Intramolecular disproportionation

The two forms, with protein from Themiste zostericola, undergo spontaneous spectral changes (28, 31) and complete loss of EPR signal (30) by a first-order rate process, with a rate constant independent of protein concentration. This change is ascribed to a remarkable intramolecular disproportionation process within the octamer ... 

Although semi-met forms of myohemerythrin (the monomeric protein from Themiste zostericola muscle) can be obtained in a similar manner to the octamer (32), the intramolecular disproportionation path is obviously now unavailable to them. In... 

The behaviour of triplet acyl-diphenylmethyl biradicals 0=C -(CH2) 2-C Ph2, generated from the Norrish type-I reaction of 2,2-diphenylcycloalkanones (CK) with various ring sizes, n = 6, 7, 9, 11, 12, 13, was the subject of a study. Eor 2,2-diphenylcycloalkanones where n = 6 and 7 an intramolecular disproportionation takes place giving rise to a diphenylalkenal (94). The primary products in the photolysis... 

The acyl-alkv biradical obtained by ring-opening of a cyclic ketone is able lo undergo intramolecular disproportionation in one of two ways. A hydrogen atom may be transferred to the acyl radical from the position adjacent to the alkyl group, and this produces an unsaturated aldehyde (4.21). Alternatively, a hydrogen may be transferred to the alkyl radical from the position adjacent to the acyl group, and this results in the formation of a ketene (4.22). Many ketenes are labile, and the use of a nucleophilic solvent or addend. 

Irradiation of the dihydropyridine Formula 486 gives rise to an intramolecular disproportionation forming Formula 487 (219). If circularly... 

In contrast, in the excited state the primary cleavage mechanism in silacyclobutanes like 5 involves the breaking of a silicon-carbon bond23. The initially formed silyl radicals 15 and 16 are stabilized by an intramolecular disproportionation reaction giving the silenes 17 and 18 and the homoallylsilane 19.17 and 18 were identified by their trapping products (20, 21) with methanol (equation 5)23. From pyrolysis of Z-5 a different set of products from 1,4-diradical disproportionation is obtained, which can be attributed to predominant cleavage of the carbon-carbon bond23. 

Interestingly, 56 cannot be obtained photochemically from the benzodisilacyclobutene 57 instead silastyrene 64 is formed as an intermediate46. 64 is produced by homolytic bond scission of the Si—Si bond in 57 followed by intramolecular disproportionation of... 

Silenes, bearing an allylic hydrogen, frequently give linear non-cyclic dimers, in which two silene molecules form a Si—Si bond39,86,88,89 107 112. This linear head-to-head dimer can be formed by intramolecular disproportionation of the initially formed biradical (path B in equation 101). Of course, an ene -reaction (path A in equation 101)... 

The products arising from the reaction of 431 with the alkyl-substituted silenes 149 and 150 suggest that the reaction occurs by a radical pathway, initiated by a homolytic Si—C bond cleavage of 431 and subsequent Si—Si bond formation giving the biradical 434. Intramolecular disproportionation of 434 gives 435, while 436 and 437 are the results of ring closure reactions without or with expulsion of tetramethylethene, respectively (equation 139)181. 

The photochemical cycloreversion of SCBs is known to be initiated from the lowest (a, a ) excited singlet state by cleavage of one of the ring Si—C(2) bonds to form a biradicaloid intermediate. This excited-state intermediate cleaves to silene and alkene, recloses to starting material, or undergoes an intramolecular disproportionation if an alkyl substituent is present at C-2 <1999CJC1136, 19890M1112>. 

A very interesting recent study describes the intramolecular disproportionation of homodinuclear and heterodinuclear fulvalene complexes in the presence of PMe3.88 Equation (14) shows one of the six reactions reported. In this case, initiation of the radical chain process was accomplished with a catalytic amount of the 19-electron reservoir complex [CpFe(C6Me6)], which reduces 13 to break the Ru—W bond and generate a 17-electron radical center (presumably at Ru). Addition of PMe3 to the Ru is followed by electron transfer to the reactant (13) to afford the zwitterionic product 14 and regenerate the radical intermediate. 

Intramolecular disproportionation of [1] could conceivably result in the formation of unsaturated aldehyde or ester. Both paths are well known to occur with cyclic alkanones (la). Abstraction of H by the acyl radical portion of [1] would produce alkenal [3] while abstraction of by the alkyl radical portion of [1] would produce ketene [4] which may be efficiently trapped in nucleophilic alcoholic solvents to yield ester [5]. The intramolecular nature of alkenal formation is supported by deuterium labeling experiments (5), while the intramolecular nature of ketene formation is supported by (a) deuterium labeling experiments (6,7), and (b) the observed decrease in ketene formation with decreasing ring size (8). 

In view of the hydride-donating potential of (23), one might anticipate similar reactivity for adducts derived from, for example, amines and thiols. Intermolecular demonstrations of such reactivity are scarce on the other hand intramolecular examples are well known, although they may be mechanistically ambiguous. The prototype reaction is the intramolecular disproportionation of glyoxal (31) to glycolic acid (32), shown in equation (19). This reaction may be induced by hydroxide. With the aid of ab initio and MNDO-SCF-MO calculations a highly bent transition state has been calculated for hydride transfer. ... 

McFadyen and Stevens (cf. Bayer344h) obtained aldehydes from the corresponding carboxylic acids in yields of up to 60% by intramolecular disproportionation of aromatic and heterocyclic l-acyl-2-(benzenesulfonyl)hydrazines induced by an alkali carbonate in ethylene glycol at 160° but this method fails for aliphatic and, / -unsaturated carboxylic acids. 

The reaction with dimethyl acetylenedicarboxylate proceeds less cleanly for 1,3-dimethylindole, possibly because this indole is a better electron donor [40c]. Seven products are obtained these are the 2-1-2 cycloadduct and the geometrical isomers of 17-19 shown in Scheme 8. The substitution products 17 dominate in polar, protic solvent and are possibly formed via photochemical electron transfer from 1,3-dimethylindole to the alkyne this substitution mechanism is discussed further in Section V. In nonpolar or aprotic media, 17 is still formed, although only as a minor product under these conditions, where electron transfer could be endothermic, it is possible that 17 is formed by a route involving intramolecular disproportionation of the triplet 1,4-biradical 20 that is also the likely precursor of the 2-1-2 photocycloadduct. The geometrical isomers 18 and 19 are the major products formed when the indole concentration is high Davis and Neckers speculate that these arise from addition of biradical 20 to 1,3-dimethylindole [40c]. However, the lifetimes of... 

Dinitrodiphenyl ether (94) was electrolytically reduced in a single wave with an uptake of eight electrons per molecule, giving rise to the bishydroxylamino intermediate (95), which underwent an intramolecular disproportionation. The resulting 2-nitroso-2 -aminodiphenyl ether (%) underwent a chemical follow-up reaction leading to dibenzo[/>/)-l,4,5-oxadiazepine (97) and phenazine (98) via two steps as shown in Scheme 16 <83CCC379>. 

Cyclization is sometimes accompanied by formation of alkenes in thermal decomposition. For the eight-membered azo compound 19, for example, alkene formation is the major reaction pathway. An intramolecular disproportionation via transition state F may account for the large amount of alkene formed in this particular case. ... 

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