Homolytic bond splitting

One-electron oxidation of [Pd (R)2(dmpe)] (R = Me or CH2SiMe3) in MeCN results in homolytic cleavage of the Pd—C bond with formation of alkyl radicals and [Pd (R)(dmpe)(NCMe)]" (118). The radical intermediate [Pd (R)2 (dmpe)] was not observed. The homolytic bond splitting of the Pd —R bond to yield Pd(II) and R radicals represents an oxidatively induced reductive elimination. 

During homolytic bond-splitting reactions, that is pyrolysis reactions, in water less tar and coke is formed than in a neat pyrolysis [57]. Sometimes this is explained by the hydrogen-donation ability of water [57]. Alternative or additional explanations may be dilution by solvation or a cage effect [57]. 

Polarization, charge distribution or atomic coefficients of the C-O bond are decisive for heterolytic bond splitting by nucleophiles [21]. Homolytic bond splitting, instead, is related to radical stability and reactivity, which in turn depends on spin density, spin polarization, delocalization of the free electron and hyper-oconjugation [22, 23]. Essential to all these radical reactions is the one-electron transfer from the trigger, typically under photochemical [24], (in the presence of sensitizer) [25], or electrochemical conditions [26]. 

It can be concluded that the reactive MMCT state is reached by radiationless deactivation from the MLCT state. Homobinuelear complexes such as Mn2(CO)io with a direct metal-metal bond are split homolytically by aa (M-M) excitation [4,7]. In heteronuclear complexes which are characterized by a polar metal-metal bond this aa excitation involves a CT from the donor to the acceptor metal. MMCT (aa ) excitation leads to a homolytic bond splitting which may also be viewed as a redox reaction.,e.g., equation (49) ... 

Adsorption of reactants on the surface of a catalyst represents the first elementary step in a catalytic reaction cycle. Chemisorption and physisorption are two kinds of adsorption, and differ according to the type and strength of bond. In physisorption, the adsorption bond is due to the rather weak van der Waals interactions between permanent or induced dipoles. Chemisorption occurs if a real chemical bond is formed between the substrate and the adsorbate. Molecules may adsorb intact or dissociate on the surface. Catalytic reactions almost always involve the dissociation of at least one of the reacting molecules. In certain highly stable molecules, such as methane or ethane, chemisorption is not possible without the rupture of a C-H bond. Dissociation of molecules on metals leads to predominantly neutral fragments (homolytic bond splitting), whereas on oxides, the dissociation... 

Comparing the reactants and the products, the reactions are apparently nonredox processes. Using a spin-trapping EPR technique it was shown [114] that irradiation of the complexes leads to an alkyl radical formation (CH3 or C2Hj). The efficiency of the homolytic metal-carbon bond splitting depends on the electronic properties of the other axial ligand. The ostensibly non-redox photoinsertions are thus a product of two redox reactions. As far as the photoreactive excited state is concerned, the metal-carbon bond is either indirectly activated by a ir-nt excitation localized on the tetrapyrrole ring [112] or there is an... 

Free radicals are made when a normal covalent bond splits in half with each atom receiving an equal share of the electron pairs. For example a hydrogen molecule H—H splits into two equal halves with each hydrogen atom receiving one electron, H. Covalent bonds that split in half in this way are said to break homolytically . Free radicals are often shown by having an asterisk on their formulae (see also Section 13.2). 

For the simplest case, i.e. compounds with the same kind of scissile bond (similar homolytic bond-dissociation energies and g energies ), the splitting pro-... 

GeneraUy, [M (por)J species (Fig. 41) are prepared by thermally or photo-chemically induced homolytic M—M, M—H, or M—C bond splitting of their corresponding M—M, M —Me or M H precursors (2). Occasionally, (elec-tro)chemical reduction or oxidation has been applied as well (128-130). 

