Migration alkenyl groups

Cis-olefins or cis./rjns-dienes can be obtained from alkynes in similar reaction sequences. The alkyne is first hydroborated and then treated with alkaline iodine. If the other substituents on boron are alkyl groups, a cis-olefin is formed (G. Zweifel, 1967). If they are cir-alkenyls, a cis, trans-diene results. The reactions are thought to be iodine-assisted migrations of the cis-alkenyl group followed by (rans-deiodoboronation (G. Zweifel, 1968). Trans, trans-dienes are made from haloalkynes and alkynes. These compounds are added one after the other to thexylborane. The alkenyl(l-haloalkenyl)thexylboranes are converted with sodium methoxide into trans, trans-dienes (E. Negishi, 1973). The thexyl group does not migrate. 

Aryl and alkenyl groups undergo this anft -migration more easily than an alkyl group. This rearrangement in combination with Sharpless asymmetric epoxidation affords a stereocontrolled route to aldols and 1,3-diols (second example). 

In Equations B2.7 and B2.8 note that an alkenyl group migrates in preference to a secondary alkyl group. 

Incorporation of an alkenyl group into the (E)-alkene synthesis and its subsequent migration followed by a protonolysis step gives (E, )-dienes (Figure B4.7). 

Alkenylalkynyldi(l,2-dimethylpropyl)boronates, when exposed to iodine and hydroxide, rearrange with selective migration of the alkenyl group to the alkynyl group. The pheromone bombykol (40) has been synthesized via this key reaction (Scheme 47). ° ... 

A diastereospecific synthesis of alkenyl enolates (13) and (15) was reported as shown in Scheme 15.1 [5]. Condensation of alkenylzirconocene (8) with lithiated sulfonyloxirane (10) affords zirconate (11), which causes migration of the alkenyl group from the zirconocene into the oxirane carbon accompanied with elimination of the sulfonyl group. Thus formed intermediate (12) regiospecifically afforded the corresponding alkenyl enolates (13) or (15) depending on the substituents on the oxirane moieties. Further reactions of these alkenyl enolates (13) and (15) with aldehydes were demonstrated. 

Zweifel and Pearson have developed a new high-yielding synthesis of 1,3-enynols (203) from acetylenes via borane intermediates (Scheme 81). The key feature of this transformation is the selective migration, with retention of configuration, of an alkenyl group from boron to carbon, (201) -> (202). The sequence can be modified to furnish 1,2,4-trienols. 

Reactions of vinylboron dihalides with benzyloxides and propargyloxides have also been examined and they proceed quite well. Isolation of racemic products from reactions employing chiral alcohols further supports a cation pathway for the alkenyl group migration. The tandem reactions of in situ generated alkoxides with vinylboron dihalides has also been accomplished (Scheme 23.73). 

---