The reactivity of substituted allenes H2CCC(R)X towards three triplet aromatic thiones has been investigated. Product analysis reveal the formation of two (2+2)‐cycloaddition products the thietanes 3, 4 (E + Z) and occasionally of one (4+2)‐cycloaddition product, thiopyran derivative (Z)‐5, generally in high overall yields. Steady state measurements show that electron donating substituents in the allenic system enhance the overall reaction rate. There is little effect of solvent polarity on the reaction rate. The correlation between the relative reaction rates and the first adiabatic ionization energy of the substituted allenes is in accordance with the formation of an exciplex between the excited thione and that π‐bond of the allenic system which is conjugated with the substituent X. From this exciplex two isomeric allylic 1,4‐biradicals (Z)‐8 and (E)‐8 are probably formed. After inter‐system crossing, ring closure gives the thietanes 3, (E)‐4, (Z)‐4 and/or thiopyran derivative (Z)‐5. The ratio in which 3, (E)‐4, (Z)‐4 and (Z)‐5 are formed is explained by considering electronic and steric factors in the proposed reaction mechanism. Experiments with an optically active allene (+)‐PhC(H)CC(H)Me support the intermediacy of a non‐chiral relatively stable allylic 1,4‐biradical. At infinite allene concentration the quantum yield approaches 1, indicating no energy loss during the reaction. Thus no measurable disproportionation of the allylic biradicals (Z)‐8 and (E)‐8 occurs.

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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.