Lattice-controlled reaction

A great number of olefinic compounds are known to photodimerize in the crystalline state (1,2). Formation of a-truxillic and / -truxinic acids from two types of cinnamic acid crystals was interpreted by Bernstein and Quimby in 1943 to be a crystal lattice controlled reaction (5). In 1964 their hypothesis on cinnamic acid crystals was visualized by Schmidt and co-workers, who correlated the crystal structure of several olefin derivatives with photoreactivity and configuration of the products (4). In these olefinic crystals the potentially reactive double bonds are oriented in parallel to each other and are separated by approximately 4 A, favorable for [2+2] cycloaddition with minimal atomic and molecular motion. In general, the environment of olefinic double bonds in these crystals conforms to one of three principal types (a) the -type crystal, in which the double bonds of neighboring molecules make contact at a distance of -3.7 A across a center of symmetry to give a centrosymmetric dimer (1-dimer) (b) the / -type crystal, characterized by a lattice having one axial length of... 

An alternative route for the generation of enantiopure oligopeptides has been elucidated recently by our group. The method comprises the self-assembly of racemic or non-racemic thio-esters or N-carboxyanhydrides of a-amino acids into either 2-D or 3-D crystalline architectures followed by lattice-controlled reactions. 

Our investigations show that 1,4-disubsituted butadiene derivatives react in layered structures under exclusive formation of 1,i-trans-polymers. A stereoregular polymer is obtained. The structure analyses of the monomer and polymer crystals of 1 show that a lattice-controlled reaction takes place. It is certainly worthwhile studying the course of the reaction more in detail, and to compare the reaction mechanism and kinetics with those of other lattice-controlled reactions, as, for example, the polymerization reactions of diolefin 12Z) and butadiyne derivatives (23). 

Diacetylene derivatives are known to polymerize in a lattice controlled reaction. Polymerization proceeds via formation of a solid solution of the polymer in the monomer lattice. For a number of compounds the formation of polymer single crystals has been demonstrated... 

Synthesis of chiral enantiomerically pure materials from non-chiral reagents has been accomplished by crystallization of the symmetrical (in solution) substrate in appropriately packed chiral single crystals, followed by a lattice controlled reaction [6]. This concept is illustrated in Scheme 1 for the generation of chiral cyclobutane polymers from non-chiral dienes packing in engineered chiral crystals [7]. 

In the latter type, the direction of the unique axis (b-axis) of the polymer coincides with that of the monomer while the directions of the other two axes do not. In the case of 3 OMe none of the directions of the axes of the polymer coincide with those of the monomer. However, the temperature effect on the reaction behaviour (see Section 3) and the continuous change of the X-ray diffraction pattern indicate a typical diffusionless crystal-lattice controlled mechanism (Hasegawa et al., 1981). 

Topochemical [24-2] photoreactions of diolehn crystals has been reviewed. The reactions clearly depart from typical solution chemistry crystal-lattice control offers a unique synthetic route into photodegradable polymers, highly strained [24-2] paracyclophanes, stereoregular polymers, and absolute asymmetric synthesis. However, achieving the desired type of crystal... 

Photochemical reactions are usually run in homogeneous solutions notwithstanding it is also possible to irradiate solid compounds directly. Examples of such reactions on a preparative scale 705) as well as a discussion on crystal lattice control on photoreactions 706) are found in the literature. Finally, specific effects of a micellar environement is also being used in photochemical reactions of preparative purposes707). 

The solid-state polymerization of diacetylenes is an example of a lattice-controlled solid-state reaction. Polydiacetylenes are synthesized via a 1,4-addition reaction of monomer crystals of the form R-C=C-CeC-R. The polymer backbone has a planar, fully conjugated structure. The electronic structure is essentially one dimensional with a lowest-energy optical transition of typically 16 000 cm-l. The polydiacetylenes are unique among organic polymers in that they may be obtained as large-dimension single crystals. 

This is one of the oldest known solid-state reactions (127) and, in the case of cinnamic acid (60), was the one first used to establish the nature of lattice control of such reactions (128,129). It has been observed in a vast variety of compound types, and has proved to be of great value synthetically. 

