Heteroatoms enhance polymerization and copolymerization of functional olefins.

Transition-metal-catalyzed[2+2+2] cycloaddition reactions that use two alkynes and a nitrile is the most straightforward and powerful strategy for the construction of multisubstituted pyridines with high atom efficiency. [1,2] The iron-catalyzed [2+2+2] cycloaddition to form pyridines remains a great challenge in this field, [3,4] although significant efforts have been made in various catalytic systems (e.g. Co,[5] Ru, [6] Rh, [7] Ni, [8] Ti, [9] Zr/Ni [10] ) in the last few decades. Guerchais and co-workers described a stoichiometric reaction between an Fe I complex (Scheme 1, structure A) and alkynes with a 73 % yield.[4a] Meanwhile, Zenneck and co-workers developed a cycloaddition reaction catalyzed by an Fe 0 complex (Scheme 1, structure B), [4b,c] however, this approach gave low chemoselectivity and had a complicated procedure for catalyst preparation. A very recent example revealed that no pyridine products were observed from alkynes under iron catalyst even when nitrile was used as the solvent.[11] Therefore, the development of a simple and highly efficient iron catalyst to exclusively generate pyridine compounds would be a useful contribution to this area. Herein, we disclose the [2+2+2] cycloaddition of diynes and unactivated nitriles at room temperature using a simple iron salt as the catalyst precursor, thus resulting in the production of pyridines with up to 98 % yield of isolated product.Two important steps are generally involved in [2+2+2] cycloaddition: 1) formation of a metallacycle intermediate by oxidative cyclization and 2) subsequent reductive elimination to produce pyridines (the "common mechanism").[2] The formation of a metallacycle intermediate [12,13] from a low-valent metal species plays a crucial role in the whole process. Inspired by an investigation by Holland and co-workers revealing that alkynes bind more tightly than phosphines to low-valent iron center, [14] we envisioned that low-valent iron catalysts generated in situ from an inorganic iron salt and phosphine ligands might initiate the reaction through ligand exchange, and thereby promote the oxidative cyclization between an alkyne and an alkyne or a nitrile followed by the formation of metallacycle intermediate (Scheme 1, Step 2). Considering that the formation of benzene rings can be somewhat inhibited in the presence of a certain amount of nitrile compounds [4a] -the nature of the ligand has a dramatic effect on the reaction product-it is possible to generate pyridines with high efficiency when the appropriate iron salt and ligand are used.Initially, diyne 1 a and benzonitrile 2 a were used as model substrates for the optimization of the cycloaddition reaction conditions, and the results are summarized in Table 1. In the first instance, we employed the iron salt FeCl 3 as the catalyst precursor, 1,2-bis(diphenylphosphino)ethane (dppe) as the ligand, and 2 a as the solvent (Table 1, entries 1-4). No desired product was observed in the absence of dppe, as expected, and only trace amounts of 3 a were obtained when FeCl 3 /dp...

The direct C-H annulation of anilines or related compounds with internal alkynes provides straightforward access to 2,3-disubstituted indole products. However, this transformation proceeds with poor regioselectivity in the synthesis of unsymmetrically 2,3-diaryl substituted indoles. Herein, we report the rhodium(III)-catalyzed C-H annulation of nitrones with symmetrical diaryl alkynes as an alternative method to prepare 2,3-diaryl-substituted N-unprotected indoles with two different aryl groups. One of the aryl substituents is derived from N=C-aryl ring of the nitrone and the other from the alkyne substrate, thus providing the indole products with exclusive regioselectivity.