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 C2‐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 α‐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.