Topochemical postulates

DSP crystal, a detailed picture of the lattice motion and related displacements was constructed and related to the topochemical postulate and the mechanism of phonon assistance. Holm and Zienty (1972) have measured the quantum yield for the overall polymerization process of a,a -bis(4-acetoxy-3-methoxybenzylidene)-p-benzenediacetonitrile (AMBBA) crystals in slurries and reported it to be 0.7 on the basis of the disappearance of two double bonds ( = 1.4 if assigned on the basis of the number of double bonds consumed). 

The prototypes of intra-solid reactions are the [2-1-2] photochemically activated cycloaddition reactions that constitute the basis of Schmidt s work on topochemical reactions [36]. It is now clear, however, that the topochemical postulate, though correct and of wide applicability, cannot account for many solid-state processes, in particular those implying high molecular or ionic mo-... 

The central problem in organic solid-state photochemistry is the preorganization of molecules satisfying the topochemical postulates. Schmidt coined the term crystal engineering for this problem of supramolecular assembly. Indeed, the importance of crystal engineering is fundamental to areas as diverse as nonlinear optics, high- /) superconductors, and the generation of polymorph forms in pharmaceuticals. 

Recent work by Len MacGillivray of the University of Iowa, USA, has used topochemistry to produce an impressive range of new solid compounds by the addition of hydrogen bonded templates in order to bring about the designer orientations of photochemically reactive molecules in the solid state.34 A simple example is the use of resorcinol (1,3-dihydroxybenzene) to position two molecules of trans 1,2 bis (4-pyridyl) ethylene with their double bonds overlapping at ca. 4 A as required by the topochemical postulate. Upon photolysis the product rcfHetrakis(4-pyridyl)cyclobutane is formed stereospecifically and in 100 % yield, Scheme 8.2. In the absence of the resorcinol template trans-l,2-bis(4-pyridyl) ethylene is... 

Figure 4 provides molecular packing of a and (3 forms of cinnamic acids. The packing arrangement nicely accounts for the observed dimers from a and 3-forms of the crystal. The above topochemical postulates rationalize the absence of reactions in the y-modification (distance of separation between C=C bonds is over 4.7 A). 

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. 

The molecular assemblies described above have inspired us, in recent years, to develop finite assemblies in the solid state that exhibit chemical reactivity. Specifically, we,69 and others,70 have been utilizing principles of molecular recognition and self-assembly to develop a method to direct the formation of covalent bonds in organic solids. The method builds on the work of Schmidt on the reactivity of cinnamic acids in the organic solid state.45 Specifically, Schmidt has described topochemical postulates that dictate geometry criteria for a [2 + 2] photodimerization to occur in a solid. The postulates state that two carbon-carbon double (C=C) bonds should be aligned in parallel and separated by a distance <4.2 A to react. 

General Considerations. Cinnamic acid solid state photodimerizations are the prototype for the topochemical postulate and reasoning [7], The a-modification of frans-cinnamic acid a-4 gives a-truxillic acid 5 (head-to-tail), while the /i-modification (/J-4) gives /i-truxinic acid 6 predominantly... 

General Considerations. In the well-known photodimerization of anthracenes in liquid solution 9, lO /lO, 9 -dimers (head-to-tail [4 + 4]) are formed in most cases. However, there have also been instances where head-to-head-photodimers (9,9 /10,10 ) are produced [19], and these were overseen previously. The solid phase photoreactions of anthracenes charged the topochemical postulate [7] for decades with hitherto unsolvable difficulties. All examples that contradict this assumption were eliminated without hesitation from the scope of topochemistry and termed to be crystal defect reactions, because the topochemically allowed processes were taken as support for topochemistry without further proof. The later provision, that the dimerizations occur within reaction cavities in the bulk of the crystal [20], did not help in this respect. A summary of the various arguments is given in Ref. 8. From examples 7 to 8 only b and perhaps c formally meet... 

Extensive crystallographic and photochemical studies by Schmidt involving polymorphs of cinammic acid led to topochemical postulates that may be used to predict whether a photoinduced [2 + 2] cycloaddition reaction will occur in the solid state (Fig. 1). - According to Schmidt, the nature of a crystal determines whether or not reaction will take place and determines the molecular structure of the product. Moreover, if a solid displays such reactivity, then the reaction will involve nearest-neighbor molecules and will occur with minimum atomic and molecular movement. ] Thus, for the [2 + 2] photoreaction, the double bonds of the reactants should, as determined by Schmidt, be separated by <4,2 A and aligned in parallel. Such separation and parallel alignment ensures that the p(z) orbitals of the double bonds are nearly colinear and overlap sufficiently to react. Notably, there... 

The combination of topochemical photoreactions with hydrothermal methods opens a window to develop efficient routes for the preparation and/or improvement of the yield of new and conventional cyclobutane stereoisomers that are difficult or impossible to access either in solution or by known solid state strategies, including the possibility of inducing chirality from achiral cyclobutane derivatives. The combination of such alternatives can be very helpful in order to overcome the limitations imposed by the topochemical postulate for obtaining regioselective photoproducts with such stereochemical requirements from crystalline assemblies. 

Interspersed among these milestones is the parallel birth of self-assembly as in the formation of self-assembled monolayers first observed as the spreading of oil on water by Benjamin Franklin in 1774, and the birth of nanochemistry (the 1818 recognition of the particle size-dependent color of colloidal gold). We can also see the evolution of crystal engineering from the early topochemical postulate and molecular engineering of von Hippel in the 1960s to... 

Abstract S ince the topochemical postulate was proposed in the solid-state reactions by Schmidt et al. based on the X-ray crystal structure analyses of related compounds, a variety of organic solid-state reactions have been studied analyzing the crystal structures of the reactant molecules in these 50 years. The postulate has been a powerful tool to understand the reaction mechanism at a rough estimate. 

This minimum movement of atoms or molecules in the solid state leading to the reaction is known as topochemical reaction. Since the structure of the product is correlated with the geometry of the reactant solid in the crystal lattice, the stereochemistry of the product obtained in a solid state reaction is, in general, predictable. Most of the photodimerization reactions reported in the literature were found to follow these topochemical postulates of Schmidt. Ai2,is,2i,26-28... 

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