Large Negative Entropy Of Activation Research Articles

Formation and isomerization of cis, cis-Ir(H) 2(--Ph)(CO)(PPh 3) 2 and cis, cis-Ir(H)(--Ph) 2(CO)(PPh 3) 2 in oxidative addition of hydrogen and phenylacetylene to trans-Ir(CO)(--Ph)(PPh 3) 2

Oxidative addition of H–R (H--Ph and H 2) to trans-Ir(--Ph)(CO)(PPh 3) 2 ( 2) gives the initial products, cis, cis-Ir(H)(--Ph) 2(CO)(PPh 3) 2 ( 3a) and cis, cis-Ir(H) 2(--Ph)(CO)(PPh 3) 2 ( 3b), respectively. Both cis-bis(PPh 3) complexes, 3a and 3b undergo isomerization to give the trans-bis(PPh 3) complexes, trans, trans-Ir(H)(--Ph) 2(CO)(PPh 3) 2 ( 4a) and cis, trans-Ir(H) 2(--Ph)(CO)(PPh 3) 2 ( 4b). The isomerization, 3b→ 4b is first order with respect to 3b with k 1=6.37×10 −4 s −1 at 25°C under N 2 in CDCl 3. The reaction rate ( k 1) seems independent of the concentration of H 2. A large negative entropy of activation (Δ S ≠=−24.9±5.7 cal deg −1 mol −1) and a relatively small enthalpy of activation (Δ H ≠=14.5±3.3 kcal mol −1) were obtained in the temperature range 15∼35°C for the isomerization, 3b→ 4b under 1 atm of H 2.

Pyramidal Inversion at Phosphorus Facilitated by the Presence of Proximate Lewis Acids. Coordination Chemistry of Group 13 Elements with the Macrocyclic Bis(amidophosphine) Ligand [P(2)N(2)] ([P(2)N(2)] = [PhP(CH(2)SiMe(2)NSiMe(2)CH(2))(2)PPh]).

Investigations on the preparation of four- and five-coordinate aluminum and gallium bis(amidophosphine) derivatives are reported. The reaction of the macrocyclic ligand precursor anti-Li(2)(THF)(2)[P(2)N(2)] ([P(2)N(2)] = [PhP(CH(2)SiMe(2)NSiMe(2)CH(2))(2)PPh]) with AlCl(3) or GaCl(3) in toluene at 25 degrees C leads to the formation of the four-coordinate species anti-MCl[P(2)N(2)] (M = Al (1), Ga (2)). An X-ray diffraction study of anti-GaCl[P(2)N(2)] shows it to be monomeric with a distorted tetrahedral geometry at Ga; only one of the phosphine donors of the [P(2)N(2)] ligand binds to the gallium, resulting in the retention of the anti-configuration. The solution NMR spectra are consistent with C(s)() symmetry. The addition of AlCl(3) or GaCl(3) to the macrocyclic ligand precursor syn-Li(2)(dioxane)[P(2)N(2)] in toluene at 25 degrees C yields the five-coordinate complexes syn-MCl[P(2)N(2)] (M = Al (3), Ga (4)). The X-ray crystal structure of syn-GaCl[P(2)N(2)] reveals a trigonal bipyramidal geometry about the metal atom, necessitating the coordination of both phosphorus atoms. The solution NMR spectra are consistent with a C(2)(v)() symmetric complex. Heating the anti complexes results in the clean conversion to the syn complexes, with pyramidal inversion observed at phosphorus. The kinetics of this inversion were studied by (1)H NMR spectroscopy and found to be first-order. Barriers to pyramidal inversion (DeltaG()) were calculated to be 29.1 and 30.1 kcal mol(-)(1) for the aluminum and gallium complexes, respectively; these barriers are approximately 2-3 kcal mol(-)(1) lower than that determined for the metal-free, protonated compounds anti- and syn-H(2)[P(2)N(2)]. It is suggested that the role that the metals play in this inversion, based on the values of DeltaG(), involves the large negative entropies of activation and thus help organize the transition state.

