Catalyst Design Principles Enabling Intermolecular Alkene-Diene [2+2] Cycloaddition and Depolymerization Reactions.

Aryl-substituted pyridine(diimine) iron complexes promote the catalytic [2 + 2] cycloadditions of alkenes and dienes to form vinylcyclobutanes as well as the oligomerization of butadiene to generate divinyl(oligocyclobutane), a microstructure of poly(butadiene) that is chemically recyclable. A systematic study on a series of iron butadiene complexes as well as their ruthenium congeners has provided insights into the essential features of the catalyst that promotes these cycloaddition reactions. Structural and computational studies on iron butadiene complexes identified that the structural rigidity of the tridentate pincer enables rare s-trans diene coordination. This geometry, in turn, promotes dissociation of one of the alkene arms of the diene, opening a coordination site for the incoming substrate to engage in oxidative cyclization. Studies on ruthenium congeners established that this step occurs without redox involvement of the pyridine(diimine) chelate. Cyclobutane formation occurs from a metallacyclic intermediate by reversible C(sp3)-C(sp3) reductive coupling. A series of labeling experiments with pyridine(diimine) iron and ruthenium complexes support the favorability of accessing the +3 oxidation state to trigger C(sp3)-C(sp3) reductive elimination, involving spin crossover from S = 0 to S = 1. The high density of states of iron and the redox-active pyridine(diimine) ligand facilitate this reactivity under thermal conditions. For the ruthenium congener, the pyridine(diimine) remains redox innocent and irradiation with blue light was required to promote the analogous reactivity. These structure-activity relationships highlight important design principles for the development of next generation catalysts for these cycloaddition reactions as well as the promotion of chemical recycling of cycloaddition polymers.

Iron-catalysed synthesis and chemical recycling of telechelic 1,3-enchained oligocyclobutanes.

Closed-loop recycling offers the opportunity to mitigate plastic waste through reversible polymer construction and deconstruction. Although examples of chemical recycling of polymers are known, few have been applied to materials derived from abundant commodity olefinic monomers, which are the building blocks of ubiquitous plastic resins. Here we describe a [2+2] cycloaddition/oligomerization of 1,3-butadiene to yield a previously unrealized telechelic microstructure of (1,n'-divinyl)oligocyclobutane. This material is thermally stable, has stereoregular segments arising from chain-end control, and exhibits high crystallinity even at low molecular weight. Exposure of the oligocyclobutane to vacuum in the presence of the pyridine(diimine) iron precatalyst used to synthesize it resulted in deoligomerization to generate pristine butadiene, demonstrating a rare example of closed-loop chemical recycling of an oligomeric material derived from a commodity hydrocarbon feedstock.

Highly active cationic cobalt(II) hydroformylation catalysts.

The cobalt complexes HCo(CO)4 and HCo(CO)3(PR3) were the original industrial catalysts used for the hydroformylation of alkenes through reaction with hydrogen and carbon monoxide to produce aldehydes. More recent and expensive rhodium-phosphine catalysts are hundreds of times more active and operate under considerably lower pressures. Cationic cobalt(II) bisphosphine hydrido-carbonyl catalysts that are far more active than traditional neutral cobalt(I) catalysts and approach rhodium catalysts in activity are reported here. These catalysts have low linear-to-branched (L:B) regioselectivity for simple linear alkenes. However, owing to their high alkene isomerization activity and increased steric effects due to the bisphosphine ligand, they have high L:B selectivities for internal alkenes with alkyl branches. These catalysts exhibit long lifetimes and substantial resistance to degradation reactions.

Pyrolysis pathways of sulfonated polyethylene, an alternative carbon fiber precursor.

Polyethylene is an emerging precursor material for the production of carbon fibers. Its sulfonated derivative yields ordered carbon when pyrolyzed under inert atmosphere. Here, we investigate its pyrolysis pathways by selecting n-heptane-4-sulfonic acid (H4S) as a model compound. Density functional theory and transition state theory were used to determine the rate constants of pyrolysis for H4S from 300 to 1000 K. Multiple reaction channels from two different mechanisms were explored: (1) internal five-centered elimination (Ei5) and (2) radical chain reaction. The pyrolysis of H4S was simulated with kinetic Monte Carlo (kMC) to obtain thermogravimetric (TGA) plots that compared favorably to experiment. We observed that at temperatures <550 K, the radical mechanism was dominant and yielded the trans-alkene, whereas cis-alkene was formed at higher temperatures from the internal elimination. The maximum rates of % mass loss became independent of initial È®H radical concentration at 440-480 K. Experimentally, the maximum % mass loss occurred from 440 to 460 K (heating rate dependent). Activation energies derived from the kMC-simulated TGAs of H4S (26-29 kcal/mol) agreed with experiment for sulfonated polyethylene (~31 kcal/mol). The simulations revealed that in this region, decomposition of radical HOSÈ®2 became competitive to α-H abstraction by HOSÈ®2, making È®H the carrying radical for the reaction chain. The maximum rate of % mass loss for internal elimination was observed at temperatures >600 K. Low-scale carbonization utilizes temperatures <620 K; thus, internal elimination will not be competitive. E(i)5 elimination has been studied for sulfoxides and sulfones, but this represents the first study of internal elimination in sulfonic acids.

Computational study of bond dissociation enthalpies for substituted ß-O-4 lignin model compounds.

