Chemistry Reference
In-Depth Information
4
Phosgenation Reactions
4.1
Classification of Phosgenation Reactions
Phosgenation (reaction) in this topic is defined as a method for inserting a chloro-
carbonyl, carbonyl, or chlorine group into an organic compound or for dehydrating
amidic groups forming cyanide, isocyanide, or carbodiimide groups using phos-
gene or a phosgene equivalent or substitute (for definitions, see Chapter 2) as re-
agent.
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 (iso-
cyanates, 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 topic is a handbook, which emphasizes its use-
fulness 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
topic, 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 classifica-
tion of phosgene reactions, and allow the simplest application of the logic of syn-
thetic planning by means of the methodology of retrosynthesis [5-7], which works
