Martin C. Schwarzer

A Stable Singlet Biradicaloid Siladicarbene: (L:)2Si

Silicon, the congener of carbon, frequently shows different chemistry than that of its sister element. It has been realized that silicon prefers a positive charge if bonded to a more electronegative atom. Compounds with silicon in lower oxidation states are very important, since they can activate small organic molecules which cannot be activated by transition metals. In 2008 Robinson et al. reported on the adduct of two N-heterocyclic carbene (NHC) molecules with disilicon A (Scheme 1). This unusual compound was prepared

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. Compounds 2 and 3 were well characterized by spectroscopic methods and single-crystal X-ray structural analysis. Density functional theory calculations were performed for 2 which indicate that this molecule possesses a singlet biradicaloid character. Moreover, we show the application of 2 in CO2 activation, which yields a zwitterionic cAAC·CO2 adduct.

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). Isoprene and phenylacetylene were introduced into the mass spectrometer via the buffer gas flow of a linear ion trap. The IMR experiments revealed a significantly higher substrate affinity of [Co(I)(dppe)](+) compared with [Co(II)Br(dppe)](+). Furthermore, the central intermediate of the solution-phase cobalt-catalyzed Diels-Alder reaction, [Co(I)(dppe)(isoprene)(phenylacetylene)](+), could be generated via IMR and examined in the gas phase. Collision activation of this complex ion delivered evidence for the gas-phase reaction of isoprene with phenylacetylene in the coordination sphere of the cobalt ion. The experimental findings are consistent with the results of quantum-chemical calculations on all of the observed Co(I) dppe complex ions. The results constitute strong analytical evidence for the formation and importance of different cobalt(I) species in regioselective Diels-Alder reactions of unactivated substrates and identify [Co(I)(dppe)](+) as the active Diels-Alder catalyst.

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 6.5 eV. The theoretical approach involves high-level quantum chemical calculation of the complete reaction path. The calculations confirm that the process takes place on the potential energy surface of the ethane dication. A surprising result of the theoretical investigation is, that the transition state of the process can be formally regarded as a H(2) molecule attached to a C(2)H(4)(2+) entity but IRC calculations show that it belongs to the reaction channel yielding C(2)H(3)(+) + H(3)(+). Experimentally measured kinetic energies of the correlated H(3)(+) and C(2)H(3)(+) ions confirm the reaction path suggested by theory.

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/PCy3-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. Interestingly, a stoichiometric amount of external base is not required for the nickel/ICy-catalyzed cross-coupling. For the mechanism and origin of the effect of the external base to be elucidated, density functional theory calculations are conducted. In the nickel/PCy3-catalyzed reactions, the activation energy for the oxidative addition of the C(aryl)-OMe bond is too high to occur under the catalytic conditions. However, the oxidative addition process becomes energetically feasible when CsF and an arylboronic ester interact with a Ni(PCy3)2/methoxyarene fragment to form a quaternary complex. In the nickel/ICy-catalyzed reactions, the oxidative addition of the C(aryl)-OMe bond can proceed more easily without the aid of CsF because the nickel-ligand bonds are stronger and therefore stabilize the transition state. The subsequent transmetalation from an Ar-Ni-OMe intermediate is determined to proceed through a pathway with lower energies than those required for β-hydrogen elimination. The overall driving force of the reaction is the reductive elimination to form the carbon-carbon bond.

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. The resulting polymer repeat unit structure was different from the starting monomers (phantom polymer) and was formed by elimination of three molecules of ammonia per repeat unit. The inter-, intra-, and inter-molecular reaction sequences led to the new structure as proved by different spectroscopic techniques (atmospheric pressure chemical-ionization mass spectrometry, and one-dimensional and two-dimensional homo- and heteronuclear correlation NMR experiments) as well as supported by quantum chemical investigations. Preliminary results regarding antibacterial use of the resulting oligoguanidine were also promising and showed its effect within 15-30 min as an antibacterial material.

Exploring suitable oligoamines for phantom ring-closing condensation polymerization with guanidine hydrochloride

Different model compounds were applied in step-growth condensation reactions with guanidine hydrochloride in order to explore the scope of tri- and oligoamine applicability for phantom ring-closing condensation polymerization processes. The selective formation of five-membered rings was proven by APCI mass spectrometry, different one- and two-dimensional homo- and heteronuclear coupling NMR techniques and investigated via quantum chemical calculations. Furthermore the possibility of ring-closing reactions among guanidine hydrochloride and merely secondary amines was precluded. The obtained oligomers were exposed to antibacterial tests and exhibited a moderate activity towards Escherichia coli.

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. In contrast, only the syn head‐to‐head dimer was formed in reasonable yield when the 4‐methylcoumarin monofunctionalized pMMA was used as the base polymer. 1D and 2D NMR spectroscopic techniques combined with some theoretical calculations helped in successfully closing one major gap concerning polymer bound 4‐methylcoumarin/1‐heptanoyl‐5‐fluorouracil heterodimers that are of potential use in photoresponsive drug delivery devices. Copyright

Enantiocontrol by assembled attractive interactions in copper-catalyzed asymmetric direct alkynylation of alpha-ketoesters with terminal alkynes: OH center dot center dot center dot O/sp(3)-CH center dot center dot center dot O two-point hydrogen bonding

Title Enantiocontrol by assembled attractive interactions in copper-catalyzed asymmetric direct alkynylation of alphaketoesters with terminal alkynes: OH center dot center dot center dot O/sp(3)-CH center dot center dot center dot O two-point hydrogen bonding Author(s) Schwarzer, Martin C.; Fujioka, Akane; Ishii, Takaoki; Ohmiya, Hirohisa; Mori, Seiji; Sawamura, Masaya Citation Chemical science, 9(14): 3484-3493 Issue Date 2018-04-14 Doc URL http://hdl.handle.net/2115/70706 Rights(URL) https://creativecommons.org/licenses/by/3.0/ Type article Additional Information There are other files related to this item in HUSCAP. Check the above URL. File Information c8sc00527c.pdfA chiral copper catalyst selects enantiofaces by assembled attractive interactions.