Allyl carbonates epoxidation

Allyhc iodides may be too reactive and, more significantly, too unstable to be useful. Even allylic bromides have rarely been used and are generally unnecessary. On the other hand, many other intrinsically less reactive allylic derivatives have been used satisfactorily. They include allylic carbonates, epoxides, acetals, ° sulfones, 2-alkoxyallyl derivatives, and even 2-azaallyl derivatives, and some representative examples are shown in Scheme 7. 

Allylic acetates are widely used. The oxidative addition of allylic acetates to Pd(0) is reversible, and their reaction must be carried out in the presence of bases. An important improvement in 7r-allylpalladium chemistry has been achieved by the introduction of allylic carbonates. Carbonates are highly reactive. More importantly, their reactions can be carried out under neutral con-ditions[13,14]. Also reactions of allylic carbamates[14], allyl aryl ethers[6,15], and vinyl epoxides[16,17] proceed under neutral conditions without addition of bases. 

Recent papers have disclosed that Pd(0) catalyzed allylic alkylations under neutral conditions are not limited to allylic carbonates or epoxides but also can be extended in many cases to the more popular allylic acetates (Eq. 5.58).sla... 

Trimethylsilyl alkyl and aryl sulfides were found to function as latent sources of a sulfide nucleophile when used in conjunction with allyl carbonates or vinyl epoxides with Pd catalysts (equation 64).223... 

Trimethylsilyl alkyl and aryl sulfides react with allyl carbonates and vinyl epoxides to deliver the alkyl or aryl sulfide to the allyl unit. These species show typical regioselectivities by (i) adding to the less substituted end of alkyl substituted allyls (equation 290) (ii) adding to the allyl terminus more distant from remote oxygen functionality (equation 291) and (iii) showing substantial endocyclic addition to methylenecyclohexane-derived allyls (equation 292).223... 

Vinyl epoxides and allylic carbonates are especially useful electrophiles because under the influence of palladium(O) they produce a catalytic amount of base since X- is an alkoxide anion. This is sufficiently basic to deprotonate most nucleophiles that participate in allylic alkylations and thus no added base is required with these substrates. The overall reaction proceeds under almost neutral conditions, which is ideal for complex substrates. The relief of strain in the three-niembered ring is responsible for the epoxide reacting with the palladium(O) to produce the zwitterionic intermediate. Attack of the negatively charged nucleophile at the less hindered end of the ic-allyl palladium intermediate preferentially leads to overall 1,4-addition of the neutral nucleophile to vinyl epoxides. 

The palladium-catalyzed [3 + 2] cycloaddition of vinylic oxirane 20a [42] and aziridine 20b [39] with the activated olefin 4a for the formation of five membered cyclic ether 21a and pyrrolidine derivative 21b has also been reported in our laboratories. The mechanistic issue is very much similar to that discussed in Scheme 9. Pd(0) catalyst added oxidatively to 20 to produce the 7r-allylpalladium complex 22. The Michael addition of a hetero nucleophile in 22 to the activated olefin 4a gives 23 which undergoes intramolecular nucleophilic attack on the inner 7r-allylic carbon atom to give the cy-clized products 21 and Pd(0) species is generated (Scheme 10). Similarly, the palladium-catalyzed [3 + 2] cycloaddition of vinylic oxirane 20a with the N-losylimincs 24 is also known (Scheme 11) [43]. Intermolecular cycloaddition of vinyl epoxides and aziridines with the heterocumulenes such as isocyanates, carbodiimides and isothiocyanates is also known [44,45]. Alper et al. reported the regio- and enatioselective formation of the thiaolidine, oxathiolane, and dithiolane derivatives by the palladium-catalyzed cyclization reaction of 2-vinylthiirane with heterocumulenes [46]. 

A study of the synthesis of chromans from allylic carbonates involving Pd-catalysed asymmetric allylic alkylation has established that the addition of acetic acid results in a pronounced increase in enantioselectivity. Furthermore, (E) allylic carbonates afford (R) chromans and the (Z) substrates the (S) heterocycle (Scheme 13) <03JA9276>. This approach to chromans has been combined with a radical epoxide cyclisation in a total synthesis of (-)-siccanin <03AG(E)3943>. 

One of die most popular reactions in organic chemistry is dissolving metal reductions [1-3], Two systems are frequently used - sodium dissolved in ammonia with alcohol and lithium dissolved in alkylamines [4]. Although calcium is seldom used, it has been successfully applied to the reduction of a variety of compounds and functional groups [5], including aromatic hydrocarbons, carbon-carbon double and triple bonds, benzyl ethers, allyl ethers, epoxides, esters, aliphatic nitriles, dithianes, als well as thiophenyl and sulfonyl groups. 

Disparlure 18 is the sex attractant of the gypsy moth. The only functional group in disparlure is the epoxide and we need an alcohol somewhere if the stereoselective introduction of this group is to be done by AE. Our disconnection strategy7 is dominated by that thought - an alcohol must be introduced somehow at one of the allylic carbons in 19. 

The presence of a bulky Z-substituent (R ), especially the substituent branched at the allylic carbon, depresses or vitiates the enantioselectivity of the reaction (Eqs. 9 and 10) [27, 28]. In contrast, epoxidation of the substrates bearing a bulky -substituent shows standard enantioselectivity (Eq. 11) [28]. On the other hand, a bulky C2-substituent only affects the enantioselectivity to some extent (Eq. 12) [28]. 

In the C-0 bond cleavage reaction of vinyl epoxides by a palladium complex, formic acid acts as a good proton and hydride donor evolving CO2 (Eqs. 3.24 and 3.25). Zero-valent palladium complex favors the attack at an allylic carbon in an Sn2 manner to give i/ -allylpalladium(II) complex with inversion of configiuation, and formate anion coordinates to the palladium center (Scheme 3.50). Then, decarboxylation of the formate affords palladium hydride, which attacks the ry -allyl moiety from the endo side. Thus, 1,2-addition of hydrogen atoms takes place regioselectively with inversion at the allylic carbon [94]. 

The proposed catalytic cycle starts by coordination of rhodium to both the oxygen and alkene component of the vinyl epoxide, forming complex 69, followed by oxidative insertion of rhodium into the allylic carbon-oxygen bond with retention of stereochemistry (Scheme 10.28). The initially formed Rh species 70 undergoes isomerization via the Jt-allyl Rh " complex 71 to the less strained species 72. Subsequent intermolecular nucleophilic Sn2 displacement with inversion affords the observed product 73 with concomitant release of Rh . 

Hydroxylation of allylic carbon Formation of epoxides and oxides Formation of diols Formation of triols... 

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