Second-generation ruthenium olefin metathesis catalysts bearing aminophosphine ligands were investigated with systematic variation of the ligand structure. The rates of phosphine dissociation (k; initiation rate) and relative phosphine reassociation (k) were determined for two series of catalysts bearing cyclohexyl(morpholino)phosphine and cyclohexyl(piperidino)phosphine ligands. In both cases, incorporating P-N bonds into the architecture of the dissociating phosphine accelerates catalyst initiation relative to the parent [Ru]-PCy complex; however, this effect is muted for the tris(amino)phosphine-ligated complexes, which exhibit higher ligand binding constants in comparison to those with phosphines containing one or two cyclohexyl substituents. These results, along with X-ray crystallographic data and DFT calculations, were used to understand the influence of ligand structure on catalyst activity. Especially noteworthy is the application of phosphines containing incongruent substituents (PRR′); detailed analyses of factors affecting ligand dissociation, including steric effects, inductive effects, and ligand conformation, are presented. Computational studies of the reaction coordinate for ligand dissociation reveal that ligand conformational changes contribute to the rapid dissociation for the fastest-initiating catalyst of these series, which bears a cyclohexyl-bis(morpholino)phosphine ligand. Furthermore, the effect of amine incorporation was examined in the context of ring-opening metathesis polymerization, and reaction rates were found to correlate well with catalyst initiation rates. The combined experimental and computational studies presented in this report reveal important considerations for designing efficient ruthenium olefin metathesis catalysts.

Chu Crystal K.    Lin Tzu-Pin    Shao Huiling    Liberman-Martin Allegra L.    Liu Peng    Robert H.Grubbs    

Journal of the American Chemical Society

2018

This Addition and Correction corrects an error in Figure 4. This correction does not impact the data analysis or general conclusions associated with this publication. (Figure Presented).

Haugan Ingrid N.    Maher Michael J.    Chang Alice B.    Lin Tzu Pin    Robert H.Grubbs     Marc Hillmyer    Frank Bates   

2018

Cyclic polymers have drawn considerable interest for their peculiar physical properties in comparison to linear polymers, despite their equivalent compositions. Synthetically, cyclic polymers can be accessed through either macrocyclic ring-closure or by ring-expansion polymerization, but the main challenge with either method is the production of highly pure cyclic polymer samples. This highlight describes advances in the area of cyclic polymer synthesis, with a particular focus on ring-expansion metathesis polymerization. Methods for characterizing cyclic polymers and assessing their purity are also discussed in order to emphasize the need for additional robust and reliable methods for synthesizing and studying topologically complex macromolecules. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018.

Edwards Julian P.    Wolf William J.    Robert H.Grubbs    

Journal of Polymer Science, Part A: Polymer Chemistry

2018

We recently reported a new method for the direct dehydrogenative C-H silylation of heteroaromatics utilizing Earth-abundant potassium tert-butoxide. Herein we report a systematic experimental and computational mechanistic investigation of this transformation. Our experimental results are consistent with a radical chain mechanism. A trialkylsilyl radical may be initially generated by homolytic cleavage of a weakened Si-H bond of a hypercoordinated silicon species as detected by IR, or by traces of oxygen which can generate a reactive peroxide by reaction with [KOt-Bu] as indicated by density functional theory (DFT) calculations. Radical clock and kinetic isotope experiments support a mechanism in which the C-Si bond is formed through silyl radical addition to the heterocycle followed by subsequent β-hydrogen scission. DFT calculations reveal a reasonable energy profile for a radical mechanism and support the experimentally observed regioselectivity. The silylation reaction is shown to be reversible, with an equilibrium favoring products due to the generation of H gas. In situ NMR experiments with deuterated substrates show that H is formed by a cross-dehydrogenative mechanism. The stereochemical course at the silicon center was investigated utilizing a H-labeled silolane probe; complete scrambling at the silicon center was observed, consistent with a number of possible radical intermediates or hypercoordinate silicates.

Wenbo Liu    Schuman David P.    Yunfang Yang    Toutov Anton A.    Yong Liang    Klare Hendrik F. T.    Nesnas Nasri    Martin Oestreich    Donna Blackmond    Scott Virgil    Banerjee Shibdas    Richard Zare    Robert H.Grubbs     Kendall Houk    Brian Stoltz   

Journal of the American Chemical Society

2017

Introduction: Repair of retinal detachment frequently requires use of intraocular gas. Patients are instructed to position themselves postoperatively to appose the intraocular bubble to the retinal break(s). We developed a novel wearable wireless positioning sensor, which provides real-time audiovisual feedback on the accuracy of positioning. Methods: Eight healthy volunteers wore the wireless sensor for 3 hours while instructed to maintain their head tilted toward the 2 o’clock meridian with no audiovisual feedback. Positioning accuracy was recorded. The subjects repeated the experiment for 3 hours with the audiovisual feedback enabled. Results: With no audiovisual feedback, the percentage of time greater than 10° out of position varied from 8.9% to 93.9%. With audiovisual feedback enabled, these percentages ranged from 9.4% to 65%. Three subjects showed significant improvement in their time out of position (P<0.01, Fisher’s exact test). Four subjects demonstrated a nonsignificant improvement, and one subject had a significant increase in time out of position with feedback (P<0.01). When pooled, all subjects demonstrated a statistically significant decrease in degrees out of position (P<0.001, Wilcoxon test) and a statistically significant improvement in total time out of position (P<0.001). Conclusion: The novel positioning sensor showed improved positioning compliance in half of the healthy volunteers during our short pilot study. Other subjects derived little benefit from the feedback. The causes for this observation are unclear. However, given the significant improvement as a group, this new technology could be beneficial to patients who struggle with postoperative positioning.

