Reactivity Relationships between Chiral Cyclic Amido and Imine Tungsten(II) Complexes

Reaction of [Tp‘W(CO)2(PhC⋮CMe)][OTf] (1) (Tp‘ = hydridotris(3,5-dimethylpyrazolyl)borate) with excess pyrrolidine affords the chiral tungsten(II) amido complex (2a). The corresponding η1-imine complex (3a) is prepared by oxidation of amido complex 2a with elemental iodine in the presence of base. The 2-methyl-1-pyrroline complex (3b) is synthesized directly by 2-methyl-1-pyrroline addition to the labile tungsten complex 1. Only one pair of enantiomers (S W R C/R W S C) of 2-methylpyrrolidine amido complex (2b) is detected when 2b is prepared by direct reaction of tungsten complex 1 with deprotonated 2-methylpyrrolidine. An alternative route to this amido complex is available through addition of MeMgBr to cationic imine complex 3a. In this case the same diastereomer predominates as in the ligand substitution route to 2b. The opposite diastereomer, S W S C/R W R C, is obtained as the major product of hydride addition to the cationic 2-methyl-1-pyrroline complex 3b. Deuteride addition to imine complex 3a yields the amido complex 2a- d 1 with a high diastereoselectivity ratio as determined by 2H NMR. Protonation of amido complex 2a using [H(OEt2)2][BAr‘4] gives the cationic pyrrolidine complex (4a).