Irradiation enantioselective reactions

In a related study, the same group investigated molybdenum-catalyzed alkylations in solution and on a solid phase [35], demonstrating that microwave irradiation could also be applied to highly enantioselective reactions (Scheme 7.15). For these examples, commercially available and stable molybdenum hexacarbonyl [Mo(CO)6] was used to generate the catalytic system in situ. The reactions in solution provided good yields (see Scheme 6.50). In contrast, the conversion rates for the solid-phase examples were rather poor. However, the enantioselectivity was excellent (>99% ee) for both the solution and solid-phase reactions. 

The enantioselective oxidative coupling of 2-naphthol itself was achieved by the aerobic oxidative reaction catalyzed by the photoactivated chiral ruthenium(II)-salen complex 73. 2 it reported that the (/ ,/ )-chloronitrosyl(salen)ruthenium complex [(/ ,/ )-(NO)Ru(II)salen complex] effectively catalyzed the aerobic oxidation of racemic secondary alcohols in a kinetic resolution manner under visible-light irradiation. The reaction mechanism is not fully understood although the electron transfer process should be involved. The solution of 2-naphthol was stirred in air under irradiation by a halogen lamp at 25°C for 24 h to afford BINOL 66 as the sole product. The screening of various chiral diamines and binaphthyl chirality revealed that the binaphthyl unit influences the enantioselection in this coupling reaction. The combination of (/f,f )-cyclohexanediamine and the (R)-binaphthyl unit was found to construct the most matched hgand to obtain the optically active BINOL 66 in 65% ee. 

In a related study Hallberg el al. also investigated molybdenum-catalysed allylic alkylations in solution and on solid phase33 demonstrating that microwave irradiation could also be applied to highly enantioselective reactions (Scheme 7.13). 

The inclusion complex 26, shown in Scheme 11, has been used as a host by Toda and coworkers to carry out a number of enantioselective reactions [231]. For example, irradiation of a 1 1 host-guest assembly of a-tropolone methyl ether 27 and (S,S)-( — )-26, in the solid state gave (lS,5R)-( — )-28 of 100% ee. The authors state that the high stereoselectivity is a result of the steric hindrance to disrotatory ring closure from one direction due to the structure of the host. This leads to the formation of only one enantiomer of the product. More details on this topic are available in Chap. 13. 

Irradiation of the 1 2 inclusion complex of cycloocta-2,4-dien-l-one (108) with 10 for 48 h gave (-)-109 of 78% ee in 55% yield [65]. The mechanism of this enantioselective reaction has been studied by X-ray analysis of the inclusion complex [66]. 

Larhed et al. investigated enantioselective Heck reactions with 2,3-dihydrofuran as alkene [86]. In the coupling with phenyl triflate, conditions previously reported by Pfaltz [87] were attempted under microwave irradiation. Interestingly, the catalytic system Pd2(dba)3/(4S)-4-t-butyl-2-[2-(diphenylphosphanyl)phenyl]-4,5-dihydro-l,3-oxazole, identified by the Swiss team, was found suitable for high-temperature microwave-assisted enantioselective Heck reactions (Scheme 76). Using a proton sponge as a base and benzene as a solvent gave the best conversions (Scheme 76). At tempera-... 

On the other hand, stereo- and enantioselective photocyclization reactions of N-benzoylformylpiperidine (74a), N-benzoylformylmorpholine (74b), and N-benzoyl-formylhexamethyleneimine (74c) were achieved by irradiation of the respective inclu-... 

Irradiation of (+ )-crystals of 96 with a 400 W high-pressure Hg-lamp, with occasional grinding with an agate mortar and pestle for 40 h at room temperature gave ( + )-97 of 93 % ee in 74 % yield. Irradiation of (—)-crystals of 96 under the same conditions gave (—)-97 of 93 % ec in 75% yield48. Purification to 100% ee can easily be achieved by recrystallization from benzene. Although the photochemical conversion of 96 into 97 on irradiation in the solid state has been reported, enantioselectivity of the reaction has not been discussed 441. 

Besides ruthenium porphyrins (vide supra), several other ruthenium complexes were used as catalysts for asymmetric epoxidation and showed unique features 114,115 though enantioselectivity is moderate, some reactions are stereospecific and treats-olefins are better substrates for the epoxidation than are m-olcfins (Scheme 20).115 Epoxidation of conjugated olefins with the Ru (salen) (37) as catalyst was also found to proceed stereospecifically, with high enantioselectivity under photo-irradiation, irrespective of the olefmic substitution pattern (Scheme 21).116-118 Complex (37) itself is coordinatively saturated and catalytically inactive, but photo-irradiation promotes the dissociation of the apical nitrosyl ligand and makes the complex catalytically active. The wide scope of this epoxidation has been attributed to the unique structure of (37). Its salen ligand adopts a deeply folded and distorted conformation that allows the approach of an olefin of any substitution pattern to the intermediary oxo-Ru species.118 2,6-Dichloropyridine IV-oxide (DCPO) and tetramethylpyrazine /V. V -dioxide68 (TMPO) are oxidants of choice for this epoxidation. 

In more recent times interest has been shown in the effects of constrained environment on the outcome of such reactions. Some enantioselectivity in the product 308 has been reported following the irradiation of benzonorbornadiene 309 in a T1Y zeolite. (—)-Ephedrine was used as the chiral inductor and sensitization brought about the reaction in 30 min. An ee of about 14% was obtained168. 

