Synthesis, Structure, and Reactivity of Chiral Rhenium Cycloalkene Complexes of the Formula ; Facile Vinylic Deprotonation of a Coordinated Alkene

Reactions of [(η5‐C5H5)Re(NO)(PPh3)(ClC6H5)]+BF−4 and (n = 5, a; 6, b; 7, c; 8, d) give cycloalkene complexes (6a – d, 94 to 79%). A crystal structure of the methylcyclopentadienyl analog of 6a shows the Re – plane and Re – PPh3 bond to be coplanar, with the vinyl protons syn to the cyclopentadienyl ligand. Reaction of 6a and tBuO−K+ leads to the vinyl complex (9a, 90%) instead of expected allyl complex (10a) Analogous reactions of 6b – d give varying mixtures of 9b – d/10b – d. HBF4 · OEt2 and 9a react to form the cyclopentylidene complex (10+ BF−4, 96%), the stability of which precludes any intermediacy in the deprotonation of 6a. A crystal structure of 11+ PF−6 shows the ReCC planes to be perpendicular to the Re – P bond. Spectroscopic features of compounds 6 are analyzed in detail, and NMR data show a high barrier to cyclopentylidene ligand rotation in 11+ BF−4 (ΔG≢ (110°C) ≥ 19 kcal/mol).