Induced Acceleration of Phosphine Exchange in Metal Carbonyls by Pendant Groups of Coordinated Polyphosphines. Two Dangling Phosphine Arms Are Much Better Than One1

The kinetics and thermodynamics of isomerization of the linkage isomers (OC)5W[η1-PPh2CH2CH(PPh2)2] (1) and (OC)5W[η1-PPh2CH(PPh2)CH2PPh2] (2) in chloroform have been studied by 31P{1H} NMR over the temperature range of 10−55 °C. Conversion of 1 to 2 is exothermic (ΔH = −12.25 ± 0.1 kJ mol-1) and is accompanied by a large decrease in entropy (ΔS = −28.2 ± 0.3 J mol-1 K-1). The reaction proceeds much faster than expected [k = (1.18 ± 0.01) × 10-5 s-1] at 25 °C, and its small activation enthalpy (ΔH⧧ = 92.6 ± 1.9 kJ mol-1) and large negative activation entropy (ΔS⧧ = −28.2 ± 6.2 J mol-1 K-1) suggest an associative mechanism. The reaction is 4 orders of magnitude faster than the isomerization of (OC)5W[η1-PPh2CH2CH2P(p-tol)2] (3) to (OC)5W[η1-P(p-tol)2CH2CH2PPh2] (4). Exchange of coordinated and dangling phosphines is faster than chelation for each of the four complexes, but chelation of 1 or 2 is much faster than chelation of 3 or 4. It appears that the second dangling phosphine arm present in 1 and 2 accel...

Azide accelerates the decay of M-intermediate of pharaonis phoborhodopsin

Natronobacterium pharaonis has retinal proteins, one of which is pharaonis phoborhodopsin, abbreviated as ppR (or called pharaonis sensory rhodopsin II, psR-II). This pigment protein functions as a photoreceptor of the negative phototaxis of this bacterium. On photoexcitation ppR undergoes photocycling; the photoexcited state relaxes in the dark and returns to the original state via several intermediates. The photocycle of ppR resembles that of bR except in wavelengths and rate. The cycle of bR is completed in 10 ms while that of ppR takes seconds. The Arrhenius analysis of M-intermediate ( ppR M) decay which is rate-limiting revealed that the slow decay is due to the large negative activation entropy of ppR. The addition of azide increases the decay rate 300-fold (at pH 7); Arrhenius analysis revealed decreases in the activation energy (activation enthalpy) and a further decrease in the activation entropy.

Synthesis and Characterization of Palladium Complexes Containing 3,3'-Annelated 2,2'-Bipyridines and 3,3'-Annelated 2,2'-Biquinolines

