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-)
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Alkene complexes [(5-C5H5)Re(NO)(PPh3)(H2C=CHR)]+BF4- (1; R = a, CH3; b, CH2CH2-CH3; C, CH(CH3)2; d, H; e, C6H5; f, C(CH3)3; 96:4 RS,SR/RR,SS Re,C configurational diastereomers) and t-BuO-K+ react in THF to give alkenyl complexes (5-C5H5)Re(NO)(PPh3)-(CH=CHR) (2; 8393% after workup). Allylbenzene complex 1g (R = CH2C6H5) gives a 89:11 mixture (91%) of 2g and allyl complex (5-C5H5)Re(NO)(PPh3)(CH2CH=CHR') (3g). The 2g:3g ratio decreases when t-BuOH solvent or (RR,SS)-1g is usedconditions that also give 2a,b/3a,b mixtures. NMR experiments show that (RS,SR)- and (RR,SS)-1 give (E)-and (Z)-2, respectively. However, the latter equilibrate (Keq> (9982):(<118)) at room temperature. Deuterium labeling shows that only one geminal =CH2 proton is abstracted from each diastereomer, as controlled by the rhenium configuration (Hs from (RS)- or (RR)-1). Deprotonation is irreversible and occurs with retention at rhenium, with a kH/kD value of 1.7, and without PPh3 dissociation. A Rh(I) catalyst shows 2a to be more stable than 3a, but no isomerization occurs under the deprotonation conditions. Low-temperature NMR spectra 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 selectivities in the preceding reactions, are analyzed in detail. 1995, American Chemical Society. All rights reserved.
