Conjugated alkadienes preparation

The procedure described here Is an example of a general method for preparing conjugated alkadienes by the palladium-catalyzed reaction of 1-alkenylboranes or boronates with vinylic halides. Hydroboratlon of l-a1kynes with catecholborane Is a standard method for obtaining (E)-l-alkenylboronates (1). Several different types of alkenylboranes and boronates (2-4) are now available as reagents for the cross-coupling reaction with vinyl halides. 

The stereo- and regioselective synthesis of conjugated alkadienes are of great importance in organic chemistry. A number of methods for the preparation of conjugated dienes and polyenes were developed uitilizing... 

Our discussion of chemical reactions of alkadienes will be limited to those of conjugated dienes. The reactions of isolated dienes are essentially the same as those of individual alkenes. The reactions of cumulated dienes are—like their preparation— so specialized that their treatment is better suited to an advanced course in organic chemistry. 

For obtaining rearranged conjugated trienes 52a-e and 53a-e on a preparative scale, the reaction mixture from the synthesis in 1 -(2, 6, 6 -trimelhyl-1 -cyclohexen-l -yl)-3-(phenylsulfinyl)-1,2-alkadienes 51a-c (200-300 mg) (both diastereomers of the vinylallene sulfoxides) after Et20 workup is allowed to stand in Et20 (15-20 mL) at r.t. for > lOh. The residue after removal of solvent is subjected to 1IPLC purification (Whatman Partisil M 9 10/50 column, 10-20% ElOAc/Skellysolve B) to afford the two conjugated trienes. From 51a yield 63% ratio (53a/52a) 81 19 from 51 b yield 91 % ratio (53b/52b) 92 8 from 51c yield 75% ratio (53c/52c) 92 8 from 51 d yield 80% ratio (53d/52d) 94 6 from 51e yield 58% ratio (53d/52d) >98 2,... 

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