Diynes and Carbon Dioxide

The [2-I-2-I-2] cycloaddition reaction of diynes 40 and carbon dioxide 41 were successfully catalysed by a NHC-nickel (Scheme 5.12) [15]. The NHC-Ni complex was prepared in situ from [NiCCOD) ] and two equivalents of carbene. Pyrones 42 were obtained in excellent yields at atmospheric pressure of CO and mild reaction conditions. 

Shono and coworkers [91] reported an interesting reductive cyclization of acetylenic ketones (XII) [Eq. (12)]. The cyclized products (XIII) were formed in good yields with some stereoselectivity. Perichon and coworkers [92] also reported that the reductive crosscoupling of 1,3-diynes with carbon dioxide gave exclusively the corresponding cis addition products in the presence of a Ni(II) complex, as shown in Eq. (13). 

A v ety of reactions are catalyzed by electrochemically generated Ni(0) (62). Electrochemical reduction of Ni(bipy)3Br2 affords a reagent that couples acid chlorides and alkyl or aryl halides to form unsymmetrical ketones (63). Symmetrical ketones are formed from alkyl halides and carbon dioxide (64). Reductive electrochemical carboxylation of terminal alkynes, enynes and diynes can be accomplished with 10% Ni(bipy)3(Bp4)2 in DMF (65-68). Terminal allies lead selectively to a-substituted acrylic acids. Electrocatalytic hydrogenation on hydrogen-active electrodes has been reviewed (69). Radical cyclizations of vinyl, alkyl and aryl radicals can be carried out by indirect electrochemical reduction with a Ni(II) complex as a mediator (70). 

The related cycloaddition of diynes with carbon dioxide has also been developed as an efficient entry to pyrones (Scheme 3-29). As observed with isocyanate couplings, initial reports described the use of phosphine complexes of Ni(0), " but more recent work with A-heterocyclic carbene ligands demonstrated considerably milder conditions and broader scope. " Additionally, the cycloaddition of diynes with nitriles efficiently proceeds to generate pyridine derivatives under similar conditions. ... 

Nickel catalysts also effectively catalyze the [2+2+2] cycloaddition reactions of some diynes with carbon dioxide. For example, diynes 23 react with carbon dioxide in the presence of Ni(COD)2 and l,3-bis-(2,6-diisopropylphenyl)imidazol at 60 °C to give the... 

Cyclopentadienylcobalt complexes are also good for co-cyclotrimerization of alkynes with other unsaturated compounds containing the carbon-heteroatom double bonds, especially when they are part of the cumulene system such as isocyanates, diimides, and carbon dioxide. The reaction conditions are essentially the same as in the previously mentioned processes. However, the biggest problem remains the selectivity for the formation of heterocycles, because of the strong competition for the formation of benzene derivatives. Whereas co-cyclotrimerization of diimides and isocyanates results in the formation of reasonable yields of the corresponding heterocycles 170 and 171 (Scheme 75), in the case of carbon dioxide the yields are generally low [108, 109]. Recently, it has been shown that the ruthenium complex 106 is capable of efficient catalysis of co-cyclotrimerization of diynes and isocyanates [110] and isothiocyanates [111] under mild reaction conditions. 

Pyridine compounds 45 can also be produced by the NHC-Ni catalysed cycloaddition between nitriles 43 and diynes 44 (Scheme 5.13) [16]. The SIPr carbene was found to be the best ligand for the nickel complex in this reaction. The reaction required mild reaction conditions and low catalyst loadings, as in the case of cycloaddition of carbon dioxide. In addition to tethered aUcynes (i.e. diynes), pyridines were prepared from a 3-component coupling reaction with 43 and 3-hexyne 23 (Scheme 5.13). The reaction of diynes 44 and nitriles 43 was also catalysed by a combination of [Ni(COD)J, NHC salts and "BuLi, which generates the NHC-Ni catalyst in situ. The pyridines 45 were obtained with comparable... 

Potentially very explosive, it may be handled and transferred by low temperature distillation. It should be stored at —25°C to prevent decomposition and formation of explosive polymers [1]. The critical pressure for explosion is 0.04 bar, but presence of 15-40% of diluents (acetylene, ammonia, carbon dioxide or nitrogen) will raise the critical pressure to 0.92 bar [2], Further data on attenuation by inert diluents of the explosive decomposition of the diyne are available [3], During investigation of the cause of a violent explosion in a plant for separation of higher acetylenes, the most important finding was to keep the concentration of 1,3-butadiyne below 12% in its mixtures. Methanol is a practical diluent [4], The use of butane (at 70 mol%) or other diluents to prevent explosion of 1,3-butadiyne when heated under pressure has been claimed [5], It polymerises rapidly above 0°C. 

A nickel(0)-catalyzed cycloaddition of carbon dioxide and symmetrical tethered diynes 649 affords cycloalkyl-fused 27/-pyran-2-ones in high yield (Equation 264) <2002JA15188>. The nickel(0)-catalyzed cycloaddition of carbon dioxide and unsymmetrical tethered diynes 650 leads to the formation of two regioisomeric 27/-pyran-2-ones 651, 652 the major regioisomeric product 651 features the larger alkyne group at C-3 (Equation 265, Table 31) <2004T7431>. 

Table 31 Ratio of products 651 652 formed during the nickei(0)-catalyzed cycloaddition of carbon dioxide and unsymmetrical tethered diynes 650 (Equation 265)...

An intramolecular version of this process has been described, leading to bicyclic 2-pyrones. Diynes in which both alkyne functions are internal and are linked by three-, four- or five-atom chains cycloadd to carbon dioxide in the presence of catalytic Ni° and various trialkylphosphines (equation 51). Terminal diynes require stoichiometric metal and give lower yields, however. Extensive studies of ligand effects on yield and chemoselectivity have established a broad scope for the process and pointed out important practical differences between it and the intermolecular reactions described above. ... 

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. 

Dienes, allenes, and alkynes react with carbon dioxide to yield cyclic lactones—the catalysts include various Ni and Pd complexes.4 With certain diynes, alternating copolymerization with C02 results in the formation of poly (2-pyrones) (Scheme 6.16). 

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... 

Louie and co-workers later used their methodology for the cycloaddition of diynes with isocyanates. In this case, SIPr was superior to IPr while isocyanates were found more reactive than carbon dioxide. Also of interest, it appeared that a 1 1 Ni/SIPr ratio was equally efficient to a 1 2 ratio. Except for some particular cases, the preparation of a large variety of highly substituted 2-pyridones were then efficiently obtained in the presence of 3 mol% catalyst under very mild conditions (Equation (10.20)). 

Louie and cowvorkers later used their methodology for the cycloaddition of diynes with isoeyanates. In this case, SIPr was superior to IPr while isocyanates were found to be more reactive than carbon dioxide. Also of interest,... 

Transition metal catalyzed hetero-[2+2+2] cycloaddition of alkynes to compounds with a multiple carbon-heteroatom bond such as isocyanates, carbon dioxide, nitriles, aldehydes, and ketones leads to heterocyclic arenes and unsaturated heterocycles. These reactions are classified into two groups coupling of diynes with multiple carbon-heteroatom... 

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