Additions to Heterocumulenes

Heterocumulenes such as R1N=C=0, RAN—C=S and R1N=S=0 react smoothly with polar organometallic intermediates [50]. Although in many procedures the counter ion is Li , other alkali metal ions or MgX might also 

The rate of RLi additions to methyl isothiocyanate seems to be strongly dependent upon the basicity of RLi [9]. Whereas phenyllithium reacts rapidly at — 90 °C, the addition of the much more weakly basic 2-thiazolyllithium (pK of 1,3-thiazole is about 29) has to be carried out at —30 to —50 °C and the reaction with lithium acetylides RC=CLi (pK acetylenes is 26 or lower) requires 0 to 20 °C (all reactions were carried out at the same 0.5 to 1 mol/1 concentrations). These temperatures indicate the conditions for other organolithium compounds and also the chance of success for other reactions. A rough estimation of the pK values of RH has to be made first, taking into account stabilizing effects of substituents. Side reactions have not yet been observed in the conversions of isothiocyanates with lithium compounds. 

With the less basic 2-thienyllithium, mainly the dithioester, 2-thienyl-C(S)SCH3 (resulting from carbophilic attack and subsequent methylation), was obtained. 2-Furyllithium also mainly gave the dithioester. By using the Grignard derivatives, the pure dithioesters were obtained in excellent yields [55]  

Catalytic or equivalent amounts of copper(I) bromide, added to the solutions of the lithium compounds, appeared to promote carbophilic attack as well [56]. 

The reactions of non-aromatic organolithium compounds with CS2 afforded high-boiling unidentified products. In these cases a very fast subsequent conjugate addition of RLi to the primary product might have taken place [9]  

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

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. 

The electron-rich and cyclically conjugated 8 rr electron system of 1,4-dihydropyrazine can be stabilized in essentially planar conformation by organosilicon substitution at the enamine nitrogen centers [1], In addition to electron transfer [1] and triorganosilyl exchange reactions [2], we have explored the possibility of inserting heterocumulenes X=C = Y into one or both of the N-Si bonds of the compounds 1 in order to functionalize this unusual ring structure [3],... 

This approach to 1,2,4-dithiazolidine derivatives 124 was investigated in detail by L abbe and co-workers on the example of a reaction of 4-alkyl-5-arylimino-l,2,3,4-thiotriazolines 123 and related compounds with isothiocyanates and other heterocumulenes. In these reactions, in addition to compounds 124, l,2,4-thiadiazolidine-3-thiones 125 are formed (Equation 17) <1996CHEC-II(4)453>. 

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 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... 

Carbodiimides also play a role as building blocks of numerous N-heterocycles. Examples are the cycloaddition reactions of carbodiimides. In the formation of [2-1-2] cycloadducts N-methyl-N -t-butylcarbodiimide is used as a marker to determine the mode of addition of isocyanates or isothiocyanates to carbodiimides. In these reactions addition across the C=N or the C=0 bond of the isocyanate or across the C=N or the C==S bond of the isothiocyanate can occur. Addition to the N-methyl-N -t-butylcarbodiimide always proceeds across the N-methyl double bond. Also, theretroreaction of the [2-1-2] cycloaddition reaction of carbodiimides can afford a different set of heterocumulenes. ... 

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