Hydrogenolysis of allyl acetates

Hydrogenolysis of allylic acetates.1 This reaction can be effected in high yield by hydrogenolysis catalyzed by Pd(OH)2 on carbon and with cyclohexene as the... 

Hydrogenolysis of allylic acetates. Allylic acetates (or carbonates or chlorides) are converted into terminal alkenes by ammonium formate using a Pd(ll) catalyst com-plexed with a trialkylphosphine, particularly P(n-Bu),. 

While in propargylic systems a vinylogous reduction is the normal reactivity, in allylic systems a double bond shift on reduction is realized more generally only when terminal alkenes are produced. Especially Interesting in this respect is the Pd-catalyzed hydrogenolysis of allylic acetates (X = OAc) with ammonium formate (equation 45). ... 

Hydrogenolysis of allylic formates. The Pd(0) catalyst generated from Pd(OAc)2 and Bu2P effects hydrogenolysis of allylic acetates or carbonates with ammonium formate to afford 1-alkenes. Allylic formates are also converted to 1-alkcncs by this Pd(0) catalyst without use of ammonium formate. 

Hydrogenolysis of allyl acetate with [PdCl2L2] (L = l, 6, 7) catalyst proceeded smoothly in heptane/aqueous sodium formate mixtures at 80 °C [57]. The catalyst, formed in situ from [Pd(OAc)2 and TPPTS, could be used for the selective removal of allylic protecting groups such closely related protecting groups as dimethyl-allylcarbamates and allyloxycarbonates could also be distinguished [58, 59]. 

The ene oxide 266 was prepared from 5-methyl-(2Z)-hexene-l-ol first by Sharpless epoxidation followed by Swem oxidation the aldehyde was submitted to Wittig reaction and the resulting alcohol was esterified to lead to ene oxide 266. Pd-catalyzed cyclization in the presence of Pd2(tba)3-CHCl3 and DPPE in THF at 25 °C gave the lactone 267 with high regio- and stereoselectivities. Introduction of the butenyl chain of the lactone was successful by Pd-catalyzed alkylation with the appropriate aUylic carbonate. The allylic substitution proceeded stereoselectively in 51% yield. The Pd-catalyzed hydrogenolysis of allylic acetate with formic acid and triethylamine afforded the lactone 268 (Scheme 62). 

Palladium-catalysed hydrogenolysis of allylic acetates and allylic phenyl ethers with ammonium formate provides a convenient method of preparation of 2-olefins [equation (5)]. The preferred catalyst is the readily available complex... 

Refluxing with zinc in ethanol reduced a-bromocinnamic acid to cinnamic acid in 80% yield [997]. Allylic chlorines in y,y,y-trichlorocrotonic acid were partly or completely hydrogenolyzed by zinc and sodium amalgam [5/9]. Hydrogenolysis of allylic bromine in oc, -unsaturated esters with zinc in acetic acid gave predominantly, y-unsaturated esters in 65-97% yields [998]. 

The hydrogenolysis of allylic ether and acetate (235 236, R=alkyl or COCHj) should also take place more easily when the compound can adopt a conformation in which the OR group can become parallel to the u orbital of the double bond (69). The same stereochemical requirement must also be necessary in the hydrogenolysis of a substituent in a benzylic position (70). 

Basic supports for the palladium catalyst can also be used to inhibit the hydrogenolysis of allyl alcohols (Eqn. 16.22) and acetates (Eqn. 16.23). The addition of quinoline to the reaction mixture has the same effect (Eqn. 16.24). 5... 

The beneficial action is thought to arise from the neutralization of acid sites on the catalyst support which may otherwise promote hydrogenolysis. In this context the alkaline-earth oxide supports CaC03 and BaC03 have been used with success to inhibit the hydrogenolysis of allylic alcohols and acetates [64,65]. 

Hydrogenolysis of allylic compounds was rather general, including halides, acetates, alcohols, ethers and amines. 

In connection with the finding that a combination of a Pd catalyst and ammonium formates is effective for hydrogenolysis of allyl esters to olefins, and of propargyl carbonates to allenes, the reactions with allyl ester of alkynyl acid and propargyl acetate under the reaction conditions shown in Scheme 4 were investigated. The results indicate that in both cases reduction of the triple bond takes preference over hydrogenolysis as sununa-rized in Scheme 5. 

Hydrogenolysis of ally acetates 80 with triethylammonium formate gives aDyl-silanes substituted at the a-position (81) (Scheme 5.20) [28]. Disilyl reagent 81a reacts with octanal in the presence of BFj-OEtj at —40°C to yield tert-butyldi-methylsilyl-substutited product 82a solely, while allylation with 81b proceeds at -78 °C to give 82b. Thus, the carbon-tin bond in 81b is cleaved much faster than the carbon-silicon bond. 

Vinylogs of benzylic alcohols, e.g. cinnamyl alcohol, undergo easy saturation of the double bond by catalytic hydrogenation over platinum, rhodium-platinum and palladium oxides [39] or by reduction with lithium aluminum hydride [609]. In the presence of acids, catalytic hydrogenolysis of the allylic hydroxyl takes place, especially over platinum oxide in acetic acid and hydrochloric acid [39]. 

Yamaguchi employed a strategy not unlike the Bartlett approach to brefeldin A, as depicted in Scheme 1.40. Keto diacid 189 was esterified and reduced with Ra(Ni) to afford a 92% yield of 245. Alcohol participation in a jyn-selective hydrolysis followed by reesterification with benzyl alcohol led to the formation of 246a and 246b in a 19 1 ratio. Protection of the alcohol as the acetate and hydrogenolysis of the benzyl ester followed by fractional recrystallization gave a 87% overall yield of 247. Acid 247 was converted to allylic alcohol 248 in 65% yield via Rosemund reduction of the derived acid chloride to the aldehyde and addition of vinylmagnesium bromide. 

Hydrogenolysis can occur using dissolving metal conditions, and is particularly useful for the cleavage of O-benzyl and N-benzyl derivatives. It is widely used as a useful alternative to catalytic hydrogenation. Treatment with sodium in ammonia cleaves the O-benzyl group, as in the conversion of 491 to 492 (in 90% yield), in Mulzer s synthesis of hastanecine. l Note that the byproduct is toluene. It is also possible to cleave other reactive C—O (or C—N) bonds. Allylic acetates, for example, are cleaved to the hydrocarbon, as shown by the reaction of 493 with lithium and ammonia to give an 81% yield of 494. ... 

The 1,5-diketone 21 is prepared by 3-butenylation of ketones to give 20 via Pd-catalyzed hydrogenolysis of the allylic acetate 19, followed by Pd-catalyzed oxidation, and is used for annulation to form the cyclohexenone 22 [19]. In this method, the 3-butenyl group is a masked methyl vinyl ketone. Preparation of the 1,7-diketone 24 was performed by oxidation of 23, and applied to the synthesis of dysidiolide (25) [23]. 

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