1.2.3 Applications

Recently highly elegant photocatalytic applications employing acridinium ions as catalysts and O₂ as oxidant have thus been reported. These methods are extremely important on a synthetically, environmental and economical point of view as they are rapid, they proceed usually at room temperature and avoid the use of a number of classical methods using (sub)stoichiometric amounts of inorganic oxidants such as chromium (IV), cobalt (III), manganese (III), cerium (IV), zirconium (IV) among others.¹³ Some of them are depicted below.¹⁴

13 Baeckvall, J.-E.; Editor Modern Oxidation Methods; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2004.

Donohoe, T. Oxidation and Reduction in Organic Synthesis; Oxford University Press: New York, N. Y., 2000. Knapp, B.

Oxidation and Reduction; Atlantic Europe: Henley-on-Thames, U.K., 1998. Hudlicky, M. ACS Monograph 186: Oxidations in Organic Chemistry; American Chemical Society: Washington, DC, 1990;.

14 For other interesting applications which are not detailed below see also Kotani, H.; Ohkubo, K.; Fukuzumi, S. Appl. Catal., B 2008, 77, 317-324. Griesbeck, A. G.; Cho, M. Org. Lett. 2007, 9, 611-613. Ohkubo, K.; Suga, K.; Fukuzumi, S. Chem.

Commun. 2006, 2018-2020. Ohkubo, K.; Nanjo, T.; Fukuzumi, S. Catal. Today 2006, 117, 356-361. Ohkubo, K.; Nanjo, T.;

Fukuzumi, S. Bull. Chem. Soc. Jpn. 2006, 79, 1489-1500. Ohkubo, K.; Nanjo, T.; Fukuzumi, S. Org. Lett. 2005, 7, 4265-4268.

Kotani, H.; Ohkubo, K.; Fukuzumi, S. J. Am. Chem. Soc. 2004, 126, 15999-16006. Fujita, M.; Shindo, A.; Ishida, A.; Majima, T.; Takamuku, S.; Fukuzumi, S. Bull. Chem. Soc. Jpn. 1996, 69, 743-749. Fujita, M.; Ishida, A.; Takamuku, S.; Fukuzumi, S. J.

Am. Chem. Soc. 1996, 118, 8566-8574.

Chapter III. Catalytic aerobic photooxidation of primary benzylic amines using hindered acridinium salts

N R

Me

(MeAcrR⁺) hν τ^-1

1MeAcrR^+*

MeAcR R'H

MeAcrR R' O₂

MeAcR R'OO R'OOH H₂O

R'H

H⁺

N

R^H2C Me

Me CHO

Me

Me Me

H⁺

Kb R'H

Scheme III-1. Mechanistic rationale for the photoinduced oxygenation of xylene to p-tolualdehyde.

For example, the 100% selective photooxygenation of p-xylene to p-tolualdehyde using acridinium ions as photocatalysts and molecular oxygen as reagent has recently been reported by Fukuzumi and co-workers (vide supra Scheme III-1).¹⁵

In this typical example, the reaction is started by photoinduced electron transfer (ket) from p-xylene (R’H) to the singlet excited state methylacridinium (¹MeAcrR^+*) to produce the acridinyl radical (MeAcrR^•) and the p-xylene radical cation (R’H^•+) which deprotonate promptly. In the absence of oxygen the p-xylenyl radical couples with MeAcrR^• to yield the adduct [MeAcrR-(CH2C6H4CH3-p)]. On the other hand, in the presence of oxygen, the p-xylenyl radical is readily trapped by singlet oxygen to give a p-xylenyl peroxyl radical (R’OO^•) that is reduced by back electron transfer from MeAcrR^• to yield p-xylenyl hydroperoxide, accompanied by regeneration of

15 Ohkubo, K.; Fukuzumi, S. Org. Lett. 2000, 2, 3647-3650.

MeAcrR⁺. At the end, the hydroperoxide decomposes to yield p-tolualdehyde. This process was further generalized for the selective oxygenation of ring-substituted toluenes.¹⁶

Scheme III-2. Mechanism for the photocatalytic oxygenation of α-methylstyrene to acetophenone.

