RESUMEN
Highly diastereo- and enantioselective, noncovalent, substrate-directable Heck desymmetrizations of cyclopentenyl olefins containing hydroxymethyl and carboxylate functional groups are presented. These conformationally unbiased cyclic olefins underwent effective arylations in yields of up to 97 %, diastereoselectivity up to >20:1, and enantiomeric excesses of up to 99 %. Noncovalent directing effects were shown to be prevalent in both Heck-Matsuda and oxidative Heck reactions, allowing the preferential formation of cis-substituted aryl cyclopentenes containing two stereocenters, including quaternary stereocenters. These results further validate the internal out-of-coordination-sphere ion-dipole interaction concept directing the reaction diastereoselectivity to the cis-Heck product. This approach is complementary to existing methods using bis-phosphine monoxide ligands to give the opposite trans-diastereoisomer. The applicability of the method is showcased by the straightforward synthesis of a potent phosphodiesteraseâ 4 inhibitor in a diastereo- and enantioselective manner. The reaction is operationally simple and has broad scope with regard to the nature of the arenediazonium salt and boronic acid employed. The mechanism and origin of stereoselectivity were investigated with control experiments and DFT calculations that fully supported the stabilizing internal out-of-coordination-sphere ion-dipole interaction between the resident functional group and cationic palladium.
RESUMEN
The regioselectivity of the NHC-Pd catalyzed Heck coupling reaction between phenyl bromide and styrene has been investigated using the density functional theory, wave-function (WF)-based methods and two different sizes of model ligands. In addition to the WF methods, the TPSS-D3, ω B97X-D, BP86-D3, and M06-L density functionals were reliable approaches to be applied, independently of the basis set. Moreover, the NCI analysis showed that weak interactions are important forces to be taken into account when exploring the regioselectivity of this reaction, mainly when a crowded NHC ligand is present. © 2017 Wiley Periodicals, Inc.
RESUMEN
New computationally driven protocols for the Heck desymmetrization of 3-cyclopenten-1-ol with aryldiazonium tetrafluoroborates were developed. These new conditions furnished remarkable product selectivity originating from a resident hydroxyl group and the critical choice of the reaction solvent. Mechanistic insights gleaned from theoretical calculations of the putative transition states predicted toluene as an adequate solvent choice to attain high enantioselectivity by strengthening the noncovalent interaction of the substrate hydroxyl group and the chiral cationic palladium catalyst. Laboratory experiments validated the theoretical predictions, and by employing 2% MeOH/toluene as solvent, the Heck-Matsuda reaction provided exclusively the cis-4-arylcyclopentenols 3a-l in good to excellent yields in enantiomeric excesses up to 99%. The performance of the new PyOx ligand (S)-4-tert-butyl-2-(3,5-dichloropyridin-2-yl)-4,5-dihydrooxazole was also successfully evaluated in the Heck-Matsuda desymmetrization of 3-cyclopenten-1-ol. The synthetic potential of these highly functionalized cis-4-arylcyclopentenols is illustrated by a gold-catalyzed synthesis of cyclopenta[b]benzofuran skeletons.
RESUMEN
In the absence of strong ancillary ligands such as phosphines or N-heterocyclic carbenes, palladium salts are selective for C-OTf cleavage in the room-temperature Suzuki couplings of chloroaryl triflates in acetonitrile. Similar "ligand-free" conditions in DMSO also promote triflate-selective Suzuki coupling of bromoaryl triflates. This triflate selectivity complements the typical preference for reaction of bromides in prior reports of Suzuki couplings using phosphine ligands. DFT calculations and additional experimental evidence are consistent with triflate-selective oxidative addition taking place at homogeneous, possibly mononuclear, anionic palladium supported by a solvent molecule.
RESUMEN
The ground and the low lying electronic states of structures on the 1,3[H, C, As] potential energy surfaces were investigated with the highly correlated theoretical approaches CCSD(T), CCSD(T)-F12b, and CASSCF/MRCI along with the series of correlation consistent (aug-cc-pVnZ, nâ¯=â¯D, T, Q, 5) basis sets. Energetic and spectroscopic parameters were obtained at the complete basis set limit, and the effect of core-valence correlation on these properties evaluated. Fundamental frequencies were also computed with the variational configuration interaction (VCI) approach. Heats of formation at 0 and 298.15â¯K were estimated for HCAs and CH, AsH, CAs, and HCAs, as well as the calculation of ionization potentials for HCAs. Comparisons of the present results with literature ones for the systems HCN/HNC, HCP/HPC highlight similarities and differences among these systems. Altogether, this investigation provides a very reliable characterization of the species on the surfaces and should guide future experimental studies on these systems.
RESUMEN
A highly efficient nickel catalysed method for the direct C2-arylation of N-heterocyclic carbenes (NHCs) is reported. This protocol enables a facile access to C2-arylated imidazolium salts (NHC-Ar)X (X = Cl, Br, I or OTf). Experimental and theoretical studies suggest the viability of a Ni(i)/Ni(ii) catalytic pathway in which the dinuclear Ni(i) species [(NHC)NiX]2 plays a key role.