The investigation of the steric hindrance of anilines by means of reactions with PCl3 and BCl3

Abstract

Sterically crowded ligands form an integral part of contemporary coordination chemistry. The most common ones include bulky anilines, which are well accessible and modifiable. Ever increasing requirements to primary amines' steric protection leads to reaching the limits of amino-group shielding. In this work, the dissymmetrical aniline Ar^t-Bu-NH₂ (1) (Ar^t-Bu = 2-C(4-t-Bu-C₆H₄)₃-4-Me-6-(CHPh₂)-C₆H₃) is designed and synthesized. Its shielding properties have been evaluated both theoretically and experimentally by the reaction with phosphorus trichloride and a base, giving monomeric chloro(imino)phosphine Ar^t-Bu-NPCl (3). The same reactivity of extremely bulky anilines Ar^H-NH₂ and Ar^Me-NH₂ (Ar^H = 2,6-[C(Me)Ph₂]₂-C₆H₃; Ar^Me = 2,6-[C(Me)(3,5-Me₂-C₆H₃)₂]₂-C₆H₃) was tested. The products [2,6-R₂-4-PCl₂-C₆H₂]-NH₂ (4^H, 4^Me) of C–H activation bearing NH₂ and PCl₂ groups simultaneously on the opposite sides of the central phenyl ring have been isolated alongside the desired products Ar^H-NH-PCl₂ (5^H) and Ar^Me-NH-PCl₂ (5^Me). Subsequent theoretical and experimental investigation has revealed that the electrophilic substitution by phosphorus trichloride is preferred over the reaction with the sterically encumbered anilide. When the reactions with phosphorus(III) compounds were conducted under forcing conditions, it was possible to isolate several imidophosphines, namely Ar^H-NPNMe₂ (8^H) and Ar^Me-NPNMe₂ (8^Me), but the steric shielding did not enable direct conversion to the chloro(imino)phosphines Ar^H-NPCl (9^H) and Ar^Me-NPCl (9^Me). For another comparison of steric hindrance, the reactions of Ar^H-NH₂, Ar^Me-NH₂ and Ar^t-Bu-NH₂ with BCl₃ and BH₃ have been carried out. Regardless of the fact that symmetrical anilines with bulkier substituents have given mixtures of unidentified products, the reactions of dissymmetrical aniline yielded the dichloroamidoborane Ar^t-Bu-NHBCl₂ (10) and Ar^t-Bu-NHBH₂ (12) as the first evidence of a terminal NHBH₂ group. The substituents in aniline 1 shield the amine group sufficiently, maintaining space for both the synthesis of amides and their subsequent reactivity.