A NEW ALKENE CARBON-HYDROGEN BOND ACTIVATION REACTION - FACILE AND STEREOSPECIFIC VINYLIC DEPROTONATION OF THE CHIRAL CATIONIC RHENIUM ALKENE COMPLEXES [(ETA(5)-C5H5)RE(NO)(PPH(3))(H2C=CHR)](BF4-)-B-+

Alkene complexes [(eta(5)-C5H5)Re(NO)(PPh(3))(H2C=CHR)](BF4-)-B-+ (1; R = a, CH3; b, CH2CH2CH3; c, CH(CH3)(2); d, H; e, C6H5; f, C(CH3)(3); greater than or equal to 96:4 RS,SR/RR,SS Re,C configurational diaster eomers) and t-BuO(-)K(+) react in THF to give alkenyl complexes (eta(5 )-C5H5)Re(No)(PPh(3))(CH=CHR) (2; 83-93% after workup). Allylbenzene c omplex 1g (R = CH2C6H5) gives a 89:11 mixture (91%) of 2g and allyl co mplex (eta(5)-C5H5)Re(NO)(PPh(3))(CH2CH=CHR') (3g). The 2g:3g ratio de creases when t-BuOH solvent or (RR,SS)-1g is used-conditions that also give 2a,b/3a,b mixtures. NMR experiments show that (RS,SR)- and (RR,S S)-1 give (E)-and (Z)-2, respectively. However, the latter equilibrate (K-eq > (99-82):(<1-18)) at room temperature. Deuterium labeling show s that only one geminal =CH2 proton is abstracted from each diastereom er, as controlled by the rhenium configuration (H-S from (RS)- or (RR) -1). Deprotonation is irreversible and occurs with retention at rheniu m, with a k(H)/k(D) value of 1.7, and without PPh(3) dissociation. A R h(I) catalyst shows 2a to be more stable than 3a, but no isomerization occurs under the deprotonation conditions. Low-temperature NMR spectr a show that t-BuO(-)K(+) initially adds to the cyclopentadienyl ligand of 1, but other data suggest this to be a nonproductive equilibrium. Possible mechanisms, and the origins of the many types of selectivitie s in the preceding reactions, are analyzed in detail.