Control of Chemoselectivity by Coordinated Water and Relative Size of Ligands to Metal Cations of Lewis Acid Catalysts for Cycloaddition of an Oxirane Derivative to an Aldehyde: Theoretical and Experimental Study

Abstract

The role played by Lewis acid catalysts in the selective cleavages of C–O and C–C bonds of oxirane derivatives with aldehydes is investigated both theoretically and experimentally. According to the different chemoselectivities, various catalysts are divided into four series: C–O selectivity, both, C–C selectivity, and none, respectively. The involvement of coordinated water molecules is crucial to rationalize the experimental observation of C–C selectivity for the Ni­(ClO4)2·6H2O-catalyzed reaction, which is supported by experiment on changing originally unreactive Ni­(OTf)2 to be an effective catalyst by mixing with water. Furthermore, the steric hindrance from the anion in Lewis acid and the water molecule have significant influence on the efficiency of catalysts. A steric parameter, α, defined as the relative ratio of ligand size to radius of the center metal cation, gives a general picture to understand the selectivities of various Lewis acid catalysts. The ineffective M­(OTf)2 type catalysts have remarkable steric hindrance with α > 4.5. Large cations (R M > 74 pm) relative to their surrounding ligands with α < 4 prefer the C–O bond cleavage path, while small cations (R M < 70 pm) with α < 4.5 lead to C–C bond breaking. An understanding of the relationship between selectivity and Lewis acid catalysts may guide the design of more selective and versatile Lewis acid catalysts for organic synthesis.

Cycloadditions to Epoxides Catalyzed by Group III–V Transition‐Metal Complexes

“…dicarboxylateo rd iketone) on one of the carbon atoms allow the coordinationo faLewis acid. [11] The coordination of the acid weakens the CÀCb ond and leads to the carbonyl ylide intermediate (Scheme4). This intermediate can then react with dipolarophiles in the intermolecular[ 3 + +2] cycloaddition.…”

“…The metallo-ylide was found to be more reactive than the corresponding ylide withoutm etal activation,h ence is an improved substrate (Scheme 4). [11] Remarkably,w hen dicarbonyl substituents are availableo n the ring, the screening of different Lewis acids for the cycloaddition of aldehydes to aryloxiranedicarboxylates revealed divergent reactivity. [12] Indeed, both CÀOa nd CÀCb onds of epoxides could be selectively cleaved by using different catalysts (i.e.…”

“…Sn(OTf) 2 2 ,a nd Ni(OTf) 2 ). [11] Ac orrelation between the radius of the metal (RM) of the Lewis acid employed and the chemoselectivity of the reactionw as noted. Am etal cation displaying as mallr adius (RM < 70 pm) would promote CÀC bond cleavage, whereas larger counterparts (RM > 74 pm), such as Sn + 2 and Hg + 2 would favor CÀOb ond cleavage.T he size of the surrounding ligands or the presence of water molecules in the coordinations phere of the cation was also found to be influential,w ith large ligandsi nhibiting cation coordination and leadingt ol ower catalytic activity.…”

“…Am etal cation displaying as mallr adius (RM < 70 pm) would promote CÀC bond cleavage, whereas larger counterparts (RM > 74 pm), such as Sn + 2 and Hg + 2 would favor CÀOb ond cleavage.T he size of the surrounding ligands or the presence of water molecules in the coordinations phere of the cation was also found to be influential,w ith large ligandsi nhibiting cation coordination and leadingt ol ower catalytic activity. [11] In the case of group III-V metals,t he reactions of aryloxiranedicarboxylates occur generally by CÀCbond opening.…”

Understanding the Mechanism of the Lewis Acid Promoted [3 + 2] Cycloaddition of Propargylic Alcohol and α-Oxo Ketene Dithioacetals

“…Functionalized five-membered carbocycles as structural motifs are often found in a large number of natural products and bioactive molecules. − The [3 + 2] cycloaddition reactions have attracted increasing interest in organic chemistry due to their wide application in the synthesis of various compounds with five-membered rings. , Among many impressive methods, transition-metal and phosphine-catalyzed [3 + 2] cycloaddition provide a powerful synthetic approach to obtain cyclopentadienes and cyclopentenes. , Despite the above significant achievements, the design and development of novel catalyst-mediated processes, beginning with easily available materials to afford densely functionalized cyclopentadienes, are still of great value. Very recently, the Bi group reported a new class of fully substituted 2,5-dialkylthio cyclopentadienes using α-oxo ketene dithioacetals as the polarized alkene component instead of classic alkylthio displacement reactions for the synthesis of carbocycles.…”