In aU cases, the mononuclear square-planar cP complexes are very reactive. As a result, the least hindered complexes containing the porphyrins OEP (octaethylporphyrinato), TPP tetraphenylporphyrinato, TTP (tetratolylporphyr-inato), and TXP [tetrakis(3,5-dimethylphenyl)porphyrinato] easily dimerize (Fig. 42) to form diamagnetic metal-metal bonded dinuclear species (in which a description as M —is indistinguishable from a antiferromagnetically coupled M°—M system). The Rlu Rh bond is, however, rather weak, and allows thermal homolytic Rh—Rh bond splitting to study the reactivity of the... 

The reactions of [Rh por)] with olefins reveal that the [Rh°(por)(olefin)] intermediates have a substantial spin density at the olefin fragment. This prompted the Wayland group to investigate attempts to initiate radical polymerization of acrylates with these species (154). Although photopromoted radical initiation (involving homolytic Rh—C bond splitting) was achieved, direct radical initiation with [Rh OEP)] or [Rh TMP)] was not observed. 

The differential curves (DTG) indicate the existence of three processes of thermal decomposition, in accordance with the type of split bonds. Thus, the secondary van der Waals bonds, physical hydrogen bonds, and the homolytic splitting of chemical bonds occur at 280, 320, and 360 °C, respectively. The intensity of these processes varies with the magnitude of the applied stress. For instance, the characteristic process of valence bonds splitting is more... 

In certain highly energetic collisions with any molecule M in the system, a bromine molecule may be dissociated in a homolytic split of the bond joining two bromine atoms. 

Only a limited number of peroxides are suitable for crosslinking purposes. The most suitable are those which form radicals through homolytic decomposition. These radicals are so aggressive that they can split hydrogen atoms from the polymer chain, producing stable peroxide decomposition products and polymer radicals. These polymer radicals can then be recombined, producing stable C-C crosslink bonds. 

Alcohols are formed as a result of reactions with the homolytic splitting of the O—O bond of hydroperoxides, for example (see Chapter 4) ... 

Having a weak O—O bond, peroxides split easily into free radicals. In addition to homolytic reactions, peroxides can participate in heterolytic reactions also, for example, they can undergo hydrolysis under the catalytic action of acids. Both homolytic and heterolytic reactions can occur simultaneously. For example, perbenzoates decompose into free radicals and simultaneously isomerize to ester [11]. The para-substituent slightly influences the rate constants of homolytic splitting of perester. The rate constant of heterolytic isomerization, by contrast, strongly depends on the nature of the para-substituent. Polar solvent accelerates the heterolytic isomerization. Isomerization reaction was proposed to proceed through the cyclic transition state [11]. 

The homolytic decomposition of diacyl peroxides proceeds via splitting of the weakest O—O bond. The acyloxy radicals formed are very unstable and a cascade of cage reactions follows this decomposition [4,42-46] ... 

Three different mechanisms of perester homolytic decay are known [3,4] splitting of the weakest O—O bond with the formation of alkoxyl and acyloxyl radicals, concerted fragmentation with simultaneous splitting of O—O and C—C(O) bonds [3,4], and some ortho-substituted benzoyl peresters are decomposed by the mechanism of decomposition with anchimeric assistance [3,4]. The rate constants of perester decomposition and values of e = k l2kd are collected in the Handbook of Radical Initiators [4]. The yield of cage reaction products increases with increasing viscosity of the solvent. 

The homolytic photochemical reaction occurs preferentially with the splitting of the weakest C—C bond. The ratio of the quantum yields depends on the energy of absorbed photon. For example, for butanone-2 photolysis, the ratio 0(Et )/4>(Me ) is equal [205] ... 

Another probable reaction of homolytic decomposition of ester hydroperoxide is the intramolecular interaction of the hydroperoxide group with the carbonyl group of ester with the formation of labile hydroxyperoxide succeeded the splitting of the weak O—O bond (see decomposition of hydroperoxides in oxidized ketones in Chapter 8). 

Reaction (26) is followed by a first order process, which has been interpreted as an homolytic splitting of the N-S bond, Eq. (27), with formation of the one-electron reduction product of NP, [Fe(CN)sNO]3, ... 

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