An other illustration of such outstanding reactions is the direct synthetic transformation of α-methylstyrene to acetophenone.¹⁷

In this case, the photoinduced electron transfer (ket) from α-methylstyrene to the singlet excited state of 10-methylacridinium ion (¹MeAcrH^+*) as shown in Scheme III-2 is followed by the formation of the radical cation-acridinyl radical pair (PhC(Me)=CH2•+

- MeAcrH^•). In the presence of oxygen, oxygen is then added to α-methylstyrene radical cation to give the peroxyl radical cation, which is converted to the more stable form that is the dioxetane radical cation 62. After that, water is added to the dioxetane radical cation to give the hydroxylated peroxyl radical 63. The back electron transfer from acridinyl radical to the peroxyl radical gives the corresponding

16 Ohkubo, K.; Suga, K.; Morikawa, K.; Fukuzumi, S. J. Am. Chem. Soc. 2003, 125, 12850-12859. Suga, K.; Ohkubo, K.;

Fukuzumi, S. J. Phys. Chem. A 2005, 109, 10168-10175.

17 Suga, K.; Ohkubo, K.; Fukuzumi, S. J. Phys. Chem. A 2003, 107, 4339-4346.

Chapter III. Catalytic aerobic photooxidation of primary benzylic amines using hindered acridinium salts

hydroperoxide and regenerates MeAcrH⁺. At last, the hydroperoxide decomposes to yield both the final product acetophenone and formaldehyde.

S S S

O O O

O2 - hν

N Me

O2 - hν

N Me

NaOH - hν

N Me

+ O₂ + CO₂

Ph

(1)

RCOOH ROOH (2)

+ +

+

Figure III-3. Photooxygenation of sulfides and carboxylic acids using acridinium ions.

Still concerning visible-light photooxygenation processes using O2 as reagent, the selective oxidation of dibenzothiophenes to dibenzosulfoxides and sulfones,^18,19 as well as the preparation of fatty hydroperoxides from their corresponding carboxylic acids²⁰ has been also achieved (Figure III-3, Equation 1 and 2 respectively).

It is to be noted that, while several synthetically useful reactions using photosensitizers and hydrogen donors as to decarboxylate and reduce carboxylic substrates have already been described, so far, this latest example is the first concerning a photoinduced decarboxylation and direct addition of molecular oxygen to produce hydroperoxide derivatives (Figure III-3, Equation 2).

Last year, a mild, general and efficient aerobic aromatization of Hantzsch 1,4-dihydropyridines (1,4-DHP) was also realized using 9-phenyl-10-methylacridinium cation as photocatalyst (see below MeAcrPh⁺). Not only 23 examples were screened but only 5 mol % catalyst was needed for the reaction to proceed effectively. Further,

18 Dibenzothiophene (DBT) and 4,6-Dimethyldibenzothiophene (DMDBT), the main sulfur-containing compounds in diesel oil, are the most difficult to remove by the current method of hydrodesulfurization (e.g., HDS or light oil). However, it has been known that DBT and DMDBT can be readily removed from diesel oil by liquid-liquid extraction after they are oxidized to sulfoxides and/or sulfones, which have greater polarity than the corresponding sulfides. Oxidation has thus been successfully carried out with H2O2 or other non-hydrogen peroxide systems but molecular oxygen mediated photooxidation may have greater potential practical value for environmental and economical respects.

19 Che, Y.; Ma, W.; Ji, H.; Zhao, J.; Ling, Z. J. Phys. Chem. B 2006, 110, 2942-2948. Che, Y.; Ma, W.; Ren, Y.; Chen, C.;

Zhang, X.; Zhao, J.; Ling, Z. J. Phys. Chem. B 2005, 109, 8270-8276.

20 Suga, K.; Ohkubo, K.; Fukuzumi, S. J. Phys. Chem. A 2006, 110, 3860-3867.

the catalyst was easily and simply recovered and reused by (i) precipitation with Et2O and (ii) filtration over Büchner funnel.²¹

N Me Ph

1MePhAcr⁺*

N

EtO₂C CO₂Et

1,4-DHP

MeAcrPh 1,4-DHP

1,4-DHP H⁺ H⁺

H₂O₂

O₂ MePhAcr⁺

HO₂ +

hν

HO₂

-Scheme III-3. Plausible mechanism for catalytic aromatization of Hantzsch 1,4-dihydropyridines.

A mechanistic rationale for the photocatalytic reactions was proposed. As illustrated in Scheme III-3, the photoexcitation of MeAcrPh⁺ allows it to abstract an electron from 1,4-DHP to generate the 1,4-DHP radical cation (1,4-DHP^+•) and MeAcrPh^•. The fast deprotonation of 1,4-DHP^+• gives the 1,4-DHP radical (1,4-DHP^•), which reacts with O₂ to afford the aromatized product along with the formation of HO2•

. The electron transfer from MeAcrPh^• to HO2•

gives HO2

and regenerates the catalyst MeAcrPh⁺ to complete the catalytic circle. All in all many other important synthetic applications have thus been reported, particularly over the last two years. But only these 5 examples have been detailed for a better understanding of the mechanism of such processes.