Heterocumulenes carbodiimides

Cycloaddition Reactions Across C=N Bonds. Cycloaddition reactions of heterocumulenes are well known reactions. Like many of the other heterocumulenes carbodiimides form cyclodimers and cyclotrimers (see Section 2.4.1). The obtained dimeric... 

A (pentamethylcyclopentadienyl)iridium chelating guanidinate complex has been conveniently prepared by treatment of [Cp IrCl2]2 with N,N, N"-th-p-tolylguanidine and base in THF at room temperature followed by recrystallization of the green product from toluene and pentane (Scheme 154). Insertion reactions of the product with heterocumulenes (diaryl carbodiimides, aryl isocyanates) have been investigated. It was found that the complex serves as highly active catalyst for the metathesis of diaryl carbodiimides with each other and for the more difficult metathesis of diaryl carbodiimides with aryl isocyanates (cf. Section V.C). ... 

In the case of the intermolecular variant, the iminophosphorane reacts with an added heterocumulene component to give a carbodiimide 50 (Scheme 27), thus forming a new electrophilic center within the molecule, which can be intramolecularly intercepted by another nucleophilic site of the intermediate. 

The synthetic route to 1,3,4-oxadiazines and 1,3,4-thiadiazines (Scheme 105) begins with the iminophosphorane of imidazolone 294, which is converted with isocyanate or isothiocyanate to carbodiimide 295. A second addition of the heterocumulenes occurs to the CH acid at the 5-position,... 

Alkyl-5-alkyliminothiatriazolines (50) decompose slowly around 40-60 "C and rapidly at 125°C with formation of sulfur, nitrogen, and carbodiimide (51) (Equation (4)). However, the carbo-diimides (51) formed react with undecomposed thiatriazoline (50), which in part explains the low yields of isolable carbodiimide. It has not been possible by trapping experiments to decide whether the decomposition involves an intermediate as addition of electron-rich alkenes or heterocumulenes induces immediate nitrogen evolution in a bimolecular reaction (see Section 4.19.5.2) <78JCS(P1)1440>. 

In contrast to 4-alkyl-5-sulfonylimino-A -l,2,3,4-thiatriazolines (58) (Section 4.19.5.1.2), 4-alkyl-5-alkylimino-A -l,2,3,4-thiatriazolines (50) react with immediate nitrogen evolution when added to electron-rich alkenes or to heterocumulenes such as enamines, carbodiimides, isocyanates, isothiocyanates, or styrene. Reaction kinetics for certain of this type of system show that they undergo bimolecular processes as described below. 

The palladium-phosphine-catalyzed cycloaddition reactions of vinyloxetanes 530 with aryl isocyanates or diaryl-carbodiimides led to 4-vinyl-l,3-oxazin-2-ones 531 or l,3-oxazin-2-imines 532, respectively (Scheme 101). In the absence of phosphine ligands (PPhs, bis(diphenylphosphino)ethane (DPPE), l,3-bis(diphenylphosphino)propane (dppp), no conversion of heterocumulenes was observed. Starting from fused-bicyclic vinyloxetanes, both types of cycloadditions proceeded in a highly stereoselective fashion, affording only the r-isomers of alicycle-condensed 1,3-oxazine derivatives <1999JOC4152>. 

N-Vinyl heterocumulenes represent a new, highly reactive 2-azadiene species, which react, in general, with electron-rich alkenes and alkynes. Accordingly, we think it is of interest to complement the utility of electron-poor 2-azadienes in [4 + 2] cycloadditions by showing some examples involving A-vinyl isocyanates, -isothiocyanates, -carbodiimides, and -ketenimines. 

The palladium complex derived from TolBINAP catalyzes cycloaddition of vinyl epoxide to a carbodiimide with high enantioselectivity (Eq. 8E.12) [166], The stereochemical course of the cycloaddition of the epoxide with the heterocumulene was noticeably influenced by the structure of... 

C02 of other monomeric stannilenes, such as bis[N-trimethylsilyl-N-2,6-diisopro-pylphenylamido]tin(II) and bis[N,N-bis(dimethylphenysilyl)amido]tin(II). Whilst the former stannilene was inert towards C02, the latter one underwent metathesis (323 K, 0.2 MPa, pentane as solvent) with the heterocumulene generating the relevant isocyanate and carbodiimide in a 4.4 1 molar ratio [117b]. 

The related 4-alky l-5-a Iky limino-1,2,3,4-th iatria/olcs 58 are much less stable than the corresponding aryl derivatives 52 and 55, and liberate nitrogen and sulfur to give the carbodiimides 59 in low yields (Equation 5). In many cases heterocumulenes 59 participate in consequent masked 1,3-dipolar reactions (see Section 6.09.6.2) with a second equivalent of compound 58 leading to complex reaction mixtures and explaining the low yields <1978J(P1)1440>. 

In the above-mentioned patents post-polymerization modification is performed in a single step. An alternative is the performance of two sequential modification steps. In the first step the polymer is reacted with tin or germanium derivatives (triphenyl tin, tributyl tin, diphenyl tin dichloride, dioctyl tin dichloride, phenyl tin trichlorde etc.). In the subsequent modification step heterocumulene compounds (ketenes, thioketenes, isocyanates, thioiso-cyanates and carbodiimides) are applied [437,438]. Specific interaction with silica is obtained by end group functionalization with vinyl monomers which contain hydroxyl or epoxy groups. The effects are observed if the PDI is below 5 [439,440]. 

