Chemoselective reduction of aldehydes and ketones by potassium diisobutyl-t-butoxy aluminum hydride (PDBBA)

t-Butoxy derivatives of DIBALH [lithium diisobutyl-t-butoxyaluminum hydride (LDBBA), sodium diisobutyl-t-butoxyaluminum hydride (SDBBA), and potassium diisobutyl-t-butoxyaluminum hydride (PDBBA)] were examined as chemoselective reducing agents of carbonyl compounds. Among them, PDBBA was found to be the most efficient for the reduction of aldehydes and ketones to the corresponding alcohols in the presence of ester, amide, and nitrile substituents at ambient temperature. In addition, the optimal conditions gave higher chemoselectivity for aldehydes in the presence of ketones.     

Lithium diisobutyl-t-butoxyaluminum hydride, a new and efficient reducing agent for the conversion of esters to aldehydes

Lithium diisobutyl-t-butoxyaluminum hydride (LDBBA), easily prepared by reaction of lithium t-butoxide with DIBALH, readily reacts with common aromatic and aliphatic esters to give the corresponding aldehydes in 74-88% yield at 0 °C. Especially, this reagent proved to be the most effective partial reducing agent for conversion of isopropyl esters to aldehydes, in most cases, with >90% yield under the same reaction temperature.     

Reaction of lithium tris(tert-butylthiolato)hydridoaluminate with selected organic compounds containing representative functional groups

The approximate rates and stoichiometry of the reaction of lithium tri(2-methyl-2-propanethioxy)aluminum hydride (LTBSA) with selected organic compounds containing representative functional groups under the standardized conditions (tetrahydrofuran, 0 °C) were studied in order to define the reducing ability of LTBSA also compared with those of the parent lithium aluminum hydride (LAH) and lithium t-butoxyaluminum hydride (LTBA).     

Facile transformation of esters to nitriles

Various esters were efficiently converted into the corresponding nitriles in good yields by the treatment with sodium diisobutyl-tert-butoxyaluminium hydride (SDBBA-H), followed by treatment with molecular iodine in aq ammonia. The present one-pot method is very efficient and practical for the conversion of various aromatic and aliphatic esters into the corresponding nitriles.     

1,4-Addition of an aryllithium reagent to diethyl ketomalonate. Scalable synthesis of ethyl 1-(hydroxymethyl)-1,3-dihydroisobenzofuran-1-carboxylate

While optimizing the synthesis of pharmaceutical building block 3 [ethyl 1-(hydroxymethyl)-1,3-dihydroisobenzofuran-1-carboxylate], we encountered an unusual addition of an aryllithium reagent to the ketone oxygen atom of diethyl ketomalonate. Compound 3 was ultimately prepared on a large scale by a two-step sequence involving (1) annulation of a functionalized Grignard reagent with diethyl ketomalonate and (2) selective mono-reduction of a geminal diester using lithium tri-tert-butoxyaluminum hydride.     

Comparative reductive desymmetrization of 2,2-disubstituted-cycloalkane-1,3-diones

Reductive desymmetrization of 2-methyl-2-substituted-cycloalkane-1,3-diones can be effected using either NaBH₄ in DME or lithium tri-tert-butoxyaluminum hydride (LTBA) in THF at −60 °C. The former is a new approach that offers slightly greater diastereoselectivity in the reduction of 2,2-disubstituted-cyclopentane-1,3-diones while LTBA is superior with 2,2-disubstituted-cyclohexane-1,3-diones. Both conditions minimize subsequent reduction to diols thereby furnishing high yields of 1,3-ketols. Particularly rapid monoreductions are observed with 2-methyl-2-nitroethylcyclopentane-1,3-dione and 2-cyanoethyl-2-methylcyclopentane-1,3-dione when treated with NaBH₄ in DME at −60 °C. As expected, diastereoselectivity varies considerably with the substitution at C-2.     

Three-membered ring formation—IV : Selective syntheses of halogenocyclopropanedicarboxylic esters possessing different alcohol residues

Selective syntheses for some 1-halogeno-2-alkyl-cis-1,2-cyclopropanedicarboxylic esters were studied. In a ring formation between α-halogenoacetic and α-propylacrylic esters, lithium hydride was a much better condensing agent than sodium hydride for the selective syntheses of cyclopropanedicarboxylic ester possessing two different alcohol residues. A greater reactivity was found at the ester group linked to carbon 1 than that linked to carbon 2 in transesterification by alkali metal t-butoxide and also in its partial hydrolysis by KOH. This is presumably ascribed to the difference in the inductive effects of alkyl group and the Cl atom.     

