Ring walk mechanism

Scheme 7.6 Ring walk mechanism of nickel insertion, via the preferential coordination of Ni(PEt3)4, followed by intramolecular oxidative addition.

The rearrangement (automerization) of Dewar thiophene 5-oxide (61), observed by NMR, occurs so much more rapidly than that of the corresponding episulfide that special mechanisms have been invoked. The one which involves a zwitterionic intermediate (Scheme 108) is favored over a pseudopericyclic sulfur-walk mechanism in which the electrons of the carbon-sulfur o--bond and the pair of electrons on sulfur exchange places as the sulfur atom migrates around the ring (80JA2861). 

In 1996, Brookhart and co-workers developed a remarkable class of Pd complexes with sterically encumbered diimine ligands (Scheme 4, S4-1, S4-2, S4-4, and S4-5). These examples are capable of mediating the co-polymerization of ethylene with methyl acrylate (MA) to furnish highly branched PE with ester groups on the polymer chain ends by a chain-walking mechanism (Scheme 10). " This represents the first example of transition metal-catalyzed ethylene/MA co-polymerization via an insertion mechanism. The mechanism for co-polymerization is by 2,1-insertion of MA and subsequent chelate-ring expansion, followed by the insertion of ethylene units. The discovery of these diimine Pd catalysts has stimulated a resurgence of activity in the area of late transition metal-based molecular catalysis. Recently, the random incorporation of MA into linear PE by Pd-catalyzed insertion polymeriza-... 

Photochemical scrambling of ring atoms can involve a ring-walk or a cyclopropene mechanism. 

Tetrazines (39) react with cyclopropenes (149) via the unisolated tricyclic intermediates (150) to form the diazanorcaradienes (151) by loss of nitrogen. Three reactions of the diazanorcaradienes have been observed (1) opening of the ring to give the 5H-diazepines (152) (2) isomerization by the walk mechanism to the diazanorcaradienes (153), which may be transformed into the 4//-diazepines (154) (3) reaction with another molecule... 

Almost all rj2-arene complexes of [Os] are amenable to both intrafacial (i.e., ring-walk) and interfacial (i.e., face-flip) isomerization mechanisms, which allow the metal to coordinate to the most thermodynamically favorable position (Figure 2). Aromatic molecules bearing a redonor group (e.g., anisole, aniline, phenol) tend to place the metal across C5-C6 in order to... 

Other routes to fluoroimidazoles have been reported, some of which are confined to specific structural motifs. For example, methyl and fluorine substituents were used in a study of the phototranspositions of the 1-methylpyrazole ring. The nitrogen walk mechanism produced ring-fluorinated imidazoles from the ring-fluorinated pyrazoles (Fig. 3.55). ° ... 

Mechanistic studies have been carried out on the formation of pyridines by ring expansion of pyrrolyl-1-methyl radicals formed by FVP of 1-aryloxymethylpyrroles at 700 °C (2015UP1). Isotopic labelling gives results consistent with a walk mechanism, and dimethylpyrrole precursor 276 gives 2,5- and 2,6-dimethylpyridines as shown in Scheme 55. The study... 

Photolysis of , 4,5-triphenyl-1,2,3-triazole 1-oxide gives the 1,3,4,5-oxatriazine (13) (80AJC2447). This is the only reported example of this monocyclic ring system, and the mechanism of its formation (Scheme 35) probably involves the intermediacy of an oxaziridine and an oxygen walk process not uncommon in the photochemistry of heterocyclic iV-oxides. 

There is another neat scheme (based on the linear recursion) applicable for hand computation on polyhex benzenoids of up to a dozen or so hexagonal rings. This John-Sachs scheme is based on a one-to-one correspondence between Kekule structures and sets of mutually self-avoiding directed walks on the graph, and indeed this correspondence was (in a special context) utilized in a statistical mechanical context in modelling collections of partly disordered polymer chains. 

Cl-symmetric catalysts such as 9 and 10 bear a Cp ring substituent pointing in the same direction as one of the chlorine atoms on the Zr center. In the active catalyst species, the chlorine is replaced by the growing polymer chain or a n-complex-bonded olefin. The coordination site on the same side as the Cp substituent (site B) is more sterically hindered than the other coordination site (site A). After insertion of a monomer coordinating at site B, the polymer chain is walked back to less sterically hindered site A by the next monomer insertion (alternating/chain migratory mechanism). But in some cases, the polymer chain can back skip to site A before the next monomer insertion (site epimerization). Norbornene, as a bulky olefin, can be rr-bonded only at site (see Chapter 2 for a further discussion of the alternating and site epimerization mechanisms with catalyst 9). 

Apart from hydrodesulphurization, reactions causing transformation or destruction of the thiophen ring are rather rare. Three interesting examples have been reported. In the first, photolysis of 3-cyanothiophen (22) in furan affords the adduct (23) as the major product. 2-Cyanothiophen behaves in a similar way, and an analysis of permutation patterns has indicated that there is a mechanism involving first the 2,5-bridged compound (24) followed by a walk of the sulphur atom. Anils (25) are converted into pyrroles (26) by triethyl phosphite a similar ring-opening by intramolecular insertion of nitrene has also been shown to occur with o-nitrophenyl-di-(2-thienyl)methanes. ... 

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