Photoinduced electrocyclization

Azabicyclo[3.2.0]hepta-3,6-dienes are the 4 5 bicyclic valence isomers of 1//-azepines. In some cases there is an equilibrium between the bicycle and the azepine, whereas with other 1//-azepines photolysis yields an azabicycloheptadiene that can be isolated and characterized. For example, ethyl 2-azabicyclo[3.2.0]hepta-3,6-diene-2-carboxylate (1), the photoinduced valence isomer of ethyl l//-azepine-l-carboxylate (2), undergoes a clean, exothermic, first order, electrocyclic ring opening (AG = 20 kJ mol-1) to the parent 1//-azepine 2 on heating at 113-143C in an inert solvent (e.g., hexadecane).101... 

A general entry into the azonine system, c.g. 4, is by photoinduced electrocyclic ring opening of the bicyclic valence isomer 3,9 11-13 which is readily prepared by earboethoxynitrene addition to cyclooctatetraene. 

Electron-transfer activation. The photoinduced electron-transfer activation of the [DBC, TCNE] complex generates the DBC+ cation radical, which undergoes a fast electrocyclic ring opening, i.e.,... 

Disubstituted 2,4-cyclohexadienones (112) undergo photoinduced electrocyclic ring opening to the transient ketene derivatives 113, which can be trapped by nucleophiles to prepare the corresponding carboxylic acid derivatives (114 equation 44)196 197 j le reaction has been employed successfully for the synthesis of various carboxylic acids, esters and amides. 

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

Early work (B-69MI51600) on 7V-substituted-l//-azepines revealed that they undergo photoinduced ring contraction to bicyclic valence tautomers as indicated in Scheme 1. Subsequently, it has been found that 3H- and 4H- azepines enter into analogous ring contractions, as do some of their oxo and benzo derivatives. These transformations, which parallel those undergone by cycloheptatriene, are often thermally reversible and occur by an orbital symmetry-controlled disrotatory electrocyclic process. 

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. 

Arylalkenes, such as stilbene derivatives, are important model compounds for the study of the E Z photoisomerization.529 559 560 The compounds absorb significantly over 250 nm, therefore direct irradiation is technically simple. Photolysis of unsubstituted stilbene in aliphatic hydrocarbons at 313 nm affords a photostationary state (PSS) consisting of 93% of (Z)-stilbene and 7% of ( )-stilbene (Scheme 6.4).112 In addition, a photoinduced 67t-electrocyclic (Section 6.1.2) formation of dihydrophenanthrene with a quantum yield of < > 0.10 competes with (Z)-stilbene isomerization ([Pg.232]

The scope and device application of the 6ji-photoinduced electrocyclization of aromatic systems continue to expand and, as in recent years, the reactions of... 

Photoinduced [,2s - -, 2s]-electrocyclization of l-acyl-l//-l,2-diazepines, like that of 3H- and 4H-diazepines, continues to provide a convenient route to dihydrodiazabicyclo[3.2.0]heptenes <86H(24)77>. However, in photolysis of l-azidocarbonyl-3,5,7-triaryl-l,2-diazepines, for example... 

The photoinduced electrocyclic ring opening reaction of cyclohexadiene provides an interesting example of a complex ultrafast reaction mediated by multiple conical intersections. For the short time domain, we derived a realistic d3Tiamical model and performed quantum d3uiamical calculations. 

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