The Synthesis and Reactions of Silylaziridines

Katampe, Ibrahim (2000). The Synthesis and Reactions of Silylaziridines. PhD thesis The Open University.

DOI: https://doi.org/10.21954/ou.ro.0000f95c

Abstract

The reaction of an α-trialkylsilylvinyl carbanion and carboxylic acid derivatives gives α,β-unsaturated carbonyl compounds which, under the conditions of the reaction undergo further conjugate additiorn. The enolate thus formed can also react further, depending upon their reactivity and that of the carboxylic acid derivative. The α-trialkylsilylvinyl carbanion reacted with the α,β-unsaturated ketone to give the 1,4 -addition product.

Further acylation of the enolate by acetic anhydride leads to attack at the oxygen while the use of alkyl chloroformates led to attack at the carbon. The reaction of an α-trialkylsilylvinyl carbanion with the α,β-unsaturated ester, formed a product arising from two conjugate additions, irrespective of the ratio of the carbanion to the ester. This product seems to be particularly stable so does not undergo further acylation.

We also examined the ability of a silicon to stabilise a positive charge in the α- or β-position. Silanes containing both vinyl and allyl groupings (bistrimethylsilylalkene) were synthesised and their reactions with electrophiles were investigated. Our observations show that the silanes prefer to react as allylsilanes.

N-substitated silyl aziridines were synthesised using both thermolytic and photolytic reactions between organic azides and allyltrialkylsilanes. The reaction of allyltrialkyl - silanes with bromine azide and the subsequent cyclization of the adduct formed by reaction with lithium aluminium hydride gave the corresponding N-unsubstituted silyl aziridine.

The silyl aziridines were shown to undergo ring-opening reactions with nucleophiles by way of attack on either of the aziridine caibons or on the silyl group. Nucleophiles such as hydrogen halides, trimethylsilyl halides and chloroformates react with the aziridines to give only the β-addition product via an “SN1”- type process. Whereas the use of sodium azide gave both the β- and α- addition products, with α -addition product (“SN2” - type process) being the major isomer. In the “SN1” type process, the intermediate β-carbonium ion formed is stabilized by hyperconjugation by the trimethylsilyl group.

Reaction of iodosobenzene with a chlorotrimethylsilylsulphonate ester gave a very reactive electrophilic intermediate which reacted with various alkenes, in a one pot synthesis to give either sultones or cyclic sulphates. The cyclic sulphates were converted to the corresponding N-benzylaziridines.

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