Intramolecular carbenoid insertions: Reactions of alpha-diazo ketones derived from furanyl-, thienyl-, (benzofuranyl)-, and (benzothienyl)acetic acids with rhodium(II) acetate

alpha-Diazo ketones tethered to furan, benzofuran, thiophene, and benzothio phene by a single methylene spacer have been shown to undergo atypical, rho dium(II) acetate catalyzed chemistry. For example, while treatment of 1-dia zo-3-(3-furanyl)-2-propanone with Rh-2(OAc)(4) resulted in the expected 2-( 4-oxo-2-cyclopentenylidene)acetaldehyde, isomeric 1-diazo-3-(2-furanyl)-2-p ropanone undergoes a vinylogous Wolff rearrangement and in the presence of water gives a mixture of 6a-methyl-2,3,3a,6a-tetrahydrofuro[2,3-b]furan-2-o ne and 2-(2-methyl-3-furyl)acetic acid. Rhodium acetate catalyzed decomposi tion of 1-diazo-3-(3-benzofuranyl)-2-propanone and 1-diazo-3-(2-benzofurany l)-2-propanone are also shown to undergo vinylogous Wolff rearrangement des pite the fact that this chemistry is not observed with homologous benzofura nyl systems. alpha-Diazo ketones derived from benzothienyl propionic acids undergo the expected cyclization with 1-diazo-4-(3-benzothienyl)-2-butanone and 1-diazo-4-(2-benzothienyl)-2-butanone giving rise to 1,2,3,4-tetrahydr odibenzo[b,d]thiophen-3-one and 1,2,3,4-tetrahydrodibenzo[b,d]thiophen-2-on e, respectively. While decomposition of 1-diazo-3-(3-benzothienyl)-2-propan one resulted in the formation of 2,3-dihydro-1H-benzo[b]cyclopenta[d]thioph en-a-one, the isomeric 1-diazo-3-(2-benzothienyl)-2-propanone gave a dimer which resulted from a [3 + 2] cycloaddition followed by a [1,3]-alkyl shift . Overall, the results from this study of intramolecular carbenoid insertio n into five-membered heteroaromatic systems show that the resultant chemist ry is dependent on the nature of the heteroatom, position of substitution, and the length of the aliphatic tether.