Bis allylic oxidation

Bis-allylic oxidation of 23 and related cyclohexa-1,4-dienes provides a convenient and general preparation of cyclohexa-2,5-dien-l-ones (Scheme 7). These cross-conjugated die-nones are substrates for a variety of photochemical rearrangement and intramolecular cycloaddition reactions. Amide-directed hydrogenations of dienones 24a and 24b with the homogeneous iridium catalyst afford cyclohexanones 25a and 25b, containing three stereogenic centers on the six-... 

Cyclohexadienones with a wide variety of alkyl substituents at C4 are readily available by alkali metal in ammonia reduction-alkylations" of methyl 2-methoxybenzoate to give cyclohexa-1,4-dienes followed by bis-allylic oxidation by pyridinium dichromate and /ert-butyl hydroperoxide. ... 

All that remains before the final destination is reached is the introduction of the C-l3 oxygen and attachment of the side chain. A simple oxidation of compound 4 with pyridinium chlorochro-mate (PCC) provides the desired A-ring enone in 75 % yield via a regioselective allylic oxidation. Sodium borohydride reduction of the latter compound then leads to the desired 13a-hydroxy compound 2 (83% yield). Sequential treatment of 2 with sodium bis(trimethylsilyl)amide and /(-lactam 3 according to the Ojima-Holton method36 provides taxol bis(triethylsilyl ether) (86 % yield, based on 89% conversion) from which taxol (1) can be liberated, in 80 % yield, by exposure to HF pyridine in THF at room temperature. Thus the total synthesis of (-)-taxol (1) was accomplished. 

DattaGupta and Singh45 report the results of bis(oxazolinyl)pyridine induced asymmetric allylic oxidation. The reaction proceeds with 59% yield and 56% ee (Scheme 8-16). 

Pfaltz and co-workers (108) reported that the allylic oxidation of cyclohexene proceeds in moderate selectivity using stoichiometric amounts of semicorrin-Cu(II) complexes. In catalytic reactions, the enantioselectivity decreased drastically. Better results were realized using bis(oxazolines) as ligands. Upon... 

Tris(oxazoline) complexes have also been investigated as ligands in the allylic oxidation reaction. Katsuki and co-workers (116) observed that Cu(OTf)2 com-plexed to the tris(oxazoline) 160 is a more selective catalyst than one derived from CuOTf, Eq. 99, in direct contrast to results observed with bis(oxazohnes) or pyridyl bis(oxazohnes) as ligands (cf. Section III.A.3). When the reaction is conducted at -20°C, the cyclopentenyl benzoate is delivered in 88% ee albeit in only 11% yield after 111 h. Larger cycloalkenes are less selective (cyclohexene 56% ee, cyclohep-tene 14% ee, cyclooctene 54% ee). 

The application of dinuclear metal catalysts to the Kharasch-Sosnovsky reaction is mechanistically intriguing due to their illustrated role in mediating biological oxidations (119). Fahmi (120) examined a variety of dinucleating ligands with Cu(MeCN)4PF6 as catalysts in the allylic oxidation of cyclohexene, Eq. 102. In these studies, early results have been inferior to those obtained from bis(oxa-zoline)-copper catalysts. 

Asymmetric allylic oxidation and benzylic oxidation (Kharasch-PSosnovsky reaction) are important synthetic strategies for constructing chiral C—O bonds via C—H bond activation.In the mid-1990s, the asymmetric Kharasch-Sosnovsky reaction was first studied by using chiral C2-symmetric bis(oxazoline)s. " Later various chiral ligands, based mainly on oxazoline derivatives and proline derivatives, were used in such asymmetric oxidation. Although many efforts have been made to improve the enantioselective Kharasch-Sosnovsky oxidation reaction, most cases suffered from low to moderate enantioselectivities or low reactivities. 

Allyl bromide and bis(trlbutyltin) oxide were obtained from Aldrich Chemical Company, Inc. 

Scheme 16 Allylic oxidation of A5-steroids catalyzed by Bi(III) salts, using r-BuOOH...

TABLE 9.34 AZIRIDINATION OF CINNAMATE ESTERS, 575 TABLE 9.35 BIS(OXAZOLINE)-MEDIATED ALLYLIC OXIDATION, 577 TABLE 9.36 HYDROSILYLATION OF ACETOPHENONE, 578... 

For specific cases such as olefin oxidation over Bi-Mo oxide combinations some information concerning the oxidation mechanism is available. The work of Adams and Jennings (2), of Sachtler (16), and of Adams (1) has led to the general acceptance of an allylic intermediate. The discoverers of the Bi-Mo catalyst system (21) showed that propene is converted to acrolein, while Hearne and Furman (9) proved that butene forms butadiene. The allylic intermediate therefore can in principle react in two different ways (1) formation of a conjugated diene... 

Solid heteropoly compounds are suitable oxidation catalysts for various reactions such as dehydrogenation of O- and N-containing compounds (aldehydes, carboxylic acids, ketones, nitriles, and alcohols) as well as oxidation of aldehydes. Heteropoly catalysts are inferior to Mo-Bi oxide-based catalysts for the allylic oxidation of olefins, but they are much better than these for oxidation of methacrolein (5). Mo-V mixed-oxide catalysts used commercially for the oxidation of acrolein are not good catalysts for methacrolein oxidation. The presence of an a-methyl group in methacrolein makes the oxidation difficult (12). The oxidation of lower paraffins such as propane, butanes, and pentanes has been attempted (324). Typical oxidation reactions are listed in Table XXXI and described in more detail in the following sections. 

Roelfes et al. prepared a non-heme iron(II) complex 26 from pentadentate ligand N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine 25 (Scheme 3.32) [126]. In the presence of H202, complex 26 reacted to a low-spin Fe(III)OOH intermediate, which was cleaved homolytically to an oxo Fe(IV) species and a hydroxy radical. Both species are capable of oxidizing various organic substrates via a radical pathway (Scheme 3.32). Under the catalysis of complex 26, cyclohexene la was oxidized with excess H202 to a mixture of products 2a, 3a and 4a. The TON was found to be solvent dependent, with higher TON in acetonitrile than in acetone (Scheme 3.32). In no case were isolated yields given and, furthermore, the allylic oxidation is limited to cyclohexene la. 

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