A second example of the use of optically pure coordinatoclathrate hosts in controlling the enantioselectivity of photochemical reactions in the crystalline state is found in the case of the a-oxoamide derivative 13, which forms a crystalline 1 1 complex with host (S,S)-8 [18,19]. Irradiation of these crystals led to the P-lactam derivative (-)-14 in 90% yield and a reported ee of 100% (Scheme 3). The X-ray crystal structure of the complex showed that oxoamide 13 adopts a helical conformation that favors the formation of a single enantiomer of photoproduct 14. The reaction is thus conformationally controlled in a way exactly analogous to the examples discussed earlier in the review. 

The same catalyst 73 also worked efficiently for catalytic asymmetric ring opening of cyelohexene oxide and meso-stilbene oxide [34] with anilines under mierowave irradiation to afford P-amino alcohols in high yield (up to 95%) and good enantioseleetivities (ee up to 55%). The reaction under mierowave was found to be 10 times faster than traditional oil-bath heating without compromising enantioselectivity. Also the ee for the produet P-amino aleohols was comparable with the values obtained at room temperature (Table 12). 

In the solid-state photoreaction of 24c, a more chemoselective reaction occurred and only p-thiolactam 25c was obtained almost quantitatively. Of particular importance is the finding that the solid-state photoreaction of 24c involves a crystal-to-crystal nature where the optically active p-thiolactam 25c is formed in specific yield. Furthermore, the X-ray crystallographic analysis revealed that the crystals of 24c are chiral, and the space group is P2j. Irradiation of crystals at 0 °C exclusively gave optically active P-thiolactam 25c, in 81% yield at 100% conversion (entry 5). As expected, the thiolactam 25c showed optical activity (81% ee). This reaction exhibited good enantioselectivity throughout the whole reaction, where a small difference was observed in the ee value from 97 to 81% ee with increasing conversion from 20 to 100% (entries 5 and 6). The solid-state photoreaction also proceeded without phase separation even after 100% reaction conversion. The crystal-to-crystal nature of the transformation was confirmed by X-ray diffraction spectroscopy. 

Maruoka and coworkers also investigated the substantial reactivity enhancement of N-spiro chiral quaternary ammonium salt and simplification of its structure, the aim being to establish a truly practical method for the asymmetric synthesis of a-amino acids and their derivatives. As ultrasonic irradiation produces homogenization (i.e., very fine emulsions), it greatly increases the reactive interfacial area, which may in turn deliver a substantial rate acceleration in the liquid-liquid phase-transfer reactions. Indeed, sonication of the reaction mixture of 2, methyl iodide and (S,S)-lc (1 mol%) in toluene-50% KOH aqueous solution at 0 °C for 1 h gave rise to the corresponding alkylation product in 63% yield with 88% ee. Hence, the reaction was speeded up markedly, and the chemical yield and enantioselectivity were comparable with those of the reaction with simple stirring (0°C for 8h 64%, 90% ee) (Scheme 5.5) [10]. 

Although these enantioselective photoreactions are limited to amide or salt derived from achiral acid and chiral amine, one enantioselective photoisomerization reaction of cobaloxime coordinated with chiral axial ligands such as 1-methylpropylamine, l-(l-naphthyl)ethylamine, and 2-phenylglycinol has been reported. For example, finely powdered (2-cyanoethyl)cobaloxime (60), suspended in liquid paraffin and spread onto a Petri dish, was irradiated to give (S)-(-)-61 of about 80% ee after displacement of the chiral auxiliary of the complex with pyridine [32],... 

Intramolecular [2 + 2] photocyclization reactions of 2- /V-(2-propcnyl)amino]cy-clohex-2-enones (67) are also controlled enantioselectively by carrying out irradiation in inclusion complex with a chiral host compound. When inclusion crystals of 67 with 12 are irradiated in the solid state, optically active 9-azatricyclo[5.2.1.0]-decan-2-ones (68) were obtained in the chemical and optical yields indicated in Table 3 [30],... 

Enantioselective photodimerization of coumarin (97) was accomplished in the inclusion complex (100) with 12a. For example, irradiation of the 1 1 inclusion compound of 97 with 12a gave the anti-head-to-head dimer (—)-98 of 96% ee in 96% yield [52], This photochemical reaction of 100 to the complex (101) of 98 was also found to proceed by a single-crystal to single-crystal manner. However, 1 2 inclusion compound of 11 and 97 gave the. vw-head-to-head dimer (99) upon irradiation in 75% yield [53],... 

Optically pure P-lactam derivatives can also be prepared by an enantioselective photocyclization reaction of 2-pyridone in its inclusion complex with a chiral host. For example, irradiation of the 1 1 inclusion complex of 1 and 4-methoxy-/V-inethyl-2-pyridone (33) in the solid state gave (—)-34 of 100% ee... 

Enantioselective triplex Diels-AIder reaction. 1,3-Cyclohexadiene is known to undergo Diels-AIder reactions with electron-rich dienes when irradiated with arene sensitizers. The reaction is presumed to involve a triplex involving the sensitizer (15, 129). The first enantioselective example of this Diels-AIder reaction has been achieved using (-)-l as sensitizer. Thus irradiation of 1,3-cyclohexadiene with trans-P-methylstyrene in the presence of (-)-l at -65° provides the endo-adduct 2 in 15% ee (equation I). The enantioselectivity decreases with an increase of the temperature, being 1% at 25°. 

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