Accessing the Rare Diazacyclobutene Motif

Abstract

A formal [2 + 2] cycloaddition of 4-phenyl-1,2,4-triazoline-3,5-dione (PTAD) with electron-rich alkynyl sulfides and selenides is described. These investigations provide a convenient method to access diazacyclobutenes in good yield while tolerating a relatively broad substrate scope of thio-acetylenes. This method provides ready access to a unique and hitherto rarely accessible class of heterocycles. A combination of dynamic NMR, X-ray crystallography, and computation sheds light on the potential aromaticity of the scaffold.

Zero-, one-, two- and three-dimensional supramolecular architectures sustained by Se…O chalcogen bonding: A crystallographic survey

“…A representative example of a zig-zag chain is shown in Fig. 9 a, for 75 [138] . Here, Se … O(carbonyl) interactions are in play, as in crystals of 76 [139] and 77 [140] .…”

Cycloaddition/Electrocyclic Ring Opening Sequence between Alkynyl Sulfides and Azodicarboxylates To Provide N,N-Dicarbamoyl 2-Iminothioimidates

“…Recently, we have reported the synthesis of stable diazacyclobutenes ( 2 ) by means of a formal [2 + 2] cycloaddition of 4-phenyl-1,2,4-triazolinedione (PTAD) and electron-rich alkynyl sulfides and selenides ( 1 ) (Scheme , eq 1) . Based on similar arguments espoused by Breton et al, , we surmised that this particular class of bicyclic diazacyclobutenes was more stable than previously studied monocyclic diazacyclobutenes − owing to the unfavorable cis – trans geometry of the imine moieties in the seven-membered heterocycle 3 expected from thermal 4π conrotatory electrocyclic ring opening of the initially formed diazacyclobutene 2 .…”

“…We began our studies by reacting electron-rich phenyl alkynyl sulfides with diethyl azodicarboxylate (DEAD) under otherwise identical conditions to our previous report (i.e., refluxing acetonitrile) . Thus, we successfully synthesized the first set of N , N -dicarbamoyl 2-iminothioimidates by varying the alkyl chain at the R 2 position on the alkynyl sulfide (Scheme ).…”

“…Based on similar arguments espoused by Breton et al, , we surmised that this particular class of bicyclic diazacyclobutenes was more stable than previously studied monocyclic diazacyclobutenes − owing to the unfavorable cis – trans geometry of the imine moieties in the seven-membered heterocycle 3 expected from thermal 4π conrotatory electrocyclic ring opening of the initially formed diazacyclobutene 2 . Indeed, the bicyclic diazacyclobutenes 2 exhibit excellent thermal stability up to >200 °C . We predicted, however, that use of acyclic azodicarboxylates ( 4 ) − in lieu of PTAD might facilitate a straightforward preparation of N , N -dicarbamoyl 2-iminothioimidates 6 via the analogous formal [2 + 2] cycloaddition with 1 to generate monocyclic diazacyclobutenes 5 , which could undergo facile 4π electrocyclic ring opening (Scheme , eq 2).…”

Water‐Involved Ring‐Opening of 4‐Phenyl‐1,2,4‐triazoline‐3,5‐dione for “Photo‐Clicked” Access to Carbamoyl Formazan Photoswitches In Situ

“…High reactivity of triazolinediones (RTADs), especially 4-methyl-1,2,4-triazoline-3,5dione [15][16][17] and 4-phenyl-1,2,4-triazoline-3,5-dione (PTAD), [14,18,19] are ascribed to the cyclic structure with further lowered LUMO energy of the N=N bond that are capable of mimicking the reactivity of singlet oxygen [20] or nitroso [21] in ene-type reactions, [22][23][24] [4 + 2] and [2 + 2] cycloadditions (Figure 1a). [25][26][27][28][29][30] Moreover, RTADs were easily attacked by amines to furnish ring-opening species which releases nitrogen to form urea [31,32] or dimerized bicyclic adduct (Figure 1b). [18.33] Engrossingly, the relatively stable PTAD derivatives are able to engage in electrophilic substitution toward electron-rich arenes (Figure 1c) [34][35][36][37] or toward alkene-modified DNA [38] in protonic solvent systems (Figure 1d).…”