Electrocyclic ring closing

In the study of substituent effect on the thermal electrocyclic ring-closing reactions of vinylallenes, two stereo-isomeric boryl-substituted vinylallenes 13 and 14 are synthesized by means of the palladium-catalyzed stannaboration of alkynes (Schemes 59 and 60).248... 

Deprotonation of Cl gives an enolate ion, which in this compound is actually a 1,3,5-hexatriene. As such it can undergo an electrocyclic ring closing. Protonation gives the product. 

An alternative end-game has the CO insert into the Cr=C bond of the allylidenechromium compound to give a Cr complex of a ketene. Electrocyclic ring closing of the ketene would then give the product. 

An electrocyclic ring-opening and an electrocyclic ring-closing... 

To summarize, when H atoms are present on the a-carbon opposite the leaving group, the electrocyclic mechanism usually operates when they are not, the semibenzilic mechanism operates. Why does the electrocyclic mechanism proceed more quickly than the more reasonable semibenzilic mechanism for eno-lizable a-haloketones Deprotonation and electrocyclic ring closing are both very rapid reactions—the latter even when a strained ring is formed—and they must simply be faster than HO- addition and migration, despite what our chemical intuition tells us. 

The electrocyclic ring closing of the allyl cation in the Favorskii rearrangement proceeds only because the cyclopropyl cation in the product is quenched... 

Problem 4.3. When phorone is treated with base, isophorone is obtained by an electrocyclic ring closing. Draw a reasonable mechanism. 

Under photochemical conditions, the electrocyclic ring closing of butadienes always proceeds by the disrotatory pathway, which is the opposite of the result under thermal conditions. The FMOs make the stereochemical result easy to understand. Under photochemical conditions, an electron is promoted from the HOMO i]ii to the LUMO 1//7, so i//i becomes the HOMO. Molecular orbital i//i has an antibonding interaction between the termini of the 77 system in the conrotatory TS but a bonding interaction between the termini of the 77 system in the disrotatory TS, so the reaction proceeds in a disrotatory fashion. 

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