Allylic systems oxidation

In the titanosilicate system, cyclic voltametric measurements had indicated (Section III.D) that the electron density at the tripodal sites is higher than at the tetrapodal sites. Hence, by analogy with the chromium and manganese complexes, we may expect the tripodal sites to favor hydrogen abstraction and allylic CH oxidation, although electron transfer and epoxidation occur preferentially on the tetrapodal sites. 

Selective oxidation of allylic alcohols.1 This zircononcene complex when used in catalytic amount can effect an Oppenauer-type oxidation of alcohols, including allylic ones, in the presence of a hydrogen acceptor, usually benzaldehyde or cyclohexanone. This system oxidizes primary alcohols selectively in the presence of secondary ones. Thus primary allylic alcohols are oxidized to the enals with retention of the configuration of the double bond in 75-95% yield. The method is not useful for oxidation of propargylic alcohols. 

Considering the excellent chemoselectivity observed in the allylic oxidation of dehydroepiandrosterone (Scheme 16), it was interesting to evaluate the selective allylic alcohol oxidation in the presence of a secondary saturated hydroxyl group using the BiCls/f-BuOOH system. This study was performed using androst-... 

The palladium catalysed substitution reaction of allylic systems has also been utilised in the formation of five membered rings. Intramolecular nucleophilic attack of the amide nitrogen atom on the allylpalladium complex formed in the oxidative addition of the allyl acetate moiety on the catalyst led to the formation of the five membered ring (3.63.). In the presence of a copper(II) salt the intermediate pyrroline derivative oxidized to pyrrole.80... 

Replacement of ligands in C3H5MoCl(CO)2(NCMe)2 by isocyanides has given the substituted products C3H5MoC1(CO)2(CNR)2 (R = alkyl) and C3H5MoC1(CO)(CNBu )3, and the reduced products [MoC1(CNBu )4]2 and m-Mo(CO)2(CNR)4 (R = Me, Et). No rationale for the loss of allyl and allyl chloride in the latter two cases was proposed (206). These reactions are rare examples of the formation of low-oxidation state metal-isocyanide complexes via reductive elimination of allyl or allyl chloride from metal-allyl species. The potential applications of mono-, bis-, and tris-n-allylic systems as precursors to low-oxidation state compounds remain to be explored. Substitution and simultaneous reduction of Mo(SBu )4 also occurred on reaction with CNBu to give Mo(SBu )2(CNBu )4 (207) (see Section IV,D,2). 

Vinyloxiranes are used for facile 7i-allyl complex formation [14], The -allylic ferralactone complex 41 was prepared by oxidative addition of Fe2(CO)9 to the functionalized vinyloxirane 40 and CO insertion. Treatment of the ferralactone complex 41 with optically active a-methylbenzylamine (42) in the presence of ZnCl2 gave the 7r-allylic ferralactam complex 45 via 44. In this case, as shown by 43, the amine attacks the terminal carbon of the allylic system and then the lactone carbonyl. Then, elimination of OH group generates the 7r-allylic ferralactam complex 45. Finally the /1-lactam 46 was obtained in 64% yield by oxidative decomplexation with Ce(TV) salt. The <5-lactam 47 was a minor product (24%). The precursor of the thienamycin 48 was prepared from 46 [15,16]. This mechanistic explanation is supported by the formation of both 7r-allyllactone and lactam complexes (49 and 51) from the allylic amino alcohol 50 [17]. 

The first step in Cr(VI) oxidations can take place to give a chromate ester (Chapter 24) but this intermediate has no proton to lose so it transfers the chromate to the other end of the allylic system where there is a proton. The chromate transfer can be drawn as a [3,3]-sigmatropic rearrangement. 

There is some interesting selectivity in this sequence. Only one of the three groups next to the alkene is oxidized and only one ( -) isomer of the enal is formed. No position next to the unsaturated ester is oxidized. All these decisions are taken in the initial cycloaddition step. The most nucleophilic double bond uses its more nucleophilic end to attack SeOi at selenium. The cycloaddition uses the HOMO (7t) of the alkene to attack the LUMO (rc of Se=0). Meanwhile the HOMO (Jt) of Se-0 attacks the LUMO (C-H a ) of the allylic system. 

Oxidation of allylic alcohols. Ruthenium(IV) oxide, particularly the hydrate, is more efficient than MnO, for oxidation of allylic alcohols to the corresponding aldehydes. Only catalytic amounts are required if the oxidation is conducted under oxygen. An antioxidant is also required to prevent further oxidation. Either system oxidizes primary allylic alcohols in high yield (76-98%) yields arc lower in oxidations of secondary allylic alcohols. 

