Temperature effects ring opening

Lithium alkyls, e.g. -butyllithium, also effect ring-opening. First, 2-lithiotetrahydrofuran is formed, which at room temperature slowly decomposes through a [3+2] cycloreversion into ethene and the lithium enolate of acetaldehyde ... 

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. 

Ring-opening of the frans-cyclobutene isomer shown takes place at much lower temperature than a similar ring-opening of the cis-cyclouiitciie isomer. Explain the temperature effect, and identify the stereochemistry of each reaction as either conrotatory or disrotatory. 

The conventional route to prepare I generally involves a high temperature melt polymerization of hexachlorocyclotriphosphazene, or trimer (IV). Recent studies have demonstrated the effectiveness of various acids and organometalllcs as catalysts for the polymerization of IV (8). Alternate routes for the preparation of chloro-polymer which do not involve the ring opening polymerization of trimer have been reported in the patent literature (9. 10). These routes involve a condensation polymerization process and may prove to be of technological importance for the preparation of low to moderate molecular weight polyphosphazenes. 

Reinhoudt et al.53) have reported the first synthesis of a monocyclic thiepin stabilized by electronic effects of the substituents. This synthesis utilizes the idea described in Section 2.3.3. 3-Methyl-4-pyrrolidinothiophene (85a) was treated in deuteriochloroform at —30 °C with dimethyl acetylenedicarboxylate. H-NMR monitoring of the reaction indicated that a [2 + 2]cycloaddition proceeded slowly at this temperature giving the 2-thiabicyclo[3.2.0]heptadiene (86a) which rearranged via ring opening of the cyclobutene moiety to the 4-pyrrolydinylthiepin (87a). At the... 

With methylcyclopentane (166, 153) as with methylcyclobutane (112), there is a leveling effect with increasing temperature so that the ring opening probabilities become more nearly equal. 

Molsidomine and pirsidomine, are both stable as solids at room temperature in the absence oflight [137]. However the ring opened metabolites SIN-1A and C87-3786 are both photo labile and sensitive to an oxidising environment resulting ultimately in the release of superoxide and NO in stoichiometric quantities [138]. Generation of these two species is an obvious problem due to the resulting formation of peroxynitrite and the generation of OH, which may initiate lipid peroxidation [139-141] (see Eq. (19)). Such concerns over the formation of peroxynitrite from SIN-1A or C87-3786 are warranted since their cytotoxic effects show close consistency with cellular studies doped with neat peroxynitrite [142, 143]. 

Stassin F, Jerome R (2003) Effect of pressure and temperature upon tin aUcoxide-promoted ring-opening polymerisation of s-caprolactone in supercritical carbon dioxide. Chem Commun 232-233... 

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