1,3-Cyclohexadiene, ring opening

Unlike other diene photopericyclic reactions, cyclohexadiene ring opening proceeds with reasonable efficiency in phenyl-substituted derivatives, although the quantum yields are generally significantly lower than those of non-arylated systems. Two examples of the conversion of 93 to 94 are shown in equation 36178,179... 

The evolution of the shapes of the SC orbitals with the progress of the cyclohexadiene ring-opening is illustrated by Fig. 5. The symmetry-unique SC orbitals tj/i-ti/s are shown as three-dimensional isovalue surfaces at the cyclohexadiene end of the IRC segment (leftmost column of orbitals), at the TS (central column of orbitals) and at the hexatriene end of the IRC segment (rightmost column of orbitals). The reflections of t /i, tj/2 and /3 in the symmetry plane which is retained throughout this IRC interval, result in /6, /5 and tj/4, respectively. 

Given the concerted, almost synchronous nature of this gas-phase reaction it might seem reasonable to suppose that the electronic mechanism would resemble those for the Diels-Alder and cyclohexadiene ring-opening reactions, described above. However, our spin-coupled calculations along the IRC reveal a somewhat different picture [3]. 

Irradiation (A, > 300 nm) of the cyclohexadiene (153a) affords a single photoproduct (154a) by way of cyclohexadiene ring opening. In comparison, the... 

Kosma K, Trushin SA, Fms W, Schmid WE. Cyclohexadiene ring opening observed with 13 fs resolution coherent oscillations confirm the reaction path. Phys Chem Chem Phys. 2009 11 172-181. 

Apparent nucleophilic attack on large, fully unsaturated rings may occur by way of attack on a valence tautomer, such as the reaction of oxepin with azide ion. Attack on the oxanorcaradiene valence tautomer leads to ring opening of the three-membered ring, and formation of 5-azido-6-hydroxy-l,3-cyclohexadiene (Section 5.17.2.2.4). 

The cyclohexadiene-hexatriene system seems to be less complicated than the cyclobutene-butadiene system. Cyclohexadiene undergoes photochemical electrocyclic ring opening ... 

Results of studies of the electrocyclic ring opening of 3 fluoro-, 3,3-difluoro-, and 3-tnfluoromethylcyclobutene are consistent with the theoretical predictions of the effect of fluonne on this reaction [142] Surpnsingly, fluonnated analogues of hexa tnene-cyclohexadiene systems undergo complex rearrangements mainly via free radical mechanisms and not by electrocyclic nng operung as expected [143] (equation 35)... 

Figure 3.24 Resonance Raman Stokes and anti-Stokes difference spectra of the photochemical ring opening of 1,3-cyclohexadiene. Anti-Stokes spectra were obtained with 284-nm pump and probe wavelengths, while the two-color Stokes spectra were generated with a 284-nm probe and a 275-nm pump. The line at 801 cm is due to the cyclohexane solvent. (Reprinted with permission from reference [122]. Copyright (1994) American Chemical Society.)...

Figure 3.25 Schematic reaction coordinates for the photochemical ring opening reactions of 1,3-cyclohexadiene. (Reprinted with permission from reference [122]. Copyright (1994) American Chemical Society.)...

Cyclohexadiene itself undergoes smooth photochemical ring opening to Z-l,3,5-hexatriene in both the gas phase (d> = 0.13)176 and in solution (d> = 0.41)71,177. As is almost always the case, extended irradiation in solution leads to the formation of a variety of isomeric products due to secondary irradiation of the Z-triene and its E-isomer (vide infra)11. 

Analogously silylated cyclohexadiene 25 having propargyl alcohol as a pendant was used in the radical intramolecular hydrosilylation followed by ionic ring opening to provide alcohol 26 in 55 % yield (Scheme 6.6) [6]. The cyclization of silyl radical 27 to radical 28 represents an example of a 5-endo-dig process. 

The SC results strongly suggest that the Diels-Alder reaction between butadiene and ethene and the ring-opening of cyclohexadiene pass through aromatic conformations, while in the case of the 1,3-dipolar cycloaddition of fulminic acid to ethyne, the reacting system remains distinctly nonaromatic throughout the course of the reaction. 

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