1- Naphthylmethyl radical

More recently, photoionization following the same mechanism as described for 132 has been reported for 1-naphthylmethyl radical (154) [147] and 4-biphenyl-methyl radical (140) [ 148] (Scheme 21). Thus, the lowest excited state of the radical forms a chloro-adduct in carbon tetrachloride, presumably as a result of charge transfer followed by trapping, while an upper excited state produced by biphotonic excitation photoionizes and the carbocation product is quenched by alcohol. 

When excited at 266 nm, all four compounds exhibited a broad blue fluorescence centered at a400 nm with lifetimes ranging from 100 to 3500 ps. Excitation with a second 355-nm laser pulse which was delayed relative to the first 266-nm pulse produced a fluorescence centered at V600 nm with a lifetime of vlO ns. Since the emission spectra (Figure 7) obtained by excitation of l-(chloromethyl)naphthalene and l-(bromomethyl)-naphthalene are superimposable in the region from 550 to 750 nm, this red fluorescence was attributed to the 1-naphthylmethyl radical. In a similar manner, the red fluorescence resulting from two-color excitation of 2-(chloromethyl)naphthalene and 2-(bromomethyl)naphthalene was attributed to the 2-naphthylmethyl radical. This two-color fluorescence technique used in conjunction with OMCDs has proven to be a powerful tool in the study of photodissociation of haloaromatic compounds. 

Photolysis of the 1-naphthyhnethyl ester of phenylacetic acid (327) in methanol, for example, affords the 1-naphthylmethyl cation carboxylate anion pair in addition to 1-naphthylmethyl radical acyloxy radical pair intermediates, which, after decarboxylation, form an adduct with methanol (328, formed along with 329) or an in-cage radical coupling product 330 (Scheme 6.146).1030 The competition between the radical and ionic pathways was found to be very dependent upon the substituents on the naphthalene ring. 

In polar solvents, a-halomethyl aromatics give rise to photochemical reactions that can be explained by both radical and ionic mechanisms. Equation 12.77 shows the results for irradiation of 1-chloromethylnaphtha-lene (119) in methanol. The most direct pathway for formation of the methyl ether 120 is heterolytic dissociation of the C-Cl bond to give a chloride ion and a 1-naphthylmethyl carbocation, the latter then undergoing nucleophilic addition by the solvent. Indeed, naphthylphenylmethyl carbocations were detected spectroscopically following laser flash photolysis of (naphthylphenylmethyl)triphenylphosphonium chlorides. On the other hand, products 121, 122, and 123 appear to be formed via the 1-naphthylmethyl radical. Therefore, an alternative source of the carbocation leading to 120 could be electron transfer from the 1-naphthylmethyl radical instead of direct photochemical heterolysis of 119.215-216 jaj-g g p. 

Similar transient behavior—formation of the charge transfer complex— was observed following irradiation of the radical derived from l-bromo-(2-bromomethyl)naphthalene, i.e., the l-bromo-2-naphthylmethyl radical (158) [99]. However, product evidence was ambiguous with respect to the production of the 1,3-biradical intermediate (Scheme 10). 

Bond Dissociation from the T State For the T state, reaction that cannot be observed for the Tj states can be expected. A bond dissociation process is one of the reactions, which can be expected for the T states. Here, we introduce the formation of naphthylmethyl radical from the T states [67],... 

The photochemical cleavage of naphthylmethyl alkanoates in methanol is reported to proceed by homolytic cleavage to naphthylmethyl radical and acyloxy radical,the latter decarboxylates in competition with electron transfer to give naphthylmethyl cation and carboxylate anion. Using known rates of electron transfer as a clock the rate constants for decarboxylation of the acyloxy radicals has been estimated.The light induced homolysis of 1-chloromethyl-naphthalene has also been studied using chemically induced dynamic electron polarisation (CIDEP) spectroscopy to detect the naphthyl-... 

