Synthesis and characterization of a novel chiral phosphole and its derivatives

A novel chiral and enantiomerically pure phosphole with two (-)-menthyl groups at the 2- and 5-positions of the phosphole ring is prepared in two steps, starting from (-)-menthylacetylene, in 67% overall yield. This is the first example of a chiral phosphole in which chiral substituents attach directly to the carbon atoms of the phosphole ring. The phosphole was characterized by X-ray crystallography, and its solid-state structure has no symmetry, with an envelope-like bent structure for the five-membered ring. The phosphole was readily oxidized into the corresponding phosphole oxide, and reaction with MCl2(cod) (M = Pt, Pd) led to two chiral phosphole-transition-metal complexes, trans-MCl2(phosphole)(2), quantitatively. Treatment of the phosphole with metallic lithium in THF cleaved the exocyclic P-Ph bond to give a chiral lithium phospholide with retention of the two (-)-menthyl substituents. The lithium phospholide was converted into a chiral diphosphole in 62% yield by reaction with BrCH2CH2Br and into a chiral monophosphaferrocene in 85% yield by reaction with [(eta (6)-mesitylene)FeCp]PF6. A reaction of the phosphole with Mn-2(CO)(10) in refluxing xylene gave a chiral phosphacymantrene in 84% yield.