RESUMEN
C-C bond forming reaction of ketone with aldehyde is well-studied for the synthesis of α, ß-unsaturated ketones, however, the reaction with two different ketones to unsaturated carbonyl compound has not yet been systematically studied. Probably due to the relatively low reactivity of ketones as electrophiles (aldol acceptors), its propensity for retro-aldol reaction. The reactions often suffer from unsatisfactory chemoselectivity (self- vs. crossed aldol products) and regioselectivity (thermodynamic vs. kinetic enolate). In this quest, we report here for the first time selective cross-coupling reaction of ketones to ß-branched ß, γ-unsaturated ketones by using ruthenium catalysis. Interestingly, single crossed aldol condensation products are formed even in reactions where a mixture of products is possible. Reaction is highly chemoselective, regioselective and produces H2 O as the only byproducts making the protocol environmentally benign. Method is compatible with a wide variety of sensitive functional group and applicable for even problematic aliphatic ketones as substrates. Notably, acetone was found as a three-carbon feedstock for the syntheses of simple ß, γ-unsaturated ketone compounds. The process can further be extended to the gram-scale reaction and late-stage functionalization of natural products. With the help of DFT calculations, several control experiments, and deuterium-labeling experiments, the mechanistic finding demonstrated that initial aldol-condensation of ketones to a ß, ß-disubstituted α, ß-unsaturated ketone, which further isomerizes to a ß, γ- unsaturated ketone via η3 -allyl ruthenium complex.
RESUMEN
The cycloisomerization of alkyne-tethered N-benzoyloxycarbamates to 2-(3H)oxazolones is described. Two catalytic systems are tailored for intramolecular 5-exo-alkyne carboxyamidation and concomitant alkene isomerization. PtCl2 /CO (5â mol%, toluene, 100 °C) promotes both carboxyamidation and alkene isomerization but has a limited substrate scope. On the other hand, FeCl3 (5â mol%, CH3 CN, 100 °C) promotes carboxyamidation effectively but a cocatalyst is required for the exocyclic alkene isomerization. Thus, a two-step one-pot protocol has been developed for a broader reaction scope, which involves FeCl3 -catalyzed carboxyamidation and base-induced alkene isomerization. Crossover experiments suggest that these reactions proceed mainly through a mechanism involving acylnitrenoid intermediates rather than carbenoid intermediates.
RESUMEN
We report herein the synthesis and characterization of nickel complexes supported by tridentate and bidentate phosphino(silyl) ancillary ligands, along with the successful application of these complexes as precatalysts for the hydroboration of terminal and internal alkenes using pinacolborane (HBPin). These reactions proceeded with low nickel loadings of 2.5-5â mol % in the absence of co-solvent, and in some cases at room temperature. Isomerization to afford exclusively the terminal hydroboration product was obtained across a range of internal alkenes, including tri- and tetra-substituted examples. This reactivity is unprecedented for nickel and offers a powerful means of achieving functionalization at a C-H position remote from the C=C double bond. Nickel-catalyzed deuteroboration experiments using DBPin support a mechanism involving 1,2-insertion of the alkene and subsequent chain-walking, which results in isotopic scrambling.
RESUMEN
Preparation of skipped dienes with a quaternary carbon center at the C-3 position remains a synthetic challenge. We report here an iridium-catalyzed formal addition of tertiary sp3 C-H bond to alkyne for the facile preparation of skipped dienes. The tertiary allylic C-H bond is cleaved regioselectively, at the site of which a new C-C bond is formed. Enantioselective construction of acyclic quaternary carbon stereocenters is also demonstrated.
Asunto(s)
Alquinos , Iridio , Carbono/química , Catálisis , Iridio/química , Estructura Molecular , Polienos , EstereoisomerismoRESUMEN
The [3,3]-sigmatropic rearrangement of allylic vinyl acetals, first investigated by Coates nearly four decades ago, is set apart from other variants of the Claisen rearrangement owing to the versatile monoprotected 1,5-dicarbonyl motif featured in the products. Unfortunately, the synthetically elusive nature of the substrates has thus far precluded the widespread application of this attractive transformation. Herein, we show that the key allylic vinyl acetals can be efficiently generated through alkene isomerization of their readily available regioisomeric counterparts (derived from allylic alcohols and α,ß-unsaturated aldehydes), thus enabling the first systematic study of the substrate scope of this rearrangement, as well as the discovery of exceptionally mild conditions for its mediation by Lewis and Brønsted acids.
RESUMEN
A new transformation pattern for enantioselective intramolecular hydroacylation has been developed involving an alkene isomerization strategy. Proceeding through a five-membered rhodacycle intermediate, 3-enals were converted to C3 - or C3 ,C5 -chirogenic cyclopentanones with satisfactory yields, diastereoselectivities, and enantioselectivities. A catalytic cycle has been theoretically calculated and the origin of the stereoselection is rationally explained.
RESUMEN
An in situ generated cationic Ir-catalyst isomerizes simple allylic silyl ethers into valuable, fully substituted aldehyde-derived silyl enol ethers. Importantly, by judicious choice of substrate, either of the two possible stereoisomers of a given enolate derivative is accessible with complete stereoselectivity. One-pot isomerization-aldol and isomerization-allylation processes illustrate the synthetic utility of this method.
RESUMEN
We report herein that thermodynamic and kinetic isomerization of alkenes can be accomplished by the combination of visible light with Co catalysis. Utilizing Xantphos as the ligand, the most stable isomers are obtained, while isomerizing terminal alkenes over one position can be selectively controlled by using DPEphos as the ligand. The presence of the donor-acceptor dye 4CzIPN accelerates the reaction further. Transformation of exocyclic alkenes into the corresponding endocyclic products could be efficiently realized by using 4CzIPN and Co(acac)2 in the absence of any additional ligands. Spectroscopic and spectroelectrochemical investigations indicate CoI being involved in the generation of a Co hydride, which subsequently adds to alkenes initiating the isomerization.
RESUMEN
Herein we establish the utility of a three-coordinate (N-phosphinoamidinate)cobalt(amido) pre-catalyst that is capable of effecting challenging alkene isomerization/hydroboration processes at room temperature, leading to the selective terminal addition of the boron group.
RESUMEN
A series of thirteen 4-arylbut-3-ene-2-amines were prepared and subjected to photosensitization experiments to interrogate their photostationary state (PS) composition of geometrical olefin isomers (E and Z). The amine PS compositions were found to depend on arene structure and temperature, while being largely independent of nitrogen substitution, solvent, or presence of triplet-quenching oxygen. Photonic efficiency of isomerization (ζp) was found to depend on amine structure, solvent choice, and presence of quencher. With the proper choice of conditions, ζp was able to closely approach the theoretical maximum value of 0.5.
RESUMEN
Pd(II)-catalyzed E/Z isomerization of alkenes is a common process-yet its mechanism remains largely uncharacterized, particularly with non-conjugated alkenes. In this work, the mechanism of Pd(II)-catalyzed E/Z isomerization of unactivated olefins containing an aminoquinoline-based amide directing group is probed using in situ kinetic analysis, spectroscopic studies, kinetic modeling, and DFT calculations. The directing group allows for stabilization and monitoring of previously undetectable intermediates. Collectively, the data are consistent with isomerization occurring through a monometallic nucleopalladation mechanism.