The copper(I)-catalyzed regioselective [3 + 2] cyclization of pyridines toward alkenyldiazoacetates leading to functionalized indolizine derivatives is reported. A broad range of pyridine derivatives (including quinoline and isoquinoline) is compatible with this cyclization reaction. The process represents the first successful example of metal-catalyzed cyclization of a π-deficient heterocyclic system with alkenyldiazo compounds.
The cycloaddition reactions of various C-heteroarylimines with α,β-unsaturated Fischer carbene
complexes have been studied. Either [3 + 3] carbocyclization or [3 + 3] and [4 + 3] heterocyclization reactions
were accomplished, depending on the structure of the imine and on the type of unsaturation of the carbene
complex. Imines derived from furan-, 4, benzofuran-, 5, and N-methylindole-2-carboxaldehyde, 7, gave C3 +
C3 cycloadducts 10−12 on reaction with alkynyl carbene complexes 2a and 3. The reaction of N-unsubstituted
pyrrole imine 8a with alkenyl carbene complexes 1 involved the ring nitrogen atom of the pyrrole unit leading
to C2N + C3 heterocycloadducts 14 and 15. Moreover, N-unsubstituted imines 8 and 9 underwent C2N2 +
C3 heterocyclization to alkynyl carbene complexes 2 and 3 to furnish zwitterionic metalpentacarbonyl containing
heterocycles 17 and 18. We have studied the thermal behavior of heterocycles 17, finding that they led to
bispyrrole 21 or indolizine 20 when heated at 120 °C in the presence and in the absence of carbon monoxide,
respectively. The results obtained show that the [3 + 3] cyclizations are promoted by a [1,2]-M(CO)5 shift,
while in the case of the [4 + 3] cyclization, a [1,3]-M(CO)5 shift follows the ring-closure step. The X-ray
analysis of compound 17b, resulting from 1,3-pentacarbonyltungsten migration, is provided.
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