Hydroborations vinylic esters, pinacolborane

Catalytic hydroboration of vinylic ethers, acetals, and esters with pinacolborane takes place smoothly in the presence of transition metal catalysts. However, a noticeable exception is the catalytic hydroboration of vinyl bromides 59 which do not furnish the expected hydroborated product under these conditions. The reaction of vinyl bromides with pinacolborane initially affords the expected /3-boronoalkylbromide 60. A fast. -elimination ensues to furnish the terminal alkene 61 and 7 -bromopinacolborane 63. The alkene 61 undergoes hydroboration with unreacted pinacolborane to provide the debrominated boronate 62. The intermediate 5-bromopinacolborane 63 cleaves the ethereal C-O bond in the solvent (THF) to provide 4-bromobutyl borate 64 as a side product (Scheme 11) <1996JA909, 2000CSP14505>. 

Vinylic ethers, acetals, and esters, also undergo catalytic hydroboration with pinacolborane without difficulty. 

The hydroboration of alkenes and alkynes to highly chemo-, regio-, and stereoselective alkyl and vinyl boronic esters with pinacolborane is catalysed by bis(imino)pyridine Fe(II) complex with activating agent tolylMgBr in THF. Preliminary mechanistic experiments suggest that an Fe(I) catalyst may be formed under the reaction conditions. ... 

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