Kinetics of the C-C bond beta scission reactions in alkyl radical reaction class

Kinetics of the β-scission in alkyl radical reaction class was studied using the reaction class transition state theory (RC-TST) combined with the linear energy relationship (LER) and the barrier height grouping (BHG) approach. All necessary parameters were derived from first-principle density functional calculations for a representative set of 21 reactions. Different error analyses and comparisons with available literature data were made. Direct comparison with available experimental data indicates that the RC-TST/LER, where only reaction energy is needed, can predict rate constants for any reaction in this reaction class with excellent accuracy. Specifically for this reaction class, the RC-TST/LER method has less than 60% systematic errors on average in the predicted rate constants when compared to explicit rate calculations.     

Reactions of the alkoxy radicals formed following OH-addition to alpha-pinene and beta-pinene. C-C bond scission reactions.

The atmospheric degradation pathways of the atmospherically important terpenes alpha-pinene and beta-pinene are studied using density functional theory. We employ the correlation functional of Lee, Yang, and Parr and the three-parameter HF exchange functional of Becke (B3LYP) together with the 6-31G(d) basis set. The C-C bond scission reactions of the beta-hydroxyalkoxy radicals that are formed after OH addition to alpha-pinene and beta-pinene are investigated. Both of the alkoxy radicals formed from the alpha-pinene-OH adduct possess a single favored C-C scission pathway with an extremely low barrier (approximately 3 kcal/mol) leading to the formation of pinonaldehyde. Neither of these pathways produces formaldehyde, and preliminary computational results offer some support for suggestions that 1,5 or 1,6 H-shift (isomerization) reactions of alkoxy radicals contribute to formaldehyde production. In the case of the alkoxy radical formed following OH addition to the methylene group of beta-pinene, there exists two C-C scission reactions with nearly identical barrier heights (approximately 7.5 kcal/mol); one leads to known products (nopinone and formaldehyde) but the ultimate products of the competing reaction are unknown. The single C-C scission pathway of the other alkoxy radical from beta-pinene possesses a very low (approximately 4 kcal/mol) barrier. The kinetically favored C-C scission reactions of all four alkoxy radicals appear to be far faster than expected rates of reaction with O2. The rearrangement of the alpha-pinene-OH adduct, a key step in the proposed mechanism of formation of acetone from alpha-pinene, is determined to possess a barrier of 11.6 kcal/mol. This value is consistent with another computational result and is broadly consistent with the modest acetone yields observed in product yield studies.     

Kinetics of the reactions of substituted benzoquinones with alkyl radicals

Conclusions Study has been made of the kinetics of the reactions of sec-decyl radicals with 2,6-di-tert-butyl-, and 2,6-diphenyl-p-benzoquinone, and 2,4- and 2,5-di-tert-butyl-o-benzoquinones at 50°C. Rate constants and inhibition coefficients have been determined for these reactions.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 2487–2491, November, 1978.     

Evidence that alkyl substitution provides little stabilization to radicals: the C-C bond test and the nonbonded interaction contradiction

Although the C-H bond dissociation energies of alkanes have been widely employed as measures of radical stability, it is shown here that the assumptions needed for that conclusion are incompatible with experimental and computational data related to C-C bond dissociation energies. Calculations at the QCISD(T)/6-311+G(d,p) level on model systems show that 1,3 nonbonded interactions in alkanes are repulsive, whereas the conventional radical stabilization analysis of bond dissociation energies requires that they become more attractive with increasing steric bulk. This result puts a severe limit on the role that radical stabilization can play and indicates that another factor must be responsible for the observed variation in the C-H bond dissociation energies of alkanes.     

对羟基苯乙酮及其类似物的烷羰C————C键的断裂

甲基、乙基、正丙基和苄基对羟苯基酮和6－羟基甲氢萘－1－酮经证实在合成乙二醇缩酮的条件下发生烷羰C－C键的断裂,而对羟基二苯酮、对羟基苯异丁酮和5－羟基二氢茚－1－酮却不发生这一断键反应。经推论这一断键反应首先是发生羟醛缩合,继以在酚羟基参与下引发烷羰C－C键断裂的;还应指出,也是由于对位羟基的参与,这些烷基对羟苯基酮不形成它们的乙醇缩酮。     

Relationship between bond dissociation energies and activation energies for bond scission reactions

Bond dissociation energies are frequently derived from values of the high pressure activation energy for bond scission reactions. The value derived depends on the transition state structure chosen for the reaction. We consider several models of the transition state and show that the variation in derived BDE values can be quite substantial, 3 to 6 kcal/mol at the high temperatures of pyrolysis kinetics. Application of the restricted Gorin model of the transition state results in BDE values in good agreement with current thermochemistry, while the other models tested result in lower to much lower values. © 1994 John Wiley & Sons, Inc.     

