Radical Reactivity of the Biradical [⋅P(μ-NTer)2 P⋅] and Isolation of a Persistent Phosphorus-Cantered Monoradical [⋅P(μ-NTer)2 P-Et

Abstract

The activation of C−Br bonds in various bromoalkanes by the biradical [⋅P(μ‐NTer)2P⋅] (1) (Ter=2,6‐bis‐(2,4,6‐trimethylphenyl)‐phenyl) is reported, yielding trans‐addition products of the type [Br−P(μ‐NTer)2P−R] (2), so‐called 1,3‐substituted cyclo‐1,3‐diphospha‐2,4‐diazanes. This addition reaction, which represents a new easy approach to asymmetrically substituted cyclo‐1,3‐diphospha‐2,4‐diazanes, was investigated mechanistically by different spectroscopic methods (NMR, EPR, IR, Raman); the results suggested a stepwise radical reaction mechanism, as evidenced by the in‐situ detection of the phosphorus‐centered monoradical [⋅P(μ‐NTer)2P‐R].< To provide further evidence for the radical mechanism, [⋅P(μ‐NTer)2P‐Et] (3Et⋅) was synthesized directly by reduction of the bromoethane addition product [Br‐P(μ‐NTer)2P‐Et] (2 a) with magnesium, resulting in the formation of the persistent phosphorus‐centered monoradical [⋅P(μ‐NTer)2P‐Et], which could be isolated and fully characterized, including single‐crystal X‐ray diffraction. Comparison of the EPR spectrum of the radical intermediate in the addition reaction with that of the synthesized new [⋅P(μ‐NTer)2P‐Et] radical clearly proves the existence of radicals over the course of the reaction of biradical [⋅P(μ‐NTer)2P⋅] (1) with bromoethane. Extensive DFT and coupled cluster calculations corroborate the experimental data for a radical mechanism in the reaction of biradical [⋅P(μ‐NTer)2P⋅] with EtBr. In the field of hetero‐cyclobutane‐1,3‐diyls, the demonstration of a stepwise radical reaction represents a new aspect and closes the gap between P‐centered biradicals and P‐centered monoradicals in terms of radical reactivity.