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Phosphonate-Directed Catalytic Asymmetric Hydroboration: Delivery of Boron to the More Substituted Carbon, Leading to Chiral Tertiary Benzylic Boronic Esters
journal contribution
posted on 2018-10-03, 00:00 authored by Suman Chakrabarty, James M. TakacsPhosphonate-directed
catalytic asymmetric hydroboration (CAHB)
of β-aryl/heteroaryl methylidenes and trisubstituted alkenes
by pinacolborane enables facile access to functionalized, chiral tertiary
benzylic boronic esters. Hydroboration is catalyzed by a chiral rhodium
catalyst prepared in situ from a Rh(I) precursor in combination with
a simple TADDOL-derived chiral cyclic monophosphite in a 1:1 ratio.
The regio- and stereochemistry arise from the combined effects of
the relative disposition of the directing group to the alkene, the
alkene substitution pattern, and the necessity of an aryl substituent
attached to the alkene. A range of aryl and heteroaryl substituents
can be accommodated, and for several chiral substrates, the reactions
are efficiently catalyst-controlled, enabling the choice of diastereomeric
products as desired. Stereospecific transformations of the chiral
boronic ester afford chiral phosphonates bearing a quaternary carbon
stereocenter. The synthetic utility of the products is further demonstrated
by α-oxidation of the phosphonate, leading to hydroxy- and oxophosphonates;
the latter readily undergo elimination/substitution reactions to unmask
the phosphonate functionality with the formation of aldehydes, alcohols,
esters, amides, acids, and ketones.
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diastereomeric productsquaternary carbon stereocenterSubstituted CarbonChiral Tertiary Benzylic Boronic Esters Phosphonate-directedbenzylic boronic estersCAHBchiral phosphonatesα- oxidationchiral substratesheteroaryl substituentschiral boronic estertrisubstituted alkenesaryl substituentStereospecific transformationsPhosphonate-Directed Catalytic Asymmetric Hydroborationchiral rhodium catalystTADDOL-derived chiral cyclic monophosphitephosphonate functionalityalkene substitution pattern
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