Kinetics and mechanism of isomerization of alkyl radicals in solution

It has been shown that in alkyl radicals the 1, 5-rearrangement of hydrogen atoms takes place. The isomerization rate constants for radicals of various structure have been estimated. There is a compensation effect for their Arrhenius parameters.

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, 1,5- . . .

     

Microwave-assisted C-C bond forming cross-coupling reactions: an overview

Among the fundamental transformations in the field of synthetic organic chemistry, transition-metal-catalyzed reactions provide some of the most attractive methodologies for the formation of C-C and C-heteroatom bonds. As a result, the application of these reactions has increased tremendously during the past decades and cross-coupling reactions became a standard tool for synthetic organic chemists. Furthermore, a tremendous upsurge in the development of new catalysts and ligands, as well as an increased understanding of the mechanisms, has contributed substantially to recent advances in the field. Traditionally, organic reactions are carried out by conductive heating with an external heat source (for example, an oil bath). However, the application of microwave irradiation is a steadily gaining field as an alternative heating mode since its dawn at the end of the last century. This tutorial review focuses on some of the recent developments in the field of cross-coupling reactions assisted by microwave irradiation.     

Decarbonylative C-C bond forming reactions mediated by transition metals

New methods for carbon-carbon (C-C) forming reactions are constantly emerging in the field of organic synthesis. In this review, a brief history followed by recent developments of decarbonylative C-C forming reactions mediated by transition metals is described. Many different substrates are presented and the review is organized by the different carbonyl precursors, such as acyl chlorides, aldehydes, anhydrides, esters and ketones, used in the respective transformations. Furthermore, the broad scope of these reactions is exhibited by the application to several reaction types (e.g. Heck-type reactions, Suzuki cross-coupling type reactions, C-H activation, etc.) as well as a natural product synthesis (e.g. muscroride A). While several examples are provided, this review marks the beginning of a new field that is still in its infancy and for what might be a new approach to achieve highly efficient reactions that come closer to meeting the standards of chemical economies (e.g. atom, redox, step, etc.) and green chemistry.     

Metallo-organic domino reactions: C-H versus C-C bond breaking

HL and MeL are prepared by condensing benzil dihydrazone with 2-formylpyridine and 2-acetylpyridine, respectively, in 1:2 molar proportions. While in a reaction with [Ru(C(6)H(6))Cl(2)]2, HL yields the cation [Ru(C(6)H(6)){5,6-diphenyl-3-(pyridin-2-yl)-1,2,4-triazine}Cl]+, MeL gives the cation [Ru(C(6)H(6))(MeL)Cl]+. Both the cations are isolated as their hexafluorophosphate salts and characterised by X-ray crystallography. In the case of HL, double domino electrocyclic/elimination reactions are found to occur. The electrocyclic reaction occurs in a C=N-N=C-C=N fragment of HL and the elimination reaction involves breaking of a C-H bond of HL. Density functional calculations on model complexes indicate that the identified electrocyclic reaction is thermochemically as well as kinetically feasible for both HL and MeL in the gas phase. For a double domino reaction, similar to that operative in HL, to occur for MeL, breaking of a C-C bond would be required in the elimination step. Our model calculations show the energy barrier for this elimination step to be much higher (329.1 kJ mol(-1)) for MeL than that for HL (96.3 kJ mol(-1)). Thus, the domino reaction takes place for HL and not for MeL. This accounts for the observed stability of [Ru(C(6)H(6))(MeL)Cl]+ under the reaction conditions employed.     

Kinetics of the gas-phase reactions of OH radicals with alkyl nitrates at 299 ± 2 K

Relative rate constants for the gas-phase reactions of OH radicals with a series of alkyl nitrates have been determined at 299 ± 2 K, using methyl nitrite photolysis in air as a source of OH radicals. Using a rate constant for the reaction of OH radicals with cyclohexane of 7.57 × 10^?12 cm³/molec·s, the rate constants obtained are (× 10¹² cm³/molec·s): 2-propyl nitrate, 0.18 ± 0.05; 1-butyl nitrate, 1.42 ± 0.11; 2-butyl nitrate, 0.69 ± 0.10; 2-pentyl nitrate, 1.87 ± 0.12; 3-pentyl nitrate, 1.13 ± 0.20; 2-hexyl nitrate, 3.19 ± 0.16; 3-hexyl nitrate, 2.72 ± 0.22; 3-heptyl nitrate, 3.72 ± 0.43; and 3-octyl nitrate, 3.91 ± 0.80. These rate constants, which are the first reported for the alkyl nitrates, are significantly lower than those for the parent alkanes, and a formula, based on the numbers of the various types of C? H bonds in the alkyl nitrates, is derived for rate constant estimation purposes.     

