Cumulated double bonds isocyanates

This reaction takes place because diimides, —N=C=N—, have reactive cumulated double-bond systems like those of ketenes, C=C=0 isocyanates, —N=C=0 and isothiocyanates, —N=C=S and are susceptible to nucleophilic attack at the central carbon. In the first step of the diimide-coupling reaction, the carboxyl function adds to the imide to give an acyl intermediate, 9. This intermediate is an activated carboxyl derivative RCO—X and is much more reactive toward an amino function than is the parent acid. The second step therefore is the aminolysis of 9 to give the coupled product and yV,N -dicyclohexylurea ... 

A representative 1,3-dipolar cycloaddition process occurs with yV-aryl-C-(trifiuoromethyl)-nitrilimines, generated from the corresponding hydrazonoyl bromides, c.g. 4. under basic conditions. which can react with dimethyl fumarate and maleate,bicyclic olefins. and dipolarophiles containing cumulative double bonds. With sodium isocyanates as the dipolarophilc the cycloaddition reaction occurs across the C = N bond, while with potassium isothiocyanate it occurs through the C = S bond. ... 

The cumulated double bonds of ketenes, isocyanates, and carbon suboxide are also very reactive. Disubstituted ketenes yield enolate ions (equation 38), which may be protonated to ketones or used in situ. The very high reactivity... 

Carbonyl diisocyanate (46) was shown to undergo a Diels-Alder type of cycloaddition with azomethines to give 2,3,6,7-tetrahydro-4/7,8/7-[l,3,5]triazino[2,l-Z>][l,3,5]oxadiazin-4,8-diones (16) (Scheme 5). A large variety of the triazino[2,l-ft][l,3,5]oxadiazines were realized by the suitable selection of dienophiles. Thus, l,3,5-triazino[2,l-Z>][l,3,5]oxadiazines (17), (18 R = NMe), and (19) and were synthesized by the reaction of carbonyl diisocyanate with alkyl or aryl isocyanates, dimethyl cyanamide, or aliphatic carbodiimides, respectively <86CB1133>. Due to the high reactivity of the cumulated double bonds, carbonyl diisocyanate (46) was also found to undergo [4 + 2] cycloadditions with cyclohexanone to yield cyclohexan-l-spiro-9 -[l,3,5]-oxadiazino-[3,4-e][l,3,5j-dioxazin-5 -spiro-l"-cyclohexane-2, 7 -dione (47) (Scheme 6) <76LA1634>. 

In addition to activated olefins, other double-bond substrates, such as carbonyl and thiocarbonyl compounds and azomethines have been used ( ). Likewise, heterocumulenes, such as carbon disulfide, isocyanates, isothiocyanates, carbodiimides, and N-sulfinylamines have been added to nitrile imides ( ). While usually reaction occurs across one of the cumulative double bonds, with carbon disulfide only the bis-adduct LVIII could be isolated ( ). 

What is the mechanism of the coupling reaction of Rgure 23.47 Remember that other cumulated double bonds (p. 512) such as ketenes (p. 515), isocyanates (p. 918), and isothiocyanates (p. 1198) all react rapidly with nucleophiles. The related DCC is no exception, and it is attacked by amines to give molecules called guanidines. Carboxylates are nucleophiles too, and will also add to DCC (Fig. 23.48). 

Cyclodimerization reactions can occur across either one of the cumulative double bonds giving rise to the formation of head-to-head or head-to-tail cyclodimers. The stable head-to-tail cyclodimers of ketenes and the head-to-head cyclodimers of isocyanates are good examples and only one type of cyclodimer is formed. In contrast, allenes often provide mixtures of cyclodimers. A super-click reaction is observed in the cyclodimerization of bis-allenes, which occurs at room temperature in the solid state upon irradiation to give the cyclodimers in quantitative yields. ... 

Oiganic isocyanates were first synthesized in 1849. Isocyanates (Table 22.1) are highly reactive heterocu-mulene chemicals. The general structure of isocyanates is R—N=C=0, which is distinct from that of cyanate (N=C—O—H). The reactivity of organic isocyanates is due to the strain in the cumulative double bonds (—N=C=0) of isocyanates (Varadarajan et al., 1985). 

Various types of triple bonds such as nitriles and cumulated double bonds such as isocyanate groups absorb in the region from 2300 to 1900 cm Various types of double bonds, including C=0, C=N, and C=C absorb from 1900 to 1550 cm Aromatic ring vibrations absorb near 1600 and 1500 cm ... 

We begin with reactions of polar organometallics with aldehydes, ketones, and v/c-dicarbonyl compounds (a-oxoaldehydes, 1,2-diketones, a-oxocarboxamides, and a-oxocarboxylic acids). Next we focus on carboxamides, lithium carboxylates, carboxylic acid esters, and acyl chlorides. Finally we turn to ketenes, (thio)isocyanates, and carbon dioxide, all featuring cumulated double bonds, and last but not least to carbon monoxide. 

Isocyanates and thioisocycanates equally harbor cumulated double bonds. As they combine virtually quantitatively with all kinds of organic lithium, sodium, or potassium compounds, they were much appreciated as trapping reagents in the early history of polar organometallic chemistry. Moreover, the products derived from iV-phenyl, yV-naphthyl, or A/-rer/-butyl substituted (thio)isocyanates crystallize extremely well. 

The silatropic process combined with the addition reaction can be realized with quite a number of compounds, including carbodiimides [Eq. (6a)], isocyanates, carbon disulfide [Eq. (6b)], or carbon dioxide [Eq. (6c)], which exhibit cumulated hetero double bonds. The first step, insertion between the P-Si bond, is followed by a silyl migration (32-35). 

The last section showed a set of reactions unified by formation of an intermediate containing two cumulated (adjacent) double bonds. Their common ancestor is allene, H2C=C=CH2- Are these molecules mere curiosities, chemical oddities of no real importance outside a textbook or exam Not at all. In 1992, broccoU was shown to contain a sulfur-containing isocyanate, an xsothiocy3.naXt, that induces formation of something called a phase II detoxication enzyme, which is involved in the metabolism of carcinogens. Eat your broccoU and study your orgo. 

Isothiocyanates (formerly known as mustard oils, 8-136) are compounds with a cumulated system of double bonds belonging to the heterocumulenes, compounds that are formally derived from the hydrocarbon allene (H2C=C=CH2). In small quantities, isothiocyanates are often accompanied by isomeric thiocyanates. Related cyanates and isocyanates do not occur in foods. 

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