Diels–Alder Reactions with the >C=P– Functionality of Annelated Azaphospholes

Abstract

The >C=P-functionality in annelated azaphospholes acts as a dienophile and undergoes Diels-Alder reactions with dienes. 1,3-Bis(alkoxycarbonyl)-2-phosphaindolizines react with dienes even in the absence of a catalyst; however, in the presence of an oxidizing agent such as sulfur or selenium, the reaction is much faster. 3-Alkoxycarbonyl-2-phosphaindolizines with an electron-withdrawing group exclusively at position 3 undergo the Diels-Alder reaction only in the presence of a Lewis acid. Density functional theory calculations reveal strong hyperconjugative interactions between the nitrogen lone pair and the >C=P-functionality of the 3-alkoxycarbonyl-2-phosphaindolizine, making it electron-rich. However, in 1,3-bis(alkoxycarbonyl)-2-phosphaindolizines, the ester group at the position 1 acts as a sink for the lone pair of electrons of nitrogen leaving the >C=P-functionality sufficiently electron-deficient to undergo the Diels-Alder reaction.

Cycloadditions of 1H‐1,3‐Benzazaphospholes with o‐Chloranil

“…The structure elucidation of 2a – 2c is based on elemental analyses, 1 H NMR spectroscopic data, conclusive 13 C NMR spectroscopic data for 2a and characteristic 31 P chemical shifts of 2a – 2c at δ = –107.8, –101.2 and –101.3 to –103.7. These phosphorus resonances fall in the range of earlier reported zwitterionic P‐2C4O‐coordinated tris‐spirocyclic bis(tetrachlorocatechol)‐phosphinates, and various types of σ 6 λ 5 ‐phosphorus compounds,[16a], [16b], [17a], which in part were structurally closer characterized by crystal structure analysis and/or assignment of the 13 C NMR signals. The chemical shifts of the 13 CH resonances of the benzene ring and the downfield shifted doublets for the 13 C q carbon nuclei of 2a ( 13 C q ‐2 δ = 201.8, 1 J = 135 Hz; 13 C q ‐3a δ = 137.6, 1 J = 185 Hz, 13 C q ‐7a δ = 142.9, 2 J = 12.7 Hz) are typical for 3H ‐1,3‐benzazaphospholes, the large one‐bond P–C coupling constants for five‐valent phosphorus .…”

“…1,2‐Dimethyl‐1,3‐benzazaphosphole exhibited different reactivity, simultaneous cycloaddition of two molecules TCBQ to give a high melting zwitterionic bis‐addition product with hexacoordinate λ 5 ‐phosphorus, which was insoluble in most common solvents and only slightly soluble in [D 6 ]acetone ( δ 31 P = –108 ppm) . Triazaphospholes, an anellated 1,3,2‐diazaphosphole and 2‐phosphaindolizines undergo similar reactions to zwitterionic products, later observed also for other P‐compounds . Herein, were report on conversions of some further 1H ‐1,3‐benzazaphospholes with o ‐chloranil to find out if this reaction is a general feature of the benzazaphosphole ring system or dependent on its substitution pattern.…”