Preparation of Isocyanates

Most of the weight of the phosgene is lost as hydrogen chloride, which is probably neutralized to form salts that are discarded. Other commonly used diisocyanates are also made by reacting the diamines with phosgene (2.2). 

The diisocyanates are reacted with diols to produce polyurethanes that end up in consumer products, such as seat cushions, mattresses, insulation, car bumpers, swim suits, floor coatings, paints, and adhesives.12 The aromatic diisocyanates are cheaper, but yield less light-stable polymers than the aliphatic ones. 

Other Ways to Make Isocyanates from Amines 

The usual method of preparation can be illustrated by that of toluenediisocyanate (2.1) a reagent that is then reacted with diols to form polyurethanes10 (see also reviews11). 

The tragedy at Bhopal13 involved methyl isocyanate. It is reacted with 1 naphthol to make a carbamate insecticide (2.3). 

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Oligomers of phosgene, such as diphosgene [503-38-8] (COCl2)2, have found use in the laboratory preparation of isocyanates. Carbamoyl chlorides, A[,A/-disubstituted ureas, dimethyl- and diphenylcarbonates, and arylsulfonyl isocyanates have also been used to convert amines into urea intermediates, which are subsequendy pyroly2ed to yield isocyanates. These methods have found appHcations for preparation of low boiling point aUphatic isocyanates (2,9,17). 

The Curtius rearrangement is a useful method for the preparation of isocyanates as well as of products derived thereof. The substituent R can be alkyl, cycloalkyl, aryl, a heterocyclic or unsaturated group most functional groups do not interfere. 

The volatility of difunctional isocyanates (such as tolylene diisocyanates, hexamethylene diisocyanate, etc.) creates many environmental problems in the urethane industry. These difficulties can be overcome by preparation of NCO-terminated oligomers with low vapor pressure. One approach is the preparation of NCO-ter-minated oligomers by partial cyclotrimerization of difunctional isocyanates. Usually this is achieved by a multi-step process which includes also deactivation of the catalyst at a certain conversion. During our work on cyclotrimerization of isocyanates we found that cyclic sulfonium zwitterions are very active cyclotrimerization catalysts (2). Recently we found that cyclic sulfonium zwitterions under certain reaction conditions act as anionic initiators. This behavior of cyclic sulfonium zwitterions permits preparation of isocyanate oligomers containing isocyanurate rings by a one-step procedure, eliminating the deactivation step. 

A few examples will show the importance of these reactions in synthetic organic chemistry. If X is an alkoxide ion, equation (1) describes the Williamson ether synthesis (2). If X is a cyanide ion, (1) describes a standard nitrile preparation (3). If X is an aromatic nucleus, and the departure of the leaving group is facilitated by A1C13, then (1) describes the Friedel-Crafts alkylation (4). With X as the cyanate ion, (1) describes the preparation of isocyanates (5) and if it is ammonia, (1) describes a common method of preparation of amines (6). In addition, nucleophilic transmethylation is a very... 

The functionalization of SAMs via ruthenium-catalyzed cross metathesis of vinyl-terminated SAMs has been reported by Lee et al.76 to install a variety of acrylic derivatives on SAMs bearing vinyl groups on their outer surface. The major drawback of this approach is the intra-SAM metathesis which causes the formation of a mixture of surface-bound products, limiting the reproducibility of the method. The formation of urethanes by the reaction of diisocyanates77 or isothiocyanates78 with hydroxyl- and amino-terminated SAMs has been reported as well. The reaction of hydroxyl-terminated SAMs with diisocyanates, allowed the preparation of isocyanate SAMs that proved to be reactive towards amines, alcohols, and water, displaying the standard chemistry of the isocyanate groups.77... 

Finally, alkyl halides are readily converted into isocyanates, in very good yields, by reaction with the silver salt of nitrocyanamide (equation 52)404. This is the first reported reaction for the preparation of isocyanates, from alkyl halides, that is of a general nature. The direct reaction of halides with metal cyanates gives very erratic results. 

Another approach to the preparation of isocyanates is the Curtius rearrangement. The reaction can be stopped at the isocyanate stage when carried out in an inert solvent such as benzene. Undecyl isocyanate (89) has been prepared from the corresponding acyl chloride via the azide in 85% yield... 

Among the more than 30 methods available for the preparation of isocyanates, phosgenation of primary amines or their hydrochlorides still remains the most popular. The method is employed on a large scale for the industrial production of mono and polyisocyantes. 

Diamine 1 was used in a mild and simple preparation of isocyanates from aliphatic amines and diphosgene (equation 20)221. 

Preparation of isocyanic acid. Isocyanic acid is prepared by the thermal depolymerization of cyanuric acid, a solid trimer available commercially (B, E, F, MCB) or prepared by hydrolysis of the less expensive cyanuric chloride (B, E, F, MCB)... 

McGhee and co-workers at Monsanto have studied the preparation of isocyanates using nontoxic carbon dioxide (instead of the poisonous carbon monoxide) with the amine (2.17).47... 

The NCO group in the 4-position is about 8 times as reactive as the NCO group in the 2-position at room temperature (25 °C). With an increase of temperature, the activity of the ortho NCO group increases at a greater rate than that of the para NCO group until at 100 °C the ortho and para NCO groups are similar in reactivity. This disparity in activity at low temperatures can be conveniently used in the preparation of isocyanate-terminated prepolymers. 

In individual cases urea derivatives may be suitable as starting materials for preparation of isocyanates ... 

Sigurdsson, S.T., Seeger, B., Kutzke, U. and Eckstein, F. (1996) A mild and simple method for the preparation of isocyanates from aliphatic amines using trichloromethyl chloroformate. Synthesis of an isocyanate containing an activated disulfide. The Journal of Organic Chemistry, 61, 3883-3884. 

The end hydroxy groups of the polymer are secondary groups and are ordinarily rather unreactive in the urethane reaction. Initially, this limitation was overcome by the preparation of isocyanate-terminated prepolymer and by the use of block copolymers with ethylene oxide. The latter products are known as tipped polyols and are terminated with primary hydroxy groups of enhanced activity. 

Pyrolysis of pyridine derivatives is a method for allylic and benzylic deamination the preparation of nitriles from aldehydes, and the preparation of isocyanates from acid chlorides or hydrazides Flash vacuum pyrolysis continues to yield interesting reactions and products. Recently, it has been reported that quite sensitive acetylene derivatives can be obtained by this method from 4-alkylideneisoxazol-5(4H)-ones by ring degradation. On the other hand, 2 carbamyl azide molecules cyclize under these conditions to form l,2,4-triazolidine-3,5-dione 1,2-ylids . 2H-Cyclohepta[b]furan-2-ones have been obtained by ring expansion of phenyl propiolates... 

This is one of the most common methods for the preparation of isocyanates. Various reported methods for obtaining isocyanates using phosgene or phosgene equivalents are discussed in Section 4.3 of the present chapter. 

Several procedures for the preparation of isocyanates with phosgene have been described [153, 159, 160]. Nevertheless, many of them require the delivery of gaseous phosgene from an external source, such as a pressurized cylinder. 

A very mild method for the preparation of isocyanates from primary amines (RNH2) and carbon dioxide (CO2) involves the use of a Mitsunobu zwitterion generated from either diisopropyl azodicarboxylate (DIAD) or di-tert-butyl azodicarboxylate and triphenylphosphine or tri-n.-butylphosphine. 

Typical procedure Preparation of isocyanates from carboxylic acids, DPPA, and 8-bis(dimethylamino)naphthalene [256] The carboxylic acid (40 mmol) was dissolved... 

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