Carbamate

Carbamates are organic compounds derived from carbamic acid (NH2COOH). A carbamate group, carbamate ester, and carbamic acids are functional groups that are inter-related structurally and often are interconverted chemically. Carbamate esters are also called urethanes.

Synthesis
Carbamic acids are derived from amines:
 * R2NH +  CO2   →   R2NCO2H

Carbamic acid is about as acidic as acetic acid. Ionization of a proton gives the carbamate anion, the conjugate base of carbamic acid:
 * R2NCO2H  →   R2NCO2-  +  H+

Carbamates also arise via hydrolysis of chloroformamides:
 * R2NC(O)Cl +  H2O   →   R2NCO2H  +  HCl

Carbamates may be formed from the Curtius Rearrangement, where isocyanates formed are reacted with an alcohol.
 * RNCO + R'OH → RNHCO2R'

Applications and occurrence
Although most of this article concerns organic carbamates, the inorganic salt ammonium carbamate is produced on a large scale as an intermediate in the production of the commodity chemical urea from ammonia and carbon dioxide.

Carbamates in biochemistry
N-terminal amino groups of valine residues in the α- and β-chains of deoxyhemoglobin exist as carbamates. They help to stabilize the protein, when it becomes deoxyhemoglobin and increases the likelihood of the release of remaining oxygen molecules bound to the protein. The influence of these carbamates on the affinity of hemoglobin for O2 is called the Bohr effect.

The ε-amino groups of the lysine residues in urease and phosphotriesterase also feature carbamate. The carbamate derived from aminoimidazole is an intermediate in the biosynthesis of inosine. Carbamoyl phosphate is generated from carboxyphosphate rather than CO2.

CO2 capture by ribulose 1,5-bisphosphate carboxylase
Perhaps the most important carbamate is the one involved in the capture of CO2 by plants since this process is relevant to global warming. The enzyme Ribulose 1,5-bisphosphate carboxylase/oxygenase fixes a molecule of carbon dioxide as a carbamate at the start of the Calvin cycle). At the active site of the enzyme, a Mg2+ ion is bound to glutamate and aspartate residues as well as a lysine carbamate. The carbamate is formed when an uncharged lysine side-chain near the ion reacts with a carbon dioxide molecule from the air (not the substrate carbon dioxide molecule), which then renders it charged, and, therefore, able to bind the Mg2+ ion.

Carbamate insecticides
The so-called carbamate insecticides feature the carbamate ester functional group. Included in this group are aldicarb, carbofuran (Furadan), carbaryl (Sevin), ethienocarb, and fenobucarb. These insecticides kill insects by reversibly inactivating the enzyme acetylcholinesterase. The organophosphate pesticides also inhibit this enzyme, although irreversibly, and cause a more severe form of cholinergic poisoning.

Fenoxycarb has a carbamate group but acts as a juvenile hormone mimic, rather than inactivating acetylcholinesterase.

The insect repellent icaridin is a substituted carbamate.

Polyurethanes
Polyurethanes contain multiple carbamate groups as part of their structure. The "urethane" in the name "polyurethane" refers to these carbamate groups; ethyl carbamate (common name "urethane") is neither a component of polyurethanes, nor is used in their manufacture. Polyurethane polymers have a wide range of properties and are commercially available as foams, elastomers, and solids. Typically, polyurethane polymers are made by combining diisocyanates, e.g. toluene diisocyanate, and diols, e.g. bisphenol-A, where the carbamate groups are formed by reaction of the alcohols with the isocyanates:
 * RN=C=O +  R'OH  →  RNHC(O)OR'

Preservatives and cosmetics
Iodopropynyl butylcarbamate is a wood and paint preservative and used in cosmetics.

In human medicine
Urethane or ethyl carbamate was once produced commercially in the United States as an antineoplastic agent and for other medicinal purposes. It was found to be toxic and largely ineffective. It is occasionally used as a veterinary medicine.

In addition, some carbamates are used in human pharmacotherapy, for example, the cholinesterase inhibitors neostigmine and rivastigmine, whose chemical structure is based on the natural alkaloid physostigmine. Other examples are meprobamate and its derivatives like carisoprodol, felbamate, and tybamate, a class of anxiolytic and muscle relaxant drugs widely used in the 60s before the rise of benzodiazepines, and still used nowadays in some cases.

The protease inhibitor darunavir for HIV treatment also contains a carbamate functional group.

Sulfur analogues
There are two oxygen atoms in a carbamate (1), ROC(=O)NR2, and either or both of them can be conceptually replaced by sulfur. Analogues of carbamates with only one of the oxygens replaced by sulfur are called thiocarbamates (2 and 3). Carbamates with both oxygens replaced by sulfur are called dithiocarbamates (4), RSC(=S)NR2.

There are two different structurally isomeric types of thiocarbamate:


 * O-thiocarbamates (2), ROC(=S)NR2, where the carbonyl group (C=O) is replaced with a thiocarbonyl group (C=S)
 * S-thiocarbamates (3), RSC(=O)NR2, where the R–O– group is replaced with an R–S– group

O-thiocarbamates can isomerise to S-thiocarbamates, for example in the Newman-Kwart rearrangement.