Synthesis and application of C₂ asymmetric phosphinines via their pyrylium salt precursors.

Abstract

The development of new chiral ligands for asymmetric catalysis is an increasingly important area of research. Though many ligands are phosphorus based, one class of phosphorus ligands, phosphinines (phosphabenzenes), have been little studied. Most studies of chiral phosphinines, especially for those used in asymmetric catalysis have involved essentially attaching achiral phosphinines to chiral auxiliaries. The synthesis of the first C₂ chiral phosphinine was accomplished by converting (+)-camphor to the corresponding pyrylium salt, and then converting the pyrylium to the phosphinine. Though several initial attempts failed at forming the necessary pyrylium salt using simpler synthetic methods, an effective route for forming the pyrylium was chosen utilizing the preformed 3-ene-1,5-dione precursor. The camphor-based phosphinine was fully characterized and applied to two asymmetric catalytic test reactions, asymmetric hydrosilylation and asymmetric hydrogenation. Though (+)-camphor provided a convenient, cost-effective, and enantiomerically pure starting material, nature provides few compounds fitting all the necessary requirements for the starting materials. Therefore, derivatized cyclohexanones were also synthesized. Specifically, pyryliums salts based on 2-methyl-2-phenylcyclohexanone were synthesized, albeit in low yield. Attempts to use the improved synthetic method developed for the camphor-based pyryliums failed at the chlorination stage. Attempts to convert the (+)-camphor chlorobenzylidene intermediate into C₁ chiral pyryliums also failed. The C₂ asymmetric phosphinine based on camphor did react with benzyne to yield a new chiral phosphabarrelene.