Regio- and chemoselective direct generation of functionalized aromatic aluminum compounds using aluminum ate base

A regio- and chemoselective direct method for generation of functionalized aromatic aluminum compounds through deprotonative directed ortho-alumination using triisobutyl(tetramethylpiperidino)aluminate (iBu3Al(TMP)Li), prepared by mixing of triisobutylaluminum (iBu3Al) and lithium tetramethylpiperidide (LTMP) in THF, has been developed. Deprotonative alumination of various functionalized benzenes with the use of iBu3Al(TMP)Li proved effective for the direct generation of various ortho-functionalized aluminum aromatic and heteroaromatic derivatives, particularly those with electrophilic functional groups such as cyano, amide, and halogen. Direct alumination, followed by electrophilic trapping (with I2), provided a convenient preparative method for 1,2- or 1,2,3-multisubstituted aromatic compounds. The functionalized aromatic aluminate intermediate also was found to undergo copper- and palladium-catalyzed C-C bond-forming reactions very efficiently and highly regio- and chemoselectively.     

Deprotonative metalation using ate compounds: synergy, synthesis, and structure building

Historically, single-metal organometallic species such as organolithium compounds have been the reagents of choice in synthetic organic chemistry for performing deprotonation reactions. Over the past few years, a complementary new class of metalating agents has started to emerge. Owing to a variable central metal (magnesium, zinc, or aluminum), variable ligands (both in their nature and number), and a variable second metallic center (an alkali metal such as lithium or sodium), "ate" complexes are highly versatile bases that exhibit a synergic chemistry which cannot be replicated by the homometallic magnesium, zinc, or aluminum compounds on their own. Deprotonation accomplished by using these organometallic ate complexes has opened up new perspectives in organic chemistry with unprecedented reactivities and sometimes unusual and unpredictable regioselectivities.     

Rozen's epoxidation reagent, CH3CN.HOF: a theoretical study of its structure, vibrational spectroscopy, and reaction mechanism

Rozen's epoxidation reagent, CH(3)CN.HOF, and a prototype epoxidation reaction employing it, have been subjected to an extensive ab initio and density functional study. Its anharmonic force field reveals a very strong red shift for the OH stretch and a strong blue shift for the HOF bend, in semiquantitative agreement with experiment. The very strong hydrogen bond (8.20 kcal/mol at the W1 level) not only serves to stabilize the reactant but also considerably lowers the barrier height for epoxidation of ethylene. Moreover, the reaction byproduct HF is found to act autocatalytically. The OH moiety acquires HO(+) character in the transition state. Our W1 benchmark data for the reaction profile allow the performance of various DFT functionals to be assessed. In general, "kinetics" functionals overestimate barrier heights, the BMK functional less so than the others. The B1B95 and TPSS33B95 meta-GGA functionals both perform very well, whereas general-purpose hybrid GGAs underestimate barrier heights. The simple PBE0 functional does reasonably well.     

Gold catalysts for pure hydrogen production in the water-gas shift reaction: activity, structure and reaction mechanism

The production of hydrogen containing very low levels of carbon monoxide for use in polymer electrolyte fuel cells requires the development of catalysts that show very high activity at low temperatures where the equilibrium for the removal of carbon monoxide using the water-gas shift reaction is favourable. It has been claimed that oxide-supported gold catalysts have the required high activity but there is considerable uncertainty in the literature about the feasibility of using these catalysts under real conditions. By comparing the activity of gold catalysts with that of platinum catalysts it is shown that well-prepared gold catalysts are significantly more active than the corresponding platinum catalysts. However, the method of preparation and pre-treatment of the gold catalysts is critical and activity variations of several orders of magnitude can be observed depending on the methods chosen. It is shown that an intimate contact between gold and the oxide support is important and any preparative procedure that does not generate such an interaction, or any subsequent treatment that can destroy such an interaction, may result in catalysts with low activity. The oxidation state and structure of active gold catalysts for the water-gas shift reaction is shown to comprise gold primarily in a zerovalent metallic state but in intimate contact with the support. This close contact between small metallic gold particles and the support may result in the "atoms" at the point of contact having a net charge (most probably cationic) but the high activity is associated with the presence of metallic gold. Both in situ XPS and XANES appear unequivocal on this point and this conclusion is consistent with similar measurements on gold catalysts even when used for CO oxidation. In situ EXAFS measurements under water gas shift conditions show that the active form of gold is a small gold cluster in intimate contact with the oxide support. The importance of the gold/oxide interface is indicated but the possible role of special sites (e.g., edge sites) on the gold clusters cannot be excluded. These may be important for CO oxidation but the fact that water has to be activated in the water gas shift reaction may point towards a more dominant role for the interfacial sites. The mechanism of the water gas shift reaction on gold and other low temperature catalysts has been widely investigated but little agreement exists. However, it is shown that a single "universal" model is consistent with much of the experimental literature. In this, it is proposed that the dominant surface intermediate is a function of reaction conditions. For example, as the temperature is increased the dominant species changes from a carbonate or carboxylate species, to a formate species and eventually at high temperatures to a mechanism that is characteristic of a redox process. Similar changes in the dominant intermediate are observed with changes in the gas composition. Overall, it is shown that reported variations in the kinetics, structure and reaction mechanism for the water gas shift reaction on gold catalysts can now be understood and rationalised.     

