Heterocumulenes, nucleophilic addition

Polyurethane Formation. The key to the manufacture of polyurethanes is the unique reactivity of the heterocumulene groups in diisocyanates toward nucleophilic additions. The polarization of the isocyanate group enhances the addition across the carbon—nitrogen double bond, which allows rapid formation of addition polymers from diisocyanates and macroglycols. 

Bardaji, M., Gimeno, M.C., Jones, P.G., Laguna, A., Laguna, M., Merchan, F. and Romeo, I. (1997) Carbon-Carbon Coupling via Nucleophilic Addition of a Gold(l) Methanide Complex to Heterocumulenes. Organometallics, 16(5), 1083-1085. 

A statement made in an earlier review (5), that the synthetic potential of cycloadditions of diazo compounds with heterocumulenes does not appear to have been extensively probed, is still valid. This may be due in part to the complexity of the reaction. Instead of a concerted [3 + 2] cycloaddition, nucleophilic addition of the diazo compound at the heterocumulene can lead to a diazonium betaine from which several different products can arise with or without conservation of the azo moiety. 

The reactions of 33 with some heterocumulenes were described (50). The products result from formal nucleophilic addition of the dmpz anion to the central carbon of the heterocumulene to give a new bidentate ligand. 

With nucleophilic bases 3,7,7a-trimethyl-2,3,5,6,7,7a-hexahydro-17f-imidazo[l,5-Zi][l,2,4]-triazole-2,5-dione (130) and -2,5-dithione (129) the [3 -H 2] cycloelimination of the heterocumulene moiety from the triazole ring is followed by nucleophilic addition of the base to give 4-substituted 4,5,5-trimethyl-2,3,4,5-tetrahydro-l/7-imidazol-2-ones (135) and -2-thiones (136), respectively (Scheme 7) <93TH 805-01, 95FES379). 

Nucleophilic addition to heterocumulenes Disulfonylmethanes as CH-acidic compounds... 

Due to their high electrophihcity, isocyanates are useful starting materials for the synthesis of heterocycles and C-C bond formation using carbon nucleophiles. A key feature of these reactions is the requirement for more than one equivalent of base. Additionally, a few examples of transition metal-catalyzed homologation of the central carbon of the heterocumulene moiety have been reported. 

Ketenes and other C=0-containing heterocumulenes also react with nucleophiles in addition reactions the C=0 double bond, however, is nonetheless retained (Section 8.2) ... 

The reverse reaction, i.e., the addition of water to the heterocumulene B with formation of carbonic acid derivatives A, is also known. With very few exceptions this reaction occurs spontaneously with catalysis by an acid or base. The hydration of a heterocumulene is a special example of an entire class of heterocumulene reactions, namely the addition of heteroatom nucleophiles H-Het3 (you have already come across such a reaction briefly in Section 6.1.1 in the context of the DCC activation of carboxylic acids). Occasionally, this addition... 

Fig. 8.11. Tandem reaction consisting of three single reactions mutually transforming heterocumulenes and heteroatom nucleophiles in a one-pot synthesis of an isothiocyanate (1) uncatalyzed addition reaction of heteroatom nucleophile (aniline) + heterocumulene (carbon disulfide) —> carbonic acid derivative (A) (2) heterolysis-initiated /3-elimination of the carbonic acid derivative (D) -> heterocumulene (F = phenylisothiocyanate) + heteroatom nucleophile (thiocar-bonic acid O-ethylester) (3) decomposition of a carbonic acid derivative (D) to a heterocumulene (carbon oxysulfide) and a heteroatom nucleophile (ethanol) via the zwitterion H.

Fig. 8.12. Mechanism of the uncatalyzed addition (starting top left and proceeding clockwise) and the acid-catalyzed addition (starting top left and then proceeding counterclockwise) of heteroatom nucleophiles to heterocumulenes.

The toxicity of methyl isocyanate comes as a surprise if its high sensitivity to hydrolysis is considered. But actually, it may reach its site of action in the human body largely undecomposed. This is due to the reversible addition of another nucleophile to this heterocumulene. The tripeptide gluthathione, which is supposed to protect the body against oxidizing agents, adds to the C=N double bond of the isocyanate by means of its thiol group whereby the thio-carbamate is formed. When the latter decomposes in a reversal to its formation reaction, it releases the intact toxic methyl isocyanate. 

The former chapter Additions of Heteroatom Nucleophiles to Heterocumulenes, Additions of Heteroatom Nucleophiles to Carbonyl Compounds and Follow-up Reactions has been spht into two separate chapters into Chapter 8 Carbonic Acid Derivatives and Heterocumulenes and Their Interconversion , whose systematic organization should represent a particularly valuable learning aid, and into Chapter 9 Additions of Het-eroatom Nucleophiles to Carbonyl Compounds and Follow-up Reactions—Condensations of Heteroatom Nucleophiles with Carbonyl Compounds. ... 

Additions of Heteroatom Nucleophiles to Heterocumulenes, Additions of Heteroatom Nucleophiles to Carbonyl Compounds and Follow-up Reactions... 

The starting materials for the industrial synthesis of urea are the nucleophile NH3 and the heterocumulene C02 (Figure 7.3). The reaction starts with the addition of NH3 to... 

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