Carbodiimide and Isocyanate Hydroboration by a Cyclic Carbodiphosphorane Catalyst.

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.

Crystal structure of 2-(2,6-diiso-propyl-phen-yl)-N,N-diethyl-3,3-dimethyl-2-aza-spiro-[4.5]decan-1-amine: a di-ethyl-amine adduct of a cyclic(alk-yl)(amino)-carbene (CAAC).

The structure of the title compound, C27H46N2, at 93 K has monoclinic (P21/n) symmetry. The title compound was prepared by treatment of 2-(2,6-diiso-propyl-phenyl)-3,3-dimethyl-2-aza-spiro-[4.5]dec-1-en-2-ium hydrogen dichloride with two equivalents of lithium di-ethyl-amide. Characterization of the title compound by single-crystal X-ray diffraction and 1H and 13C NMR spectroscopy is presented. Formation of the di-ethyl-amine adduct of the cyclic(alk-yl)(amino)-carbene (CAAC) was unexpected, as deprotonation using lithium diiso-propyl-amide results in free CAAC formation.

Processing Effects on the Self-Assembly of Brush Block Polymer Photonic Crystals.

The self-assembly of poly(dimethylsiloxane)-b-poly(trimethylene carbonate) (PDMS-b-PTMC) bottlebrush block polymers was investigated under different processing conditions. Small-angle X-ray scattering (SAXS) and UV/Visible spectroscopy provided insight into the self-assembly and structure in response to heating and applied pressure. In the absence of applied pressure (i.e., before annealing), the PDMS-b-PTMC bottlebrush block polymers are white solids and adopt small, randomly oriented lamellar grains. Heating the materials to 140 °C in the absence of applied pressure appears to "lock in" the isotropic, short-range-ordered state, preventing the formation of the long-range-ordered lamellar structure responsible for photonic properties. Applying modest anisotropic pressure (3 psi) between parallel plates at ambient temperature orients the short-range lamellar grains; however, applied pressure alone does not produce long-range order. Only when the bottlebrush block polymers were heated (>100 °C) under modest pressure (3 psi) were long-range-ordered photonic crystals formed. Analysis of the SAXS data motivated analogies to liquid crystals and revealed the potential self-assembly pathway. These results provide insight into the structure and self-assembly of bottlebrush block polymers with low glass transition temperature side chains in response to different processing conditions.

Disentangling Ligand Effects on Metathesis Catalyst Activity: Experimental and Computational Studies of Ruthenium-Aminophosphine Complexes.

Second-generation ruthenium olefin metathesis catalysts bearing aminophosphine ligands were investigated with systematic variation of the ligand structure. The rates of phosphine dissociation ( k1; initiation rate) and relative phosphine reassociation ( k-1) 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]-PCy3 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 (PR1R'2); 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.

Design, Synthesis, and Self-Assembly of Polymers with Tailored Graft Distributions.

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 < khomo < 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 < r1/r2 < 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.

Application of Bottlebrush Block Copolymers as Photonic Crystals.

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.

Control of Grafting Density and Distribution in Graft Polymers by Living Ring-Opening Metathesis Copolymerization.

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 (PLAx-ran-DME1-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.

Electrophilic Activation of Silicon-Hydrogen Bonds in Catalytic Hydrosilations.

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 -H2 SiRR' 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.

Significant Cooperativity Between Ruthenium and Silicon in Catalytic Transformations of an Isocyanide.

Complexes [PhBP3]RuH(η(3)-H2SiRR') (RR' = Me,Ph, 1a; RR' = Ph2, 1b; RR' = Et2, 1c) react with XylNC to form carbene complexes [PhBP3]Ru(H)═[C(H)(N(Xyl)(η(2)-H-SiRR'))] (2a-c; previously reported for 2a,b). Reactions of 1a-c with XylNC were further investigated to assess how metal complexes with multiple M-H-Si bonds can mediate transformations of unsaturated substrates. Complex 2a eliminates an N-methylsilacycloindoline product (3a) that results from hydrosilylation, hydrogenation, and benzylic C-H activation of XylNC. Turnover was achieved in a pseudocatalytic manner by careful control of the reaction conditions. Complex 1c mediates a catalytic isocyanide reductive coupling to furnish an alkene product (4) in a transformation that has precedent only in stoichiometric processes. The formations of 3a and 4 were investigated with deuterium labeling experiments, KIE and other kinetic studies, and by examining the reactivity of XylNC with an η(3)-H2SiMeMes complex (1d) to form a C-H activated complex (6). Complex 6 serves as a model for an intermediate in the formation of 3a, and NMR investigations at -30 °C reveal that 6 forms via a carbene complex (1d) that isomerizes to aminomethyl complex 7d. These investigations reveal that the formations of 3a and 4 involve multiple 4-, 5-, and 6-coordinate silicon species with 0, 1, 2, or 3 Ru-H-Si bonds. These mechanisms demonstrate exceptionally intricate roles for silicon in transition-metal-catalyzed reactions with a silane reagent.

Lewis acid-base interactions between platinum(ii) diaryl complexes and bis(perfluorophenyl)zinc: strongly accelerated reductive elimination induced by a Z-type ligand.

Z-type interactions between bis(perfluorophenyl)zinc and platinum(ii) diaryl complexes supported by 1,10-phenanthroline (phen), 2,2'-bipyridine (bpy), and bis(dimethylphosphino)ethane (dmpe) ligands are reported. In the solid state, the nature of the Pt-Zn interaction depends on the bidentate ligand; the phen-supported complex exhibits an unsupported Pt-Zn bond, while the dmpe derivative features additional bridging aryl interactions. A strongly accelerated rate of reductive elimination is observed for phen- and bpy-supported complexes, while aryl exchange between Pt and Zn is observed for the dmpe complex.

Lewis Acidity of Bis(perfluorocatecholato)silane: Aldehyde Hydrosilylation Catalyzed by a Neutral Silicon Compound.

Bis(perfluorocatecholato)silane Si(cat(F))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(cat(F))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 NBu4BAr(F)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.

A remote Lewis acid trigger dramatically accelerates biaryl reductive elimination from a platinum complex.

A strategy for the control of electron density at a metal center is reported, which uses a remote chemical switch involving second-sphere Lewis acid binding that modulates electron density in the first coordination sphere. Binding of the Lewis acid B(C6F5)3 at remote nitrogen positions of a bipyrazine-diarylplatinum(II) complex accelerates biaryl reductive elimination by a factor of 64,000.