Influence des interactions de torsion dans les reactions d'addition sur les cyclohexanones. interpretation generale du deroulement sterique des reactions des hydrures ou des organomagnesiens sur les cyclohexanones

The addition of allyl and butenyl Grignard reagents to 4-t-butylcyclohexanone affords a greater proportion of the equatorial alcohols than is formed from the corresponding saturated Grignard reagents (propyl and s-butyl). In the case of the allyl Grignard reagent, the equatorial alcohol is the major product. Lithium aluminium hydride reduction in ether at 0° of cis and trans 4-t-butyl-2-methyl-cyclohexanone and cis and trans 4-t-butyl-2-chlorocyclohexanone affords mixtures of epimeric alcohols in ratios of equatorial to axial OH 8.4:1, 20:1, 4.2:1, and >70:1 respectively. These results fit in well with the view that polar interactions and torsional strain are both important in determining the steric course of the reaction.     

ESR study of the thermal decomposition of di-tert-butoxy-tert-butyl alumotrioxide formed in the reaction of tri-tert-butoxyaluminum with tert-butyl hydroperoxide

Tri-tert-butoxyaluminum reacts with tert-butyl hydroperoxide to produce di-tert-butoxy-tert-butyl alumotrioxide, which decomposes heterolytically to form singlet dioxygen and homolytically with the O—O bond cleavage. The Bu^tOO^·, (Bu^tO)₂AlOO^·, Bu^tO^·, and (Bu^tO)₂AlO^· radicals were identified by ESR using spin traps. These findings confirm the formation of aluminum-containing trioxide. The above radicals initiate alkylarene oxidation by the tri-tert-butoxyaluminum—tert-butyl hydroperoxide system. The carbon-centered and alkylperoxy radicals originated from the oxidized substrates were identified.     

Enantioselective hydrosilylation of prochiral ketones catalyzed by chiral BINAP-copper(I) complexes

The CuCl/NaOt-Bu/BINAP system was found to efficiently catalyze the hydrosilylation of aryl alkyl ketones with excellent enantioselectivities by using phenyl methyl silane as a stoichiometric hydride source. High enantiomeric excesses (up to 97%) and excellent yields (up to 99%) were obtained.     

Investigation of main group promoted carbon dioxide reduction

The reduction of carbon dioxide (CO₂) is of interest to the chemical industry, as many synthetic materials can be derived from CO₂. To help determine the reagents needed for the functionalization of carbon dioxide this experimental and computational study describes the reduction of CO₂ to formate and CO with hydride, electron, and proton sources in the presence of sterically bulky Lewis acids and bases. The insertion of carbon dioxide into a main group hydride, generating a main group formate, was computed to be more thermodynamically favorable for more hydridic (reducing) main group hydrides. A ten kcal/mol increase in hydricity (more reducing) of a main group hydride resulted in a 35% increase in the main group hydride's ability to insert CO₂ into the main group hydride bond. The resulting main group formate exhibited a hydricity (reducing ability) about 10% less than the respective main group hydride prior to CO₂ insertion. Coordination of a second identical Lewis acid to a main group formate complex further reduced the hydricity by about another 20%. The addition of electrons to the CO₂ adduct of ^tBu₃P and B(C₆F₅)₃ resulted in converting the sequestered CO₂ molecule to CO. Reduction of the CO₂ adduct of ^tBu₃P and B(C₆F₅)₃ with both electrons and protons resulted in only proton reduction.     

New chiral lithium aluminum hydrides based on biphenyl-2,2′-bisfenchol (BIFOL): structural analyses and enantioselective reductions of aryl alkyl ketones

A series of new chiral lithium aluminum hydrides based on BIFOL (biphenyl-2,2′-bisfenchol) and various alkyl alcohols (i.e., methanol, n-butanol, tert-butanol yielding BIFAl-H's) was synthesized and characterized by single crystal X-ray analyses. These investigations point to alkoxide redistribution for BIFAl-H-(O-tBu) (biphenyl-2,2′-bisfenchol aluminum hydride) species. The new BIFAl-H reagents are suitable to reduce aryl alkyl ketones with up to 62% ee. Computational transition structure analyses help to explain the experimentally observed enantioselectivities.     

