Reactive Bond Research Articles

Efficient Asymmetric Synthesis of P-Chiral Phosphine Oxides via Properly Designed and Activated Benzoxazaphosphinine-2-oxide Agents

A general, efficient, and highly diastereoselective method for the synthesis of structurally and sterically diverse P-chiral phosphine oxides was developed. The method relies on sequential nucleophilic substitution on the versatile chiral phosphinyl transfer agent 1,3,2-benzoxazaphosphinine-2-oxide, which features enhanced and differentiated P-N and P-O bond reactivity toward nucleophiles. The reactivities of both bonds are fine-tuned to allow cleavage to occur even with sterically hindered nucleophiles under mild conditions.

Monomers for Adhesive Polymers 12 Synthesis and Free‐Radical Homo‐ and Copolymerization of 2‐Ethoxycarbonylallyl 5‐(1,2‐dithiolane‐3‐yl)‐pentanoate

AbstractThe 2‐ethoxycarbonylallyl 5‐(1,2‐dithiolane‐3‐yl)‐pentanoate monomer (AODS) includes in its molecular structure CC and SS reactive bonds allowing it to behave as a bi‐functional monomer, possessing two groups, however, with different reactivity for use in polymer chain building. The polymerization‐specific features of this monomer are the absence of auto‐acceleration and polymer chain crosslinking. Polymerization proceeds readily through most free‐radical initiators. One exception, carboxy‐peroxides are rapidly decomposed without the production of free radicals. AODS is partially converted to a gel without the consumption of double bonds during monomer dissolution in certain organic solvents and after being mixed in solution with carboxy‐peroxides. The determined AODS‐co‐MMA copolymerization parameters are r1 = 2.61, r2 = 0.23 if Luperco peroxide is used as a polymerization initiator, and r1 = 2.71, r2 = 0.38 if AIBN is used. magnified image

Cross-linking of ROMP derived polymers using the two-photon induced thiol–ene reaction: towards the fabrication of 3D-polymer microstructures

Ring opening metathesis polymerization (ROMP) has become an important tool for the synthesis of highly defined polymers and various polymer architectures. In the present work, the residual double bonds in ROMP derived polymeric materials were exploited for a photoinduced thiol–ene reaction in order to achieve a selective cross-linking of the macromolecules. Besides the photoinduced thiol–ene reaction, which was investigated by means of FT-IR measurements, also the resulting change in the refractive index was studied by spectroscopic ellipsometry. To demonstrate the versatility of this reaction for the realization of 3D polymeric microstructures, films of poly(norbornene dicarboxylic acid, dimethylester) were structured via two-photon induced cross-linking. The accessibility and reactivity of the polynorbornene main chain CC double bonds in the thiol–ene reaction pave the way towards novel strategies for the realization of polymer 3D microstructures.

An alternative picture of alkali-metal-mediated metallation: cleave and capture chemistry

This perspective article takes an alternative look at alkali-metal-mediated chemistry (exchange of a relatively inert C-H bond for a more reactive C-metal bond by a multicomponent reagent usually containing an alkali metal and a less electropositive metal such as magnesium or zinc). It pictures that the cleavage of selected C-H bonds can be accompanied by the capturing of the generated anion by the multi (Lewis acid)-(Lewis base) character of the residue of the bimetallic base. In this way small atoms or molecules (hydrides, oxygen-based anions) as well as sensitive organic anions (of substituted aromatic compounds, ethers or alkenes) can be captured. Cleave and capture reactions which occur in special positions on the organic substrate are also included.

Early-Age Tensile Strength and Calcium Hydroxide Content of Concrete Containing Low-Calcium Fly-Ash

The effects of using fly-ash in concrete on the early-age tensile strength and the effectiveness of reactive bond breakers are concerns for applications such as tilt-up panels and liftslabs, however, experimental data is scarce in literature. Fifteen concrete mixtures with up to 40% low-calcium fly-ash were tested. The early-age tensile strength of fly-ash concrete was found to correlate well with compressive strength. The calcium hydroxide (CH) content on concrete surface was found to vary in a wide range because of the varying distribution of aggregates. Generally, the CH content decreased with the increase of fly-ash. The CH content increased with age in the control concrete while the change was relatively small for the fly-ash concretes up to the age of 7 days. Thus, the reduction of CH content in concrete containing high volume low-calcium fly-ash may have significant effect on the performance of bond breakers.

