Allyhc oxidation

The oxidation reactions were performed in a 25 mL ronnd bottom flask. In a typical reaction the catalyst (0.5 % Fe mol) was added to 0.125 M olefin solntion in acetone then dry TBHP (3.5 M in CH2CI2 or in PhCl) was added in one step, and the reaction mixture was stirred and heated in an oil bath at 40°C for 7 h. For the allyhc oxidation of cyclohexene with isotopically labelled oxygen ( 02) the following procedure was carried out the suspension of the catalyst (0.5% Fe mol) in cyclohexene (4 mL, 0.125 M) was frozen and the air in the reactor was evacuated and replaced by an oxygen (21% mol) - argon (79% mol) mixture. Then, the suspension was allowed to warm at room temperatnre and 1.3 mmol of degasified TBHP was added to the solution and the reaction mixtnre was stirred at 40°C for 3 h. 

The role of oxygen on the allyhc oxidation of cyclohexene over the FePcCli6-S/TBHP catalytic system was determined by using 2 labelled oxygen. Since more than 70% of the main cyclohexene oxidation products, 4,11, and 12, had labelled oxygen, we can assure that molecular oxygen acts as co-oxidant. However, under the reaction conditions the over-oxidation of 4 seems to be unavoidable. Labelled 2, 3- epoxy-l-cyclohexanone (13), 2-cyclohexen-l, 4-dione (14), and 4-hydroxy-2-cyclohexen-l-one (15) were detected as reaction products. 

The low effectiveness of the [Rh2"( Jt-OAc)4] system [31] in the oxygenation of alkenes has been attributed to its high oxidation potential to form the mixed-valence complex [Rh2" " ( Ji-OAc)4] on the basis of a relationship between the abihty of the complexes to transfer one electron and the effectiveness of the catalysts in allyhc oxidation, suggested by Kochi [33]. 

Additional results of the enhancement in phenol conversion (to dihydroxy benzenes) and oxidation of aUyl alcohol (to glycidol and allyhc oxidation products) catalyzed by TS-1 in various solvents are illustrated in Fig. 46. In solvents with high dielectric constants, the heterolytic cleavage of the 0—0 bond... 

Reaction of this W04 - with H2O2 produces peroxocomplexes, which in an aqueous methanohc medium epoxidize allyHc alcohols. The reactivity of our system agrees well with that of tungstate salts, dissolved in a single polar Liquid phase. The alkaline nature of the LDH support seems however to prevent solvolysis reactions. In the epoxidation of (homo)allyhc alcohols, selectivities are therefore better with the W04 -LDH A than with the homogeneous W salts [2,31. However, for some of the simple olefins, allyhc oxidation is not neghgible. 

This allyhc oxidation can be suppressed by working with the hpophihc catalyst C. Hydrophobic olefins such as cydooctene are expected to interact favorably with the aromatic p-tosylate on the peroxoW-LDH surface. The transfer of the electrophihc oxygen to the olefin then takes place in an essentially hydrophobic compartment of the reaction system, as in the Venturello-Ishii reactions. In such an environment the olefin is enriched with respect to hydrogen peroxide and this improves the effidency of the oxidant use. Another potential advantage of our system C is that there is no need for a chlorinated solvent to constitute a second, apolcir hquid phase. 

Finally, along the lines of the simple monoterpenes, once the allyhc oxidations have occurred (or in the case of the carbocations, water has added to produce alcohols), additional oxidation is possible and the products are commonly found in nature. Some of these compounds are presented in Figure 11.20. 

Quantitative Analysis of All llithium Initiator Solutions. Solutions of alkyUithium compounds frequentiy show turbidity associated with the formation of lithium alkoxides by oxidation reactions or lithium hydroxide by reaction with moisture. Although these species contribute to the total basicity of the solution as determined by simple acid titration, they do not react with allyhc and henzylic chlorides or ethylene dibromide rapidly in ether solvents. This difference is the basis for the double titration method of determining the amount of active carbon-bound lithium reagent in a given sample (55,56). Thus the amount of carbon-bound lithium is calculated from the difference between the total amount of base determined by acid titration and the amount of base remaining after the solution reacts with either benzyl chloride, allyl chloride, or ethylene dibromide. 