Synthesis of metal/polymer nanocomposite by UV-radiation curing

Nanocomposite polymers containing bismuth nanoparticles (2 wt%) have been obtained by photopolymerization of acrylic resins. The bismuth nanoparticles have been synthesized by reduction of BiCl₃ with t-BuONa activated sodium hydride. In situ t-BuONa stabilization protects the metallic particles against aggregation. Transmission electron microscopy (TEM) analysis has shown that the bismuth nanoparticles are well dispersed in the acrylic resin. The curing process was followed quantitatively by infrared spectroscopy through the decrease upon UV exposure of the IR bands characteristic of the functional groups. The bismuth nanoparticles were found to have no detrimental effect on the photopolymerization kinetics. Dynamic mechanical analysis (DMA) has shown that the viscoelastic properties of the nanocomposite photopolymer are significantly modified in comparison with corresponding UV-cured polymer. The addition of metal nanoparticles was found to greatly reduce the gloss of UV-cured coatings.     

Exploiting in situ hydride trapping in tungsten coil atomizer for Se and As determination in biological and water samples

A flow injection hydride manifold was coupled to a 150 W tungsten coil electrothermal atomizer for in situ hydride collection followed by selenium and arsenic determination by ET AAS. Rhodium (200 μg), thermally reduced over the double layer tungsten atomizer, was very efficient at collecting selenium or arsenic hydrides. Prior to analysis, biological samples were digested in closed-vessels microwave digestion system. Prior to the hydride formation, both selenium and arsenic were reduced to valence state (IV) and (III), respectively. The detection limit was 35 ng L⁻¹ for selenium and 110 ng L⁻¹ for arsenic. Sample throughput was 70 h⁻¹ using 30 s of hydride trapping time. Method accuracy was evaluated by analyzing biological-certified reference materials from the National Institute of Standard and Technology (SRM-1577a and SRM-1577b “bovine liver” and RM-8414 “bovine muscle powder”) and from the International Agency for Energy Atomic (A-13 “animal blood”) and one water-certified reference material from the National Institute of Standard and Technology (SRM-1640 trace elements in natural water). By applying a t-test, there was no significant difference at the 95% probability level between the results obtained with the proposed method and those certified values.     

Selective oxo-functionalisation of C–H bond with t-BuOOH catalysed by [RuIII(amp)(bipy)Cl] complex (H2amp=N-(hydroxyphenyl)salicyldimine; bipy=2,2′bipyridyl)

[Ru^III(amp)(bipy)Cl] complex (1) has been synthesised and characterised by physico–chemical methods. Complex-1 is found to be an effective catalyst in the oxidation of cyclohexene to cyclohexene-1-ol, cyclohexane to cyclohexanol and cyclohexanone, stilbenes to stilbene epoxides and benzaldehyde upon reaction with tert-butylhydroperoxide (t-BuOOH). A high valent Ru(V)-oxo species formed as a catalytic intermediate in the reaction of complex-1 with t-BuOOH is proposed as the source of oxygen in the oxidised product. Kinetic data suggests that the formation Ru(V)-oxo is substitution controlled. The results of the product distribution in the present investigation clearly indicate the high electrophilic nature of Ru=O bond in [Ru^V(amp)(bipy)O]⁺ intermediate complex which leads to high affinity for atomic hydrogen/hydride abstraction.     

Comparative ballistic efficiency of solid composite propellants: which plasticizer/polymer combination is the energetically preferred binder?

The relative efficacy of real energetic plasticizers and polymers for model solid composite propellants comprising 25% aluminum hydride, 50% dinitramide ammonium salt and 25% binder (20% a plasticizer and 5% a polymer) has been estimated. The quantitative dependence of the efficiency of plasticizers on the value of their enthalpy of formation ΔH_t⁰, the oxygen coefficient α, percentage of hydrogen %H and density d has been revealed. 3,4-Dinitrofurazan tested as a plasticizer for the binder provides effective impulse values at the 3^rd stage up to ～2 s higher than those for other plasticizers.     

Evaluation of metal hydride thermograms by a differential-correction technique

A least-squares linear-Taylor differential-correction technique has been used for the rapid evaluation of thermogravimetric curves obtained during the decomposition of magnesium hydride, iron—titanium hydride and lanthanum—nickel hydride. For magnesium hydride and iron—titanium hydride the Avrami—Erofe'ev equation fits the experimental data, thus indicating that nucleation is the rate-determining step under thermogravimetric conditions. For lanthanum—nickel hydride a combination of the Avrami—Erofe'ev equation and the phase boundary movement equation fits the data up to a fractional decomposition of 0.8. For magnesium hydride decomposition the activation energy E and the pre-exponential factor Z are dependent on the hydrogen pressure (E = 101.2 kJ mole^?1 and Z = 8.96 × 10⁷ at 0.30 MPa, while E = 66.3 kJ mole^?1 and Z = 4.77 × 10⁷ at 0.11 MPa). For iron—titanium hydride (E = 28.4 kJ mole^?1) and lanthanum—nickel hydride (E = 13.4 kJ mole^?1) the values are independent of pressure.     

