First Order In Substrate Research Articles

Intramolecular catalysis. Part 4. The intramolecular Cannizzaro reaction of biphenyl-2,2′-dicarbaldehyde, [α,α′-2H2]biphenyl-2,2′-dicarbaldehyde and 4,4′-or 5,5′- or 6,6′-disubstituted biphenyl-2,2′-dicarbaldehydes

Biphenyl-2,2′-dicarbaldehyde, along with its α,α′-[2H2] isomer, and a series of 4,4′-, 5,5′- and 6,6′-disubstituted biphenyl-2,2′-dicarbaldehydes have been shown to undergo Cannizzaro reactions in 30%(v/v) dioxane-water containing base. The reaction has been shown to be intramolecular and of the third order; first order in substrate and second order in base. The rate coefficients have been measured at several temperatures and activation parameters evaluated. Studies of solvent isotope, solvent and salt effects have been made. The kinetic isotope effect, kH/kD, was found to be ca. 1.8. The effect of 4,4′- and 5,5′-disubstitution was assessed by use of the Hammett equation to give ρ at 30 °C to be ca. 4.6. Severe steric effects were noted for 6,6′-dimethyl substitution. The alkaline hydrolysis of the corresponding ε-lactones of 2′-hydroxymethylbiphenyl-2-carboxylic acids was studied under the same conditions. The intermediacy of the lactones in the intramolecular Cannizzaro reaction can be excluded. The evidence for the Cannizzaro reaction indicates a mechanistic pathway involving rate-determining hydride transfer from the dianion of a hydrated formyl group to the second formyl group. An estimate is made of effective concentration of the intramolecular hydride transfer.

Ease of C–N bond cleavage in four-membered rings

The hydrolysis of an azetidin-2-ylideneammonium salt gives a β-lactam and an aminoamide by competitive endo- and exo-cyclic C–N bond cleavage, the product ratio being pH and buffer dependent; the rate law for hydrolysis shows a third order term which is first order in substrate, hydroxide, and carbonate ions, which is interpreted as kinetically equivalent to general acid catalysed breakdown of the ionised tetrahedral intermediate and as evidence for non-facile C–N bond cleavage in the four-membered ring.

A multistep mechanism of nucleophilic substitution of vitamin B 1 in methanol

Thiamin (1) and an analog (2) having a pyridine ring in place of the thiazole portion react with amine nucleophiles in methanol at 71.5°C to give substitution products 3. The nucleophiles replace the thiazole and pyridine rings. The course of the substitution reactions which were followed by PMR are first order in substrate and zero order in amine. Proposed is a multistep mechanism analogous to that observed for second-order reactions between sulfite ion, for example, and thiamin, its derivatives, and analogs. The conjugate acid of the substrate forms a sigma adduct with methoxide ion. Following fragmentation to a resonance-stabilized cation the amine nucleophile becomes incorporated into this charged intermediate. Loss of methoxide ion generates the aromatic substitution product. Substrate protonation and methoxide ion addition are kinetically equivalent to the addition of solvent, a process that cannot be detected kinetically. Addition of the amine occurs in a fast step. Overall then, the scheme is only first order in substrate as required by the experimental observations.

Phosphinerhodium complexes as homogeneous catalysts: XV. Hydrogenation of ketones with PPh 3-containing rhodium catalysts

Wilkinson-type complexes containing PPh 3 ligands and modified by Et 3N can be successfully used as catalysts for the homogeneous hydrogenation of ketones. The reaction is first order in substrate and rhodium and the rate does not depend on the partial pressure of hydrogen. Excess of the ligand (PPh 3/Rh > 3) does not inhibit the reduction. The reaction between HRh(PPh 3) 3 and the ketone is regarded as the rate-determining step, and this is followed by a fast reaction with H 2. The activity of the catalyst is not very sensitive to the structure of the substrate.

Oxidation by cobalt(III) acetate. Part 5. Kinetics and mechanism of the oxidative cleavage of hydrobenzoins

The mechanism of the oxidative cleavage of hydrobenzoin by cobalt(III) acetate in acetic acid has been studied by following the kinetics of the reaction. The rate is first order in cobalt(III) acetate, inverse first order in cobalt(II) ion, and first order in substrate. DL-Hydrobenzoin is oxidized 4.5 times faster than the meso-isomer. A mechanism involving a bidentate complex formed from the reaction of cobalt(III) acetate dimer with glycol is discussed.

