Hydroboration trans

The dominant factors reversing the conventional ds-hydroboration to the trans-hydroboration are the use of alkyne in excess of catecholborane or pinacolborane and the presence of more than 1 equiv. of EtsN. The P-hydrogen in the ris-product unexpectedly does not derive from the borane reagents because a deuterium label at the terminal carbon selectively migrates to the P-carbon (Scheme 1-5). A vinylidene complex (17) [45] generated by the oxidative addition of the terminal C-H bond to the catalyst is proposed as a key intermediate of the formal trans-hydroboration. 

A formal trans-hydroboration of terminal alkynes with catecholborane and pinacolborane to yield cis-l-alkenylboronates has also been carried out in the presence of Rh(I) and Ir(I). The dominant factors for reversing the conventional cis-hydroboration to the trans-hydroboration were the use of NEts and the use of bulk phosphines such as P Pr3 and PCy3, and an excess of alkyne in front of the boron reagent [46]. 

Miyaura and co-workers have reported the rhodium- or iridium-catalyzed trans-hydroboration of terminal alkynes, where hydroboration ofthe vinybdene complexes is considered as a key reaction path Ohmura, T., Yamamoto, Y. and Miyaura, N. (2000) Journal of the American Chemical Society, 122, 4990. 

Recently Miyaura and co-workers have reported a trans-hydroboration of terminal alkynes using [Rh(COD)Cl]2[PCPr)3]4 or [Ir(COD)Cl]2[P( Pr)3]4 (eq 13). Mechanistic studies via deuteriumlabeling show that after the oxidative addition of the alkyne to the metal, the acetylenic deuterium undergoes migration to the S-carbon resulting in the formation of a vinylidene metal complex. Oxidative addition of borane to the metal complex and 1,2-... 

Indirect trans-hydroboration using alkynyl bromides... 

Rhodium and Iridium catalyzed trans-hydroboration of alkynes... 

In order to access the (Z) diastereomer, Lopez has reported a formal trans-hydroboration of terminal enyne using pinacolborane in the presence of a Rh(I) complex and triethylamine [180] (Scheme 93) The yield is highly influenced by the presence of substituents on the enyne substrate. 

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