Pyridines via Fischer Carbene complexes.- W(CO)6-Mediated Cycloadditions of Enamines to Alkynes via Alkynylcarbene and 1,2-Dihydropyridin-2-ylidene Complexes

Rudolf Aumann; Klaus Roths; Matthias Grehl

doi:10.1055/s-1993-22566

Synlett 1993; 1993(9): 669-671
DOI: 10.1055/s-1993-22566

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Pyridines via Fischer Carbene complexes.- W(CO)₆-Mediated Cycloadditions of Enamines to Alkynes via Alkynylcarbene and 1,2-Dihydropyridin-2-ylidene Complexes

Rudolf Aumann^* , Klaus Roths, Matthias Grehl

^*Organisch-Chemisches Institut der Universitét Münster, Orléans-Ring 23, D-48149 Münster, Germany

Abstract

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Pyridinium salts 6 and pyridines 7 are obtained by the W(CO)₆-mediated cycloaddition of the alkyne LiC≡CPh 1 to 4-amino-pent-3-en-2-ones MeC(NHR)=CH-COMe 3a,b [R = H, Me] or the 4-amino-2-iminopent-3-ene 3c [R = Me, X = NMe]. The reaction involves the activation of 1 by formation of an alkynylcarbene complex (CO)₅W=C(OEt)C≡CPh 2, which adds 3 to give 6-amino-1-tungstena-1,3,5-trienes 4. Compounds 4 cyclize spontaneously by the elimination of EtOH and form 1,2-dihydropyridin-2-ylidene complexes 5. Protonation of 5 takes place at the W,C bond and leads to the disengagement of pyridinium salts 6 and the recovery of W(CO)₆. A pyridine 7 is generated from 6 (R = H) after treatment with a base. 1,2-Dihydropyridin-2-ylidene complexes 5 are also obtained by the aminolysis of pyran-2-ylidene complexes 8.

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