Propanols, hydrogen abstraction

Reaction step 5 in Scheme 3.1 can be rnled ont becanse the flnoranil ketyl radical (FAH ) reaches a maximum concentration within 100 ns as the triplet state ( FA) decays by reaction step 2 while the fluoranil radical anion (FA ) takes more than 500 ns to reach a maximum concentration. This difference snggests that the flnoranil radical anion (FA ) is being produced from the fluoranil ketyl radical (FAH ). Reaction steps 1 and 2 are the most likely pathway for prodncing the flnoranil ketyl radical (FAH ) from the triplet state ( FA) and is consistent with the TR resnlts above and other experiments in the literatnre. The kinetic analysis of the TR experiments indicates the fluoranil radical anion (FA ) is being prodnced with a hrst order rate constant and not a second order rate constant. This can be nsed to rnle ont reaction step 4 and indicates that the flnoranil radical anion (FA ) is being prodnced by reaction step 3. Therefore, the reaction mechanism for the intermolecular hydrogen abstraction reaction of fluoranil with 2-propanol is likely to predominantly occur through reaction steps 1 to 3. 

Scheme 3.1 Possible reaction steps in the hydrogen abstraction reaction of fluoranil with 2-propanol. Note FA= fluoranil, (CH3)2CHOH = 2-propanaol, FAH = fluoranil ketyl radical, FA = fluoranil radical anion.

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. 

It should be noted at tins point that the mechanism of photoreduction in amine solvents is highly likely to be quite different from that in hydrogen donor solvents. In the former class of solvents, electron transfer seems to prevail. StericaJly hindered nitrobenzenes are not capable of hydrogen abstraction from hydrogen donors as ethers or 2-propanol (see Section A. 1.4) but are efficiently photoreduced in di- or triethylamine ). 

This rate constant is twenty five times higher than the rate constant of hydrogen abstraction from 2-propanol Ah = 0-8 X 10 1 mole t s t t4) ... 

Upon irradiation (366 nm) in 2-propanol, 1-nitronaphthalene is inefficiently photoreduced (Ah<10 1 moles i s i) despite its high triplet yield 3). With tributylstannane, a marked increase of the hydrogen abstraction rate (<3 x 10 ... 

The phosphorescence of a 5 X lO" M solution of biacetyl in de-aerated 2-propanol at room temperature could be quenched completely by 1 a,d,e (10 8 M) 84). In all three cases, the corresponding photoreduction products 2a,d,e emerge from analogous preparative scale biacetyl sensitized runs. Since 2e is also formed, steric hindrance to hydrogen abstraction from solvent cannot be too effective when a (probably longer-lived) triplet is populated, whereas it might be effective in the direct photolysis ot 1 e 88) where isomerisation competes with reduction probably in the (short-lived) singlet state. 

The exact values for the rate constants for hydrogen abstraction by triplet benzophenone are not yet entirely certain. Three groups338-338 have reported a value of 108M-1 sec-1 for abstraction from 2-propanol in concentrated 2-propanol, while the combination of the data of three other groups333,338 339 for dilute benzene solutions yields a value of only 105M-1 sec-1. This discrepancy could well reflect a solvent effect such as that found in studies of the reactivity of alkoxy radicals.340 However, the hundredfold difference between the reported rates for attack of triplet benzophenone on toluene338,338 undoubtedly reflects experimental problems, because both values were measured in aromatic solvents. 

A quenching study at one concentration of 2-propanol (2.6Af) in benzene indicates that the rate constant for hydrogen abstraction is about 1 x 106Af 1 sec-1,451 similar to that for benzophenone. A fact long known is that sulphonate substituents in the 2 and 6 positions do not affect quantum yields of photoreduction, whereas in the 1 and 5 positions they cut O from 1.0 down to 0.01-0.05.216... 

The chlorine atom in 4-chloropyridine can be replaced photochemically by the dimethyl ketyl radical755. Irradiation of 4-chloropyridine in a 4 1 mixture of 2-propanol and water gives a low yield (2%) of 2-(4 -pyridyl)-2-propanol. Sensitization by benzophenone increases the yield to 25%, but the product is now accompanied by 6% of diphenyl-(4-pyridyl)methanol. The major product is believed to be formed via hydrogen abstraction from 2-propanol by photoexcited pyridine. Protonated pyridines do not undergo this abstraction process, and accordingly the product yield decreases under acidic conditions. The radical (259) formed from the pyridine will combine (at position 4) with the dimethyl ketyl radical (260) and elimination of HC1 from the adduct (261) completes the reaction (equation 194). 

An interesting example of selectivity with respect to hydrogen abstraction by hydroxyl radicals is demonstrated in Fig. 6-25. 2-Propanol is an effective OH radical scavenger that is used in many kinetic studies (e.g. Hislop and Bolton,... 

Fig. 6.25 Selectivity of hydrogen abstraction in the reaction of OH radicals with 2-propanol Asmus et al., 1973).