The synthesis and characterization of a number of PdCl2(N⁀N) and Pd(N⁀N)(fn) complexes (N⁀N = 3,3'-annelated 2,2'-bipyridines and 3,3'-annelated 2,2'-biquinolines, fn = fumaronitrile) have been described. Despite the variation in dihedral angle and distance between the nitrogen lone pairs in these series of ligands, they are all capable of forming stable chelates. An investigation was made as to whether the 3,3'-trimethylene-2,2'-biquinoline (3-bq) and 3,3'-tetramethylene-2,2'-biquinoline (4-bq) ligands could serve as C2-chiral auxiliaries. The ligand 3-bq in Pd(3-bq)(fn) turned out to be conformationally rigid on the NMR time scale, but the diastereomers appeared to be indistinguishable, as could be concluded from the 1H NMR spectra. However, Pd(4-bq)(fn) exists in two diastereomeric forms and their ratio (57 : 43) could be determined from 1H NMR spectra. Assignment of the major and minor isomers was done with a 2D NOESY experiment. Epimerization of the diastereomers occurred on the laboratory time scale which was deduced from a spin saturation transfer experiment and also from the observation that diastereomerically enriched crystalline material of Pd(4-bq)(fn) at low temperature showed a ratio of 80 : 20 which deteriorated to a ratio of 57 : 43. From the spin saturation transfer experiment, the thermodynamic parameters for the inversion could be determined (∆H≠ = 29.1 kJ mol-1 and ∆S≠ = -136.3 J K-1 mol-1). This large negative entropy of activation favours an epimerization process via inversion of the coordinated 4-bq ligand over a mechanism via alkene dissociation/recoordination. The structures of the complexes have been studied in solution and in the solid state and the following X-ray crystal structures have been determined: (i) [dichloropalladium(II)](3,3'-trimethylene-2,2'-bipyridine-κ2N), monoclinic crystals, space group P21/n, with a = 8.4230(7) Å, b = 16.425(2) Å, c = 9.5836(7) Å, β = 106.316(7)°, Z = 4, final R = 0.033 for 2 309 reflections, (ii) [3,3'-bis(methoxycarbonyl)-2,2'-bipyridine-κ2N][dichloropalladium(II)], triclinic crystals, space group P1, with a = 7.340(4) Å, b= 10.012(3) Å, c= 11.954(3) Å, α = 98.10(2)°, β = 104.03(3)°, γ = 107.33(5)°, Z = 2, final R = 0.039 for 4 273 reflections, (iii) [chloromethylpalladium(II)](4,5-diazafluorene-κ2N), orthorhombic crystals, space group Pmcn, with a = 6.713(1) Å, b = 10.120(1) Å, c = 17.187(3) Å, Z = 4, final R = 0.048 for 949 reflections.

Kinetic Study of the Insertion of Norbornadiene into Palladium−Carbon Bonds of Complexes Containing the Rigid Bidentate Nitrogen Ligand Bis(arylimino)acenaphthene

The ionic [Pd(C7H8C(O)R)(Ar-BIAN)]X (R = Me, Et, iPr, Ph; X = Cl, Br, I; Ar = p-An, p-FC6H4, p-BrC6H4, p-Tol, Ph, o,o‘-Me2C6H3, o,o‘-iPr2C6H3) complexes (1b−12b), bearing the bidentate nitrogen ligand bis(arylimino)acenaphthene (Ar-BIAN), have been synthesized via reaction of the corresponding neutral acylpalladium complexes Pd(C(O)R)X(Ar-BIAN) (1a−12a) with norbornadiene (nbd). For the first time, an extensive kinetic study of this migratory alkene insertion into acyl−palladium bonds of neutral complexes containing α-diimine ligands has been carried out. It has been found that under pseudo-first-order circumstances these reactions follow the rate law kobsd = k1 + k2[nbd], which shows that these reactions proceed via a pathway independent of alkene concentration (k1 pathway) and a pathway dependent on alkene concentration (k2 pathway). The dramatic decrease of the rate constants k1 and k2 upon increasing the steric bulk of the BIAN ligand and the large negative entropy of activation and low enthalpy of ac...

The kinetics and mechanism of the reaction of trimethyl phosphate with benzylideneacetophenones

The kinetics and mechanism of the equilibrium reaction of benzylideneacetophenones 4 with trimethyl phosphate (TMP) to yield, 2,2,2-trimethoxy-2,3-dihydro-1,2λ5-oxaphospholes 5 are reported. The secondorder rate constants for the formation of 5 were determined using a polarographic method and the rate constants for the decomposition reaction were calculated from the equilibrium constants. The linear Hammett plots in some cases and the curved ones in others indicate that the rate-determining step depends on the character of the ring substituents in 4 and 5. A thermodynamic study of the forward reaction showed a relatively small enthalpy and a large negative entropy of activation in agreement with the existence of a strongly polarised and/or crowded structure in the transition state. The proposed mechanism, in cases where electron-donating substituents (Y) or hydrogen are on the aromatic ring next to the carbonyl carbon or where electron-donating substituents (X) are on the aromatic ring next to the double bond, involves nucleophilic attack of phosphate on the carbonyl carbon atom in the rate-limiting step to form intermediate I1 followed by a ring closure to an oxaphosphirane I2 and ring enlargement to the oxaphospholene 5. The use of electron-withdrawing substituents (both for X and Y) results in a change in the reaction pathway in which phosphate attacks the carbon atom β to the carbonyl group in the rate-limiting step followed by a fast ring closure of I3 to oxaphospholene 5.

