Polyurethanes (PUs) have many applications resulting from their preeminent properties, but being commonly used toxic catalysts, and the lack of processability for PU thermosets cause limitations. Herein, we report a new class of the PU-like dynamic covalent polymers, poly(oxime-urethanes) (POUs), which are prepared from the uncatalyzed polyaddition of multifunctional oximes and hexamethylene diisocyanate (HDI) at ambient temperature. Kinetics studies reveal that almost complete polymerization (∼99% conversion) can be achieved in 3 h at 30 °C in dichloromethane (DCM), the most effective among the solvents evaluated, producing linear POUs with comparable molecular weights to the catalyzed PUs. We find that the oxime-carbamate structures are reversible at about 100 °C through oxime-enabled transcarbamoylation via a thermally dissociative mechanism. The cross-linked POUs based on oxime-carbamate bonds show efficient catalyst-free healable/recyclable properties. Density functional theory (DFT) calculations suggest that the fast oxime-urethanation and the mild thermoreversible nature are mediated by the characteristic nitrone tautomer of the oxime. Given widespread urethane-containing materials, POUs are of promising potential in applications because of the excellent mechanical performances, facile preparation, and dynamic property without using catalysts.

The asymmetric O-H insertion reaction is an ideal synthetic strategy for preparing optically pure alpha-alkoxy, alpha-aryloxy, and alpha-hydroxy carboxylic acid derivatives, which are valuable building blocks for the construction of natural products and other biologically active molecules. Surprisingly, to date there have been no reports of significant levels of enantiocontrol in the O-H insertions using chiral dirhodium(II) catalysts, which are powerful for asymmetric C-H insertions. Only recently, through the use of chiral copper catalysts, have highly enantioselective insertions of alpha-diazocarbonyl compounds into O-H bonds been achieved. To explain these interesting phenomena, density functional theory calculations have been conducted. The results show that in the Cu(I)-catalyzed system, the [1,2]-H shift process (the stereocenter formation step) favors the copper-associated ylide pathway. This ensures that when a chiral copper complex is used as the catalyst, the stereocenter forms in a chiral environment, which is the prerequisite for achieving enantioselectivity. In contrast, the free-ylide pathway is favored in the Rh(II)-catalyzed system. This significant difference renders the copper(I) complexes more competent than the dirhodium(II) complexes in catalytic asymmetric O-H insertions. In addition, it has been found for the first time that in transition-metal-catalyzed X-H insertions, water acts as an efficient proton-transport catalyst for the [1,2]-H shift.

Through the joint forces of computation and experiment, the detailed mechanism of the Lu phosphine catalyzed (3 + 2) cycloaddition of allenoates and alkenes has been elucidated. The overall potential energy surface of the Lu (3 + 2) reaction has been computed. More importantly, theory and experiment have confirmed that a trace amount of water plays a critical role in assisting the process of [1,2] proton shift in the Lu reaction.

With the aid of computations and experiments, the detailed mechanism of the phosphine-catalyzed [3+2] cycloaddition reactions of allenoates and electron-deficient alkenes has been investigated. It was found that this reaction includes four consecutive processes: 1) In situ generation of a 1,3-dipole from allenoate and phosphine, 2) stepwise [3+2] cycloaddition, 3) a water-catalyzed [1,2]-hydrogen shift, and 4) elimination of the phosphine catalyst. In situ generation of the 1,3-dipole is key to all nucleophilic phosphine-catalyzed reactions. Through a kinetic study we have shown that the generation of the 1,3-dipole is the rate-determining step of the phosphine-catalyzed [3+2] cycloaddition reaction of allenoates and electron-deficient alkenes. DFT calculations and FMO analysis revealed that an electron-withdrawing group is required in the allene to ensure the generation of the 1,3-dipole kinetically and thermodynamically. Atoms-in-molecules (AIM) theory was used to analyze the stability of the 1,3-dipole. The regioselectivity of the [3+2] cycloaddition can be rationalized very well by FMO and AIM theories. Isotopic labeling experiments combined with DFT calculations showed that the commonly accepted intramolecular [1,2]-proton shift should be corrected to a water-catalyzed [1,2]-proton shift. Additional isotopic labeling experiments of the hetero-[3+2] cycloaddition of allenoates and electron-deficient imines further support this finding. This investigation has also been extended to the study of the phosphine-catalyzed [3+2] cycloaddition reaction of alkynoates as the three-carbon synthon, which showed that the generation of the 1,3-dipole in this reaction also occurs by a water-catalyzed process.

A computational study with the Becke3LYP density functional was carried out to elucidate the mechanisms of Au(I)-catalyzed reactions of enynyl acetates involving tandem [3,3]-rearrangement, Nazarov reaction, and [1,2]-hydrogen shift. Calculations indicate that the [3,3]-rearrangement is a two-step process with activation free energies below 10 kcal/mol for both steps. The following Nazarov-type 4pi electrocyclic ring-closure reaction of a Au-containing dienyl cation is also easy with an activation free energy of 3.2 kcal/mol in CH2Cl2. The final step in the catalytic cycle is a [1,2]-hydride shift, and this step is the rate-limiting step (with a computed activation free energy of 20.2 kcal/mol) when dry CH2Cl2 is used as the solvent. When this tandem reaction was conducted in wet CH2Cl2, the [1,2]-hydride shift step in dry solution turned to a very efficient water-catalyzed [1,2]-hydrogen shift mechanism with an activation free energy of 16.4 kcal/mol. Because of this, the tandem reaction of enynyl acetates was found to be faster in wet CH2Cl2 as compared to the reaction in dry CH2Cl2. Calculations show that a water-catalyzed [1,2]-hydrogen shift adopts a proton-transport catalysis strategy, in which the acetoxy group in the substrate is critical because it acts as either a proton acceptor when one water molecule is involved in catalysis or a proton-relay stabilizer when a water cluster is involved in catalysis. Water is found to act as a proton shuttle in the proton-transport catalysis strategy. Theoretical discovery of the role of the acetoxy group in the water-catalyzed [1,2]-hydrogen shift process suggests that a transition metal-catalyzed reaction involving a similar hydrogen shift step can be accelerated in water or on water with only a marginal effect, unless a proton-accepting group such as an acetoxy group, which can form a hydrogen bond network with water, is present around this reaction's active site.

