Radical homolysis

A number of other chemistries which involve C-O bond cleavage have been reported.226 22 Druliner226 has reported on systems where NCO, 112, 113 or related species is the persistent radical. Homolysis rates for these systems were stated to he suitable for MMA polymerization at ambient temperature. The use of NCO has also been studied by Grande et al. z most recently for AA polymerization.2 0 Although control during AA homopolymerization was poor the process yielded NCO- terminated PAA that could be used to make PAA-block-PMMA.230... 

The ability of radicals to propagate by abstraction is a key feature of radical chain reactions, which we shall come to later. There is an important difference between homolysis and abstraction as a way of making radicals homolysis is a reaction of a spin-paired molecule that produces two radicals abstraction is a reaction of a radical with a spin-paired molecule that produces one new radical and a new spin-paired molecule. Radical abstractions like this are therefore examples of your first radical reaction mechanism they are in fact substitution reactions at H and can be compared with proton removal or even with an Sfj2 reaction. 

The C—H distance in acetylene is 1.08 A, even shorter than in ethylene (1.103 A) because of their greater s character, sp orbitals arc smaller than sp orbitals,-and 5p-hybridized carbon forms shorter bonds than 5/ -hybridized carbon. The C—H bond dis.sociation energy in acetylene is not known, but we would expect it to be even greater than in ethylene. Oddly enough, the same sp hybridization that almost certainly makes cleavage of the C—H bond to form free radicals homolysis) more difficult, makes cleavage to form ions heterolysis) easier, as we shall see (Sec. 8.10). 

Ugly facts, unfortunately, sometimes invalidate a beautiful mechanism. The Jacobsen epoxidation sometimes proceeds with loss of configurational purity of acyclic alkenes. This feature of the reaction can be explained by invoking radicals. Homolysis of the Mn C bond in the manganaoxetane intermediate would give a Mn(III) 1,4-diradical complex, and attack of the alkyl radical on O with displacement of Mn(II) would give the epoxide and regenerate the catalyst. 

The first process, H-abstraction, may initiate an important generic reaction of amino acids (Fig. 4). Radicals attributed to the a-carbon have been identified by electron spin resonance (ESR) of peroxidized proteins (22). Further reaction with O2 (hypothetical) would lead to amino acid hydroperoxides. A different pathway to amino acid hydroperoxide has been proposed by Yong and Karel (23), but their proposal involves an indirect pathway to the a-carbon radical. Homolysis of the hydroperoxy group would afford an amino acid oxy radical susceptible to p-scission via Reaction G. Thus, p-scission between the a-carbon and the amino group may explain the increase in amide content of protein that has been peroxidized in dry systems, as well as the coincident protein chain scission observed... 

Figure 8.21 Probable pathways for racemization during LA formation via radical pathways at asymmetric carbon atoms in PLLA (a) Alkyl-oxygen radical-homolysis and LA formation (b) Acyl-oxygen radical-homolysis and LA formation (c) Enolization 1272], (Reproduced with permission from ref. 1272], Copyright Elsevier, 2003.)...

Radical reaction (Section 10.IB) A reaction involving radicals. Homolysis of covalent bonds occurs in radical reactions. 

The racemization in the pyrolysis of PLLA has been discussed in some reports [7, 37, 54, 68]. To explain this phenomenon, Kopinke et al. [7] proposed that an ester-semiacetal tautomerization occurred in the lactate unit during the pyrolysis. This speculation was based on the observation of more than two different diastereoisomers of each cyclic oligomer as pyrolysates. Moreover, it is suggested that possible radical homolysis pathways cause the racemization through ring-forming processes [7, 37]. 

The more stable the radical the lower the energy required to generate it by C—H bond homolysis... 

As the table indicates C—H bond dissociation energies m alkanes are approxi mately 375 to 435 kJ/mol (90-105 kcal/mol) Homolysis of the H—CH3 bond m methane gives methyl radical and requires 435 kJ/mol (104 kcal/mol) The dissociation energy of the H—CH2CH3 bond m ethane which gives a primary radical is somewhat less (410 kJ/mol or 98 kcal/mol) and is consistent with the notion that ethyl radical (primary) is more stable than methyl... 

In discussing mechanism (5.F) in the last chapter we noted that the entrapment of two reactive species in the same solvent cage may be considered a transition state in the reaction of these species. Reactions such as the thermal homolysis of peroxides and azo compounds result in the formation of two radicals already trapped together in a cage that promotes direct recombination, as with the 2-cyanopropyl radicals from 2,2 -azobisisobutyronitrile (AIBN),... 

Irradiation of ethyleneimine (341,342) with light of short wavelength ia the gas phase has been carried out direcdy and with sensitization (343—349). Photolysis products found were hydrogen, nitrogen, ethylene, ammonium, saturated hydrocarbons (methane, ethane, propane, / -butane), and the dimer of the ethyleneimino radical. The nature and the amount of the reaction products is highly dependent on the conditions used. For example, the photoproducts identified ia a fast flow photoreactor iacluded hydrocyanic acid and acetonitrile (345), ia addition to those found ia a steady state system. The reaction of hydrogen radicals with ethyleneimine results ia the formation of hydrocyanic acid ia addition to methane (350). Important processes ia the photolysis of ethyleneimine are nitrene extmsion and homolysis of the N—H bond, as suggested and simulated by ab initio SCF calculations (351). The occurrence of ethyleneimine as an iatermediate ia the photolytic formation of hydrocyanic acid from acetylene and ammonia ia the atmosphere of the planet Jupiter has been postulated (352), but is disputed (353). 

Hydroperoxides are photo- and thermally sensitive and undergo initial oxygen—oxygen bond homolysis, and they are readily attacked by free radicals undergoing induced decompositions (eqs. 8—10). 

Chemical Properties. Acychc di-Z f/-alkyl peroxides efftciendy generate alkoxy free radicals by thermal or photolytic homolysis. 

The alkyl peroxyesters undergo homolysis, thermally and photochemically, to generate free radicals (168,213,229—232) ... 

The amount of induced decomposition that occurs depends on the concentration and reactivity of the radical intermediates and the susceptibility of the substrate to radical attack. The radical X- may be formed from the peroxide, but it can also be derived from subsequent reactions with the solvent. For this reason, both the structure of the peroxide and the nature of the reaction medium are important in determining the extent of induced decomposition, relative to unimolecular homolysis. 

The radical X is formed by homolysis of the X—R bond either thermally or photolytically. In the reactions of alcohols with lead tetraacetate evidence suggests that the X—R bond (X = 0, R = Pb(OAc)3) has ionic character. In this case the oxy radical is formed by a one electron transfer (thermally or photochemically induced) from oxygen to lead. 

In most cases the carbon radical formed in the hydrogen abstraction step 2 will react with the radical R formed in the homolysis of the X—R bond. However, a cage reaction does not seem to be involved in this step. This has been established in the nitrite photolysis and probably applies to hypohalites as well. In the lead tetraacetate reaction, the steps following the oxyradical formation leading to tetrahydrofuran derivatives are less clear. 

The homolysis of tertiary hypochlorites for the production of oxy radicals is well known." The ease with which secondary hypohalites decompose to ketones has hampered the application of hypohalites for transannular reactions. However the tendency for the base-catalyzed heterolytic decomposition decreases as one passes from hypochlorites to hypobromites tohypoidites. Therefore the suitability of hypohalites for functionalization at the angular positions in steroids should increase in the same order. Since hypoidites (or iodine) do not react readily with ketones or carbon-carbon double bonds under neutral conditions hypoiodite reactions are more generally applicable than hypochlorite or hypobromite decompositions. 

---