Photochemistry_of_Organic

Photocycloaddition Reactions of Two Aromatic Moieties

Although intermolecular photocycloaddition of two benzene rings in the condensed phase has not been observed, this reaction is common for polycyclic aromatic hydrocarbons.805,812 For example, anthracene-9-carbonitrile (231) in acetonitrile undergoes efficient [4 þ 4] photocycloaddition with anthracene to give an adduct 232 in 94% chemical yield (Scheme 6.90).833 This process is thermally reversible.

CN

hν

CN +

∆

Scheme 6.90

However, when benzene or other aromatic rings are constrained in close proximity,

photocycloaddition may result in the formation of unusual cage compounds (see also Special Topic 6.6).725,805 For example, [34](1,2,3,5)cyclophane (233) undergoes [6 þ 6]

photocycloaddition to give the hexaprismane derivative 234 in 7% chemical yield834 (Scheme 6.91).

hν

Scheme 6.91

Case Study 6.14: Synthesis of cage compounds – octahedrane

Whereas [2.2]paracyclophane (235) is almost insensitive to irradiation, two benzene units in the diazacyclophane derivative 236 were found to photocyclize to the octahedrane 237 in one step (Scheme 6.92).835 The nitrogen atom is apparently the key to the starting compound reactivity here. The bridges hold the aromatic rings closer together and through-bond coupling between the benzene p-orbitals and the C N s- orbitals are thought to facilitate an enhanced interaction between the rings in the excited state.

Experimental details.835 A solution of diazacyclophane (236; 50 mmol) in benzene (10 ml) in a quartz tube was purged with nitrogen and irradiated in a Rayonet photoreactor (Figure 3.10) equipped with 16 fluorescent lamps (lirr ¼ 300 nm, 25 W each) at room temperature for 36 h. The solution was concentrated under reduced pressure and the residue was separated repeatedly by preparative TLC to afford octahedrane in 33% chemical yield.

Scheme 6.94

In-cage ion pairs can also be formed by initial carbon–halogen photoinduced homolysis (see also Section 6.6.2), followed by an electron transfer step (C–X

homolysis, Scheme 6.93). Chlorobenzene, for example, is converted photochemically to phenol in aqueous solutions (Scheme 6.95).848,849 The hydrogen atom cannot be

abstracted from water because of the high hydrogen–oxygen bond dissociation energy (DO–H ¼ 498 kJ mol 1); nevertheless, the high solvent dielectric constant promotes an in-cage electron transfer pathway.

Scheme 6.95

Photosubstitution may proceed by direct attack of a nucleophile on the singlet or triplet excited state of an aromatic molecule to form a s-complex (SN2Ar ; Scheme 6.93),836,838

analogous to the Meisenheimer complex intermediate recognized in thermal SN2Ar

reactions. A s-complex is also known to be formed in the photochemical nucleophile olefin combination aromatic substitution process (photo-NOCAS).837,850 This reaction

involves a regioselective interconnection of three reactants: an aromatic electron acceptor (usually an aryl nitrile), an electron donor [olefin (alkene); acting as a p-nucleophile] and an auxiliary nucleophilic species such as methanol. For example, excitation of 1,4- dicyanobenzene (242) to its lowest excited singlet state, which can also be co-sensitized, promotes electron transfer from an alkene 243 to give a contact radical ion pair (Scheme 6.96).851 The alkene radical cation is then attacked by methanol to form the corresponding b-methoxyalkyl radical 244 after deprotonation. In the last step of the mechanism, this radical adds itself to the 1,4-dicyanobenzene radical anion at the ipso position, forming a s-complex 245 and finally 246 in 17% chemical yield.

