Synthesis and characterization of a novel chiral phosphole and its derivatives

A novel chiral and enantiomerically pure phosphole with two (-)-menthyl gro ups at the 2- and 5-positions of the phosphole ring is prepared in two step s, starting from (-)-menthylacetylene, in 67% overall yield. This is the fi rst example of a chiral phosphole in which chiral substituents attach direc tly to the carbon atoms of the phosphole ring. The phosphole was characteri zed by X-ray crystallography, and its solid-state structure has no symmetry , with an envelope-like bent structure for the five-membered ring. The phos phole was readily oxidized into the corresponding phosphole oxide, and reac tion with MCl2(cod) (M = Pt, Pd) led to two chiral phosphole-transition-met al complexes, trans-MCl2(phosphole)(2), quantitatively. Treatment of the ph osphole with metallic lithium in THF cleaved the exocyclic P-Ph bond to giv e a chiral lithium phospholide with retention of the two (-)-menthyl substi tuents. The lithium phospholide was converted into a chiral diphosphole in 62% yield by reaction with BrCH2CH2Br and into a chiral monophosphaferrocen e in 85% yield by reaction with [(eta (6)-mesitylene)FeCp]PF6. A reaction o f the phosphole with Mn-2(CO)(10) in refluxing xylene gave a chiral phospha cymantrene in 84% yield.