Carbon dioxide heterocumulene

Iminophosphoranes have also proved to be key intermediates because they react with carbonyl compounds, isocyanates, isothiocyanates, acid chlorides, carbon dioxide, and carbon disulfide to give a wide range of imines and heterocumulenes, which are intermediates to pteridines. The preparation of iminophosphoranes has been investigated... 

Carbon dioxide (C02) can interact easily with several N-nucleophiles. Such interaction results in an activation of the heterocumulene, and has a great synthetic relevance as it is a key step towards the carboxylation or, more generally, the car-bonylation of the N-donor substrate and the synthesis of a variety of N-carbonyl compounds. 

The mechanism of ring formation from monoalkyne and heterocumulenes, catalysed by Ni(0) complexes, Lx Ni(0), has been proposed to involve one-step cycloaddition scheme (10) [103] and scheme (11) [104, 105] show the formation of the 2-pyrone ring in the alkyne reaction with carbon dioxide and the 2-pyridone ring in the alkyne reaction with isocyanate respectively ... 

However, the most important goal that might be reached by the application of coordination catalysts for the polymerisation of heterounsaturated monomers is the possibility of the enchainment of heterounsaturated monomers, not susceptible to homopropagation, via their copolymerisation with heterocyclic monomers. This concerns primarily the coordination copolymerisation of carbon dioxide and oxacyclic monomers such as epoxides, leading to aliphatic polycarbonates [8 12]. Representative examples of the copolymerisations of heterocyclic monomers and hardly homopolymerisable heterocumulenes, in the presence of coordination catalysts, are listed in Table 9.4 [1]. 

Figure 8.3 relates the decomposition of free carbonic acid (B) via an isomeric zwitterion (C) into the heterocumulene carbon dioxide and the heteroatom nucleophile water (top-most line) with four entirely analogous decomposition reactions. The scheme shows (from top to bottom) ... 

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 use of Diels-Alder-type cycloaddition reactions is the most intensively investigated cycloaddition approach to the design of ladder polymers in a concerted process. Another methodology was published by Tsuda and coworkers [52, 53, 54]. They developed a nickel (0)-catalyzed [2 + 2 -l- 2] cycloaddition copolymerization of cyclic diynes 38 with heterocumulenes (like carbon dioxide or isocyanates 39). The soluble ladder-type products - poly(2-pyrone)s and poly(2-pyridone)s 40 - possess molecular weights M of up to 60000, corresponding to a Dp > 200. Unfortunately, the products formed were contaminated by nickel salts originating from the catalyst used Ni(COD)2. 

Cumulenes are compounds with two adjacent perpendicular double bonds (Figure 1.14). Each double bond can be considered separately. The central carbon is sp hybridized, and the two CH2S lie in perpendicular planes. In heterocumulenes, heteroatoms have replaced one or more of the carbons, as in carbon dioxide, 0=C=0. 

In order to avoid the [2-f 2] cycloaddition and homodimerization side reactions, the use of a heterocumulene bearing only one (central) carbon atom is the obvious choice. If, additionally, the groups X and Y are chosen to be identical, there is no possibility for isomers with respect to the hetero double bond involved in the cycloaddition. One substrate fulfilling these requirements is carbon dioxide, which was employed in some early reactivity studies. ... 

Transition-metal-catalyzed hetero-[2 + 2 + 2]-cy-cloaddition of alkynes with carbon—heteroatom multiple bonds, such as isocyanides, carbon dioxide, nitriles, aldehydes, and ketones, provides heteroare-nes and unsaturated heterocycles. This reaction can be categorized into two groups one is the reaction of l,a>-diynes 397 with carbon—heteroatom multiple bonds, and the other is reaction of the alkynes 399, having a carbon—heteroatom multiple bond with alkynes as illustrated in Scheme 127. The reaction of 1,6 -diynes 397 proceeds through formation of the metalacyclopentadiene intermediate 398 followed by insertion of a carbon—heteroatom multiple bond, such as heterocumulenes (route a),189 nitriles (route b),190 and carbonyls (route c).191 On the other hand, the... 