In the absence of dipolarophiles the intermediate loses sulfur to give the carbodiimide however, the intermediate may be trapped with a number of alkenes, heterocumulenes and other reagents such as ynamines, ketenes, isocyanates, isothiocyanates, carbodiimides, ketenimines, sulfonylimines, imines, nitriles, thiocarbonyl compounds, Wittig reagents and the already mentioned enamines (80AG277). Contemporary knowledge of the chemistry of these sulfonyliminothiatriazolines is mainly due to the meticulous work of L abbe and his coworkers (80AG277). 

The palladium-catalyzed [3 + 2] cycloaddition of vinylic oxirane 20a [42] and aziridine 20b [39] with the activated olefin 4a for the formation of five membered cyclic ether 21a and pyrrolidine derivative 21b has also been reported in our laboratories. The mechanistic issue is very much similar to that discussed in Scheme 9. Pd(0) catalyst added oxidatively to 20 to produce the 7r-allylpalladium complex 22. The Michael addition of a hetero nucleophile in 22 to the activated olefin 4a gives 23 which undergoes intramolecular nucleophilic attack on the inner 7r-allylic carbon atom to give the cy-clized products 21 and Pd(0) species is generated (Scheme 10). Similarly, the palladium-catalyzed [3 + 2] cycloaddition of vinylic oxirane 20a with the N-losylimincs 24 is also known (Scheme 11) [43]. Intermolecular cycloaddition of vinyl epoxides and aziridines with the heterocumulenes such as isocyanates, carbodiimides and isothiocyanates is also known [44,45]. Alper et al. reported the regio- and enatioselective formation of the thiaolidine, oxathiolane, and dithiolane derivatives by the palladium-catalyzed cyclization reaction of 2-vinylthiirane with heterocumulenes [46]. 

By Aza-Wittig Reaction. In the reaction of heterocumulenes with iminophos-phoranes, carbodiimides with different substituents on nitrogen are obtained. The reaction of iminophosphoranes 61 with isocyanates (X=0) or isothiocyanates (X=S) to give carbodiimides with two different substituents 62 was discovered by Staudinger and Hauser in 1921.102... 

Symmetrically substituted carbodiimides are also obtained from iminophosphoranes and carbon dioxide or carbon disulfide, involving isocyanates or isothiocyanates as intermediates. Instead of the heterocumulenes di-t-butylcarbonate is also used in the reaction with iminophosphoranes. For example, bis(l-naphthyl)carbodiimide is obtained in 64 % yield using this procedure. 

The insertion reaction of heterocumulenes can be used to syuthesize carbodiimides having two different substituents. For example, the carbamate 157 derived from an isocyanate and bis(tributyltin)oxide can be reacted with an isothiocyanate to give a carbodiimide 158 with substituents derived from the isocyanate and the isothiocyanate. ... 

The reaction of alkyl halides with trimethylsilylcarbodiimide affords carbodiimides with adjacent heterocumulene groups. For example, reaction of Q -chlorosulfilimines 169 with N-trimethylsilyl-N -phenylcarbodiimide 170 affords the functional carbodiimide 171. ... 

Aliphatic carbodiimides often react with heterocumulenes to form six-membered ring 1 2 cycloadducts. For example, DCC reacts with N-p-toluenesulfonyl-N -cyclohexylcarbodiimide to give the 1 2 cycloadduct 207, mp 123-124 °C in 93 % yield. The initially formed polar intermediate 206 reacts with a second sulfonylcarbodiimide molecule to give the final product. ... 

Heterocumulenes undergo insertion reactions with numerous substrates. In general, carbodiimides react faster than isocyanates and isothiocyanates, in that order. Insertions of carbodiimides into metal-hydrogen, metal-halogen, metal-mitrogen, metal-oxygen and metal-sulfur bonds are reported. Also insertions of carbodiimides into carbon-hydrogen bonds are known. 

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. ... 

An important reaction of carbodiimides leading to bicycUc N-heterocycles is the aza-Wittig reaction. Often carbodiimides with unsaturated substituents are generated in situ and subsequent cyclization reactions afford the N-heterocycles. Many of these reactions are reported by Molina, Saito ° and Wamhoff. In the aza-Wittig reaction iminophosphoranes react with heterocumulenes (CO2, CS2, RNCO and RNCS) to generate carbodiimides, which often undergo subsequent reactions. 

In the course of our studies on organic and macromolecular syntheses with lanthanoid alkoxide, we have foimd that a lanthanoid complex, formed by the addition of a lanthanoid isopropoxide (LnlUPrla) to a heterocumulene bearing nitrogen atom (R-N=C=X isocyanates and carbodiimides), served as a novel COj carrier for the rapid carboxylation of active methylene compounds (Y-CHj-Z) under mild conditions such as room temperatxue and atmospheric pressure (Scheme 1). 

The final heterocumulenes containing nitrogen are the carbodiimides, which are less reactive as acceptors than the isocyanates. Tributyltin(IV) methoxide forms an adduct with diarylcarbodiimides ... 

Eq. (5.11)) [18]. This method has also been applied to aldehydes [16, 18]. A variation of the electrophile gave a cyclic compound (Eq. (5.12)). Similarly, the acyllithium, which is generated in situ, can be trapped by dialkyl disulfides (Eq. (5.13)) [19] and also by heterocumulenes such as carbon disulfide [20], isocyanates [21], isothiocyanate [21], and carbodiimides [22]. 

---