The chemistry of small ring compounds. Part 30. Reactions of arylcyclopropanone hemiacetals with bases and acids

Abstract

AbstractThe 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.