Photochemistry_of_Organic

Scheme 6.192 shows the o-nitrobenzyl moiety used as a photolabile linker in the solid-phase oligosaccharide synthesis on a polystyrene (PS) support.1208 After the synthesis of the protected oligosaccharide 404, PS attached to the photoremovable group is removed by photolysis and the final product is obtained by hydrogenolysis. Such a strategy could be promising for combinatorial synthesis of oligosaccharide libraries.

Scheme 6.192

Other applications of photoremovable protecting groups are presented in Case Studies 5.3, 6.24, 6.26 and 6.30.

Photochemical switches are also discussed in Special Topic 6.15 (Scheme 6.161) and Special Topic 6.19 (Scheme 6.207). Here we illustrate the photoswitching process, which can control the geometry of biomolecules.1101 When an azobenzene-derived cross-linker in the DNA-recognition helix of the transcriptional activator MyoD is irradiated at 360 nm, the linker predominantly attains a Z-configuration that significantly stabilizes the helix (Scheme 6.193).1209 Reverse isomerization can proceed either thermally or photochemically at a different wavelength; therefore, the process is photochromic (Special Topic 6.15).

Case Study 6.30: Photoactivatable compounds – chromatic orthogonality

The regioselective control of two independent photochemical processes simply by choosing the wavelength of monochromatic light has been demonstrated on the heptanedioic acid diester 406 (Scheme 6.195).1211 One carboxylate group is protected by the photoremovable (Special Topic 6.18) 4,5-dimethoxy-2-nitrobenzyl (2-nitroveratryl) group, which is selectively released by 420 nm irradiation to yield 407. The other carboxylate moiety is protected by the 30,50-dimethoxybenzoin group, selectively liberated by 254 nm illumination to give 408. Both products were obtained as the corresponding carboxylic acids and were methylated (esterified) by (trimethylsilyl)diazomethane in the subsequent step. The 2-nitroveratryl group absorbs at 420 nm exclusively (Figure 6.13); therefore, no photoreaction of the other chromophore can take place by irradiation at this wavelength. This is assuming that energy transfer is not a complicating factor. Since both chromophores absorb at 254 nm (Figure 6.13), differences in the absorbance times excited-state quantum yield determine the selectivity of 30,50-dimethoxybenzoin release. The orthogonal approach in the field of protecting groups is a strategy allowing the selective deprotection of multiple protecting groups by changing the reaction conditions. Because the irradiation wavelength is the only variable employed for the deprotection, this method has been termed chromatic orthogonality.

Scheme 6.195

Experimental details.1211 A solution of the diester (406) (20 mmol) in acetonitrile (10 ml) degassed by purging with nitrogen was irradiated at a selected wavelength (254 or 420 nm) (Figure 3.28) for 24 h. The solvent was evaporated and a crude product (acid) was esterified by (trimethylsilyl)diazomethane (200 mmol in 2 ml of a benzene–ethanol mixture). The chemical yields were determined by NMR spectroscopy.

Scheme 6.199

Photorelease (Special Topic 6.18) of carboxylates from phenacyl esters represents the

reaction, which may involve an electron transfer step induced by amine excitation1227 (Scheme 6.196b) or hydrogen abstraction from a suitable H-atom donor1228,1229 (see also

Section 6.3.1). For example, upon irradiation, the singlet excited N,N-dimethylaniline (DMA) (ES ¼ 398 kJ mol 1)157 transfers an electron to 412 to form a radical ion pair

(Scheme 6.200).1230 In the subsequent steps, the carboxylate ion 413, a phenacyl radical (414) and a DMA radical cation (415) are released and 414 abstracts hydrogen from 415 to form an iminium ion 416 that is hydrolysed by traces of water to give N-methylaniline. The electron transfer step was proposed to be exergonic by 60–85 kJ mol 1.

