Allyl carbonates cleavage

The selective cleavage shows clearly that the oxirane C-O bond at the allylic carbon atom can be cleaved easier than the benzylic one and the oxet-ane ring. 

Electrochemical oxidation in methanol of both a-pinene 17 and p-pinene 18 leads to opening of the four membered ring by cleavage of one allylic carbon-... 

The synthesis of (S)-ibuprofen (S)-34 utilizing allylic alkylation was undertaken to determine the stereochemical course of this process. The reaction of the enantiomeri-cally enriched allylic carbonate (S)-32 (95% ee) with the requisite aryl zinc bromide (Scheme 10.7) [32], under optimized reaction conditions, furnished the 3-aryl propenyl derivative (R)-33 in 90% yield (2° 1°=10 1) with inversion of configuration (100% cee). The synthesis of (S)-ibuprofen (S)-34 was then completed through the oxidative cleavage of the aUcene (R)-33 in 74% yield [33]. 

This electrochemical oxidation mediated by NHPI was applicable to benzylic carbons, allylic carbons, deprotection of acetals, oxidative cleavage of cyclic acetals and amide to afford benzoylated compounds, enones ", carbonyl compounds, -hydroxyethyl esters and imides, respectively (equations 31-35). 

In contrast to the case of allylsilanes, anodic oxidation of disubstituted olefins provides in general four regioisomeric products because all the allylic carbon-hydrogen bonds can be cleaved. In the case of allylsilane, the cleavage of a C—Si bond takes place... 

Needless to say, /i-keto esters are important compounds in organic synthesis. Their usefulness has been considerably expanded, based on Pd-catalysed reactions of allyl / -keto carboxylates 399. Cleavage of the allylic carbon-oxygen bond and subsequent facile decarboxylation by the treatment of allyl / -keto carboxylates with Pd(0) catalysts generate the 7i-allylpalladium enolates 400, 401. These intermediates undergo, depending on the reaction conditions, various transformations which are not possible by conventional methods. Thus new synthetic uses of / -keto esters and malonates based on Pd enolates have been expanded. These reactions proceed under... 

Transition-metal-mediated C-O bond cleavage reactions are interesting in view of environmentally benign halogen-free chemical processes [59]. Zerovalent ruthenium complexes are also active toward C-O bond-deavage reactions, and a number of catalytic processes have been developed in this respect. For example, Ru(l,5-COD)(l,3,5-COT) catalyzes allylic alkylation of carbon nucleophiles with allylic carbonates in basic solvent (Scheme 14.24) [60]. 

A six-membered cyclic allylic carbonate 102 undergoes a palladium-catalyzed decarboxylative C-C bond cleavage to afford dienic carbonyl compound 104 [122]. Decarboxylation of the allylic carbonate moiety provides the driving force for production of the intermediate five-membered hetero-palladacycle 103, from which formal reductive cleavage takes place. 

The experimental results that both branched and linear allylic ethers are obtained in the palladium-catalyzed decarboxylation of branched allylic carbonate indicate occurrence of direct oxidative addition involving the C-0 bond cleavage followed by the nucleophilic attack of the alkoxide liberated on either the substituted or non-substituted terminus of the allylic ligand (Scheme 4) [1]. 

In this reaction, cleavage of the C-O bond in allylic carbonates promoted by Ru(0) to form r 3-allylruthenium intermediate with liberation of C02 is proposed. 

Recently, interesting processes for the direct activation of the C-O bonds in allylic alcohols have been realized by the promotion of C02 [24]. In this process the effect of C02 was interpreted by formation of hydrogen allyl carbonate by the reaction between allyl alcohol and C02. The hydrogen allyl carbonate thus produced is more susceptible to the allyl-0 bond cleavage than in the parent allyl alcohol in interaction with a Pd(0) catalyst as shown in Scheme 5. Combination of... 

Indeed, 79a, which must exist predominantly in the conformation 79b to avoid A(1 3) strain with the methyl substituent on the olefinic carbon, allows approach of the hydroboration reagent on the face syn to the methyl substituent on the allylic carbon and, thus, the product 80 is formed with a selectivity of 8 1 after oxidative cleavage of the cjc b bond. A similar analysis applies to 81 and guides us to consider 81b as the conformer preferred to 81a. Hydroboration syn-to-methyl on the allylic carbon followed by oxidative cleavage of the crc B bond leads to 82 as the predominant product. Indeed, 82a and 82b are formed in 12 1 diastereoisomeric ratio [37]. 

Cyclic allylic disulfide readily underwent the R-ROP due to the facile cleavage of the bond between allylic carbon and sulfur atom, resulting in the production of polysuffides with exo-methylene groups (3, 41, 42). Acrylate-based cyclic allylic sulfide also showed a high radical polymerizability (3, 43). The copolymerization of acrylate-based cyclic allylic sulfide with MMA and styrene produced the corresponding copolymers bearing sulfide and ester moieties in the polymer backbone (43). 

Allylic carbonates also undergo oxidative addition, cleavage of the allyl-0 bond, and loss of CO2 can occur to give a species formulated as the allyl-Pd-alkoxide (equation 48). In the presence of a suitably acidic carbon acid (p/fal5 or lower), the carbanion is generated and nucleophile addition can occur. The overall process requires only a catalytic amount ofPd° no base or other stoichiometric reagents is necessary. ... 

Another important mechanism for the allylic carbon-halogen bond cleavage involves prior r/ -coordinafion of the C=C double bond to the transition metal followed by Sn2- or SN2 -type nucleophilic addition (Scheme 3.8). 

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]. 

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