C=O bond, allylation

The reaction proceeds at room temperature and is rationalized invoking oxidative addition of a Pd(0) species upon the allylic C - O bond of 67, followed by decarboxylation to form an oxapalladacyclopentane intermediate 66 (Pd in place of Ni), which undergoes a facile b-C elimination to finally give an co-dienyl aldehyde 68 (Scheme 17). Recently, it has been revealed that a combination of Ni(cod)2 and a phosphine ligand also catalyzes the same... 

In spite of the successful application of catalytic transfer hydrogenolysis in the case of benzylic compounds, this method is not suitable for the cleavage of the allylic C-O bonds. 

The Pd-catalyzed reaction of 5-vinyl-l,3-oxazolidin-2-ones 260 at 65-70°C and 65 atm of CO in ethanol gave 5-lactams 263 in fairly good to high yields (Scheme 38).As Scheme 38 illustrates, this decarboxylative carbonylation reaction is likely to involve (i) the cleavage of the allylic C-O bond of 260 to form tr-allyl-Pd complex syn-2(A, (ii) isomerization to anti-2(A, (iii) decarboxylation forming 7r-allyl-Pd-amine complex 262, and (iv) CO insertion to 262 followed by aminolysis to yield 5-lactam 263. 

Considerable use has also been made of allyl carbonates as substrates for the allylation of Pd enolates.9 The reaction of Pd° complexes with allyl enol carbonates119,120 proceeds by initial oxidative addition into the allylic C—O bond of the carbonate followed by decarboxylation, yielding an allylpalladium enolate, which subsequently produces Pd° and the allylated ketone (equation 22). In like fashion, except now in an intermolecular sense, allyl carbonates have been found to allylate enol silyl ethers (equation 23),121 enol acetates (with MeOSnBu3 as cocatalyst) (equation 24),122 ketene silyl acetals (equation 25)123 and anions a to nitro, cyano, sulfonyl and keto groups.115,124 In these cases, the alkoxy moiety liberated from the carbonate on decarboxylation serves as the key reagent in generating the Pd enolate. 

Ethyl-2-(sulfonylmethyl)- and 2-(cyanomethyl)-allyl carbonates133 as well as (methoxycarbo-nyl)methylallyl carbonates136 serve as substrates for the [3 + 2] cycloaddition. Oxidative addition into the allylic C—O bond of the carbonate, followed by decarboxylation, gives a 2-substituted allylpalladium al-koxide. The alkoxide then deprotonates the C—H a to the electron-withdrawing substituent at the 2-position of the allyl. This anion then undergoes a Michael addition to an a,(3-unsaturated ketone or ester, followed by intramolecular allylation of the anion of the Michael product (Scheme 2). 

The mechanisms proposed for these reactions are all quite analogous, and only the intramolecular cases will be considered in detail (Scheme 5). Oxidative addition by Pd° into the allylic C—O bond of the allyl 0-ketocaiboxylate produces an allylpalladium caxboxylate. This species then undergoes decarboxylation to yield an allylpalladium enolate (oxa-ir-allyl), which subsequently eliminates a 0-H to form the enone and provide an allyl-Pd-H. Reductive elimination from the allyl-Pd-H yields propene and returns Pd to its zero oxidation state. A similar mechanism can be imagined for the alkenyl allylcarbonate. Oxidative addition by the Pd° forms an allylpalladium carbonate, which decarboxylates again to give an allylpalladium enolate (oxa-ir-allyl). 0-Hydride elimination and reductive elimination complete the process. The intermolecular cases derive the same allylpalladium enolate intermediates, only now as the result of bimolecular processes. 

A similar in situ generation of a sulfide nucleophile can be accomplished by the reaction of an allylic carbonate with a Pd° catalyst and PhSSiMe3. Following oxidative addition by the Pd° into the allylic C—O bond to form the ir-allyl complex, CO2 is lost from the carbonate counterion, generating... 

While investigating trisubstituted allylic ethers, Woerpel and coworkers observed insertion of silylene into the allylic C-O bond (Scheme 7.56).136 The product formation was dependent on the identity of the catalyst. Silver trifluor-oacetate promoted the formation of allylic disilane 201, whereas copper(I) triflate... 

