Alkene, Diene, and Alkyne Synthesis

Halogenation of vinylboranes generates vinyl halides. Both cis and trans halides are available by modification of the reaction conditions. When trans-alkenyl boronic acid (73, derived from 1-octyne) was treated with iodine and sodium hydroxide, trans-1-iodo-l-octene (74) was formed in 90%. When the boronic acid was treated with iodine and then with base, the (Z)-alkenyl iodide (75) was produced.Vinylboranes derived from internal alkynes lead to cis-trans mixtures with both of these procedures. Boronic acids derived from alkynes and catecholborane give the (Z)-bromide on addition of bromine followed by sodium hydroxide.  

There is an alternative method for converting alkynes to vinyl iodides using B-iodo-9-BBN. Reaction of cyano alkyne 76 with reagent, followed by protonolysis with acetic acid gave a 74% yield of 77 in Hodgson s synthesis of (-)-periplanone B. 7 x is procedure is more direct and avoids some potential problems with previous procedures. 

The halogenolysis reaction makes possible the following transformations  

Vinylboranes are versatile intermediates, and there are modifications that convert them to alkenes. Hydro-boration of a terminal alkyne such as cyclohexyl acetylene (78) with dicyclohexylborane gave ( )-alkenyl-borane (79). Subsequent treatment with iodine and sodium hydroxide resulted in transfer of an alkyl group from boron to the alkenyl carbon, generating cis-1,2-dicyclohexylethene (80) in 77% yield (92% isomeric 

In both of these alkene syntheses, the B- C alkyl shift proceeded with retention of configuration. Both alkene-forming reactions have been used for the synthesis of prostaglandin intermediates, in which the stereoselectivity of the processes is valuable. 

---