Topochemical photodimerization

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. 

In the crystal of 1,4-dicinnamoylbenzene (1,4-DCB) (see Fig. 12), the distances between the intermolecular photoadductive carbons are 3.973 and 4.086 A for one cyclobutane ring, and 3.903 and 3.955 A for the other. The two topochemical pathways may occur competitively in a single crystal of 1,4-DCB at the initial stage of reaction. Then, both intramolecular photodimerization and intermolecular photopolymerization of the diolefinic mono-cyclobutane intermediate occur competitively to give tricyclic dimer 21,22,23,24-tetraphenyl-l,4,ll,14-tetraoxo-2(13),12(13-diethanol, [4.4] para-cyclophane or oligomers (Hasegawa et al., (1985). On photoirridation at room temperature the 1,4-DCB crystal gives >90% of the tricylic... 

Many derivatives of quinones, cinnamic acids, and mucconic acids photodimerize in solid phases to give results 16> that in many cases are not in agreement with the general PMO rule of head-to-head reaction. However, it is clear that those reactions are controlled by topochemical effects, i.e. the geometry and proximity of the reactants in the solid phase. 135> Consequently, PMO theory will not be useful for calculating reactions of that type. 

Photodimerization of cinnamic acids and its derivatives generally proceeds with high efficiency in the crystal (176), but very inefficiently in fluid phases (177). This low efficiency in the latter phases is apparently due to the rapid deactivation of excited monomers in such phases. However, in systems in which pairs of molecules are constrained so that potentially reactive double bonds are close to one another, the reaction may proceed in reasonable yield even in fluid and disordered states. The major practical application has been for production of photoresists, that is, insoluble photoformed polymers used for image-transfer systems (printed circuits, lithography, etc.) (178). Another application, of more interest here, is the use that has been made of mono- and dicinnamates for asymmetric synthesis (179), in studies of molecular association (180), and in the mapping of the geometry of complex molecules in fluid phases (181). In all of these it is tacitly assumed that there is quasi-topochemical control in other words, that the stereochemistry of the cyclobutane dimer is related to the prereaction geometry of the monomers in the same way as for the solid-state processes. 

Figure 1 summarizes the chemical structures of the topochemically polymerizable 1,3-diene monomers providing stereoregular 1,4-trans polymer (Scheme 6) [ 16]. Most of the polymerizable monomers contain benzyl, naphthylmethyl, and long alkyl-chain substituents in their chemical structures. The (ZyZ)-, (E,Z)-, and ( , )-muconic and sorbic acids as well as the other diene carboxylic acids are used as the ester, amide, and ammonium derivatives. In contrast to this, the carboxylic acids themselves have crystal structures unfavorable for polymerization while they undergo [2-1-2] photodimerization, as has already been described in the preceding sections.

The synthesis of cis-1,4 polymers was also tried by e use of monomers with an s-cis conformation. The solid-state photopolymerization of pyridone derivatives, which is a six-membered cyclic diene amide and is a tautomer of 2-hydroxypyridine, was attempted [100]. Pyridones make hydrogen-bonded cocrystals with a carboxylic acid in the crystalline state. Because the cyclic structure fixes its s-cis conformation, if the polymerization proceeds, a cis-2,5 polymer would be obtained. Actually, however, the photopolymerization did not occur, contrary to our expectation, but [4-1-4] photodimerization proceeded when the carbon-to-carbon distance for the dimerization was small (less than 4 A) [101]. A closer stacking distance of the 2-pyridone moieties might be required for the topochemical polymerization of cychc diene monomers. 

In the absence of defects, the reactivity of organic solids is mainly determined by molecular packing. Reactions in which the crystal structure holds sway over intrinsic molecular reactivity are said to be topochemically controlled (Thomas, 1974). A classic example of a topochemically controlled organic reaction in the solid state is the photodimerization of rrans-cinnamic acids studied by Schmidt et al. (see Ginsburg,... 

An interesting example of crystal engineering and topochemical reactivity is provided by 2-benzyl-5-benzylidene-cyclopentanone(BBCP) derivatives which undergo single crystal-single crystal photodimerization (Jones et al, 1980). In the monomer... 

A between translationally related molecules to give a dimer of mirror symmetry (m-dimer) and (c) the y-type crystal, which is photochemically stable because no double bonds of neighboring molecules are within 4 A. On the basis of mechanistic and crystallographic results it has been established that in a typical topochemical photodimerization, transformation into the product crystal is performed under a thermally diffusionless process giving the space group quite similar to that of the starting crystal (5,6). 

Recent studies on the photodimerization of olefinic crystals several examples, which deviate from accepted topochemical principles, have been... 

