Photochemistry_of_Organic

hν

ArSO2OR ArSO2OR ArSO2 + OR Et3N

Scheme 6.217

For example, photolysis of methyl 3-O-benzyl-2,6-dideoxy-4-O-(p-tolylsulfonyl)- a-D-arabino-hexopyranoside (457) in the presence of a high concentration of triethylamine gives the glycoside 458 (Scheme 6.218).1289

Scheme 6.218

Experimental details.1289 A solution of 457 (0.74 mmol) and triethylamine (2 mmol) in methanol was purged with nitrogen for 1 h and irradiated in a Rayonet photochemical chamber equipped with 16 low-pressure mercury discharge lamps (254 nm) (Figure 3.10) for 12 h. After irradiation, methanol was distilled off under reduced pressure and the product was purified by column chromatography in 90% chemical yield.

Homolytic fission of the C S bond is a primary process following the excitation of sulfoxides. For example, the sulfoxide 459 produces a 1,5-biradical, which can either rearrange to the sulfenate ester 460 or revert to the starting material (Scheme 6.219).1290

Scheme 6.219

6.5.3Problems

1.Explain the following concepts and keywords: photofragmentation; photoextrusion.

2.Suggest the mechanisms for the following reactions:

(a)

[ref. 1291]

(b)

[ref. 1292]

3.Predict the major photoproduct(s):

(a)

S

hν

H

SH

[ref. 1293]

(b)

[ref. 1294]

6.6 Halogen Compounds

Halogen substitution has little effect on the absorption spectra of conjugated compounds. The heavier halogens bromine and iodine may, however, substantially increase the rate of ISC, especially when the parent compound has a long excited-state lifetime. The s,s absorption of haloalkanes reaches into the near-UV region, increasingly with the heavier halogens and geminal dior trisubstitution. Halogenated aliphatic solvents have relatively low cut-off wavelengths (see also Table 3.1), for example, 2,2,2-trifluorethanol (<190 nm),

dichloromethane ( 230 nm), chloroform ( 250 nm), carbon tetrachloride ( 265 nm) and bromoform ( 300 nm).158

In this section, we discuss separately the chemical reactions that follow photofragmentation of a halogen molecule and the reactions of excited halogen-containing organic molecules. In the first case, a light-absorbing halogen molecule, such as chlorine (Table 6.18, entry 1), is homolytically cleaved to give two halogen atoms, which take part in the subsequent radical chain reactions (halogenations) with, for example, saturated or unsaturated hydrocarbons. Since the dissociation energies of halogen molecules are relatively low, visible light is sufficient to promote the initial step.

Table 6.18 Photoprocesses involving halogen compounds

aX ¼ halogen atom; Nu ¼ nucleophile; Ar ¼ aryl.

Homolytic fission of the carbon–halogen bond is typical of excited organic alkyl and aryl halides (entry 2). Various subsequent reactions may then follow, including radical reactions such as hydrogen abstraction or rearrangements or electron transfer within the caged radical pair. The resulting ion pair intermediates can be attacked by nucleophiles, although direct nucleophilic attack on an excited aryl halide (see Scheme 6.93) is also possible (entry 3).

6.6.1Halogen Compounds: Photohalogenation

Recommended review articles.155,1295,1296

Selected theoretical and computational photochemistry references.1297–1301

Photohalogenation – a photoinitiated reaction between a halogen donor, such as a halogen molecule and a substrate – proceeds typically via a radical chain mechanism; therefore, the reaction quantum yield, closely related to the average chain length, is greater than unity.155

The absorption spectra of chlorine and bromine molecules have lmax at 330 nm (« ¼ 65 l mol 1 cm 1) and 420 nm (« ¼ 1651 l mol 1 cm 1), respectively.155 Since their

dissociation energies are 243 and 192 kJ mol 1, which correspond to photons of 492 and 623 nm wavelength, respectively, they readily undergo homolytic fission upon excitation, even when using visible (incandescent) low-power radiant sources.

In the photochlorination of alkanes, chlorine atom formation is followed by various propagation and termination radical reactions (Scheme 6.220). The hydrogen abstraction step is usually exothermic and irreversible. Hydrogen atoms are abstracted at rates that are in

the order primary < secondary < tertiary C H bonds; the activation energies for abstraction may vary from 4 kJ mol 1 in the first to 0.4 kJ mol 1 in the last case.155 Such small

differences indicate that selectivity disappears at higher temperatures. Hydrocarbon rearrangements are not common, but every possible monochlorinated product is usually formed.1295 The quantum yields may exceed 106 in the absence of radical trapping agents.

R = alkyl

Scheme 6.220

Aromatic solvents and carbon disulfide often increase the chlorination selectivity due to the formation of p- or s-complexes between the solvent molecules and the chlorine atom. Such an interaction decreases the reactivity and hence increases the selectivity toward the hydrogen atoms. For example, photochlorination of adamantane (461) gives a higher production of 1-chloroadamantane in CS2 than in benzene (Scheme 6.221).1302

Scheme 6.221

Scheme 6.223

Experimental details.1305 A solution of progesterone 464 (6 mmol), bromine (6 mmol) and cyclohexane oxide (9 mmol) in CCl4 (120 ml) was exposed to light from a tungsten lamp (incandescent; visible light; 100 W) (Figure 3.9). The product precipitated after 15 h and was filtered off. Column chromatography of the remaining filtrate gave an additional amount of the starting material. The overall chemical yield was 87%.

N-Bromosuccinimide (NBS) (466) is a common bromination agent, used mainly for substitution reactions in the allylic or benzylic positions. The mechanism involves excitation of NBS generating either a bromine atom (Scheme 6.224a) or a succinimidyl

radical1306 (Scheme 6.224b) as one of the chain propagation steps, depending on the solvent and conditions used and, to some extent, on the reactivity of the substrate.1307

Scheme 6.224

Rose oxide (467), a valuable perfume additive, is manufactured by an ene reaction (Section 6.7.3) of citronellol (468) (derived from lemon grass) with oxygen by

irradiation in the presence of Methylene Blue as a triplet photosensitizer (Scheme 6.227).1311,1312

Scheme 6.227

Photopolymerization, that is, a polymerization process initiated by photochemical production of the reactive species – initiators (Section 6.8.1), is largely used in industry. Polymers can be synthesized by photoinitiation, but can also be modified by photochemical cross-linking (photochemical hardening or UV curing; photoimaging; photolithography; see Special Topic 6.27).

The most important and widespread applications of photochemistry are, of course, photoimaging techniques, such as photography and xerography (Special Topic 6.32).1313 Some other large-scale applications, such as environmental remediation of anthropogenic pollutants by photocatalytic processes (Special Topic 6.28) and potential applications such as photoelectrochemical cells (Special Topic 6.31) and artificial photosynthesis (Special Topic 6.26), are also discussed in the following text.

6.6.2 Organic Halogen Compounds: Photofragmentation, Photoreduction

and Nucleophilic Photosubstitution

hν, Nu

R-X R-Nu

Recommended review articles.836,838–842, 849,852,1314–1321

Selected theoretical and computational photochemistry references.1322–1333