Photochemistry_of_Organic

Case Study 6.4: Optical information storage – photochromic diarylethenes

Many derivatives of 1,2-bis(hetero)arylethenes have received special attention because of their possible use for data storage and molecular device applications.624 For example, irradiation of the bisheteroarylethene derivative 69, which has an optically active ( )- or ( þ )-menthyl group at position 2 of the benzo[b]thiophene ring, in cold toluene at 450 nm leads to the photostationary formation of a pair of diastereomers 70 (the stereogenic centres ( ) are shown), via a 6p-electron electrocyclization, with a very high diastereomeric excess (de <87%) (Scheme 6.25).625 Both diastereomers selectively return to the open-ring forms upon irradiation with 570 nm light due to their much longer wavelength absorption. The whole process can be repeated several times; thus 69 displays photochromic (Special Topic 6.15) properties.

R = (-)-menthyl or (+)-menthyl

Scheme 6.25

Experimental details.625 A toluene solution of 69 (not degassed) was irradiated with a mercury lamp (500 W) or a xenon lamp (Figure 3.9). The desired wavelengths were obtained by passing the light through cut-off and interference filters. The optical purity of the products was determined using HPLC with a chiral column.

Sigmatropic Photorearrangements

Pericyclic intramolecular reactions, involving both the formation of a new s-bond between atoms previously not directly linked and the breaking of an existing s-bond via a cyclic transition state, are called sigmatropic rearrangements. Concerted607 photorearrangements of the 4n electron systems, according to the Woodward–Hoffmann orbital symmetry rules,336 are preferred because they involve supra-supra (suprafacial) bonding; only longer 4n þ 2 electron systems may allow the supra-antara mechanism. This is demonstrated by the photochemistry of 2-(2-methyl-3-phenylcyclohexylidene)malononi- trile (71), which gives 72 as the exclusive product by a 4p-electron [1,3]-allylic shift in 25% isolated chemical yield (Scheme 6.26).626

An interesting example of a 6p-electron [1,5]-hydrogen shift in the diisopropylidenecyclobutane 73 to give the cyclobutene 74 is believed to occur in an antarafacial fashion as predicted by the orbital symmetry rules (Scheme 6.27).627

a small amount of cyclooctatetraene (80) via the intramolecular [2 þ 2] cycloaddition (Section 6.1.5) (Scheme 6.31).648

hν

sens

hν

80

Scheme 6.31

The biradical mechanism is commonly proposed to explain the reaction regioselectivity.629,630,632,633 Two alternative ring-opening processes for the 1,4-biradical 81,

obtained from the photolysis of 82, may lead to two 1,3-biradicals, 83 and 84 (Scheme 6.32). The latter, which is thermodynamically more stable because the unpaired electron is delocalized on the benzhydryl moiety, should be a precursor to the diphenylvinylpropene 85. Indeed, the irradiation of 82 afforded 85 as a sole product.655

8381 84

Scheme 6.32

Case Study 6.6: Photobiology – natural photoproduction of erythrolide A

Erythrolide A (86) has been thought to be produced naturally from the diterpenoid 87 by a light-induced di-p-methane rearrangement (Scheme 6.33), because both compounds were isolated from the Caribbean octocoral Erythropodium caribaeorum.656 In order to confirm this hypothesis, the potential precursor 87 was irradiated under various conditions and 86 was obtained in relatively high chemical yields.

Scheme 6.33

Experimental details.656 A benzene solution of 87 in a quartz vessel was irradiated with a medium-pressure mercury lamp (benzene filters to cut off the wavelengths below 280 nm) (Figure 3.9) to give an 87% yield of 86 in 3 h. In comparison, sunlight irradiation of 87 in 5% methanolic seawater in a glass vessel produced 86 in 37% yield in 8 days.

The following reactions formally belong to Sections 6.3 and 6.4, because absorption by the C¼O and C¼N chromophores is largely responsible for the photochemistry; however, discussion of their mechanistic pathways is better suited here. An analogous photochemical rearrangement of b,g-enones, involving 1,2-acyl migration to

give cyclopropane derivatives,657 is termed the oxa-di-p-methane rearrangement.631,632,635,643,658,659 The reaction generally occurs from the lowest excited triplet

state (T1, p,p ), possibly via two biradical intermediates (Scheme 6.34). A competing sigmatropic (Section 6.1.2) 1,3-acyl shift occurs from the excited singlet (S1, n,p ) state; therefore, direct excitation to the singlet state should be avoided.

Scheme 6.34

Case Study 6.7: Organic synthesis – substituted cyclopropanes

Oxa-di-p-methane rearrangement leads to cyclopropane derivatives, compounds that are otherwise difficult to synthesize. The diphenylenal 88, for example, is converted to the cyclopropyl aldehyde 89 by triplet sensitization (Scheme 6.35).660 The photoproduct can be further transformed to other compounds, for example a diphenylvinylcyclopropane derivative 90.

