Organoboron compounds in new synthetic reactions

A general and convenient method for the stereoand regiospecific synthesis of conjugated alkadienes, alkenynes, arylated alkenes, and other olefinic compounds is described. The reaction of (E)or (Z)-lalkenyldisiamylboranes, or 2-(E)-l-alkenyl-l ,3,2-benzodioxaboroles easily obtainable by hydroboration, with either (E)or (Z)-l-alkenyl halides in the presence of a catalytic amount of tetrakis(triphenylphosphine)palladium and bases such as sodium alkoxides gives the corresponding (E,E)-, (_,_)-, or (Z,Z)-conjugated alkadienes stereoand regiospecifically, while retaining the configurations of both the starting alkenylboranes and haloalkenes. The reaction of (E)and (Z)-l-alkenyldisiamylboranes with 1-haloalkynes similarly provides a stereoand regiospecific synthesis of conjugated ()_ and (Z)-alkenynes. A mechanism of this crosscoupling reaction, which involves the transmetallation between a 1-alkenylborane and an alkoxopalladium(II) complex generated through the metathetical displacement of a halogen atom from RPd(II)X with sodium alkoxide, is proposed. The versatility of this method has been demonstrated by the stereospecific synthesis of natural products bearing conjugated alkadiene or alkenyne structures, e.g., trans(C10)-allofarnesene, bombykol and its three geometrical isomers, humulene, and trisporol B.