The new ONIOM (our own n-layered integrated molecular orbital and molecular mechanics) approach has been proposed and shown to be successful in reproducing benchmark calculations and experimental results. ONIOM3, a three-layered version, divides a system into an active part treated at a very high level of ab initio molecular orbital theory like CCSD(T), a semiactive part that includes important electronic contributions and is treated at the HF or MP2 level, and a nonactive part that is handled using force field approaches. The three-layered scheme allows us to study a larger system more accurately than the previously proposed two-layered schemes IMOMO, which can treat a medium size system very accurately, and IMOMM, which can handle a very large system with modest accuracy. This three-layered scheme has been applied to activation barriers for the Diels−Alder reaction of acrolein + isoprene, acrolein + 2-tert-butyl-1,3-butadiene, and ethylene + 1,4-di-tert-butyl-1,3-butadiene. In general, the results for both geometry optimizations and single point energy calculations agree well with benchmark predictions and experimental results. The scheme has also been applied to the transition state for the oxidative addition of H2 to Pt(P(t-Bu)3)2. The activation energy of this 83-atom reaction is predicted to be 14.2 kcal/mol with the ONIOM3(CCSD(T):MP2:MM3) method.

We have investigated the olefin polymerization mechanism of hafnium catalysts supported by a pyridyl-amide ligand with an ortho-metalated naphthyl group. Ethylene-alpha-olefin copolymers from these catalysts have broad molecular weight distributions that can be fit to a bimodal distribution. We propose a unique mechanism to explain this behavior involving monomer modification of the catalyst, which generates multiple catalyst species when multiple monomers are present. More specifically, we present evidence that the hafnium alkyl cation initially undergoes monomer insertion into the Hf-naphthyl bond, which permanently modifies the ligand to generate new highly active olefin polymerization catalysts. Under ethylene/octene copolymerization conditions, a plurality of new catalysts is formed in relative proportion to the respective monomer concentrations. Due to the asymmetry of the metal complex, two "ethylene-inserted" and eight "octene-inserted" isomers are possible, but it is a useful approximation to consider only one of each in the polymerization behavior. Consequently, gel permeation chromatography data for the polymers can be fit to a bimodal distribution having a continuous shift from a predominantly low molecular weight fraction to predominantly higher molecular weight fraction as [octene]/[ethylene] is increased. Theoretical calculations show that such insertions into the Hf-aryl bond have lower barriers than corresponding insertions into the Hf-alkyl bond. The driving forces for this insertion into the Hf-aryl bond include elimination of an eclipsing H-H interaction and formation of a stabilizing Hf-arene interaction. These new "monomer-inserted catalysts" have no beta-agostic interaction, very weak olefin binding, and olefin-insertion transition states which differ on the two sides by more than 4 kcal/mol. Thus, the barrier to site epimerization is very low and high polymerization rates are possible even when the chain wags prior to every insertion. Experimental evidence for aryl-insertion products is obtained from reactions of ethylene (13C2H4 NMR studies) or 4-methyl-1-pentene (4M1P) using relatively low monomer/catalyst ratios. Quantitative generation of monomer-inserted products is complicated by slow initiation kinetics followed by fast polymerization kinetics. However, NMR evidence for reaction with 13C2H4 was observed in situ at low temperature, and the attachment of monomer to ligand was confirmed by GC/MS and 13C NMR after quenching. Furthermore, a 4M1P-appended ligand was isolated from a polymerization reaction (50:1 monomer:catalyst) by column chromatography followed by multiple recrystallizations. One isomer was characterized by X-ray crystallography, which unequivocally shows a 4-methylpentyl substituent at the 2-position of the naphthyl, consistent with 1,2-insertion into the Hf-aryl bond. NMR suggests a second diastereomer (not isolated) is formed from a 1,2-insertion of opposite stereoselectivity.

This report describes a method for the deoxyfluorination of phenols with sulfuryl fluoride (SOF) and tetramethylammonium fluoride (NMeF) via aryl fluorosulfonate (ArOFs) intermediates. We first demonstrate that the reaction of ArOFs with NMeF proceeds under mild conditions (often at room temperature) to afford a broad range of electronically diverse and functional group-rich aryl fluoride products. This transformation was then translated to a one-pot conversion of phenols to aryl fluorides using the combination of SOF and NMeF. Ab initio calculations suggest that carbon-fluorine bond formation proceeds via a concerted transition state rather than a discrete Meisenheimer intermediate.

The mechanism of diimine−Ni-catalyzed ethylene polymerization reaction has been studied theoretically using the B3LYP density functional method. The chain initiation reaction proceeds with the coordination of ethylene to the model active catalyst [L2NiCH3]+, L2 = (HNCH)2, followed by ethylene insertion into the metal−alkyl bond with a rate-determining 11.7 kcal/mol free energy barrier to form a γ-agostic intermediate, which with a small barrier rearranges to a more stable β-agostic intermediate and then forms an olefin alkyl complex upon coordination of the next ethylene. Linear polymer propagation takes place from this olefin alkyl complex, the resting state in the catalytic cycle, via the same insertion, rearrangement, and coordination pathway. An alternative pathway from the olefin alkyl complex passes over a 14−15 kcal/mol barrier for β-hydride elimination and reinsertion for branched polymer propagation. These energetics suggest that the Ni(II)-catalyzed reaction is expected to produce more linear than methyl-branched polymers, and that higher temperature increases and higher ethylene pressure decreases the branching. Hydrogenolysis is an energetically favorable termination pathway, proceeding via coordination of a hydrogen molecule to the metal center, followed by H−H activation through a four-centered “metathesis-like” transition state and reductive elimination of alkane. A comparison with zirconocene-catalyzed ethylene polymerization shows that the Ni(II)-catalyzed polymerization should be slightly slower and should give more branching.

