Diynes/carbon dioxide, cycloaddition

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. 

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... 

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)...

The increased reactivity of isocyanates, relative to carbon dioxide, was reflected in the wider range of cycloaddition partners. For example, terminal diynes as well as nontethered alkynes (e.g., 3-hexyne) were also successfully converted to 2-pyridones rather than undergoing rapid telomerization to aromatic by-products. Importantly, the cycloaddition of an asymmetrical... 

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. 

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... 

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... 

---