The Chemistry of Silylaziridines

Kyle, Patricia Ann (1994). The Chemistry of Silylaziridines. PhD thesis The Open University.



A series of new and generally applicable routes to silylaziridines containing a silyl group in the 2-position has been developed with a view to examining the chemical reactivity of such species. Ring opening reactions, ring preserving reactions and fluorodesilylation reactions have been examined in detail. The study also includes an examination of the conjugate addition reactions of some vinylsilanes.

Three new routes to silylaziridines are presented, these are summarized below.

1. The thermal reaction between phenyl azide and functionalized vinylsilanes. This reaction proceeds via an intermediate triazoline and was used to prepare silylaziridines with an N-phenyl and an α-carbomethoxy substituent. Different 2-silyl groups were introduced by this method: triethylsilyl and dimethylphenylsilyl.

2. The lithium aluminium hydride reduction of bromoazides derived firom vinylsilanes. This route was used to prepare N-unsubstituted aziridines, in some cases stereoselectively. Two methods were employed to prepare the bromoazides, one using an aqueous N-bromosuccinimide/sodium azide as an in situ bromine azide source, and another using bromine azide generated from bromine and sodium azide in dichloromethane.

3. The photolysis of ethyl or methyl azidoformate in the presence of vinylsilanes. This reaction proceeds via a nitrene intermediate and was used to prepare several substituted silylaziridines.

All of the aziridines were subjected to reactions with electrophilic agents including hydrogen halides, trimethylsilyl halides, trifluoroacetic acid, and the triflates; trifluoromethanesulphonic acid, methyl trifluoromethanesulphonic acid and trimethylsilyl trifluoromethanesulphonic acid with a view to comparing their chemistry from a global perspective. Mainly products of ring opening were observed, however, a richer chemistry is displayed by the more functionalized silylaziridines, i.e., those containing a 1-phenyl and a 2-carboethoxy substituent, where an enamine (trifluoromethanesulphonic acid), an aziridinium salt (trimethylsilyl trifluoromethanesulphonic acid) were produced. Some structure-activity relationships have been drawn.

A study was conducted to find suitable conditions for the fluorodesilylation of silylaziridines. It was found that only silylaziridines that contained activating substituents could be desilylated. Silylaziridines having an N-phenyl and an α-carbomethoxy substituent were active in desilylation reactions, but, only when the silyl group contained a phenyl group (dimethylphenylsilyl) could electrophiles be trapped. Benzaldehyde and hexanal were active reagents in this respect, however, other aldehydes, or ketones could not be trapped. A mechanism involving the aziridinyl anion and the fluorosilane as a loosely associated ion pair is proposed on the basis of the results obtained.

The study also includes an examination of the conjugate addition reactions of some vinylsilanes. α-Lithiovinyltrimethylsilane reacts with ethyl chloroformate, ethyl acetate, acetic anhydride and dimethylcarbamyl chloride to produce α,β-unsaturated carbonyl compounds which act as Michael acceptors to the starting material, α-lithiovinyltrimethylsilane. The fate of the resultant enolate is dependant on the properties of the enolate and the electrophile.

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