Carbonyldiimidazole

1,1'-Carbonyldiimidazole, or CDI, (formula (C3H3N2)2CO), an organic compound, is a white crystalline solid. It is often used for the coupling of amino acids for peptide synthesis and as a reagent in organic synthesis.

Preparation
CDI can be prepared straightforwardly by the reaction of phosgene with four equivalents of imidazole under anhydrous conditions. Removal of the side product, imidazolium chloride, and solvent results in the crystalline product in ~90% yield.


 * 4 C3H4N2 +  C(O)Cl2  →   (C3H3N2)2CO  +  2 [C3H3N2H2]Cl

In this conversion, the imidazole serves both as the nucleophile and the base. An alternative precursor 1-(trimethylsilyl)imidazole requires more preparative effort with no corresponding advantages.

CDI hydrolyzes readily to give back imidazole:
 * (C3H3N2)2CO +  H2O  →   2 C3H4N2  +  CO2

The purity of CDI can be determined by the amount of CO2 that is formed upon hydrolysis (since the gas is formed essentially on a 1:1 molar ratio).

Use in synthesis
CDI is mainly employed to convert alcohols and amines into carbamates, esters, and ureas.



Acid Derivatives
One common extension of this scheme lies in the transacylation reaction of acids that is promoted by CDI. The reactivity of CDI is similar to that of acid chlorides, but it is more easily handled and its reactions have a wider scope in synthesis. An early application of this type of reaction was noted in the formation of imidazole peptide (and in general carboxylic acid) derivatives (with CO2 formation as a driving force).



In the realm of peptide synthesis, this product may be treated with an amino acid or peptide ester (or amino acid hydrochloride in water) to release the imidazole group and couple the peptides. The side products, carbon dioxide and imidazole, are relatively innocuous. Racemization of the amino acids also tends to be minimal, due to mild reaction conditions.

CDI can also be used for esterification, although alcoholysis requires heat or the presence of a potent nucleophiles as sodium ethoxide,) and other strong bases like NaH. This reaction has generally good yield and wide scope (though forming the ester from tertiary alcohols when the acid reagent has a relatively acidic α-proton is troublesome, since C-C condensations can occur, though this itself may be a desirable reaction).  A similar reaction involving thiols and selenols can yield the corresponding esters.  The alcohol reaction can be used to form glycosidic bonds, as well.

Similarly, an acid can be used in the place of an alcohol to form the anhydride. The equilibrium is best shifted in the favor of the anhydride by utilizing an acid in a 2:1 ratio that forms an insoluble salt with the imidazole, such as trifluoro- or trichloroacetic acid (and thus removes the free imidazole from the reaction). Symmetric anhydrides can thus be formed by replacing this trifluoro- or trichloroacetyl group with the acid that was used to form the original reagent.

Another related reaction is the reaction of formic acid with CDI to form the formylized imidazole. This reagent is a good formylating agent and can regenerate the unsubstituted imidazole (with formation of carbon monoxide) upon heating.

Yet another reaction involves the acylation of triphenylalkelynephosphoranes.

(C6H5)3P+-CHR-COR' + (C6H5)3P=CHR → (C6H5)3P=CR-COR' + (C6H5)3P+-CH2R
 * (C6H5)3P=CHR + R'-CO-Im → (C6H5)3P+-CHR-COR' + Im-

These can undergo the Wittig reaction to form α,β unsaturated ketones or aldehydes.

The reagent can even undergo reaction with peroxide to form the peroxycarboxylic acid, which can react further to form diacyl peroxides. The imidazole group is also reduced by LiAlH4 to form aldehydes from the carboxylic acid (rather than amines or alcohols). The reagent can also be reacted with Grignard reagents to form ketones.

A C-C acylation reaction can occur with a malonic ester-type compound, in the following scheme useful for syntheses of macrolide antibiotics.



Other Reactions
The N-phenylimino derivative of CDI can be formed in a Wittig-like reaction.



CDI can act as a carbonyl equivalent in the formation of tetronic acids or pulvinones from hydroxyketones and diketones in basic conditions.



An alcohol treated with at least 3 equivalents of an activated halide (such as allyl bromide or iodomethane) and CDI yields the corresponding bromide with good yield. Bromination and iodination work best, though this reaction does not preserve the stereochemistry of the alcohol. In a similar context, CDI is often used in dehydration reactions.