Cascade reaction of imines with phenylsulfinylallene. X-ray structure of the product and its formation mechanism

The structure of the product derived from the reaction of a dihydropyridine derivative with phenylsulfinylallene has been clarified by a single crystal X-ray analysis and the formation mechanism is discussed on the basis of the reaction-path calculations by semiempirical and ab initio molecular orbital methods.     

Synthesis,structure, and properties of Schiff base iodobismuthate and its alteration in DMSO solution

New hybrid iodobismuthates (C₁₂H₁₈N₂)(BiI₄)₂ (1, C₁₂H₁₈N₂ is N,N´-bis(1- methylethylidene)-1,4-benzоdiammonium dication) and (C₆H₄(NH₃)₂) [Bi₂I₈?2DMSO]?4DMSO (2, C₆H₄(NH₃)₂ is p-phenylenediammonium dication) were synthesized and their crystal structures were established. Compound 1 crystallizes in the space group P2₁/n, a = 7.632(4), b = 13.471(6), c = 14.556(5) Å, β = 93.57(4)°; compound 2 crystallizes in the space group P\(\overline 1 \), a = 9.208(4), b = 12.203(5), c = 13.600(5) Å, α = 110.20(3)°, β = 97.28(4)°, γ = 110.04(4)°. In the crystal structure of compound 1, one-dimensional infinite BiI ₄ ^– polyanions are linked into a three-dimensional structure by N,N'-bis(1-methylethylidene)- 1,4-benzоdiammonium dications through N–H…I and C–H…I hydrogen bonds. Compound 1 is stable up to 200 °С, optical studies showed that it has a band gap of 2.15 eV. The recrystallization of compound 1 from DMSO is accompanied by the transformation of chain BiI ₄ ^– anions to binuclear [Bi₂I₈?2DMSO]^2– anions, while the azomethine cation decomposes to form a p-phenylenediammonium dication.     

The reaction between diacetyl and hydrogen peroxide: its mechanism and kinetic constants

The reaction between diacetyl and hydrogen peroxide in aqueous solution was investigated using an adiabatic reaction calorimeter. The kinetic constants are calculated using procedures based on the temperature-time behaviour of the reaction system.The mechanism of the reaction proposed fits very well with the reaction-rate equations used and the observed temperature-time data.     

New crystalline aluminum alkoxide oxide fluorides: evidence of the mechanism of the fluorolytic sol-gel reaction

This study reports three new crystalline aluminum isopropoxide oxide fluorides with molar ratios of Al:F equal to 1:1 and 1:1.25. These are the first three representatives isolated without the incorporation of external donor molecules. Compound 1 Al(4)F(4)(μ(4)-O)(μ-O(i)Pr)(5)[H(O(i)Pr)(2)] contains a tetranuclear unit consisting of two different five fold coordinated AlFO(4)-units, with F exclusively in the terminal position. Compound 2, Al(4)F(4)(μ(4)-O)(μ-O(i)Pr)(5)[H(O(i)Pr)(2)]·Al(5)F(5)(μ(5)-O)(μ-O(i)Pr)(8), contains both a tetranuclear unit (as in 1) and a pentanuclear Al-unit. Al-atoms in the latter are five- and six fold coordinated. Compound 3, Al(16)F(20)(μ(4)-O)(4)(μ-O(i)Pr)(20)·2((i)PrOH), exhibits a slightly higher fluorination degree and contains an oligomeric chain of four F-linked tetranuclear Al-units. In addition to X-ray structure analysis, compound 1 was characterized by different solid state MAS NMR techniques, including (27)Al triple quantum MAS NMR and (1)H, (1)H→(13)C CP, (19)F and (27)Al MAS NMR. On the basis of the collected data, a reliable decomposition of (27)Al single pulse MAS NMR spectra and an unambiguous assignment of the resonances to the respective structural AlFO(4)-units are given. The new crystalline aluminum isopropoxide oxide fluorides are direct evidence of the fluorolytic sol-gel mechanism previously discussed.     