Atomistic modelling of CVD synthesis of carbon nanotubes and graphene

We discuss the synthesis of carbon nanotubes (CNTs) and graphene by catalytic chemical vapour deposition (CCVD) and plasma-enhanced CVD (PECVD), summarising the state-of-the-art understanding of mechanisms controlling their growth rate, chiral angle, number of layers (walls), diameter, length and quality (defects), before presenting a new model for 2D nucleation of a graphene sheet from amorphous carbon on a nickel surface. Although many groups have modelled this process using a variety of techniques, we ask whether there are any complementary ideas emerging from the different proposed growth mechanisms, and whether different modelling techniques can give the same answers for a given mechanism. Subsequently, by comparing the results of tight-binding, semi-empirical molecular orbital theory and reactive bond order force field calculations, we demonstrate that graphene on crystalline Ni(111) is thermodynamically stable with respect to the corresponding amorphous metal and carbon structures. Finally, we show in principle how a complementary heterogeneous nucleation step may play a key role in the transformation from amorphous carbon to graphene on the metal surface. We conclude that achieving the conditions under which this complementary crystallisation process can occur may be a promising method to gain better control over the growth processes of both graphene from flat metal surfaces and CNTs from catalyst nanoparticles.

Early-age tensile strength and calcium hydroxide content of concrete containing low-calcium fly-ash

The effects of using fly-ash in concrete on the early-age tensile strength and the effectiveness of reactive bond breakers are concerns for applications such as tilt-up panels and liftslabs, however, experimental data is scarce in literature. Fifteen concrete mixtures with up to 40% low-calcium fly-ash were tested. The early-age tensile strength of fly-ash concrete was found to correlate well with compressive strength. The calcium hydroxide (CH) content on concrete surface was found to vary in a wide range because of the varying distribution of aggregates. Generally, the CH content decreased with the increase of fly-ash. The CH content increased with age in the control concrete while the change was relatively small for the fly-ash concretes up to the age of 7 days. Thus, the reduction of CH content in concrete containing high volume low-calcium fly-ash may have signifi cant effect on the performance of bond breakers.

Synthesis, characterization, Co–S bond reactivity of a vitamin B12 model complex having pentafluorophenylthiolate as an axial ligand

Heptamethyl (aquo)(pentafluorophenylthiolate)cobyrinate perchlorate, [(H2O)(C6F5S)Cob(III)7C1ester]ClO4, was synthesized as a B12 model complex having a thiolate ligand in the axial position. The axial ligand change in heptamethyl (diaquo)cobyrinate diperchlorate, [(H2O)2Cob(III)7C1ester](ClO4)2, from H2O to C6F5S(-) afforded the B12-thiolate complex. The B12-thiolate model complex was characterized by UV-vis, NMR and ESI-mass spectroscopies. The coordination of C6F5S(-) to the cobalt center affected the spectroscopic properties of the corrin ring through the electronic interaction between the axial ligand (C6F5S(-)) and the equatorial ligand (corrin). The photolysis of the B12-thiolate model complex led to the homolytic cleavage of the Co(iii)-S bond to form the Co(II) complex and the phenyl thiyl radical. The thermolysis of the B12-thiolate model complex also led to the homolytic cleavage of the Co(III)-S bond. Furthermore, the reactivity of the Co(III)-S bond of the B12-thiolate model complex was applied to the catalytic oxidation of C6F5SH to C6F5S-SC6F5.

