Exploring the chemistry of backbone amino(chloro)phosphanyl-substituted imidazole-2-thiones

Abstract

Backbone (R′₂N)₂P-substituted imidazole-2-thiones 2a–c [(R′₂N)₂P-IMS^R,R; IMS^R,R = 1,3-dialkylimidazole-2-thione-4-yl, a: R = ⁱPr, R′ = Et; b: R = Mes, R′ = Et, c: R = Me, R′ = ⁱPr] were treated with PCl₃ to synthesize R′₂N(Cl)P-substituted imidazole-2-thiones 3a–c [R′₂N(Cl)P-IMS^R,R]. The P-chloro compounds 3a,b were used to explore the chemistry further, i.e. nucleophilic substitution at phosphorus using Ph₂CHLi or fluorenyl lithium afforded compounds 4a–c [R′₂N(R′′)P-IMS^R,R; a: R = ⁱPr, R′ = Et, R′′ = PH₂CH; b: R = Mes, R′ = Et, R′′ = Flu c: R = ⁱPr, R′ = Et, R′′ = Flu]. Compound 4c was used to access the P-borane adduct 5 [R′₂N(R′′)(BH₃)P-IMS^R,R; R = ⁱPr, R′ = Et, R′′ = Flu] and phosphinoyl-substituted imidazolium hydrogensulfate 6 [[R′₂N(R′′)(O)P-IMH^R,R]HSO₄; IMH^R,R = 1,3-dialkylimidazolium-4-yl, R = ⁱPr, R′ = Et, R′′ = Flu]. Compounds 4a,b were transformed into the P-Cl compounds 7a,b [R′′(Cl)P-IMS^R,R; a: R = ⁱPr, R′′ = Ph₂CH; b: R = Mes, R′′ = Flu] which represent potential starting materials for backbone phosphaalkenyl substituted imidazole-2-thiones. Compound 7a yielded the NHC-stabilized phosphenium salt 8 [[R′′(IMe₄)P-IMS^R,R]Cl; R = ⁱPr, R′ = Et, R′′ = Ph₂CH, IMe₄ = 1,3,4,5-tetramethylimidazole-2-ylidene] in reaction with 1,3,4,5-tetramethylimidazole-2-ylidene (IMe₄). Attempts to deprotonate 7a,b using ⁿBuLi revealed that nucleophilic substitution of P–Cl to give the P-ⁿBu product occurred preferentially, and dehydrochlorination to the phosphaalkene was not observed.