This study reveals the effect of an anthracene moiety on the resulting Cu(i) complexes and their photo- and electrochemical properties. As a result, unprecedented excited state lifetimes were found for such Cu(i) photosensitizers containing an extended π-system.
Remarkably active (down to 0.001% Al) catalysts for ring-opening polymerization of cyclohexene oxide under neat reaction conditions are reported. High molecular weight polymers with uniform dispersity are produced. Kinetic data from NMR studies and MALDI-TOF MS data of the polymers provide some mechanistic insight.
Critical to advancing the uptake of olefin metathesis in leading contexts, including pharmaceutical manufacturing, is identification of highly active catalysts that resist decomposition. Amines constitute an aggressive challenge to ruthenium metathesis catalysts. Examined here is the impact of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU), morpholine, n -butylamine, and triethylamine on Ru metathesis catalysts that represent the current state of the art, including cyclic alkyl amino carbene (CAAC) and N-heterocyclic carbene (NHC) complexes. Accordingly, the amine-tolerance of the nitro-Grela catalyst RuCl 2 (H 2 IMes)(=CHAr) ( nG ; Ar = C 6 H 4 -2-O i Pr-5-NO 2 ) is compared with that of its CAAC analogues nGC1 and nGC2 , and the Hoveyda-class catalyst RuCl 2 ( C2 )(=CHAr′) HC2 (Ar′ = C 6 H 4 -2-O i Pr). In C1 , the carbene carbon is flanked by an N -2,6-Et 2 C 6 H 3 group and a CMePh quaternary carbon; in C2 , by an N -2- i Pr-6-MeC 6 H 3 group and a CMe 2 quaternary carbon. The impact of 1 equiv amine per Ru on turnover numbers (TONs) in ring-closing metathesis of diethyl diallylmalonate was assessed at 9 ppm Ru, at RT and 70 °C. The deleterious impact of amines followed the trend NEt 3 ∼ NH 2 n Bu ≪ DBU ∼ morpholine. Morpholine is shown to decompose nGC1 by nucleophilic abstraction of the methylidene ligand; DBU, by proton abstraction from the metallacyclobutane. Decomposition was minimized at 70 °C, at which nGC1 enabled TONs of ca. 60 000 even in the presence of morpholine or DBU, vs ca. 80 000 in the absence of base. Unexpectedly, H 2 IMes catalyst nG delivered 70–90% of the performance of nGC1 at high temperatures, and underwent decomposition by Brønsted base at a similar rate. Density functional theory (DFT) analysis shows that this similarity is due to comparable net electron donation by the H 2 IMes and C1 ligands. Catalysts bearing the smaller C2 ligand were comparatively insensitive to amines, owing to rapid, preferential bimolecular decomposition.
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
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