Cleavage of allyl carboxylates

Cleavage of allyl carboxylates (general procedure) f To a solution of NaHTe (prepared in situ from tellurium powder (1.30 g, 1.0 mmol) and NaBH4 (0.90 g, 2.4 mmol) in EtOH (20 mL) buffered with deoxygenated HOAc (1.2 mL in 5 mL EtOH)) is added the allyl carboxylate (0.005 mol). The mixture is refluxed under N2 for 2 h, then filtered, the filtrate... 

Direct evidence for the oxidative addition of allyl carboxylates to a zero-valent ruthenium complex with the allyl-0 bond cleavage, has been obtained by the reaction of Ru(cod)(cot) (cot = 1,3,5-cyclooctatriene) with allyl carboxylates in the presence of tertiary phosphine under ambient conditions to give Ru(OCOR)()7 -C3H5)L3 [55], where Ru( 7 -C8Hio)L3 acts as an intermediate for the reaction (Scheme 3.29) [56]. 

Oxidative addition of vinyl carboxylates with the vinyl-0 bond cleavage is much less explored to date than that of allyl carboxylates. Reaction of vinyl... 

C-0 bond cleavage of alkenyl carboxylates such as vinyl and allyl carboxylates can also be achieved by transition metal hydrides. For example, a divalent ruthenium dihydride complex c 5-RuH2(PPh3)4 reacts with vinyl acetate to give c 5-Ru(H)(OAc)(PPh3)4 accompanied by evolution of ethylene (Scheme 3.31)... 

Similar to carboxylates and carbonates, the C-0 bond cleavage of allylic ethers with low-valent metal complexes relatively is well documented (Eq. 3.15). Typical example is demonstrated in the reaction of group 10 metal complexes such as Pd or Ni resulting in the clean allyl-0 bond cleavage [76]. 

Nickel peroxide is a solid, insoluble oxidant prepared by reaction of nickel (II) salts with hypochlorite or ozone in aqueous alkaline solution. This reagent when used in nonpolar medium is similar to, but more reactive than, activated manganese dioxide in selectively oxidizing allylic or acetylenic alcohols. It also reacts rapidly with amines, phenols, hydrazones and sulfides so that selective oxidation of allylic alcohols in the presence of these functionalities may not be possible. In basic media the oxidizing power of nickel peroxide is increased and saturated primary alcohols can be oxidized directly to carboxylic acids. In the presence of ammonia at —20°, primary allylic alcohols give amides while at elevated temperatures nitriles are formed. At elevated temperatures efficient cleavage of a-glycols, a-ketols... 

A crystal structure of the C02 derivative of (8), K[Co(salen)( 71-C02)], haso been reported in which the Co—C bond is 1.99 A, the C—O bonds are both equivalent at 1.22 A and the O-C-O angle is 132°.125 Carboxylation of benzylic and allylic chlorides with C02 in THF-HMPA was achieved with (8) electrogenerated by controlled-potential electrolysis,126 in addition to reductive coupling of methyl pyruvate, diethyl ketomalonate and / -tolylcarbodiimide via C—C bond formation. Methyl pyruvate is transformed into diastereomeric tartrates concomitant with oxidation to the divalent Co(salen) and a free-radical mechanism is proposed involving the homolytic cleavage of the Co—C bond. However, reaction with diphenylketene (DPK) suggests an alternative pathway for the reductive coupling of C02-like compounds. 

Bis(homoallylic) alkoxides also undergo this 3-cleavage. The substrates (1) are available by reaction of a methyl carboxylate with 2 equiv. of allyl- or methallyl-magnesium chloride. The alcohols when heated at 80° in a slurry of KH in HMPT are converted into 3.7- and a,3-unsaturated ketones (2 and 3) in 75-85% yield. 

The same basic strategy was applied to the synthesis of the smaller fragment benzyl ester 28 as well (Scheme 4). In this case, aldehyde 22 prepared from (S)-2-hydroxypentanoic acid [9] was allylated with ent-10 and tin(IV) chloride, and the resulting alcohol 23 was converted to epimer 24 via Mitsunobu inversion prior to phenylselenenyl-induced tetrahydrofuran formation. Reductive cleavage of the phenylselanyl group, hydrogenolysis of the benzyl ether, oxidation, carboxylate benzylation, and desilylation then furnished ester 28. 

Allyl esters, carbonates, and carbamates readily undergo C-O bond cleavage upon reaction with palladium(O) to yield allyl palladium(II) complexes. These complexes are electrophilic and can react with nucleophiles to form products of allylic nucleophilic substitution. Linkers based on this reaction have been designed, which are cleavable by treatment with catalytic amounts of palladium complexes [165,166], For the immobilization of carboxylic acids, support-bound allyl alcohols have proven suitable (Figure 3.12, Table 3.7). 

The allyl cation generated by the electrocyclic cleavage of dibromocyclopropanes (cf. Section 4.7.3.7.1) has been trapped by a carboxyl group in a highly efficient synthesis of furanones (equation 111) and pyranones.236 Acid-catalyzed openings of dihalocyclopropanes also give similar results.237... 

---