Reactions of coordinated ligands, Part III(1). Kinetics of lodination of malonate and pyruvate in malonato(pentaammine)-cobalt(III), malonato-bis-(ethylenediamine)cobalt(III) and pyruvato(pentaammine)cobalt(III) ions

The kinetics of iodination of malonate and pyruvate in the title complexes are reported at 35.0 °C and I=0.3 M. The reaction is first order in substrate and zeroth order in [I2]. This result is commensurate with rate determining enolisation of the active methylene and methyl groups of the malonate and pyruvate respectively. The reaction is catalysed by H2O, OH− and by the buffer anions used. The rate data suggest that the malonate methylene group in the [Co(en)2-O2CCH2CO2]2+ chelate is considerably more active towards electrophilic substitution than is the case in [Co(NH3)5O2CCH2CO2]2+.

Kinetics and mechanism of ruthenium(III) catalyzed oxidation of tellurium(IV) by cerium(IV)

The reaction is first order in substrate and catalyst and zero order in cerium(IV). The rate decreases with increasing [H+] as well as with increasing ionic strength. ΔH‡ and ΔS‡ have been found to be 44.8 kJ mol−1 and 161.8 JK−1 mol−1 respectively. A mechanism is proposed.

Reactions of metal carbonyl complexes. VIII. Kinetics and mechanisms of the substitution reactions of cis-Mn(CO)4LBr (L = PPh3, AsPh3, SbPh3) with group VA bidentate ligands

The substitution reactions of cis-Mn(CO)4LBr (L = PPh3, AsPh3, SbPh3) with diphos, diarsine, and dipy (A—A) take place with the loss of one CO group and L to form fac-Mn(CO)3(A—A)Br. The observed reaction rates in CHCl3 solution are first order in substrate but depend on the nature of L and A—A, and in some cases the concentration of A—A as well. When L = PPh3 and A—A = diphos, and when L = AsPh3 and A—A = diphos, diarsine, or dipy, the rates are independent of [A—A]. For these reactions, an SN1 dissociative mechanism involving the rupture of one of the Mn—CO bonds as the rate-determining step is proposed. The observed positive entropies of activation (∼10 e.u.) for these reactions are supportive of the proposed mechanism. The reactions of cis-Mn(CO)4(SbPh3)Br with A—A are essentially independent of both the nature and the concentration of A—A. However, for these reactions, an SN1 dissociative mechanism involving the rupture of the Mn—SbPh3 bond as the rate-determining step is proposed; the observed positive ΔS≠ values (∼12 e.u.) are in accord with such a mechanism. The rates when L = PPh3 and A—A = diarsine or dipy are dependent on both the nature and the concentration of A—A. A mechanism involving rapid pre-equilibrium steps between the parent molecule and the intermediates, Mn(CO)3(PPh3)Br and Mn(CO)4Br, is proposed to account for the complex kinetic behaviour found for these reactions. These results are particularly noteworthy because octahedral first-row transition metal complexes generally undergo substitution reactions which are independent of the concentrations of the entering nucleophiles.

A kinetic study of osmium tetroxide catalysed oxidation of methyl digol and ethyl digol by hexacyanoferrate (III) in aqueous alkaline medium

The osmium tetroxide catalysed reactions of alkaline hexacyanoferrate. (III) with methyl digol and ethyl digol were investigated kinetically. The reactions are zero order in hexajanoferrate (III) and of first order in substrate and osmium tetroxide concentrations. With an increase in [math] the rate increased and the reaction showed positive intercept at Y-axis. An inner-sphere mechanism involving complex formation, which then disproportionates to form the products, is suggested.

Kinetics of the oxidation of p-aminobenzoic acid catalyzed by horseradish peroxidase compounds I and II.

The kinetics of p-aminobenzoic acid oxidation catalyzed by horseradish peroxidase Compounds I and II was investigated intensively as a function of pH at 25 degrees in aqueous solutions of ionic strength 0.11. All of the rate data were collected from single turnover experiments involving reactions of a single enzyme compound. In reactions of both compounds, deviations from first order behavior with respect to the enzyme were observed at high pH values which were explained in terms of a free radical interaction of product with the enzyme. The effect could be eliminated with sufficient excess of substrate. Kinetic behavior which deviated from first order in substrate, observed at low pH, was explained by a mechanism involving an enzyme-substrate complex which reacted with an additional molecule of substrate but at a slower rate. The pH dependence of the second order rate constants for the reaction of p-aminobenzoic acid with free Compounds I and II is similar to results obtained for the comparable reactions of ferrocyanide, suggesting similar proton-transfer mechanisms for both reducing substrates. The reduction of Compound II by p-aminobenzoic acid appeared to be influenced by two ionizable groups on the enzyme which affect the electronic environment of the heme. The lack of influence of substrate ionizable groups on the rate of the Compound II reaction indicated that potential differences in reactivities of NH2C6H4COO- and NH2C6H4COOH were levelled by the diffusion-controlled limit in the acid region of pH. The reduction of Compound I by p-aminobenzoic acid was not diffusion-controlled and the rate-pH profile could be explained in terms of three acid ionizations, two on the substrate and one on Compound I.

