Ruthenium‐catalyzed [2+2+2] Cycloaddition of Diynes with Nitriles in Pure Water

Abstract

The transition metal-catalyzed [2+2+2] cycloaddition of two alkyne molecules and a nitrile, particularly involving catalysis by Co, Ru, Rh, Ni, Ti, and Fe species, [1][2][3][4][5][6] has been widely investigated as a valuable tool for the synthesis of substituted pyridines. [7,8] However, highly diluted conditions and techniques of slow addition are generally required to suppress the competing side reactions, such as dimerization and trimerization of the diynes. [9a] Despite substantial advances, metal-catalyzed [2+2+2] cycloaddition remains a formidable challenge. Thus, new tactics should be applied to the reaction, aimed at attenuating or controlling the side reactions. It has been shown that the Diels-Alder reaction can be intensified in aqueous media, [9b] and we expect that water can also expedite the [2+2+2] cycloaddition.Although the [2+2+2] cycloaddition reaction has been widely explored in the last few decades, there are only limited examples of the use of water as solvent (Scheme 1). Heller and co-workers reported that acetylene and nitriles in water efficiently undergo co-cyclization under visible-light irradiation in the presence of CpCo(COD) (COD: 1,5-cyclooctadiene) and its derivatives, resulting in excellent product selectivities and moderate yields. [10] Parsons and Jerome reported a reaction of hexyne and acetonitrile in supercritical water at 374 8C, affording the corresponding pyridine isomers in a yield of 15 %. [11] Eaton and co-workers showed that a water-soluble cobalt complex catalyzes the [2+2+2] cycloaddition of two alkyne molecules and a nitrile in a mixture of water and methanol at 85 8C, with a yield of up to 99 %. However, hydrophilic functional groups on the alkynes were necessary. [12] In 2002, Butenschçn reported another cobalt complex with pendant phosphane substituents on the cyclopentadienyl ring. These substituents made the cobalt complex stable in water, thus allowing the preparation of pyridine derivatives at room temperature in a mixture of water and ethanol. [13] In aqueous medium, acceleration of the cycloaddition by water should outweigh the lowered efficiency and reaction rate due to the highly dilute reaction conditions. [9] In addition, water can induce rate enhancements, [14] chemoselectivity, [15] and stereoselectivity [16] in numerous synthetic transformations. However, to the best of our knowledge mixed solvents or photochemical activation were necessary in previous works, and the cycloaddition of diynes and a nitrile in neat water without irradiation has never been reported. The development of a new catalytic system to circumvent these restrictions is still highly desirable.Generally, two crucial problems are encountered in the [2+2+2] cycloaddition of alkynes and nitriles in water: 1) the poor solubility of the metal precursor and substrates in pure water, and 2) easy hydrolysis of the nitriles in water under the reaction conditions. [17] To overcome these issues, we surmised that a water-soluble ligand might connect the metal precursor and substrates in th...

Eco-friendly synthesis of pyridines via rhodium-catalyzed cyclization of diynes with oximes

“…Therefore, chemists endeavored to develop elegant and clean technologies to improve traditional method with relatively environment-friendly benign solvent. In 25 this context, we reported ruthenium-catalyzed cycloaddition of diynes and nitriles in pure water under mild conditions [9] . However, limited scope of nitriles suppress the development of this method and bestir us to design a new strategy to afford pyridines.…”

Ruthenium‐Catalyzed C−C Bond Cleavage of 2H‐Azirines: A Formal [3+2+2] Cycloaddition to Fused Azepine Skeletons

“…In addition, the O‐tethered diyne 1 u reacted with 2 H ‐azirine 2 h to deliver 3 u in moderate yield. Unfortunately, unlike our previous observation in the ruthenium‐catalyzed [2+2+2] cycloaddition, the methylene‐linked diyne was completely unreactive, thus indicating that the Thorpe–Ingold effect was crucial for this transformation ( 3 v ). Furthermore, when the unsymmetrical diyne 1 w , bearing a methyl group at one alkyne terminus, was subjected to the reaction, the azepine compound 3 w was obtained albeit with low yield.…”

Transition metal-catalyzed [2 + 2 + 2] cycloaddition of nitrogen-linked 1,6-diynes: a straightforward route to fused pyrrolidine systems

“…The most commonly used Ru-complex in general, and particularly in this chemistry, are commercially available Cp*Ru(cod)Cl, though only a few synthetic applications were reported (Scheme 30). [62][63][64][65][66][67] 3.1.4. Iron.…”