Reaction Pathways in Solid-State Processes. 2. Carbon-13 NMR and X-ray Crystallography of Cyanobullvalene and Bullvalenecarboxylic acid

Shape‐Shifting Molecules: Unveiling the Valence Tautomerism Phenomena in Bare Barbaralones

We report a state‐of‐the‐art spectroscopic study of an archetypical barbaralone, conclusively revealing the valence tautomerism phenomena for this bistable molecular system. The two distinct 1‐ and 5‐substituted valence tautomers have been isolated in a supersonic expansion for the first time and successfully characterized by high‐resolution rotational spectroscopy. This work provides irrefutable experimental evidence of the [3,3]‐rearrangement in barbaralones and highlights the use of rotational spectroscopy to analyze shape‐shifting mixtures. Moreover, this observation opens the window toward the characterization of new fluxional systems in the isolation conditions of the gas phase and should serve as a reference point in the general understanding of valence tautomerism.

Shapeshifting molecules: the story so far and the shape of things to come

Shapeshifting molecules exhibit rapid constitutional dynamics while remaining stable, isolable molecules, making them promising artificial scaffolds from which to explore complex biological systems and create new functional materials. However, their structural complexity presents challenges for designing their syntheses and understanding their equilibria. This minireview showcases (1) recent applications of highly dynamic shapeshifting molecules in sensing and distinguishing complex small molecules and (2) detailed insights into the adaptation of tractable bistable systems to changes in their local environment. The current status of this field is summarised and its future prospects are discussed.

Shape-selective crystallisation of fluxional carbon cages

Dynamic covalent rearrangements of fluxional carbon cages, such as bullvalenes and barbaralanes, impart 'shapeshifting' molecular properties. Here, a series of five barbaralanes each interconvert dynamically between two constitutional isomers in solution, but resolve to single isomers upon crystallisation. Unexpectedly, the minor solution-phase isomers are resolved in two instances. Through dynamic NMR, crystallographic and DFT analyses, we show that the isomer observed in the solid state is not a direct consequence of the equilibrium distribution in solution or any specific noncovalent interactions. Rather, the dynamic preferential crystallisation is dictated by differences in molecular size and shape.

Synthesis of Barbaralones and Bullvalenes Made Easy by Gold Catalysis

The gold(I)-catalyzed oxidative cyclization of 7-ethynyl-1,3,5-cycloheptatrienes gives 1-substituted barbaralones in a general manner, which simplifies the access to other fluxional molecules. As an example, we report the shortest syntheses of bullvalene, phenylbullvalene, and disubstituted bullvalenes, and a readily accessible route to complex cage-type structures by further gold(I)-catalyzed reactions.

Chiral crystallization: freezing a rapid Cope rearrangement in the solid state

Homotropylidene and its derivatives undergo a fast Cope rearrangement. A homotropylidene system (cyclooctatetraene dimer) is reported that racemizes rapidly by such a valence isomerization in solution at room temperature. In the solid state, however, this rearrangement is frozen, and the single crystals precipitating from the racemic solution contain only one enantiomer. Since very large crystals can be grown, the racemic solution in principle can be converted into a single enantiomer by crystallization.

Bond-shift rearrangement in solid Li(3)P(7)(monoglyme)(3): a (31)P MAS NMR study

The (31)P MAS NMR spectrum of solid Li(3)P(7)(monoglyme)(3) has been reinvestigated over a wide temperature range (-70 to +77 degrees C) and under conditions of better resolution (Larmor frequency of 162 MHz and spinning rate of approximately 30 kHz) than previously measured (121 MHz and 13 kHz). At low temperatures three spinning sideband (ssb) manifolds are observed: a singlet (centered at -45 ppm relative to 85% H(3)PO(4)) due to the apical atom (A) of the P(7)-cage trianion; a 1 : 1 : 1 triplet (at -110, -117, and -124.5 ppm) due to the negatively charged equatorial (E) atoms, and a one to two doublet (at -161 and -168.5 ppm) due to the basal (B) atoms. These results are consistent with the P(7) cage having nearly, but not perfect, C(3v) symmetry. The compound appears to be well ordered in the solid state with very little structural dispersity. On heating, the NMR lines broaden and eventually coalesce into a single ssb manifold. This behavior is ascribed to bond-shift rearrangement similar to the Cope rearrangement in bullvalene. A MAS 2D exchange experiment and a quantitative analysis of the 1D NMR lineshapes indicate that, unlike in solution where the rearrangement involves a single bond shift at a time, in the solid the process involves a succession of two bond shifts: The first leads to an intermediate species in which the rearranged P(7) cage is inverted, while in the subsequent step a second bond shift takes place that also restores the original orientation of the cage in the lattice. The overall effect of the double bond shift is equivalent to cyclic permutation of the phosphorus atoms within the five member rings of the P(7)-cage. The quantitative analysis of the dynamic lineshapes shows that this cyclic permutation proceeds at a different rate in one ring (k(d)(1)) than in the other two (k(d)(2,3)). The kinetic parameters for these processes are E(a)(1)=18.7 kJ/mol, E(a)(2,3)=58.0 kJ/mol, k(d)(1)(17 degrees C)=k(d)(2,3)(17 degrees C)=10(4) s(-1). No indications for independent threefold molecular jumps of the P(7) cage were found.