The reaction of the group-13 alkyls ER3 (E = Al, Ga, In; R = CH2t-Bu, CH2 SiMe3) with the platinum-complex [(dcpe)Pt(H)(CH2t-Bu)]

The reactions of the sterically demanding group-13 alkyls ER₃ (E = Al, Ga, In; R = CH₂t-Bu, CH₂SiMe₃) with the platinum-complex [(dcpe)Pt(H)(CH₂t-Bu)] were re-investigated. The bimetallic compounds [(dcpe)Pt(ER₂)(R)] (3: E = Ga, R = CH₂SiMe₃; 5: E = In, R = CH₂t-Bu; dcpe = bis(dicyclohexylphosphino)ethane) with direct σ(Pt-E) bonds were obtained by oxidative addition of an E-C bond to the coordinatively unsaturated fragment [(dcpe)Pt] produced in situ by thermolysis of the starting complex [(dcpe)Pt(CH₂t-Bu)(H)]. The single crystal structure determination reveals a Pt-Ga bond length of 2.376(2) Å and a Pt-In bond length of 2.608(1) Å. All new compounds were characterised by elemental analysis, ³¹P and ¹⁹⁵Pt NMR spectroscopy. Interestingly, the Pt-Ga compound 3 slowly transforms into the platinum alkyl/hydride isomer {(dcpe)Pt(μ₂-H)[CH₂Si(CH₃)₂ CH₂Ga(CH₂SiMe₃)₂]} (4) during crystallization from solution at room temperature. The X-ray single crystal structure analysis revealed both complexes 3 and 4 coexisting in the unit cell in a 1:1 relation. The inaccessibility of analytically pure samples of the Pt-Al complex {(dcpe)Pt[Al(CH₂t-Bu)₂](CH₂t-Bu)} (6), postulated as intermediate of the reaction of [(dcpe)Pt(H)(CH₂t-Bu)] with Al(CH₂t-Bu) on the basis of ³¹P and ¹⁹⁵Pt NMR data, is attributed to an enhanced tendency to isomerisation into the alkyl/hydride Pt/Al congener of 4. A brief DFT analysis of the bonding situation of the model complex [(dhpe)Pt(GaMe₂)(Me)] (1M) revealed, that the contribution of π(Pt-Ga) back-bonding is negligible.     

A Reversible Proton Relay Process Mediated by Hydrogen‐Bonding Interactions in [FeFe]Hydrogenase Modeling

A reversible and temperature‐dependent proton‐relay process is demonstrated for a Fe₂ complex possessing a terminal thiolate in the presence of nitrogen‐based acids. The terminal sulfur site (S^t) of the complex forms a hydrogen‐bond interaction with N,N‐dimethylanilinium acid at 183 K. The Fe₂ core, instead, is protonated to generate a bridging hydride at 298 K. Reversibility is observed for the tautomerization between the hydrogen‐bonded pair and the Fe–hydride species. X‐ray structural analysis of the hydrogen‐bonded species at 193 K reveals a short N(H)???S^t contact. Employment of pyridinium acid also results in similar behavior, with reversible proton–hydride interconversion. DFT investigation of the proton‐transfer pathways indicates that the pK_a value of the hydrogen‐bonded species is enhanced by 3.2 pK_a units when the temperature is decreased from 298 K to 183 K.     

Synthesis of tetramic acids with a benzo[f]indolizine skeleton. Transannular rearrangements in pyrazino[1,2-b]isoquinolin-4-ones

Pyrazino[1,2-b]isoquinoline-1,4-diones (3) having a bulky activating group at the N(2)-position were rearranged to tetramic acids with a benzo[f]indolizine skeleton (8) in the presence of K^tBuO or LHMDS as bases. This rearrangement was diasteroselective for the 6,11a-trans-isomers of the starting compounds. 1-Hydroxy-pyrazino[1,2-b]isoquinolin-4-one (7) afforded a 1-trichloroacetamido derivative (14) after treatment with trichloroacetonitrile and a catalytic amount of sodium hydride as a base, through two subsequent base-promoted transannular rearrangements. In summary, the combination of functions in the piperazine ring of the starting tricyclic compounds conferred to them new reactivities that imply different base-promoted transannular rearrangements and led to unexpected transformations.