ChemInform Abstract: Thermal Cycloaddition of Substituted Thiobenzophenones with Phenylallene: Mechanism and Linear Free Energy Relationships of Reaction Rates.

Abstract

075ChemInform Abstract The reaction rates for the thermal cycloaddition of thiobenzophenones (I) with phenylallene (II) to products such as (III) and (IV) and occasionally an unidentified product (V), whose structure (probably either (VII) or (VIII)) is not established, and (VI) are determined and analysed by linear free energy correlations. Separate correlation of electron-donating and electron-withdrawing substituentsgives rise to two lines with different slopes. This indicates that the thione system is relatively more sensitive to electron-donating substituents than to electron-withdrawing substituents. The contribution of resonance effect is 69% and the contribution of inductive effect is 31%. An inverse secondary isotope effect on the rate constant of disappearance of unsubstituted thione (I) during the reaction with (II), Ph-CD=C=CH2, and Ph-CD=C=CD2 is observed; an isotope effect of 1.20 ±0.02 is observed in the case of allene CX3-O-CH=C=CH2 (X: -H, -D). Solvent effects for unsubstituted (I) + (II) are studied also. The optically active allene (+)-Ph-CH=C=CH-tBu (28% e.e.), when reacted with xanthene-9-thione gives products with no measurable optical activity indicating a non-chiral intermediate.