Preparation of 1-Alkenes by Hydrogenolysis with Formates

Many applications of 1-alkene formation have been reported. The allylic acetate 427 was converted to exo methylene 428 by removal of the acetoxy group [162]. 

Highly regioselective hydrogenolysis of the aldol product 433 proceeded smoothly in DMF to produce the terminal alkenes 434a and 434b in high yield 

Hydrogenolysis of vinyloxirane is regioselective to give homoallylic alcohol 435. Hydrogenolysis of the alkenyloxirane 436 is regio- and stereospecific and proceeds by overall inversion of configuration of the allylic C—O bond to afford the homoallylic alcohol 437 [167]. Sato applied the reaction to the synthesis of an A-ring precursor of vitamin D. The protected homoallylic alcohol 439 was obtained at room temperature from the alkenyloxirane 438 in 90% yield [168]. 

Regioselectivity in hydrogenolysis of internal allylic systems depends on the bulkiness of substituents. The alkene 441 was obtained regioselectively from tert-carbonate 440. The hydride attacked the congested tert-carbon, but not i ec-carbon [169]. 

On the other hand, the a-oriented r-allylpalladium 448, formed from the (Z)--carbonate 447, has an unstable anti structure, which rotates to form the more stable syn structure 450 before decarboxylahon. By this rotation, Pd moves from the a-side to y3-side as shown by 449, and hydride transfer occurs from side as shown by 451, and the natural conhguration 452 was constructed. 

---