Deprotonation and anionic rearrangements of organometallic compounds. 5. Kinetic vs. thermodynamic deprotonation in reactions of cyclopentadienylrhenium acyl and alkyl complexes with strong bases
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Reaction of η5-C5H5)Re(NO)(PPh3)(COR) (la, R = CH3; lb, R = CH2C6H5; lc, R = C6H5; Id, R = H) with Li+-N(CH(CH3)2)2(LDA; THF, -78 °C) and then CH3I gives (η5-C5H4COR)Re(NO)(PPh3)(CH3) (2a-d, 50-78%). Deuterium-labeling experiments indicate that 1 is initially deprotonated to (η5-C5H4Li)Re(NO)(PPh3)(COR) (3), and 31P NMR shows a subsequent rapid rearrangement (3a: <3 min, -95 °C) to Li+ [(η5-C5H4COR)Re(NO)(PPh3)]-(4). Crossover experiments show 3 → 4 to be intramolecular. Reaction of (η5-C5H5)Re(NO)(PPh3)(CH2C6H5) with n-BuLi/TMEDA (-78 °C, 5 min) gives (η5-C5H4Li)Re(NO)(PPh3)(CH2C6H5) (6), which does not rearrange upon warming and yields (η5-C, H4CH3)Re(NO)(PPh3)(CH2C6H5) (7, 74%) upon addition of CH3I. When (+)-(S)-(η5-C5H5)Re(NO)(PPh3)(CH3) ((+)-(S)-8, >98% ee) is treated with /n-BuLi/TMEDA and then (RCO)2O (R = CH3, C6H5CH2, C6H5), (-)·(S)·2a, (-)·(S)·2b, and (-)·(S)-2c, all ≥98% ee, are obtained. Experiments with optically active 1 then show that 3a → 4a proceeds with >90% retention of configuration at rhenium and that 3b → 4b and 3c → 4c proceed with lesser degrees of retention and inversion, respectively. Pentamethylcyclopentadienyl complexes (η5-C5Me5)Re(NO)(PPh3)(COCH2R) (10a, R = H; 10b, R = C6H5) and n-BuLi react to give enolates Li+(η5-C5Me5)Re(NO)(PPh3)(C(O-)==CHR), which yield (η5-C5Me5)Re(NO)(PPh3)(COCH(R)CH3) (72-82%) when treated with CH3I. When lb is treated with the weaker base Li+-N(Si(CH3)3)2, E-enolate Li+(η5-C5H5)-Re(NO)(PPh3)(C(O-)=CHC6H5(17) forms and gives (SR, RS)-(η5-C5H5)Re(NO)(PPh3)(COCH(C6H5)CH3) ((SR, RS)- 16) with 96% diastereoselectivity when treated with CH3I. The factors controlling the stereoselectivity and site of deprotonation in these reactions, and the relationship of 3 → 4 to anionic organic rearrangements, are discussed. © 1986, American Chemical Society. All rights reserved.
author list (cited authors)
Heah, P. C., Patton, A. T., & Gladysz, J. A.