Synthesis of Chiral 2-Phospha[3]ferrocenophanes and their Behaviour as Organocatalysts in [3+2] Cyclization Reactions

Planar chiral 2-phospha[3]ferrocenophanes have been prepared via a stereoselective three-step synthesis. The key step is the lithiation of the 1,1'-disubstituted ferrocene 11 bearing (S)-2-(methoxymethyl)pyrrolidines as the chiral ortho-directing groups. The diastereoselectivity of these reactions has been mastered by an appropriate choice of the metallating agent, so as to afford a suitable access to C-2-symmetrical, tetrasubstituted ferrocenes. These compounds have been converted into the enantiomerically pure 2-phospha[3]-ferrocenophanes 16, via the corresponding acetates and their reactions with primary phosphines. Phosphines 16 have been used as nucleophilic catalysts in model cyclization reactions. Unlike 2-phospha[3]-ferrocenophanes with stereogenic alpha-carbons, the planar chiral derivatives 16 proved to be suitable catalysts for these processes. Thus, for instance, phosphine 16c successfully promotes the enantioselective [3+2]annulations of allenes and enones into functionalized cyclopentenes (ees up to 96%). Among others, spirocyclic derivatives have been obtained in good yields and ees in the range 77-85%. The robustness of this catalyst has been demonstrated by recycling experiments.