Photolysis of the naphthylmethyl esters (365) gives naphthylmethyl radicals and phenylacetoxyl radical. The radical pair may transfer an electron to yield phenyl acetate and naphthylmethyl cation which is quenched by nucleophilic solvent. Alternatively, the radical pair may escape their solvent cage and so yield free radical derived products. Pincock has estimated the rate constant for the electron transfer in the radical pair for different X-substituents on the naphthalene by monitoring the ratio of free radical products to ionic products produced in the photolysis reaction, and has correlated this rate constant with the free energy of the electron transfer reaction in the radical pair. The results are discussed in terms of the Marcus theoretical relationship between reaction rate constant and equilibrium constant. 

Fig. U. Hme-resoWed emission of the 1- and 2-naphthjtaiethyl radicals observed u waveiengths longer than 570 nm. 1-Napbthylmethyl radical emission induced by a 3S5-nm pulse (a) 60 ps after 266 excitation, (b) 320 ps after excitation, and (c) 2-naphthylmethyl radical observed 60-ps after 266-nm excitation.

It will be noted that the non-planar propeller form lacks a plane of symmetry and that the production of optically active products by way of an intermediate radical might therefore be possible. Attempts to make precursory optically active phenylxenyl-a-naphthylmethyl halides failed, but the corresponding active thioglycolic acid has been prepared. It is racemized by the reaction with triphenylmethyl free radicals.19 Various other reactions believed to involve radical intermediates also give inactive products ... 

Apart from lengthening the chain by one carbon atom, this reaction provides a compound with two readily modified functional groups. Many other halogen compounds offer a more complex picture. Radical-derived products are sometimes obtained, but in other cases a heterolytic mechanism must operate, for example to give a methyl ether when naphthylmethyl iodides are irradiated in methanol (5.651. 

Grollmann U, Schnabel W (1980) On the kinetics of polymer degradation in solution, 9. Pulse radiolysis of polyethylene oxide). Makromol Chem 181 1215-1226 Hamer DH (1986) Metallothionein. In Richardson CC, Boyer PD, Dawid IB, Meister A (eds) Annual review of biochemistry. Annual Reviews, Palo Alto, pp 913-951 Held KD, Harrop HA, Michael BD (1985) Pulse radiolysis studies of the interactions of the sulfhydryl compound dithiothreitol and sugars. Radiat Res 103 171-185 Hilborn JW, PincockJA (1991) Rates of decarboxylation of acyloxy radicals formed in the photocleavage of substituted 1-naphthylmethyl alkanoates. J Am Chem Soc 113 2683-2686 Hiller K-O, Asmus K-D (1983) Formation and reduction reactions of a-amino radicals derived from methionine and its derivatives in aqueous solutions. J Phys Chem 87 3682-3688 Hiller K-O, Masloch B, Gobi M, Asmus K-D (1981) Mechanism of the OH radical induced oxidation of methionine in aqueous solution. J Am Chem Soc 103 2734-2743 Hoffman MZ, Hayon E (1972) One-electron reduction of the disulfide linkage in aqueous solution. Formation, protonation and decay kinetics of the RSSR radical. J Am Chem Soc 94 7950-7957... 

Pincock and DeCosta [96] have recently described photoinduced bond cleavage reactions in a series of naphthylmethyl esters to produce both ionic and radical products. The authors attribute their results to excited state homolysis to form a caged radical pair and ET between the radicals to form an ion pair. The rates of ET have been rationalized by the Marcus theory which shows a Marcus inverted region where ET becomes slow when AG T is highly exothermic. In this case ET appears to follow homolytic bond cleavage rather than prior to bond breaking. 

Di-(l-naphthylmethyl)sulphone forms an excimer but does not react to give an intramolecular cycloaddition product like the corresponding ether but rather fragments to give sulphur dioxide and (l-naphthyl)methyl radicals (Amiri and Mellor, 1978). I-Naphthylacetyl chloride has a very low quantum yield of fluorescence and this is possibly due to exciplex formation between the acyl group and the naphthalene nucleus (Tamaki, 1979). Irradiation leads to decarbonylation. It is known that acyl chlorides quench the fluorescence of aromatic hydrocarbons and that this process leads to acylation of the aromatic hydrocarbon (Tamaki, 1978a). The decarboxylation of anhydrides of phenylacetic acids [171] has been interpreted as shown in (53), involving... 

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