Methanol hydrogen abstraction

Two types of addition to pyrimidine bases appear to exist. The first, the formation of pyrimidine photohydrates, has been the subject of a detailed review.251 Results suggest that two reactive species may be involved in the photohydration of 1,3-dimethyluracil.252 A recent example of this type of addition is to be found in 6-azacytosine (308) which forms a photohydration product (309) analogous to that found in cytosine.253 The second type of addition proceeds via radical intermediates and is illustrated by the addition of propan-2-ol to the trimethylcytosine 310 to give the alcohol 311 and the dihydro derivative 312.254 The same adduct is formed by a di-tert-butyl peroxide-initiated free radical reaction. Numerous other photoreactions involving the formation by hydrogen abstraction of hydroxyalkyl radicals and their subsequent addition to heterocycles have been reported. Systems studied include 3-aminopyrido[4,3-c]us-triazine,255 02,2 -anhydrouri-dine,256 and sym-triazolo[4,3-fe]pyridazine.257 The photoaddition of alcohols to purines is also a well-documented transformation. The stereospecific addition of methanol to the purine 313, for example, is an important step in the synthesis of coformycin.258 These reactions are frequently more... 

Nitro compounds have also been reported to undergo photocyclizations. The intermediacy of an isoxazoline in the photorearrangement of o-nitro-benzaldehyde to o-nitrosobenzoic acid is now in doubt,318 but intramolecular hydrogen abstraction by an excited nitro group in nitrobenzene derivatives can result in the formation of heterocycles. 4-tm-Butyl-3-methoxy-2,6-dinitrotoluene (384) on irradiation in methanolic sodium hydroxide solution... 

Combination of BP with 2-propanol or amines induces homopolymerization alone. The rate constants of BP 3 - isopropylamine and triethylamine are 2.95 10 and 2.42 1()9m-1s-1, respectively(22) whereas that of BP 3 - isooctane as a model of OPP is 1.0 lO M s l (24). Also hydrogen abstraction from 2-propanol(k=1.0 106 M s"1) (25) is much more efficient than that from aliphatic hydrocarbons. Even methanol is more reactive (k=2.8 10% - s - -) (25) than OPP towards BP 3. The aforementioned results and the finding that surface grafting does not occur in methanol are well interpreted by the following elementary reactions. 

Recent studies conducted by the same group revealed that the radical cation of toluene generated by photoinduced electron transfer can be deprotonated in a protic cosolvent and thus efficient trapping by electrophilic alkenes is feasible, yielding benzylation products. Secondary hydrogen abstraction by the benzyl radical from methanol generates hydroxymethyl radicals, which can also be used for preparative hydroxymethylation of alkenes (Scheme 18) [24],... 

The quenching of a triplet carbene reaction with methanol is frequently used as the standard means of probing the singlet-triplet gap. It is widely believed that singlet carbenes insert readily into the O—H bonds of methanol, while the triplet states undergo hydrogen abstraction from the C—H bonds." The behavior of diarylcar-... 

During photolysis in methanol, fluoro- and methoxyarenes are formed without products resulting from hydrogen abstraction from the solvent.249... 

The UV irradiation of quinoxaline in methanol yields radicals, not by hydrogen abstraction, but by protonation of the first singlet excited state, followed by exiplex formation.82 Irradiation of quinoxaline in acidified methanol furnishes 2-methylquinoxaline, and the reaction is suggested to go through a pathway involving electron-transfer from the solvent to an excited state of the protonated quinoxaline (Scheme 3).83... 

Kinetic investigations have appeared in the literature since 1965. A redox mechanism is generally accepted [254], and has been confirmed by pulse experiments which demonstrated the equal activity of the catalyst in the presence and absence of oxygen. The results of Pernicone [254] and Liberti et al. [187] seem to indicate that the rate-determining step is either hydrogen abstraction from methanol or desorption of formaldehyde. 

Methyl ethyl ketone is unique, in that long and irreproducible induction periods were observed on occasion, reaction ensued only after 7 hours and then was completed within 10 minutes. During the long induction period the only detectable product was methanol. No convincing reason can be advanced to account for this anomalous behavior. The virtual absence of ethylene from the products of the low temperature slow combustion of methyl ethyl ketone strongly suggests that the low-temperature mechanism proceeds almost exclusively by further oxidation of the radicals produced by hydrogen abstraction from the parent ketone. 

A series of substituted 2,2,4,4-tetramethylcyclobutanones in which the substituent is alkyl, aryl, hydroxy, amino, or imino yield on photolysis in methanol or ethanol the corresponding alkoxytetrahydrofuran [Eq. (112)] in addition to a cyclopropane derivative formed by decarbonylation.429 Earlier examples of this process occurring in 7,7-dimethyl[3.2.0]bicyclohept-2-en-6-one430 and in (+)-cyclocam-phanone431 have been reported. In all cases, intramolecular hydrogen abstraction is unfavorable, and a carbene is postulated as the intermediate species. 

Gassman and Cryberg solvolyzed 125 in a number of solvents containing methanol.170 The predominant products were 126, 127, and 128. The first two, 126 and 127, are solvolysis products derived from rearrangement to electron-deficient nitrogen. But 128 almost surely results from hydrogen abstraction from the solvent. The proposed mechanisms for their formation are shown in Scheme 11. 

The photodecomposition of oxiran and alkyloxirans both in the gas phase and in solution has been extensively investigated. Processes arising by carbon-oxygen bond homolysis and hydrogen abstraction have been reported, and the subject has been reviewed in detail elsewhere.48 The most recent studies include the photoaddition of methanol to alicyclic epoxides, a process that appears to be promoted by acid,49 and the interesting if unusual photochemically induced conversion of the epoxyalcohol (56) to sugiresinol dimethyl ether (57).50... 

The photosensitized 1-4 addition of alcohols to hexenopyranosuloses first reported by B. Fraser Reid and coworkers [58 a] has been developed with other studies on photoadditions of oxycarbinyl species such as polyols, acetals, dioxolanes, aldehydes. A mechanistic study on this photoaddition has been recently detailed [58 b] showing that the important photochemical event is hydrogen abstraction from methanol, for example, to form the hydroxymethyl radical. 

Treatment of di-O-formyl-D-erythrose with methanolic hydrogen chloride has yielded a much higher proportion of the 0 isomer J. N. Baxter and A. S. Perlin, Chem. Inst. Canada, Abstracts 4th Western Conference, Sept. 4, 1958, p. 23. 

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