Template Synthesis of 1,4,7-Triphosphacyclononanes

Iron(II) templates based on a [(η5-CpR)Fe]+ core have been employed for the successful synthesis
of 1,4,7-triphosphacyclononane derivatives (9-aneP3R‘3) from a range of appropriately functionalized
coordinated diphosphines and monophosphines. 1,2-Diphosphinoethane (1,2-dpe) or (2-phosphinoethyl)phenylphosphine (Phdpe) undergo a base-catalyzed Michael-type addition to trivinylphosphine, divinyl(benzyl)phosphine, or divinyl(phenyl)phosphine in [(η5-CpR)Fe(diphosphine)(monophosphine)]+ complexes
(2a−j) to give [(η5-CpR)Fe(9aneP3R‘3)]+ derivatives (4a−j) containing coordinated triphosphacyclononanes
bearing one (with Phdpe) or two (with 1,2-dpe) secondary phosphine donors. The rates of macrocyclization
show a dependence on the nature of the substituent(s) R on the cyclopentadienyl ligand with increased
rates being observed along the series R = H5 < (Me3Si)H4 < 1,3-(Me3Si)2H3 ≈ Me5. For coupling reactions
with trivinylphosphine, a pendant vinyl function remains in the macrocyclic product (4a−g) which is readily
hydrogenated to the corresponding ethyl derivatives (5a−g). Further functionalization of coordinated
secondary phosphines in the initially formed macrocycles (5a−g) is achieved by proton abstraction followed
by addition of the appropriate alkyl halide electrophile and gives rise to tritertiary-triphospha-cyclononanes
(7a−g, 7l, 7m). All new complexes have been fully characterized by spectroscopic and analytical methods
in addition to the structural determination by single-crystal X-ray techniques of [{η5-(Me3Si)2C5H3)Fe(9-aneP3H2C2H3)]PF6, 4c, and [(η5-Me3SiC5H4)Fe(9-aneP3Et3)]BF4, 7b. 1,4,7-Triethyl-1,4,7-triphosphacyclononane is released from its metal template (7a, 7b) by treatment with either H2O2 or Br2/H2O to give the
trioxide 9-aneP3(O)3Et3 (8). Attempts to recover the trivalent phosphorus species, 1,4,7-triethyl-1,4,7-triphosphacyclononane, from the trioxide by reduction proved unsuccessful.