Reversible Carbon-Carbon Bond Cleavage of a 3-Vinyl-1-Cyclopropene by Rh(I). Molecular Structures of Two Sterically Crowded 1,2,3,5-η-Pentadienediyl Complexes of Rh(III) |
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Authors: | Bernadette T Donovan James W Egan Jr Russell P Hughes Paul P Spara Hernando A Trujillo Arnold L Rheingold |
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Affiliation: | 1. Department of Chemistry, Dartmouth College, Hanover, NH 03755, USA;2. Department of Chemistry and Biochemistry, University of Delaware, Newark, DE 19716, USA |
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Abstract: | 1,2,3-Tri-tert-butyl-3-vinyl-1-cyclopropene 6a reacts with RhCl(C2H4)]2 to give the dimeric 1,2,3,5-η-pentadienediyl complex 7a . The cyclopentadienyl derivative of this complex, 8a , is obtained by reaction of 7a with T1(C5H5). Crystal structures of both 7a and 8a were determined: 7a ; orthorhombic, Pbcn, a = 28.136 (4) Å, b = 10.637 (2) Å, c = 12.154 (2) Å, V = 3637.6 (1.2) Å3, and Z = 4 : 8a ; triclinic, P 1 -bar, a = 9.906 (2) Å, b = 9.736 (2) Å, c = 12.126 (2) Å, α = 76.07 (2)°, β = 78.21 (2)°, γ = 65.34 (2)°, V = 1024.6 (3) Å3, and Z = 2. Treatment of 7a with one equivalent of PMe3 per Rh center results in regeneration of the vinylcyclopropene, demonstrating the reversibility of the ring opening reaction. Deuterium labelling studies show that both the ring opening and closing reactions proceed with retention of configuration at the vinyl olefin. Thus the trans-deuterated vinylcyclopropene 6b reacts with RhCl(C2H4)2]2 to give 7b in which deuterium is located exclusively in the syn position. Treatment of 7b with T1(C5H5) yields 8b . Treatment of 7b with one equivalent of PMe3 per Rh center results in formation of 6b as the sole organic product. Unlike their triphenyl relative 2 , the tri-Bu complexes 7a , 7b, 8a , and 8b are stereochemically rigid in solution at room temperature, and do not undergo an η3 → η1 → η3 isomerization on the NMR time scale. However, after prolonged heating, syn-anti site exchange of H and D slowly takes place with 8b . |
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