Syntheses, Structures, Reactivities, and Basicities of Quinolinyl and Isoquinolinyl Complexes of an Electron Rich Chiral Rhenium Fragment and Their Electrophilic Addition Products.
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abstract
Reactions of Li+ [(η5 -C5 H5 )Re(NO)(PPh3 )]- with 2- and 4-chloroquinoline or 1-chloroisoquinoline give the corresponding σ quinolinyl and isoquinolinyl complexes 3, 6, and 8. With 3 and 8 there is further protonation to yield HCl adducts, but additions of KH give the free bases. Treatment of 3 with HBF4 ⋅OEt2 or H(OEt2 )2+ BArf- gives the quinolinium salts [(η5 -C5 H5 )Re(NO)(PPh3 )(C(NH)C(CH)4 C(CH)(CH))]+ X- (3-H+ X- ; X- =BF4- /BArf- , 94-98 %). Addition of CF3 SO3 CH3 to 3, 6, or 8 affords the corresponding N-methyl quinolinium salts. In the case of [(η5 -C5 H5 )Re(NO)(PPh3 )(C(NCH3 )C(CH)4 C(CH)(CH))]+ CF3 SO3- (3-CH3+ CF3 SO3- ), addition of CH3 Li gives the dihydroquinolinium complex (SRe RC ,RRe SC )-[(η5 -C5 H5 )Re(NO)(PPh3 )(C(NCH3 )C(CH)4 C(CHCH3 )(CH2 ))]+ CF3 SO3- ((SRe RC ,RRe SC )-5+ CF3 SO3- , 76 %) in diastereomerically pure form. Crystal structures of 3-H+ BArf- , 3-CH3+ CF3 SO3- , (SRe RC , RRe SC )-5+ Cl- , and 6-CH3+ CF3 SO3- show that the quinolinium ligands adopt Re⋅⋅⋅C conformations that maximize overlap of their acceptor orbitals with the rhenium fragment HOMO, minimize steric interactions with the bulky PPh3 ligand, and promote various π interactions. NMR experiments establish the Brønsted basicity order 3>8>6, with Ka (BH+ ) values >10 orders of magnitude greater than the parent heterocycles, although they remain less active nucleophilic catalysts in the reactions tested. DFT calculations provide additional insights regarding Re⋅⋅⋅C bonding and conformations, basicities, and the stereochemistry of CH3 Li addition.
