Abstract: 

Electophilic cyclizations are common reactions to form substituted heterocycles. Here investigations into two systems and the mechanistic findings are presented. In the first part haloboration versus cyclization is explored. Haloboration was found to be reversible and could be intercepted to form zwitterionic sulfonium borates which were analyzed by NMR spectroscopy and X-ray crystallography.

The second part describes the isolation of intermediates in electrophilic cyclizations by Pd(II). Analysis of the formed sulfonium heterocycles by NMR spectroscopy and X-ray crystallography revealed that they can be best described as endocyclic sulfonium ylide stabilized carbenes.