157 research outputs found
Organocatalytic Lewis base functionalisation of carboxylic acids, esters and anhydrides via C1-ammonium or azolium enolates
This tutorial review highlights the organocatalytic Lewis base functionalisation of carboxylic acids, esters and anhydrides via C1-ammonium/azolium enolates. The generation and synthetic utility of these powerful intermediates is highlighted through their application in various methodologies including aldol-lactonisations, Michael-lactonisations/lactamisations and [2,3]-rearrangements.Publisher PDFPeer reviewe
Streptavidin-hosted organocatalytic aldol addition
In this report, the streptavidin-biotin technology was applied to enable organocatalytic aldol addition. By attaching pyrrolidine to the valeric motif of biotin and introducing it to streptavidin (Sav), a protein-based organocatalytic system was created, and the aldol addition of acetone with p-nitrobenzaldehyde was tested. The conversion of substrate to product can be as high as 93%. Although the observed enantioselectivity was only moderate (33:67 er), further protein engineering efforts can be included to improve the selectivity. These results have proven the concept that Sav can be used to host stereoselective aldol addition
Transfer hydrogenations catalyzed by streptavidin-hosted secondary amine organocatalysts
Here, the streptavidinâbiotin technology was applied to enable
organocatalytic transfer hydrogenation. By introducing a biotintethered pyrrolidine (1) to the tetrameric streptavidin (T-Sav), the
resulting hybrid catalyst was able to mediate hydride transfer from
dihydro-benzylnicotinamide (BNAH) to a,b-unsaturated aldehydes.
Hydrogenation of cinnamaldehyde and some of its aryl-substituted
analogues was found to be nearly quantitative. Kinetic measurements revealed that the T-Sav:1 assembly possesses enzyme-like
behavior, whereas isotope effect analysis, performed by QM/MM
simulations, illustrated that the step of hydride transfer is at least
partially rate-limiting. These results have proven the concept that
T-Sav can be used to host secondary amine-catalyzed transfer
hydrogenations
Recent advances in homogeneous borrowing hydrogen catalysis using earth-abundant first row transition metals
The review highlights the recent advances (2013-present) in the use of earth-abundant first row transition metals in homogeneous borrowing hydrogen catalysis. The utility of catalysts based on Mn, Fe, Co, Ni and Cu to promote a diverse array of important CâC and CâN bond forming reactions is described, including discussion on reaction mechanisms, scope and limitations, and future challenges in this burgeoning area of sustainable catalysis
Nickel-Catalyzed Intramolecular Alkene Difunctionalization by Ball-Milling
A mechanochemical nickel-catalyzed intramolecular difunctionalization reaction of alkene tethered aryl halides with alkyl halides is herein described. This method allows for synthesis of 3,3-disubstituted heterocycles, namely oxindoles, with shorter reaction times than solution-phase counterparts. Additionally, this process is solvent minimized, with DMA used in liquid-assisted grinding (LAG) quantities and circumvents the need for chemical activation of the terminal reductant (manganese) through mechanical grinding. The process can be scaled up to yield over a gram of product and modest enantioinduction is possible by utilizing a chiral PyrOx ligand. (Figure presented.)
FLP-catalyzed transfer hydrogenation of silyl enol ethers
Herein we report the first catalytic transfer hydrogenation of silyl enol ethers. This metal free approach employs tris(pentafluorophenyl)borane and 2,2,6,6âtetramethylpiperidine (TMP) as a commercially available FLP catalyst system and naturally occurring Îłâterpinene as a dihydrogen surrogate. A variety of silyl enol ethers undergo efficient hydrogenation, with the reduced products isolated in excellent yields (29 examples, 82% average yield)
Magnesiate addition/ring-expansion strategy to access the 6-7-6 tricyclic core of hetisine-type C20-diterpenoid alkaloids
A synthetic strategy to access the fused 6â7â6 tricyclic core of hetisine-type C20-diterpenoid alkaloids is reported. This strategy employs a DielsâAlder cycloaddition to assemble a fused bicyclic anhydride intermediate, which is elaborated to a vinyl lactone-acetal bearing an aromatic ring in five steps. Aromatic iodination is followed by magnesiumâhalogen exchange with a trialkyl magnesiate species, which undergoes intramolecular cyclization. Subsequent oxidation provides the desired 6â7â6 tricyclic diketoaldehyde, with carbonyl groups at all three positions for eventual CâN bond formation and subsequent elaboration
Manganese-catalyzed N-alkylation of sulfonamides using alcohols
An efficient manganese-catalyzed N-alkylation of sulfonamides has been developed. This borrowing hydrogen approach employs a well-defined and bench-stable Mn(I) PNP pincer precatalyst, allowing benzylic and simple primary aliphatic alcohols to be employed as alkylating agents. A diverse range of aryl and alkyl sulfonamides undergoes mono-N-alkylation in excellent isolated yields (32 examples, 85% average yield)
Deoxycyanamidation of alcohols with N-Cyano-N-phenyl-p-methylbenzenesulfonamide (NCTS)
The first one-pot deoxycyanamidation of alcohols has been developed using N-cyano-N-phenyl-p-methylbenzenesulfonamide (NCTS) as both a sulfonyl transfer reagent and a cyanamide source, accessing a diverse range of tertiary cyanamides in excellent isolated yields. This approach exploits the underdeveloped desulfonylative (NâS bond cleavage) reactivity pathway of NCTS, which is more commonly employed for electrophilic C- and N-cyanation processes
Synthesis and reactivity of N-allenyl cyanamides
N-Allenyl cyanamides have been accessed via a one-pot deoxycyanamidationâisomerization approach using propargyl alcohol and N-cyano-N-phenyl-p-methylbenzenesulfonamide. The utility of this novel class of allenamide was explored through derivatization, with hydroarylation, hydroamination, and cycloaddition protocols employed to access an array of cyanamide products that would be challenging to access using existing methods
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