Highly regio- and stereoselective synthesis of alkenylboronic esters by copper-catalyzed boron additions to disubstituted alkynes

Abstract

The copper-catalyzed addition of bis(pinacolato)diboron to internal alkynes in the presence of methanol generates alkenylboron compounds with high levels of regio- and stereoselectivities. The catalytic efficiency is increased by using monodentate phosphine ligands, especially P(p-tolyl)(3) and a range of internal alkynes was borylated in good yields.

Diastereodivergent Synthesis of Trisubstituted Alkenes through Protodeboronation of Allylic Boronic Esters: Application to the Synthesis of the Californian Red Scale Beetle Pheromone

“…[17] It should be noted that protection of the hydroxyalkyne as silyl ether 11 prior to "hydroboration" resulted in lower regioselectivity. The Z-vinyl boronic esters were prepared by a Cu I -catalyzed formal "hydroboration" of hydroxy alkyne 9 (Scheme 2).…”

Origin of Regioselectivity in the Copper-Catalyzed Borylation Reactions of Internal Aryl Alkynes with Bis(pinacolato)diboron

“…On the other hand, when a bulky tert-butyl group is substituted into the alkyl group (substrate 1_t-Bu), the reaction produces an α-borylated product (7α_t-Bu) in which the boryl moiety is attached at the C α of the aryl group with exclusive regioselectivity. In our previous report, 61 the stereochemistry of 1_t-Bu was wrongly assigned as the E isomer on the basis of NOE data of the α-product of the (trifluoromethyl)phenyl-substituted substrate.…”

ChemInform Abstract: A trans‐Selective Hydroboration of Internal Alkynes.

“…The ensuing trans-selective hydroboration opens a practical new entry into E-configured alkenylboron derivatives, which could previously only be made by indirect routes (Scheme 1). [11] When applied to bulky alkynes of the type ArCCtBu (Ar = aryl), the corresponding E-configured alkenylboronates are formed; upon the smallest decrease in size, however, the system relaxes and returns to the usual synaddition pathway.Herein, we outline the first broadly applicable, functionalgroup-tolerant, and highly stereoselective trans hydroboration of internal alkynes. [6,7] In fact, only very few exceptions are documented where this rule is formally violated.…”

“…Thus, terminal alkynes were recently shown to lead to a net anti addition when reacted with catecholborane (cat-BH) or pinacolborane (pin-BH) in the presence of certain rhodium, iridium, or ruthenium complexes as the catalysts (Scheme 2). [11] When applied to bulky alkynes of the type ArCCtBu (Ar = aryl), the corresponding E-configured alkenylboronates are formed; upon the smallest decrease in size, however, the system relaxes and returns to the usual synaddition pathway. As a consequence, it is the alkyne proton itself, rather than the hydrogen from the borane reagent, that ends up anti to the boron moiety in the product, as unequivocally shown by labeling studies.…”

“…[8,9] This unusual outcome results from an initial rearrangement with formation of metal vinylidene complexes B as reactive intermediates. [11] When applied to bulky alkynes of the type ArCCtBu (Ar = aryl), the corresponding E-configured alkenylboronates are formed; upon the smallest decrease in size, however, the system relaxes and returns to the usual synaddition pathway. [8][9][10] For this very reason, the method does not work with internal alkynes, for which trans hydroborations remain basically unknown.…”