A novel protocol toward imidazolidines has been accomplished using tosylhydrazones and hexahydro-1,3,5-triazines as the substrates under metal-free reaction conditions. Importantly, the role of LiOtBu has been confirmed not only to release the diazo but also to promote the cycloaddition. Further mechanistic investigations reveal that the reaction proceeds through a stepwise [2 + 1 + 2] process.