Irradiation of aromatic compounds in the presence of a good electron donor (nucleophile) may promote electron transfer from this species to the excited aromatic

substrate in order to form an anion-radical intermediate, which releases the leaving group (SR–N1Ar , where R ¼ radical, – ¼ anion; Scheme 6.93).836,838,840,852 In contrast, electron

transfer from the excited aromatic substrate to good electron acceptors, followed by the reaction of the cation-radical intermediate thereby formed with a nucleophile, is possible when an electron-donating group is present on the aromatic substrate (SR þ N1Ar , where þ ¼ cation; not shown).836

halogen ion.849 The resulting phenyl radical 251 couples with 248 (as a nucleophile) to form the radical anion 252, which transfers an electron to iodobenzene in the propagation step of the chain mechanism. Aryl iodides are more susceptible to such reactions than aryl bromides or chlorides.

Case Study 6.15: Mechanistic photochemistry – regioselectivity of photosubstitution

The possibility of aryl–nitrogen bond formation by photosubstitution reaction has been investigated for use in photoaffinity labelling854 experiments (see also Special Topic 6.16 in Section 6.4.2).838 4-Nitroanisole (253) presents an interesting model of an aromatic substrate, which could, in the excited triplet state, undergo nucleophilic substitution in the presence of an amine (Scheme 6.98).855 The reaction was highly regioselective; n-hexylamine (254) gave rise to methoxy substitution with a maximum quantum yield of F ¼ 0.018, while ethyl glycinate substituted the nitro group (255) with F ¼ 0.008. Experiments with radical scavengers provided evidence that the former reaction occurs via electron transfer from the amine to the excited triplet state of 4-nitroanisole to form a radical ion pair, which undergoes a bimolecular SN2Ar reaction. Compared with alkylamine, the ionization potential of ethyl glycinate is too high to allow for efficient electron transfer to the triplet nitroanisole; furthermore, SN2Ar reactions are known to prefer the transition state that leads to the least stable s-complex,325 which is NO2 group substitution in the present case.

Scheme 6.98

Experimental details.855 The quantum yields of the reactions shown in Scheme 6.98 were obtained by simultaneous irradiation of the corresponding 4-nitroanisole– amine solutions in methanol–water (20:80, v/v) and actinometer (Section 3.9.2) solutions (aqueous potassium ferrioxalate) in UV cells, placed in a merry-go-round apparatus (Figure 3.30). The samples were irradiated by passing the light from a

3.Predict the major photoproduct(s):

(a)

hν

[ref. 859]

(b)

OMe

hν

+

CN

[ref. 860]

(c)

[ref. 861]

6.3 Oxygen Compounds

Oxygen is more electronegative than carbon but the nonbonding lone pairs (doubly occupied np-orbitals) of oxygen substituents ( OH, OR, O ) act as a mesomeric electron donor. The absorption spectra of alternant hydrocarbons are not much affected by the inductive effect, but the conjugative interaction shifts their p,p transitions to longer wavelengths. The 1Lb bands are shifted more strongly than the 1Lb bands and the shifts increase along the series OH, OR, O . Electronic excitation of phenols involves a substantial amount of electron transfer from oxygen to the aromatic ring, particularly to the meta-positions.

Carbonyl compounds also have two nonbonding lone pairs on the oxygen atom. In organic chemistry texts, these are sometimes shown as two equivalent sp2-hybridized lobes (rabbit s ears). While hybridization has no effect on the total energy, the two degenerate nsp2 orbitals are inappropriate as a basis set to discuss one-electron properties such as ionization potentials or n,p transitions. Rather, the symmetry-adapted lone pairs

Figure 6.4 The n,p transition of formaldehyde

(see Section 4.4) nsp and np should be used (Figure 6.4). Although situated on the electronegative oxygen atom and therefore at lower energy than a nonbonding orbital on carbon, the np lone pair is higher in energy than the bonding p-MO. The first ionization potential of simple ketones is attributed to electron ejection from the np-orbital that is situated well above the nsp-orbital having 50% s-character.

The lowest excited state of simple ketones and aldehydes corresponds to excitation of an electron from the np lone pair to the p -MO. The transition is forbidden in compounds of C2v symmetry. The local symmetry is the same in compounds such as acetaldehyde, so that n,p transitions of ketones and aldehydes are generally weak, « 20–50 M 1 cm 1, and they are easily overlooked in absorption spectra or hidden by the red edge of stronger p,p absorption. The nature of the lowest excited state is, however, decisive for the photophysical properties and the photochemical reactivity of carbonyl compounds; the reactivity of n,p excited ketones is comparable to that of alkoxy radicals (see below).

