David L. Pole

The Structure of a Seven-Bladed Propeller: C7Ph7+ is Not Planar

The first nondisordered structure of a free tropylium cation has been unequivocally determined by single-crystal X-ray diffraction. The solid-state structure of the heptaphenylcycloheptatrienyl cation, C7Ph7+, in [C7Ph7+][CF3CO2−]⋅2 CF3CO2H reveals a severely crowded system in which the central ring is nonplanar, and the propeller blades (the peripheral phenyl substituents) are arranged in a slightly tilted fashion about the seven-membered ring (right).

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

Abstract 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.

Reactions of nucleophilic carbenes with enols

The formal insertion of dimethoxycarbene (1a) into the acidic C–H bond of pentane-2,4-dione (9a), methyl acetoacetate (9b), 3-methylpentane-2,4-dione (9c) and 1,3-diphenylpropane-1,3-dione (9d) is reported as well as the insertion of 3-benzoyloxazolidin-2-ylidene (1b) into 9c. The β-dicarbonyl compounds 9 are known to be equilibrated with their corresponding enols in benzene solution and the insertions appear to proceed by proton abstraction from the enol tautomers of 9 to generate enolate anions and either a dimethoxymethyl cation (from 1a) or a 3-benzoyloxazolidin-2-ium cation (from 1b). Collapse of these ion pairs at the carbon atom of the enolate yields the major product. Formal insertion of 1a into the O–H bond of the enol tautomer of anthrone (12) is also reported.