Ruthenium-Based Heterocyclic Carbene-Coordinated Olefin Metathesis Catalysts

The fascinating story of olefin (or alkene) metathesis (eq 1) began almost five decades ago, when Anderson and Merckling reported the first carbon-carbon double-bond rearrangement reaction in the titanium-catalyzed polymerization of norbornene. Nine years later, Banks and Bailey reported “a new disproportionation reaction . . . in which olefins are converted to homologues of shorter and longer carbon chains...”. In 1967, Calderon and co-workers named this metal-catalyzed redistribution of carbon-carbon double bonds olefin metathesis, from the Greek word “μετάθεση”, which means change of position. These contributions have since served as the foundation for an amazing research field, and olefin metathesis currently represents a powerful transformation in chemical synthesis, attracting a vast amount of interest both in industry and academia

Discrete local altitude sensing device Patent

Device for use in descending spacecraft as altitude sensor for actuating deceleration retrorocket

Synthesis and Structures of Fused N-Heterocylic Carbenes and Their Rhodium Complexes

New procedures for the synthesis of N-heterocyclic carbenes with multiple fused rings have been developed utilizing a key ring-closing metathesis step. Rhodium complexes were obtained via the pentafluorophenyl carbene adducts. Solid-state structural behaviors of the new carbene ligands were analyzed via X-ray crystallography

Acoustic Changes during Passage Reading in Speakers with Parkinson\u27s Disease

Purpose: The purpose of this study was to evaluate speech changes in Parkinson’s disease (PD) while reading a passage, using both local (i.e., segment level) and global (i.e., utterance level) acoustic measures. Methods: 20 speakers participated in the study (10 PD, 10 neurologically healthy controls). The speakers were asked to read The Caterpillar passage in a conversational mode. A total of five acoustic measures were included (local: vowel duration, Euclidean distance between corner vowels and schwa; global: articulation rate, F0/intensity range). These acoustic measures were compared between two sentences located in the two positions within the paragraph, initial and final. Results: The findings indicated (1) overall speech differences between the two groups such as increased vowel duration and reduced vowel contrast and (2) speech differences between the beginning and end of the passage such as increased articulation rate toward the end. In addition, the results revealed that unlike control speakers, speakers with PD did not show a greater F0 and intensity range in the end compared to the beginning of the passage, which points a limited capability of prosody modulations in PD and its apparent pattern toward the end of passage reading. Discussion: Findings of this study support the notion that within- or across-task acoustic variation should be considered in speech sampling in clinical practice and research

Synthesis and Activity of Six-Membered Cyclic Alkyl Amino Carbene–Ruthenium Olefin Metathesis Catalysts

Ru–cyclic alkyl amino carbene (Ru–CAAC) olefin metathesis catalysts perform extraordinarily in metathesis macrocyclization and ethenolysis, but previous studies have been limited to the use of five-membered CAAC (CAAC-5) ligands. In this work, we synthesized a different group of ruthenium catalysts with more σ-donating and π-accepting six-membered CAAC (CAAC-6) ligands, and their metathesis activity was probed through initiation studies, ring-closing metathesis (RCM), cross-metathesis, and ethenolysis. These catalysts display higher initiation rates than analogous Ru–CAAC-5 complexes but demonstrate lower activity in RCM and ethenolysis

The application of catalytic ring-closing olefin metathesis to the synthesis of unsaturated oxygen heterocycles

The development of general approaches to carbon-carbon bond formation represents an important ongoing challenge for synthetic organic chemists. One efficient method for constructing carbon-carbon double bonds, the transition metal alkylidene-catalyzed olefin metathesis reaction (eq 1), has been the focus of intense interest in recent years from the standpoint of both mechanism and polymer synthesis; in contrast, use of this transformation in organic synthesis has been limited. As part of a broader program directed toward establishing transition metal alkylidenes as versatile reagents for organic chemistry, we report the successful application of catalytic olefin metathesis to the generation of a variety of unsaturated oxygen heterocycles

Synthesis and Activity of Ruthenium Olefin Metathesis Catalysts Coordinated with Thiazol-2-ylidene Ligands

A new family of ruthenium-based olefin metathesis catalysts bearing a series of thiazole-2-ylidene ligands has been prepared. These complexes are readily accessible in one step from commercially available (PCy_3)_2Cl_2Ru CHPh or (PCy_3)Cl_2Ru CH(o-iPrO−Ph) and have been fully characterized. The X-ray crystal structures of four of these complexes are disclosed. In the solid state, the aryl substituents of the thiazole-2-ylidene ligands are located above the empty coordination site of the ruthenium center. Despite the decreased steric bulk of their ligands, all of the complexes reported herein efficiently promote benchmark olefin metathesis reactions such as the ring-closing of diethyldiallyl and diethylallylmethallyl malonate and the ring-opening metathesis polymerization of 1,5-cyclooctadiene and norbornene, as well as the cross metathesis of allyl benzene with cis-1,4-diacetoxy-2-butene and the macrocyclic ring-closing of a 14-membered lactone. The phosphine-free catalysts of this family are more stable than their phosphine-containing counterparts, exhibiting pseudo-first-order kinetics in the ring-closing of diethyldiallyl malonate. Upon removing the steric bulk from the ortho positions of the N-aryl group of the thiazole-2-ylidene ligands, the phosphine-free catalysts lose stability, but when the substituents become too bulky the resulting catalysts show prolonged induction periods. Among five thiazole-2-ylidene ligands examined, 3-(2,4,6-trimethylphenyl)- and 3-(2,6-diethylphenyl)-4,5-dimethylthiazol-2-ylidene afforded the most efficient and stable catalysts. In the cross metathesis reaction of allyl benzene with cis-1,4-diacetoxy-2-butene increasing the steric bulk at the ortho positions of the N-aryl substituents results in catalysts that are more Z-selective

Ruthenium Olefin Metathesis Catalysts Bearing an N-Fluorophenyl-N-Mesityl-Substituted Unsymmetrical N-Heterocyclic Carbene

Two new ruthenium-based olefin metathesis catalysts, each bearing an unsymmetrical N-heterocyclic carbene ligand, have been synthesized and fully characterized. Their catalytic performance has been evaluated in ring-closing metathesis, cross metathesis, and ring-opening metathesis polymerization reactions

Nonproductive Events in Ring-Closing Metathesis Using Ruthenium Catalysts

The relative TONs of productive and nonproductive metathesis reactions of diethyl diallylmalonate are compared for eight different ruthenium-based catalysts. Nonproductive cross metathesis is proposed to involve a chain-carrying ruthenium methylidene. A second more-challenging substrate (dimethyl allylmethylallylmalonate) that forms a trisubstituted olefin product is used to further delineate the effect of catalyst structure on the relative efficiencies of these processes. A steric model is proposed to explain the observed trends

Effects of NHC-Backbone Substitution on Efficiency in Ruthenium-Based Olefin Metathesis

series of ruthenium olefin metathesis catalysts bearing N-heterocyclic carbene (NHC) ligands with varying degrees of backbone and N-aryl substitution have been prepared. These complexes show greater resistance to decomposition through C−H activation of the N-aryl group, resulting in increased catalyst lifetimes. This work has utilized robotic technology to examine the activity and stability of each catalyst in metathesis, providing insights into the relationship between ligand architecture and enhanced efficiency. The development of this robotic methodology has also shown that, under optimized conditions, catalyst loadings as low as 25 ppm can lead to 100% conversion in the ring-closing metathesis of diethyl diallylmalonate