Site-selective iodine-magnesium exchange reaction of 1,4-diiodo-1,3-alkadienes by an organomagnesium ate complex,and its application to synthesis of polysubstituted styrenes and phenols

When 1,4-diiodo-1,3-alkadienes were treated with i-PrBu2MgLi, highly site-selective iodine-magnesium exchange reaction took place to afford, after the reaction with electrophiles, a variety of 1-iodo-1,3-alkadiene derivatives. Synthetic utility of the reaction was demonstrated by an efficient preparation of polysubstituted styrenes and phenols.     

Epoxide polymers: Synthesis,stereochemistry, structure,and mechanism

Epoxide polymerization studies have yielded technically important catalysts and polymers. The polymers were studied by cleaving them with Group IA organometallics to monomer, dimer, and trimer glycol fragments. The identification of these glycol fragments has established that the crystalline polymers from the cis- and trans-2,3-epoxybutanes are respectively racemic and meso-diisotactic and that the amorphous polymer from the cis-oxide is disyndiotactic. These studies also showed that the amorphous fraction from propylene oxide polymerization with coordination catalysts contains substantial head-to-head and tail-to-tail segments. This work has led to a much better understanding of the mechanism of epoxide polymerization. These facts were established: (1) epoxides polymerize with inversion of configuration of the ring-opening carbon atom; (2) monosubstituted epoxides polymerize largely by attack on the primary carbon with a coordination catalyst; and (3) two or more metal atoms must be involved in the coordination polymerization of epoxides.     

Coenzyme biosynthesis: enzyme mechanism, structure and inhibition

This review highlights five key reactions in vitamin biosynthesis and in particular focuses on their mechanisms and inhibition and insights from structural studies. Each of the enzymes has the potential to be a target for novel antimicrobial agents.     

Nitrogen-doped carbon nanotubes: growth, mechanism and structure

Nitrogen-doped bamboo-structured carbon nanotubes have been successfully grown using a series of cobalt/molybdenum catalysts. The morphology and structure of the nanotubes were analysed by transmission electron microscopy and Raman spectroscopy. The level of nitrogen doping, as determined by X-ray photoelectron spectroscopy, was found to range between 0.5 to 2.5 at.%. The growth of bamboo-structured nanotubes in the presence of nitrogen, in preference to single-walled and multi-walled nanotubes, was due to the greater binding energy of nitrogen for cobalt in the catalyst compared to the binding strength of carbon to cobalt, as determined by density functional theory.     

On the early stage of aluminum oxidation: An extraction mechanism via oxygen cooperation

We propose a barrierless mechanism for describing the oxidation of Al(111) in which oxygen atoms located on the outer surface extract aluminum atoms of the surface layers through local cooperation of other pre-adsorbed oxygen atoms. We show the details of this complex chemical process that kinetically competes with the non-destructive formation of an oxygen monolayer onto the Al surface, thus elucidating the initial aluminum oxidation regime. We demonstrate that further stripping of the complete surface Al layer is consistent with both (i) the formation of a defective alumina structure and (ii) an oxide capping layer preventing further oxidation at low temperature.     

Ab initio study of aluminum chemical vapor deposition from dimethylaluminum hydride: a gas phase reaction mechanism

The effect of preheating of dimethylaluminum hydride (DMAH) as a gas on the epitaxial growth in aluminum chemical vapor deposition (Al-CVD) is studied theoretically. The chemical changes of DMAH in the gas phase such as unimolecular decomposition reactions, bimolecular reactions and polymerizations are treated using ab initio molecular orbital method (MP2/6-31G**) and density functional theory (B3P86/LanL2DZ). The gas phase equilibrium composed of the previous reaction products under the usual experimental conditions for Al-CVD is also investigated in detail as the initial stage of the CVD process. From the energetics point of view, unimolecular decomposition reactions and bimolecular reactions hardly occur, however, polymerizations of DMAH take place readily at the low temperatures found in Al-CVD. A large amount of DMAH-dimer and a small amount of DMAH-monomer and trimer coexist in the equilibrium state.