Diffusion coefficients (D) of various radicals created by the photoinduced hydrogen abstraction reactions from alcohols (ethanol and 2-propanol) as well as those of the parent molecules are measured by using the transient grating (TG) method. Dependence of D on the viscosity, molecular size, and temperature are investigated, and the results are interpreted in terms of microscopic aggregation of the radicals with solvents or solutes. 

The photoreduction of aromatic ketones by tertiary amines is reported [38] to proceed at rates which are substantially faster than those observed for the corresponding photoinduced hydrogen abstraction from, e.g. alcohols. A limit case is given by fluorenone, the photoreduction of which does not occur in alcohol, ether or alkane solution, but readily takes place in the presence of amines, tertiary amines being the most effective [39,40]. Xanthone has also been reported to be easily photoreduced by iV,A-dimethylaniline [41], but not by 2-propanol [42]. However, the oxidation of tertiary amines photosensitized by fluorenone and xanthone is much less efficient than when sensitized by benzophenone, apparently because of lower rates of hydrogen abstraction [43]. Fluorenone/tertiary amine systems have been used successfully to photoinitiate the polymerization of MMA, St, MA and AN [30,38,44] and rather similar results have been obtained in the photoinitiated polymerization of MA by the benzophenone/EtsN system [45]. Thus, the great variety of substrates participating in exciplex formation has been readily extended to polymer-based systems. 

It has also been shown in radical substitution at the 2-position of a series of 4-substituted (CN, MeO, Me) protonated pyridines, that the cyclopropyl radical is the least nucleophilic of the cycloalkyl radicals This low nucleophilicity is consistent with the observed difficulty in oxidizing the cyclopropyl radical by Cu ". The lack of reactivity of the 2-phenylcyclopropyl radical, generated by the thermal decomposition of the 2-phenyl-cyclopropanepercarboxylic acid, towards the 0-0 peracid bond to yield 2-phenylcyclo-propanol is also in line with the radical s weak nucleophilicity However from a study of relative rates of hydrogen abstraction to olefin addition of the cyclopropyl radical to a variety of olefins (Table 7) Stefani and coworkers concluded that the cyclopropyl radical was decidedly nucleophilic. 

A study of the photodeconjugation reaction of the ester (136) in the presence of a variety of optically active amines such as (IR,2S)-l-phenyl-2-isopropylamino propanol has shown that the product can be obtained with an enantiomeric excess of about 70 X. Irradiation of the alkynylketones (137) in alcohols (methanol, ethanol or propanol) affords the furan derivatives (138) by a free radical hydrogen abstraction path. Excitation of the cyclohexenone derivatives (139) yields the cyclized products (140) the structures of which were verified by X-ray crystallography. The formation of the amide products is reminiscent of a Norrish Type II process. 

In the first step the spectroscopically detectable ketyl radical 36 is formed, which then recombines to form benzopinacol (37) (Weiner, 1971). The same pinacol is obtained by reacting benzophenone with 2-propanol, since the dimethylketyl radical (38) produced in the hydrogen abstraction step is a strong reductant and transfers a hydrogen atom to the excess benzophenone to form another molecule of the diphenylketyl radical (36). 

Naphthyl ketones, which are generally unreactive toward photoreduction by 2-propanol, are efficiently reduced by amines. Electron transfer (cf. Section 5.4.4) is considerably faster than hydrogen abstraction. Thus, the reaction cannot be quenched using common triplet quenchers although it proceeds from the triplet state. 

The reactions of biacetyl in fluorocarbons and mineral oil parallel the gas phase except that quantum yields were appreciably reduced 61>. Hydrogen abstraction is the major process in less inert solvents but products containing an acetyl group (e. g. acetylcyclohexane) were observed 20> to an appreciable extent from irradiations in cyclohexane, ether, and dioxane cyclohexene, ethylbenzene, and 2-propanol reacted only by H-abstraction. Photoirradiation of biacetyl and phenylacetic or phenoxyacetic acid in acid medium produced 12> benzyl methyl ketone... 

The photocatalyzed addition of 2-propanol to the chiral menthyl monoesters of maleic acid exhibits modest levels of stereoselectivity (1,4-induction)24. The reaction proceeds via 2-propanoyl radicals that add regio- and stereoselectively to the double bond. Hydrogen abstraction and subsequent lactonization gives two diastcreomeric terebic acid derivatives. 

Propanol. 2-Propanol was chosen as a model for a simple alcohol. Irradiation under vacuum gave hydrogen and acetone as major products along with smaller amounts of methane, acetaldehyde, carbon monoxide, tert-butyl alcohol, and ethanol. These minor products must arise mainly from acetone. Acetaldehyde may be formed either by direct photolysis of 2-propanol, hydrogen atom abstraction by acetyl radical, or /3-scission of the 2-propyloxy radical. The formation of tert-butyl alcohol implies the presence of methyl and 2-hydroxy-2-propyl radicals. 

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