Oxygen Atom Transfer, Coupled Electron-Proton Transfer, and Correlated Electron-Nucleophile Transfer Reactions of Oxomolybdenum(IV) and Dioxomolybdenum(VI) Complexes.

The oxo-Mo(IV) complexes LMoO(S(2)PR(2)-S,S') [L = hydrotris(3,5-dimethylpyrazol-1-yl)borate; R = Me, Et, Pr(i)(), Ph] were prepared by reacting MoO(S(2)PR(2))(2) and KL in refluxing toluene. The dioxo-Mo(VI) complexes cis-LMoO(2)(S(2)PR(2)-S) (R = Pr(i)(), Ph) were prepared by oxidation of the oxo-Mo(IV) complexes or by reaction of LMoO(2)Cl with NaS(2)PR(2). Oxygen atom transfers from Me(2)SO to LMoO(S(2)PR(2)) were first-order with respect to Me(2)SO and complex; the overall second-order rate constants at 40 degrees C range from 9.0(1) x 10(-)(5) M(-)(1).s(-)(1) for LMoO(S(2)PMe(2)) to 2.08(5) x 10(-)(4) M(-)(1).s(-)(1) for LMoO(S(2)PPr(2)); activation parameters were in the ranges DeltaH() = 63(1) to 73(1) kJ.mol(-)(1), DeltaS() = -88(1) to -111(1) J.K(-)(1).mol(-)(1), and DeltaG() = 100(2) kJ.mol(-)(1) for LMoO(S(2)PMe(2)) to 98(2) kJ.mol(-)(1) for LMoO(S(2)PPr(2)). Oxygen atom transfer from pyridine N-oxide to LMoO(S(2)PPr(2)) was also second-order with a rate constant of 1.54(5) x 10(-)(3) M(-)(1).s(-)(1) at 40 degrees C, DeltaH() = 62(1) kJ.mol(-)(1), DeltaS() = -90(1) J.K(-)(1).mol(-)(1), and DeltaG() = 90(1) kJ.mol(-)(1). The second-order rate laws and large negative entropies of activation are consistent with associative mechanisms for the above reactions. Oxygen atom transfer from LMoO(2)(S(2)PPr(2)) to PPh(3) was first-order with respect to reactants, with an overall second-order rate constant of 2.5(3) x 10(-)(4) M(-)(1).s(-)(1) at 30 degrees C. In toluene at 40 degrees C, all the above complexes catalyzed the oxidation of PPh(3) by Me(2)SO, with turnover rates of ca. 0.9 mol of PPh(3)/(mol of catalyst/h). Reduction of LMoO(2)(S(2)PR(2)) by SH(-) led to the generation of the dioxo-Mo(V) anions [LMoO(2)(S(2)PR(2)-S)](-), which were slowly converted to the analogous oxothio-Mo(V) complexes [LMoOS(S(2)PR(2)-S)](-). Dioxygen reacted with [LMoOS(S(2)PPr(2))](-) to produce the oxothio-Mo(VI) complex LMoOS(S(2)PPr(2)-S). The (hydroxo)oxo-Mo(V) complexes LMoO(OH)(S(2)PR(2)-S) were formed upon reduction of LMoO(2)(S(2)PR(2)) with PPh(3) in wet (3-5 M H(2)O) tetrahydrofuran or upon ferrocenium oxidation of LMoO(S(2)PR(2)) in wet tetrahydrofuran. In dry solvents, LMoO(S(2)PR(2)) were oxidized to the corresponding cations, [LMoO(S(2)PR(2)-S,S')](+), which reacted with water to form LMoO(OH)(S(2)PR(2)). The Mo(V) complexes have been characterized by EPR spectroscopy.

