Photochemistry_of_Organic

Scheme 6.138

For example,3n,p excited 1,4-benzoquinone affords spiro-oxetanes in the presence of alkenes exclusively, whereas 1,4-naphthoquinone, in which the energies of the 3n,p and 3p,p states are close, produces both spiro-oxetanes and cyclobutanes.990 Introduction of electron-donor substituents on quinones further destabilizes the 3n,p state.

[2 þ 2] Photocycloaddition Reactions

Cycloaddition reactions of triplet excited 1,4-quinones to ground-state alkenes occur either via a triplet exciplex intermediate, which collapses to a triplet biradical,1000 or via separated radical ion intermediacy.990 The existence of biradical intermediates has been proven by measurements of chemically induced dynamic nuclear polarization (CIDNP) (Special Topic 5.3), for example in the reaction of 1,4-benzoquinone (313) with norbornadiene (314) yielding two products, the spiro-oxetane 315 and the spiro-oxolane 316 (Scheme 6.139).1001 Interestingly, quadricyclane (317) provides the same reaction as norbornadiene.

Scheme 6.139

The hydroxy group at position 2 in 1,4-naphthoquinone (318) is involved in [3 þ 2] addition, producing the 2,3-dihydronaphtho[2,3-b]furan-4,9-dione 319 in 50% chemical yield (Scheme 6.140).1002 The reaction proceeds by a two-step process from a triplet excited quinone via exciplex (Section 2.2.3), intramolecular electron transfer (Section 5.2) and possibly ionic intermediate 320 formation.

O

319

Scheme 6.140

Hydrogen Abstraction

Hydrogen abstraction is a typical reaction of triplet excited quinones.994 The first step involves hydrogen atom transfer to form a pair of radicals, similar to that occurring in simple triplet ketones (Section 6.3.1). Alternatively, hydrogen can also be abstracted in a two-step process, which is initiated by electron transfer from the hydrogen atom source to a quinone to form a radical ion pair, followed by proton transfer. The radical mechanism of this reaction is demonstrated in the following example: 9,10-phenanthrenequinone (321) in the triplet state abstracts the hydrogen atom from an aldehyde to form a triplet radical pair that recombines to give an acylated product 322 (Scheme 6.141).1003

Scheme 6.141

H2O

O

326

Scheme 6.144

Esters of Carboxylic Acids and Lactones

Aliphatic esters of carboxylic acids absorb below 200 nm;1013,1017 therefore, their photochemistry is usually associated with other chromophores. Photolysis of arylmethyl esters affords products derived from either homolytic or heterolytic cleavage via the excited singlet state (Scheme 6.145). The resulting radical or ionic intermediates readily undergo subsequent radical or polar reactions, respectively. The bond dissociation energies of the acyl [O CC(O)] bond are relatively low (e.g. DO C ¼ 355 or 313 kJ mol 1 for ethyl acetate or phenyl benzoate, respectively874); therefore the energy of photons with wavelengths below 330 nm is sufficient for s-bond homolytic fission.

Scheme 6.145

Photolysis of the 1-naphthylmethyl 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.

Scheme 6.146

It was discussed in Section 6.2.3 that substituted aromatic compounds in the lowest excited singlet state display behaviour contrasting with those in the ground state, which is related to differing charge distributions in the two electronic states. For example, benzene derivatives substituted with an electron-withdrawing group in the meta position are more

reactive towards photochemical solvolysis than their para-substituted analogues (meta effect).1031–1033 Scheme 6.147 shows the difference in photoreactivity of 3-methoxybenzyl

acetate (meta) (331) and 4-methoxybenzyl acetate (para) (332) in dioxane.327 Irradiation of the meta-derivative leads predominantly (35%) to the product of heterolysis (333), whereas the para-derivative affords products of homolysis (334) only.

Scheme 6.147

Case Study 6.26: Biology – photoactivatable compounds

Esters of carboxylic acids have been utilized as photoremovable protecting groups (PPGs) (see also Special Topic 6.18) in organic synthesis and biology.1013,1015,1034,1035

For example, the 4-hydroxyphenacyl protecting group has proven to be an efficient tool for analysis of fast biological processes. Its esters are usually water soluble and stable in aqueous solutions in the dark and the photoproducts are nontoxic. The photolysis of the 4-hydroxy-3-methoxyphenacyl ester of g-amino butyric acid (GABA) (335)

(Scheme 6.148) allows for the spatially and temporally controlled rapid release of GABA (336) in CA1 neurons of acute hippocampal brain slices.1036 It has been

demonstrated that the deprotection mechanism proceeds via the excited triplet state (see Case Study 5.3).

Experimental details.1036 CA1 hippocampal neuron from a 7-day-old rat, bathed in a solution of 335 (200 mM), was irradiated by UV flashes from a mercury arc lamp (100 W) coupled to fused-silica optical fibres using transmitting condensers (Figure 3.28).1037 The light-emitting end of the fibre was positioned above the brain slice, producing an approximately 15 25 mm spot. The duration of the light pulse (2 ms) was controlled with an electromechanical shutter. The pulse elicited fast membrane currents due to the specific stimulation of GABA A-receptors (ligand-gated chloride channels) because of the in situ GABA photorelease.

The photodecarboxylation of lactones usually involves the homolytic fission of the C O bond attached to the carbonyl group. Strained ring systems, such as cyclopropane, can be obtained by intramolecular coupling of the resulting biradical intermediate.1025 Scheme 6.149 shows the photodecarboxylation of the a-santonin derivative 337, possessing a g-lactone ring, to give 338 in 56% chemical yield.1038

Scheme 6.149

Photolysis of aromatic lactones and anhydrides, such as naphthalene-1,2-dicarboxylic anhydride (339), gives arynes (in this case 340)1039 as reactive intermediates.1040 Matrix isolation spectroscopy (Section 3.10) then allows their study along with other exotic molecules, which are formed as primary or secondary (photo)products (Scheme 6.150).

Scheme 6.150

The photo-Fries rearrangement, analogous to the Lewis acid-catalysed (ground state) Fries reaction, is a typical reaction of the aryl esters of carboxylic acids, but also of aryl carbonates, carbamates, sulfonates and other related compounds.1012 Scheme 6.151 shows the photorearrangement of phenyl acetate, which cleaves homolytically at the carbonyl– oxygen single bond from the excited singlet state to give a radical pair. Subsequent hydrogen abstraction by phenyloxy radical from a hydrogen atom donor ([H]) affords phenol, while in-

cage recombination of the radical pair provides 2- or 4-hydroxyacetophenone. It has also been reported that some aryl esters react from their upper triplet states.1012,1041

H O

O

Scheme 6.151

Case Study 6.27: Supercritical CO2 chemistry – photo-Fries reaction

Supercritical solvents, compounds at a temperature and pressure above their thermodynamic critical points, are interesting reaction media because their properties,