ChemInform Abstract: RATES AND MECHANISMS OF SUBSTITUTION REACTIONS OF PLATINUM DITHIOLATO‐COMPLEXES

AbstractDie Pt‐dithiolatokomplexe (I) wurden durch Reaktion von Tetrachloroplatinat mit den entsprechenden Liganden im molaren Verhältnis 1 22 dargestellt.

Deoxygenation of quinoxaline N-oxides and related compounds

The alkali induced deoxygenation of 3-(α-hydroxyalkyl)-quinoxaline-1-oxides is shown to be first order in substrate and in hydroxide ion. Examples are given to illustrate the synthetic utility of this reaction for the synthesis of quinoxaline and quinoline derivatives of type 4, 5, 8, and 11. The mechanism of the reaction is related to the mechanism of deoxygenation of heteroaromatic N-oxides by sodium dithionite.

Reactions of metal carbonyl complexes. II. Kinetics and mechanism of cis-cyclooctene substitution in π-C 5H 5Mn(CO)(CS)(C 8H 14 by triphenylphosphine

Kinetic data have been obtained for the replacement of cis-cyclooctene in π-C 5H 5Mn(CO)(CS)(C 8H 14 by triphenylphosphine to form π-C 5H 5Mn(CO)(CS)PPh 3. The observed reaction rates in methylcyclohexane at 50–70°C are first order in substrate and independent of the concentration of PPh 3 suggesting a mechanism in which the rate of dissociation of the olefin is the rate-determining step. The high positive entropy of activation is suportive of the proposed S N1 dissociative mechanism. The kinetic results suggest that CS is a better π-acceptor ligand than CO, in agrement with other studies on transition metal thiocarbonyl complexes.

Rates and mechanisms of substitution reactions of platinum dithiolato-complexes

Kinetics and mechanisms of substitution reactions of square-planar platinum(II) dithiolato-complexes [Pt(S–S)2]2–[S–S2–=(NC)2C:CS22–(cet), S(NC)C:C(NC)S2–(ct), NO2CH:CS22–(net), S(O)C:C(O)S2–(dta), NC–N:CS22–(cimd), and NC(Ph)C:CS22–(npet)], with both uni- and bi-dentate nucleophiles have been studied in aqueous solution. The reactions are second order, first order in substrate and first order in nucleophile. Activation parameters have been measured for a number of the reactions. The results permit a comparison of the reaction mechanisms and relative reactivities of dithiolato-complexes of nickel, palladium, and platinum.

Kinetics of ethylene polymerization and butadiene polymerization catalyzed by solutions of hydridochlorotris(triphenylphosphine)ruthenium(II)

The kinetics of ethylene polymerization and butadiene polymerization, catalyzed homogeneously by solutions of HRuCl(PPh 3) 3 in N,N-dimethylacetamide at 50–85 °C, are reported. The reaction rates are first order in catalyst, and are first order in substrate for the ethylene system, but are independent of the substrate concentration in the diene system. The data are interpreted in terms of a common mechanism analogous to that of Ziegler catalysts, where, following coordination of the monomer to metal, the polymer chain grows by insertion of the monomer at a polarized metal-carbon bond. The differences in kinetics are accounted for by the stronger complexing ability of the diene. The activation parameters obtained show that the butadiene polymerization rate is higher because of a more favorable entropy of activation. The catalyst becomes deactivated due to decarbonylation of the solvent, and possibly due to an intramolecular hydrogen transfer process involving a phenyl ring of the phosphine ligand.