Oxidation and Reduction Reactions of Hydroperoxo Cobalt Macrocycles

The reduction of LCoOOH2+ (L = L' = [14]aneN4 and L2 = Me6[14]aneN4) by Cr2+, Fe2+, V02+, V2+, Ti3+, and Co(tim)*+ has been studied in acidic aqueous solution. The L'CoOOH2+/Fe2+ reaction takes place with a second-order rate constant 4.8 L mol-' s-' at 25 C and 0.10 M H@+. It is accompanied by a large negative entropy of activation, -122 J mol-' K-I. In the presence of a large excess of Fez+, this reaction proceeds with a 1:2 stoichiometry and produces L'Co3+ and Fe3+. A Fenton mechanism is proposed for this and other reduction reactions of LCoOOH2+. The oxidation of LCoOOH2+ by R~(bipy)3~+, Fe(~hen)3~+, IrChj2-, VO2+, and Fe3+ has also been investigated. The outer-sphere electron transfer reactions with Ru(bipy)j3+, Fe(~hen)3~+, and proceed with a 1:2 stoichiometry. The reaction of L'CoOOH2+ with Fe3+ produces 0 2 with a second-order rate constant 2.74 L mol-' s-' at 25 C and 0.10 M H@+. Unlike the reduction reactions, the kinetics of the oxidation of L'CoOOH2+ are acid-dependent.

Oxidation of thiosulfate by manganese(III) and nickel(III) complexes: A kinetic and mechanistic investigation

AbstractThe kinetics of the oxidation of thiosulfate ion by [Mn(Y)]− (H4Y = trans‐cyclohezane‐1,2‐dimine‐N,N,N′,N′‐tetraacetic acid) and [Ni(L1)]2+ (HL1 = 15‐amino‐3‐methyl‐4,7,10,13‐tetraazapentadec‐3‐en‐2‐one oxime) have been investigated at 20.0, 30.0, and 40.0°C in aqueous solution and found to follow the general rate law −d/dt [complex] = kox[S2O32−] [complex]. The large negative entropy (ΔS‡) of activation for the reactions and the results of Marcus calculation provide support for inner‐sphere mechanism to be operative. © 1995 John Wiley & Sons, Inc.

Kinetic studies on the pyramidal inversion of optically active selenonium imides

AbstractKinetic studies were carried out on the thermal epimerization by pyramidal inversion of optically active (S)‐(−)Se‐ and (R)‐(+)Se‐4‐[(‐)‐menthyloxycarbonyl]phenyl(2′,4′,6′‐triisopropylphenyl)selenonium‐N‐toluene‐4″‐sulphonimides (1). The first‐order rate constants for the pyramidal inversion of (S)‐(−)Se‐1 and (R)‐(+)Se‐1 were determined and the activation enthalpies and entropies were calculated. A fairly large difference was found in the activation parameters between optically active (S)‐(−)Se‐1 and (R)‐(+)Se‐1, i.e. a large negative activation entropy was obtained in the case of (R)‐(+)Se‐1.