The biopolymer lignin is a potential source of valuable chemicals. Phenethyl phenyl ether (PPE) is representative of the dominant ß-O-4 ether linkage. DFT is used to calculate the Boltzmann-weighted carbon-oxygen and carbon-carbon bond dissociation enthalpies (BDEs) of substituted PPE. These values are important for understanding lignin decomposition. Exclusion of all conformers that have distributions of less than 5% at 298 K impacts the BDE by less than 1 kcal mol(-1). We find that aliphatic hydroxyl/methylhydroxyl substituents introduce only small changes to the BDEs (0-3 kcal mol(-1)). Substitution on the phenyl ring at the ortho position substantially lowers the C-O BDE, except in combination with the hydroxyl/methylhydroxyl substituents, for which the effect of methoxy substitution is reduced by hydrogen bonding. Hydrogen bonding between the aliphatic substituents and the ether oxygen in the PPE derivatives has a significant influence on the BDE. CCSD(T)-calculated BDEs and hydrogen-bond strengths of ortho-substituted anisoles, when compared with M06-2X values, confirm that the latter method is sufficient to describe the molecules studied and provide an important benchmark for lignin model compounds.

Cytochrome P450: the active oxidant and its spectrum.

This Forum focuses on the nature of the elusive oxidizing intermediate in P450 catalysis. The identity of this species has reemerged as a topic of contentious debate. It was recently reported that laser flash photolysis (LFP) can be used to generate P450 compound I (P450-I) quantitatively. Kinetic analyses of the reaction of the LFP-generated intermediate with substrates have been suggested to indicate that compound I is not the active oxidant in P450 catalysis. We evaluate these claims via an analysis of the UV/visible spectrum of the LFP-generated intermediate. The techniques of singular value decomposition and target testing are used to obtain the spectrum of P450-I in a model-independent manner from stopped-flow data of the reaction of P450 with m-chloroperbenzoic acid. It is shown that the LFP-generated spectrum bears no similarity to the P450-I spectrum. One may conclude that the LFP-generated intermediate is not P450-I.

Structural analysis of the Mn(IV)/Fe(III) cofactor of Chlamydia trachomatis ribonucleotide reductase by extended X-ray absorption fine structure spectroscopy and density functional theory calculations.

The class Ic ribonucleotide reductase from Chlamydia trachomatis ( Ct) uses a stable Mn(IV)/Fe(III) cofactor to initiate nucleotide reduction by a free-radical mechanism. Extended X-ray absorption fine structure (EXAFS) spectroscopy and density functional theory (DFT) calculations are used to postulate a structure for this cofactor. Fe and Mn K-edge EXAFS data yield an intermetallic distance of approximately 2.92 A. The Mn data also suggest the presence of a short 1.74 A Mn-O bond. These metrics are compared to the results of DFT calculations on 12 cofactor models derived from the crystal structure of the inactive Fe 2(III/III) form of the protein. Models are differentiated by the protonation states of their bridging and terminal OH X ligands as well as the location of the Mn(IV) ion (site 1 or 2). The models that agree best with experimental observation feature a mu-1,3-carboxylate bridge (E120), terminal solvent (H 2O/OH) to site 1, one mu-O bridge, and one mu-OH bridge. The site-placement of the metal ions cannot be discerned from the available data.

A mechanistic study of the alkaline hydrolysis of diaryl sulfate diesters.

Nearly all of the reported studies of reactions of sulfate diesters are for dialkyl or alkyl aryl diesters, which undergo reaction by carbon-oxygen bond fission. Sulfuryl transfer reactions of sulfate diesters (RO-SO(2)-OR') proceeding by attack at sulfur have been little explored. When both ester groups are aryl groups the hydrolysis reaction (sulfuryl transfer to water) occurs by way of attack at sulfur. The alkaline hydrolysis of diaryl sulfate diesters was shown to obey first-order kinetics with respect to [(-)OH] and proceed through S-O bond fission, in a mechanism that is most likely concerted. Activation parameters for 4-chloro-3-nitrophenyl phenyl sulfate and 4-nitrophenyl phenyl sulfate gave the following respective values: Delta H(++) = 88.0 +/- 0.1 and 84.83 +/- 0.06 kJ mol(-)(1) and Delta S(++) = -37 +/- 1 and -50.2 +/- 0.5 J mol(-)(1) deg(-)(1). The dependence of the second-order rate constant for hydrolysis on leaving group pK(a) was analyzed giving a beta(lg) slope of -0.7 +/- 0.2 and a Leffler alpha parameter value of 0.36. A (15)k kinetic isotope effect (KIE) for the hydroxide attack on 4-nitrophenyl phenyl sulfate of 1.0000 +/-0.0005 and an (18)k(lg) KIE value of 1.003+/-0.002 were obtained.

A nonhydrolyzable analogue of phosphotyrosine, and related aryloxymethano- and aryloxyethano-phosphonic acids as motifs for inhibition of phosphatases.

Nonhydrolyzable analogues of both stereoisomers of phosphotyrosine, and a series of related aryloxy (or thio) methyl and aryloxy (or thio) ethyl phosphonic acids of the general formula RX-(CH(2))(n)-PO(3)H(2) (where X=O or S and n=1 or 2), have been tested as nonhydrolyzable mimetics of phosphatase substrates. These compounds were tested against a panel of phosphatases (two alkaline phosphatases, a protein-tyrosine phosphatase, and two serine/threonine phosphatases) with different active site motifs. The compounds exhibit competitive inhibition toward all enzymes tested, with the best inhibition expressed toward the Ser/Thr phosphatases. The stereoisomers of the phosphotyrosine analogues exhibited an unexpected difference in their inhibitory properties toward the protein-tyrosine phosphatase from Yersinia. The K(i) for the d isomer is 33-fold lower than that of the l isomer, and is more than an order of magnitude lower than the reported K(m) of the substrate l-phosphotyrosine.