Rearrangement of aromatic N-nitroamines. Part VII. The acid-catalysed transformation of N-methyl-N-nitro-9-aminoanthracene

Treatment of N-methyl-N-nitro-9-aminoanthracene with sulphuric acid (ca. 7N) yielded 10-nitroanthrone (90%) together with nitrous acid (5%). The reaction which involved intramolecular shift of the nitro-group, was first order in substrate its rate was linearly correlated with the h0 function; but, unexpectedly, the reaction had a solvent isotope effect (kD2O/kH2O) of less than unity.

Mechanism of benzidine and semidine rearrangements. Part XXIII. Kinetics and products of acid-catalysed conversion of 4-phenyl-; 4-nitro-; 2,2′-diphenyl-; and 4,4′-difluoro-substituted hydrazobenzenes

4-Phenyl-, 4-nitro-, and 2,2′-diphenyl-substituted hydrazobenzenes underwent rearrangements and concurrent disproportionations that were first order in substrate and second order in acid. The 4,4′-difluoro-compound also reacted first order in substrate but the order in acid rose from one to two with increasing acidity. These kinetic forms, the relative rates, and the nature of products could be rationalised by a polar transition-state mechanism that had been previously proposed.

The kinetics and mechanism of the reaction of cyclopentadienylidenetriarylphosphoranes with cyano-olefins. Part III. Elimination of hydrogen cyanide from 2-(1-phenyl-1,2,2-tricyanoethyl)cyclopentadienylidenetriphenylphosphorane; an E1CB reaction via an ion-pair

Kinetic data on the base-catalysed elimination of hydrogen cyanide from 2-(1-phenyl-1,2,2-tricyanoethyl)-cyclopentadienylidenetriphenylphosphorane in benzene, benzene–acetonitrile, or acetonitrile solvent are reported. The elimination is of the first order in substrate and base (tertiary amine) and the second-order rate constants (k2) increase with increasing polarity of the medium. The deuterium isotope effect (k2H/k2D) is ca. 1·0 in each solvent system and a linear Bronsted plot is observed with a number of tertiary amines over a range of five pKa units. Activation parameters are reported for the reaction with triethylamine and the kinetic results are interpreted in terms of a E1CB reaction involving fast proton loss to form an ion-pair, followed by rate-determining elimination of cyanide ion from the ion-pair intermediate.

Kinetic studies of the substitution reactions of the cis-dihalotetracarbonylmanganate(I) ions

The substitution reactions of (C2H5)4[cis-Mn(CO)4X2], where X = Br or I, with a variety of ligands, L, were observed to take place with the loss of halide ion, X−, to form cis-Mn(CO)4LX and (C2H5)4NX. The reaction rates in s-tetrachloroethane were first order in substrate, independent of both the nature and the concentration of L, and decreased with increasing atomic weight of the halogen. A dissociative mechanism involving the breaking of a manganese–halogen bond as the rate-determining step is proposed. It is postulated that the order of reactivity of the substrates is most probably the result of an entropy rather than an enthalpy effect. The substitution reactions of (C2H5)4N[cis-Mn(CO)4BrI] in s-tetrachloroethane were also studied kinetically and found to be first order in substrate and independent of the nature and concentration of L. The products of these reactions, however, were mixtures of cis-Mn(CO)4LBr and cis-Mn(CO)4LI, and (C2H5)4NBr and (C2H5)4NI. A solvent study of the reaction between (C2H5)4N[cis-Mn(CO)4Br2] and P(C6H5)3 indicated that the rate of reaction tended to increase with increasing polarity of the solvent.

Kinetics of Solvolyses of Various N-Alkyl-N-nitrosoureas in Neutral and Alkaline Solutions

The N- alkyl- N -nitrosoureas possess antitumor activity but readily decompose at the pH of biological fluids with half-lives about 20 minutes at pH 7 and 37.5°. The effect of substituents, R, on the kinetics of solvolysis of synthesized R─N─(NO)─CO─NH 2 was determined in the phosphate buffer region, pH 6–7.8, as a basis for the proper design of biologically effective compounds. Spectrophotometric techniques were used over a range of several temperatures. The solvolytic rates were first order in substrate and hydroxyl ions and the log k -pH profile was linear with a slope of + 1.0. No solvent or general acid-base catalysis was observed, and salt effects were negligible. No significant differences in reactivities, entropies, or heats of activation were observed for R = methyl, ethyl, and butyl. The 2-methylpropyl was slightly more reactive. When R = allyl, phenethyl, and benzyl, a significant increase in reactivities was observed. The unsaturated substituents favored hydroxyl ion attack as expected. However, the cyclohexyl compound also had high reactivity to hydroxyl ion attack.