Abstract
The arylcyclopropanone methyl hemiacetals (aryl = C6H5, p-ClC6H4 and p-CH3C6H4) are quantitatively isomerised by sodium methoxide or tert-amines to propionic esters. The ring fission is rapid and occurs exclusively at the C1-C2 position, leading to products derived from the most stable incipient carbanion.
The hemiacetals react with acids via a Woodward-Hoffmann cleavage of the C1-C3 cyclopropyl bond, affording a mixture of 1- and 3-substituted arylacetones, the ratio of which depends on the nature of the acid. This acid-catalysed C2-C3 ring-fission only occurs when an aryl group is present. The unsubstituted cyclopropanone hemiacetal isomerises in (weak) acid to the propionic ester (C1-C2 rupture).
The transformation of arylcyclopropanone methyl hemiacetals in acidified methanol to α-aryl-α-methoxy-acetones, is first order in the hemiacetal. From the rate constants a Hammett ρ value of −0.7 is obtained, thus showing a small interaction of the aryl group with the remote reaction centre. This is considered to support a mechanism in which arylcyclopropanone is an essential intermediate. Subsequent protonation of this ketone with simultaneous C2-C3 bond rupture gives the substituted 2-hydroxyallylic cation. For this step an approximate ρ value of −2.9 is calculated, thus indicating that this step is much more sensitive to the presence of a (para) substituent in the aromatic ring. Further evidence for the occurrence of an intermediate arylcyclopropanone is the alkoxy exchange in the starting material when CD3OD or n-propanol is used as solvent. The proposed mechanism is also consistent with the observed stability of the corresponding arylcyclopropanone acetals under similar acidic conditions.