The (2 + 2) photocyclodimerization of substituted olefins has provided some of the most striking examples of crystal-lattice control of the stereochemistry of a reaction. This may be exemplified by a selection of derivatives of 5-phenylbutadienoic acid (61), for which it is observed that the solid-state photobehaviors of the amide 62, the methyl ester 63, and the dichlorophenyl ester 64 differ entirely from one another each affords a single stereo- and regioisomer in high yield, but with different starting materials giving different types of products (130). In solution, irradiation of 63 or other photoactive dienes yields... 

These and other experiments imply that even reactions that proceed via bulky transition states can take place in the clathrate if the initiation is photochemical, perhaps as a result of local deformation of the host lattice, and presumably as a result of the large energy dissipation involved. However, even here there is some lattice control of the reaction pathway. 

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. 

These schemes have been frequently suggested [105-107] as possible mechanisms to achieve the chirally pure starting point for prebiotic molecular evolution toward our present homochiral biopolymers. Demonstrably successftd amplification mechanisms are the spontaneous resolution of enantiomeric mixtures under race-mizing conditions, [509 lattice-controlled solid-state asymmetric reactions, [108] and other autocatalytic processes. [103, 104] Other experimentally successful mechanisms that have been proposed for chirality amplification are those involving kinetic resolutions [109] enantioselective occlusions of enantiomers on opposite crystal faces, [110] and lyotropic liquid crystals. [Ill] These systems are interesting in themselves but are not of direct prebiotic relevance because of their limited scope and the specialized experimental conditions needed for their implementation. 

Eq. 2-248) [Braun and Wegner, 1983 Hasegawa et al., 1988, 1998]. This polymerization is a solid-state reaction involving irradiation of crystalline monomer with ultraviolet or ionizing radiation. The reaction is a topochemical or lattice-controlled polymerization in which reaction proceeds either inside the monomer crystal or at defect sites where the product structure and symmetry are controlled by the packing of monomer in the lattice or at defect sites, respectively. 

Chemical reactions in the sohd state have intrinsic features different from those for reactions performed in solution or in the gaseous state. For example, sohd-state organic reactions often provide a high regio- or stereoselectivity because the reactions and the structiue of a product are determined by the crystal structure of the reactant, i.e., the reaction proceeds under crystaUine lattice control [1-8]. When the reactant molecules are themselves crystalhne (molecular crystals) or are included in host crystals (inclusion compounds), the rate and selectivity of the reaction are different from those obtained in an isotropic reaction medium. 

Other careful computer simulations [33] focused more attention on the two diffusion-controlled reactions A + B —> 0 and A + A — 0, on both fractal and Id lattices. In the case of the Euclidean space it was well demonstrated that achievement of the theoretical limit of a = 0.25 is a quite long-time... 

Fig. 16. Nanophase PtyTiOa catalysts (a) finely dispersed Pt/TiOa at room temperature, (b) In situ dynamic catalyst activation in hydrogen imaged at 300°C. The (111) lattice atomic spacings (0.23 nm) are clearly resolved in the platinum metal particle (P) under the controlled reaction conditions, (c) The same particle of platinum (P) imaged at 450°C, also in H2. Catalyst deactivation with growth of the support oxide monolayer indicated by a larger arrow, and the development of nm-scale single-crystal clusters of platinum metal (which show no coating as they emerge) with 0.2-nm lattice spacings indicated by smaller arrow (87).

In conclusion, the four-center photopolymerization is a novel type of topochemical reaction which is crystal-lattice controlled with respect to the whole set of elementary processes44 including initiation, propagation and crystallization of polymer. 

In contrast to solutions, solid drugs have a fixed conformation resulting in topochemical reactions. The majority of photoreactions in the solid state, described in the literature, deal with lattice-controlled examples and photodimerizations. A precondition for these reactions is the parallel position of the double bond of two adjacent molecules in the crystal lattice as shown by the example of the trimorphic, frans-cinnamic acid. Irradiation of the a- and the 5-modifications causes the formation of a-truxillic acid and (i-truxinic acid, respectively, whereas the y-modification is photostable due to the distance of the double bonds fixed by the lattice (Fig. 8) (10). 

The alkali metal-graphite compounds are extremely reactive. They ignite in air and may react explosively with water. In the controlled reaction with water or alcohol only alkali hydroxide and hydrogen result there is no acetylene or any other hydrocarbon. Fredenhagen concluded from this that the compounds could not be carbides. Mercury dissolves the alkali metal out of the lattice. When treated with liquid ammonia, CgMe gives up only a third of the alkali metal and takes in its place two molecules of ammonia (see Section IIIA4). 

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