Homolysis reactions

Hargreaves has suggested that the insolubilization of some closely related polymers is due to photolytic homolysis of the endoperoxide 0-0 bond and subsequent generation of carbon-centered radicals from the O radicals (19). There are several facts that make this an extremely unlikely explanation for the data described here these include the quantitative insufficiency of the maximum amount of endoperoxide reaction obtainable with a few hundred mJ/cm2 dose (homolysis quantum yield <0.5 (46), and extinction coefficient 1 (M cm)-1 (47)), and the synthetic utility of such homolysis reactions in related molecules in the presence of good hydrogen atom donors (implying facile epoxide formation) (48). Clearly the crosslinking observed under N2 is not accounted for by this mechanism. 

The pathway corresponding to heterolytic metal ligand cleavage can be evaluated by suppressing the homolysis reaction... 

On the other hand, the homolysis reaction, an unfavorable equilibrium, can be drawn to the right by adding scavenging 2+... 

In some systems it appears that the initiation step differs from the usual two-step sequence of Eqs. 3-13 and 3-14. Thus in the t-butyl hydroperoxide-styrene system only a minor part of the initiation occurs by the first-order homolysis reaction (Eq. 3-26f), which accounts for the complete decomposition of t-butyl hydroperoxide in the absence of styrene. Homolysis of the hydroperoxide occurs at a much faster rate in the presence of styrene than in its absence. The increased decomposition rate in the t-butyl hydroperoxide-styrene system occurs by a molecule-induced, homolysis reaction which is first-order in both styrene and hydroperoxide [Walling and Heaton, 1965]. The initiation reaction may be written as... 

Table III summarizes a recent study comparing equilibrium constants of homolysis and volumes of activation and reaction for the formation and homolysis reaction of transient complexes with metal-carbon -bonds containing different metal centers (46). Previously the large volumes of activation for the homolysis reaction that were measured for Cr111 (56), Co111 (73), Ni111 (43) (15-26 cm3 mol-1) were interpreted as indication for an ShI mechanism (i.e., due to bond breakage and to the breakup of the solvent cage due to the separation of the aliphatic radicals from the Lm 2Mre center) (127). But as an I mechanism for the forward (formation)...

Re—Re, Re—M, and Re—C Bond Homolysis Reactions Photochemistry of Re(I) Diimine Tetracarbonyl Complexes Photochemical Ligand Substitution Reaction of/ac-[Re(Diimine)... 

For Red) dimers with a Re—Re bond, photoexcitation induces homolysis of the Re—Re bond. The two rhenium(O) metal centers in Re2(CO)io each donate one electron from their d 2 orbitals to make a a M—M bond. Upon photoirradiation, the bonding a-orbital (ob) electron is excited into the antibonding a-orbital (qz ), which leads to metal-metal bond cleavage. It has been also reported that a CO ligand elimination proceeds competitively with the metal-metal bond homolysis reaction. When a cyclohexane solution of Re2(CO)io was irradiated by the 355-nm laser light, the ratio of the reaction quantum 5nelds of Re—Re bond... 

It has been also reported that Re—R bond homolysis can be promoted by the photoexcitation of mononuclear rhenium(I) complexes with a Re—alkyl bond, /ac-[Re(CO)3(a-diimine)R] (R = alkyl ligand). This type of the reaction also proceeds via a state, which is produced by the relaxation from the MLCT excited state. In the cases of R = ethyl (Et) and benzyl (Bz), the homolysis reaction proceeds efficiently. However, the reaction efficiency is much lower in the case R = methyl (Me) because the transition from MLCT to requires extra energy (Scheme 1). Using transient absorption spectroscopy, the state was... 

Pulse radiolysis has been used to study the transient formation and decomposition of cobalt-alkyl bonds in aqueous solution in the same manner as it has been used for chromium alkyls. And as for chromium alkyls, bond homolysis is a major decomposition pathway (28). For bond formation reactions, pulse radiolysis shows that they are assisted by increases in pressure. This feature results from the homolysis having a larger activation volume than the bond formation reaction, resulting in a significantly negative overall reaction volume for the process (29). In general for all of these metal-alkyl bond homolysis reactions of the aquo complexes, steric hindrance facilitates the reaction. Ligand effects also play a role, but the factors involved are more subtle. 

The A5 term also appears to be sensitive to solvation effects in the two-bond concerted homolysis reaction. This is particularly evident for R is (CH3)jC in Table 91. It should be noted that some, but not all, of the entropies of activation in chlorobenzene solvent are more positive than predicted from the AH vs. AS plot for 38 peresters. For example, from the parameters given above, AS for R is (CH3)3C is calculated to be 6.32+0.19 eu in chlorobenzene where AH = 30.0 kcal.mole". This is in contrast to the reported value of 11.1 eu. Induced decomposition would be the most likely explanation for this disparity in AS values. Indeed, kinetic data for the decomposition of bicyclic peresters in chlorobenzene and cumene suggests that induced decomposition is more important in chlorobenzene (see section 13.4.9). 

Kinetic data for the decomposition of mono- and diperesters of oxalic acid are given in Table 109. The three benzyl mono-peresters did not give a linear plot with either or or ct substituent constants Instead, the response of the substituents was intermediate between these two constants. If a three-bond homolysis reaction... 

Activation parameters for monoperoxycarbonates (Table 111) indicate a one-bond homolysis reaction. In comparison to di-t-butylperoxyoxalate where multibond homolysis occurs, the rate coefficient for di-f-butyl monoperoxycarbonate is 10 times slower at 45 °C . The rate of decomposition of di-t-butyl diperoxy-... 

Kinetic data for thermal decomposition of the related dialkyl peroxydicarbonates are given in Table 112. Variation of the substituent groups R has little effect on the rate coefficients or the activation energy. In addition the activation energies are in the range of those reported for benzoyl peroxides. This suggests a one-bond homolysis reaction. Activation energies for dialkyl peroxydicarbonates with... 

Relatively few rate constants are available for the alkyl homolysis reactions mainly because clean sources of the alkyl radical have proved difficult to find. Consequently, the data are not always reliable, but some check is available [64, 65] from thermochemical and kinetic data for the reverse reaction. Direct photolysis of azo-compounds and mercury-photosensitized decomposition of alkanes have so far provided the most reliable (although old) data [64]. For good results, the method depended on precise product analysis in the early stages of reaction, with equation (1.9) used to determine where Rabs and Rr r are the initial rates of formation... 

RO radicals undergo only two reactions C—C homolysis reaction with O2 to give an aldehyde or ketone and an HO2 radical isomerization to give a hydroxyalkyl radical when the ring transition state involves little strain energy. These possibilities are illustrated below for 2-pentoxy radicals. 

However, H atom addition to alkenes is ca. IVOkJmoP exothermic, so that C—C homolysis reactions are enhanced through the chemically activated alkyl radicals formed. Relative yields of products are thus very dependent on pressure. 

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