Molecular rearrangements, solid-state

In this connection, in order to judge the level of these molecular rearrangements, the solid state X-ray structures of ferrocene and ferrocenium ion could be compared. Unfortunately, the molecular disorder caused by the rotation of the cyclopentadienyl rings in ferrocene means that the comparison procedure is far from simple and, in fact, the first results were interpreted in terms of a staggered conformation of the two cyclopentadienyl rings. It is now believed that the eclipsed conformation is the more stable (with a rotation angle of about 10°).2 However, as the rotational barrier is notably low (about 4 kJ mol-1), the conformation that one observes is probably that imposed by crystal packing forces. 

Toda et al. reported that the topotactic and enantioselective photodimerization of coumarin and thiocoumarin takes place in single crystals without significant molecular rearrangements [49]. Molecular motion needs to be called upon to explain the photochemically activated cycloaddition reaction of 2-benzyl-5-benzylidenecyclopentanone. The dimer molecules, once formed, move smoothly in the reactant crystal to form the product crystal [50]. Harris et al. investigated the reactivity of 10-hydroxy-10,9-boroxophenanthrene in the solid state and the mechanism of the solid-state reaction was characterized by both X-ray diffraction and thermal analysis [51]. It was demonstrated that the solution chemistry of 10-hydroxy-10,9-boroxophenanthrene is different from that in the solid state, where it undergoes dimerization and dehydration to form a monohydride derivative. 

Garcia-Garibay M (2003) Engineering carbene rearrangements in crystals from molecular information to solid-state reactivity. Acc Chem Res 36 491-498... 

Figure 14 Schematic diagram of solid state rearrangement. Straight lines represent mean molecular planes and F indicates the Pd(CF3COCHCOCF3) terminus. The low temperature form is shown on the left...

In the first case, achiral IV-methacryloylthiobenzanilide 39 formed (E,Z)-conformation of the imide moiety and crystallized in a chiral fashion. The solid-state photoreaction gave optically active 3-lactam 40. The dynamic molecular rearrangement for cyclization was elucidated on the basis of direct comparison of the absolute configuration of both the starting material and the photoproduct (Scheme 11).[25][38]... 

This rearrangement involves considerable molecular flexibility (motions) and thus it is expected that the rate would depend upon the physical properties of the medium, such as the solvent viscosity and solvent interaction. In solid state and at 77 K such a rearrangement would not have been significant. Further design experiments using specific derivatives of GAV as the reactant given in a later section unequivocally confirm this rather unusual radical rearrangement. 

Lippmaa et al. (86) have successfully studied solid state Si NMR of several trimethylsilyl silicate samples using proton-enhanced nuclear induction spectroscopy and magic angle spinning. They showed that the high resolution Si NMR of solids can be useful for structural studies of certain soluble and insoluble silicates, because many of these compounds with well defined molecular structures in the solid state tend to undergo very complicated rearrangements and condensation in solution. (93)... 

Cryochemical research in the past 15 years has established the existence of the low-temperature limit of rate constants of various solid-state chemical reactions with transfer of atoms and molecular fragments of different masses over distances comparable with intermolecular ones, from H-atoms transfer under intramolecular rearrangement to organic radicals and halogen atom... 

Polymerization in the solid state holds promise for making highly ordered polymers by highly selective reaction pathways. In these polymerization processes, the molecular rearrangement within the constraints of the monomer crystal lattice plays an important role. In many polymerization reactions, these molecular rearrangements produce sufficient strain in the lattice to disrupt the crystal. 

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