Discovery and Synthetic Value of a Novel, Highly Crowded Cyclopentadienylphosphane Ph 2 P‐Cp TM H and Its Ferrocenyl‐Bisphosphane dppf TM

Base‐catalysed condensation of Ph 2 P–C 5 H 5 ( 1 ) with an excess of acetone leads to a fulvene‐like diphenyl(4,4,6‐trimethyl‐4,5‐dihydropentalen‐2‐yl)phosphane Ph 2 P‐C 11 H 13 ( 3 ) as a product of double condensation. Carbometallation of 3 with MeLi, followed by aqueous work‐up, results in formation of a new cyclopentadienylphosphane bearing a highly sterically demanding, anellated 1,1,3,3‐tetramethylcyclopentane moiety ( 4 , Ph 2 P‐Cp TM H). It reacts with chalcogene oxidants (H 2 O 2 , S 8 , Se) to form the corresponding phosphane chalcogenides Ph 2 P(=X)Cp TM H, X = O ( 5 ), S ( 6 ), Se ( 7 ) in high yields. Quaternization of 4 with MeI gives the phosphonium salt 8 as a single isomer in high yield. Dehydrohalogenation of 8 by reaction with n BuLi gives Cp TM ‐phosphonium ylide Ph 2 P(Cp TM )Me ( 9 ). An alternative protocol towards 9 that includes deprotonation of 8 with benzylpotassium followed by P ‐alkylation is superior and gives 9 in more than 95 % yield. Staudinger reaction of 4 with t BuN 3 gives only P ‐amino‐cyclopentadienylidenephosphorane Ph 2 P(Cp TM )NH t Bu ( 10 ), whereas with Me 3 SiN 3 only the tautomeric P ‐imino‐cyclopentadienylphosphane Ph 2 P(NSiMe 3 )Cp TM H ( 11 ) was isolated. Hydrolysis of 11 with wet MeCN leads to the new parent P ‐amino‐cyclopentadienylidenephosphorane Ph 2 P‐(Cp TM )NH 2 ( 12 ). Treatment of 4 with benzylpotassium followed by transmetallation with FeCl 2 leads to the sterically most crowded ferrocenyl‐bisphosphane [{Ph 2 P‐Cp TM } 2 Fe] ( 13 , dppf TM ) in high yield. Its X‐ray diffraction analysis reveals an anti ‐orientation of phosphane functionalities at both cyclopentadienyl rings. However, upon reaction of dppf TM with [PdCl 2 (MeCN) 2 ], a constrained syn ‐orientation is achieved in the product [{dppf TM }PdCl 2 ] ( 14 ). Halogen exchange by reaction of 14 with NaI leads to the corresponding [{dppf TM }PdI 2 ] ( 15 ). Molecular structures of 4 , 9 , 13 and 15 have been confirmed by XRD studies.