Examples of five-membered heterocyclic rings prepared by aza-Wittig and related methods include the alkaloid leucettamine B (206) and pyrrolo[2,3-b]pyridines (207) (Scheme20). The aza-Wittig reactions of the iminophosphorane (208) with methyl isocyanate, carbon dioxide, or carbon disulfide give heterocumulenes (209), (210), and (211), respectively, which react with amines or ammonia to provide a route to the framework of aplysinopsin alkaloids (212). ... 

Scheme 3 depicts preparative routes to NHC I from cyclic precursors. The extrusion of heterocumulenes, such as carbon dioxide from suitable precursors II, the deprotonation of hetarenium salts HI, the a-ehmination... 

The reaction is useful in the synthesis of acycHc imines [122-124] and heterocumulenes [112-117] and in the intramolecular formation of carbon-nitrogen double bonds in heterocycHc synthesis [112-117]. On the other hand aza-Wittig type reactions of iminophosphoranes with carbon dioxides, carbon disulphides, isocyanates, isothiocyanates and ketenes render access to functionalized heterocumulenes as highly reactive intermediates able to undergo a plethora of heterocycUzation reactions [112-117]. 

Coordination of carbon dioxide to a metal center results generally in marked modification of the bond properties (CO bond lengths, OCO angle, etc.) of CO2 ligand and provides the heterocumulene in an activated form (Chap. 2). Conversion of CO2 to C02 anion is another important mode of activating carbon dioxide molecule, and represents the first of the reactive steps leading to the partial or full reduction of the heterocumulene (Chap. 8). 

The reaction of carbon dioxide with Li, Na, K, and Cs in rare gas and nitrogen matrices leads to the reduction of the heterocumulene with formation of M C02 species. Two geometric isomers of Li C02 were isolated in solid argon or krypton... 

An important issue is the regioselectivity of the insertion reaction in those cases where more than one reaction site is available for carbon dioxide insertion or, more generally, for reaction with the heterocumulene. Cowan and Trogler [112] found that trani-Pt(H)(NHPh)(PCy3)2 reacted reversibly with CO2 to give a carbamate rather than a formate or metallo-acid complex. Other studies have established that... 

Carbon dioxide can also form carbamates by reaction with amides of non-metallic elements [78-84, 136-144]. Most examples reported in the literature deal with amides of silicon, germanium, and phosphorus. Among these, the insertion of the heterocumulene into the P-N bond of hexaalkylphosphorus triamides P(NR2)3, which affords phosphocarbamates species of formula P(NR2)3 (02CNR2) t, has received particular attention for its potential in chemical synthesis, because this reaction opens a route to the phosgeneless synthesis of carbamate esters [138,139] and ureas [140]. 

Methylenecyclopropanes have been carboxylated with gaseous carbon dioxide in the presence of a stoichiometric amount of nickel(O) complexes [150]. Depending on the polarity of the reaction solvent and the nature of amine ligand (amidine or guanidine base), several reactions pathways have been observed, which may involve either oxidative coupling of CO2 with the C-C double bond of the substrate (see Sect. 5.3) or opening of the three-membered ring and further reaction with the heterocumulene. 

Schenk S, Notni J, Kohn U, Wermann K, Anders E (2006) Carbon dioxide and related heterocumulenes at zinc and lithium cations bioinspired reactions and principles. Dalton Trans 4191 206... 

Schindler et al. carried out a kinetic analysis for the reaction of isoprene (ip) with carbon dioxide and Ni(bpy)(cod) [103]. Ni(bpy)(cod) did not react directly with CO2 (see also Sect. 5.2), but reacted reversibly with isoprene to give Ni(bpy)(ip). The kinetic results support a mechanism where, in a fast preequilibrium step, one of the bonds between Ni and cod is cleaved and, in a following step, ip coordinates to the free coordination site (Scheme 5.23). At low temperatures Ni(bpy)(ip) can further react, through a consecutive equilibrium, with another ip molecule to give Ni(bpy)(ip)2. However, in the presence of CO2, the nickel-ip complex, Ni(bpy)(ip), reacts irreversibly with the heterocumulene to form the product complex (3-5-q )-3-methyl-3-pentenylato)nickel bipyridine (Scheme 5.23). The activation parameters for the latter step were calculated to be A// = 25 7 kJ/mol and = 184 24 J/(mol K). The large negative activation entropy indicates that the carboxylation reaction proceeds in an associative way, during which CO2... 

