Hydroboration trans, formal

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]. 

Morphine has a B/C cis ring fusion, and formal inversion of the C-14 chiral center leads to a series of analgesically active B/C trans-4,5-epoxymor-phinans. Japanese workers013,114) have described a preparation of B/C trans-4,5-epoxymorphinans by the hydroboration of 8-dihydrodesoxycodeine, and the same group has synthesized B/C trans-morphine and B/C trans-codeine... 

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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