Transition metal catalysis plays a central role in contemporary organic synthesis. Considering the tremendously broad array of transition-metal-catalyzed transformations, it is remarkable that the underlying elementary reaction steps are relatively few in number. Here we describe an alternative to the organometallic transmetallation step that is common in many metal-catalyzed reactions such as Suzuki-Miyaura coupling. Specifically, we demonstrate that vinyl boronic ester ate complexes, prepared by combining organoboronates and organolithium reagents, engage in Pd-induced metallate rearrangement wherein 1,2-migration of an alkyl or aryl group from boron to the vinyl α-carbon occurs concomitantly with C-Pd σ-bond formation. This elementary reaction enables a powerful cross-coupling reaction in which a chiral palladium catalyst merges three simple starting materials – an organolithium, an organoboronic ester, and an organotriflate – into chiral organoboronic esters with high enantioselectivity.
Under the influence
of a chiral palladium catalyst, 1,1-bis(pinacolboronate)
esters undergo asymmetric cross-coupling with bromoalkenes to generate
nonracemic allyl boronates with high levels of enantioselectivity.
The so-formed allyl boronates may be oxidized with hydrogen peroxide
to provide secondary allylic alcohols or with nitrosobenzene to furnish
nonracemic tertiary allylic alcohols. Mechanistic experiments suggest
the operation of a pathway involving outer-sphere stereoinvertive
transmetalation.
A Pt-catalyzed enantioselective hydrosilylation of alkenylboronates is described. This reaction occurs with high regio- and enantioselectivity, providing a convenient route to chiral non-racemic geminal silylboronates. These compounds are useful reagents in stereoselective synthesis.
Multicomponent reactions and heterocyclic chemistry are important concepts of organic synthesis, especially in the pharmaceutical industry. A one-pot, multicomponent Biginelli condensation reaction to synthesize dihydropyrimidine derivatives from multiple unknowns is investigated as a discovery-based experiment in a second semester, secondyear undergraduate organic chemistry laboratory course. Three 1,3-dicarbonyl compounds, two aryl aldehydes, and urea are utilized to provide six different unknown dihydropyrimidine derivatives with average yields ranging from 63− 79%. Students identify their products using 1 H NMR, 13 C NMR, DEPT, and IR spectroscopic data. The experiment provides an opportunity to discuss multicomponent reactions, carbonyl condensations with amines, enol chemistry, and interpretation of spectra, while being completed in a 3-h laboratory period.
Vinyl boronic ester ate complexes, synthesized from organoboronates and organolithium reagents, are involved in palladium‐induced metallate rearrangements wherein an 1,2‐migration of an alkyl/aryl group from boron to the vinyl α‐carbon takes place simultaneously with a C—Pd σ‐bond formation.
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