Photochemically initiated decarboxylation

A variety of photocatalyzed decarboxylation reactions on Ti02 powder including the decomposition of acetate to methane and carbon dioxide and the breakdown of benzoic acid yielding predominantly CO2 have been reported by Bard and coworkers (23,24). Evidence for the occurrence of these "photo-Kolbe" reactions has stimulated the search for other organic reactions that might be photochemically initiated by excitation of semiconductors and extensive work in this area is in progress (25). 

Two pathways can be considered to explain the formation of the [3]rotane derivative either addition of the carbene to methyl acrylate to give a y-lactone which subsequently undergoes thermal or photochemical decarboxylation, or initial decarboxylation of the dispiro-y-lac-tonylidene to a dispiro[2,0.2.1]hept-7-ylidene which is then trapped by methyl acrylate to give the product. 

The use of the hypervalent iodine reagent [bis(trifluoroacetoxy)iodo]benzene has been reported to be effective in the synthesis of C-nucleoside-like compounds. Radical decarboxylation of a suitably protected uronic acid, initiated photochem-ically, followed by addition of a heterocyclic base provided the C-nucleoside in high yield.154 The mode of action involves initial radical formation of 122 (Scheme 33), followed by introduction of the base and radical coupling.155 The anomeric selectivity was high in some examples, and low in others—lepidine gave the highest proportion of the ( anomer. Isolated yields were poor to moderate. 

Griesbeck, A.G., Henz, A., Kramer, W., Lex, J., Nerowski, F., and Oelgemoller, M. (1997) Synthesis of medium- and large-ring compounds initiated by photochemical decarboxylation of W-phthalimidoalkanoates. Helvetica Chimica Acta, 80, 912-933. 

Benzoin derivatives are used as initiators for the photochemical curing of printing inks, lacquers and other surface coatings, since the intermediate radicals in a reaction such as (4.7) can be diverted to initiate the polymerization of vinyl monomers. The use of an unsvm-metrical ketone (4.8) also shows that discrete radicals are produced in the cleavage reaction, since the ratio of hydrocarbon products is close to that expected tor a random combination of separated radicals. Esters that give rise to similar stabilized radicals undergo loss of carbon dioxide (decarboxylation) by a closely related mechanism, and this has proved useful in making quite strained cyclic systems bv irradiation of readily prepared cyclic diesters (4.9). 

We focus initially on the photochemical behaviour of complexes of Fe(III) with simple carboxylic acids and give particular attention to oxalic acid. This compound is prevalent in atmospheric aerosols [28], provides a simple example of environmentally important light-mediated ligand-to-metal charge transfer (LMCT) processes which result in ligand decarboxylation [27] and is used to initiate the degradation of contaminants both in the absence and presence of added hydrogen peroxide (via the so-called modified photo-Fenton process [29,30]). In addition, the photochemistry of Fe(III)-oxalate complexes has been studied in detail, as it is the basis of... 

For e radical pair, 72a, derived from 70a, decarboxylation to the more stable phenoxyl radical should be much more rapid, probably in the nanosecond time domain. Therefore, any initially formed radical pair will be rapidly converted to either the ion pair, 71a, by electron transfer or to the new radical pair, 73a, by decarboxylation. Coupling of this radical pair then gives the other three products the yield of the product 74, Eq. (29), does not necessarily reflect its primary photochemical yield because it is rapidly converted to the other two coupling products by irradiation in methanol (Sec. IV.D.2). 

Sulfamethoxazole failed to produce any trappable radicals with an array of different spin traps, but naproxen afforded the EPR spectrum shown in Figure 2.11 when irradiated with 330 nm UV-R in the instrument cavity in the presence of 2-methyl-2-nitroso-propane (MNP). The spectrum contains contributions from di-t-butyl nitroxide, a known photoproduct of MNP. The H-atom adduct MNP-H also evident can arise by several different mechanisms, including the trapping of an H atom by MNP the reaction of MNP with an electron followed by protonation and the direct reduction of MNP by an excited state species. In view of the flash photolysis results, it was concluded that photoionization was the major precursor of MNP-H. The third radical corresponded to a C-centered radical carrying a single H atom, leading to the postulate of a decarboxylation reaction as the primary photochemical step. Confirmation of the participation of free radical intermediates came from the initiation of the free radical polymerization of acrylamide with rates as shown in Table 2.1. 

