Ring-chain tautomerism atoms

However, the introduction of sterically hindered substituents at the p-car bon atom of nitroalkene (42) completely changes the ring-chain tautomerism of conjugated nitroalkenes. Apparently, steric hindrance caused by two bulky Bu groups in product (42a) (Scheme 3.47) prevents effective conjugation of the jt systems of the C,C double bond and the nitro group, thus causing its deviation from the plane of the C=C bond as a result of which isomer (47a) becomes thermodynamically more favorable. 

The process under consideration could be discussed in terms of ring-chain tautomerism of nitronates (378=381) (see the lower part of Scheme 3.215), the more so that such examples were documented for proton analogs of similar SENA (492). However, both nitronates ((378) and (381)), which were prepared by independent syntheses, are quite stable, and therefore their isomerization in the presence of a silyl Lewis acid should involve ring-chain tautomerism of cations. Evidently, cyclization of nitronate (378) is attributed to high electrophilicity of the carbon atom of the carbonyl group, provided that this group is not involved in the conjugation chain. 

The formation of five-membered cyclic nitronates (404) is explained in terms of ring-chain tautomerism of cationic intermediates A (A=A ). The presence of the alkoxy substituent (R4) at the C-6 atom could stabilize the open form (cation A7), which finally leads to the formation of functionalized five-membered cyclic nitronates (404) probably with the participation of water. 

Ring-chain tautomerism can occur when a substituent group can interact with an NH group or nitrogen atom of a heterocyclic ring. 

Bicyclic 5-5 Systems, One Ring Junction N Atom Three Extra Heteroatoms 2 1 8.05.4.2 Ring-Chain Tautomerism... 

In contrast, BTDOs 5 exhibit high thermal stability, which is attributed to the favorable positioning of the oxygen atoms, preventing ring-chain tautomerism and subsequent decomposition. Elimination of the highly stable N2 molecule is also impossible. Instead, they decompose via the loss of N2O, which is, however, not as favorable thermodynamically <2002EJO2342>. 

A large variety of ring-chain tautomerism studies (H-atom transfer and cycliza-tion) have been reported. The equilibria are usually slow on the NMR timescale, and the tautomers are structurally quite different to be discriminated by NMR spectroscopy. Usually, NMR signals characterizing the tautomers are readily available to be integrated to calculate partitions of tautomers and thermodynamics (fCp = [Ring]/[Chain] — AG° = RTlnfCp). 

Scheme 5. Ring-chain and iminium-enamine tautomerism for the interconversion of per-hydropyrido[2,l-b][l,3]oxazines. a and /3 refer to the down vs. up orientation, respectively, of the alkyl substituents on C-2 and C-9 and of the hydrogen atom on C-9a.

Evidently, the cleavage of the weak endocyclic N-0 bond is the driving force of the ring opening. The nucleophilicity of the /V-oxide oxygen atom in nitronates facilitates the backward cyclization reaction (in the case of minimization of steric hindrance). With regard to the above mentioned one cannot exclude the tautomerism between cyclic nitronates 100 and open (or chain) isomers 101. 

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