Quintuple Bond Reactivity toward Group 16 and 17 Elements: Addition vs Insertion

The low valent, coordinatively unsaturated, and formally quintuply bonded bimetallic aminopyridinato chromium complex 1 was investigated regarding its reactivity toward group 16 and 17 elements. Reaction of 1 with O(2) yielded a dimeric Cr oxo complex 2, a compound with a high formal oxidation state carrying both bridging and terminal oxo ligands. Reactions with the higher homologues of the group lead to the formation of dimeric Cr(II) complexes in which E(2)(2-) ligands were formed [E = S (3), Se (4), and Te (5)]. Here the quintuply bonded dichromium unit formally undergoes an addition reaction. Reaction of 1 with the homo diatomic molecules of the group 17 elements leads to products in which the Cr-Cr quintuple bond is inserted into the corresponding X(2) molecule [X = Cl (6), Br (7), and I (8)]. Complex 1 was also found to insert into the S-S and Se-Se bonds of 1,2-diphenyldisulfane or the corresponding selenium compound (complexes 9 and 10, respectively). All the compounds have been characterized by NMR and elemental analysis. Additionally, eight of the complexes have been characterized by X-ray analysis. The bimetallic Cr(II) complexes feature metal-metal distances between 1.8369(18) and 1.918(12) Å.

Quantitative study of the reactivity of fly ash in geopolymerization by FTIR

Fourier transform infrared (FTIR) spectroscopy has been applied to analyse the environments of Al–O and Si–O bonds in fly ash, which are used as raw materials of geopolymer synthesis. It is noted that the relative intensities of the bands at around 1000, 910 and 700 cm−1 are much higher in fly ash with higher reactivity, as reflected by the compressive strength of geopolymer. Deconvolution analysis of the band from 400 to 1400 cm−1 shows that the cumulative area of these three resolved bands, together with the band at ∼1090 cm−1, which is assigned to the asymmetric stretching of Si(Al)–O–Si, is proportional to the reactivity of fly ash. If it is assumed that the area of the resolved bands is proportional to the concentration of the corresponding bonds, a general indication is therefore that fly ash containing more reactive bonds will exhibit higher reactivity in geopolymerisation. FTIR spectroscopy in combination with particle size analysis provides a fast approach to predict the reactivity of fly ash, from the perspective of aluminosilicate glass chemistry.

Tuning the substrate selectivity of meta-cleavage product hydrolase by domain swapping

meta-Cleavage product (MCP) hydrolases can catalyze relatively low reactive carbon-carbon bond hydrolysis of products, which are derived from the meta-cleavage of catechols. The strict substrate selectivity of MCP hydrolases attracts an interest to understand the determinants of substrate specificity. Compared with conventional site-directed mutagenesis, domain swapping is an effective strategy to explore substrate specificity due to the large-scale reorganization of three-dimensional structure. In the present study, the hybrid MCP hydrolases BphDLidA and MfphALidD were constructed by exchanging the lid domain of two parental enzymes MfphA and BphD. The residues Gly130/Ala196 (MfphA) and Gly136/Ala211 (BphD) were selected as crossover points according to structural disruption score analysis and molecular dynamics simulations. It was shown that the hybrid enzymes exhibited similar substrate selectivity with the parent enzyme providing the lid domain. Docking studies suggested that the lid domain may play a key role in determining substrate specificity by reshaping the active pocket and modulating the orientation of the substrate.

Carbon–Oxygen Bond Activation in Esters by Platinum(0): Cleavage of the Less Reactive Bond

Reaction of Pt(dippe)(NBE)2 (NBE = norbornene) with a variety of esters leads to a product in which the less reactive C–O bond is cleaved to give Pt(dippe)(O2CR)(R′). A further disproportionation reaction occurs to eliminate R′2 and produce Pt(dippe)(O2CR)2. Details of the mechanisms of these reactions are explored.

