DFT computational study of the mechanism of allyl halides carbonylation catalyzed by nickel tetracarbonyl

A theoretical investigation at the DFT(B3LYP) level on the carbonylation reaction of allyl bromide catalyzed by nickel tetra-carbonyl Ni(CO)(4) is discussed. The computational results show the following: (i) Three main steps characterize the catalytic cycle: (a) an oxidative addition step, (b) a carbonylation step, and (c) a reductive elimination step where the acyl product is obtained and the catalyst is regenerated. (ii) Both Ni(CO)(3) and Ni(CO)(4) complexes can behave as "active" catalytic species. (iii) The oxidative addition leads to the formation of either eta(3) or eta(1)-allyl nickel complexes, which are involved in a fast equilibrium. (iv) The carbonylation occurs much more easily on the eta(1) than on the eta(3) intermediates.     

New chiral phosphine-phosphite ligands in the enantioselective palladium-catalyzed allylic alkylation

A series of chiral phosphine-phosphite ligands 1-6 have been synthesized and used in the enantioselective palladium-catalyzed reaction of rac-1,3-diphenyl-2-propenyl acetate with dimethyl malonate as nucleophile. Ligands 1a, 2, 3, 5a, 6a, and 6b have been synthesized starting from racemic tert-butylphenylphosphinoborane. The use of dynamically resolved Li phosphide (-)-sparteine provided the optically pure ligands. Crystals of the allylpalladium (6a) complex were obtained, suitable for X-ray crystal structure determination. The X-ray crystal structure of the allylpalladium (6a) complex revealed a longer palladium-carbon bond distance trans to the phosphine moiety indicating that the attack of the nucleophile takes place at the carbon trans to the phosphine moiety. This was confirmed by the fact that the phosphine moiety did not affect the enantioselectivity directly. Under mild reaction conditions, enantioselectivities up to 83% were obtained (25 degrees C) with ligand 1e. Systematic variation of the ligand bridge and the phosphite moiety showed that the configuration of the product is controlled by the atropisomerism of the biphenyl substituent at the phosphite moiety. The conformation of the biphenyl group, in turn, is controlled by the substituent at the chiral carbon in the bridge. Ligands with large bite angles yielded higher enantioselectivities.     

Intercalation of Benzamide into Kaolinite

Well-crystallized kaolinite (K) was initially reacted at 60 degrees C with a water/dimethylsulfoxide (DMSO) mixture and the resulting intercalation derivative (K-DMSO) was characterized by powder X-ray diffractometry (PXRD), thermal analysis (simultaneous TG and DSC), and Fourier-transformed infrared spectroscopy (FTIR). Benzamide crystals were then melted with the K-DMSO derivative at 140 degrees C for 4 days, when a gradual displacement of DMSO by benzamide was observed within the interlayer spacing of the modified kaolinite. The resulting material, after extensive washing with acetone, was characterized and compared to the results obtained previously for the K-DMSO composite. Benzamide intercalation proceeded by gradual displacement of DMSO molecules until completion. The structural stabilization of the K-BZ derivative was explained through the establishment of hydrogen bonds between the carbonyl oxygen atoms of the intercalated benzamide and aluminol groups present at the surface of the kaolinite layer. The interlamellar spacing of K-BZ was shown to be possibly occupied by benzamide molecules that were located at a 68 degrees orientation in relation to the layer surface. Unlike most intercalation molecules such as DMSO, variations in the interplanar spacing of kaolinite were consistent with the nonkeying of any other part of the molecule between the aluminosilicate interlayers. Copyright 2000 Academic Press.     

