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Herbert Mayr

Publications and source records attributed to Herbert Mayr.

35 records · Page 2Linked to original sources
PubMed2003

Ambident reactivity of the thiocyanate anion revisited: can the product ratio be explained by the hard soft acid base principle?

Laser flash photolysis and stopped-flow methods have been employed to determine the kinetics of the reactions of benzhydrylium ions with both termini of the thiocyanate ion. In contrast to previous investigations which reported sulfur/nitrogen ratios of k(S)/k(N) = 2-10 for the reactions of carbocations with SCN(-), values of k(S)/k(N) = 10(3)-10(4) are now derived from absolute rate constants. This discrepancy is explained by the fact that the data determined in this investigation are the first which refer to activation-controlled attack of carbocations at both termini of the thiocyanate ion, while previous reactivity ratios included diffusion-controlled reactions. It is concluded that the selectivities of the reactions of carbocations with the thiocyanate ion cannot be explained by the hard soft acid base principle.

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PubMed2003

Nucleophilic reactivities of carbanions in water: the unique behavior of the malodinitrile anion.

The kinetics of the reactions of nine carbanions 1a-i, each stabilized by two acyl, ester, or cyano groups, with benzhydrylium ions in water were investigated photometrically at 20 degrees C. Because the competing reactions of the benzhydrylium ions with water and hydroxide ions are generally slower, the second-order rate constants of the reactions of the benzhydrylium ions with the carbanions can be determined with high precision. The rate constants thus obtained can be described by the Ritchie equation, log(k/k(0)) = N(+) (eq 1), which allows us to calculate Ritchie N(+) parameters for a series of stabilized carbanions, for example, malonate, acetoacetate, malodinitrile, etc., and compare them with those of other n-nucleophiles in water (hydroxide, amines, azide, thiolates, etc.). Because the Ritchie relationship (eq 1) is a special case of the more general relationship log k = s(N + E) (eq 4), the reactivity parameters N and s for the carbanions 1a-i can also be calculated and compared with the nucleophilic reactivities of a large variety of n-, pi-, and sigma-nucleophiles, including reactivities of carbanions in dimethyl sulfoxide. While the acyl and ester substituted carbanions are approximately 3 orders of magnitude less reactive in water than in dimethyl sulfoxide, the malodinitrile anion (1i) shows almost the same reactivity in both solvents. Correlations between the nucleophilic reactivities of carbanions with the pK(a) values of the corresponding CH acids reveal that the malodinitrile anion (1i) is considerably more nucleophilic than was expected on the basis of its pK(a) value. This deviation is assigned to the exceptionally low Marcus intrinsic barriers of the reactions of the malodinitrile anion (1i).

Journal Article↗
PubMed2003

Role of electron-transfer processes in reactions of diarylcarbenium ions and related quinone methides with nucleophiles.

Second-harmonic alternating current voltammetry has been used to determine one-electron reduction potentials of 15 diarylcarbenium ions and 5 structurally analogous quinone methides, which have been employed as reference electrophiles for the development of nucleophilicity scales. A linear correlation (r(2) = 0.993) between the empirical electrophilicity parameters E and the reduction potentials in acetonitrile (E = 14.091E degrees (red) - 0.279) covering a range of 1.64 V (or 158 kJ mol(-)(1)) has been observed. For a large number of nucleophiles, it has been demonstrated that the observed activation free energies of the electrophile-nucleophile combinations are 61-195 kJ mol(-)(1) smaller than the free energy change of electron transfer from nucleophile to electrophile, which definitely excludes outer-sphere electron transfer occurring during these reactions.

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PubMed2003

How nucleophilic are diazo compounds?

The kinetics of the reactions of benzhydryl cations with eight diazo compounds 1 a-g were investigated photometrically in dichloromethane. The nucleophilicity parameters N and slope parameters s of these diazo compounds were derived from the equation log k (20 degrees C)=s (E+N) and compared with the nucleophilicities of other pi systems (alkenes, arenes, silyl enol ethers, silyl ketene acetals). It is shown that the nucleophilic reactivities of diazo compounds cover more than ten orders of magnitude, being comparable to that of styrene on the low reactivity end and to that of enamines on the high reactivity end. The rate-determining step of these reactions is the electrophilic attack at the diazo-carbon atom to yield diazonium ions, which rapidly lose nitrogen.

Journal Article↗
PubMed2003

Electrophilicity parameters for benzylidenemalononitriles.