To control the stereochemistry of 1,3-dipolar cycloaddidon reacdons, chiral auxiliaries are introduced into either the dipole-part or dipolarophile A recent monograph covers this topic extensively ° therefore, only typical examples are presented here. Alkenes employed in asymmetric 1,3-cycloaddidon can be divided into three main groups (1) chiral allyhc alcohols, f2 chiral amines, and Hi chiral vinyl sulfoxides or vinylphosphine oxides. 

A family of interesting polycychc systems 106 related to pyrrolidines was obtained in a one-pot double intermolecular 1,3-dipolar cycloaddition, irradiating derivatives of o-allyl-sahcylaldehydes with microwaves in toluene for 10 min in presence of the TEA salt of glycine esters [71]. A very similar approach was previously proposed by Bashiardes and co-workers to obtain a one-pot multicomponent synthesis of benzopyrano-pyrrolidines 107 and pyrrole products 108 (Scheme 37). The latter cycloadducts were obtained when o-propargylic benzaldehydes were utihzed instead of o-allyhc benzalde-hydes, followed by in situ oxidation [72]. 

Scheme 10.8 outlines the application of rhodium-catalyzed allyhc amination to the preparation of (il)-homophenylalanine (J )-38, a component of numerous biologically active agents [36]. The enantiospecific rhodium-catalyzed allylic amination of (l )-35 with the lithium anion of N-benzyl-2-nitrobenzenesulfonamide furmshed aUylamine (R)-36 in 87% yield (2° 1° = 55 1 >99% cee) [37]. The N-2-nitrobenzenesulfonamide was employed to facilitate its removal under mild reaction conditions. Hence, oxidative cleavage of the alkene (R)-36 followed by deprotection furnished the amino ester R)-37 [37, 38]. Hydrogenation of the hydrochloride salt of (l )-37 followed by acid-catalyzed hydrolysis of the ester afforded (i )-homophenylalanine (R)-3S in 97% overall yield. 

The proposed mechanism for allyhc acetoxylation of cyclohexene is illustrated in Scheme 15. Pd -mediated activation of the allyhc C - H bond generates a Jt-allyl Pd intermediate. Coordination of BQ to the Pd center promotes nucleophilic attack by acetate on the coordinated allyl ligand, which yields cyclohexenyl acetate and a Pd -BQ complex. The latter species reacts with two equivalents of acetic acid to complete the cycle, forming Pd(OAc)2 and hydroquinone. The HQ product can be recycled to BQ if a suitable CO catalyst and/or stoichiometric oxidant are present in the reaction. This mechanism reveals that BQ is more than a reoxidant for the Pd catalyst. Mechanistic studies reveal that BQ is required to promote nucleophilic attack on the Jt-allyl fragment [25,204-206]. 

Recent advances in alcohol oxidations by rhodium and iridium complexes have mainly focused on Oppenauer-type oxidations or reactions in which this type of oxidation is an intermediate step. An independent result is the oxidation of allyhc (Eq. 9) and benzyUc alcohols with f-BuOOH to the corresponding a,/l-unsaturated ketones [38] with [Rh2(p.-OAc)4]. The reactions were carried out at room temperature in dichloromethane and yields of up to 92% (by GC) in 24-48 h have been described. 

N-Methylated y-amino-p-hydroxy acids are accessible by the usual synthetic sequences, i.e. aldol condensation or y-amino-P-oxo ester reduction, starting from the corresponding N-methylated a-amino acids, but are obtained with low diastereoselectivity. 61-63 Alternatively, Brown allylboration of the ALBoc-ALMe amino aldehyde 16 (R1 = Bzl, X=Boc, Y = Me) gives the allyhc N-methylated intermediate 27 in 64% yield and 90% de (Scheme 12). 64 Oxidative cleavage of the alkenol is performed using the two-step ozonolysis and sodium chlorite oxidation sequence. 

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