Structures and stereochemical non-rigidity of Si-substituted N-(dimethylsilylmethyl)- and N-(methylphenylsilylmethyl)amides and -lactams

Eleven new silicon-substituted N-(dimethylsilylmethyl)- and N-(methylphenylsilylmethyl)amides and -lactams bearing a chiral carbon in the amide or lactam fragment, and containing the OSiC3X (X = Hal, OTf) coordination fragment have been synthesized and their structures determined in solution by spectroscopic means. These structures are consistent with the hypervalency model. Quantum chemical calculations adequately reflect correlations between the type of monodentate ligand X and the geometric parameters of the N–C–O–Si–X fragments.The activation parameters (ΔG298#,ΔH#,ΔS#) for enantiomerization and diastereomerization in these new compounds and the other related compounds were determined by the dynamic NMR (DNMR) method using full line-shape analysis. The free activation energy values (ΔG298#) in the absence of external nucleophiles vary from 9 to 27 kcal mol−1. The entropies of activation (ΔS#) are negative (−20 to −50 cal mol−1 K−1) in all cases except for the chloride derivatives of 4-phenyl-2-pyrrolidone and 4-oxazolidinone that have weaker intramolecular O → Si coordination. Irregular mechanisms of permutational isomerization were proposed on the basis of the DNMR data and the results of quantum-chemical calculations carried out by ab initio (HF) and DFT (PBE, B3PW91, 6-311++G(d,p)). Depending on the coordination environment at silicon, the mechanisms proposed involve either the dissociation of the Si–X bond followed by the Berry pseudorotation or similar in the intermediate or the cleavage of intramolecular O–Si bond with subsequent inversion at the silicon atom. The apparently simple pseudorotation mechanism involving only the pentacoordinate structures 1–21 does not appear to be favoured in any of the examples studied.

Graphical abstractEleven new silicon-substituted N-(dimethylsilylmethyl)- and N-(methylphenylsilylmethyl)amides and -lactams bearing a chiral carbon in the amide or lactam fragment, and containing the OSiC3X (X = Hal, OTf) coordination fragment have been synthesized and their structures determined in solution by spectroscopic means. The activation parameters (ΔG298#,ΔH#,ΔS#) for enantiomerization and diastereomerization were determined by the dynamic NMR (DNMR) method using full line-shape analysis.Figure optionsDownload full-size imageDownload as PowerPoint slide