Dimethylpyrrolidine and its analogs are efficient stereocontrol elements, since the C2 axis of the pyrrolidine makes the conformation about the carbonyl-nitrogen bond irrelevant. In radicals such as 27, both conformations provide essentially the same stereochemical environment for the radical center. An example of the use of dimethylpyrrolidine, is shown in Fig. 4. In this transformation, a /cr/-butyl ETOC ester is reacted with the acryloyl carboxamide of dimethylpyrrolidine and the addition product is isolated in excellent yield and selectivity. The propagation sequence involves addition of a tert-h xty radical to the acrylamide, trapping of the adduct radical, 27, by the PTOC ester, and decarboxylation of the pivaloyl carboxy radical. This transformation may be initiated thermally or photochemically, and photoinitiation at reduced temperatures gives product with higher diastereoselectivity. A... 

Diacyl peroxides are sources of alkyl radicals because the carboxyl radicals that are initially formed lose CO2 very rapidly. In the case of aroyl peroxides, products can be derived from either the carboxyl radical or the radical formed by decarboxylation. The decomposition of peroxides can also be accomplished by photochemical excitation. 

The azoperester, t-butyl 4-methyl-4-(t-butylazo)peroxypentanoate, cleaves at the azo group on excitation to yield the y-perester radical (231). Cyclisation of (231) by attack on the peroxy linkage to yield the corresponding y-lactone occurs sufficiently slowly (1.5 x 10 s at 22 °C) that the azoperester precursor may be used as a photochemical bifunctional initiator. In contrast thermolysis occurs at the peroxy linkage. Decarboxylation and cyclisation of y-azo radical (230) occur so rapidly that an azo-containing polymer would not be produced.Diarylfur-oxans (232) yield 1,2-diarylacetylenes in low yield on irradiation. ... 

Decarboxylation. Della and Patney recommend a two-step method for decarboxylation of bridgehead carboxylic acids. The first step is the Cristol-Firth modification of the Hunsdiecker reaction (I, 657, improved by use of methylene bromide as solvent). The second is photochemical reduction of the bromides by tri- -butyltin hydride (I, 1192-1193) with azobisisobutyronitrile as initiator. Yields are 80-95% in the first step and 80-90% in the second step. 

Bis(acyloxy)iodo]arenes are commonly used as efficient initiators of radical processes. Under photochemical conditions or heating these reagents undergo decarboxylative decomposition generating alkyl... 

Of greater interest is the sonolysis of lead tetraacetate (LTA) in the presence of styrene. i In this reaction, several products are formed, 1-phenylpropanol acetate via a radical pathway, and gem- and vie- diacetates via polar mechanisms (Fig. 27). 2 Conventionally, the free radical process is initiated photochemically. The cleavage of LTA to an acetoxyl radical is followed by decarboxylation and the resulting methyl radical adds to the C=C double bond. As expected, the radical process is favored by sonication. These reactions can be considered as an example of sonochemical switching. The mechanism is still unclear, and LTA was found to be stable when sonicated in the absence of styrene.53... 

Not surprisingly, decarboxylation is a common reaction of / -lactones and can be initiated either photochemically or thermally. The low-temperature photolysis of 2-oxabicyclo[2,2,0]hex-5-en-3-one (159), prepared photochemically from a-pyrone, has been used as a solid matrix preparation of cyclobutadiene. An alternative to the Wittig reaction for the stereospecific introduction of double bonds has been reported by Adam and co-workers. Thermal decarboxylation of the /S-lactone (160), prepared by cyclization of the corresponding /3-hydroxy-acid with benzenesulphonyl chloride, takes place with retention of initial geometry and without isomerisation of the... 

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