Preparation of Conjugated Dienes by 1,4-Elimination

When allylic compounds 346 are treated with Pd(0) catalyst in the absence of any nucleophile, 1,4-elimination (dehydropalladation) is a sole reaction path as shown by 347, and conjugated dienes are formed as a mixture of (E) and (Z) isomers [164,165], From terminal allylic compounds, terminal conjugated dienes are formed. 

Allylic amines are difficult to cleave, but they can be cleaved by using a cationic Pd complex and DPPB as a ligand. As an example, N, /V-dicthylgcranylamine (348) is converted 349, and its regioselective dehydropalladation affords the myrcene-type conjugated diene 350, and the reaction is applied to commercial production of the fragrant compound 351, called kovanol [166]. 1,4-Elimination of allylic amines is also possible as their amine salts using AcOH as a solvent [167], 

The allyl chloride 353 is prepared from diprenyl ether (352), and its Pd-catalysed 1,4-elimination in the presence of AcONa affords the conjugated diene 354. Citral (355) is obtained by the consecutive Claisen and Cope rerrangements of 354 [168], 

The reaction has been successfully applied to the synthesis of the precursor 366 of provitamin D which has the homoannular conjugated diene in the B ring [169]. Treatment of 7a-carbonate 364 with the Pd catalyst at 40 °C affords the 5,7-diene 366 regioselectively in good yield. No heteroannular diene 367 is detected. In the intermediate complex 365, the /f-oricnted 7(j-allylpalladium undergoes facile syn elimination of the 8 ft-hydrogen to afford 366 exclusively. 

However, a mechanistically interesting result which contradicts the expected anti addition -syn elimination mechanism of Pd-catalysed 1,4-elimination of allylic compounds has been reported. This is the elimination of the cyclic allylic carbonate 368 which afforded diene 371, but not diene 370, as expected from the anti addition-syn elimination mechanism. The selective formation of 371 is explained by oxidative addition with inversion to genearate 369, followed by elimination of the anti H of 369, namely anti elimination occurs [170]. 

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