Kinetics of the reactions of stabilized carbanions (derived from nitroethane, diethyl malonate, ethyl cyanoacetate, ethyl acetoacetate, acetyl acetone) with benzylidenemalononitriles have been determined in dimethyl sulfoxide solution at 20 degrees C. The second-order rate constants are employed to determine the electrophilicity parameters E of the benzylidenemalononitriles according to the correlation equation log k (20 degrees C) = s(E + N). Comparison with literature data shows that this equation allows the semiquantitative prediction of the reactivities of benzylidenemalononitriles toward a wide variety of nucleophiles, including carbanions, enamines, amines, water, and hydroxide.

Journal Article↗
PubMed2003

Structure-nucleophilicity relationships for enamines.

The kinetics of the reactions of benzhydryl cations with 22 enamines, three pyrroles, and three indoles were investigated photometrically in dichloromethane. The nucleophilicity parameters N and slope parameters s of these electron-rich pi-systems were derived from equation log k (20 degrees C)=s(E+N) and compared with the nucleophilicities of other pi-systems (silyl enol ethers, silyl ketene acetals) and carbanions. It is shown that the nucleophilic reactivities of enamines cover more than ten orders of magnitude, comparable to enol ethers on the low reactivity end and to carbanions on the high reactivity end. Since the products of N-attack are thermodynamically less stable than the reactants, the observed rate constants refer to the formation of the carbon bond;carbon bonds. In some cases, equilibrium constants for the formation of iminium ions were measured, which allow one to determine the intrinsic rate constants of these reactions.

Journal Article↗
PubMed2003

How constant are Ritchie's "constant selectivity relationships"? A general reactivity scale for n-, pi-, and sigma-nucleophiles.

The kinetics of 82 reactions of benzhydrylium ions (Ar(2)CH(+)) with n-nucleophiles has been determined at 20 degrees C. Evaluation by the equation log k = s(N + E) delivered the reactivity parameters N and s for 15 n-nucleophiles (water, hydroxide, amines, etc.). All nucleophiles except water (s = 0.89) and (-)SCH(2)CO(2)(-) (s = 0.43) have closely similar slope parameters (0.52 < s < 0.71), indicating that the reactions of most n-nucleophiles approximately follow Ritchie's constant selectivity relationship (s = constant). The different slope parameter for water is recognized as the main reason for the deviations from the Ritchie relationship reported in 1986. Correlation analysis of the rate constants for the reactions of benzhydrylium ions with the n-nucleophiles (except H(2)O) on the basis of Ritchie's equation log k = N(+) + log k(0) yields a statistically validated set of N(+) parameters for Ritchie-type nucleophiles and log k(0) parameters for benzhydrylium ions. The N and s parameters of the n-nucleophiles derived from their reactions with benzhydrylium ions were combined with literature data for the reactions of these nucleophiles with other carbocations to yield electrophilicity parameters E for tritylium, tropylium, and xanthylium ions. While the E parameters for tropylium and xanthylium ions appear to be generally applicable, it is demonstrated that the E parameters of tritylium ions can be used to predict reactivities toward n-nucleophiles as well as hydride transfer rate constants but not rates for the reactions of tritylium ions with pi-nucleophiles. It is now possible to merge the large data sets determined by Ritchie and others with our kinetic data and present a nucleophilicity scale comprising n- (e.g., amines), pi- (e.g., alkenes and arenes), and sigma-nucleophiles (e.g., hydrides).

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PubMed2003

Pi-nucleophilicity in carbon-carbon bond-forming reactions.

Which electrophiles react with which nucleophiles? The correlation log k(20 degrees Celsius) = s(E + N), in which electrophiles (carbocations, metal-pi-complexes, diazonium ions) are characterized by one (E) and nucleophiles are characterized by two parameters (N, s), proved to be applicable for a wide variety of electrophile-nucleophile combinations. Since the introduction of this correlation in 1994 (Angew. Chem., Int. Ed. Engl. 1994, 33, 938-957), numerous new reagents have been characterized, and in 2001 (J. Am. Chem. Soc. 2001, 123, 9500-9512), a new method of parametrization was proposed that facilitates a continuous extension of the data sets without the need for reparametrization of existing data. This Account adjusts the N and s parameters of all presently characterized pi-nucleophiles (arenes, alkenes, organometallics) to the new parametrization and illustrates how to employ the resulting reactivity scales for analyzing synthetic and mechanistic problems in organic and macromolecular chemistry. Predictions of absolute rate constants, inter- and intramolecular selectivities, and analyses of reaction mechanisms are discussed. We outline how new compounds can be added to the scales and present our view on the scope and limitations of this approach to polar organic reactivity.

Journal Article↗
PubMed2003

Clinical and angiographic outcome after cutting balloon angioplasty.

