Alkenes electrocyclizations

The photochemistry of alkenes and dienes has already been mentioned in connection with the principles of orbital symmetry control in electrocyclic and cycloaddition processes in Section 13.2. Cycloadditions are considered, from a synthetic viewpoint, in Chapter 6 of Part B. This section will emphasize unimolecular photoreactions of alkenes and dienes. 

In 1909, Patemo and Chieffi noted that mixtures of tri- or tetra-substituted olefins and aldehydes formed trimethylene oxides when exposed to sunlight. Biichi later repeated Patemo s experiments by irradiating 2-methyI-2-butene in the presence of benzaldehyde, butyraldehyde, or aeetophenone and rigorously purifying and identifying the resulting products. The reaction thus bears the name of its two primary pioneers and has come to represent any photo-catalyzed [2 + 2] electrocyclization of a carbonyl and an alkene. 

Fora [4 + 2 -7r-electron cycloaddition (Diels-Aldei reaction), let s arbitrarily select the diene LUMO and the alkene HOMO. The symmetries of the two ground-slate orbitals are such that bonding of the terminal lobes can occur with suprafacial geometry (Figure 30.9), so the Diels-Alder reaction takes place readily under thermal conditions. Note that, as with electrocyclic reactions, we need be concerned only with the terminal lobes. For purposes of prediction, interactions among the interior lobes need not be considered. 

Thermal and photochemical cycloaddition reactions always take place with opposite stereochemistry. As with electrocyclic reactions, we can categorize cycloadditions according to the total number of electron pairs (double bonds) involved in the rearrangement. Thus, a thermal Diels-Alder [4 + 2] reaction between a diene and a dienophile involves an odd number (three) of electron pairs and takes place by a suprafacial pathway. A thermal [2 + 2] reaction between two alkenes involves an even number (two) of electron pairs and must take place by an antarafacial pathway. For photochemical cyclizations, these selectivities are reversed. The general rules are given in Table 30.2. 

The other commonly quoted industrial photochemical process is the production of vitamin D3 involving a photochemical electrocyclic ring opening followed by a thermal 1,7-hydride shift (Scheme 7.2). This is a further example of a successful low quantum yield process in this case there is no viable thermal alternative. Vitamin A acetate has also been produced commercially using a photochemical isomerization process to convert a mixed tetra-alkene precursor to the all-trans form. 

Uchida and Irie have reported a photochromic system based on ESIPT to an alkene carbon.82 They observed that vinylnaphthol 121 isomerizes to the ring-closed 123 when irradiated with 334 nm light ( = 0.20, Eq. 1.34). The reaction is photoreversible since irradiation of 123 (at400 nm) regenerates the starting vinylnaphthol. The authors proposed a mechanism in which ESIPT from the naphthol OH to the [3-alkenyl carbon gives intermediate o-quinone methide 122, which undergoes subsequent electrocyclic... 

The different electron distribution in the excited state also may lead to other types of reactions. As an example, alkenes and polyenes display a low intermole-cular reactivity, but undergo extremely fast rearrangements, since the tt bonding character dramatically diminishes in the excited state. Thus, free rotation becomes feasible and, where appropriate, electrocyclic and sigmatropic processes take place (Figure 3.3). 

Photoinduced electrocyclization of a carbonyl with an alkene to form polysubsti-tuted oxetane ring systems... 

Some interesting new chemistry has been produced on the well-known 1,5-electrocyclization reaction of alkene-conjugated nitrile ylides but the greatest... 

Intramolecular [2n + 27r] cycloadditions leading to cyclobutanes formally belong to valence isomerizations. Alkene/alkcne, ketene/alkene, and allene/alkene cycloadditions have received detailed attention. These rearrangements provide powerful methods for the synthetic arsenal. An example is the facile synthesis of bieyelo[3.2.0]heptenones 3 by intramolecular ketene/alkene cycloaddition after electrocyclic ring opening of cyclobutenoncs l.89... 

Electrocydic processes are one major group of concerted reactions far alkenes, that is, reactions in which all the bond-making and bond-breaking events occur in a single reaction step. In electrocyclic... 

The 6-electron system required for electrocyclic ring-closure can be made up from any combination of aromatic and alkene double... 

As mentioned on pages 317 and 324, the 1,3-dipolar electrocyclization of thiocarbonyl ylides leads to thiirane derivatives, which represents an excellent method for the preparation of those three-membered rings. Typically, thiiranes are isolated as the final products, but in some instances they are produced as intermediate compounds which spontaneously desulfurize to give alkenes [twofold extrusion (47,48)]. 

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