Photopericyclic Reactions of Conjugated Dienes and Trienes

Predistortion amplified in the excited state

“…Such substituents pretwist the 2,3 single bond. The twist is C 2 symmetric (conrotatory, which supports the initial path to bicyclization [3,48]), and antarafacial for the H-shift, whereas the ring closure would be disrotatory (see also the brief summary of these and other aspects on p. 222-223 in [49]). Obviously this is again a case for Fig.…”

“…From these and a large number of other polycyclic cyclohexadiene systems, from temperature and wavelength dependences, Dauben et al concluded that the selection of the path is again controlled by the conformers [48]; and from inspection of the stereochemistry by X-ray diffraction or from conformational analysis by molecular models, they inferred that the conrotatory ring opening occurs most efficiently from the half-chair conformation with a C2-C3 twist angle around 20 or more, whereas the disrotatory ring closure to cylocbutenes takes place from nearly planar conformations with a C2-C3 twist angle of 10 or smaller (see [42,48,49]) (some authors [44,50] prefer the torsion of the C5-C6 single bond as a criterion for the ring puckering).…”

“…This includes -electrocyclic ring opening of cyclohexadiene and derivatives (even large molecules -although with relatively localized excitation -such as 7-dehydrocholesterol [40,[51][52][53]), as described above; ring closure of dienes to form cyclobutenes [54,55] and the corresponding ring opening [56], ring opening of cyclooctatriene to octatetraene [57], -formation of bicyclo [110]alkanes from dienes [48,55,58]. Also formation of bicyclo[3.1.0]alkenes from trienes must be ultrafast, as it can compete with cis-trans isomerization [28,41,42,48,49,59], -[2 + 2]-cycloaddition to cyclobutanes [60][61][62], including also the dimerization of cyclohexadiene, which at high concentration can compete with the ultrafast ring opening [2] ([2 + 2]-cycloaddition in a divinylcyclophane takes longer -13.5 ps [63] -perhaps due to unfavourable geometry and/or too much delocalization of the excitation), -sigmatropic hydrogen 1,3-and 1,7-migration [55,64-68] and a suggested antarafacial H 1,5-migration (Scheme 4 and context in [69], proposed geometry confirmed in Fig. 13 of [70]), sigmatropic alkyl 1,3-migration [60].…”

Forward and Backward Pericyclic Photochemical Reactions Have Intermediates in Common, Yet Cyclobutenes Break the Rules

“…The product ratio cis,cis-COD/ cis,trans-COD is 2.5 when cis,cis-COD is excited, [18,34] whereas it is 1.3 if BCO7 is the reactant; [9,35] see in this context also the discussion on p. 220 of ref. [18]. Our mechanism is hence compatible with more observations than Schemes 2 b and c.…”

“…within the IS). Scheme 2 c shows a third interpretation-"adiabatic" ring opening-in which excited CB produces excited ccD, which then partly isomerizes to ctD; [9,15,18] this mechanism provides a way for CB to open non-stereospecifically without formally breaking the WH rules. The goal of this work was to check these ideas by monitoring the forward and backward paths in a time-resolved experiment.…”