K. Peter C. Vollhardt

On the Nature of Nonplanarity in the [N]Phenylenes

The unusual solid-state nonplanarity seen in the X-ray structures of the [N]phenylenes (an example is shown here) is shown to be most likely the result of crystal-packing-induced deformations. These conclusions are based on a study of published and eight new structure determinations, comparison with data on napththalenes and anthracenes obtained from the Cambridge Structural Database, and ab initio calculations.

Tandem palladium-, cobalt-, and nickel-catalyzed syntheses of polycyclic π-systems containing cyclobutadiene, benzene, and cyclooctatetraene rings

Abstract A sequence is described in which Co I serves as a catalyst in the co-cyclization of 1,2-diethynylarenes (in turn derived by Pd II -catalyzed alkynylation of the corresponding haloaromatics) with alkynes to furnish substituted biphenylenes and Ni 0 is used to dimerize the latter to the corresponding tetrabenzocyclooctatetraenes. The strategy provides a novel synthetic entry into novel polycyclic π-systems of synthetic and theoretical interest.

Mechanism of Thermal Reversal of the (Fulvalene)tetracarbonyldiruthenium Photoisomerization: Toward Molecular Solar–Thermal Energy Storage

In the currently intensifying quest to harness solar energy for the powering of our planet, most efforts are centered around photoinduced generic charge separation, such as in photovoltaics, water splitting, other small molecule activation, and biologically inspired photosynthetic systems. In contrast, direct collection of heat from sunlight has received much less diversified attention, its bulk devoted to the development of concentrating solar thermal power plants, in which mirrors are used to focus the sun beam on an appropriate heat transfer material. An attractive alternative strategy would be to trap solar energy in the form of chemical bonds, ideally through the photoconversion of a suitable molecule to a higher energy isomer, which, in turn, would release the stored energy by thermal reversal. Such a system would encompass the essential elements of a rechargeable heat battery, with its inherent advantages of storage, transportability, and use on demand. The underlying concept has been explored extensively with organic molecules (such as the norbornadiene-quadricyclane cycle), often in the context of developing photoswitches. On the other hand, organometallic complexes have remained relatively obscure in this capacity, despite a number of advantages, including expanded structural tunability and generally favorable electronic absorption regimes. A highly promising organometallic system is the previously reported, robust photo-thermal fulvalene (Fv) diruthenium couple 1 {l_reversible} 2 (Scheme 1). However, although reversible and moderately efficient, lack of a full, detailed atom-scale understanding of its key conversion and storage mechanisms have limited our ability to improve on its performance or identify optimal variants, such as substituents on the Fv, ligands other than CO, and alternative metals. Here we present a theoretical investigation, in conjunction with corroborating experiments, of the mechanism for the heat releasing step of 2 {yields} 1 and its Fe (4) and Os (6) relatives. The results of the combined study has enabled a rigorous interpretation of earlier and new experimental measurements and paint a surprising picture. First-principles calculations were employed based on spin unrestricted density functional theory (DFT) with a non-empirical gradient corrected exchange-correlation functional. Ultrasoft pseudopotentials were used to describe the valence-core interactions of electrons, including scalar relativistic effects of the core. Wavefunctions and charge densities were expanded in plane waves with kinetic energies up to 25 and 200 Rydberg, respectively. Reaction pathways were delineated with the string method, as implemented within the Car-Parrinello approach. This method allows for the efficient determination of the minimum energy path (MEP) of atomistic transitions and thus also saddle points (transition states, TSs), which are the energy maxima along the MEP. All geometries were optimized until all forces on the atoms were less than 0.02 eV/{angstrom}. The calculated structures of 1 and 2 were in good agreement with their experimental counterparts.

Auxiliary silicon in regioselective cobalt catalyzed protoberberine syntheses

Abstract η 5 -Cyclopentadienyl dicarbonyl cobalt catalyzes the cocyclization of 1,2-bis(propargyl)-1,2,3,4-tetrahydroisoquinolines 4b and 5 with alkynes to provide a novel synthetic entry into the tetrahydroprotoberberine nucleus. By judicious choice of trimethylsilyl substituents, regiocontrol in the D-ring can be achieved. Reaction of 4b with benzonitrile in the presence of the catalyst furnishes the rare isoquino[2, l- b ]-2,6-naphthyridine framework, also regioselectively.

X‐ray Transient Absorption and Picosecond IR Spectroscopy of Fulvalene(tetracarbonyl)diruthenium on Photoexcitation

Caught in the light: The fulvalene diruthenium complex shown on the left side of the picture captures sun light, causing initial Ru-Ru bond rupture to furnish a long-lived triplet biradical of syn configuration. This species requires thermal activation to reach a crossing point (middle) into the singlet manifold on route to its thermal storage isomer on the right through the anti biradical.

Practical synthesis of two annelated optically active cyclopentadienes from the chiral pool and their transition metal complexes

Abstract Starting from camphor or tartaric acid the two optically active annelated cyclopentadienes 3 and 5 are readily synthesized and complexed to transition metals.

Catalytic co-cyclisation of α,ω-cyanoalkynes with alkynes: a versatile chemo- and regio-selective synthesis of 2,3-substituted 5,6,7,8-tetrahydroquinolines and other cycloalka[1,2-b]pyridines

α,ω-Cyanoalkynes are catalytically cocylised with alkynes in the presence of dicarbonyl(cyclopentadiecyl)-cobalt to furnish [b]annuiated pyridines chemo- and regio-selectively.

Oligomerization of alkynes by the RhCl3-aliquat 336 catalyst system: Part 1. Formation of benzene derivatives

Abstract The cyclo-oligomerization of several terminal and internal alkynes under phase transfer conditions by the RhCl 3 -Aliquat® 336 catalyst is described. The kinetics of 1-heptyne cyclotrimerization at 90 °C in 1,1,2,2-tetrachloroethane/water were found to follow the second-order rate law d[arene]/d t = − k [alkyne][RhCl 3 ] when the molar ratio of substrate:rhodium chloride:quat was approximately 25:1:1. The activation energy of E a = 12.5 kcal mol −1 suggests that the catalysis is both chemically and diffusion controlled. Product analysis is compatible with a mechanism that involves rhodacyclopentadiene rather than metal cyclobutadiene intermediates.

Benzocyclynes adhere to Huckel's rule by the ring current criterion in experiment (1H NMR) and theory (NICS)

Abstract The experimental 1H NMR chemical shift values of six benzocyclynes (benzodehydroannulenes) have been assigned and, in combination with nucleus-independent chemical shift (NICS) calculations, shown to indicate diatropic behavior for 4n+2, paratropic behavior for 4n π systems.

Silicon-Hydrogen Bond Activation and Hydrosilylation of Alkenes Mediated by CpCo Complexes: A Theoretical Study

Using DFT techniques, we show that triplet cyclopentadienylcobalt activates Si-H bonds to generate singlet silylcobalt hydrides without the intervention of sigma-silanes. The cobalt is configurationally unstable, as evidenced by the diastereoisomerization of derivatives bearing chiral silyl ligands. Inversion at the metal proceeds in the singlet state via a bridging hydride. We demonstrate that a two-state mechanism for the transformation of silyl hydride cobalt complexes into disilyl dihydride cobalt species is feasible. Our calculations predict that catalytic hydrosilylation of alkenes should be achievable in the coordination sphere of cyclopentadienylcobalt.