Abstract
We report a stereoselective regiodivergent hydroboration of aryl,alkyl-disubstituted internal allenes with pinacolborane (HBpin) in the presence of cobalt catalysts generated from bench-stable Co(acac)2 and bisphosphine ligands. An interesting correlation between the regioselectivity of this hydroboration and the bite angles of bisphosphine ligands was identified. When hydroboration was conducted with cobalt catalysts containing bisphosphines with medium bite angles (e.g. 98° for dppb and 96° for dppf), HBpin was selectively added to the alkyl-substituted double bond. However, HBpin was selectively added to the aryl-substituted double bond when the reactions were conducted with cobalt catalysts containing bisphosphines with large bite angles (e.g. 111° for xantphos and 114° for Nixantphos). A range of internal allenes underwent these Co-catalyzed hydroboration reactions in a regiodivergent manner to yield the corresponding (Z)-alkenylboronates in high isolated yields and with high regioselectivity. These reactions show good functional group compatibility and can be readily scaled up to gram scales without using a dry box. In addition, the comparison of regioselectivity between the Co-catalyzed hydrosilylation and hydroboration reactions of the same allene substrate suggests that this Co-catalyzed regiodivergent hydroboration of allenes proceeds through a Co-Bpin intermediate. Deuterium-labeling experiments suggest that the Co-Bpin intermediates react with allenes to form allylcobalt species which then react with HBpin to release (Z)-alkenylboronate products.
Highlights
Alkenylboronates are synthetically versatile building blocks in organic chemistry because they are stable and non-toxic, and can undergo a range of catalytic cross-coupling reactions to produce stereode ned multisubstituted alkenes.[1]
The hydroboration of allenes with HBpin has been studied with platinum and copper catalysts,[3] and these reactions have been limited to terminal allenes with a single example employing an internal allene (Scheme 1A).3b Alternatively, formal hydroboration of terminal allenes with B2(pin)[2] in the presence of MeOH as a proton source has been achieved with copper catalysts (Scheme 1B).3b,4 To the best of our knowledge, metal catalysts for the hydroboration of 1,3
The catalysts generated by the combination of Co(acac)[2] and a series of bisphosphine ligands were subsequently evaluated, and the majority of these cobalt catalysts were highly active for the hydroboration of 1a, except the catalysts containing dppe and dppp ligands
Summary
Alkenylboronates are synthetically versatile building blocks in organic chemistry because they are stable and non-toxic, and can undergo a range of catalytic cross-coupling reactions to produce stereode ned multisubstituted alkenes.[1]. In the presence of ligands with large bite angles, such as xantphos and Nixantphos, the hydroboration of 1a afforded the (Z)-alkenylboronate 3a as the major product in high isolated yields with high selectivity (entries 12 and 13).
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