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Forward and Backward Pericyclic Photochemical Reactions Have Intermediates in Common, Yet Cyclobutenes Break the Rules
Author(s) -
Fuß Werner,
Schmid Wolfram E.,
Trushin Sergei A.,
Billone Paul S.,
Leigh William J.
Publication year - 2007
Publication title -
chemphyschem
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.016
H-Index - 140
eISSN - 1439-7641
pISSN - 1439-4235
DOI - 10.1002/cphc.200600639
Subject(s) - pericyclic reaction , cyclobutene , chemistry , bicyclic molecule , electrocyclic reaction , cycloheptene , diene , photochemistry , excited state , isomerization , conical intersection , cyclobutanes , cope rearrangement , computational chemistry , ring (chemistry) , stereochemistry , physics , molecule , organic chemistry , atomic physics , cyclobutane , natural rubber , catalysis
Photochemical pericyclic reactions are believed to proceed via a so‐called pericyclic minimum on the lowest excited potential surface (S 1 ), which is common to both the forward and backward reactions. Such a common intermediate has never been directly detected. The photointerconversion of 1,3‐butadiene and cyclobutene is the prevailing prototype for such reactions, yet only diene ring closure proceeds with the stereospecificity that the Woodward–Hoffmann rules predict. This contrast seems to exclude a common intermediate. Using ultrafast spectroscopy, we show that the excited states of two cyclobutene/diene isomeric pairs are linked by not one, but by two common minima, p* and ct*. Starting from the diene side (cyclohepta‐1,3‐diene and cycloocta‐1,3‐diene), electrocyclic ring closure passes via the pericyclic minimum p*, whereas ct* is mainly responsible for cis–trans isomerization. Starting from the corresponding cyclobutenes (bicyclo[3.2.0]heptene‐6 and bicyclo[4.2.0]octene‐7), the forbidden isomer is formed from ct*. The path branches at the first ( S 2 / S 1 ) conical intersection towards p* and ct*. The fact that the energetically unfavorable ct* path can compete is ascribed to a dynamic effect: the momentum in CC twist direction, acquired—such as in other olefins—in the Franck–Condon region of the cyclobutenes.