containing an electron-withdrawing group 2-316-2-318 followed by reaction with a double nucleophile 2-319 is assumed to account for the production of a broad palette of various heterocycles 2-321 and 2-323 via 2-320 and 2-322, respectively. Dihydrofurans, piperazines, morpholines and diazepanes were obtained during the process. 

This reaction achieves an umpolung cyclization in vhich a terminal alkyne is hydrated and undergoes an intramolecular Michael addition according to the mechanism depicted in Scheme 6.34. 

Triphenylphosphine was employed as a nucleophilic catalyst for the umpolung addition of azoles (225) to the electron-deficient allenes (226 R1 = H, R2 = OEt, R3 = H, Et) to afford the addition products (227). This organocatalytic methodology has been extended to addition-cyclization reactions between electron-deficient allenes or alkynes and pyrrole-2-carboxaldehyde in the presence of a catalytic amount of tri-butylphosphine, giving the substituted indolizine-7-carboxylates (228 R2 = OEt, Me R3 = H, Et).265... 

Figure 2.39 Summary of orbital interactions involved in "LUMO umpolung" in alkynes.

The reactivity of activated C-C double bonds with NHC has been reviewed more specifically. This report details the umpolung reaction involving Michael acceptors, the use of carbenes in Morita-Baylis-Hillman as well as in various cycloadditions. The catalysis of alkyne cycloaddition with nitrile oxide is also covered. 

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