Proton transfer vs. oligophosphine formation by P–C/P–H σ-bond metathesis: decoding the competing Brønsted and Lewis type reactivities of imidazolio-phosphines†
Studies of the protonation and alkylation of imidazolio-phosphides and deprotonation of imidazolio-phosphines reveal a complex behaviour that can be traced back to an interplay of Brønsted-type proton transfers and Lewis-type P–P bond formation reactions. As a consequence, the expected (de)protonation and (de)alkylation processes compete with reactions producing cyclic or linear oligophosphines. A careful adjustment of the conditions allows us to selectively address each reaction channel and devise specific synthesis methods for primary, secondary and tertiary imidazolio-phosphines, imidazolio-alkylphosphides, and cyclic oligophosphines, respectively. Mechanistic studies reveal that oligophosphines assemble in sequential P–P bond formation steps involving the condensation of cationic imidazolio-phosphines via σ-bond metathesis and concomitant elimination of an imidazolium ion. Imidazolio-phosphides catalyse these transformations. Computational model studies suggest that the metathesis proceeds in two stages via an initial nucleophilic substitution under expulsion of a carbene, and a subsequent proton transfer step that generates an imidazolium cation and provides the driving force for the whole transformation. As energy barriers are predicted to be low or even absent, different elementary steps are presumed to form a network of mutually coupled equilibrium processes. Cyclic oligophosphines or their dismutation products are identified as the thermodynamically favoured final products in the reaction network.