The stereoselective oxidation of hydrocarbons represents one of the most
significant advances in synthetic chemistry over the last fifty years1–3. Inspired by nature, chemists have developed
enantioselective dihydroxylations, epoxidations, and other oxidations of
unsaturated hydrocarbons. More recently, the catalytic enantioselective allylic
C–H oxidation of alkenes has emerged as a powerful chemical strategy,
streamlining the production of pharmaceuticals, natural products, fine chemicals
and other functional materials4–7. Allylic
functionalization provides a direct path to chiral synthons with a newly formed
stereocenter from petrochemical feedstocks while preserving the olefin
functionality as a handle for further elaboration. Various metal-based catalysts
have been discovered for the enantioselective allylic C–H oxidation of
simple alkenes with cyclic or terminal double bonds8–16. However, a general and selective allylic oxidation
remains elusive with the more common internal alkenes. Here, we report the
enantioselective, regioselective, and E/Z selective allylic
oxidation of unactivated internal alkenes via a catalytic asymmetric hetero-ene
reaction with a chalcogen-based oxidant. This method represents the first
example of selectively converting unsymmetrical internal alkenes into allylic
functionalized products with high stereoselectivity and regioselectivity.
Stereospecific transformations of the multifunctional allylic oxidation products
highlight the potential for rapidly converting internal alkenes into a broad
range of enantioenriched structures that can be utilized in the synthesis of
complex target molecules.
The enantioselective formation of stereocenters proximal to unprotected heterocycles has been accomplished. Thus, vinyl boronic acids are added to heterocycle-appended enones via a modified-BINOL catalyst. Catalyst design was key to enable a general reaction. High yields and useful er's are observed for a host of common heteroaryls.
A hydrazone-based carbene/alkyne cascade produced a variety of bridged and fused polycyclic products. NaOSiMe3 is a superior base for conversion of hydrazones to diazoalkanes. A key mechanistic intermediate, a ring-fused cyclopropene, has been isolated and characterized.
Alumina-supported platinum catalysts, both with and without ceria, were prepared by supercritical fluid deposition and evaluated for activity for water-gas shift reaction. The organometallic precursor, platinum(II) acetylacetonate, was deposited from solution in supercritical carbon dioxide. Analysis of the catalysts by high resolution scanning transmission electron microscopy indicated that platinum was present in the form of highly dispersed metal nanoparticles. Pretreatment of the alumina-supported ceria in hydrogen prior to the deposition of the platinum precursor resulted in more platinum nucleated on ceria than non-pretreated alumina-supported ceria but varied in both particle size and structure. The ceria-containing catalyst that was not pretreated exhibited a more uniform particle size, and the Pt particles were encapsulated in crystalline ceria. Reaction rate measurements showed that the catalyst was more active for water-gas shift, with reaction rates per mass of platinum that exceeded most literature values for water-gas shift reaction on Pt-CeO x catalysts. The high activity was attributed to the significant fraction of platinum/ceria interfacial contact. These results show the promise of supercritical fluid deposition as a scalable means of synthesizing highly active supported metal catalysts that offer efficient utilization of precious metals.
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