DEPROTONATION AND ANIONIC REARRANGEMENTS OF ORGANOMETALLIC COMPOUNDS .5. KINETIC VS THERMODYNAMIC DEPROTONATION IN REACTIONS OF CYCLOPENTADIENYLRHENIUM ACYL AND ALKYL COMPLEXES WITH STRONG BASES
Academic Article
Overview
Identity
Additional Document Info
View All
Overview
abstract
Reaction of 5-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 (5-C5H4COR)Re(NO)(PPh3)(CH3) (2a-d, 50-78%). Deuterium-labeling experiments indicate that 1 is initially deprotonated to (5-C5H4Li)Re(NO)(PPh3)(COR) (3), and 31P NMR shows a subsequent rapid rearrangement (3a: <3 min, -95 C) to Li+ [(5-C5H4COR)Re(NO)(PPh3)]-(4). Crossover experiments show 3 4 to be intramolecular. Reaction of (5-C5H5)Re(NO)(PPh3)(CH2C6H5) with n-BuLi/TMEDA (-78 C, 5 min) gives (5-C5H4Li)Re(NO)(PPh3)(CH2C6H5) (6), which does not rearrange upon warming and yields (5-C, H4CH3)Re(NO)(PPh3)(CH2C6H5) (7, 74%) upon addition of CH3I. When (+)-(S)-(5-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 (5-C5Me5)Re(NO)(PPh3)(COCH2R) (10a, R = H; 10b, R = C6H5) and n-BuLi react to give enolates Li+(5-C5Me5)Re(NO)(PPh3)(C(O-)==CHR), which yield (5-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+(5-C5H5)-Re(NO)(PPh3)(C(O-)=CHC6H5(17) forms and gives (SR, RS)-(5-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.