New computational evidence for the catalytic mechanism of carbonic anhydrase

Some aspects of the catalytic mechanism of HCA have been investigated. Either a zinc-bound water or a zinc-bound hydroxide has been considered as a nucleophile attacking CO ₂. No reaction path exists in the former case, while a transition state for the nucleophilic attack has been located in the latter (barrier of 7.6 kcal mol⁻¹). This activation energy is determined by the breaking of the hydrogen-bond network that shields the zinc-bound hydroxide when the CO ₂ molecule approaches the reaction center. No ambiguity exists about the mechanism for the internal rearrangement of the zinc–bicarbonate complex. The rotation pathway (Lindskog mechanism) proposed by many authors is too energy demanding since it causes the breaking of the hydrogen-bond network around the bicarbonate. The only possible rearrangement mechanism is a proton transfer (Lipscomb) that occurs in two steps (each step corresponding to a double proton transfer) and involves the Thr199 residue as a proton shuttle. Electronic Supplementary Material The online version of this article (doi:) contains supplementary material, which is available to authorized users. Contribution to the Fernando Bernardi Memorial Issue.     

The mechanism of transition metal catalyzed carbonylation of allyl halides: A theoretical investigation

The mechanism of the carbonylation reaction of allyl halides catalyzed by nickel (Ni(CO)₄) and palladium (Cl₂Pd(PPh₃)₂) complexes is theoretically investigated at the DFT level using the hybrid B3LYP functional. The favored reaction path to carbonylation corresponds, for both catalysts, to a direct attack of the halogen on the metal. This affords η¹ intermediates that can undergo the final carbonylation step. It is also possible to obtain the acyl product (β,γ-unsaturated acyl halides) from η² and/or η³ intermediates. However, in this case, the barrier of the rate-determining step to carbonylation is much higher. Since a channel on the potential surface connects rather easily the η² or η³ intermediates to the η¹ intermediates, an alternative and competitive path leading to the acyl products can originate from the η² or η³ intermediates, follow the η²/η³ → η¹ transformation, then undergo the final carbonylation step.     

A DFT computational study of the bis-silylation reaction of acetylene catalyzed by palladium complexes

In this paper we have investigated at the DFT(B3LYP) level the catalytic cycle for the bis-silylation reaction of alkynes promoted by palladium complexes. A model-system formed by an acetylene molecule, a disilane molecule, and the Pd(PH(3))(2) complex has been used. The most relevant features of this catalytic cycle can be summarized as follows: (i) The first step of the cycle is an oxidative addition involving H(3)Si-SiH(3) and Pd(PH(3))(2). It occurs easily and leads to the cis (SiH(3))(2)Pd(PH(3))(2) complex that is 5.39 kcal mol(-1) lower in energy than reactants. (ii) The transfer of the two silyl groups to the C-C triple bond does not occur in a concerted way, but involves many steps. (iii) The cis (SiH(3))(2)Pd(PH(3))(2) complex, obtained from the oxidative addition, is involved in the formation of the first C-Si bond (activation barrier of 18.34 kcal mol(-1)). The two intermediates that form in this step cannot lead directly to the formation of the final bis(silyl)ethene product. However, they can isomerize rather easily (the two possible isomerizations have a barrier of 16.79 and 7.17 kcal mol(-1)) to new more stable species. In both these new intermediates the second silyl group is adjacent to the acetylene moiety and the formation of the second C-Si bond can occur rapidly leading to the (Z)-bis(silyl)ethene, as experimentally observed. (iv) The whole catalytic process is exothermic by 41.54 kcal mol(-1), in quite good agreement with the experimental estimate of this quantity (about 40 kcal mol(-1)).     

Asymmetric Phase‐Transfer‐Catalyzed Intramolecular N‐Alkylation of Indoles and Pyrroles: A Combined Experimental and Theoretical Investigation

Asymmetric phase‐transfer catalysis (PTC) has risen to prominence over the last decade as a straightforward synthetic methodology for the preparation of pharmacologically active compounds in enantiomerically pure form. However, the complex interplay of weak nonbonded interactions (between catalyst and substrate) that could account for the stereoselection in these processes is still unclear, with tentative pictorial mechanistic representations usually proposed. Here we present a full account dealing with the enantioselective phase‐transfer‐catalyzed intramolecular aza‐Michael reaction (IMAMR) of indolyl esters, as a valuable synthetic tool to obtain added‐value compounds, such as dihydro‐pyrazinoindolinones. A combined computational and experimental investigation has been carried out to elucidate the key mechanistic aspects of this process.     