The cutting balloon is a new device for coronary angioplasty, that, by the combination of incision and dilatation of the plaque, is believed to be promising for treatment of in-stent restenosis. The purpose of the study was to evaluate the safety and efficacy of CBA. We reviewed the immediate and 6-month follow-up angiographic and clinical outcome of 147 patients (109 men and 38 women) with a mean age of 67.3 +/- 10 undergoing this procedure at eight interventional centers in Austria. The target lesions treated with CBA were in-stent restenosis in 61% of patients, stenosis after balloon angioplasty in 8% of patients, and native lesions in 33% of patients. Sixty-five percent of the patients included had multivessel disease. Lesion type was A in 18% of patients, B1 in 31% of patients, B2 in 39% of patients, and C in 12% of patients. The degree of stenosis was 87% +/- 9%, the length of the target lesion treated with CBA was 8.8 +/- 5.1 mm. Target vessel was left circumflex artery in 22 cases, right coronary artery in 36 cases, and left anterior descending artery in 89 cases. The overall procedural success rate was 90.5%. "Stand-alone" CBA was performed in 63% of patients, the procedure was combined with coronary stenting in 16% of patients, and with balloon angioplasty in 21% of patients. Coronary complications occurred in eight cases (5.4%) with coronary dissection in seven (total dissection rate of 4.7%) and urgent bypass surgery in one case (0.7%). No further complications such as death, occlusion, or perforation of coronary arteries, embolization, or thrombosis were observed. Six-month clinical follow-up revealed q-wave myocardial infarction in 2.7% of patients, aortocoronary bypass surgery in 8.5% of patients, and repeated percutaneous coronary intervention in 17% of patients (11.5% with stenting). Six-month angiographic follow-up of patients with recurrent angina showed target lesion restenosis (> 50% diameter stenosis) in 14% of patients, late lumen loss with < or = 50% diameter stenosis in 6% of patients and progression of "other than target" lesions with > 50% diameter stenosis in 14% of patients. This series demonstrates the safety and feasibility of cutting balloon angioplasty in patients with complex coronary artery disease and in-stent restenosis.

Adult↗
PubMed2002

Relationships between carbocation stabilities and electrophilic reactivity parameters, E: quantum mechanical studies of benzhydryl cation structures and stabilities.

Quantum chemical calculations of the affinities of benzhydryl cations (XC(6)H(4))(2)CH(+) for the methyl anion, hydroxide, and hydride anion have been performed up to the B3LYP/6-311++G(3df,2pd)//B3LYP/6-31G(d,p) level and compared with rate and equilibrium constants in solution. An excellent linear correlation between the empirical electrophilicity parameter E (by log k = s(N + E); Mayr, H.; Bug, T.; Gotta, M. F.; Hering, N.; Irrgang, B.; Janker, B.; Kempf, B.; Loos, R.; Ofial, A. R.; Remennikov, R.; Schimmel, H. J. Am. Chem. Soc. 2001, 123, 9500-9512) and the calculated methyl anion affinities ranging over 46.5 kcal mol(-1) is found that reproduces the electrophilicity parameters E which range from -10 to +6 with a standard deviation of +/-0.26 (11 points). The calculated OH(-) and H(-) affinities correlate with the calculated CH(3)(-) affinities with a slope of 1.00, indicating that the relative anion affinities of benzhydrylium ions are independent of the nature of the reference base. Linear correlations of the experimental pK((R)(+)) values and chloride ion affinities in solution with the calculated anion affinities in the gas phase indicate that solvation attenuates the differences of carbocation stabilization in the gas phase but does not affect the relative differences. Application of Marcus theory shows that only reaction series with alpha = partial differential DeltaG(++)/ partial differential Delta(r)G degrees = 0.5 corresponding to a slope parameter of s = 0.67 can have constant intrinsic barriers. The slope parameters s found for pi-nucleophiles and C-H hydride donors (s approximately equal 1) are interpreted by a decrease of the intrinsic barriers with increasing electrophilicities of the carbocations. On the other hand, a value of s < or = 0.67 as found for many n-nucleophiles as well as for Si-H, Ge-H, or Sn-H hydride donors is indicative of intrinsic barriers which are constant or slightly increase with increasing electrophilicity of the carbocation.

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PubMed2002

Reactions of carbocations with unsaturated hydrocarbons: electrophilic alkylation or hydride abstraction?

