Cycloaddition of 1,3-butadiynes: efficient synthesis of carbo- and heterocycles

Cycloaddition reactions of alkynes are elegant, atom-efficient transformations for the synthesis of carbo- and heterocycles, mostly aromatic, involving the construction of challenging skeletons of complex molecules. Therefore significant efforts have recently been devoted to the development of novel methodologies, efficient strategies and different catalytic systems to broaden the scope of these reactions. We summarize in this review the recent advances in the cycloaddition reactions of 1,3-butadiynes to provide facile and reliable approaches to various functionalized carbo- and heterocycles.

Highly Regioselective Synthesis of Substituted Isoindolinones via Ruthenium-Catalyzed Alkyne Cyclotrimerizations

(Cyclooctadiene)(pentamethylcyclopentadiene)ruthenium chloride [Cp*RuCl(cod)] has been used to catalyze the regioselective cyclization of amide-tethered diynes with monosubstituted alkynes to give polysubstituted isoindolinones. Notably, the presence of a trimethylsilyl group on the diyne generally led to complete control over the regioselectivity of the alkyne cyclotrimerization. The cyclization reaction worked well in a sustainable non-chlorinated solvent and was tolerant of moisture. The optimized conditions were effective with a diverse range of alkynes and diynes. The 7-silylisoindolinone products could be halogenated, protodesilylated or ring opened to access a range of usefully functionalized products.

Fine-tuning the reactivity and stability by systematic ligand variations in CpCo(I) complexes as catalysts for [2+2+2] cycloaddition reactions

CpCo(I)-olefin-phosphite and CpCo(I)-bisphosphite complexes were systematically prepared and their reactivity in [2+2+2] cycloaddition reactions compared with highly active [CpCo(H(2)C=CHSiMe(3))(2)] (1). Whereas 1 is an excellent precursor for the synthesis of [CpCo(olefin)(phosphite)] complexes (2 a-f), [CpCo(phosphite)(2)] complexes (3 a-e) were prepared photochemically from [CpCo(cod)]. The complexes were evaluated in the cyclotrimerization reaction of diynes with nitriles yielding pyridines. For [CpCo(olefin)(phosphite)], as well as some of the [CpCo(phosphite)(2)] complexes, reaction temperatures as low as 50 °C were sufficient to perform the cycloaddition reaction. A direct comparison showed that the order of reactivity for the complex ligands was olefin(2)>olefin/phosphite>phosphites(2). The complexes with mixed ligands favorably combine reactivity and stability. Calculations on the ligand dissociation from [CpCo(olefin)(phosphite)] proved that the phosphite is dissociating before the olefin. [CpCo(H(2)C=CHSiMe(3)){P(OPh)(3)}] (2 a) was investigated for the co-cyclization of diynes and nitriles and found to be an efficient catalyst at rather mild temperatures.

Ruthenium-catalyzed regioselective [2 + 2 + 2] cyclotrimerization of trifluoromethyl group substituted internal alkynes

It found that the Ru(3)(CO)(12) coordinated with 2-(diphenylphosphino)benzonitrile (2-DPPBN) to effectively catalyze the [2 + 2 + 2] cyclotrimerization of the trifluoromethyl group substituted internal alkynes in high yields with up to >98% regioselectivity. Isolation of a ruthenacyclopentadiene was successful and confirmed that the complex is a reaction intermediate.