1,3-Cyclohexadiene, photochemical reactions

Trauner and colleagues [39] recently found a striking contrast in the thermal and catalyzed reactions of a triene. Thermal reaction of a trienolate readily underwent disrotatory electrocyclization to afford cyclohexadiene (delocalization band in Scheme 8) in accordance with the Woodward-Hoffmann rule. Surprisingly, treatment of the trienolate with Lewis acid did not result in the formation of the cyclohexadiene but rather gave bicyclo[3.1.0]hexene in a [4n +2nJ manner (pseudoexcitation band in Scheme 8). The catalyzed reaction is similar to the photochemical reaction in the delocalization band. 

The bora-2,5-cyclohexadienes 25, 43, and 44 have been used in experimental studies. Photochemical reaction of 25 and 43 with Fe(CO)s produces the robust complexes 45 (29) and 46 (92) with 7)5-divinylborane structures (92), whereas thermally induced complex formation of 44 with Fe2(CO)9 is accompanied by ligand isomerization and affords complex 47 (67). We note in passing that S(UB) = 38.8 ppm for 47 is at rather low field. The only strictly comparable boraolefin known is l-methoxy-6-(trimethylsilyl)bora-2,4-cyclohexadiene [8(nB) = 47.1 ppm] (26). On this basis, the high field shift upon complexation (only 8.3 ppm) indicates weak Fe-B interaction. 

The literature of mechanistic aromatic photochemistry has produced a number of examples of [4 + 4]-photocycloadditions. The photodimerization of anthracene and its derivatives is one of the earliest known photochemical reactions of any type97. More recently, naphthalenes98, 2-pyridones" and 2-aminopyridinium salts100 have all been shown to undergo analogous head-to-tail [4 + 4]-photodimerization. Moreover, crossed [4+4]-photocycloaddition products can be obtained in some cases101. Acyclic 1,3-dienes, cyclohexadienes and furan can form [4 + 4]-cycloadducts 211-214 with a variety of aromatic partners (Scheme 48). 

The photochemistry of all these compounds is of interest with respect to the well-known photochemistry of cyclohexadiene (1.3) derivatives, for instance, the photochemical reactions of 2.4.6-triaryl-cyclohexadien(2.4)-on-ol(2) series of Perst and Dimroth or in a more comprehensive context the photochemical transformation of ergosterol, luniisterol, isopyrocalciferol and pyrocalciferol which were first studied by Windaus and Dimroth and fully investigated by Veluz, Havinga and Dauben... 

If we analyze the case of hexatriene to cyclohexadiene conversion, the situation is just the reverse. The thermal reaction should be disrotatory and the photochemical reaction conrotatory. The butadiene belongs to (4ri)n system and hexadiene to (4n +2)rc system, and a generalization of the systems may be attempted. 

On the positive side, there are photochemical reactions that are essential for human health. One of these is the formation of vitamin D (the antirachitic vitamin) by irradiation of ergosterol. This photochemical reaction is an electro-cyclic ring opening of the cyclohexadiene ring of ergosterol of the type described in Section 28-2D. The product, previtamin D2, subsequently rearranges thermally to vitamin D2 ... 

Fig. 5.10 HOMO for photochemical reaction Here the electron gets promoted to % in cyclohexadiene.

If production of an oxidizing hole in the da orbital is the important factor in the photochemical reaction, then electrochemical veneration of such a hole should produce a highly reactive intermediate mat would mimic the initial step in the 3(da po) photoreaction. Several of the binuclear complexes undergo reversible one-electron oxidations in noncoordinating solvents (22-24). The complex Rh2(TMB)42+ possesses a quasireversible one-electron oxidation at 0.74 V (Electrochemical measurements for [Rh2(TMB)4](PF6)2 CH2CI2/TBAPF6 (0.1 M), glassy carbon electrode, 25°C, SSCE reference electrode). Electrochemical oxidation of Rh2(TMB)42+ in the presence of 1,4-cyclohexadiene exhibits an enhanced anodic current with loss of the cathodic wave, behavior indicative of an electrocatalytic process (25). Bulk electrolysis of Rh2(TMB)42+ in an excess of 1,4-cyclohexadiene results in the formation of benzene and two protons (Equation 4). 

In the course of their synthesis of Vitamin B12, R. B. Woodward and co-workers were puzzled by the failure of certain cyclic products to form from apparently appropriate starting materials—in particular, the stereochemistry of interconversions of cyclohexadienes with conjugated trienes in thermal and photochemical reactions. Woodward, in collaboration with Roald Hoffmann (ca. 1965), discovered that the course of such reactions depended on identifiable symmetries of the participating molecular orbitals. The principle of conservation of orbital symmetry can be stated thus ... 