Benzhydryl cations were used as reference electrophiles to determine the hydride donor reactivities of unsaturated hydrocarbons. The kinetics of the reactions were followed by UV-vis spectroscopy and conductivity measurements, and it was found that the second-order rate constants for the hydride transfer processes were almost independent of the solvents or counterions employed. The rate constants correlate linearly with the previously published empirical electrophilicity parameters E of the benzhydrylium ions. Therefore, the linear free energy relationship log k(20 degrees C) = s(E + N) could be employed to characterize the hydride reactivities of the hydrocarbons by the nucleophilicity parameters N and s. The similarity of the slopes s for hydride donors and pi-nucleophiles allows a direct comparison of the reactivities of these different functional groups based on their nucleophilicity parameters N. Since nucleophilicity parameters of -5 < N < 0 have been found for a large variety of allylic and bisallylic hydride donors, a rule of thumb is derived that hydride transfer processes may compete with carbon-carbon bond-forming reactions when carbocations are combined with olefins of pi-nucleophilicity N < 0.

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PubMed2002

Rate-equilibrium relationships in hydride transfer reactions: the role of intrinsic barriers.

A literature survey on the kinetics of hydride abstractions from CH-groups by carbocations reveals a general phenomenon: Variation of the hydride acceptor affects the rates of hydride transfer to a considerably greater extent than an equal change of the thermodynamic driving force caused by variation of the hydride donor. The origin of this relationship was investigated by quantum chemical calculations on various levels of ab initio and DFT theory for the transfer of an allylic hydrogen from 1-mono- and 1,1-disubstituted propenes (XYC=CH-CH(3)) to the 3-position of 1-mono- and 1,1-disubstituted allyl cations (XYC=CH-CH(2)(+)). The discussion is based on the results of the MP2/6-31+G(d,p)//RHF/6-31+G(d,p) calculations. Electron-releasing substituents X and Y in the hydride donors increase the exothermicity of the reaction, while electron-releasing substituents in the hydride acceptors decrease exothermicity. In line with Hammond's postulate, increasing exothermicity shifts the transition states on the reaction coordinate toward reactants, as revealed by the geometry parameters and the charge distribution in the activated complexes. Independent of the location of the transition state on the reaction coordinate, a value of 0.72 is found for Hammond-Leffler's alpha = deltaDeltaG/deltaDelta(r)G degrees when the hydride acceptor is varied, while alpha = 0.28 when the hydride donor is varied. The value of alpha thus cannot be related with the position of the transition state. Investigation of the degenerate reactions XYC=CH-CH(3) + XYC=CH-CH(2)(+) indicates that the migrating hydrogen carries a partial positive charge in the transition state and that the intrinsic barriers increase with increasing electron-releasing abilities of X and Y. Substituent variation in the donor thus influences reaction enthalpy and intrinsic barriers in the opposite sense, while substituent variation in the acceptor affects both terms in the same sense, in accord with the experimental findings. Marcus theory is employed to treat these effects quantitatively.

Journal Article↗
PubMed2002

[Angiographic results of "atypical" chest pain].

BACKGROUND AND AIMS: Coronary angiography permits evaluation of coronary artery morphology and coronary pathology. It represents an accurate method of defining stenotic coronary lesions. Chest pain may be caused by coronary artery disease as well as by other cardiac and noncardiac disorders. However, sensitivity of clinical evaluation and noninvasive diagnostic assessment in detection of coronary artery disease is limited. Noninvasive diagnostic strategies give inconsistent results in about 10-30%. Here coronary angiography is regarded as an accurate method for appropriate diagnosis. Ist sophisticated apparatus, cost, and invasiveness necessitate well-considered application of this procedure. Therefore, it appears important to analyze coronary angiograms in patients with the referral diagnosis of "atypical" chest pain with inconsistent noninvasive testing or impossibility to perform noninvasive assessment. PATIENTS AND METHODS: We analyzed records of 1,000 consecutive patients (625 men, 375 women, mean age 63.1 years), who underwent coronary angiography at our institution from January 5, 1998 to May 5, 1998. RESULTS: 49 patients (17 women, 32 men; mean age 59 years) were referred due to "atypical" chest pain. 21 (42.9%, nine women, twelve men) of these 49 patients had normal coronary arteries at angiography. 21 (42.9%) patients showed coronary artery disease with a diameter stenosis > 50%. In seven (14.2%) patients, coronary sclerosis with a diameter stenosis < 50% could be observed. Only five (29.4%) of the 17 women but 16 of the 32 men (50%) had coronary artery disease with a diameter stenosis > 50% (p < 0.01). CONCLUSIONS: In unselected patients referred for coronary angiography due to "atypical" chest pain and inconsistent noninvasive testing or impossibility to perform noninvasive assessment. 42.9% had coronary artery disease with a diameter stenosis > 50%. Angiographic evaluation of symptomatic patients with "atypical" signs and symptoms and inconsistent noninvasive testing seems to be appropriate.

Aged↗