H2O

O

HO R + Ph +

O

Scheme 6.200

Aromatic Nitriles

Aromatic nitriles are typical electron acceptors; their electron acceptor abilities increase with increase in the number of electron-withdrawing cyano groups and with

increasing size of the aromatic moiety.670 An analogous model to that described for electron donors (amines) in Scheme 6.196 can also be applied for strong electron acceptors, such as aromatic nitriles. In the first scenario, a ground-state electron acceptor is involved in the electron transfer reaction. For example, an auxiliary electron-donating sensitizer (phenanthrene, 417; ES ¼ 346 kJ mol 1)157 is excited to its singlet state, which then transfers an electron to an (auxiliary) acceptor (1,4-dicyanobenzene, 418) to produce a radical cation 419 (Scheme 6.201).1231 This species then accepts an electron from the substrate (1,1-diphenylethane, 420) to give the key intermediate – the radical cation complex 421 – which reacts with a nucleophile (methanol) to afford an antiMarkovnikov product 422 in 70% chemical yield. This very useful approach is termed redox photosensitization (see also Section 6.8),1212 because it permits electron transfer processes in alkene substrates possessing relatively high standard potentials of oxidation.

Ph Ph

+417

OMe

422

Scheme 6.201

According to the second scenario, an auxiliary electron acceptor, for example 9,10-dicyanoanthracene (DCA) in the excited state, is used as a co-sensitizer.670 DCA absorbs well in the near-UV region and, in the excited singlet state (ES ¼ 284 kJ mol 1), it

can accept an electron from N-methyl-N-(trimethylsilylmethyl)aniline (423) to give an amine radical cation 424 (Scheme 6.202).1232,1233 This intermediate subsequently attacks

the C¼C bond of 4,4-dimethylcyclohex-2-enone (425) and undergoes desilylation upon nucleophilic attack by the solvent. The coupling radical species 426 finally cyclizes to 427 (the overall chemical yield is 10%).

[{(bpy)2Ru(dpp)}2RhCl2]5+

Figure 6.14 Hydrogen production catalyst

using photocatalysts and solar radiation. Although heterogeneous photocatalysts are relatively effective1237 (Special Topic 6.26), homogeneous photocatalysts for hydrogen production are also promising. For example, the [{(bpy)2Ru(dpp)}2RhCl2]5 þ (bpy ¼ 2,20- bipyridine, dpp ¼ 2,3-bis-2-pyridylpyrazine) complex (Figure 6.14) was shown to undergo photoexcitation (Ru ! dpp) and electron collection at the rhodium centre, having the ability to be reduced by two electrons by converting Rh3 þ to Rh þ .1238 The complex, in an acetonitrile–water solution and in the presence of N,N- dimethylaniline (DMA) (an auxiliary electron donor), was found to produce hydrogen photocatalytically when excited with visible light (470 nm) using an LED array. The quantum yield of the process was 0.01, assuming that two photons were used to produce hydrogen (in general: 2H2O þ 4hn ! 2H2 þ O2).

Special Topic 6.19: Molecular machines

The great demand for miniaturization of components in electrotechnical, medicinal or material applications has led to the development of a highly multidisciplinary scientific and technological field called nanotechnology to produce devices with critical dimensions within the range 1–100 nm. The ultimate solution to miniaturization is logically a functional molecular machine, an assembly of components capable of

performing mechanical motions (rotation or linear translation) upon external stimulation, such as photoactivation.1103,1104,1239–1244 This motion should be

controllable, efficient and occur periodically within an appropriate time-scale; therefore, it involves photochromic behaviour discussed in the Special Topic 6.15. Such devices can also be called photochemical switches (Special Topics 6.18 and 6.15). Here we show two examples of molecular machines: a molecular rotary motor and a molecular shuttle.

Photochemical E–Z isomerization (Section 6.1.1) is responsible for continuous unidirectional rotary motion along a carbon–carbon double bond of 433, a chiral helical alkene mounted on the surface of gold nanoparticles through two octanethiol linkers (Scheme 6.206).1245 Thanks to the considerable conformational flexibility of