Solvent shifts are useful as a criterion to identify n,p transitions in absorption spectra because hydrogen bonding of protic solvents with the carbonyl oxygen stabilizes the np lone pair and gives rise to a hypsochromic shift of the n,p absorption bands; see the positions of the n,p absorption band of acetone in heptane and water (Figure 6.5, top). This contrasts with p,p transitions that tend to be shifted bathochromically in polar solvents. Also, the photophysical and photochemical properties often serve to identify the nature of the lowest excited state. Lone-pair interaction in biacetyl splits the two np- orbitals giving rise to two n,p transitions at ~v ¼ 2.23 and 3.54 mm 1. In the spectrum of 1,4-naphthoquinone in methanol, the n,p band is barely detectable as a shoulder on the red edge of the p,p absorption.

The position of the p,p transitions in conjugated ketones may be estimated from the corresponding bands in hydrocarbons of the same topology; the inductive effect of oxygen on p,p transitions in alternant hydrocarbons is small (Equation 4.26). Thus the p,p transitions of acetophenone are at about the same position as those of styrene (Figure 6.1). However, the intensity of the first, parity-forbidden absorption band of styrene is increased by the inductive effect in acetophenone (Section 4.7) and the n,p band is, of course, missing in the isoelectronic hydrocarbon.

The symmetry of n,p excited configurations is different from that of the p,p configurations (the wavefunctions are antisymmetric and symmetric, respectively, with respect to the plane of the R2CO moiety), so the two sets do not interact in CI calculations (Section 4.7) for planar molecules. The shifts of n,p transitions with increasing conjugation tend to be smaller than those of the p,p transitions. This is readily understood in terms of HMO or PMO theory: increased conjugation reduces the HOMO–LUMO gap by lowering the p -MO and raising the p-MO, but only the lowering of the p -MO affects

Figure 6.5 Absorption spectra of prototype ketones.280 Top: benzophenone (EtOH, ––), acetophenone (EtOH, —), acetone (n-heptane, – – –), acetone (water, ....). Bottom: 1,4-

benzoquinone (hexane, ––), 1,4-naphthoquinone (MeOH, - - -), anthraquinone (cyclohexane,

—), biacetyl (hexane, ....)

the n,p transition. Hence the lowest singlet state is generally of p,p character in large systems.

Intersystem crossing (ISC) to the triplet manifold is especially fast and efficient when a p,p triplet state is energetically close to or slightly below the lowest singlet state of n,p character (El Sayed s rules, Figure 2.8). This is the case for acetophenone, benzophenone, 1,4-benzoquinone and 9,10-anthraquinone. In these compounds, ISC takes place within a few picoseconds so that their quantum yields of ISC are near unity. ISC in saturated ketones is much slower ( 10 8 s 1), because the lowest 3p,p state lies well above the 1n,p state. The energy gap separating singlet and triplet p,p configurations is much larger than that for n,p states, especially in molecules of alternant topology (Section 4.7, Equation 4.33). Therefore, the situation S1(n,p ) but T1(p,p ), as in donor-substituted acetophenone and benzophenone derivatives, is not uncommon.

The shifts of n,p transitions in related chromophores can be estimated using perturbation theory (Equations 4.13 and 4.14). The np-orbital is sensitive mostly to inductive perturbations. The p -orbital is shifted both by inductive and mesomeric interactions; its AO coefficient at the carbonyl C atom is large (Figure 6.4). These qualitative considerations are supported by the data given in Table 6.5.

Table 6.6 lists the most important phototransformations discussed in this section. The carbonyl compounds (entries 1–6) are typical representatives of the photolabile compounds. Their reactions played an essential role in revealing the mechanisms of some primary photochemical steps. Thanks to their excellent absorption properties, thermal stability, usually uncomplicated synthesis and reaction diversity, they represent popular starting material in applied synthetic or material photochemistry and in photobiochemistry. It is