Modelling nucleophilic substitution in solution by multi-nuclear NMR spectroscopy

In response to the need for a modelling technique not reliant on crystalline materials, solution phase mapping has been developed. A series of closely related, stable hypervalent complexes are synthesized differing only in leaving group or in ligand substituent group. We can therefore produce complexes containing a range of frozen 'nucleophile'-silicon structures which may be analysed in deuterochloroform solution.

Ligands are chosen so that the degree of substitution exhibited by a complex may be obtained directly from NMR information. The technique relies on the degree of substitution measurably affecting the values of the ring ¹³C chemical shifts in the ligand. Pyridone (A), thiopyridone (B) and most recently quinoline (C) -based ligands have been successfully used.

In order to scale the change in the chemical shifts, compounds representing the limits of the substitution must also be prepared. For example, species modelling zero substitution and complete substitution in series C are compounds D and E respectively.

By comparison of each ring-carbon chemical shift in a complex C with the limiting values it holds in D and E, the percentage silicon-oxygen bonding can be calculated on the basis of each carbon. By taking the mean of the percentages expressed by each ring carbon in a given complex, a value for the overall percentage silicon-oxygen bonding displayed by that complex is obtained.

Each horizontal line of the following graph represents the ¹³C chemical shifts of one quinoline complex plotted against the percentage Si-O bonding calculated for that complex. It shows how individual carbon resonances 'drift' either up or down field as the modelled substitution progresses.

The ²⁹Si NMR chemical shift is particularly sensitive to its coordination number. The more negative the chemical shift, the more pentacoordinate the silicon. Experience has shown that for complexes of this type a minimum value of d -40 represents maximum penta-coordination and that d +28 is representative of a fully tetracoordinate silicon centre.

Therefore from the combined ¹³C and ²⁹Si NMR data, the changing coordination state of the silicon can be monitored as the percentage Si-O bonding increases as though an S_N2 reaction were taking place.

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