Photocondensation

Wan, P Hennig, D. Photocondensation of o-hydroxyhenzyl alcohol in an alkaline medium synthesis of phenol-formaldehyde resins. J. Chem. Soc., Chem. Commun. 1987, 939-941. 

With two y,8 double bonds, two a,/3 double bonds, and the possibilities of cis and trans ring fusions with syn and anti configurations, 20 isomeric dimers are possible. Surprisingly, only one product is formed in a head-to-tail fashion. The sole product of the irradiation of the 3,5-diene-7-ketosteroid (76), however, is the head-to-head dimer. The specificity and mode of addition arise presumably through the effect of the specific environment of the chromaphore. The dimerization of (75) is believed to involve the addition of the a,fi double bond of a photoexcited molecule to the less hindered y,8 double bond of a ground state molecule. The photocondensation of (76) with cyclopentene, in which steric hindrance should not be a controlling factor, was found to yield a cyclobutane product involving the a,/ bond of the steroid in contrast to dimerization across the y,8 bond. 

It has recently been shown that organic photoconductor-liquid crystal sandwich cells can in theory act as dynamic scattering devices 164> and the technical possibilities ought to be tested. In this context, it should be noted that dyes can be used in two-layer photocondensers (consisting e.g. of phthalocyanine and a ferroelectric ceramic), which are very sensitive to light and have a response time of lO-4 to 10-3 Sec 165). 

Condensation reactions of aniline and of other bases with aromatic aldehydes to give Schiff s bases are accelerated by ultraviolet light in buffered media. For example, o-phenylenediamine reacts with formic acid to yield benzimidazole and acetylacetone reacts with guanidine to form 2-amino-4,6-dimethylpyrimidine.162 Similarly, lobelanine is formed by the photocondensation of glutaraldehyde, benzoylacetic acid, and methylamine hydrochloride.163... 

Irradiations of l,3,2-dioxaphosph(V)oles in various solvents with and without sensitizers have also been described67. 2,2,2-Trimethoxy-4,5-dimethyl-1,3,2-dioxaphospho-lene (32) added acetone under photo-irradiation to yield the oxaphospholane 33 (equation 15)68. Using slightly damp acetone, the cyclic phosphate 34 is obtained. However it could be shown that the oxetane 35 is formed as an intermediate in the photocondensation... 

A very different process of photopolymerization relies on the reaction of photocondensation which is an addition of two molecules to form a longer adduct. There is no radical intermediate in this case and one photon (at least) is required for each step in the polymerization process. Some male-imide derivatives can be polymerized in this way, to form an insoluble crystalline polymer. It should be noted that the monomer molecules must have two reactive groups, one at each end, so that the polymer chains can extend in principle indefinitely (Figure 6.12). 

Figure 6.12 The principle of a photocondensation polymerization reaction. With bichromophoric monomers long chains of polymers can be formed...

This SET-induced photocondensation has been used to internally cyclize enones bearing the a-silylamino functionality in the molecule. This leads to cyclic, bicyclic and spirobicyclic compounds.82,338... 

Photocondensation reactions of unsaturated ketones. Tetrahedron Letters 1964, 3695. 

The pathway described in Scheme 26 is essentially the same as that proposed previously for the solution photolysis. The X-ray analysis of the complexes (Scheme 25, Ar = 1-naphthyl or 3-indolyl) demonstrated parallel, alternate stacking of D and A with the inteiplanar distance 3.4 A. The C---C distance relevant to the photocondensation reaction was found to be 3.75 A for Ar = 1-naphthyl and 4.50 A for Ar = 3-indolyl. The longer distance for the latter explains its negligible photoreactivity [53a]. However, because the authors in this paper again [39b,40a,41] assumed a duplicate excitation by one photon in their mechaius-tic discussion, the details appear problematic. 

Workers at Nankai University and Ryukoku University continued this work and studied surprisingly many solid mixtures. They reported photocondensation... 

Photocondensation Polymerization.—This term is used in the sense that the polymerization requires the absorption of a photon for every propagation step, as distinct from the photoinitiation of the free-radical chain addition polymerization discussed above. The photodimerization reaction (14) of maleimides in... 

This chapter contains a review of some photodimerizations and photocondensations leading to the formation of substituted cyclobutanes. Since the literature in this field is indeed vast, it would be impossible to present here a comprehensive review of all the reactions reported to lead to such products. Instead, it is our goal to present a general survey of the various types of compounds known to produce cyclobutane compounds upon irradiation and to discuss mechanisms which have been proposed to account for some of these products. 

Water as Reaction Medium. Being transparent to the near-ultraviolet spectrum of sunlight, pure water can serve as an inert medium in which pesticide transformations take place. For example, Henderson and Crosby (28) have shown that suspensions of the chlorinated hydrocarbon insecticide dieldrin (I), although essentially insoluble in water, undergo a photocondensation reaction to give photodieldrin (II) (Equation 2). 

These groups are further subdivided into various reactions including photooxidation, hydroperoxide formation and conversion, Norrish Type 1 II and other carbonyl and carboxyl group reactions, photo-Fries rearrangement, photosubstitution, photoaddition, photoelimination, photodimerization, photocondensation, and others. This paper will not elucidate these mechanisms except to note issues regarding spectral irradiance. 

Cyclic imides with carbon-carbon double bonds are susceptible to polymerization by a radical mechanism or by several other mechanisms, such as photocondensation, Diels-Alder addition, or nucleophilic substitution. Although these latter reactions yield step-growth polymers, they will be considered in this part because traditionally they are considered addition polyimides. Furthermore, in many cases they are indeed cross-linkable through polymerizable imide double bonds. 

Although the investigation of polymaleimides and copolymaleimides has served to solve several theoretical aspects on radical polymerization of these unsaturated rings [265,266] and to give a great deal of data on maleimide polymers, they have not yet found end-use applications. The same is true for polyimides synthesized by the so-called photocondensation method, that consists of step-growth polymerization of bisimides with benzene or alkylbenzenes [267-269]. The reaction is induced by UV radiation (Scheme 52). 

Photocondensation is a procedure limited to monomers that do not easily polymerize by a photoinitiated radieal mechanism. Otherwise the UV radiation would induce the formation of chain growth homopolymers. 

Eaton, P.E., Photocondensation reactions of unsaturated ketones. Tetrahedron Lett., 3695, 1964. Sydnes, L.K. and Stensen, W., Regioselective photocycloaddition of 2-cyclopentenone to some allenes, Acta Chem. Scand. Ser. B, 40, 657,1986. 

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