Topochemical analysis

This strengthens the case for treating structure analysis as a particular field of analytical chemistry despite the fact that, from the philosophical point of view, structure analysis can be considered as distribution analysis (topochemical analysis) of species in atomic dimensions. Structure analysis of solids follows a similar scheme like that given above. The characteristics of molecules are then linked with those of crystals and elementary cells. 

In this chapter the topochemical [2+2] photoreactions of diolefin crystals are reviewed from the viewpoints of organic photochemistry, analysis of reaction mechanism, and crystallography as well as in terms of synthetic polymer chemistry and polymer physics. 

The analysis of the regioselective reactivity of olefins in identical topochemical environments by three computational methods concludes that both steric factors (cavity and potential energy) and electronic factors (perturbation energy from orbital interactions) play important cooperative roles in determining which C—C double bond in a molecule reacts first in [2-1-2] photodimerization. The steric factor is considered to be effective in the movement of olefins at an early stage of the reaction, whereas the electronic factors are effective in the adduction of olefins at a later stage of the reaction. 

Similar behaviour has been observed in the photoreaction of methyl a-cyano-4-[2-(2-pyridyl)ethenyl]cinnamate (7 OMe) crystals in which the yield of [2.2] paracyclophane reached 65% on irradiation at — 78°C (see Scheme 10 p. 153) (Hasegawa et al., 1989b). From the crystal structure analysis of the same type of [2.2] paracyclophane, which is topochemically derived from alkyl a-cyano-4-[2-(4-pyridyl)ethenyl]cinnamate crystals, a highly strained molecular shape is confirmed in which two phenylene rings are severely bent (Maekawa et al., 1991b). 

All these types of behaviour are reasonably interpreted by crystallographic analysis of these compounds, based on topochemical consideration. 

The physical organic chemistry of very high-spin polyradicals, 40, 153 Thermodynamic stabilities of carbocations, 37, 57 Topochemical phenomena in solid-slate chemistry, 15, 63 Transition state analysis using multiple kinetic isotope effects, 37, 239 Transition state structure, crystallographic approaches to, 29, 87 Transition state structure, in solution, effective charge and 27, 1... 

There are a number of possible explanations for the formation of more than one photodimer. First, due care is not always taken to ensure that the solid sample that is irradiated is crystallographically pure. Indeed, it is not at all simple to establish that all the crystals of the sample that will be exposed to light are of the same structure as the single crystal that was used for analysis of structure. A further possible cause is that there are two or more symmetry-independent molecules in the asymmetric unit then each will have a different environment and can, in principle, have contacts with neighbors that are suited to formation of different, topochemical, photodimers. This is illustrated by 61, which contrasts with monomers 62 to 65, which pack with only one molecule per asymmetric unit. Similarly, in monomers containing more than one olefinic bond there may be two or more intermolecular contacts that can lead to different, topochemical, dimers. Finally, any disorder in the crystal, for example due to defective structure or molecular-orientational disorder, can lead to formation of nontopochemical products in addition to the topochemical ones formed in the ordered phase. This would be true, too, in those cases where there is reaction in the liquid phase formed, for example, by local melting. 

McBride and co-workers have studied extensively the reactions of such free-radical precursors as azoalkanes and diacyl peroxides (246). By employing a variety of techniques, including X-ray structure analysis, electron paramagnetic resonance (EPR), and product studies, and comparing reactions in the crystal and in fluid and rigid solvents, they have been able to obtain extremely detailed pictures of the solid-state processes. We will describe here some of the types of lattice control they have elucidated, and the mechanisms that they suggest limit the efficacy of topochemical control. 

Heating experiments on Nautilus shell material (see page 10) indicates a retardation in the rates of thermal decomposition of mineral and organic phase. This has been attributed to the development of a topochemical boundary at the contact protein-mineral. X-ray diffraction analysis gives no evidence for structural alteration of the original aragonite during the thermal treatment up to 200 °C. 

X-ray structure analysis and topochemical photopolymerization of 2,5-dimethoxyphenyl-and quinone-substituted octa-3,5-diynes. Tetrahedron 2000, 56, 6781-6794. 

Most solid state work published in recent years has dealt primarily with a molecular analysis of product formation that seems to arise from the intuitive appeal of the topochemical postulate. Problems associated with phase changes can sometimes be neglected if reactions are carried out to sufficiently low conversion values. However, since preferential reactions at defect sites may be a problem, the involvement of nontopochemical reactions at defect sites should be experimentally documented and avoided. Changes in reaction rates and product selectivity have also been associated with internal stress [54], with sample melting, or with surface effects [62]. In contrast, the mechanisms and consequences of phase transformation have been studied much less. Phase changes depend on the properties of the ensemble and, as suggested in Scheme 5, they are affected by composition, temperature, pressure and whether or not equilibrium is achieved throughout the reaction. 

One reason that salts of general structure 7 were chosen for investigation is that molecular mechanics calculations showed the benzocyclohexadienone framework to be planar. This planarity translates into an equal opportunity of forming either enantiomer of photoproduct 8, or to put it another way, the photochemistry of these salts is conformationally unbiased with respect to enantioselec-tivity. As a result, any ee in the crystalline state would have to be due to an essentially pure topochemical effect. Only one of the salts investigated (7b) gave a respectable ee in the solid state, and unfortunately, it has not been possible to obtain an X-ray crystal structure of this material, thus precluding an analysis of the specific topochemical factors responsible for asymmetric induction in this case. 

Irradiation of 1 1 host-guest crystals of coumarin 85a with (/ ,/ )-(— )-6a derived from tartaric acid gave the (— )-antvdtedd-to-head dimer 86a of 96% ee [88]. Enantiospecific photodimerization of thiocoumarin 85b gave optically pure (+ )-anti-head-to-head dimer 86b when the 1 1 complex with R,R)- — )-6b was used. X-ray structure analysis revealed that the distance between the two ethylenic double bonds was short enough (3.59 and 3.42 A for 85a and 3.73 and 3.41 A for 85b) for addition to occur and topochemically [89]. Further, both reactions were found to proceed via a single crystal-to-single crystal transformation. 

X-ray crystallographic studies during the course of the reaction have demonstrated that the reaction is a typical topochemical process involving a direct rearrangement of the monomer crystal to the polymer crystal having an extended rigid rod-like structure. By x-ray analysis and DSC on the thermal depolymerization of the polymer crystal, a reversible topochemical process has been demonstrated for monomer and polymer crystals. 

From the viewpoint of the topochemical process, the diolefinic compounds prepared so far are classified into three groups photopolymerizable, photooligomerizable1, and photostable crystals. Among these, nine kinds of monomer crystals and their polymers have been subjected to structure analysis so far. The obtained crystal data are compiled in Table 4. 

Morphological changes, which are classified into four groups, have been correlated to the degree of topotactic control. Thermal analysis has been studied on DSP poly-DSP in some detail. Two main endothermic peaks of as-polymerized poly-DSP crystals are characterized as thermal depolymerization in the crystalline state and crystal melting point followed by thermal depolymerization in the molten state. From the results of the studies on the heat treatment of as-polymerized polymer crystals, a reversible topochemical processe has been established. 

---