Oxidation with singlet oxygen is subject to steric effects but can show poor regioselectivity. For example, (+)-3-carene (52) is oxidized to produce a mixture of all three possible regioisomeric hydroperoxides with oxygenation occurring at the face of the allyl system opposite the gem-dimethylcyclo-propane unit in each case (equation 22). ... 

The syntheses can be carried out in the absence of other ligands or solvents, and so the oxidative addition product, CH2=CHCH2NiCl, being coordinatively deficient, converts to the t-allylic system and dimerizes to help fill its open sites. 

Functionalized allylic systems are not only often part of target molecules but also have potential as versatile synthetic intermediates. They can frequently be used to generate multifunctional compounds by further elaboration of the C=C bond, namely by reduction (e.g. hydrogenation) or oxidation (e.g. to alcohols, diols, oxiranes, aldehydes or ketones) or by diverse addition reactions. As such reactions can very often be performed with remarkable stereocontrol by means of the allylic functionality, especially in intramolecular reactions, the potential of functionalized allylic systems is immense. 

In accordance with this model one finds diastereoselectively anti products on reaction of aldehydes with ( )-allyl compounds, whereas allyl systems with the (Z)-configuration give mainly syn products and it is even possible to effect asymmetric induction. As the double bond of the product can be oxidatively cleaved to a CW3 group, the reaction can be regarded as a stereoselective aldol reaction, an aspect which explains the widespread interest in this type of reaction. With heterosubstituted allylic anions it is sometimes possible to effect predominantly y-attack with different electrophiles by the choice of the heteroatom.2 For instance it is well known that with sulfur substituents like —SR, —SOR or —SOjR the a-attack dominates, but doubly lithiated allenethiol possesses high y-reactivity and can be used as a homoenolate anion equivalent in reaction with electrophiles such as alkyl halides (Scheme 7). ... 

Other double-bond isomerizations (e.g., those that occur in the course of Pd-catalyzed hydrogenolyses or hydrogenations) proceed by insertion of the alkene into a M-H bond followed by /3-hydride elimination. Wilkinson s catalyst, though, lacks a Rh-H bond into which an alkene can insert. The reaction may proceed by oxidative addition to an allylic C-H bond, then reductive elimination at the other end of the allylic system. 

The direct nucleophilic attack of anilines occurs at both termini of the allylic system of the cationic 73-allylpalladium(II) complex 16 generated by oxidative addition of allylic compounds to a Pd(0) complex. 

The anions generated from geometrically pure allylic diphenylphosphine oxides, e.g. (102), react with carbonyl compounds to afford conjugated dienes in which the geometry of the original allyl system is preserved. ... 

You saw in the last section that the preparation of allylic sulfides by displacement of, say, a halide from an allylic system is likely to give the more stable allylic sulfide whichever allylic halide is used and that the corresponding allylic sulfoxides are made by oxidation of the sulfide. The same is true of phosphorus compounds 73 used in chapter 15 to make dienes of fixed configuration. If R = OR, 73 is an allylic phosphonate for the Horner-Wadsworth-Emmons reaction but if R = Ph, 73 is a phosphine oxide for the Wittig-Horner reaction. 

It can be seen that, under similar conditions, the reactivity of the dialkyl sulfides is directly linked to their molecular size Et2 S > Pr2 S > Bu2 S, and saturated sulfides are more reactive than allyl or aryl sulfides Pr2 S > Me S Allyl > Allyb S > Me S Ph > Ph2 S. These results can be explained, first, if we take into account the relative easiness of thioethers accessibility to the Ti active sites of the catalytic species located in the zeolite framework. The diffusion of the bulkier molecules, such as Ph2S is very difficult even inside the large pores of Ti-beta zeolite. Secondly, the reactivity of thioethers is in agreement with the nucleophilicity of the sulfur atom, so that alkyl sulfides are more easily oxidized than allyl or aryl sulfides by H2O2 (an electrophilic oxidant) in agreement with reported results [1-9]. It must be pointed out, that in the case of allyl methyl sulfide and di-allylsulfide, the epoxidation of the allyl system is not observed under our experimental conditions. 

In general, the catalytic cycle for the transition-metal catalyzed allylic substitution reactions involves initial attack of the metal at the double bond followed by oxidative insertion into the antiperiplanar C-0 bond to afford the Ti-allyl system. At this point, depending on whether soft or hard nucleophiles are used, however, the alkylation reaction proceeds through distinctly different pathways (Scheme 10). With soft nucleophiles, where Pd is often the metal center of choice. 

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