DFT computations have been executed aimed at illuminating the variety of pathways by which pyridones react with alkynes in the presence of [CpCoL(2)]: NH-2-pyridones furnish N-dienylated ligands (N-H activation pathway), N-methyl-2-pyridones are converted into ligated cyclohexadienes ([2+2+2] cocycloaddition pathway), and N-alkynyl-2-pyridones may undergo either [2+2+2] cocycloaddition or C-dienylation (C-H activation), depending on the length of the tether. The calculations predict the formation of the experimentally observed products, including their regio- and stereochemical make up. In addition, the unusual regiochemical outcome of the all-intramolecular [2+2+2] cycloaddition of N,N'-dipentynylpyrazinedione was rationalized by computation, which led to the discovery of a new mechanism.

Cobalt-mediated [2+2+2] cycloaddition versus C-H and N-H activation of 2-pyridones and pyrazinones with alkynes: a theoretical study

Iminophosphorane-mediated annelation of a pyridine ring into a preformed pyridine one: synthesis of naphthyridine, pyrido[1,2-c]pyrimidine and pyrido[1,2-c]quinazoline derivatives

The (3Z)-1,3,5-hexatriene (1) to 1,3-cyclohexadiene (2) interconversion is the prototypical example of the six-electron (6e−) electrocyclization reaction as defined by Woodward and Hoffmann (WH).1,2 Among the early observations, the complementary stereochemical results (Figure 1) obtained for thermal (disrotatory, dis) and photochemical (conrotatory, con) processes in the vitamin D system (vide infra) provided experimental bases leading to the formulation of the WH rules. Despite its theoretical significance, the thermal variant of the 6e− electrocyclization3 has found relatively little use in synthesis. By contrast, the photochemical counterpart (photocyclization of stilbenes and analogs) constitutes a powerful synthetic procedure for the formation of aromatic compounds.4 More recently, new ways of inducing electrocyclic processes have been discovered, such as those which proceed by reductive electron transfer5 or by electrical discharge.6 The former appears to mimic stereochemically the photochemical process while the latter is considered to involve vibrationally excited (thermal) ground states. The potential utility of these new methods for inducing 6e− electrocyclizations remains virtually unexplored.

1,3-Cyclohexadiene Formation Reactions

https://apps.dtic.mil/sti/pdfs/ADA183824.pdf

Catalytic Synthesis of Oligosilazanes. 2

[reaction: see text] In prosecution of previous work on the thermal cyclization of 1-aryl-4-methanesulfonyl-2-nitro-3-phenylsulfonyl-1,3-butadienes (7), the 3-unsubstituted derivatives 8, deriving from the initial ring opening of 3-nitrothiophene (2), have been likewise found herein to undergo cyclization, followed by aromatization, in analogous mild experimental conditions, leading to the ring-fused homo- or heteroaromatic nitro derivatives 10. The concerted electrocyclic nature of the process is strongly supported by the outcome of tests based on the variation of the polarity of the solvent or of the electron density on the aryl of 8. Thus, the successful application of the process to the non-phenylsulfonyl-activated 8 significantly widens the scope of a synthetically valuable overall ring-opening/ring-closing procedure from nitrothiophenes. Support to the recently renewed interest in thermal 6pi electrocyclizations as a tool for the construction of the benzene ring is furthermore provided.

From beta-nitrothiophenes to ring-fused nitrobenzenes: an overall ring-enlargement process via a facile, aromatization-driven, thermal 6pi electrocyclization.

James L. Conger

Papers

Pyrolysis of 1-phenyl-1,3-butadienes. New synthesis of 1,2-dihydronaphthalenes