Photochemistry_of_Organic

Alkyl Halides

Simple alkyl chlorides and bromides do not absorb above 250 nm, whereas the absorption band of an n,s transition in methyl iodide, for example, has a maximum at lmax ¼ 258 nm.1334 Direct irradiation of alkyl halides results in homolytic cleavage of the C X bond (X ¼ halogen). In general, the excitation energy (ES) in alkyl iodides or

bromides (380–475 kJ mol 1) is higher than the C X bond dissociation energies (220–300 kJ mol 1).1314 The quantum yield of the fragmentation does not have to be

unity even in the gas phase because of possible concurrent non-radiative or radiative energy dissipation processes.1335 The photoreaction usually involves the excited singlet state and reaction is initiated by homolytic cleavage, possibly followed by electron transfer within the caged radical pair (Scheme 6.228). The radical species undergo radical reactions, such as hydrogen abstraction or rearrangement, while the resulting carbocation can be attacked by nucleophiles (nucleophilic photosubstitution; SN1 ).

Scheme 6.228

Irradiation of primary alkyl bromides or iodides affords typically a mixture of reduction and elimination products. For example, a radical pair produced by photolysis of the halooctanes 469 undergoes in the initial stage either competing diffusion and hydrogen abstraction from the solvent molecules (to give 470) or electron transfer giving rise to elimination product (471) – a favoured process in case of alkyl iodides (Scheme 6.229).1336 Formation of the nucleophilic substitution product 472 is not observed, because solvent does not participate in carbocation generation.

Scheme 6.229

Photolysis of norbornan-1-yl (tertiary) halides (473) in methanol affords both reduction and the nucleophilic photosubstitution products 474 and 475, respectively (Scheme 6.230).1336

scavenger affords the major nucleophilic substitution product 479 and small amounts of the radical-derived reduction product cyclohexene (480) (Scheme 6.232).1339 In the absence of zinc, the ketal 481 is apparently obtained via an acid-catalysed addition of methanol to 479. In contrast, vinyl bromide (478, X ¼ Br) mainly provides the photoreduction product 480, which parallels closely the photobehaviour of the saturated analogues.1314

Scheme 6.232

Aryl Halides

Simple aromatic halides display two principal superimposing absorption bands, p,p and s,s , tailing over 250 nm. Intersystem crossing (ISC) to T1 can be fast and efficient due to

increased spin–orbit coupling attributed to the heavy-atom effect (Section 4.8) of halogen atoms (Br, I).842,849,1316 The subsequent C X bond homolysis may afford aryl radicals and

halogen atoms as the primary products (Scheme 6.233), which requires that the energy of the reactive excited state be greater than the corresponding C X dissociation energy. Fission is, therefore, thermodynamically unfavourable for aryl fluorides, for example

fluorobenzene (ET ¼ 353 kJ mol 1; DC F ¼ 526 kJ mol 1), and for most aryl chlorides, for example 1-chloronaphthalene (ET ¼ 248 kJ mol 1; DC Cl ¼ 403 kJ mol 1).849 In the latter

Scheme 6.233

Scheme 6.239

Nucleophilic photosubstitutions are typical reactions of aromatic halides. The variability of the process in terms of mechanism is presented in detail, along with many examples, in Section 6.2.3; therefore only one example of the reaction is mentioned here. Multiple photocyanation of hexachlorobenzene (492) takes place in aqueous acetonitrile containing a cyanide ion to give pentacyanophenolate (493) (Scheme 6.240).1347 The reaction involves the triplet state apparently via an SN2Ar (Scheme 6.93) pathway.

Scheme 6.240

Case Study 6.35: Synthesis of cage compounds – cubane

Photolysis of 1,4-diiodocubane (494) in nucleophilic solvents, such as methanol, was found to provide effectively the substitution product 495 (Scheme 6.241).1348 It was argued that the reaction proceeds via the formation of a caged radical pair and electron transfer to give iodide ion and a carbocation, which is trapped by the solvent (SN1 pathway). Hyperconjugative stabilization in cubyl cation cannot assist much from an extremely strained olefin cubene. However, the authors expected that photochemical fission of the C I bond would still provide sufficient energy for production of the cation intermediate.

Scheme 6.241

Experimental details.1348 1,4-Diiodocubane (494; 0.2 mmol) suspended in dry methanol (6.0 ml) in a quartz tube equipped with a magnetic stirrer was irradiated in a Rayonet reactor equipped with 16 low-pressure mercury lamps (lirr ¼ 254 nm) (Figure 3.10) under a nitrogen atmosphere at 40 C. About 32% monosubstituted 1-iodo-4-methoxycubane was detected by GC after 6 h. Further irradiation gave a significant production of l,4-dimethoxycubane (495; 50%).

Hypervalent Iodine Compounds

Hypervalent iodine compounds, especially organoiodine(III) species, are commonly used in synthetic organic chemistry, but photochemical applications are still rare.1349 Direct photolysis of diphenyliodonium triflate (496), for example, affords a singlet excited species that undergoes a heterolytic cleavage leading to phenyl cation and iodobenzene. In addition, intersystem crossing leads to the triplet state, which is homolytically fragmented to give a caged pair of iodobenzene radical cation and phenyl radical. The primary isolated products in H-atom donating solvents are 2-, 3- and 4-iodobiphenyl, iodobenzene, benzene and trifluoromethanesulfonic acid (Scheme 6.242).1350 Such a photochemically generated strong acid has been successfully used as an initiator of cationic polymerizations1351 (Section 6.8.1).

496

Scheme 6.242

Alkyl Hypohalites

Hypohalites represent another class of photolabile organic halogen compounds, which exhibit two absorption peaks above 250 nm; alkyl hypochlorites have lmax at 260 and320 nm, while the absorption of alkyl hypobromites and hypoiodites is bathochromically shifted.1319 Irradiation of alkyl hypohalites leads to a homolytic cleavage to give alkoxyl and halogen radicals with a high photochemical efficiency (Scheme 6.243).1352 These primary intermediates can subsequently undergo various reactions, such as hydrogen abstraction, rearrangement or fragmentation.

hν

R OX R O + X

Scheme 6.243

Alkyl hypochlorites and hypobromites are relatively stable molecules; alkyl hypoiodites can only be prepared in situ, usually by the reaction of alcohols with metal acetates or oxides and iodine or by the reaction of alcohols with a hypervalent iodine compound and iodine.1319 Alkyl hypochlorites and hypoiodites can be utilized in reactions that parallel those of the organic nitrites (Barton reaction; Section 6.4.2). For example, the photochemistry of the steroidal hypoiodite 497, prepared by the reaction of the corresponding alcohol with iodine oxide (I2O) generated from mercury(II) oxide and molecular iodine in situ, affords a new five-membered ring via an O–I bond fission, 1,5- hydrogen abstraction and substitution (Scheme 6.244).1353

Scheme 6.244

6.6.3Problems

1.Explain the following concepts and keywords: photoinduced radical chain mechanism; photonitrosylation; nucleophilic photosubstitution; photodehalogenation.

2.Suggest the mechanisms for the following reactions:

(a)

[ref. 1354]

6.7.1Molecular Oxygen: Ground State and Excited State

Recommended review articles.135,136,1358–1363

Selected theoretical and computational photochemistry references.135,136,1364

Ground-state Molecular Oxygen

Ground-state molecular oxygen (dioxygen), an open-shell triplet (biradical) denoted by spectroscopists 3Sg, is one of the most abundant and important life-supporting species on Earth. It can participate in some photochemically initiated reactions, sometimes called