Brodie Frank L.    Ramirez David A.    Pandian Sundar    Woo Kelly Y.    Balakrishna Ashwin    De Juan    Choo Hyuck    Robert H.Grubbs    

Clinical Ophthalmology

2017

Olefin metathesis is an incredibly valuable transformation that has gained widespread use in both academic and industrial settings. Lately, stereoretentive olefin metathesis has garnered much attention as a method for the selective generation of both E- and Z-olefins. Early studies employing ill-defined catalysts showed evidence for retention of the stereochemistry of the starting olefins at low conversion. However, thermodynamic ratios E/Z were reached as the reaction proceeded to equilibrium. Recent studies in olefin metathesis have focused on the synthesis of catalysts that can overcome the inherent thermodynamic preference of an olefin, providing synthetically useful quantities of a kinetically favored olefin isomer. These reports have led to the development of stereoretentive catalysts that not only generate Z-olefins selectively, but also kinetically produce E-olefins, a previously unmet challenge in olefin metathesis. Advancements in stereoretentive olefin metathesis using tungsten, ruthenium, and molybdenum catalysts are presented.

Montgomery T. Patrick    Ahmed Tonia    Robert H.Grubbs    

Angewandte Chemie - International Edition

2017

The ideal of living polymerization has defined research in polymer chemistry over the past 50 years. In this Perspective, we present the case that this concept has enabled the treatment of polymers as organic molecules, rather than impure mixtures of species, and allowed the translation of methods developed by synthetic organic chemists into ever more accessible living and/or controlled polymerization methods. The concurrent development of rapid analytical methods for screening new polymerization methods for living characteristics, chiefly size exclusion chromatography, has greatly aided in the expansion of living polymerization methods.

Robert bernard Grubbs    Robert H.Grubbs    

Macromolecules

2017

Voltammetry of [5,10,15-tris(pentafluorophenylcorrole)]Mn(III) was investigated in four different ionic liquids (ILs): 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide (BMIm-TFSI); 1-ethyl-3-methylimidazolium ethylsulfate (EMIm-EtOSO3); 1-ethyl-3-methylimidazolium triflate (EMIm-OTf); and 1-ethyl-3-methylimidazolium tetracyanoborate (EMIm-TCB). We found that MnIV/III E values depend on IL counter anion: OTf–< EtOSO < TFSI < TCB. In EMIm-TCB and BMIm- TFSI, reversible, diffusion-controlled Mn reactions occurred, as evidenced in each case by the ratio of anodic to cathodic diffusion coefficients over a range of scan rates. Axial coordination was evidenced by a cathodic to anodic diffusion coefficient ratio greater than one, an increasing cathodic to anodic peak current ratio with increasing scan rate, and a split Soret band in the UV-vis spectrum of the complex.

McNicholas Brendon J.    Blumenfeld Carl    Kramer    Robert H.Grubbs     Jay jay Winkler    Harry Gray   

Russian Journal of Electrochemistry

2017

Grafting density is an important structural parameter that exerts significant influences over the physical properties of architecturally complex polymers. In this report, the physical consequences of varying the grafting density (z) were studied in the context of block polymer self-assembly. Well-defined block polymers spanning the linear, comb, and bottlebrush regimes (0 ≤ z ≤ 1) were prepared via grafting-through ring-opening-metathesis polymerization. ω-Norbornenyl poly(d,l-lactide) and polystyrene macromonomers were copolymerized with discrete comonomers in different feed ratios, enabling precise control over both the grafting density and molecular weight. Small-angle X-ray scattering experiments demonstrate that these graft block polymers self-assemble into long-range-ordered lamellar structures. For 17 series of block polymers with variable z, the scaling of the lamellar period with the total backbone degree of polymerization (d∗ ∼ N) was studied. The scaling exponent α monotonically decreases with decreasing z and exhibits an apparent transition at z ≈ 0.2, suggesting significant changes in the chain conformations. Comparison of two block polymer systems, one that is strongly segregated for all z (System I) and one that experiences weak segregation at low z (System II), indicates that the observed trends are primarily caused by the polymer architectures, not segregation effects. A model is proposed in which the characteristic ratio (C), a proxy for the backbone stiffness, scales with N as a function of the grafting density: C ∼ N. The scaling behavior disclosed herein provides valuable insights into conformational changes with grafting density, thus introducing opportunities for block polymer and material design.

Lin Tzu-Pin    Chang Alice B.    Luo Shao-Xiong    Chen Hsiang-Yun    Byeongdu Lee    Robert H.Grubbs    

ACS Nano

2017

Olefin metathesis reactions with 3E-1,3-dienes using Z-selective cyclometalated ruthenium benzylidene catalysts are described. In particular, a procedure for employing 3E-1,3-dienes in Z-selective homodimerization and cross-metathesis with terminal alkenes is detailed. The reaction takes advantage of the pronounced chemoselectivity of a recently reported ruthenium-based catalyst containing a cyclometalated NHC ligand for terminal alkenes in the presence of internal E-alkenes. A wide array of commonly encountered functional groups can be tolerated, and only a small excess (1.5 equiv) of the diene coupling partner is required to achieve high yields of the desired internal E,Z-diene cross-metathesis product. Computational studies have been performed to elucidate the reaction mechanism. The computations are consistent with a diene-first pathway. The reaction can be used to quickly assemble structurally complex targets. The power of this cross-metathesis reaction is demonstrated by the concise syntheses of two insect pheromones.

Luo Shao-Xiong    Cannon Jeffrey S.    Taylor Buck L. H.    Keary Engle    Kendall Houk    Robert H.Grubbs    

Journal of the American Chemical Society

2016