Chain termination ketones

An alternative chain-terminating decomposition of the tetroxide, known as the Russell mechanism (29), can occur when there is at least one hydrogen atom in an alpha position the products are a ketone, an alcohol and oxygen (eq. 15). This mechanism is troubling on theoretical grounds (1). Questions about its vaUdity remain (30), but it has received some recent support (31). 

Chain termination has been discussed by Drent. At higher temperatures both ester and alkyl chain ends are formed, thus leading to polymer chains with two esters, two ketones, or one of each as end groups.468... 

The chain termination by reactions of tertiary alkylperoxyl radicals is complicated by their decomposition with production of ketone and the alkyl radical, for example ... 

Russell [179] proposed the following mechanism of chain termination by primary and secondary peroxyl radicals with coordinated decomposition of formed tetroxide to alcohol, ketone, and 02 ... 

The decay of the biradical produces ketone molecule in the triplet state, which is an emitter of light [222], The CL intensity was proved to be propotional to the rate of chain initiation, which is equal to the rate of chain termination. The observed luminescence spectra were found to be identical with the spectra of the subsequent ketone in the triplet state. The intensity of CL (/chi) produced by oxidized hydrocarbon is the following ... 

The rate of oxidation decreases with dilution of methyl ethyl ketone by a nonpolar solvent—namely, benzene. The rate constants for both chain propagation and chain termination also drop (Table I), and the activation energies increase (Table II) because the elementary reactions of chain propagation and termination represent interaction between two dipoles occurring at a rate which depends on the dielectric constant of the medium. 

Then, a major part of these radicals would be consumed by chain termination to form corresponding alcohols and ketones. Yokohata and Tsuda (23, 24, 25) have suggested that the silent discharge reaction can well be interpreted in terms of radiolysis caused by the ionizing radiation of high LET (Linear Energy Transfer), and this would help explain the short chains of this oxidation. 

C-Terminal ketone derivatives of peptides have been used as effective inhibitors for a variety of proteases including serine, cysteine, and aspartyl proteases. 271 This class of peptides includes diazomethyl ketones (Section 15.1.2), halomethyl ketones (Section 15.1.3), and fluoromethyl ketones (Section 15.1.4). In general, the A -amino group and side chain must be protected. The diazomethyl ketones serve as good intermediates for conversion into chloromethyl and bromomethyl ketones. Fluoromethyl ketones, the most widely known class of peptide haloketones, can also be prepared from diazomethyl ketones or by halogen-exchange reactions. Other methods for the synthesis of fluoromethyl ketones are described in Section 15.1.4. 

Even when the reductive processes, so evident in metal-stimulated processes, are avoided, several side reactions can still cause reductions in the yield of the desired a-arylated ketones. The first, abstraction of 3-hydrogen atoms from the enolate ion by the aryl radical, has already been mentioned (Section 2.2.2.1) and is sometimes a serious, chain-terminating process.43 5 This abstraction reaction, however, appears to be quite unpredictable. 3-Hydrogen abstraction from the enolate of 2,4-dimethyl-3-pentanone (PriCOPr Table 1) which severely disrupts the reaction with iodobenzene, does not prevent high-yielding reactions of the same enolate (and those from other ketones with a-branching) with many other substrates. In in-... 

This will certainly be reasonable in view of the small amounts of ketone produ( ed at low temperatiU es. It is certainly to bo expected that kbf < at low temperatures, while at the higher temperatures (Ac) is so low that it is probably unimportant in chain-termination processes. 

The reaction yields paraffins, olefins, and oxygenated products such as alcr ols. aldehydes, ketones, acids, and esters. As is usual for an oligomerization, a more or-less complicated product mixture has to be expected rather than the selective formation of individual products. The molecular weight distribution found can be described well by simple equations, originally developed for polymerization processes, considering the probability of chain grouch and chain termination. 

It seems to be well established that reactions (l)-(4) are the chain initiation and propagation steps of the decomposition. The question of chain termination is, however, far from being clear. In their fundamental paper. Rice and Herzfeld suggested that the recombination of the methyl and acetonyl radicals to give methyl ethyl ketone was the probable termination step. This leads to an overall order of one which is in agreement with the observations at high pressures. 

Scheme 1.57. Chain scission (13) or elimination (14) reactions of alkoxy radicals in polypropylene resulting in terminal (13) or in-chain (14) ketone groups as well as alkyl radicals that may react further with oxygen.

Termination of the radical chain reaction As the reaction proceeds, autoxidation is followed by an autoretardation stage, resulting in a standstill before the hydrocarbon is completely consumed. This autotermination is called the chain termination reaction and dominates in this final phase of the oxidation process such that degradation comes to a halt. Termination may be effected by the combination of radical species such as peroxy radicals to yield ketones and alcohols. Reaction sequence (4.14) ... 

Ketone oxidation chains terminate when two peroxy radicals react with each other. This is the main reaction of chain termination if the ketone contains no inhibitor and the oxygen pressure is sufficiently high for fast conversion of R- to R02-. The values of kt measured by the chemiluminescence technique [81] are shown in Table 10. 

Both the Japanese Synthetic Rubber Company and Nippon Zeon have reported that anionically prepared elastomers that are functionally terminated by active lithium can be chain terminated with Michler ketone, benzophenone, and a variety of enamide groups. Moreover, these chains can be terminated with silicone or tin metals. Chain end functionalization did not change the viscoelasticity of the polymer chains but rather dramatically improved the elastomer-filler interaction and, therefore, reduced its hysteretic properties. 

A second type of chain-termination is caused by ketones and aldehydes that might contaminate the solvents. For instance dichloromethane often contains formaldehyde. Such carbonyl compounds react with the N-terminal dipeptide portion of a chain to give imidazolidinones ... 

---