Asymmetric hydrogenation of quinolines catalyzed by chiral cationic η(6)-arene-N-tosylethylenediamine-Ru(II) complexes have been investigated. A wide range of quinoline derivatives, including 2-alkylquinolines, 2-arylquinolines, and 2-functionalized and 2,3-disubstituted quinoline derivatives, were efficiently hydrogenated to give 1,2,3,4-tetrahydroquinolines with up to >99% ee and full conversions. This catalytic protocol is applicable to the gram-scale synthesis of some biologically active tetrahydroquinolines, such as (-)-angustureine, and 6-fluoro-2-methyl-1,2,3,4-tetrahydroquinoline, a key intermediate for the preparation of the antibacterial agent (S)-flumequine. The catalytic pathway of this reaction has been investigated in detail using a combination of stoichiometric reaction, intermediate characterization, and isotope labeling patterns. The evidence obtained from these experiments revealed that quinoline is reduced via an ionic and cascade reaction pathway, including 1,4-hydride addition, isomerization, and 1,2-hydride addition, and hydrogen addition undergoes a stepwise H(+)/H(-) transfer process outside the coordination sphere rather than a concerted mechanism. In addition, DFT calculations indicate that the enantioselectivity originates from the CH/π attraction between the η(6)-arene ligand in the Ru-complex and the fused phenyl ring of dihydroquinoline via a 10-membered ring transition state with the participation of TfO(-) anion.

Transition-metal-catalyzed alkene oligomerizations usually yield mixtures of alkenes in the C 4 -C 26 range.[1] It is tedious to separate these mixtures into pure compounds. Consequently, much effort from both industrial and academic communities has been devoted to the search for more efficient catalysts that will produce terminal alkenes of specific lengths. The trimerization of ethene to 1-hexene is of special industrial significance since 1-hexene is an important comonomer in the preparation of linear low-density polyethylene.[2] Most catalysts known today are based on chromium compounds. [3,4] Recently, Sen and co-workers discovered that [TaCl 3 R 2 ], generated in situ by the reaction of TaCl 5 with alkyl metal compounds such as CH 3 Li, can also function as trimerization catalysts under mild conditions (47.6 atm, 40-60 8C).[5] It has been postulated that metallacycles are involved in alkene trimerizations, [3e,f, 6-8] but there has not been detailed information about the potential energy surfaces (PESs) and the structures of the transition states (TSs) and intermediates. Here we report MP2 and B3LYP calculations on the detailed mechanism of the olefin trimerization catalyzed by Sen's catalyst [TaCl 3 (CH 3 ) 2 ]. [9,15] We also explored why this catalyst selectively trimerizes rather than dimerizes or polymerizes alkenes, and have uncovered a novel mechanism for the decomposition of dialkyl complexes of tantalum.Dialkyltantalum complexes [TaCl 3 (R 1 )(R 2 )] (R 1 and R 2 are alkyl groups) are most stable as trigonal bipyramids with two electronegative Cl ligands in the axial positions (TB-1 conformation, see 1 in Figure 1 and Supporting Information). The TSs and intermediates (except 9, see below) involved in the trimerization process adopt similar TB-1 conformations.The insertion of ethene into 1 proceeds directly via TS2 to give 3 (Figure 1). No complex of ethene with 1 was found, in contrast to the usual Cossee mechanism.[16] The lack of ethene complexation may be explained by the crowded TB-1 conformation, the coordinately saturated Ta center of 1, and the lone-pair-p repulsion between Cl ligands and the incoming ethene. Subsequently, 3 undergoes b elimination to afford 5 with liberation of methane. This step is a novel agosticassisted hydride shift (see below), and no minimum for a metal-hydride intermediate could be found. A similar mechanism was proposed by Negishi et al. for the transformation of dialkylzirconocenes to zirconocene-alkene complexes. [17a,b] The catalytic cycle shown in Figure 2 starts with the coordination of a second ethene molecule to the tantalumethylene complex 6, producing 7. Subsequently, facile ring closure via TS8 transforms 7 to a tantalacyclopentane intermediate, which initially has the TB-1 conformation of 9* but easily converts into the more stable 9 with a pseudosquare-pyramidal structure. Intermediate 9 is transformed to tantalacycloheptane 11 by ethene insertion via TS10. The activation energy (in terms of DE 0 ) of this step is 13.0 kcal mol À1 (25.6 kcal mol À1 ...

DFT calculations have been applied to investigate the reaction mechanism of rhodium dimer, [Rh(CO)2Cl]2, catalyzed intermolecular (5 + 2) reactions between vinylcyclopropanes and alkynes. The catalytic species is Rh(CO)Cl and the catalytic cycle is through the sequential reactions of cyclopropyl cleavage of vinylcyclopropane, alkyne insertion (rate-determining step), and a migratory reductive elimination.