Hydrosilation reactions represent an important class of chemical transformations and there has been considerable recent interest in expanding the scope of these reactions by developing new catalysts. ...A major theme to emerge from these investigations is the development of catalysts with electrophilic character that transfer electrophilicity to silicon by Si‐H activation. This type of mechanism has been proposed for catalysts ranging from Group 4 transition metals to Group 15 main group species. Additionally, other electrophilic silicon species, such as silylene complexes and η3‐H2SiRR′ complexes, have been identified as intermediates in hydrosilation reactions. In this Review, different types of catalysts are compared to highlight the range of hydrosilation mechanisms that feature electrophilic silicon centers. The importance of these catalysts to the development of new hydrosilation reactions is also discussed.
On active duty: Catalysts based on elements from Groups 4–15 and that operate through electrophilic silane activation pathways are currently intensively investigated for hydrosilylations. This Review surveys these catalysts and describes several related mechanisms for electrophilic Si‐H activation (σ‐H‐Si coordination, M(H)2=SiRR′ formation, η3‐H2SiRR′ binding).
Brush block copolymers are a class of comb polymers that feature polymeric side chains densely grafted to a linear backbone. These polymers display interesting properties due to their dense ...functionality, low entanglement, and ability to rapidly self‐assemble to highly ordered nanostructures. The ability to prepare brush polymers with precise structures has been enabled by advancements in controlled polymerization techniques. This Feature Article highlights the development of brush block copolymers as photonic crystals that can reflect visible to near‐infrared wavelengths of light. Fabrication of these materials relies on polymer self‐assembly processes to achieve nanoscale ordering, which allows for the rapid preparation of photonic crystals from common organic chemical feedstocks. The characteristic physical properties of brush block copolymers are discussed, along with methods for their preparation. Strategies to induce self‐assembly at ambient temperatures and the use of blending techniques to tune photonic properties are emphasized.
Brush block copolymers have emerged as promising components in the bottom‐up synthesis of photonic crystals from common organic chemical feedstocks. This polymer architecture permits fabrication of photonic crystal films under ambient conditions driven by self‐assembly. Photonic properties can be controlled by the backbone length of the brush block copolymer or by utilizing blending techniques.
Bis(perfluorocatecholato)silane Si(catF)2 was prepared, and stoichiometric binding to Lewis bases was demonstrated with fluoride, triethylphosphine oxide, and N,N′-diisopropylbenzamide. The potent ...Lewis acidity of Si(catF)2 was suggested from catalytic hydrosilylation and silylcyanation reactions with aldehydes. Mechanistic studies of hydrosilylation using an optically active silane substrate, R-(+)-methyl-(1-naphthyl)phenylsilane, proceeded with predominant stereochemical retention at silicon, consistent with a carbonyl activation pathway. The enantiospecificity was dependent on solvent and salt effects, with increasing solvent polarity or addition of NBu4BArF 4 leading to a diminished enantiomeric ratio. The medium effects are consistent with an ionic mechanism, wherein hydride transfer occurs prior to silicon–oxygen bond formation.
We report hydroboration of carbodiimide and isocyanate substrates catalyzed by a cyclic carbodiphosphorane catalyst. The cyclic carbodiphosphorane outperformed the other Lewis basic carbon species ...tested, including other zerovalent carbon compounds, phosphorus ylides, an N‐heterocyclic carbene, and an N‐heterocyclic olefin. Hydroborations of seven carbodiimides and nine isocyanates were performed at room temperature to form N‐boryl formamidine and N‐boryl formamide products. Intermolecular competition experiments demonstrated the selective hydroboration of alkyl isocyanates over carbodiimide and ketone substrates. DFT calculations support a proposed mechanism involving activation of pinacolborane by the carbodiphosphorane catalyst, followed by hydride transfer and B−N bond formation.
A cyclic carbodiphosphorane catalyzes the hydroboration of carbodiimide and isocyanate substrates. DFT calculations support a mechanism involving activation of pinacolborane by the carbodiphosphorane catalyst, followed by hydride transfer and B−N bond formation.
Ruthenium benzylidene complexes containing a carbodicarbene (CDC) ligand are reported. Mechanistic studies indicate that the CDC ligand can dissociate under relatively mild conditions to afford ...active olefin metathesis catalysts. These catalysts were found to be effective at ring-closing metathesis (RCM) and ring-opening metathesis polymerization (ROMP) reactions.
We report the use of a cyclic carbodiphosphorane catalyst for ketone and imine hydroboration reactions. Ketone hydroboration reactions are particularly rapid, typically reaching completion within 15 ...min using a 1 mol % catalyst loading at 25 °C. To our knowledge, this represents the first use of a carbodiphosphorane as an organocatalyst. The carbodiphosphorane exhibited superior catalytic activity in comparison to other neutral carbon nucleophiles tested, including an N-heterocyclic carbene, an N-heterocyclic olefin, and phosphorus ylides.