Reactions of [{N -,N,C naphthyl-}HfMe][MeB(C 6 F 5 ) 3 ] (2) precatalyst with a series of R-olefins have been investigated in order to intercept the active polymerization species generated by an in situ modification of the precursor by insertion of a single monomer unit into the Hf-C Aryl bond. In all cases the first migratory insertion of monomer occurs into the Hf-C Aryl bond rather than the Hf-C Alkyl bond. A low-temperature polymerization with 170 equiv of 1-hexene activated with tris-(pentafluorophenyl)borane (FAB) allows for the complete NMR characterization of a Hf-C Alkylaryl methyl cation. This structure agrees with DFT studies of the kinetically favored diastereomer, although a number of other structures are more stable thermodynamically. Attempts to trap an inserted catalyst through the stoichiometric addition of 2-vinylpyridine or 3-ethoxy-1-propylene led to complicated reactions, but in both cases, experimental and computational data suggest that both processes initiate through insertion of the substrate into the Hf-C Aryl bond. In addition, an activated Hf-dibutyl complex is studied in an attempt to minimize differences in the rates of initiation and propagation, as a butyl group is a reasonable mimic for a propagating polymer chain. Tritylborate activation of this complex cleanly generates 1 equiv of 1-butene in close proximity to the Hf-butyl cation via β-hydride abstraction. This reaction results in formation of isotactic poly(1-butene) with a fairly high molecular weight rather than butene oligomers. The observed molecular weight is consistent with a small fraction of active species, and quenching studies show a similar fraction of butene-modified ligand. These results are consistent with slow insertion into the Hf-C Aryl bond followed by fast polymerization kinetics, with the latter rate constant being 3 orders of magnitude faster than the former.

The integrated molecular orbital-molecular mechanics (IMOMM) method adopting the B3LYP: MM3 combination has been used to study the full catalysts in the diimine-M (M ) Ni, Pd) catalyzed ethylene polymerization reaction. These results have been compared with previous molecular orbital calculations on model systems (model). There is a lowering of the migratory insertion activation barriers when including substituent effects from 9.9 (model) to 3.8 (IMOMM) kcal/mol for nickel and from 16.2 (model) to 14.1 (IMOMM) kcal/mol for palladium. Steric interactions decrease the complexation energy which leads to a lowering of the barrier. The -H transfer process which involves the reaction n-propyl -agostic f olefin hydride f isopropyl -agostic is the likely mechanism leading to branching of polyethylenes. In the nickel system, the olefin-hydride intermediate lies 13.6 (model) or 14.5 (IMOMM) kcal/mol above the n-propyl -agostic species, indicating that this pathway is unlikely for unsubstituted or substituted nickel diimine catalysts. For palladium, where the olefin-hydride intermediate resided 5.4 kcal/mol above the -agostic species in model B3LYP predictions, IMOMM reduces this difference to almost zero, suggesting branching to be more prominent with bulky substituents. Although -H transfer is more likely for substituted palladium, the formation of the 5-coordinate intermediate is not possible due to steric effects and thus an associative chain termination process is not possible for substituted palladium while it likely can occur for unsubstituted Pd catalysts.

Not as radical as you think: The free-radical hydrostannylation of alkynes has been extensively studied and while every published mechanism involves solely radical intermediates, this appears not to be correct. Trace molecular oxygen is necessary for any radical-mediated hydrostannylation to occur with a wide selection of alkynes, thus leading to a proposed hybrid single-electron transfer/radical propagation mechanism. AIBN=2,2'-azobis(2-methylpropionitrile).

A new and less expensive G2-type approach, G2MS, which can be used for accurate energy prediction for up to seven to eight atoms has been proposed and tested against the standard G2 data set. The results compare well with other G2 methods. The G2MS method performs an extrapolation of correlation and basis set effects, while the integrated MO+MO (IMOMO) method provides an extrapolation of electronic and steric effects from a small model to a large real system. Thus, using G2MS as the high-level method in IMOMO is a natural approach to accurate energy predictions for large molecular systems. The G2MS method predicts activation energies for ethylene + butadiene and ethylene + cyclopentadiene of 23.9 and 18.5 kcal/mol, respectively. The IMOMO(G2MS:MP2) method has been used to obtain accurate activation barriers for a number of Diels−Alder reactions, including the dimerization of butadiene where the calculated value of 23.5 kcal/mol is within 1 kcal/mol of two experimental values. For the addition of acrylic acid to 2,4-pentadienoic acid, a nearly quantitative agreement in the branching ratio for the product regio- and stereoisomers has been obtained. Calculations of the activation barriers for larger Diels−Alder reaction systems were performed, including the reaction of maleic anhydride with isoprene and 2-tert-butyl-1,3-butadiene, where the conformation of the reactant diene is found to be an important factor in determining the activation energy.