Ring opening and closing

Rate constants have been determined for the forward and reverse reactions shown in equation (5) in which LL = (a-dimethylaminophenyl)-dimethylarsine bonded through nitrogen and arsenic, L = this ligand bonded only through arsenic, and X = SCN, SeCN, NO2, NJ, or pyridine. 

The rate constant involving ring opening, k2, is little affected by the nature of X, but ki is much affected thereby. A dissociative mechanism is believed to operate in the ring closure, mer to trans, direction.  

Solvent effects on the kinetics of ring closure and ring opening have been studied for reaction (6), 

Ring closure of monodentate carbonatorhodium(III) complexes is dealt with in a general review of formation and dissociation of this class of complexes/ 

In neutral or basic solution, the 0-bonded isomer of [Rh(NH3)5(urea)f isomerizes to the N-bonded form and undergoes hydrolysis in parallel reaction pathways. Interconversion of the 0-bonded and N-bonded isomers is also an important feature of hydrolysis in acid solution.  

8 Coordination Numbers 6 and Above Other Inert Centers 

Kinetics of these processes have been studied for rhodium(III)-chelating-arsine-halide complexes in the reactions of equation (40). 

The reader is referred to Section 5.7.6.2 for discussion of nitrito to nitro linkage isomerization kinetics at rhodium(III).  

---

Metathesis of alkenes has been reviewed in terms of cross-metathesis, ring opening and closing, disproportionation, transmutation, and self-metathesis.34 A review on catalytic processes involving ft -carbon elimination has summarized recent progress on palladium-catalysed C-C bond cleavage in various cyclic and acyclic systems.35... 

The photochemical reactivity of alkenes is also of great interest [1,2]. Studies in this area have led to an expansion of the synthetic utility of these substances. Typical photochemical reactions include cis-trans isomeriza-tions, inter- and intramolecular cycloadditions, photooxidations, and electrocyclic ring opening and closing of conjugated dienes and polyenes. Many of these photoreactions have thermal counterparts. In contrast,... 

For the 1,2-dimethylenecyclobutane rearrangement, Gajewski and Shih 53> have demonstrated preferred conrotatory ring opening and closing. For the allene-allene cycloaddition, then, the sequence would be disrotatory motion as two allenes approach to form the perpendicular biallylene intermediate, followed by conrotatory closure of that species. 

Quantum mechanical calculations on the 2//-pyran ring opening and closing agree with the different mechanisms for photocoloration and photodecoloration experimentally observed in spiropyrans. The photobleaching proceeds from the 7 state, and mechanistically is very different from thermal bleaching, which proceeds from the S0 state and has a much lower activation of energy barrier. 

Ring opening and closing of adenine Paper Fujii, Itaya and Saito 77H(6)1627 ... 

Figure 9 Coupling of mobility with the glass transition in a polymer system. Mobility is the reciprocal of the correlation time for the type of motion indicated. Correlation times were evaluated from D-NMR relaxation data. (A) Reorientation or fluctuation of the chain axis. (O) Rotation about the chain axis. ( ) the 180° ring flips of side chain aromatic rings. Open and closed symbols refer to different degrees of deuteration. (Reproduced with permission from [4], Original data from [52].)...

Figure 1.22 The Fischer glycoside synthesis, illustrated by reaction of methanol and glucose. The ring opening and closing reactions are shown as stereospecific, by analogy with transglycosylations, whereas the displacements at Cl of the pyranosides is nonstereopsecific.

The acid- and base-catalysed hydrolysis of the cyclic carbonate of 2,2-diethylpropane-l,3-diol (36) is accompanied by oxygen exchange of the exocyclic (carbonyl) oxygen (Sarel et al., 1960). In acid solution multiple ring opening and closing could perhaps account for the incorporation of oxygen. In base, however, the results are still not clearly understood. 

Certain pyrylium salts react with primary amines to undeigo a process of ring opening and closing, which results in the replacement of oxygen by nitrogen. This can be a useful approach to pyridinium ions, as found in Scheme 6.31. 

---