Control over polymer sequence and architecture is crucial to both understanding structure–property relationships and designing functional materials. In pursuit of these goals, we developed a new ...synthetic approach that enables facile manipulation of the density and distribution of grafts in polymers via living ring-opening metathesis polymerization (ROMP). Discrete endo,exo-norbornenyl dialkylesters (dimethyl DME, diethyl DEE, di-n-butyl DBE) were strategically designed to copolymerize with a norbornene-functionalized polystyrene (PS), polylactide (PLA), or polydimethylsiloxane (PDMS) macromonomer mediated by the third-generation metathesis catalyst (G3). The small-molecule diesters act as diluents that increase the average distance between grafted side chains, generating polymers with variable grafting density. The grafting density (number of side chains/number of norbornene backbone repeats) could be straightforwardly controlled by the macromonomer/diluent feed ratio. To gain insight into the copolymer sequence and architecture, self-propagation and cross-propagation rate constants were determined according to a terminal copolymerization model. These kinetic analyses suggest that copolymerizing a macromonomer/diluent pair with evenly matched self-propagation rate constants favors randomly distributed side chains. As the disparity between macromonomer and diluent homopolymerization rates increases, the reactivity ratios depart from unity, leading to an increase in gradient tendency. To demonstrate the effectiveness of our method, an array of monodisperse polymers (PLA x -ran-DME 1‑x ) n bearing variable grafting densities (x = 1.0, 0.75, 0.5, 0.25) and total backbone degrees of polymerization (n = 167, 133, 100, 67, 33) were synthesized. The approach disclosed in this work therefore constitutes a powerful strategy for the synthesis of polymers spanning the linear-to-bottlebrush regimes with controlled grafting density and side chain distribution, molecular attributes that dictate micro- and macroscopic properties.
Grafting density and graft distribution impact the chain dimensions and physical properties of polymers. However, achieving precise control over these structural parameters presents long-standing ...synthetic challenges. In this report, we introduce a versatile strategy to synthesize polymers with tailored architectures via grafting-through ring-opening metathesis polymerization (ROMP). One-pot copolymerization of an ω-norbornenyl macromonomer and a discrete norbornenyl comonomer (diluent) provides opportunities to control the backbone sequence and therefore the side chain distribution. Toward sequence control, the homopolymerization kinetics of 23 diluents were studied, representing diverse variations in the stereochemistry, anchor groups, and substituents. These modifications tuned the homopolymerization rate constants over 2 orders of magnitude (0.36 M–1 s–1 < k homo < 82 M–1 s–1). Rate trends were identified and elucidated by complementary mechanistic and density functional theory (DFT) studies. Building on this foundation, complex architectures were achieved through copolymerizations of selected diluents with a poly(d,l-lactide) (PLA), polydimethylsiloxane (PDMS), or polystyrene (PS) macromonomer. The cross-propagation rate constants were obtained by nonlinear least-squares fitting of the instantaneous comonomer concentrations according to the Mayo–Lewis terminal model. In-depth kinetic analyses indicate a wide range of accessible macromonomer/diluent reactivity ratios (0.08 < r 1/r 2 < 20), corresponding to blocky, gradient, or random backbone sequences. We further demonstrated the versatility of this copolymerization approach by synthesizing AB graft diblock polymers with tapered, uniform, and inverse-tapered molecular “shapes.” Small-angle X-ray scattering analysis of the self-assembled structures illustrates effects of the graft distribution on the domain spacing and backbone conformation. Collectively, the insights provided herein into the ROMP mechanism, monomer design, and homo- and copolymerization rate trends offer a general strategy for the design and synthesis of graft polymers with arbitrary architectures. Controlled copolymerization therefore expands the parameter space for molecular and materials design.
The silicon and zinc Lewis acids Si(cat)2 (cat = catecholato), Si(catF)2 (catF = tetrafluorocatecholato), and Zn(C6F5)2 bind to the remote ligand site of a 2,2′-bipyrimidyl–platinum diaryl ...complex. This platinum complex provides a platform to systematically evaluate electronic and reactivity differences triggered by Lewis acid binding. The electron density of the bipyrimidine ligand is substantially depleted upon Lewis acid binding, as evidenced by UV–vis spectroscopy and cyclic voltammetry. Biaryl reductive elimination studies allowed quantification of the effect of Lewis acid binding on reactivity, and Lewis acid binding accelerated reductive elimination rates by up to 8 orders of magnitude. Kinetics experiments in combination with DFT studies support an unusual mechanism featuring complete dissociation of the Lewis acid-coordinated bidentate bipyrimidine ligand prior to reductive elimination.
This report describes rapid changes to instructional materials, assessment, and technology use in a flipped college-level organic chemistry course in response to the sudden COVID-19 campus closures ...and midsemester transition to remote learning. The instructors modified in-class instructional materials to accommodate students’ preference for text-based over audiovisual engagement in the remote environment. The pros and cons of three approaches to administering remote organic chemistry exams are presented. Issues related to equitable incorporation of technology and maintaining academic integrity are discussed.
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