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Cycloaddition Reactions of Substituted Allenes with Triplet Excited Aromatic Thiones: Product Formation and Reaction Mechanism
Author(s) -
Kamphuis J.,
Grootenhuis P.D.J.,
Ruijter A.P.,
Visser R.G.,
Bos H.J.T.
Publication year - 1985
Publication title -
israel journal of chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.908
H-Index - 54
eISSN - 1869-5868
pISSN - 0021-2148
DOI - 10.1002/ijch.198500081
Subject(s) - chemistry , cycloaddition , allene , thiopyran , photochemistry , allylic rearrangement , excited state , singlet state , reaction mechanism , steric effects , medicinal chemistry , computational chemistry , stereochemistry , organic chemistry , catalysis , physics , nuclear physics
The reactivity of substituted allenes H 2 CCC(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)CC(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.