|TITLE||Asymmetric Synthesis of Borylalkanes via Copper-Catalyzed Enantioselective Hydroallylation|
As organoboranes are valuable reagents in organic synthesis due to their versatility in functionalization, the demand for stereochemically well-defined organoboronates is increasing. While useful methods to prepare enantioenriched organoboron compounds have been developed, such as metal-catalyzed borylation and stoichiometric lithiation−borylation, the search for novel and more powerful asymmetric synthetic methods continues.
In general, the creation of simple alkylboron compounds with high enantiocontrol presents a greater challenge than the production of boron compounds with a biasing or directing substituent. Transition-metal-catalyzed C−C bond formation through allylic substitution is one powerful method to construct molecules. Among allylation methods, copper-catalyzed allylic substitution, or addition of an organometallic nucleophile to an electrophile with an allylic leaving group has proven to be widely applicable and versatile in synthesis.
A typical approach to the synthesis of organoboron compounds by copper-catalyzed allylic substitution is the installation of the C−B bond using a diboron as the nucleophile, to yield α-chiral allylboron compounds. Recently, the use of 1,1-diborylalkanes as the organometallic component to yield organoboron products by SN2′-selective substitution was reported by the Cho, Hoveyda, and Xiao group, respectively. However, regioand enantioselective substitution was only reported with 1,1- diborylmethane, which afforded β-chiral (from B), primary organoboron compounds. All of these methods discard one B moiety from the diboron starting material for activation and depend on the use of stereochemically well-defined prochiral allylic electrophiles to create stereogenic centers during the addition.
Herein they report a highly enantioselective synthesis of alkyl organoboron compounds starting from easily preparable alkenylboron compounds, by an efficient addition of B-α-chiral organocopper species to allyl phosphates. The keys to successful transformation are (1) highly regio- and enantioselective hydrocupration to borylalkenes and (2) subsequent asymmetric C−C bond formation with high efficiency.
They believe this protocol provides easy access to chiral secondary homoallylic alkylboron compounds with high enantioselectivity; and efforts to expand the use of B-α-chiral organocopper species in catalytic bond formations are currently underway.
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