Abstract
We report the first enantioselective Rh-catalyzed Markovnikov hydroboration of unactivated terminal alkenes. Using a novel sp2–sp3 hybridized diboron reagent and water as a proton source, a broad range of alkenes undergo hydroboration to provide secondary boronic esters with high regio- and enantiocontrol.
Highlights
Brown asymmetric hydroboration of alkenes, reported in 1961, holds an important position in the history of asymmetric synthesis as the first example of a chemical transformation in which high enantioselectivity was conferred by a small molecule.[1]
A general catalytic asymmetric method for the generation of secondary alkylboronic esters from the abundant feedstock of aliphatic terminal alkenes[13] remains an unmet challenge, which is addressed in this paper
Introduction of a proton source was not sufficient to favor a protodemetalation pathway: addition of isopropyl alcohol to the standard Nishiyama diboration conditions with 4-phenyl-1-butene (2a) as substrate did not lead to the desired hydroboration product and diboration product 5 was formed exclusively (86%, 98:2 er, Table 1, Entry 1)
Summary
AReactions conducted with 0.38 mmol 2a. bYields determined by GC analysis by using biphenyl as an internal standard; yields of isolated product in parentheses. cThe branched/linear ratio (rr) was determined by GC analysis of the crude reaction mixture. dDetermined by chiral SFC analysis following oxidation of 3a. e5 mol % NaOt-Bu was used as an additive; reaction conducted at 60 °C; diboration product 5 was isolated in 86% yield, 98:2 er. fReaction conditions: 5 mol % catalyst 1a, 1.5 equiv boron source 4, 6 equiv proton source, 1 M concentration, 40 °C, 16 h. gDiboration product 5 was isolated in 6% yield, 81:19 er. Mechanistic Studies aReactions were conducted with 0.38 mmol of 2 Quoted yields are those of isolated product and are based on an average of values obtained from two experiments. Of the alkene into the rhodium−boron bond is the first irreversible step of the catalytic cycle, followed by rapid protodemetalation, or alternatively, reversible migratory insertion occurs before a rate-determining binding of a water molecule to the rhodium center, followed by rapid intramolecular protodemetalation. To differentiate between these two pathways, the reaction was conducted under the standard reaction conditions using a 1:1 mixture of H2O and D2O (Scheme 4D).
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