Model Reaction of Self-Condensation of Sulfenic Acids. Kinetic Investigation for the Reaction of Methyl Benzenesulfenate with trans-Decalin-9-sulfenic Acid and 2-Methyl-2-propanesulfenic Acid

Abstract Reaction of 2-methyl-2-propanesulfenic acid (1) with methyl arenesulfenates (2) gave S-aryl 2-methyl-2-propanethiosulfinates quantitatively, and kinetic investigation was carried out. Second-order rate dependence (first-order in methyl benzenesulfenate (2a) and first-order in the sulfenic acid (1)) was observed, and a small activation enthalpy (ΔH‡ = 28 kJ mol−1) and a large negative activation entropy (ΔS‡ = −130 J K−1 mol−1) were obtained. Kinetics for the reaction of trans-decalin-9-sulfenic acid (3) with the sulfenates 2 under acidic conditions in methanol–water gave the observation of the second-order dependence, a small activation enthalpy (ΔH‡ = 30.5 kJ mol−1), a large negative activation entropy (ΔS‡ = −140 J K−1 mol−1), a large negative Hammett ρ-value (ρ = −1.39) and no solvent isotope effect (kMeOH/kMeOD = 1.10—1.14 in the range of 15.64—29.98 °C). Kinetics for acidic hydrolysis of the sulfenate 2 under the same conditions afforded the similar activation parameters (ΔH‡ = 40.7 kJ mol−1, ΔS‡ = −200 J K−1 mol−1 at [H2O] = 0.80 × 10−3mol dm−3) and a larger negative Hammett ρ-value (ρ = −2.22) than that for the reaction of the sulfenic acid 3 with the sulfenate 2. From these results, structures of the transition state for both the self-condensation of sulfenic acid and the hydrolysis of the sulfenate ester 2 were discussed. The reaction of sulfenate esters 2 with stable sulfenic acids is a good model for studying the self-condensation of sulfenic acids.

Kinetics and mechanism of addition of tertiary phosphines and phosphites to the dicarbonyl(η5-cyclopentadienyl)-(η-ethene)iron cation

Kinetic studies of the addition of a range of tertiary phosphine and phosphite nucleophiles PR3 to the cation [Fe(cp)(CO)2(η-C2H4)]+1(cp =η5-C5H5)[equation (i)] revealed the general rate law, Rate =k1[Fe][PR3]. The second-order rate constants k1 decrease markedly down the order P(C6H4OMe-2)3 > PBun3 > P(C6H4OMe-4)3 > P(C6H4Me-4)3 > P(C6H4Me-4)Ph2 > PPh3 > P(C2H4CN-2)Ph2 > P(C2H4CN-2)3 > P(C6H4Cl-4)3 > P(OBun)3. This reactivity order parallels that of decreasing electron availability at the phosphorus centre, as shown quantitatively by the good correlation between log k1 and the Tolman Σχ values. An excellent fit to the Hammett and Bronsted equations is also observed for reaction (i) with the nucleophiles P(C6H4X-4)3. The moderate Bronsted slope α of [Fe(cp)(CO)2(η-C2H4)]++ PR3→[Fe(cp)(CO)2(C2H4PR3)]+(i) 0.46 establishes the importance of phosphine basicity in determining nucleophilicity towards the ethene ligand in cation 1. These results, together with the large negative entropy of activation with PPh3(ΔS‡1=–103 J K–1 mol–1), are interpreted in terms of direct addition (k1) of the phosphorus nucleophiles to the ethene ligand in 1 and suggest a transition state in which there is build-up of positive charge on the phosphorus centre and considerable phosphorus–carbon bond formation.

Kinetics and mechanisms of the redox reactions of the hydroperoxochromium(III) ion

