Regioselectivity of the Base‐Induced Ring Cleavage of 1‐Oxygenated Derivatives of Cyclobutabenzene

Oxy anions 3 generated from 1,2‐dihydrocyclobutabenzen‐1‐ones 1 through addition of a charged nucleophile or from 1‐hydroxy‐1,2‐dihydrocyclobutabenzenes 2 by deprotonation with base lead to stable products through distal and/or proximal cleavage of the strained four‐membered ring via benzyl carbanion 4 and/or aryl carbanion 5 . A systematic study of this process reveals the relative stability of the two isomeric carbanions 4 and 5 as a key factor in determining the course of the ring‐cleavage reaction. While benzyl carbanions 4 can be trapped with carbon electrophiles, attempts at trapping aryl carbanions 5 with electrophiles other than H + failed. In protic solvents, the magnesium salt of the tertiary alcohol 2 shows an increased rate of proximal cleavage as compared to its alkali salts. From this, we conclude that, in contrast to benzyl carbanions 4 , free aryl carbanions 5 are of transient existence only. Proximal C,C‐bond cleavage seems to occur either through protonation of 5 from a fast, reversible equilibrium 3 ⇌ 5 in which 3 strongly predominates, or in protic solvents possibly even through a rate‐limiting protonation of 3 at the aromatic C‐atom, bypassing free anion 5 altogether. Thus, additional factors other than just the relative stability of isomeric carbanions 4 and 5 are of importance in determining the regiochemistry of the base‐induced C,C‐bond cleavage in ketones 1 and in alcohols 2 .