Photochemical C2-C6 cyclization of enyne-allenes: detection of a fulvene triplet diradical in the laser flash photolysis

Enynone-enabled migratory insertion and Schmittel cyclization cascade for the synthesis of furan-fused fluorenes

A new method for the synthesis of furan-/benzo-fused fluorenes through copper(i)-catalyzed coupling of conjugated enynones or α-diazocarbonyl compounds with dialkynylbenzenes has been developed.

Curtin-Hammett-Driven Intramolecular Cyclization of Heteroenyne-Allenes to Phenanthridine-Fused Quinazoliniminiums

Intramolecular cyclization of the heteroenyne-allene 2-((biphenyl-2-ylimino)methyleneamino)benzonitrile 1 to phenanthridine-fused quinazoliniminium salt PQ in the presence of a Lewis acid at room temperature involves formation of two new bonds: a C-C bond and a C-N bond. In this article, density functional theory (B3LYP and M06-2X) was employed in conjunction with 6-311G* basis set to gain insights into the mechanism of this cyclization reaction. The solvent effects were considered using Polarizable Continuum Model with nitromethane as the solvent. Our calculations show that C-C bond formation precedes the C-N bond formation. Precisely, the mechanism involves initial protonation of 1 at N_α and N_β of the carbodiimide to form rapidly equilibrating conformers of the tautomers 2a,b and 3a,b. The Curtin-Hammett principle is invoked to determine the course of the reaction from these protonated species, which predicts that the intramolecular Friedel-Crafts type N-acylation (C-C bond formation) occurs between the protonated carbodiimide and biphenyl ring of the isomer 3b to form phenanthridinium cation 6b via transition state TS3_b6_b. Once 6b is formed, it converts to its most stable tautomers 8R and 9a. Once again, the Curtin-Hammett principle suggests that intramolecular nucleophilic attack is preferred from the tautomer 8R, where phenanthridine N-atom (N_β) attacks the protonated nitrile group (C-N bond formation) and results in the formation of intermediate 11 via TS8_R11. 11 then tautomerizes to the most stable cation 13. The coordination of the latter with the chloride anion yields the phenanthridine-fused heterocyclic salt PQ with overall release of energy. The pathways involving protonation at the nitrile (N_γ) of 1 were found to be energetically unfavorable and thus insignificant to the mechanism of cyclization.

Platinum-Catalyzed Cascade Rearrangement Reaction of 1,5-Diynyl Esters: Unusual Regioselective 1,5-Hydride Migration

A highly regioselective sequential 1,3-acyloxy migration/pentannulation/1,5-hydride migration reaction is disclosed which provides an efficient access to (E)-2-vinyl-3-oxo-1-methyleneindenes under neutral and mild reaction conditions. The migrated hydrogen atom was derived from an unactivated alkyl group, and the long-range 1,5-H shift was confirmed through related deuterium experiments.

Mechanistic Investigation on the Formation of 2-Halo-3-aryl-4(3H)-quinazoliniminium Halides from Heteroenyne-allenes: A Computational Study

Intramolecular cyclization of the heteroenyne-allene, 2-((phenylimino)methyleneamino)-benzonitrile (1) in the presence of HCl to produce 2-chloro-3-phenyl-4(3H)-quinazoliniminium chloride (Qz) involves the formation of two new bonds: a C-Cl bond and a C-N bond. We propose five pathways for this reaction. Four of these pathways involve chloride capture to form the C-Cl bond prior to the intramolecular nucleophilic attack to form the C-N bond, while one pathway involves ring closure to form the C-N bond prior to C-Cl bond formation. All calculations were carried out at B3LYP and MP2 levels of theory and employed the 6-311G* basis set. The solvent effects were considered using a Polarized Continuum Model with dichloromethane as the solvent. The calculations at both levels show that the mechanism involves initial protonation of 1, preferentially at Nα to give 2 which rapidly captures the chloride ion to form 5. This intermediate is protonated at the -CN group to form 8ROT, which then tautomerizes to its more stable isomer 9ROT. The latter undergoes intramolecular nucleophilic attack from Nβ to the protonated -CN group to form the cyclized intermediate 12, which tautomerizes to its most stable isomer 13. The coordination of Cl(-) ion present in the solution with 13 gives the final product Qz.

Clean photodecomposition of 1-methyl-4-phenyl-1H-tetrazole-5(4H)-thiones to carbodiimides proceeds via a biradical

The photochemistry of 1-methyl-4-phenyl-1H-tetrazole-5(4H)-thione (1a) and 1-(3-methoxyphenyl)-4-methyl-1H-tetrazole-5(4H)-thione (1b) was studied in acetonitrile at 254 and 300 nm, which involves expulsion of dinitrogen and sulfur to form the respective carbodiimides 5a,b as sole photoproducts. Photolysis of the title compounds in the presence of 1,4-cyclohexadiene trap led to the formation of respective thioureas, providing strong evidence for the intermediacy of a 1,3-biradical formed by the loss of dinitrogen. In contrast, a trapping experiment with cyclohexene provided no evidence to support an alternative pathway of photodecomposition involving initial desulfurization followed by loss of dinitrogen via the intermediacy of a carbene. Triplet sensitization and triplet quenching studies argue against the involvement of a triplet excited state. While the quantum yields for the formation of the carbodiimides 5a,b were modest and showed little change on going from a C(6)H(5) (1a) to mOMeC(6)H(4) (1b) substituent on the tetrazolethione ring, the highly clean photodecomposition of these compounds to a photostable end product makes them promising lead structures for industrial, agricultural, and medicinal applications.