Anomalous C-C bond cleavage in sulfur-centered cation radicals containing a vicinal hydroxy group

1,3-dithianyl cation radicals having alpha-hydroxy-neopentyl or similar groups in position 2, which are generated via oxidative photoinduced electron transfer, undergo anomalous fragmentation necessitating refinement of the accepted mechanism. Experimental and computational data support a rationale in which proton abstraction from the hydroxy group in the initial cation radical does not cause a Grob-like fragmentation, but rather produces a neutral radical species, the alkoxy radical, that undergoes fragmentation in either direction, i.e., cleaving the C-C bond to dithiane or to the tertiary alkyl group.     

Dipyridylketone binding and subsequent C-C bond insertion reactions at cyclopentadienylrhodium

UV irradiation of a solution of (eta5-C5H5)bis(ethene)rhodium in the presence of 2,2'-dipyridylketone yields (eta5-C5H5)Rh(kappa2-N,O-2,2'-dipyridylketone) which inserts ethene to yield a bicyclic oxametallacyclopentane.     

C-C and C-H bond activation reactions in N-heterocyclic carbene complexes of ruthenium

Thermolysis of Ru(PPh3)3(CO)H2 with the N-heterocyclic carbene bis(1,3-(2,4,6-trimethylphenyl)imidazol-2-ylidene) (IMes) results in C-C activation of an Ar-CH3 bond in one of the mesityl rings of the carbene ligand. Upon addition of IMes to Ru(PPh3)3(CO)H2 at room temperature in the presence of an alkene, C-H bond activation is observed instead. The thermodynamics of these C-C and C-H cleavage reactions have been probed using density functional theory.     

Rate constants for the reactions of alkyl radicals with 1,4-cyclohexadiene

An electron paramagnetic resonance (EPR ) technique was used to show that simple alkyl radicals readily abstract hydrogen from 1,4-cyclohexadiene. Rate constants for the reaction were ca. 10⁴–10⁵ M^?1 s^?1 at 300 K and activation energies 5–7 kcal mol^?1. For the stabilized radicals, allyl and benzyl, the rate constants were <10² M^?1 s^?1 at 300 K. The data suggest that 1,4-cyclohexadiene could be used as an effective trap to probe rearrangement reactions of carbon centered radicals and biradicals.     

beta-scission of the N-O bond in alkyl hydroxamate radicals: a fast radical trap

[reaction--see text ] The rate of the beta-scission of the N-O bond in the alkyl hydroxamate radical is faster than 2 x 10(8) s(-)(1). This reaction may be useful as a radical trap.     

Addition reactions of alkyl and carboxyl radicals to vinylidene fluoride

The addition reactions of alkyl radicals CF3* and CH3* and carboxyl radicals C2H5O*, C2H5OCOO*, CF3COO*, and CH3COO* to a vinylidene fluoride (VDF) molecule are studied using ab initio calculations. These radicals were selected because they are intermediate or final products of diacyl peroxides decomposition in the initiation reactions of VDF polymerization. Two combinations of methods for energetics and structure optimization are applied: QCISD/6-311G(d,p)//HF/6-31G(d) and B3LYP/6-311G+(3df, 2p)//B3LYP/6-31G(d). It is found that the formed bond length of the product, the forming bond length of the transition state, and the attack angle of the product structures are not sensitive to the level of theory even though the attack angle of the transition state structures is. Early transition states are obtained upon attack at both high-substituted and nonsubstituted carbon atom VDF ends. Kinetic and thermodynamic control rules play different roles on governing the reactivity of the addition with the studied radicals. Both theoretical methods yield the same trends for the preferential attack site in terms of regioselectivity, barrier energies, and reaction enthalpies. It is shown that the addition reactions of the intermediate radicals C2H5OCOO*, CF3COO*, and CH3COO* of the decomposition of diethyl peroxydicarbonate, trifluoroacetyl peroxide, and diacetyl peroxide initiators yield smaller energy barriers than the additions of the corresponding final radicals, C2H5O*, CF3*, and CH3*; therefore, the reactions of the intermediate radicals should not be ignored when analyzing the initiation process of the VDF polymerization using those initiators.     

New C-N and C-C bond forming reactions catalyzed by titanium complexes

Titanium(IV) complexes with two basic dimethylamido ligands can be efficient catalysts for alkyne hydroamination by primary amines. Readily prepared pyrrolyl ligands have proven to be especially useful in generating active catalysts for hydroamination and hydrohydrazination. From these reactions, new methodologies for the synthesis of imines, hydrazones, alpha,beta-unsaturated imines, pyridines, indoles, and pyrroles have developed. In addition, a new reaction that is mechanistically related to hydroamination, alkyne iminoamination, has been discovered. Iminoamination, which is a multicomponent coupling between an amine, alkyne, and isonitrile, provides unsymmetrical alpha, beta-unsaturated beta-iminoamines in a single, titanium-catalyzed step.