Hydrogenolysis of Allylic Compounds

The Pd-catalyzed hydrogenolysis of allylic compounds by various hydrides gives alkenes. From terminal allylic compounds, either 1-alkenes or 2-alkenes are formed depending on the hydride sources [360a]. 

The formation of 1-and 2-aIkenes can be understood by the following mechanism. In the presence of formate anion, the 7r-allylpalladium complex 572 is converted into the 7r-allylpalladium formate 573. The most interesting feature is the attack of the hydride from formate to the more substituted side of the (T-allylic system by the cyclic mechanism shown by 574 to form the 1-alkene 575[367]. The decarboxylation and hydride transfer should be a concerted 

In addition to the preparation of l-alkenes, the hydrogenolysis of allylic compounds with formate is used for the protection and deprotection of carboxylic acids, alcohols, and amines as allyl derivatives (see Section 2.9). 

Various terminal allylic compounds are converted into l-alkenes at room temperature[362]. Regioselective hydrogenolysis with formate is used for the formation of an exo-methylene group from cyclic allylic compounds by the formal anti thermodynamic isomerization of internal double bonds to the exocyclic position[380]. Selective conversion of myrtenyl formate (579) into /9-pinene is an example. The allylic sulfone 580 and the allylic nitro compound 

581 are prepared from cyclic ketones. Treatment of these allylic compounds with triethylammonium formate generates e.xro-methylene compounds. The preparation of 582 by regioselective hydrogenolysis of the allylic acetate has been applied to the syntheses of gomisin A and schizandrin[381]. 

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Besides formic acid, many other reducing agents, such as LiAlH4, borohy-drides, hydrosilanes, and tin hydrides have been used for the hydrogenolysis of allylic compounds [7]. 

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

Pd-catalyzed hydrogenolysis of allylic compounds with various hydride sources is important not only for the preparation of alkenes, but also for a deprotection of aUyl-derived protecting group. The latter will be discussed in Sect. V.2.3.2. 

Pd-catalyzed hydrogenolysis of allylic compounds with formates is an efficient and mild method. The most important feature of the hydrogenolysis is that the hydride generated from the palladium formate attacks the more substituted side of the allylic system to give less substituted olefins. Various terminal allylic compounds are converted to 1-alkenes. - ... 

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). 

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

This chapter surveys the reduction of saturated alkyl halides to alkanes. Reductive -eliminations of vicinal dihalides to alkenes are also described briefly. Reduction of vinyl and aryl halides is covered in this volume. Chapter 4.5 hydrogenolysis of allyl and benzyl halides is covered in this volume. Chapter 4.7, and reduction of a-halo-substituted carbonyl compounds CX—CO to carbonyl compounds CH—CO is covered in this volume. Chapter 4.8. 

Hydrogenolysis of Allyl and Benzyl Halides and Related Compounds... 

Hydrogenolysis of Allylic and Propargylic Compounds with Various Hydrides. 

Furthermore, the regioselective hydrogenolysis can be extended to internal allylic systems. In this case, clean differentiation of a tertiary carbon from a secondary carbon in an allylic system is a problem. The regioselectivity in the hydrogenolysis of unsymmetrically substituted internal allylic compounds depends on the nature and size of the substituents. The less substituted alkene 596 was obtained from 595 as the main product, but the selectivity was only... 

Reduction of vinylic and allylic compounds without hydrogenolysis may present a problem. The ratio of olefin saturation to hydrogenolysis depends importantly on catalyst, temperature, solvent, and pH. In both classes of compounds, hydrogenolysis is favored by polar solvents, acid, and elevated temperatures hydrogenation, by nonpolar solvents and low temperatures. 

A frequent problem is selective reduction of an acetylene to the olefin in the presence of other easily reducible functions. Usually the reaction can be done without difficulty because of the relatively strong and preferential adsorption of the acetylenic function on the catalyst. Functions adjacent to the triple bond may cause special problems if the resulting allylic compound is itself susceptible to facile hydrogenolysis (18,23). The product composition is, as expected, sensitive to steric effects (82). 

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