Abstract. 1,2,3-Tri-tert-butyI-3-vinyl-l-cyclopropene 6a reacts with [RhC1(CzH4)z]z to give the dimeric 1,2,3,5-q-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, 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 q3-q1+q3 isomerization on the NMR time scale. However, after prolonged heating, syn-anfi site exchange of H and D slowly takes place with 8b.
The above results suggest the following conclusions: (a) The carboxylate of Asp99 is able to carry out its function without forming H bonds to The phenolic hydroxyl of neither Tyr-52 nor Tyr-73 is catalytically essential even though they are absolutely conserved in groups I and I1 PLA2 sequences. (c) The aromatic rings of both residues are required, possibly for structural reasons. (d) If the H-bonding network shown in Figure The transition-metal-promoted reactions of cyclopropenes are of great synthetic utility and mechanistic interest.' We have demonstrated that 1,2,3-triphenyl-3-vinyl-l-cyclopropene undergoes a facile C-C cleavage reaction promoted by Pt(O), Rh(I),
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