David J. Parker

Modelling SN2 nucleophilic substitution at silicon by structural correlation with X-ray crystallography and NMR spectroscopy

Abstract The X-ray crystal structures of four 1-(halodimethylsilylmethyl)-2-quinolinones have been measured and used to model the reaction profile for nucleophilic substitution at silicon. Similar structural correlations have been performed in solution, the percentage Siue5f8O bond formation being obtained from the 13 C chemical shifts of the quinolinone carbons and the extent of pentacoordination from the 29 Si chemical shift of the silicon. Excellent agreement is obtained between the two methods confirming the validity of the NMR technique for structural correlation in solution.

Modelling nucleophilic substitution at silicon using hypervalent silicon compounds based on di and tri halosilanes

Abstract The 29 Si- and 13 C-NMR spectral parameters of a range of N -(amidomethyl)-polyhalosilanes have been measured in solution and the extent of Siue5f8O bond formation and pentacoordination determined. The larger the number of electronegative groups attached to the silicon the greater the extent of pentacoordination achievable by the silicon. Thus with trichlorosilanes a ‘tight’ pentacoordinate silicon is observed where the Siue5f8O bond is almost fully formed.

Modelling nucleophilic substitution at silicon using solution 19F-NMR chemical shift, 1JSiF and 2JCF coupling constant data of pentacoordinate silicon compounds. Correlation with other magnetic nuclei and X-ray structures

Abstract Solution 1 J Siue5f8F and 2 J Cue5f8F NMR coupling constant and 19 F-NMR chemical shift data have been analysed in a series of pentacoordinate silyl monofluoride complexes used to model nucleophilic substitution at silicon. Patterns in the data reveal strong correlations between both coupling constants and 19 F-NMR chemical shifts and the degree of substitution displayed by each. Excellent correlation is obtained between the new data and our previous 13 C- and 29 Si-NMR studies to further confirm the validity of the NMR technique for structural correlation in solution. By pooling the X-ray crystal structures of a large number of compounds of this class from both our own results and the literature we are able to provide a possible explanation for some of the trends in the NMR data we observe in solution.

Chemoselective methylation of amides and heterocycles using chloromethyldimethylsilyl chloride

Abstract The reaction of chloromethyldimethylsilyl chloride with an amide generates a pentacoordinate N-(amidomethyl)-halosilane that can be desilylated with cesium fluoride to give the N-methyl amide. This provides a selective method for the monoalkylation of amides in the presence of other, more nucleophilic groups.

The reactions of dialkyl and diarylethoxysilanes with T6 silsesquioxane cages: X-ray crystallographic studies of the mono-T6D1 and bis-T6D2 insertion ring expansion products

Successful ring-expanding insertion reactions of T6 silsesquioxane cages using dialkyl and diarylethoxysilanes have been performed to give the first reported mixed T6D1 and T6D2 silsesquioxane cages. The reactions of hexacyclohexylsilsesquioxane (T6) with dialkyl and diaryldiethoxysilanes give predominantly T6D2 bis-insertion compounds while the reaction of T6 with dimethylethoxysilane gives one T6D1 mono-insertion product and various T6D2 bis-insertion products as isolable components. Three of the ring-expanded products are chiral and it has been shown from their X-ray crystal structures that the pairs of enantiomers, formed as racemic mixtures, co-crystallise together. As well as comparing these structures with related ones in the literature, the possible mechanism of their formation is discussed.

The X-ray structure of a protonated hypervalent silanol

Careful hydrolysis of a pentacoordinate chlorosilane gives a npentacoordinate protonated silanol that resembles the intermediate in the naqueous hydrolysis of silanes.

Modelling nucleophilic substitution at silicon in solution, using hypervalent silicon compounds based on 2-pyridones

Halodimethylsilylmethyl derivatives of 2-thiopyridones have been prepared. The N–CH2 isomer is favoured with the 6-methylthiopyridone. 13C and 29Si chemical shifts have been used to calculate the extent of sulfur–silicon bond formation and the extent of pentacoordination. The results are consistent with the oxygen analogues and reveal that as expected sulfur is a poorer nucleophile than oxygen. The unsubstituted thiopyridone and the 5-trifluoromethyl derivative favour the S–CH2 isomer. Again the mapping of nucleophilic substitution by nitrogen is in line with sulfur and oxygen nucleophiles, but in this series nitrogen is a poorer nucleophile than expected. The results are discussed in terms of steric strain, the preferences for alkylation of the pyridones and the bond strength of coordination to silicon.