Beyond the alternatives that switch the mechanism of the 1,3-dipolar cycloadditions from concerted to stepwise or vice versa: a literature review

For several decades, the concerted or stepwise character of the mechanism of the 1,3-dipolar cycloaddition reaction has been one of the most debated issues in the field of organic chemistry. The significance of this problem is due to the fact that in a catalyst-free 1,3-dipolar cycloaddition, when the mechanism switches from concerted to stepwise, the stereospecificity is lost and thus unwanted stereoisomers may emerge. The first proposals about the mechanism of the 1,3-dipolar reaction were due to Huisgen (concerted model) and subsequently by Firestone (two-step diradical channel) in the 1960s. After a decade of debate, most researchers accepted the concerted model for the reaction, but during these years, researchers reported some examples of the stepwise mechanism for catalyst-free 1,3-dipolar cycloadditions. This review attempts to find a number of factors that could influence the reaction channels and switch the mechanism from concerted to stepwise, or vice versa.