Synthesis of Acyclic, Cyclic, and Heterocyclic Alkenes

As already mentioned, the internal triple bond is hydroborated (90-96%) with equimolar quantity of 9-BBN, while twofold excess of terminal alkyne is used for monohydroboration (90-100%) with 9-BBN [1]. Protonolysis ofB-vinyl-9-BBN derivatives provides the corresponding alkenes. Thus, protonolysis of the product from 5-decyne and 9-BBN yields only ds-5-decene (Eq. 24.1X 

This method is utilized [1] for the synthesis of deuterated alkenes (Eq. 24.2) with tram geometry, which otherwise are prepared only by tedious methods. 

An alternative and more efficient method for synthesis of fraws-vinyl-9-BBN was developed by Soderquist [2], where 1 equiv of 9-BBN is sacrificed instead of the terminal alkyne. The process involves the dihydroboration of terminal alkyne (1 equiv) with 2 equiv of 9-BBN. The 1,1-dibora product is then treated with 1 equiv of PhCHO for 2 h at 25 °C, and B-PhCH20-9-BBN and vinyl-9- 

The use of 1-naphthaldehyde instead of benzaldehyde renders the distillative separation of trans-vinylboranes, a greatly simplified process in most cases. 

The vinylboranes that are not formed efficiently even with large excess of terminal alkynes, however, are prepared conveniently from 1,1-dibora adducts and are smoothly converted to frans-l-deuterio-l-alkenes with CDjCOOD or AcOD at 0 °C (Eq. 24.3). 

---