Yasuda H, Okamoto T, Matsuoka Y, Nakamura A, Kai Y, Kanehisa N, Kasai N (1989) Diverse reaction courses in the controlled carbometalation of heterocumulenes with zirconium-diene complexes and molecular structures of carbon dioxide, isocyanate and ketene 1 1 and 1 2 inserted compounds. Organometallics 8 1139-1152... 

Silylmethyl-Substituted Heterocumulenes. (Trimethylsilyl-methyl)iminotriphenylphosphorane is obtained from a one-pot reaction of the azide and triphenylphosphine (eq 3). The reactions of the phosphorane with carbon dioxide or carbon disulfide (eq 5) give trimethylsilylmethyl isocyanate or trimethylsilylmethyl isothiocyanate in 68 and 94% yields, respectively. ... 

The chemical reactivity of the cumulenes under discussion ranges from highly reactive species to almost inert compounds. While some cumulenes can only be generated in a matrix at low temperatures, others are indefinitely stable at room temperature. For example, sulfines and sulfenes are only generated in situ, but some cumulenes with bulky substituents are sometimes isolated at room temperature for example, C=C=S was detected in interstellar space by microwave spectroscopy, and its spectrum was later verified by matrix isolation spectroscopy. In contrast, some cumulenes, such as carbon dioxide and carbon disulfide, are often used as solvents in organic reactions or in the extraction of natural products. The reactivity of some center carbon heterocumulenes in nucleophilic reactions is as follows isocyanates > ketenes > carbodiimides > isothiocyanates. However these reactivities do not relate to the reactivities in cycloaddition reactions. Often reactive cumulenes are isolated as their cyclodimers. Aromatic diisocyanates are more reactive than aliphatic diisocyanates in nucleophilic as well as cycloaddition reactions. 

Carbon dioxide is the most abundant heterocumulene on earth. The total amount of carbon dioxide in the atmosphere and in the oceans is estimated to represent lO tons of carbon. However its industrial use is currently not extensive. Examples of its industrial use include the production of urea, the Kolbe-Schmitt synthesis of salicylic acid, methanol synthesis and the synthesis of cyclic carbonates. Future industrial uses of carbon dioxide can be anticipated because it will become readily available by sequestering from the air. 

Similar [2+2] cycloadducts are also obtained from 46 and benzophenone and fluorenone. The reaction of titana allenes with carbon dioxide, ketenes and isocyanates proceeds across the C=0 bonds of the heterocumulenes to give the [2+2] cycloadducts 48. ... 

Various heterocumulenes can be used as synthetic equivalents of the synthon. In reaction with isocyanates, isothiocyanates, carbon dioxide, or carbon disulfide by the Wittig reaction, the iminophosphates 231, obtained firom o-(l-pyrrolyl)phenyl azide, form o-pyrrolylphenylheterocumulenes, which undergo cyclization to condensed pyrroloquinoxalines 155, 209k, 210, and 232 (Scheme 3.72) (Molina et al. 1989, 1990). 

Direct addition of CO2 across a Zr-Ir bond leading to stoichiometric reduction of carbon dioxide to formate was shown by Bergman and co-workers.[Gp2Zr(/t-NBu )IrCp ] 766 reacts rapidly with CO2 and heterocumulenes. Insertion of CO2 into an M-H bond, leading to a unique heterobimetallic formato complex and stoichiometric conversion of the formato complex to 766 and Li formate by addition of base, was also demonstrated. Different reactions gave the products listed in Scheme 109 and a possible mechanism for their formation was suggested. 

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