Discovery and Synthetic Value of a Novel, Highly Crowded Cyclopentadienylphosphane Ph2P-Cp™H and Its Ferrocenyl-Bisphosphane dppf™

Base-catalysed condensation of Ph2P-C5H5 (1) with an excess of acetone leads to a fulvene-like diphenyl(4,4,6-trimethyl-4,5-dihydropentalen-2-yl)phosphane Ph2P-C11H13 (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, Ph2P-Cp (TM) H). It reacts with chalcogene oxidants (H2O2, S-8, Se) to form the corresponding phosphane chalcogenides Ph2P(=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 nBuLi gives Cp (TM)-phosphonium ylide Ph2P(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 tBuN(3) gives only P-amino-cyclopentadienylidenephosphorane Ph2P(Cp (TM))-NHtBu (10), whereas with Me3SiN3 only the tautomeric P-imino-cyclopentadienylphosphane Ph2P(NSiMe3)Cp (TM) H (11) was isolated. Hydrolysis of 11 with wet MeCN leads to the new parent P-amino-cyclopentadienylidenephosphorane Ph2P-(Cp (TM))NH2 (12). Treatment of 4 with benzylpotassium followed by transmetallation with FeCl2 leads to the stericatty most crowded ferrocenyl-bisphosphane [{Ph2PCp (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 [PdCl2(MeCN)(2)], a constrained syn-orientation is achieved in the product [(dppf (TM)}PdCl2 (14). Halogen exchange by reaction of 14 with NaI leads to the corresponding [dpp (TM))PdI2] (15). Molecular structures of 4, 9, 13 and 15 have been confirmed by XRD studies.