Regioselective and Stereoselective Reactions of 2-Butyne Bound to a Resolved Chiral Tungsten(II) Center

Abstract

Deprotonation of chiral complexes of the type Tp‘(CO)(I)W{RC⋮CCH2R‘} at the propargyl carbon followed by alkylation is regioselective and stereoselective. Photolysis liberates the elaborated alkynes from the metal complex. Resolution of metal diastereomers, with amido ligands, Tp‘(CO)(NHR)W{CH3C⋮CCH3} (NH2R = ((S)-(−)-α-methylbenzylamine), has been accomplished by fractional crystallization. Conversion to an amine complex followed by iodide replacement of H2NCHMePh produces separate enantiomers of Tp‘(CO)(I)W{CH3C⋮CCH3}. Methylation followed by benzylation of each enantiomer affords Tp‘(CO)(I)W{CH3C⋮CCHMeCH2Ph}. The alkynes (−)-CH3C⋮CCHMeCH2Ph and (+)-CH3C⋮CCHMeCH2Ph have been released from each of these enantiomerically enriched complexes in optically active form, as assayed by optical rotation and by 1H NMR with chiral shift reagents. The barrier to alkyne rotation in Tp‘(CO)(I)W{RC⋮CCH2R‘} complexes has been probed by variable-temperature 1H NMR, and extended Huckel molecular orbital calculations have been performed on model complexes. The origin of regioselectivity and stereoselectivity is considered in light of the X-ray structures of Tp‘(CO)(I)W{CH3C⋮CCHMe(CH2)4I} (7) and Tp‘(CO)(I)W{Me(PhCH2)HCC⋮CCHMeCH(OH)Ph} (11). The absolute configuration of Tp‘(CO)W{CH3C⋮CCH3}(NHCHMePh) ((+)-14(SS)) has been established by X-ray analysis with the resolving amine as reference.