Vibrational circular dichroism spectroscopy in the C-D, XY, and XYZ stretching region

Vibrational circular dichroism (VCD) spectroscopy is a powerful technique for structural analysis of chiral molecules, but information available from VCD spectra of large molecular systems can be limited by severe overlap of vibrational bands. While common chiral molecules do not absorb in the 1900-2400 cm-1 region, observation of VCD signals in this spectrally-isolated region is possible for molecules containing C-D, XY, and XYZ chromophores. Thus, a strategic introduction of these chromophores to a target molecule may produce VCD signals informative for molecular structures. VCD spectroscopy in the 1900-2400 cm-1 region is a rather unexplored research field and its basic properties remain to be investigated. This perspective article discusses insight obtained so far on the usefulness and physicochemical aspects of VCD spectroscopy in this region with briefly summarizing previous experimental VCD studies including classic examples as well as our recent results. We show that anharmonic effects such as overtones and combination bands often complicate VCD patterns. On the other hand, some molecules exhibit characteristic VCD signals that can be well interpreted by harmonic DFT spectral calculations for structural analysis. This article also discusses several examples of the use of this region for studying solute-solvent interactions and for VCD signal augmentation.

Scope and limitations of absolute configuration determination of allenic natural products using the CCC stretching VCD signal

The allene functional group in natural products isolated so far exists in a non-racemic form, but its axial chirality is difficult to elucidate. Allenes exhibit a characteristic antisymmetric CCC stretching mode at around 1950 cm^-1, and their VCD properties have not been studied in detail. This work, for the first time, applied VCD spectroscopy to allenic natural products and allenic molecules with other asymmetric centers focusing on the antisymmetric CCC stretching mode. This vibrational mode yielded a negligibly weak VCD signal for several molecules, but in the presence of electron-withdrawing and/or conjugating substituents, it generated a stronger one. Its sign was found to be influenced by the nature of substituents. These findings should deepen the understanding of the VCD properties of the allene functional group and should be useful for future studies of chiral allenes.

Asymmetric Synthesis of Fluorinated Allenes by Rhodium-Catalyzed Enantioselective Alkylation/Defluorination of Propargyl Difluorides with Alkylzincs

The reaction of propargyl difluorides R¹ CF₂ C≡CR² with alkylzincs R³ ZnCl giving axially chiral fluorinated allenes R¹ FC=C=CR² R³ with high enantioselectivity (up to 99 % ee) was found to be catalyzed by a chiral diene/rhodium complex. A key step in the catalytic cycle is selective elimination of one of the enantiotopic fluorides at the β-position of an alkenyl-Rh intermediate, which is generated by regioselective addition of R³ -Rh onto the triple bond of the starting difluorides.

Synthesis of (difluoromethyl)naphthalenes using the ring construction strategy: C-C bond formation on the central carbon of 1,1-difluoroallenes via Pd-catalyzed insertion

The insertion of 1,1-difluoroallenes was carried out to form a C-C bond exclusively on their central carbon. o-Bromophenyl-bearing 1,1-difluoroallenes underwent intramolecular insertion in the presence of a palladium catalyst. Regioselective C-C bond formation occurred to form a six-membered carbocycle, leading to pharmaceutically and agrochemically promising difluoromethylated naphthalenes.

Vibrational circular dichroism under the quantum magnifying glass: from the electronic flow to the spectroscopic observable

A chemically intuitive method to analyse and interpret vibrational circular dichroism spectra based on the vibrational transition current density.

Copper-catalyzed radical approach to allenyl iodides

A copper-catalyzed 1,4-addition reaction of arylsulfonyl iodides with 1,3-enynes afforded various allenyl halides in high yields under mild conditions via a radical mechanism.

Coarctate and Möbius: The Helical Orbitals of Allene and Other Cumulenes

As brought to the attention of the community by Hendon et al. and noted by previous workers, the π orbitals of the equilibrium geometry odd-carbon (even number of double bonds = n) [n]cumulenes may be written in either rectilinear or helical form. We trace the origins and detailed composition of the helical orbitals of cumulenes, which emerge in the simplest Hückel model and are not much modified in advanced computations. For the α,ω-disubstituted even [n]cumulenes, the helical representation is obligatory as the symmetry is reduced from D_2d to C₂. A relationship is apparent between these helical orbitals of the even [n]cumulenes, seen as a Herges coarctate system, and the corresponding Möbius cyclic polyene orbitals. The twist of the orbitals varies in interesting ways along the helix, and so does the contribution of the component atomic orbitals. Though the electronic structures of even [n]cumulenes and Möbius cyclopolyenes are closely related, they differ for higher n in intriguing ways; these are linked to the constrained rotation of the basis orbitals along the helical twist itinerary. Relations are constructed between the level patterns of the π-systems of even [n]cumulenes and ideas of Hückel and Möbius aromaticity.

Aluminium fluoride – the strongest solid Lewis acid: structure and reactivity

Highly Lewis acidic aluminium fluorides are interesting heterogeneous catalysts for many reactions, especially C–H and C–F bonds can be activated at room temperature.

Stereodivergent hydrodefluorination of gem-difluoroalkenes: selective synthesis of (Z)- and (E)-monofluoroalkenes

We have developed a novel approach for the stereodivergent hydrodefluorination of gem-difluoroalkenes using copper(i) catalysts to obtain stereodefined monofluoroalkenes. Both (Z)- and (E)-terminal monofluoroalkenes were obtained by hydrodefluorination of gem-difluoroalkenes in the presence of copper(i) catalysts and diboron or hydrosilane, respectively, with high stereoselectivity. DFT calculations were conducted to elucidate the stereoselectivity.

Vibrational optical activity as probe for intermolecular interactions

A detailed VCD spectroscopic analysis of well-selected chiral model systems can give valuable and unprecedented insights into intermolecular interactions such as solvation or reactant–substrate binding in catalysis.

Synthesis of a rhodium(i) germyl complex: a useful tool for C–H and C–F bond activation reactions

The rhodium(i) germyl complex [Rh(GePh₃)(PEt₃)₃] is a useful tool for C–F and C–H bond activation reactions. For instance, treatment with hexafluoropropene results in the formation of two isomeric C–F activation products [Rh{(E)-CFCF(CF₃)}(PEt₃)₃] and [Rh{(Z)-CFCF(CF₃)}(PEt₃)₃] in a 3 : 1 ratio.

Competing reaction pathways of 3,3,3-trifluoropropene at rhodium hydrido, silyl and germyl complexes: C–F bond activation versus hydrogermylation

The reactivity of the Rh complexes [Rh(L)(PEt₃)₃] (L = H, Si(OEt)₃, GePh₃) towards CH₂CHCF₃ was investigated which involve C–F bond activation and germylation reactions.

Improving selectivity in catalytic hydrodefluorination by limiting SNV reactivity

Competition of HM, SBM and S_NV in hydrodefluorination can lead to low selectivity which can be improved via solvent change.

Synthesis of monofluoroalkenes through selective hydrodefluorination of gem-difluoroalkenes with Red-Al®

A practical approach for the selective hydrodefluorination of gem-difluoroalkenes using Red-Al® as reductant at room temperature in CH₂Cl₂ was reported. Monofluoroalkenes were obtained in moderate to high yields with good E-selectivity.

A theoretical study of potentially observable chirality-sensitive NMR effects in molecules

Derivatives of fluorocyclopropene have large antisymmetric parts of the fluorine nucleus shielding tensor, thus they are promising molecules for direct determination of chirality by NMR analysis.