The reactions of the hydroperoxochromium(III) ion, (H[sub 2]O)[sub 5]CrO[sub 2]H[sup 2+] (CrO[sub 2]H[sup 2+]), with Fe[sup 2+], VO[sup 2+], V[sup 2+], Cu[sup +], Ti[sup 3+], Co([14]aneN[sub 4])[sup 2+], Co(Me[sub 6][14]aneN[sub 4])[sup 2+], Co(tim)[sup 2+], and [Ru(NH[sub 3])[sub 6]][sup 2+] have been studied in acidic aqueous solution. The reactions are accompanied by large negative entropies of activation, [minus]110 J mol[sup [minus]1] K[sup [minus]1] for Fe[sup 2+] and [minus]85 J mol[sup [minus]1] K[sup [minus]1] for Ti[sup 3+]. All the reactions studied follow an isokinetic relationship in that [delta]H[sup [double dagger]] is a linear function of [delta]S[sup [double dagger]]. The same is true for the analogous reactions of H[sub 2]O[sub 2]. It is proposed that the reactions of CrO[sub 2]H[sup 2+] take place by an inner-sphere, Fenton-type process yielding pentaaquaoxochromium(IV), (H[sub 2]O)[sub 5]CrO[sup 2+] (CrO[sup 2+]), an an intermediate. The reactivity of CrO[sub 2]H[sup 2+] as an oxygen transfer reagent is about 20 times greater than that of H[sub 2]O[sub 2]. For example, the reactions with (en)[sub 2]CoSCH[sub 2]CH[sub 2]NH[sub 2][sup 2+] to yield (en)[sub 2]CoS(O)CH[sub 2]CH[sub 2]NH[sub 2][sup 2+] have rate constants 20.5 [+-] 0.4 M[sup [minus]1] s[sup [minus]1] (CrO[sub 2]H[sup 2+]) and 1.36 M[sup [minus]1] s[sup [minus]1] (H[sub 2]O[sub 2]), bothmore » in 0.1 M HClO[sub 4] at 25[degrees]C. The chromyl ion, CrO[sup 2+], oxidizes CrO[sub 2]H[sup 2+] to CrO[sub 2][sup 2+] with a rate constant of (1.34 [+-] 0.06) [times] 10[sup 3] M[sup [minus]1] s[sup [minus]1] in 0.10 M HClO[sub 4] in H[sub 2]O and 266 [+-] 10 M[sup [minus]1]s[sup [minus]1] in D[sub 2]O.« less

Kinetics of ion‐exchange on montmorillonite clays

AbstractThe kinetics for the exchange of Li+, K+, Rb+, and Cs+ for Na+ as the exchangeable cation on bentonite and montmorillonite K10 and KSF have been studied using conductimetric stoppedflow. Dilute aqueous suspensions of the clays, of particle sizes of a few micrometers, were used, so that diffusion was fast and the rate‐determining step was the substitution of one cation by another on the lattice surface. The kinetics were treated in terms of relaxation from equilibrium. Relaxation times ranged from 100 to 250 ms, and forward rate constants from 30 to 500 M−1 s−1. The reactions had very low activation enthalpies (7–25 kJ mol−1) and were only slow enough to be studied by the stopped‐flow technique because of the large negative entropies of activation (−120 to −170 J K−1 mol−1). © 1993 John Wiley & Sons, Inc.

Inner Rotation of the Exocyclic C=C Bonds of Methyl 2-Cyano-(2-methylcyclohepta[b]thiophen-8-ylidene)acetate and Methyl 2-Cyano-(1,2-dimethylcyclohepta[b]pyrrol-8-ylidene)acetate

Abstract The rotational barrier of the exocyclic C=C bonds of heptafulvenes condensed with a hetero aromatic ring is dependent on electron donating ability of heterocycles and on the steric repulsion around the exocyclic C=C bond. The large negative entropies of activation are consistent with an extensively polarized transition state.

Hydration of bis(pentamethylphenyl)- and bismesityl-ketenes leading to ene-1,1-diols (enols of carboxylic acids)

The reaction of the sterically hindered diarylketenes (Ar = Me5C6 or 2,4,6-Me3C2H2) with water is pH independent over the range 1–9 and is not subject to strong buffer catalysis. The primary products formed are the corresponding ene-1,1-diols which result from addition across the CO (rather than the CC) of the ketene. These ‘enols of carboxylic acids’ are relatively long lived owing to slow protonation of the β-carbon (because of steric hindrance caused by the o-Me groups) and evidence for their structures in solution is presented. On attempted isolation of the ene-1,1-diols facile oxidation to a stable free radical, as well as ketonisation to the corresponding acids, occurs. Evidence is presented in terms of the large negative entropy of activation (ca. 200 J K–1 mol–1), solvent isotope effects and the absence of significant general-base catalysis, that (despite the sterically hindered nature of these ketenes) the characteristics of their hydration are typical of other ketenes and that this most likely involves an initial concerted reaction with a water oligomer. Updated ab initio calculations are also presented, which are consistent with this view.

