Go with the flow: A simple flow reactor has been developed to accommodate highly absorbing [RuL3]2+ photosensitizers for light‐starved photoredox reactions (see picture). The use of vessels having a thin diameter increases the efficiency of the reaction. This methodology has been applied to the divergent synthesis of C‐glycoconjugates.
Reaction of biphepPt(CO 3 ) (biphep ) 2,2′-bis(diphenylphosphino)-1,1′-biphenyl) with BINOL or HN(Tf)CHPhCHPhOH (TfNO) yielded square-planar biphepPtX 2 (X 2 ) BINOL, N(Tf)CHPhCHPhO) complexes as a mixture of diastereomers (∼1:1). BiphepPtCl 2 also reacted with Na 2 BINOL to generate biphepPt(BINOL) as a 1:1 mixture of diastereomers. With racemic BINOL or TfNO ligands, the mixtures were prone to isomerize to thermodynamic diastereomer mixtures (BINOL, 95:5; TfNO, >97:3) by an X 2 -X 2 ligand-ligand exchange mechanism that was rapid at room temperature. With enantiopure ligands the X 2 -X 2 ligandligand exchange process was degenerate and nonproductive. However, thermolysis of 1:1 mixtures of enantiopure biphepPt(BINOL) diastereomers (92-122 °C) cleanly established thermodynamic equilibrium by a process that involves biphenyl atropisomerism (∆H q ) 27(2) kcal mol -1 , ∆S q ) -5(5) eu). Two mechanisms for this process were considered, concerted stereoinversion via a planar seven-membered metallacycle, and one-arm-off prior to a biphenyl isomerization (anti disposed PPh 2 units). In pyridine, a third mechanism for atropisomerism was identified and proposed to involve a five-coordinate pyridine intermediate (not observed) with an enhanced phosphine dissociation rate. Pyridine lowered the isomerization temperature of enantiopure complexes by ∼50 °C. X-ray structures of the thermodynamically favored biphepPt(TfNO) ((()-4a) and the thermodynamically less favored biphepPt(BINOL) (λ(S)-5b) diastereomers were obtained, and a stereochemical model to explain the diastereoselectivity was formulated.
A high-yielding fluorination of (triphos)Pt-R+ has been achieved using an array of F+ sources, with XeF2 yielding R–F in minutes. The C–F coupling proved to be a stereoretentive process that proceeds via a concerted reductive elimination from a putative dicationic Pt(IV) center. The larger the steric congestion of the (triphos)Pt–Csp3+ complexes, the more efficient the fluorination, seemingly a result of sterically accelerated C–F reductive elimination along with simultaneous deceleration of its competing processes (β-H elimination).
The conversion of readily available cellulosic biomass to valuable feedstocks and fuels is an attrative goal but a challenging transformation that requires the cleavage of multiple nonactivated CO bonds. Herein, the Lewis acid trispentafluorophenylborane (B(C6 F5 )3 ) is shown to catalyze the metal-free hydrosilylative reduction of monosaccharides and polysaccharides to give hydrocarbons with reduced oxygen content. The choice of the silane reductant influences the degree of deoxygenation, with diethylsilane effecting the complete reduction to produce hexanes while tertiary silanes give partially deoxygenated tetraol and triol products.
The synthesis of transition metal active sites by the molecular imprinting of polymerizable metal complexes into highly cross-linked organic polymers is described. The emphasis of this Account is on the synergy between the long-term goals associated with new catalyst development and the more short-term goal of addressing fundamental questions in coordination chemistry, particularly emphasizing stereochemistry and structure. An argument is presented that the latter is necessary for ultimately achieving the more difficult but more important goal of designer catalysts that achieve reaction selectivities and reactivities not obtainable with traditional homo- or heterogeneous catalysts.
In light of diminishing petroleum feedstocks, there is significant interest in developing carbohydrate defunctionalization reactions. In this context we have examined the use of iridium pincer catalysts for the hydrosilylative reduction of sugars, and we report herein complete reduction of silyl-protected glucose to a mixture of hexane isomers.
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