Triflate, trimethylsilyl bromination

Unsaturated amides are easily converted to imidates by 0-alky la ting reagents such as trimethyl-oxonium telrafluoroborate or trimethylsilyl triflate. Nucleophilic attack by the thus formed imino group on an halonium ion followed by hydrolysis gives a lactam. Thus an unsaturated dimethyl imidate was treated with bromine in dichloromethane at 0 °C and cyclized with total regio- and stereoselectivity to give 1, since the tertiary cation forces the cyclization towards the formation of a six-membered ring. The trans addition to the double bond was shown by successive conversion to a m-aziridine 2126. 

Bromination. Electron-rich aromatic compounds undergo nuclear bromination readily with the dibromohydantoin. To improve its reactivity toward less reactive arenes (e.g., t>-anisaldehyde) the addition of trimethylsilyl triflate has positive effects. Noteworthy are the different reaction profiles of 2-allylphenol in the presence of M SiOTf 2-bromomethyldihydrobenzofuran is the product in the presence of MejSiOTf. 

Propargylic trimethylsilanes, now readily accessible by reaction of lithium acetylides with (trimethylsilyl)methyl halides or triflate, are emerging as useful synthetic intermediates. The general pattern of reactivity involves attack by an electrophile with cleavage of the carbon-silicon bond. For example, treatment of propargylic trimethylsilanes with TFA or bromine leads to allenes or 3-bromoallenes respectively, " and reactions with acetals furnish 4-alkoxy-allenes" (e.g. Scheme 101). 

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