In addition to the influence on the dimer morphology, the presence of water molecules strikingly affect apparent photoreaction rate and temperature dependence of the rate (12). Since the topochemical reaction deteriorates pronouncedly at reaction temperatures close to the melting point of the starting crystal, maximal reaction rate is necessarily observed at a specific temperature for individual crystals, for example, at ca. 20"C for a -form crystal of cinnamic acid (mp 132°C) (13). In an aqueous dispersant the apparent maximal rate of photodimerization of 1 is observed about 15°C while the temperature for maximal rate in a non-aqueous dispersant is about 35 °C. The... 

In spite of the unfavorable topochemical arrangement (the reactive double bonds are rotated by 65° with respect to each other and the center-to-center double-bond distance is 3.83 A, see Figure 22), photodimerization occurs in crystals of 19 to give the syn head-tail isomer. On the other hand, 20 which also has nonideal topological arrangement of double bonds in the crystalline state (the distance between the centers of adjacent double bonds is 3.93 A the double bonds are rotated and make an angle of 28° when projected down the... 

The stereochemistry of photodimerization in the solid state and solution has been reported for several halogenated derivatives of t-1 (Table 2) (59-62). Solid state photodimerization of stilbenes, like other alkenes, is subject to topochem-ical control viz, the two reactive double bonds must be parallel and separated by < 4.2 A (63). The photostability of t-1 in the solid state (39b,59) is consistent with its reported crystal packing (64). The halogenated stilbenes 15-20 serve to illustrate the variety of stereochemical outcomes observed for solution and solid state dimerization (eq. 11). 

It is well established, in a qualitative sense, that chemical reactions occurring in crystals are subject to restrictive forces, not found in solution, which limit the allowable range of atomic and molecular motions along the reaction coordinate. This often leads to differences, either in the product structures or the product ratios, in going from solution to the solid state. This was first demonstrated in a systematic way by Cohen and Schmidt in 1964 in their studies on the solid state photodimerization of cinnamic acid and its derivatives (1 ). This work led to the formulation of the famous topochemical principle which states, in... 

Topochemically controlled solid reactions are governed by the crystal structure of the solid. Even if the intrinsic reactivity allows for a reaction, the topochemical restrictions circumvent a conversion. The photodimerization of solid anthracene was observed electron microscopically at dislocations [85], The reason for this is the possibility of molecule rotation at the dislocation, and not only the high pressure. 

Figure 9 Temperature dependence of topochemical photodimerization of films of polymer, 23a. (a) HMW polymer, (b) LMW polymer, (c) Unfractionated polymer. Relative reactivities were obtained by monitoring the change in absorbance (A0 — At) of the C=C stretching band at times t relative to t = 0 (/Iq). (Reprinted with permission from Ikeda et al. [54]. Copyright 1990 American Chemical Society.)...

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. 

Homogeneous topochemical reactions are quite rare. The topochemical polymerization of diacetylenes and with special precautions some four center photodimerizations are examples for this reaction mode. 

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). 

Theocharis, C. R., and Jones, W. Topotactic and topochemical photodimerization of benzylidenecyclopentanones. In Organic Solid State Chemistry. (Ed., Desiraju, G. R.) Ch. 2., pp. 47-68. Elsevier Amsterdam, Oxford, New York, Tokyo (1987). 

Enantiomeric Purification of Partially Enriched Mixtures of Enantiomers via Topochemical Photodimerization... 

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. 

When after formation of photoproduct at one of the two sites the crystal was heated and cooled the photoreaction was reversed. We were able to conclude with the help of the packing calculations of sec. 3.2 and other experimental results that those impurity molecules that reacted could not be those that replaced anthracenes in the regular lattice, but must be at dislocations or regions of abnormal packing, where the host and guest were in face-to-face positions. The reaction is thus belongs to the class of the photodimerizations of anthracene, and 9-cyanoanthracene, not of dimers formed topochemically in the normal lattice. 

It was first reported that the topochemical photopolymerization of diolefin crystals gave rise to cracks and deformation [7]. An atomic force microscopic (AFM) study made possible the observation that the photodimerizations of trans-cinnamic acids and anthracenes in the crystalline state induced surface morphological changes at the tens and hundreds of nanometers level by the transportation and rebuilding of the surface molecules [8]. The appearance of a surface relief grating on the single crystal of 4-(dimethylamino)azobenzene was demonstrated by repeated irradiation with two coherent laser beams [9]. 

Recent studies of the topochemical [2+2] photodimerization have shown several examples which deviate from the accepted concept of the topochemical reactions enunciated by Schmidt. This anomalous behavior has been reviewed in a recent publication (11). 

The intermolecular reactive bonds, separated by 4.023(9) A for C(7) C(I4) and by 4.097( 10) A for C(8)"-C(13), are nonparallel, while these distances are within those of normal reactive bonds. The topochemical [2+2] photodimerization of the double bonds oriented nonparallel has been reported for several olefinic crystals in recent papers (11,13b,21a-d). 

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