Base cleavage of trimethylsilyl-1,2-dihydrobenzocyclobutenes, and its significance for the acidities of the 1-, 3-, and 4-positions of 1,2-dihydrobenzocyclobutene (“benzocyclobutene”).

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The rates of cleavage of the 1-, 3-, and 4-trimethylsilyl derivatives of 1,2-dihydrobenzocyclobutene (“benzocyclobutene”) (I) by potassium hydroxide in dimethyl sulphoxide—water have been measured spectrophotometrically. The 3-derivative is 10.6 times as reactive as its 4-isomer, and 8.5 times as reactive as its closest analogue 1,2-dimethyl-3-trimethylsilylbenzene. The results are consistent with enhanced electronegativity of the 3-carbon atom [and thus enhanced acidity of the 3-position of (I)], which is associated with the effect of the ring-strain on the hybridization of the orbitals of the atoms of the 4-membered ring. The reactivity of the 1-derivative is much greater than that of its 3- and 4-isomers, and rather similar to that of its analogue, α-(trimethylsilyl)ethylbenzene; any enhancement of the electronegativity of the 1-carbon atom appears to be balanced out by the less effective delocalization of a lone-pair from the 1-carbon into the aromatic ring. In the strongly-basic medium used (0.05 M KOH in dimethyl sulphoxide containing 3 wt. % of water), phenyltrimethylsilane is some 9 times as reactive as its tin analogue; this unusual order of reactivity may be associated with a smaller contribution from proton-transfer to the separating carbon atom in a medium of low hydroxylic content.