Stereospecificity of acetylene reduction catalyzed by nitrogenase.

In addition to catalyzing the reduction of dinitrogen to ammonia, the metalloenzyme nitrogenase catalyzes the reduction of a number of alternative substrates, including acetylene (C(2)H(2)) to ethylene (C(2)H(4)) and, in certain cases, to ethane (C(2)H(6)). The stereochemistry of proton addition for C(2)D(2) reduction to C(2)D(2)H(2) catalyzed by the Mo-dependent nitrogenase has been used to probe substrate binding and proton addition mechanisms. In the present work, the C(2)D(2) reduction stereospecificity of altered MoFe proteins having amino acid substitutions within the active site FeMo-cofactor environment was examined by Fourier transform infrared (FTIR) spectroscopy. Altered MoFe proteins examined included those having the alpha-subunit 96(Arg) residue substituted by Gln, Leu, or Ala, the alpha-subunit 69(Gly) residue substituted by Ser, and the alpha-subunit 195(His) residue substituted by Asn. The stereochemistry of proton addition to C(2)D(2) does not correlate with the measured K(m) values for C(2)H(2) reduction, or with the ability of the enzyme to reduce C(2)H(2) by four electrons to yield C(2)H(6). Instead, the electron flux through nitrogenase was observed to significantly influence the ratio of cis- to trans-1,2-C(2)H(2)D(2) formed. Finally, the product distribution observed for reduction of C(2)H(2) in D(2)O is not consistent with an earlier proposed enzyme-bound intermediate. An alternative model that accounts for the stereochemistry of C(2)H(2) reduction by nitrogenase based on a branched reaction pathway and an enzyme-bound eta(2)-vinyl intermediate is proposed.     

The reduction of osmium carbonyl clusters by nitriles

The clusters Os₆(CO)₁₈ Os₇(CO)₂₁ and Os₈(CO)₂₃ are reduced to the anoins [Os₆(CO)₁₈]^2?, [Os₇(CO)₂₀]^2? and [Os₈(CO)₂₂]^2?, respectively, by the action of nitriles RCN (R = Me, Et, CH₂C(Me)); the kinetics of the reaction of Os₆(CO)₁₈ with EtCN have been examined and reveal a third order dependence on nitrile concentration.     

芳杂环腈及其聚合物XV. 不同结构芳香族腈的聚合及其聚合物结构的测定

本文研究了十二种不同取代基的芳香族腈在Lewis酸-金属体系催化下的聚合反应。结果表明, 取代基的吸电子能力越强, 聚合反应速率越快。其速率常数与Hammett方程中的取代基特性常数σ值呈线性关系, 此外, 对聚合物的结构也进行了测定。     

Photostimulated reduction of nitriles by SmI2

Despite their high electron-withdrawing strength, nitriles are not good electron acceptors and therefore are hard to reduce. In this work, using photostimulation in the visible region, we examined the reactivity of aliphatic and aromatic, mono- and dicyano compounds in reaction with SmI(2). A proton donor that complexes efficiently with SmI(2) must be used. Maximum yield was obtained at ca.0.2 M MeOH. Aromatic nitriles were more reactive than aliphatic nitriles, which exhibited negligible yields. Phenylacetonitrile presents an intermediate reactivity. The mechanism of the reaction involves coordination of the SmI(2) to the lone pair of the nitrile nitrogen followed by an inner sphere electron transfer. Surprisingly, m-dicyanobenzene was less reactive than the monocyano derivative benzonitrile. This was traced to the lower ability of the dicyano compound to coordinate to the SmI(2) due to, as was shown by quantum mechanical calculations, its lone pair having an energy significantly lower than that of benzonitrile. It is noteworthy that at the SmI(2) initial concentration used (0.04M), light penetrates only the 0.4 mm outer layer of the reaction mixture. Therefore the photostimulation effect observed was due to irradiation of only 4% of the total reaction volume, implying that under optimal conditions the effect should be 25 times larger.     

