Homolysis carbonyls

Thermal decomposition of dihydroperoxides results in initial homolysis of an oxygen—oxygen bond foUowed by carbon—oxygen and carbon—carbon bond cleavages to yield mixtures of carbonyl compounds (ketones, aldehydes), esters, carboxyHc acids, hydrocarbons, and hydrogen peroxide. 

Iron(II) alkyl anions fFe(Por)R (R = Me, t-Bu) do not insert CO directly, but do upon one-electron oxidation to Fe(Por)R to give the acyl species Fe(Por)C(0)R, which can in turn be reduced to the iron(II) acyl Fe(Por)C(0)R]. This process competes with homolysis of Fe(Por)R, and the resulting iron(II) porphyrin is stabilized by formation of the carbonyl complex Fe(Por)(CO). Benzyl and phenyl iron(III) complexes do not insert CO, with the former undergoing decomposition and the latter forming a six-coordinate adduct, [Fe(Por)(Ph)(CO) upon reduction to iron(ll). The failure of Fe(Por)Ph to insert CO was attributed to the stronger Fe—C bond in the aryl complexes. The electrochemistry of the iron(lll) acyl complexes Fe(Por)C(0)R was investigated as part of this study, and showed two reversible reductions (to Fe(ll) and Fe(l) acyl complexes, formally) and one irreversible oxidation process."" ... 

Several carbonyl-containing peroxide additives have been shown to increase the initial rate of the nonoxidative photo-dehydrochlorination of PVC (54). In studies with polymeric ketones unrelated structurally to PVC, the excited singlet and triplet states of the carbonyl groups in these polymers were found to sensitize 0-0 homolysis at rates approaching diffusion control (55). Similar reactions may well occur in oxidized vinyl chloride polymers. 

On photolysis, dinuclear single metal-to-metal-bonded carbonyls undergo homolysis of the metal-to-metal bond ... 

Figure 27. Mechanism of radiation induced chain scission in PMMA. Homolysis of the mainchain-carbonyl carbon bond is indicated as the initial step. Acylcarbon-oxygen, sigma bond homolysis also occurs but rapid decarbonylation ultimately leads to the same indicated products.

These authors also showed that the indolizidine skeleton can be prepared from cyclopropyl dipolarophiles (Scheme 1.16). The cycloaddition of alkyhdenecyclo-propanes 67 with various nitrones (e.g., 68) afforded the expected isoxazolidine adducts 69 and 70, commonly forming the C(5) substituted adducts 70 (97,105-108) predominantly but not exclusively (109-111). Thermally induced rearrangement of the spirocyclopropyl isoxazolidine adduct 70 afforded the piperidinones 71 (107,108). These authors propose reaction via initial N—O bond homolysis of 70 to diradical 72 followed by ring expansion through relief of the cyclopropyl ring strain forming the carbonyl of a second diradical intermediate 73, which cyclizes to afford the isolated piperidinone 71. 

A carbonyl ylide is undoubtedly implicated in the singlet-derived photorearrangement of the epoxydiene (88) to the cyclopropene (89)75 a proposed pathway is outlined in Scheme 4. Carbon-oxygen bond homolysis is preferred, however, on triplet-sensitized irradiation, leading to the formation... 

Not surprisingly, the introduction of a carbonyl group into the heterocycle has a profound effect on the photochemistry of the system. The formation of photoproducts arising by a Type I reaction involving 2,3-bond homolysis... 

The final step is homolysis of the OH bond in the radical to afford the carbonyl product plus a second hydrogen radical, which is converted by the oxidant into a proton [Eq. (6)]. Equation (4) is the slow or rate-determining step removal of the second hydrogen atom is rapid. 

Several mechanisms have been suggested to produce the energy required to populate an excited carbonyl, which is at least 290-340 kj mol-1 [8]. Direct homolysis of hydroperoxides [9, 10], disproportion of alkoxy radicals [11] and /2-scission of alkoxy radicals [12] are all exothermic enough. However, the most widely accepted mechanism has been the highly exothermic (460 kj mol-1) bimolecular termination of primary or secondary alkyl per-oxyl radicals, i.e. the Russell mechanism (Scheme 2). It proceeds via an intermediate tetroxide to give an excited carbonyl, an alcohol, and oxygen [13, 14]. 

Susuki and Tsuji reported the first Kharasch addition/carbonylation sequences to synthesize halogenated acid chlorides from olefins, carbon tetrachloride, and carbon monoxide catalyzed by [CpFe(CO)2]2 [101]. Its activity is comparable to or better than that of the corresponding molybdenum complex (see Part 1, Sect. 7). Davis and coworkers determined later that the reaction does not involve homolysis of the dimer to a metal-centered radical, which reduces the organic halide, but that radical generation occurs from the dimeric catalyst after initial dissociation of a CO ligand and subsequent SET [102]. The reaction proceeds otherwise as a typical metal-catalyzed atom transfer process (cf. Part 1, Fig. 37, Part 2, Fig. 7). 

In addition to the above mentioned homolysis, peroxides can also be ionically decomposed leaving behind by-products. Primary and secondary peroxyesters decompose into the corresponding carbonic acids and carbonyl compounds during the hydrolysis reaction of tertiary peroxyesters leading to the formation of carbonic acids and hydroperoxides to give ... 

Undissociated hydrogen peroxide behaves, to some extent, as a nucleophile, being about 104 times more nucleophilic than water. For example, hydrogen peroxide readily adds to carbonyl bonds giving rise to hydroxyhydroperoxides (peracetals and perketals). Such compounds are often used as polymerization initiators on account of their radical decomposition at moderate temperatures (0-0 bond homolysis). Neutral hydrogen peroxide can also react with activated acyl compounds such as anhydrides to give peroxyacids. 

The photolysis of Mu2(CO)io is representative of the behavior of many bimetallic carbonyl compounds. In solution and in the gas phase strongly wavelength dependent photochemistry has been observed. At low energy, Mn-Mn bond homolysis predominates while at high energy CO-loss is observed. 

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