: The unpolarized absorption and circular dichroism spectra of the fundamental vibrational transitions of the chiral molecule, 4-methyl-2-oxetanone, are calculated ab initio. Harmonic force fields are obtained using Density Functional Theory (DFT), MP2, and SCF methodologies and a 5S4P2D/3S2P (TZ2P) basis set. DFT calculations use the Local Spin Density Approximation (LSDA), BLYP, and Becke3LYP (B3LYP) density functionals. Mid-IR spectra predicted using LSDA, BLYP, and B3LYP force fields are of significantly different quality, the B3LYP force field yielding spectra in clearly superior, and overall excellent, agreement with experiment. The MP2 force field yields spectra in slightly worse agreement with experiment than the B3LYP force field. The SCF force field yields spectra in poor agreement with experiment.The basis set dependence of B3LYP force fields is also explored: the 6-31G* and TZ2P basis sets give very similar results while the 3-21G basis set yields spectra in substantially worse agreements with experiment. jg

Ab initio calculation of vibrational absorption and circular dichroism spectra using density functional force fields

This Review summarizes the advancements in Pd-catalyzed C(sp3)–H activation via various redox manifolds, including Pd(0)/Pd(II), Pd(II)/Pd(IV), and Pd(II)/Pd(0). While few examples have been reported in the activation of alkane C–H bonds, many C(sp3)–H activation/C–C and C–heteroatom bond forming reactions have been developed by the use of directing group strategies to control regioselectivity and build structural patterns for synthetic chemistry. A number of mono- and bidentate ligands have also proven to be effective for accelerating C(sp3)–H activation directed by weakly coordinating auxiliaries, which provides great opportunities to control reactivity and selectivity (including enantioselectivity) in Pd-catalyzed C–H functionalization reactions.

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Palladium-Catalyzed Transformations of Alkyl C-H Bonds.

Organic reactions that involve the direct functionalization of non-activated C–H bonds represent an attractive class of transformations which maximize atom- and step-economy, and simplify chemical synthesis. Due to the high stability of C–H bonds, these processes, however, have most often required harsh reaction conditions, which has drastically limited their use as tools for the synthesis of complex organic molecules. Following the increased understanding of mechanistic aspects of C–H activation gained over recent years, great strides have been taken to design and develop new protocols that proceed efficiently under mild conditions and duly benefit from improved functional group tolerance and selectivity. In this review, we present the current state of the art in this field and detail C–H activation transformations reported since 2011 that proceed either at or below ambient temperature, in the absence of strongly acidic or basic additives or without strong oxidants. Furthermore, by identifying and discussing the major strategies that have led to these improvements, we hope that this review will serve as a useful conceptual overview and inspire the next generation of mild C–H transformations.

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Mild metal-catalyzed C–H activation: examples and concepts

It is well-recognized that N-heterocyclic carbene (NHC) ligands have provided a new dimension to the design of homogeneous catalysts. Part of the success of this type of ligands resides in the limitless access to a variety of topologies with tuned electronic properties, but also in the ability of a family of NHCs that are able to adapt their properties to the specific requirements of individual catalytic transformations. The term “smart” is used here to refer to switchable, multifunctional, adaptable, or tunable ligands and, in general, to all those ligands that are able to modify their steric or electronic properties to fulfill the requirements of a defined catalytic reaction. The purpose of this review is to comprehensively describe all types of smart NHC ligands by focusing attention on the catalytically relevant ligand-based reactivity.

Smart N-Heterocyclic Carbene Ligands in Catalysis

Discovered in 2005, cyclic (alkyl)(amino)carbenes (CAACs) are among the most nucleophilic (σ donating) and also electrophilic (π-accepting) stable carbenes known to date. These properties allow them to activate a variety of small molecules and enthalpically strong bonds, to stabilize highly reactive main-group and transition-metal diamagnetic and paramagnetic species, and to bind strongly to metal centers, which gives rise to very robust catalysts. The most important results published up to the end of 2013 are briefly summarized, while the majority of this Review focuses on findings reported within the last three years.

Cyclic (Alkyl)(amino)carbenes (CAACs): Recent Developments

A Singlet Phosphinidene Stable at Room Temperature

Pushing Electrons—Which Carbene Ligand for Which Application?

A versatile methodology, involving readily available starting materials, allows for the synthesis of stable hemilabile bidentate cyclic (alkyl)(amino)carbenes (CAACs) featuring alkene, ether, amine, imine, and phosphine functionalities. The stability of the free carbenes has been exploited for the synthesis of copper(I) and gold(I) complexes. It is shown that the pendant imine moiety stabilizes the gold(III) oxidation state and enables the C–C bond oxidative addition of biphenylene to the corresponding cationic gold(I) complex. The latter and the corresponding copper(I) complex show high catalytic activity for the hydroarylation of α-methylstyrene with N,N-dimethylaniline, and the copper(I) complex promotes the anti-Markovnikov hydrohydrazination of phenyl acetylene with high selectivity.

Synthesis of Hemilabile Cyclic (Alkyl)(amino)carbenes (CAACs) and Applications in Organometallic Chemistry.

The application of environmentally benign, cheap, and economically viable oxidation procedures is a key challenge of homogeneous, oxidative alkane functionalization. The typically harsh reaction conditions and the propensity of dioxygen for radical reactivity call for extraordinary robust catalysts. Mainly three strategies have been applied. These are (1) the combination of a catalyst responsible for C–H activation with a cocatalyst responsible for dioxygen activation, (2) transition-metal catalysts, which react with both hydrocarbons and molecular oxygen, and (3) the introduction of very robust main-group element catalysts for C–H functionalization chemistry. Herein, these three approaches will be assessed and exemplified by the reactivity of chelated palladium (N-heterocyclic carbene) catalysts in combination with a vanadium cocatalyst, the methane functionalization by cobalt catalysts, and the reaction of group XVII compounds with alkanes.

Alkane C–H Functionalization and Oxidation with Molecular Oxygen

Singlet fission (SF) is a photophysical downconversion pathway, in which a singlet excitation transforms into two triplet excited states. As such, it constitutes an exciton multiplication generation process, which is currently at the focal point for future integration into solar energy conversion devices. Beyond this, various other exciting applications were proposed, including quantum cryptography or organic light emitting diodes. Also, the mechanistic understanding evolved rapidly during the last year. Unfortunately, the number of suitable SF-chromophores is still limited. This is per se problematic, considering the wide range of envisaged applicability. With that in mind, we emphasize uncommon SF-scaffolds and outline requirements as well as strategies to expand the chromophore pool of SF-materials.

Dominik Munz

Papers

A Singlet Phosphinidene Stable at Room Temperature

Pushing Electrons—Which Carbene Ligand for Which Application?

Synthesis of Hemilabile Cyclic (Alkyl)(amino)carbenes (CAACs) and Applications in Organometallic Chemistry.

Alkane C–H Functionalization and Oxidation with Molecular Oxygen

Unconventional singlet fission materials.