Preparation of Iodide-Thiolate Bridged Binuclear Ni-Ni Complexes via Versatile Reaction of Labile Ni(II)-S(thiolate) Bond

The reaction between Fe(CO)4I2and Ni(SR)2(dppe) affords NiI2(dppe) due to the nucleophilic attack of iodide on the labile Ni-S(thiolate) bonds. The iodide-dithiolate-bridged binuclear Ni-Ni complexes [(dppe)Ni(µ-I)(µ-pdt)Ni(dppe)]I is readily prepared from the reaction between [NiI2(dppe)] and [Ni(pdt)(dppe)] [dppe = 1,2-bis(diphenyl phosphino)-ethane; pdt = 1,3-propane-dithiolate] in CH2Cl2as a result of attack on Ni-I bond by the lone pairs of electrons on thiolato sulfur donors. The reaction between [FeCp(CO)2I] and [Ni(pdt)(dppe)] in CH2Cl2processes extremely slowly. However, upon metathesis with NH4PF6, the iodide-thiolate bridged binuclear Ni-Ni complexes [(dppe)Ni(µ-I)(µ-pdt)Ni(dppe)]PF6is formed from the reaction of iodide and the Ni(II)-S bonds. The reaction between [NiCl2(dppe)] and NH4PF6and [Ni(pdt)(dppe)] gives a binuclear complex [(dppe)Ni(µ-pdt)Ni(dppe)]PF6without a halide-bridge. These results suggest that the reactivity of Ni-SR bonds in the Ni-thiolate-phosphine complexes is tunable with regard to the electronic environment of second metal ion and the different reactivity of iodide moiety. Electrochemical and crystallographic results are also analyzed for relevant compounds.

Revealing the stereospecific chemistry of the reaction of Cl with aligned CHD3(ν1 = 1)

The concept of geometrical constraints and steric hindrance in reactions is implanted deeply in a chemist's 'chemical intuition'. However, until now a true three-dimensional view of these steric effects has not been realized experimentally for any chemical reaction in full. Here we report the complete three-dimensional characterization of the sterics of a benchmark polyatomic reaction by measuring the dependence of the product state-resolved angular distributions on the spatial alignment of the reactive bond in a crossed molecular beam experiment. The results prove the existence of two distinct microscopic reaction mechanisms. Detailed analysis reveals that the origin of the stereodynamics in the HCl(ν = 0) + CD(3)(0(0)) product channel can be captured by a textbook line-of-centres collision model. In contrast, a time-delay pathway, which includes a sharp switch from in-plane to out-of-plane scattering in the forwards direction, appears to be operative in forming the excited HCl(ν = 1) + CD(3)(0(0)) product pair.

Full Exploration of the Diels-Alder Cycloaddition on Metallofullerenes M3N@C80 (M=Sc, Lu, Gd): The D5h versus Ih Isomer and the Influence of the Metal Cluster

In this work a detailed investigation of the exohedral reactivity of the most important and abundant endohedral metallofullerene (EMF) is provided, that is, Sc(3)N@I(h)-C(80) and its D(5h) counterpart Sc(3)N@D(5h)-C(80) , and the (bio)chemically relevant lutetium- and gadolinium-based M(3)N@I(h)/D(5h)-C(80) EMFs (M = Sc, Lu, Gd). In particular, we analyze the thermodynamics and kinetics of the Diels-Alder cycloaddition of s-cis-1,3-butadiene on all the different bonds of the I(h)-C(80) and D(5h)-C(80) cages and their endohedral derivatives. First, we discuss the thermodynamic and kinetic aspects of the cycloaddition reaction on the hollow fullerenes and the two isomers of Sc(3)N@C(80). Afterwards, the effect of the nature of the metal nitride is analyzed in detail. In general, our BP86/TZP//BP86/DZP calculations indicate that [5,6] bonds are more reactive than [6,6] bonds for the two isomers. The [5,6] bond D(5h)-b, which is the most similar to the unique [5,6] bond type in the icosahedral cage, I(h)-a, is the most reactive bond in M(3)N@D(5h)-C(80) regardless of M. Sc(3)N@C(80) and Lu(3)N@C(80) give similar results; the regioselectivity is, however, significantly reduced for the larger and more electropositive M = Gd, as previously found in similar metallofullerenes. Calculations also show that the D(5h) isomer is more reactive from the kinetic point of view than the I(h) one in all cases which is in good agreement with experiments.