Car-Parrinello molecular dynamics study of the initial dinitrogen reduction step in Sellmann-type nitrogenase model complexes

We have studied reduction reactions for nitrogen fixation at Sellmann-type model complexes with Car-Parrinello simulation techniques. These dinuclear complexes are especially designed to emulate the so-called open-side FeMoco model. The main result of this work shows that in order to obtain the reduced species several side reactions have to be suppressed. These involve partial dissociation of the chelate ligands and hydrogen atom transfer to the metal center. Working at low temperature turns out to be one necessary pre-requisite in carrying out successful events. The successful events cannot be described by simple reaction coordinates. Complicated processes are involved during the initiation of the reaction. Our theoretical study emphasizes two experimental strategies which are likely to inhibit the side reactions. Clamping of the two metal fragments by a chelating phosphane ligand should prevent dissociation of the complex. Furthermore, introduction of tert-butyl substituents could improve the solubility and should thus allow usage of a wider range of (mild) acids, reductants, and reaction conditions.     

The first example of a nitrile hydratase model complex that reversibly binds nitriles

Nitrile hydratase (NHase) is an iron-containing metalloenzyme that converts nitriles to amides. The mechanism by which this biochemical reaction occurs is unknown. One mechanism that has been proposed involves nucleophilic attack of an Fe-bound nitrile by water (or hydroxide). Reported herein is a five-coordinate model compound ([Fe(III)(S(2)(Me2)N(3)(Et,Pr))](+)) containing Fe(III) in an environment resembling that of NHase, which reversibly binds a variety of nitriles, alcohols, amines, and thiocyanate. XAS shows that five-coordinate [Fe(III)(S(2)(Me2)N(3)(Et,Pr))](+) reacts with both methanol and acetonitrile to afford a six-coordinate solvent-bound complex. Competitive binding studies demonstrate that MeCN preferentially binds over ROH, suggesting that nitriles would be capable of displacing the H(2)O coordinated to the iron site of NHase. Thermodynamic parameters were determined for acetonitrile (DeltaH = -6.2(+/-0.2) kcal/mol, DeltaS = -29.4(+/-0.8) eu), benzonitrile (-4.2(+/-0.6) kcal/mol, DeltaS = -18(+/-3) eu), and pyridine (DeltaH = -8(+/-1) kcal/mol, DeltaS = -41(+/-6) eu) binding to [Fe(III)(S(2)(Me2)N(3)(Et,Pr))](+) using variable-temperature electronic absorption spectroscopy. Ligand exchange kinetics were examined for acetonitrile, iso-propylnitrile, benzonitrile, and 4-tert-butylpyridine using (13)C NMR line-broadening analysis, at a variety of temperatures. Activation parameters for ligand exchange were determined to be DeltaH(+ +) = 7.1(+/-0.8) kcal/mol, DeltaS(+ +) = -10(+/-1) eu (acetonitrile), DeltaH(+ +) = 5.4(+/-0.6) kcal/mol, DeltaS(+ +) = -17(+/-2) eu (iso-propionitrile), DeltaH(+ +) = 4.9(+/-0.8) kcal/mol, DeltaS(+ +) = -20(+/-3) eu (benzonitrile), and DeltaH(+ +) = 4.7(+/-1.4) kcal/mol DeltaS(+ +) = -18(+/-2) eu (4-tert-butylpyridine). The thermodynamic parameters for pyridine binding to a related complex, [Fe(III)(S(2)(Me2)N(3)(Pr,Pr))](+) (DeltaH = -5.9(+/-0.8) kcal/mol, DeltaS = -24(+/-3) eu), are also reported, as well as kinetic parameters for 4-tert-butylpyridine exchange (DeltaH(+ +) = 3.1(+/-0.8) kcal/mol, DeltaS(+ +) = -25(+/-3) eu). These data show for the first time that, when it is contained in a ligand environment similar to that of NHase, Fe(III) is capable of forming a stable complex with nitriles. Also, the rates of ligand exchange demonstrate that low-spin Fe(III) in this ligand environment is more labile than expected. Furthermore, comparison of [Fe(III)(S(2)(Me2)N(3)(Et,Pr))](+) and [Fe(III)(S(2)(Me2)N(3)(Pr,Pr))](+) demonstrates how minor distortions induced by ligand constraints can dramatically alter the reactivity of a metal complex.     

