Chemistry Reference
In-Depth Information
phenyl group; the reagent used to prepare the corresponding ''active'' ester is di-p-
chlorotetrafluorophenyl carbonate (see Section 4.3.3.2).
6.1.3
Deductions
Each of these criteria is determined by its intrinsic value, although among several
criteria, one '' leading'' criterion generally predominates. Nevertheless, by adjusting
the criteria with respect to each other, the optimal phosgenation reagent can be
selected.
6.2
Comparison of Reagents in Phosgenation Reactions
In the following, the same or similar phosgenation reactions with various phosge-
nation reagents are compared with regard to certain criteria. They are presented in
the order of Chapter 4.
6.2.1
Criteria for Comparison of Phosgenation Reagents
Yield is still the primary criterion in chemistry because, indirectly, it contains some
other preparative criteria such as reactivity and selectivity, and also has a significant
bearing on costs. Therefore, yield will be the '' leading'' criterion in our comparison
of same and similar reactions accomplished with various phosgenation reagents.
Further criteria to be taken into consideration are purity, handling, and work-up,as
well as safety, toxicity, and environmental impact.
6.2.2
Phosgenation Reactions
6.2.2.1
Chlorocarbonylation
For chlorocarbonylations, only phosgene, diphosgene, and triphosgene are able to
react with alcohols to afford the desired products, namely chloroformates and car-
bamoyl chlorides (see Section 4.2).
Benzyl chloroformate 1
To begin our comparison of phosgenation reagents, an example is presented in
which a marked influence of reaction conditions on product formation is observed.
Benzyl chloroformate (Z
Cl) 1 can be prepared from benzyl alcohol with either
phosgene (for a safe source, see Chapter 7) [4] or triphosgene [5]. Reaction with
phosgene at
a
20 to 0
C without a base proceeds extremely well in the desired
manner to afford 1 in high purity and, even after distillation, in a high yield of 97%
without the occurrence of benzyl chloride 2 [4] (see Section 4.2.1).
