Electrocyclic reactions photochemical conversion

Electrocyclic reactions can be brought about by heat, by ultraviolet irradiation and sometimes by use of metal catalysts. The thermal reaction is generally not reversible and as written above cyclobutenes have been converted to 1, 3 dienes by heating between 100° and 200°C. But the photochemical conversion can be carried out in either direction. Generally 1, 3 dienes can be converted to cyclobutenes rather than the reverse because the dienes because of n electrons are strong absorbers of light at the used wavelengths. 

A silicon version of the well-studied electrocyclic interconversion of C4H6 (cyclobutene, bicyclobutane, butadiene, etc.) has been investigated both experimentally23,101 and theoretically.102,103 Irradiation of 51 in 3-methylpentane (X >420nm) gives a 1 9 mixture of 51 and bicyclo[1.1.0]tetrasilane 88 at the photostationary state as determined by UV-vis spectroscopy.23 When the mixture is left for 12 h in the dark at rt, 51 is produced quantitatively [Eq. (58)]. The photochemical conversion of 51 to 88 and the thermal reversion can be repeated more than 10 times without any appreciable side reactions. 

Electrocyclic reactions are a type of pericyclic rearrangement reaction where the net result is a conversion of one Tt-bond into a G-bond (Scheme 9.16). These reactions can be either photochemically or thermally induced. Selected examples of the potential of photoinduced 6jt-electrocyclization for organic synthesis are shown below. 

In the skin of animals, 7-dehydrocholesterol is converted to vitamin D, by the reaction sequence that follows. The first step in this process, the conversion of 7-dchy-drocholesterol to pre-cholecalciferol, requires light. This is an electrocyclic reaction and must occur by a conrotatory motion to avoid the formation of a highly strained trans double bond in one of the rings. Conrotation involving three pairs of electrons must occur photochemically to be allowed. 

The forward reaction can be induced photochemically and the back reaction normally thermally. The products 8, 9 formed in this conversion can only be written in a zwitterionic form. This is the basic difference from spiropyrans or spiroxazines which also undergo a 67t-electrocyclic reaction, yielding, however, an electroneutral molecule. 

Photochemical conversion of stilbenes to phenanthrenes via a six 7t-elec-tron conrotatory cyclization according to an electrocyclic mechanism to the dihydrophenanthrenes and subsequent dehydrogenation is a very famous and useful synthetic reaction (103). 

There are also examples of electrocyclic reactions that follow the stereochemical outcomes (conrotatory vs. disrotatory) expected for reactions under orbital symmetry control. For example, the photochemical ring opening of Eq. 16.24 should be a six-electron, conrotatory process, and indeed the product has the predicted trans double bond. An important biological example of such a process is the photochemical conversion of ergosterol to pre-vitamin D (Eq. 16,25), a key event in the synthesis of vitamin D. 

An electrocyclic reaction is defined as the thermal or photochemical conversion of an acyclic conjugated system into a ring system by formation of a o bond between the ends of the conjugated system in a concerted process, or the reverse of this reaction. These reactions are reversible in nature. 

To summarize, for a linear polyene with 4, 8, 12,. ..Jt electrons involving in its thermal electrocyclic reaction, the reaction will follow a conrotatory pathway conversely, its photochemical electrocyclic reaction will adopt a disrotatory pathway. On the other hand, for a linear polyene electrocyclic reaction with 6, 10, 14,. .. r electrons involved, the pathways of its thermal and photochemical electrocyclic reactions will be disrotatory and conrotatory, respectively. 

Diphenylcyclohexadiene shows divergent photochemical behavior, depending on whether the reaction is induced by direct irradiation or by photosensitization. On direct irradiation, the electrocyclic conversion to 1,1-diphenylhexatriene is dominant, whereas a... 

An important group of conjugated diene/triene systems are those in the vitamin D series. The key reactions in the commercial manufacture of vitamin D (and probably also in its formation in skin exposed to daylight) are a photochemical, conrotatory electrocyclic ringopening in the provitamin, and a thermal 1,7-shift of hydrogen in the previtamin so formed (2.23). High conversions to the vitamin are not normally possible because all three species absorb appreciably at the... 

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