Abstract
The selective transformation of 1-alkenes into E-olefins is a long-standing challenge in olefin metathesis. Density functional theory (DFT) calculations predict high E-selectivity for catalysts incorporating a bidentate, dianionic thio-indolate ligand within a RuXX’(NHC)(py)(= CHR) platform (NHC = N-heterocyclic carbene; py = pyridine). Such complexes are predicted to yield E-olefins by favoring anti-disposed substituents in the transition state expected to be rate-determining: specifically, that for cycloreversion of the metallacyclobutane intermediate. Three pyridine-stabilized catalysts Ru21a-c were synthesized, in which the thio-indolate ligand bears a H, Me, or Ph substituent at the C2 position, and the NHC ligand is the unsaturated imidazoline-2-ylidene Me2IMes (which bears N-mesityl groups and methyl groups on the C4,5 backbone). Single-crystal X-ray diffraction analysis of Ru21c confirms the ligand orientation required for E-selective metathesis, with the thio-indolate sulfur atom binding cis to the NHC, and the indolate nitrogen atom trans to the NHC. However, whereas the new complexes mediated metathetic exchange of their 2-thienylmethylidene ligand in the presence of the common metathesis substrates styrene and allylbenzene, no corresponding self-metathesis products were obtained. Only small amounts of 2-butene (73% (Z)-2-butene) were obtained in self-metathesis of propene using Ru21a. Detailed DFT analysis of this process revealed that product release is surprisingly slow, limiting the reaction rate and explaining the low metathesis activity. With the barrier to dissociation of (Z)-2-butene being lower than that of (E)-2-butene, the calculations also account for the observed Z-selectivity of Ru21a. These findings provide guidelines for catalyst redesign in pursuit of the ambitious goal of E-selective 1-alkene metathesis.Graphic abstract
Highlights
E-olefins, with substituents trans-disposed across the double bond, are important structural features in molecular entities ranging from antibiotics [1] and anticancer therapeutics [2, 3] to precision polymers [4]
Seminal computational mechanistic work showed that the MCB intermediates and the associated transition states of the preferred dissociative reaction pathways adopt trigonal bipyramidal (TBP) geometries, in which the η2-Cα-Cβ-Cα ring occupies the equatorial plane [61]
Based on considerations of the geometries of stereoretentive metathesis catalysts [77] and on catecholthiolate modifications aimed at increasing the share of E-isomeric product [56], a thio-indolate ligand scaffold was designed to exert steric pressure on the β-substituent of the MCB, and the MCB-like transition states for cycloaddition and cycloreversion
Summary
E-olefins, with substituents trans-disposed across the double bond, are important structural features in molecular entities ranging from antibiotics [1] and anticancer therapeutics [2, 3] to precision polymers [4] Such compounds have been generated from aldehydes via stoichiometric approaches such as the Wittig [5], Horner-WadsworthEmmons [6] and Julia [7] olefination reactions. The most direct, atom-economic, and elegant solution is offered by catalysts that enable selective synthesis of the singleisomer target [33, 35,36,37]. Building on insights obtained in earlier modifications of stereoretentive catalysts [56], we explore a new family of thio-indolate catalysts for which E-selectivity is predicted on the basis of density functional theory (DFT) calculations
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