Bennett has shown that sugar-derived substrates can also be used for the preparation of highly functionalised cyclopentanes.82,83 For example, treatment of iodide 75 with Sml2 initiated a highly diastereoselective construction of cyclopentane 76 (Scheme 5.50). These reactions require the presence of HMPA and a proton source to avoid competing 1,4-reduction of the a,p-unsaturated ester and cleavage of the allylic C-O bond.82,83... 

The acetate unit is particnlarly convenient, since it is obtained from the corresponding alcohol and is relatively stable toward solvolysis, Sn2 reactions, and so on. Many other electronegative units are also effective, including halide and tosylate. Less obvious is the high reactivity of allylic sulfones and allylic nifro compounds toward the oxidative addition. Three special cases are outlined here. First, an allylic epoxide can undergo rapid oxidative addition of the allylic C-O bond under mild conditions, leading to the zwitterion or equivalent (Scheme 13). Proton abstraction can lead... 

In contrast, the related cyclizations used to form caibocyclic systems have been of limited synthetic value due to the acid sensitive nature of the derived allylic ether product (equation 1 Figure 11). The sensitivity of these caibocycles may be rationalized by considering the poor orbital oveilap between the allylic C—O bond with the adjacent ir-bond in a five-, six- or seven-membered ring. ... 

Activation of Allylic C-O Bonds by Transition Metal Complexes... 

The cleavage of allylic C-O bond in allylic organic compound on interaction with a low valent transition metal complex proceeds by net oxidative addition to give r)3-allyltransition metal complexes (Eq. 3). 

The other factor to enhance the ease of the allylic C-O bond cleavage is the interaction of the transition metal with the C=C bond in the allylic entity. The... 

Sn2 type interaction of the metal with the allylic double bond will facilitate the cleavage of the allylic C-O bond, thus making the process kinetically more favorable. 

Zhou et al. [18] reported the use of calcium in ethylenediamine for reductive cleavage of dihydropyrans (Scheme 4.6). In 5,6-dihydro-2H-pyrans 19, the allylic C-O bond is cleaved selectively to give a mixture of alkenyl alcohol 20 and saturated alcohol 21 in a 98 2 ratio with an overall yield of 58-60%. Application of the same reducing agent to 3,4-dihydro-2H-pyran (22), however, led to tetrahydro-pyranyl pentyl ether 23 in 51% yield. This involves reduction followed by an addition process. 

Two possible mechanisms are outlined in Scheme 2. Insertion in the allylic C-O bond to form a 7t-allylic nickel alkoxide would be followed by reduction of the carbon nickel bond (path a). An alternative sequence involves a hydronickela-tion of the complexed olefin followed by a P-elimination of the oxygen bridge (path b). 

Cleavage of allylic C-O bonds (Eq. 80),135 reduction of conjugated double bonds,136 137 and reductive dehalogenations138 139 occasionally intervene when reducing functionalized sulfones with Na/Hg. These side reactions are dependent on the substrates and reaction conditions, and should not be considered as general limitations. 

Hydrogenolysis of vinyloxirane is regioselective to give homoallylic alcohol 435. Hydrogenolysis of the alkenyloxirane 436 is regio- and stereospecific and proceeds by overall inversion of configuration of the allylic C—O bond to afford the homoallylic alcohol 437 [167]. Sato applied the reaction to the synthesis of an A-ring precursor of vitamin D. The protected homoallylic alcohol 439 was obtained at room temperature from the alkenyloxirane 438 in 90% yield [168]. 

A related procedure for the cleavage of allyl ethers has been developed by Ogasawara. Treatment of an allyl ether with diisobutylaluminum hydride and catal5hic [l,3-bis(di-phenylphosphino)propane]dichloronickel leads to facile cleavage of the allylic C—O bond (Scheme A mechanism similar to that proposed for the hydroalumina-... 

Chiral organometaUic reagents offer the broadest range of possibilities for enantiodivergent reactions. The oldest and most extensively studied cases involve displacement reactions of allylic C—O bonds to give regioisomeric products under... 

In a somewhat different sequence involving allylic C—O bond cleavage, regiodivergent RRM has been observed in the reaction of dihydrofuran (R,S)-111 with ethylmagnesium chloride and a chiral zirconium catalyst 112 (Scheme 6.24) [44]. The formation of 113 and 114 occurs via carbometaUation of the alkene followed... 

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