Tetramethylene diradical singlet

Since singlet tetramethylene diradicals are the presumed intermediates in a variety of reactions, they have been the subject of a large number of experimental investigations. Ab... 

Let us first do a reality check we ll test the ability of some model chemistry methods to perform geometry optimizations on singlet 1,3-propanediyl or trimethylene (CH2-CH2-CH2) and on singlet 1,4-butanediyl or tetramethylene ( CH2CH2CH2CH2 ), simple singlet diradicals. 

In the study of photochemical cycloadditions of singlet or triplet (sensitized) diphenylvinylene carbonate with vinyl ethers [110], however, Lewis clearly proposed cycloaddition either directly from the exciplex, or via collapse of the exciplex to a diradical tetramethylene intermediate. 

Numerous [2 + 2] cycloadditions which may proceed through dipolar intermediates (tetramethylenes which may be represented as resonance hybrids of singlet diradical and zwitterionic forms) react with partial or even complete loss of the stereochemical relationships present in starting materials. Reactions between the cis and trans isomers of l,2-bis(trifluoromethyl)-l,2-dicyanoethylene and cis- and trans-propenyl n-propyl ether are not completely stereoselective. The trans electrophilic alkene (107) and the cis isomer of the enol ether (108), for instance, give rise to products (109), (110) and (111), the last having trifluoromethyl groups in cis relationship one to the other. ... 

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