N-Isocyanates, N-Isothiocyanates and Their Masked/Blocked Derivatives: Synthesis and Reactivity

Abstract

In contrast to <i>C</i>-substituted isocyanates which are frequently used reagents in synthesis, nitrogen-substituted isocyanates (<i>N</i>-isocyanates) remained a scientific curiosity in the literature for over a century. The amphoteric character of these reactive intermediates, which results in their propensity to dimerize, prevented the development of their full synthetic potential. In this review, the pioneering work that led to the development of reliable methods for their synthesis, established the existence of such species by spectroscopy, and allowed a delineation of their basic reactivity is presented. The high reactivity of these intermediates has been controlled through the use of masked precursors to regulate their concentration via a reversible equilibrium, a strategy which is also used in industrial processes utilizing <i>C</i>-isocyanates. This control led to the development of more complex reactions, including reaction cascades that allow the rapid synthesis of heterocycles possessing the N–N–C=O motif, which is widely present in agrochemicals and pharmaceuticals. This review covers the literature of <i>N</i>-isocyanates and their sulfur analogues, <i>N</i>-isothiocyanates, until April 2016. 1 Introduction 2 Formation of <i>N</i>-Isocyanates 2.1 Rearrangement of Carbamoyl Azides 2.2 Ring Opening of Heterocycles 2.3 Thermolysis of Hydrazine Derivatives 3 Formation of <i>N</i>-Isothiocyanates 4 Isolation and Observation 4.1 Isolation and Observation of <i>N</i>-Isothiocyanates 4.2 Isolation and Observation of <i>N</i>-Isocyanates 5 Reactivity 5.1 Reactivity of <i>N</i>-Isothiocyanates 5.2 Reactivity of <i>N</i>-Isocyanates 6 Conclusion