Effect of viscous dissipation on the mixed convection heat transfer from an exponentially stretching surface

The mixed convection flow and heat transfer from an exponentially stretching vertical surface in a quiescent fluid is analyzed using similarity solution technique. Wall temperature and stretching velocity are assumed to have specific exponential function forms. The influence of buoyancy along with viscous dissipation on the convective transport in the boundary layer region is analyzed in both aiding and opposing flow situations. The flow is governed by the mixed convection parameter Gr/Re². The velocity and temperature inside the boundary layer are observed to be influenced by the parameters like Prandtl number Pr, Gebhart number Gb. Significant changes are observed in non-dimensional skin friction and heat transfer coefficients due to viscous dissipation in the medium. The flow and temperature distributions inside the boundary layer are analyzed and the results for non-dimensional skin friction and heat transfer coefficients are discussed through computer generated plots.     

New model for a theoretical density functional theory investigation of the mechanism of the carbonic anhydrase: how does the internal bicarbonate rearrangement occur?

A theoretical density functional theory (DFT, B3LYP) investigation has been carried out on the catalytic cycle of the carbonic anhydrase. A model system including the Glu106 and Thr199 residues and the "deep" water molecule has been used. It has been found that the nucleophilic attack of the zinc-bound OH on the CO(2) molecule has a negligible barrier (only 1.2 kcal mol(-1)). This small value is due to a hydrogen-bond network involving Glu106, Thr199, and the deep water molecule. The two usually proposed mechanisms for the internal bicarbonate rearrangement have been carefully examined. In the presence of the two Glu106 and Thr199 residues, the direct proton transfer (Lipscomb mechanism) is a two-step process, which proceeds via a proton relay network characterized by two activation barriers of 4.4 and 9.0 kcal mol(-1). This pathway can effectively compete with a rotational mechanism (Lindskog mechanism), which has a barrier of 13.2 kcal mol(-1). The fast proton transfer found here is basically due to the effect of the Glu106 residue, which stabilizes an intermediate situation where the Glu106 fragment is protonated. In the absence of Glu106, the barrier for the proton transfer is much larger (32.3 kcal mol(-1)) and the Lindskog mechanism becomes favored.     

3-bromozinc propenyl esters: an experimental and theoretical study of the unique stereocrossover observed in their addition to aromatic and aliphatic aldehydes

We report the results of a combined experimental and theoretical study on the reaction of 3-bromopropenyl acetate in the presence of zinc with three different aldehydes (i.e., benzaldehyde, 2-methylpropanal, and cyclohexanecarboxaldehyde). A 80% de in favor of the anti product has been experimentally observed with both saturated aldehydes, while for benzaldehyde, a 1:1 syn/anti ratio has been found. DFT computations show the existence of three eta1-allylic organozinc complexes [gamma-(Z)-5a, gamma-(E)-5a, and alpha-5a], very close in energy. Only gamma-(Z)-5a and gamma-(E)-5a lead to the observed product. The computational investigation of the reaction of these allylic organozinc complexes with benzaldehyde and 2-methylpropanal demonstrates in both cases the existence of two competitive reaction paths leading to the syn and anti adducts, respectively. An anti preference has been found for 2-methylpropanal with both gamma-(Z)-5a and gamma-(E)-5a species (a diastereoselectivity larger than 80% is predicted), in agreement with the experiment. With benzaldehyde, while the reaction of gamma-(Z)-5a retains an anti-stereopreference (de = 70%), that involving gamma-(E)-5a is characterized by two degenerate transition states. In this case, the agreement between computations and experiments would be satisfactory under the assumption that the initial oxidative addition affords the gamma-(E)-5a zinc complex only. Additional MP2 computations have demonstrated that pi-stacking interactions can play a significant role in determining the relative energy of the transition states leading to the syn and anti products.