Electrocychc ring opening

From the colorless state it can be switched with light of short wavelength (A = 380 nm) via an electrocycHc ring opening and cis/trans rotation of one half of the molecule into a state with violet/purple color. The reverse reaction is effected by visible light (A = 580 nm). Since the system is metastable, one of the two reaction directions is matched by a rival thermal reaction, the thermoreversion. This progresses, however, in the case of benzospiropyran, at room temperature by a factor of 10 slower than the light-induced reaction. 

Stoichiometrically protonated cycloheptadienones appear to undergo electrocycHc ring-opening/iing-closure processes R. Noyoii, Y. Ohnishi and M. Kato, J.Am. Chem. Soc., 94, 5105 (1972). 

The product is clearly derived from a [3 -I- 2] cycloaddition. Disconnection of the appropriate bonds leads back to an azomethine ylide intermediate, which can be formed by electrocychc ring opening of the aziridine. 

Sheradsky and Moshenberg attempted the preparation of (88) by electrocychc ring opening of the dione (89). Irradiation of (89) did indeed give the triazonine (M), but this was too reactive to be isolated. When the irradiation was conducted in dichloromethane solution, (88) rearranged to (90), but in methanol, transannular addition of the solvent to (88) occurred, leading ultimately to a mixture of (91) and (92) (Scheme 2) <84JOC587>. 

For example, during the thermal electrocychc ring opening of 3,4-dimethylcyclobutene, the trans-isomsi (1) yields only (2 ,4 )-hexa-2,4-diene (2) and the cii-isomer (3) yields only (2 ,4Z)-hexa-2,4-diene (4). On irradiation, however, the results are opposite. Cychzation of the 2 under photochemical conditions yields the cii-product (3) (Scheme 2.2). 

It would be difficult, at this stage, to present a properly argued preference between schemes A and B in the Introduction as the more faithful representation for the disrotatory electrocychc ring-opening of cyclohexadiene. Chemical intuition (at least that of the authors) would suggest scheme B, but scheme A is on the textbook pages and, as has been shown by the SC description of the 1,3-dipolar addition of fulminic acid to ethyne, a heterolytic scheme remains a definite possibility. 

The SC descriptions of the electronic mechanisms of the three six-electron pericycUc gas-phase reactions discussed in this paper (namely, the Diels-Alder reaction between butadiene and ethene [11], the 1,3-dipolar cycloaddition of fulminic acid to ethyne [12], and the disrotatory electrocychc ring-opening of cyclohexadiene) take the theory much beyond the HMO and RHF levels employed in the formulation of the most popular MO-based treatments ofpericyclic reactions, including the Woodward-Hoffmann rules [1,2], Fukui s frontier orbital theoiy [3] and the Dewar-Zimmerman model [4-6]. The SC wavefunction maintains near-CASSCF quality throughout the range of reaction coordinate studied for each reaction but, in contrast to its CASSCF counterpart, it is veiy much easier to interpret and to visualize directly. 

The bisacridinium salt 39 (lucigenin), on oxidation with an alkaline H2O2 solution, displays an intensely green chemiluminescence. The light emission is due to the excited state of N-methylacridone (41), formed by an electrocycHc ring opening of the initially formed dioxetane (40) (cf p. 55) [223] ... 

Ultrafast dynamics in bacteriorhodopsin We consider the ultrafast dynamics in bacteriorhodopsin (bR), which is a light-harvesting system. A related class of reactions is electrocychc ring opening or closure as discussed in Section 8.2.1. Then we turn briefly to quantum chemistry to recognize that even the simple process of photoassisted rotation about a double bond is not quite so simple. 

ElectrocycHc ring opening of the cyclobutene rings in adducts 422 and 424 produces the bridged oxepine products 423 and 425 in high yields (Scheme 89 2000CEJ3706). 

Banwell MG, Harvey JE, JoUiffe KA. rt-Allyl cation cychsa-tions initiated by electrocychc ring-opening of gem-dihalocyclopropanes application to the first total syntheses of the crinine-t3fpe alkaloids maritinamine and epi-maritinamine. J. Chem. Soc., Perkin 1. 2001 2002-2005. 

---