Intrazeolite photooxygenations

Oxidations are indisputably one of the most important chemical transformations known and improvement in oxidation technology would have a significant impact 

That was the situation encounter in 1986 when Pettit and Fox56 reported nearly identical selectivity in the singlet oxygen ene reaction of 1-methylcylclohexene, 5, using either a soluble sensitizer (Ru(bpy)3+/MeOH) or a zeolite-embedded sensitizer (Ru(bpy)f+ NaY) in a methanol slurry (Fig. 8). 

Conventional singlet oxygen sensitizers such as Thiazine dyes, 2 and 3, have been reported to promote Wagnerova Class II electron-transfer photooxygenations.60 For example, the participation of singlet oxygen in the reaction depicted in Fig. 9 

Despite the fact that Model C65 does not explicitly invoke complexation of the interstitial cation with the alkene linkage, extensive experimental61,62 and computational66 precedent suggests that complexation is thermodynamically favorable. The interaction between the alkene and the cation serves to anchor the substrate near the supercage wall where the cation is located (Fig. 1). The proximity to the framework wall has experimental consequences as revealed by examination of the 

A striking feature of the intrazeolite singlet oxygen ene reaction is the rate enhancement often observed in these reactions.57,67 This rate enhancement is nicely accounted for by stabilization of the incipient perepoxide as depicted by Model C in Fig. 12. This rate enhancement can be used to promote the ene reaction at the expense of other reaction modes.68 An interesting example of this was reported by Stratakis and Rabalakos.69 Photooxygenations of alkenylarenes, 11 and 12 (Fig. 15) are dominated by [2 + 2] and [4 + 2] reactions in solution but react predominately by the ene mode in intrazeolite reactions. 

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The effect of cations on ene regiochemistry of several intrazeolite photooxygenations have been examined83 (Fig. 20). The effect is modest but the unique... 

The majority of intrazeolite photooxygenations have been conducted in NaY,84 85 however, one study in the pentasil zeolite ZSM-5 demonstrates that steric confinement effects can play important roles.84 A comparison of the reactions of a series of tri-substituted alkenes in isooctane, NaY, and in ZSM-5 is given in Fig. 21. The reactions... 

Fig. 28 A. Intrazeolite photooxygenation of a -chlorosulfide that requires a hydroperoxy sulfonium ylide and B. The formation of an extended cation complexed persulfoxide that can potentially inhibit hydroperoxy sulfonium ylide formation.

The most notable feature of these intrazeolite photooxygenations (Fig. 30) is that the oxygen CT band experiences a dramatic bathochromic shift in comparison to solution. This was detected initially by recording the product growth as a function of irradiation wavelength (laser reaction excitation spectrum)98,110 and was later verified by direct observation using diffuse reflectance UV-Vis spectroscopy.111 For example, 2,3-dimethyl-2-butene CT-absorbance is shifted to lower energy by more than 300 nm... 

Stratakis, M., Nencka, R., Rabalakos, C., Adam, W. and Krebs, O. (2002). Thionin-sensitized intrazeolite photooxygenation of trisubstituted alkenes substituent effects on the regioselectivity as probed through isotopic labeling. J. Org. Chem. 67, 8758-8763... 

Stratakis, M., Sofikiti, N., Baskakis, C. and Raptis, C. (2004). Dye-sensitized intrazeolite photooxygenation of 4-substituted cyclohexenes. Remote substituent effects in regioselectivity and diastereoselectivity. Tetrahedron Lett. 45, 5433-5436... 

Charge transfer (CT) intrazeolite photooxygenation, 253-254, 255, 257 Cw-effect, in singlet oxygen ene reaction, 240, 243... 

HERON reactions of, 75-76, lit reaction of azide with, 85/ structure, 44, 45/ 46/ 47 Frei intrazeolite photooxygenation, 253/ 255, 258, 261, see also Wagnerova Class II intrazeolite photooxygenation... 

Wagnerova Class I intrazeolite photooxygenation, 233-253 of alkanes, 234—235 of alkenes, 235-243 regiochemistry, 236, 237, 243, 244 steric confinement effects, 237/ 246-247 Wagnerova Class II intrazeolite photooxygenation, 253-261 of alkanes, 256/ 258-259 of alkenes, 253-257, 253/ 256/ charge-transfer (CT) complexes in, 253-254, 255, 257... 

ZSM-5, 228, 229, 246, 247-248 Zeolites, oxygenations in Wagnerova Class I intrazeolite photooxygenation, 233-253... 

Figure 8 Products from diastereomeric perepoxides formed in intrazeolite photooxygenation of 2.

IV. SELECTIVE FORMATION OF ALLYLIC HYDROPEROXIDES BY DYE-SENSITIZED INTRAZEOLITE PHOTOOXYGENATION... 

For the achievement of mass balances in intrazeolite photooxygenation reaction, >80%, loading levels of 0.1-0.3 adsorbed molecules per zeolite supercage have been successfully used in the past. However, the recent observation by Pace and Clennan, that replacing the solvent hexane with peril uorohexane was very crucial for the efficiency of the reaction, allowed the zeolite medium to be used for preparative scale photooxygenation reactions (500 mg of alkene), without loss of the product selectivity or the reduction of the mass balance. 

SCHEME 42. Type I and Type II intrazeolite photooxygenation processes... 

Type 1 intrazeolite photooxygenation of alkenes has been also reported to give mainly allylic hydroperoxides (Scheme 42). In this process, the charge transfer band of the alkene—O2 complex within Na-Y was irradiated to form the alkene radical cation and superoxide ion. The radical ion pair in turn gives the allylic hydroperoxides via an allylic radical intermediate. On the other hand, for the Type II pathway, singlet molecular oxygen ( O2) is produced by energy transfer from the triplet excited state of a photosensitizer to 02. 

SCHEME 44. Intermolecular kinetic isotope effect in the intrazeolite photooxygenation of 1-phenyl-3 -methyl- 2-butene... 

Additionally, in order to elucidate the energy reaction profile in the intrazeolite photooxygenation of trisubstituted alkenes, the competing photooxygenation of l-phenyl-3-methyl-2-butene and its geminal methyl deuteriated analogues (Scheme 44) was studied . ... 

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