Using of Different Nano Scale Energetic Material Systems for Reactive Bonding

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Synthesis and Characterization of Silver Nanoparticle Composite with Poly(<I>p</I>-Br-phenylsilane)

The one-pot synthesis and characterization of silver nanoparticle-poly(p-Br-phenylsilane) composites have been carried out. The conversion of silver(+1) salt to stable silver(0) nanoparticles is promoted by poly(p-Br-phenylsilane), Br-PPS possessing both possible reactive Si-H bonds in the polymer backbone and C-Br bonds in the substituents. The composites were characterized using XRD, TEM, FE-SEM, and solid-state UV-vis analytical techniques. TEM and FE-SEM data show the formation of the composites where large number of silver nanoparticles (less than 30 nm of size) are well dispersed throughout the Br-PPS matrix. XRD patterns are consistent with that for fcc-typed silver. The elemental analysis for Br atom and the polymer solubility confirm that the cleavage of C-Br bond and the Si-Br dative bonding were not occurred appreciably at ambient temperature. Nonetheless, TGA data suggest that some sort of cross-linking was occurred at high temperature. The size and processability of such nanoparticles depend on the ratio of metal to Br-PPS. In the absence of Br-PPS, most of the silver particles undergo macroscopic aggregation, which indicates that the polysilane is necessary for stabilizing the silver nanoparticles.

Full-Dimensional Quantum Dynamics Study of the H + H2O and H + HOD Exchange Reactions

The initial state-selected time-dependent wave packet approach is employed to study the H' + H(2)O → H'OH + H and H' + HOD → H'OD + H, HOH' + D exchange reactions with both OH bonds in the H(2)O reactant and OH(D) bond in the HOD reactant treated as reactive bonds. The total reaction probabilities for different partial waves, as well as the integral cross sections, which are the exact CC (coupled-channel) results, are first obtained in this study for the H(2)O(HOD) reactant initially in the ground rovibrational state. Because of the shallow C(3v) minimum along the reaction path, the reaction probabilities for the three reactions present several resonance peaks, with one dominant resonance peak just above the threshold. The cross sections for the H' + HOD → HOH' + D reaction are substantially smaller than those for the H' + H(2)O → H'OH + H and H' + HOD → H'OD + H reactions, indicating that the H'/H exchange reactions are much more favored. In the CC calculations, the resonance peaks in the reaction probabilities diminish quickly with the increase in total angular momenta J, resulting in the existence of a clear step-like feature just above the threshold in the cross sections for the title reactions, which manifests the signature of shape resonances in these reactions. In the CS calculations, the resonance peaks on reaction probabilities persist in many partial waves, and thus the resonance structures can no longer survive the partial-wave summation and are washed out completely in the CS cross sections for the title reactions.

Shape resonance in the H + D2O → D + HOD reaction: a full-dimensional quantum dynamics study

We present exact coupled-channel (CC) results of full-dimensional quantum mechanical calculations for the H + D2O → D + HOD exchange reaction which provide the first evidence for a shape resonance in this reaction, employing the initial state-selected time-dependent wave packet method with both OD bonds in the D2O reactant treated as reactive bonds. Due to the shallow C3v minimum along the reaction path, the signature of shape resonance is manifested as the existence of a distinct step-like feature in the CC integral cross sections of the exchange reaction just above the threshold. Our study also shows that the current CC integral cross sections are in very good agreement with the experimental results, and a bit larger than those calculated in the previous quantum dynamics study, where the one reactive bond and centrifugal sudden (CS) approximations were invoked, revealing that the one reactive bond approximation overestimated, while the CS approximation underestimated the integral cross sections.

LOW COST PROCESSING OPTIONS FOR CA-α-SIALONS

The aim of this work is to investigate the potential for lower cost production of Ca -α- SiAlON materials. In order to reduce the costs typically associated with the production of these materials, we have investigated Reaction Bonding (RB) as an alternative processing route, as well as alternative raw materials for the source of calcium by recycling waste mussel shells from the New Zealand aquaculture industry. Full transformation to Ca -α- SiAlON was achieved at 1400°C in the reaction bonded materials, whereas the conventional silicon nitride based materials required temperatures of 1800°C to achieve full transformation. The relative density of the sintered samples was low but, as a production route for SiALON powders or for materials for low tech applications, the reaction bonding / mussel shell combination offers significant cost benefits.