Photoinduced Nucleophilic Addition and Protonation Reactions

Upon direct irradiation in inert solvents, aliphatic alkenes undergo E–Z or other isomerization reactions (Sections 6.1.1 and 6.1.2). The same excited states responsible for such

transformations, particularly the Rydberg p,R(3s) singlet state, are involved in nucleophilic addition or photoprotonation reactions in protic media.662,663,671 For example, the p,R(3s)

state of tetramethylethene (93), having cation-radical character of the central bond, is readily attacked by a nucleophile (such as methanol) to form a solvated electron and a radical intermediate 94 that disproportionates to a mixture of the ethers 95 (30%) and 96 (37%) (Scheme 6.37).671 The solvated electrons produced are trapped by the solvent molecules.

Scheme 6.37

Similarly, direct irradiation of cycloalkenes results in nucleophilic trapping of the p,R(3s) excited state.662 For example, photolysis of dimethylcyclobutene (97) in methanol at 228 nm affords three methoxy-substituted products via disproportionation reactions (Scheme 6.38).672

OCH3 OCH3 OCH3

or +

Scheme 6.38

In contrast, arylalkenes or -alkynes readily undergo acid-catalysed Markovnikov

addition on direct irradiation in water to form the corresponding alcohols and ketones, respectively (Scheme 6.39).673,674 The initial and at the same time rate-limiting step is the

protonation of a more electron-rich p,p excited singlet state (in contrast to aliphatic alkenes), occurring over 10 orders of magnitude more rapidly than protonation of the corresponding ground-state molecule. This step is followed by hydration to form the corresponding alcohols in the case of alkenes or enols675 in the case of alkynes.

Case Study 6.8: Asymmetric synthesis – diastereoselective photosensitized polar addition

Singlet photosensitized polar addition of methanol to (R)-(þ)-limonene (102) in nonpolar solvents afforded a mixture of the diastereomeric ethers 103 and 104 and the rearrangement product 105 (Scheme 6.42).677 The diastereomeric excess (de) of the photoadduct was optimized by varying the solvent polarity, reaction temperature and nature of the sensitizer. The first step of the reaction is the Z–E photoisomerization (Section 6.1.1) of 102 to a highly strained E-isomer, followed by protonation and methanol addition. The initial formation of a carbocation via the protonation step has been excluded under those reaction conditions. The Markovnikov-oriented methanol attack on the less-hindered (Rp)-(E)-102 compared with that of (Sp)-(E)-102 explains why 103 can be obtained in up to 96% de upon sensitization with methyl benzoate in a methanol solution. The hypothesis that Z–E isomerization of the cyclohexene moiety affords a strained (reactive) alkene, whereas isomerization of the exocyclic double bond does not, was supported by the observation of an exclusive nucleophilic addition to the cyclohexene double bond.

Scheme 6.42

Experimental details.677 A diethyl ether solution containing limonene (102; 5 mM) and a sensitizer (2 mM) was irradiated under an argon atmosphere in a quartz tube immersed in a cooling (methanol/ethanol) bath at 75 C using a high-pressure mercury lamp (300 W) through a Vycor sleeve (>250 nm) (Figure 3.9). The reaction mixture was evaporated in vacuo to give a residue, from which the products were isolated by preparative gas chromatography in about 10% chemical yield.

Electron Transfer to Excited Alkenes

Quenching of singlet state alkenes (as electron acceptors) by amines (as electron donors) may proceed via the formation of exciplex or radical ion pair [see the photoinduced electron transfer (PET) process in Section 5.2] intermediates, which may regenerate the ground-state reactants via back electron transfer or undergo various chemical reactions.665–667 For example, singlet excited stilbene in the presence of trimethylamine forms an exciplex–radical ion pair intermediate in polar aprotic solvents, followed by proton transfer (oxidation of an alkylamine, i.e. formation of the radical cation on the nitrogen atom, significantly increases the acidity of the a-CH bonds) and radical coupling reactions (Scheme 6.43).678

Scheme 6.43

Case Study 6.9: Mechanistic photochemistry – photocyclization of N,N- dimethylaminoalkylstyrenes

The photochemical intramolecular cyclization of N,N-dimethylaminoalkylstyrenes in nonpolar solvents was found to be affected by the alkyl chain length.679 Whereas 106 (having a dimethylene interchromophore spacer) undergoes efficient intramolecular addition to form a single five-membered ring adduct 107 via a 1,5-biradical, 108 (having a tetramethylene interchromophore spacer) produces the six-membered ring diastereomers 109 and 110 in an 8:1 molar ratio via a 1,6-biradical intermediate (Scheme 6.44). A mechanistic study indicated that highly regioselective intramolecular H-atom transfer occurs via least-motion pathways from the lowest-energy folded conformations of the singlet exciplex intermediates.

Experimental details.679 Hexane solutions of a styrylamine (106 or 108; 0.01 M) in Pyrex (transparent over 280 nm) test-tubes were irradiated to >95% conversion (GC) under nitrogen using a Rayonet reactor fitted with 16 lamps (21 W; lirr ¼ 300 nm; Figure 3.10). Products were isolated in >80% chemical yield by either preparative