Propargyl hydrogenolysis

Hydrogenation of the 1-propargyl and 1-allyl derivatives of 63 (R = H) over Pd/CaC03 or Pd/C gave the 1-propyl compound,103 273 whereas 2-propargyloxy-4-oxo-4A/-pyrido[l,2- ]pyrimidine yielded the 2-propyloxy compound.273 Catalytic hydrogenation of the 2-alkoxy- and 2-aralkoxy-4-oxo-4A/-pyrido[l,2- ]pyrimidines was accompanied by the hydrogenolysis of the substituents in position 2.112,114... 

Palladium(0)-catalysed selective allene formation by hydrogenolysis is a convenient synthetic route to allenes from propargylic alcohols. The method offers a new and efficient preparative method for allenes from easily available propargylic alcohols after converting them to formates or carbonates. The best catalyst for smooth hydrogenolysis is prepared by mixing Pd(OAc)2 or Pd(acac)2 with -Bu3P in a ratio... 

Palladium-Catalyzed Hydrogenolysis of Allylic and Propargylic Compounds with Various Hydrides. Tsuji, J. Mandai, T. Synthesis 1996,1. 

Ammonium formate hydrogenolysis of 7t-allyl-palladium(II) complexes formed via oxidative addition of Pd(0) to propargylic carbonate esters is a useful method for preparing disubstituted alkynes, as depicted below. This method of deoxygenation avoids the use of expensive and toxic reagents often associated with the free-radical Barton-McCombie deoxygenation. °... 

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). ... 

Acetylenic carbinols, which are manufactured from aldehydes or ketones and acetylenes, can be partially hydrogenated, with the exception of propargyl alcohol itself. Hydrogenolysis is a minor side reaction unless the hydrogenation is completed to the alkane. Nickel boride (PI or P2) catalysts achieve the saturation of propargyl and allylic compounds in the presence of double bonds without hydrogenolysis, such as in 2 an intermediate in the total synthesis of the sesquiterpene sesquicarene ... 

The propargyl ether of 3-benzyloxyphenol was cyclised in hot diethylaniline to 7-benzyloxychrom-3-ene which with 3-hydroxy-4-chloromercurimethylenedioxy benzene in acetonitrile containing lithium chloropalladite afforded acemic benzylmaakiain. Hydrogenolysis, formation of the allyl ether and its... 

Catalytic hydrogenation of alkynes takes place in a stepwise manner, and both the alkene and the alkane can he isolated. Complete reduction of alkynes to the saturated compound is easily accomplished over platinum, palladium or Raney nickel. A complication which sometimes arises, particularly with platinum catalysts, is the hydrogenolysis of hydroxyl groups a- to the alkyne (propargylic hydroxyl groups) (7.15). 

Allenes 169 and alkynes 170 are prepared by hydrogenolysis of propargyl compounds with several hydrides. Triethylammonium formate is used most conveniently under mild conditions [45]. Chromium tricarbonyl-complexed phenylallene 172 was prepared from the carbonate 171 [46]. The alkyne 174 was obtained selectively from the propargyl formate 173 having an amino group [47]. 

Reaction of propargyl formate 175 affords two products 179 and 181 depending on the kind of intermediates being subjected to hydrogenolysis with formate. When 5-exo cyclization of 176 and 3-exo cyclization of 177 to give the cyclopropane 178 occurred before the hydrogenolysis, the bicyclo[3.1.0]hexane 179 was obtained. On the other hand, the cyclopentane 181 was formed by hydrogenolysis of 176, and the cyclopentane 181 was obtained by Pd-catalyzed cyclization of 180 [3]. 

Pd-catalyzed hydrogenolysis of allylic compounds with formates is an efficient and mild method. The hydride generated from the palladium formate attacks the more substimted side of the allylic system to give less substituted olefins in contrast to the case with other hydride sources. Pd-catalyzed hydrogenolysis of propargylic compounds affords either aUenes or alkynes depending on the structure of the propargylic compounds. 

Alkynyl cyclic carbonate 13 underwent hydrogenolysis at the propargylic C—O bond along with the reduction of the alkyne to give cw-alkenol 14 (Scheme 6)1 ... 

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 Allylic and Propargylic Compounds with Various Hydrides. 

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