Influence of Steric Factors on Chiral Lithium Amide Aggregates

The solution structures of three mixed aggregates dissolved in toluene-d(8) consisting of the lithiated amides derived from (S)-N-isopropyl-1-((triisopropylsilyl)-oxy)propan-2-amine, (R)-N-(1-phenyl-2-((triisopropylsilyl)oxy)-ethyl)propan-2-amine, or (S)-N-isobutyl-3-methyl-1-((triisopropylsilyl)oxy)butan-2-amine and n-butyllithium are characterized by various NMR experiments including diffusion-ordered NMR spectroscopy with diffusion coefficient-formula weight correlation analyses (D-FW) and other one- and two-dimensional NMR techniques. We report that steric hindrance of R-1 and R-2 groups of the chiral lithium amide controls the aggregation state of the mixed aggregates. With a less hindered R-2 group, lithium (S)-N-isopropyl-1((triisopropylsilyl)oxy)propan-2-amide forms mostly a 2:2 ladder-type mixed aggregate with n-butyllithium. Increase of steric hindrance of the R-1 and R-2 groups suppresses the formation of the 2:2 mixed aggregate and promotes formation of a 2:1 mixed aggregate. We observe that lithium (S)-N-isobuty1-3-methyl-1-((triisopropylsilyl)oxy)butan-2-amide forms both a 2:2 mixed aggregate and a 2:1 mixed trimer with n-butyllithium. Further increase in the steric hindrance of R-1 and R-2 groups results in the formation of only 2:1 mixed aggregate as observed with lithium (R)-N-(1-phenyl-2-((triisopropylsilyl)oxy)ethyl)propan-2-amide.