Intramolecular carbenoid insertions into thiophene: Reactions of 1-diazo-3-(2-thienyl)-2-propanone and 1-diazo-3-(3-thienyl)-2-propanone

Treatment of 1-diazo-3-(2-thienyl)-2-propanone with catalytic rhodium (II) acetate results in cyclopropanation followed by acid-catalyzed ring opening and tautomerization to yield 5,6-dihydro-4H-cyclopenta[b]thiopen-5-one. Under the same conditions, however, the isomeric 1-diazo-3-(3-thienyl)-2-propanone generates a cyclopropane intermediate which undergoes [4+2] cycloreversion, isomerization and Diels-Alder dimerization to give a complex spiro-disulphide. While the 2-substituted thiophene behaves like other homologous members of the thienyl series, the isomeric 3-substituted thiophene undergoes chemistry seen previously with analogous furanyl compounds. The insight into the mechanistic underpinnings provided by preliminary molecular modeling at a PM3 level is discussed.