A generic approach for the catalytic reduction of nitriles

The scope of nickel boride mediated reduction of nitriles has been extended further to allow the preparation of Boc protected amines via a mild catalytic process. It is noteworthy that the toxicity of this procedure is greatly reduced due to its catalytic nature in nickel(II) chloride used in combination with excess sodium borohydride. The protocol is marked by its resilience towards air and moisture and hence an easy and general practical protocol.     

Enantioselective reactions catalyzed by synthetic enzymes. A model for chemical evolution

Polyleucines of various lengths act as enantioselective catalysts in the aldol condensation between cyclohexanone and various aromatic aldehydes. Polyleucine and other polyamino acids behave as synthetic enzymes in the epoxidation of chalcone and other electron-deficient alkenes. Both reactions are of considerable prebiotic significance.     

Thermochemistry and quantum chemical calculations of two dibenzocycloalkane nitriles

The energies of combustion and fusion of 5-cyano-5H-dibenzo[a,d]cycloheptene (1) and (5E,11E)-dibenzo[a,e]cyclooctene-5,11-dicarbonitrile (2) were measured by means of microbomb calorimetry and DSC, respectively. The derived enthalpies of formation in solid state are 320 ± 18 for nitrile 1 and 470 ± 31 kJ mol⁻¹ for nitrile 2, respectively. The experimental enthalpies of formation are discussed in relationship with values calculated at the G3(MP2)//B3LYP level of quantum chemical theory, by means of group additivity and isodesmic reactions. The two nitriles are not stabilized by dibenzoannelation.     

A mild and efficient method for the reduction of nitriles

A simple and useful method for the reduction of nitriles into the corresponding amines using a tetramethyldisiloxane/titanium(IV) isopropoxide reducing system is described. The synthetic approach is straightforward and provides primary amines as hydrochloride salt in almost quantitative yield. Other advantages of this method, such as easy-to-handle hydride source, inert by-products, that is, TiO₂ and oligomeric siloxanes, make it very attractive to prepare primary amines.     

Application of flow chemistry to the reduction of nitriles to aldehydes

Reduction of nitriles to aldehydes with diisobutylaluminium hydride (DIBAL-H) is an important transformation in organic chemistry. But the use of this reaction is limited for the lack of reproducibility due to the instability of the intermediates formed. In the current article we disclose the improvement of the reaction applying continuous flow technology.     

The role of phosphorus in chemical evolution

In this tutorial review we consider the role of phosphorus and its compounds within the context of chemical evolution in galaxies. Following an interdisciplinary approach we first discuss the position of P among the main biogenic elements by considering its relevance in most essential biochemical functions as well as its peculiar chemistry under different physicochemical conditions. Then we review the phosphorus distribution in different cosmic sites, such as terrestrial planets, interplanetary dust particles, cometary dust, planetary atmospheres and the interstellar medium (ISM). In this way we realize that this element is both scarce and ubiquitous in the universe. These features can be related to the complex nucleosynthesis of P nuclide in the cores of massive stars under explosive conditions favouring a wide distribution of this element through the ISM, where it would be ready to react with other available atoms. A general tendency towards more oxidized phosphorus compounds is clearly appreciated as chemical evolution proceeds from circumstellar and ISM materials to protoplanetary and planetary condensed matter phases. To conclude we discuss some possible routes allowing for the incorporation of phosphorus compounds of prebiotic interest during the earlier stages of solar system formation.