Enantiocontrol by assembled attractive interactions in copper-catalyzed asymmetric direct alkynylation of α-ketoesters with terminal alkynes: OH···O/sp-CH···O two-point hydrogen bonding combined with dispersive attractions.

Copper-catalyzed asymmetric direct alkynylation of α-ketoesters with terminal alkynes with chiral prolinol-phosphine ligands, most preferably (α,2)-1-(2-dicyclohexylphosphinobenzyl)-α-neopentyl-2-pyrrolidinemethanol, afforded various enantioenriched chiral propargylic tertiary alcohols. Quantum-chemical calculations using the BP86 density functional including Grimme's empirical dispersion correction [DF-BP86-D3(BJ)-PCM(BuOH)/TZVPP//DF-BP86-D3(BJ)/SVP] show the occurrence of OH···O/sp-CH···O two-point hydrogen bonding between the chiral ligand and the carbonyl group of the ketoester in the stereo-determining transition states. Combined with the hydrogen-bonding interactions orienting the ketoester substrate, dispersive attractions between the chiral ligand (-cyclohexyl groups) and the ketoester in the favored transition states, rather than steric repulsions in the disfavored transition state explain the enantioselectivity of the asymmetric copper catalysis.

Combined Theoretical and Experimental Studies of Nickel-Catalyzed Cross-Coupling of Methoxyarenes with Arylboronic Esters via C-O Bond Cleavage.

Nickel(0)-catalyzed cross-coupling of methoxyarenes through C-O bond activation has been the subject of considerable research because of their favorable features compared with those of the cross-coupling of aryl halides, such as atom economy and efficiency. In 2008, we have reported nickel/PCy-catalyzed cross-coupling of methoxyarenes with arylboronic esters in which the addition of a stoichiometric base such as CsF is essential for the reaction to proceed. Recently, we have also found that the scope of the substrate in the Suzuki-Miyaura-type cross-coupling of methoxyarenes can be greatly expanded by using 1,3-dicyclohexylimidazol-2-ylidene (ICy) as the ligand.

Asymmetric Synthesis of β-Lactams through Copper-Catalyzed Alkyne-Nitrone Coupling with a Prolinol-Phosphine Chiral Ligand.

Prolinol-phosphine chiral ligands enabled highly enantioselective copper-catalyzed intermolecular alkyne-nitrone coupling (Kinugasa reaction) to produce 1,3,4-trisubstituted chiral β-lactams. A high level of enantiocontrol was achieved not only with aryl- or alkenylacetylenes but also with alkylacetylenes, which were important but unfavorable substrates in the previously reported protocols. Two-point hydrogen bonding between the chiral ligand and the nitrone oxyanion consisting of O-H⋅⋅⋅O and C(sp )-H⋅⋅⋅O hydrogen bonds is proposed.

Cobalt catalysis in the gas phase: experimental characterization of cobalt(I) complexes as intermediates in regioselective Diels-Alder reactions.

In situ-formed cobalt(I) complexes are proposed to act as efficient catalysts in regioselective Diels-Alder reactions of unactivated substrates such as 1,3-dienes and alkynes. We report the first experimental evidence for the in situ reduction of CoBr2(dppe) [dppe = 1,2-bis(diphenylphosphino)ethane] by Zn/ZnI2 to [Co(I)(dppe)](+) by means of electrospray MS(n) experiments. Additionally, the reactivities of Co(II) and Co(I) dppe complexes toward the Diels-Alder substrates isoprene and phenylacetylene were probed in gas-phase ion/molecule reactions (IMRs).

A singlet biradicaloid zinc compound and its nonradical counterpart.

Metal ions with radical centers in their coordination sphere are key participants in biological and catalytic processes. In the present study, we describe the synthesis of the cAAC:ZnCl2 adduct (1) using a cyclic alkylaminocarbene (cAAC) as donor ligand. Compound 1 was treated with 2 equiv of KC8 and LiB(sec-Bu)3H to yield a deep blue-colored dicarbene zinc compound (cAAC)2Zn (2) and the colorless hydrogenated zinc compound (cAACH)2Zn (3), respectively.

Phantom ring-closing condensation polymerization: towards antibacterial oligoguanidines.

The first example of phantom ring-closing condensation polymerization for the synthesis of oligoguanidines is presented. A new oligoguanidine with a ring structure was achieved in one step by the condensation reaction of a triamine, like diethylenetriamine, with guanidine hydrochloride. The condensation reaction proceeded by selective ring-closure towards the formation of five-membered rings in the oligomer backbone.

Unusual mechanism for H3(+) formation from ethane as obtained by femtosecond laser pulse ionization and quantum chemical calculations.

The formation of H(3)(+) from saturated hydrocarbon molecules represents a prototype of a complex chemical process, involving the breaking and the making of chemical bonds. We present a combined theoretical and experimental investigation providing for the first time an understanding of the mechanism of H(3)(+) formation at the molecular level. The experimental approach involves femtosecond laser pulse ionization of ethane leading to H(3)(+) ions with kinetic energies on the order of 4 to 6.

Polymer-bound 4-methylcoumarin/1-heptanoyl-5-fluorouracil photodimers: NMR elucidation of dimer structure.

Heterodimers based on the polymer-bound chromophore 4-methylcoumarin and the prodrug 1-heptanoyl-5-fluorouracil, synthesized by photochemical [2 + 2]-cycloaddition are promising photoresponsive drug depots. Drug release experiments are one possibility to deliver proof of a successful reversible drug immobilization, whereas NMR spectroscopy is a potent tool for further structural characterization of these polymer-bound heterodimers. In case of the random copolymer poly(methyl methacrylate-co-7-(2'-methacryloyloxyethoxy)-4-methylcoumarin) three dimers have been identified of which the syn head-to-tail was the predominant one.