A New Access To Enantioselective Allylic Phosphine Oxides

In the recent past, chiral phosphorous compounds have received considerable attention as a result of their increasing applications in metal-catalyzed or organocatalytic reactions and medical chemistry. Such compounds are typically prepared by using a stoichiometric amount of starting materials, chiral auxiliaries or by resolution of the racemic phosphines. The enantioselective construction of P-C bonds has attracted considerable interest and witnessed significant progress recently. To date most of the successful examples of such process are limited to the use of phosphites [(RO)2P(O)H] as nucleophiles. Recently, the use of secondary phosphines (R2PH) as nucleophiles has been successfully applied in the asymmetric hydrophosphination. However, little examples have been reported for the use of other phosphorous nucleophiles such as secondary phosphine oxides [R2P(O)H] in the construction of chiral phosphorous compounds. Thus, the development of efficient catalytic methods for the construction of P-C bonds emplying phosphine oxides is currently of pressing importance. On the other hand, allylic phosphine oxides are valuable intermediates for chiral tertiary allylphosphines or chiral alkenes. However, the only previous report on the preparation of optically allylic phosphine oxides involved the asymmetric hydroarylation of diphenylphosphinyl-allenes with arylboronic acids by the catalysis of chiral rhodium complexes. In this case, both the catalyst and substrates are not easy to be prepared, along with the using of expensive metal reagents. Thus, the development of easy and simple catalytic methods for the construction of optically allylic phosphine oxides seems to be of great importance.Recently, the metal-free organic Lewis base-catalyzed substitution of Morita-Baylis-Hillman (MBH) adducts has emerged as a powerful tool for the construction of multifunctional products in one step. We question whether we can synthesize the allylic phosphine oxides by the substitution of MBH carbonates with phosphine oxides through the catalysis of a chiral organic base. Fortunately, the strategy has been sucessfully employed, affording the desired adducts allylic phosphine oxides in high enantioselectivities.