A synthetic cycle for the ruthenium-promoted formation of 1H-phosphindoles from phosphaalkynes.

Beginning with inexpensive and commercially available starting materials, a rational synthesis for the new phosphaalkyne Ph3C-C[triple bond]P (1) is presented. Coordination of 1 to group 8 transition metal centers furnishes the eta1-complexes [MH(dppe)2(Ph3CC[triple bond]P)]OTf, where M = Fe (3) or Ru (4) (dppe = bis-1,2-diphenylphosphinoethane). Treatment of 3 or 4 with a strong acid cyclizes the coordinated phosphaalkyne and is the first example of an electrophilic aromatic substitution reaction in which the electrophile is a low coordinate phosphorus. With the aid of DFT calculations, we were able to gain a more thorough understanding of the energetics and mechanism of this new cyclization reaction. Thermolysis of the iron-3,3-diphenyl-3H-phosphindole adduct (6) in CH3CN results in quantitative formation of the free 3H-phosphindole (7). Alternatively, when ruthenium-3,3-diphenyl-3H-phosphindole adduct (5) is irradiated, a photochemical rearrangement occurs furnishing 2,3-diphenyl-1H-phosphindole (9). Mechanistic work is presented that provides an explanation for this transformation. Compounds 1, 3, 5, and 9 have been characterized by single X-ray diffraction studies. Finally, a synthetic cycle for the conversion of 1 to 1H-phosphindole 9 has been developed that recycles the ruthenium cation [RuH(dppe)2]+.