Electrocyclic ring closing reactions

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... 

It is easier to see the FMO interactions in electrocyclic ring closing reactions than in ring opening reactions. However, the TS s for ring closing and ring open-... 

Okay, so a cycloaddition reaction was 2ji—2a, and we just saw that sigmatropic reactions are la la, so what about a pericyclic reaction that is Ijt la That s an electrocyclic reaction. Like most pericylic reactions, electrocyclic reactions can either make or break a a-bond. If a a-bond is made in the process, it s an electrocyclic ring-closing reaction. If the a-bond is consumed in the process, the process is named an electrocyclic ringopening reaction, but it s the same process. 

One noteworthy example of an electrocyclic ring-closing reaction is the Nazarov cyclization, which converts divinyl ketones into cyclopentenes, usually in the presence of an acid. 

Bromo-l-aryl-substituted indenes have been reported to be synthesized from 1,2-diaryl-substituted gem-dibromocyclopropanes via a domino sequence involving silver(I)-promoted ionization and 2a -disrotatory electrocyclic ring-opening followed by a 4a -conrotatory electrocyclic ring-closing reaction of the allylic carbocation intermediate (Scheme 46). ... 

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. 

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. 

Disrotatory (Section 30.2) A term used to indicate thatp orbitals rotate in opposite directions during electrocyclic ringopening or ring-closing reactions. 

Electrocyclic ring opening and ring closing reactions of alternating polyenes are controlled by orbital symmetry. 

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... 

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