STEREOSELECTIVE RHODIUM-PROMOTED RING-CLOSURE OF AN ETA(4)-1,3-PENTADIENEDIYL LIGAND TO AN ETA(4)-1,3-CYCLOPENTADIENE, WITH SUBSEQUENT REGIOSPECIFIC ENDO-H MIGRATION - MOLECULAR-STRUCTURE OF [RH(ETA-5-C5H5)((1-4-ETA)-C5H3-1,2,EXO-5T-BU3)]

In contrast to its indenyl derivative, which undergoes ring closure to a 1,2,3-tri-tertbutylcyclopentadiene complex, dimeric bis[(mu-chloro)((1,3-5-eta)-1,2,3-tri-tert-butylpentadienediyl)rhodium] (5a) undergoes ring closure accompanied by rearrangement to give 1,4,5-tri-tert-butylcyclopentadiene dimer 9a on heating or prolonged standing in solution. Studies using syn-deuterated 5b have shown that the majority of the deuterium resides in the endo position of 9b, although some scrambling into both the alpha- and beta-positions was observed. The rearrangement and scrambling can both be explained by a combination of [1,5]-exo-hydride shifts around the cyclopentadiene ring and endo-hydride migrations to the metal and back to the ring. Similarly, in the presence of ethylene, scrambling of the deuterium into the ethylene occurs and may be explained by hydride migrations between the metal and coordinated ethylene. To verify the structure of 9a, its cyclopentadienyl derivative 10a was prepared and subjected to crystallographic study: C22H35Rh, triclinic, P1BAR, a = 8.705(3) angstrom, b = 9.481(4) angstrom, c = 13.722(6) angstrom, alpha = 71.77(3)-degrees, beta = 86.43(3)-degrees, gamma = 71.95(3)-degrees, V = 1022.0(7) angstrom3, Z = 2.