Phosgenation Reactions
4.1 Classification of Phosgenation Reactions
Phosgenation (reaction) in this book is defined as a method for inserting a chlorocarbonyl, carbonyl, or chlorine group into an organic compound or for dehydrating amidic groups forming cyanide, isocyanide, or carbodiimide groups using phosgene or a phosgene equivalent or substitute (for definitions, see Chapter 2) as reagent.
There are several areas of uncertainty regarding the literature classification of phosgene chemistry. The most common classification is based on the nature of the reaction center and usually lists the reagents, or the products of reaction (isocyanates, carbonates, carbamates, ureas, chloroformates, acid chlorides, isonitriles, nitriles, carbodiimides, etc.) [1, 2].
A recent monograph [3] classifies phosgene reactions into two main categories: those that introduce the building block ''carbonyl'' as a structural unit and those involving phosgene and its derivatives as reagents. This classification focuses on the formal reaction product structure and defines phosgene as a ''carbonyl'' unit carrier or, more simply, emphasizes the role of phosgene as a ''dehydroxylating'' agent. An alternative criterion starts from the traditional classification of functional groups and their preparation [4], and emphasizes phosgene as a carbonic acid derivative (dichloride of carbonic acid).
Every classification is somewhat arbitrary. The key factors are its accuracy and its meaning in a wider context. This book is a handbook, which emphasizes its usefulness for both the chemist who is strongly interested in phosgene chemistry planning syntheses and the chemist working in the laboratory. When searching for a method or procedure to perform a certain synthetic strategy, both need a simple way of finding it. Therefore, the contents of this chapter, the main chapter of the book, are arranged in terms of the building or transforming of functional groups that appear in the resulting products. The four main reactions of phosgene, namely chloroformylation, carbonylation, chlorination, and dehydration, forming the functional groups chlorocarbonyl, carbonyl (isocyanate, carbonate, carbamate, urea), chloro, cyano, isocyano, and carbodiimide, constitute the backbone of our classification of phosgene reactions, and allow the simplest application of the logic of synthetic planning by means of the methodology of retrosynthesis [5-7], which works