Sensitized photooxygenation VI. Kinetic studies on the singlet oxygenation of 6‐ethyl‐3, 4‐dihydro‐2H‐pyran‐5‐carboxylic acid ethyl ester and 6‐isopropyl‐3,4‐dihydro‐2H‐pyran‐5‐carboxylic acid ethyl ester

AbstractKinetic studies of the singlet oxygenation of the title compounds were performed according to Monroe's method. The reaction rate increases with temperature decreasing, leading to a negative activation enthalpy and a large negative activation entropy. These data are interpreted as the evidence for the intermediacy of an exciplex. The solvent effect on the reaction rate suggests that the “dioxetane” path involves a transition state or an intermediate with significant zwitterionic character. The electronic effect of the substituent is obvious, with electron‐withdrawing substituent retarding the reaction and electron‐donating substituent increasing the reaction rate. However, steric bulkiness at the 6‐position does not play an important role in the reaction rate.

Kinetics and mechanism of the addition of triphenylphosphoniocyclopentadienide to tetrachloro-p-benzoquinone

The kinetics and mechanism of the reaction of triphenylphosphoniocyclopentadienide 1 with tetrachloro-p-benzoquinone (chloranil 2a) to yield 2,3,5-trichloro-6-(3-triphenylphosphoranylidene-cyclopenta-1,4-dienyl)-p-benzoquinone 3 in dichloromethane solution are reported. The rate limiting step was found to be electrophilic attack of tetrachloro-p-benzoquinone on the aromatic cyclopentadiene ring of the ylide. A thermochemical study of the system showed a small enthalpy and a large negative entropy of activation in agreement with the existence of a highly polar betaine as ah intermediate in the reaction. The proposed mechanism involves bimolecular transfer of a proton to a molecule of ylide. No evidence was found either for the formation of a π-complex or for acid catalysis of the reaction.

Kinetics, isotope effects of the reaction of 1-(4-nitrophenyl)-1-nitroalkanes with DBU in tetrahydrofuran and chlorobenzene solvents

The kinetics of the reaction of[Formula: see text](R = Me, Et, i-Pr; NPNE, NPNP, MNPNP respectively; L is H or D) with 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) base in tetrahydrofuran (THF) and chlorobenzene (CB) solvents are reported. The products of these proton transfer reactions are ion pairs absorbing at λmax = 460–480 nm. The equilibrium constants in THF were [Formula: see text]and in CB [Formula: see text]for NPNE, NPNP, MNPNP respectively. The thermodynamic parameters of the reactions are also quoted. The substrate reacts with DBU in both THF and CB solvents in a normal second-order proton transfer reaction. In the case of deuteron transfer, isotopic D/H exchange is much faster than internal return. The reactions show low values of enthalpy of activation ΔH* = 14.3, 18.1, 24.2 and 13.0, 15.1, 18.6 kJmol−1 for NPNE, NPNP, and MNPNP in THF and CB respectively, and large negative entropies of activation −ΔS* = 141, 139, 146; 140, 146, 160 J mol−1 deg−1 for the same sequence of substrates and solvents. The kinetic isotope effects are large, (kH/kD)20°c = 12.2, 13.0, 10.1; 12.9, 12.0, 10.2 for the above sequence of substrates and solvents, and show no difference with changes in either steric hindrance of the C-acids or polarity of the solvents. Keywords: proton transfer, kinetic isotope effect.