Ring closures mechanism

The direct 1-6 ring closure mechanism is the major one for paraffins with at least six carbon atoms in a linear chain [129-131). This mechanism is evidenced by the initial formation of benzene from n-hexane, but not from... 

Modeling studies where R = R = Me provide evidence in favor of a silaimine intermediate however, efforts to trap the silaimine species were unsuccessful. Thus, the support for a specific ring closure mechanism remains inconclusive. 

Scheme III shows Liberman s associative ring closure mechanism 19). The participation of surface hydrogen atoms (26) in the cyclization-ring-opening complex is noteworthy. The other atoms of the C5 ring are claimed as lying on the metal linked to it by physisorption forces.

A comparison of the cyclization rates of alkanes and alkenes may help to distinguish between associative and dissociative ring closure mechanisms, just as in the case of Cg dehydrocyclization of hexane and hexenes. 

This reaction is directly analogous to that found with 1-benzoxepin and the valence tautomeric cyclobutene product obtained from (46) and was formed by a concerted disrota-tory ring closure mechanism. 

Treatment of acylated chloroenamines with sodium borohydride in methanol resulted in the formation of bicyclic lactams124 (Scheme 93). The final isomerization is the result of a proton catalyzed ring-opening-ring closure mechanism. 

There is thus a certain analogy between the proposed end-to-end ring closure mechanism and the condensation reaction, which has been shown to proceed parallel to addition polymerization as shown in scheme below [245,246] ... 

Such a ring cleavage and ring closure mechanism is supported by the rearrangement of 5-cyanouracils into 6-aminouracils in the presence of sodium hydroxide and amines (Scheme 139) (see also Scheme 143). 

The ring-closure mechanism of 2-chloroethanol has been studied on the basis of kinetic and equilibrium chlorine isotope effects. Epoxidation of the terminal double bond of farnesyl acetate has been achieved via the bromohydrin, obtained with NBS. A stereospecific method has been elaborated for the preparation of 1-alkynyloxiranes, starting from the monotosylate ester of acetylenic diols. 1-Alkynyloxiranes are also formed from a-hydroxy quaternary ammonium salts in alkaline medium (Eq. 57). ... 

This reaction, like dicarbene recombination, also has its analog in coordination chemistry, that is, reductive elimination of tetramethylene and pentamethylene ligands from platinum complexes yields cyclobutane and cyclopentane, respectively (777). According to this direct ring closure mechanism, the observed selectivity for dehydrocyclization of n-alkanes on metals (nonformation of quaternary-secondary and tertiary-secondary C-C bonds in reactions of type A and B) should be interpreted in terms of simple steric effects. However, although, in the case of platinum, the concepts of steric hindrance could account for the change of selectivity that occurs with decreasing metal particle size (i.e., cyclization of n-hexane takes place on... 

Even though it has been shown that the S fANRORC) mechanism did not operate in the amination of 4-phenylpyrimidine (29) in m-xylene (83RTC367), a ring-opening and ring-closure mechanism cannot be excluded in the pres-... 

Malonamide and similar compounds under phase-transfer catalytic conditions react via a ring-opening/ring-closure mechanism with 3-benzyl-6-methyl-2TM,3-oxazine-2,4(3//)-dione to give 3-benzyl-5-methylpyrido[4,3-i/]pyriinidine-2,4,7(177,3//,6//)-trione in 90% yield.525... 

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