Apply the Woodward-Hoffmann rules to the electrocyclic reaction of hex-atriene to cyclohexadiene considering the appropriate Hiickel MO s. Determine whether the mechanism is conrotatory or disrotatory for both thermal and photochemical reactions. 

The interconversions of the corresponding dimethylcyclohexadienes and the 2,4,6-octatrienes are also stereospecific (Fig. 29.13). Here, too, thermal and photochemical reactions differ in stereochemistry. If we examine the structures closely, we see something else the stereochemistry of the triene-cyclohexadiene interconversions is opposite to that of the diene-cyclobutene inlerconversions. For the thermal reactions, for example, cis methyl groups in the cyclobutene become cis and trans in the diene cis methyl groups in the cyclohexadiene are trans and trans in the related triene. 

Barton has suggested 35> that in photochemical reactions of cyclic conjugated olefins, ring fission will predominate in rings of n annular atoms containing (n/2)-l double bonds and valence tautomerization will occur in other systems. Consequently, it was anticipated that 1,3,5-cyclo-octatriene upon photolysis should give rise to an acyclic tetraene in the same manner that 1,3-cyclohexadiene opens 35> 36> to 1,3,5-hexatriene. When 1,3,5-cyclooctatriene (55) was irradiated in solution (ether or pentane) 37> 39> two isomerization products were isolated, bicyclo[4.2.0]-octa-2,7-diene (56) and tricyclo[5.1.0.0 4> 8]oct-2-ene (57). The formation of 56 is not exceptional. The formation of 57 has been visualized by... 

As the open triene closes to the cyclohexadiene (Rg. 20.18), the only way to create a bonding interaction between the end carbons is to close in a disrotatory fashion from 4>3, and in a conrotatory fashion from <1>4. So, we predict that the thermal interconversion will occur in a disrotatory way, and that the photochemical reaction must involve conrotation. 

FIGURE 20.18 For the hexatriene-cyclohexadiene system, in order to produce a bonding interaction, the thermal reactions must take place in a disrotatory fashion and the photochemical reactions in a conrotatory way. 

Thermal reactions are generally not reversible (although there are some exceptions). Some cyclobutenes can be converted to 1, 3-diene by heating to 100-200 C. Photochemical reaction can be carried out in either direction. Some 1, 3-dienes can be converted to cyclobutenes rather than reverse reaction. Cyclohexadiene to 1 3, 5-trienes conversions cause thermal ring closure rather than photochemical ring opening. These reactions are highly stereospeciflc. 

Predictions for 1, 3, 5-Hexatriene cyclohexadiene interconversions These predictions can be made on the similar grounds as for 1, 3-butadiene cyclobutene interconversion. Photochemical reaction is feasible by conrotatory mode whereas thermal reaction follow disrotatory mode of ring closure as is explainable by Fig. 4.1. and Fig. 4.2, respectively. 

Problem 5.1 Analyse the cyclohexadiene hexatriene interconveision by the Zimmerman method. What is the interaction (i.e. Huckel or Mobhis) at the transition state of the thermochemical and photochemical reactions ... 

Shima, K., Kubota, T., and Sakurai, H., Organic photochemical reactions. XXIV. Photocycloaddition of propanal to 1,3-cyclohexadiene, Bull. Chem. Soc. Jpn., 49, 2567, 1976. 

In contrast to the commonly observed, highly selective photochemical reactions, thermal rearrangements of endoperoxides often lead to more complex mixturex of products.A naturally occurring compound, the terpene endoperoxide ascaridole (3a) rearranges into isoascaridole (4a) upon irradiation (long-wavelength excitation at 366 nm) while similar cyclohexadiene endoperoxides 3 result in mixtures of bis-epoxides 4 and epoxyketones 5 upon thermolysis (Scheme 5). ... 

The most striking feature of electrocyclic reactions is their stereochemistry. For example, (2 ,4Z,6 )-2,4,6-octatriene yields only c/s-5,6-dimethyl-l,3-cyclo-hexadiene when heated, and (2 ,4Z,6Z)-2,4,6-octatriene yields only trnns-5,6-dimethyl-l,3-cyclohexadiene. Remarkably, however, the stereochemical results change completely when the reactions are carried out under what are called photochemical, rather than thermal, conditions. Irradiation, or photolysis,... 

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