Cycloaddition reactions light promotion

The [2 + 2] cycloadditions are widely used reactions. There are basically three situations in which [2 + 2] cycloadditions are seen when the reaction is promoted by light, when one of the components is a ketene (R2C=C=0) or another cumu-lene (e.g., RN=C=0), or when one of the components has a 1t bond between C and a second-row or heavier element (e.g., Ph3P=CH2 or Cp2Ti=CH2). 

The reaction of l,l-bis(2-thienyl)ethanol 221 with TONE at room temperature afforded a white crystalline compound identified as cycloadduct 223 in quantitative yield (90H1873). The formation of 223 is easily understood as a [4 + 2]-cycloaddition of l,l-bis(2-thienyl)ethylene 222, formed by dehydration of alcohol 221 and TCNE. The reaction was unusually fast, it took place in less than 15 minutes and did not need any source of heat or light to be promoted. Since other vinylthiophenes participate in Diels-Alder reactions to give aromatic cycloadducts under rather severe conditions (Section II,A,5), the authors suggested that the reaction... 

It is proposed that these cycloadditions are also reactions of the alkyne triplet state, and this seems reasonable in the light of the non-concerted nature of the reaction indicated by the fact that dimethyl acetylenedicarboxylate gives a mixture of the same four stereoisomers of the bicyclohexane product with either cis- or /ra i5-but-2-ene (equation 44) . This is in contrast to the stercospecific nature of the Lewis acid promoted cycloaddition of alkenes and alkynes to give cyclobutenes . ... 

Reason 1 [2 + 2] Cycloadditions do not occur under thermal conditions unless one of the components is a ketene or has a heavy atom. No light is specified in this reaction, and ambient light is not sufficient to promote a [2 + 2] cycloaddition. Reason 2 Both 77 bonds that participate in the purported [2 + 2] cycloaddition have trans H substituents. In the product, though, the two bottom H atoms of the cyclobutane are cis. Therefore, one of the tt bonds would have to be reacting antarafacially, which is geometrically impossible. 

Ruthenium (II) polypyridyl complexes promote the photoredox reactions such as Diels-Alder reaction (see formulae 29 31) and azomethine ylide formation followed by [3 + 2] cycloaddition (32-34) by use of visible light. Yoon found 2,2 -bipyrazine ligand is better than 2,2 -bipyridine. Bach reported enantioselective intramolecular [2 + 2] and intermolecular [3 + 2] photocycloaddition by use of chiral hydrogen bond templates (37) and (41). ... 

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