Ring opening reactions cationic assistance

It appears that strong Lewis acids such as boron trifluoride can bring about a cationic ring-opening reaction without assistance from a co-catalyst, whereas weaker Lewis acids, such as stannic chloride, require co-catalysts. 

The first four compounds have lower j values than methyl bromide because each is quite reactive in itself and therefore is not very dependent on help from the nucleophile /-toluenesulfonate is a good leaving group and does not need much assistance to begin to depart ring strain in propiolactone and in the mustard cation make a ring-opening Sv2 reaction very favorable and the transition state of benzyl chloride is stabilized by resonance and therefore is easily reached. 

Under these conditions, silver-assisted electrocyclic ring opening provided the haloallyl cation, which was subsequently trapped by isocyanate anion. Interception of the cationic species with isocyanate was successful since bromide was removed from the reaction mixture as a precipitate (AgBr). Finally, treatment of intermediate 9 with methanol furnished the desired carbamate in 96% yield. This example demonstrates the usefulness of the silver(I)-mediated process. Removal of free halide from the reaction mixture affords a long-lived cationic species that can be captured by a different nucleophile, such as solvent, the silver(I) counteranion, or an intramolecular nucleophile. This reactivity has been exploited in many different ways throughout the years and is examined in greater detail later in this chapter. 

Microwave-assisted living cationic ring-opening showed an enhanced polymerization rate in comparison to the reaction in common organic solvents the ionic liquid was eSiciently recovered and reused in new reaction cycles, completely avoiding the use of volatile organic compounds [20]. 

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