Concerted photoreactions

Adapted from W.G. Herkstroeter and G.S. Hammond, Energy transfer study by kinetic spectrophotometry , Journal of the American Chemical Society, Volume 88. American Chemical Society 

A concerted reaction is a chemical reaction in which all bond breaking and bond formation occurs in a single step in which reactive intermediates are not involved. Concerted photoreactions tend to be stereospecific, occurring from the vertical excited state. 

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Outline the important features of the concerted photoreactions of C=C compounds, including the excited state involved, the stereochemistry and the factors which determine the direction of photochemical change. 

An alternative mechanism occurs for concerted photoreactions in which the bond-breaking and bond-forming processes occur together. Such reactions proceed via a single transition state ... 

A cycloaddition reaction produces a ring of atoms by forming two new G-bonds, for example the formation of a cyclobutane dimer from two alkene molecules. The direct photoreaction involves the concerted reaction of the singlet Jtpt ) excited state of one alkene with the ground state of the other. Stereospecific reactions in which the dimers preserve the ground-state geometry occur when liquid cis- or trans-but-2-ene are irradiated at low temperature ... 

Bergmark theorized that photosolvolysis of Z-34 in methanol resulted in the release of the chlorine atom and the formation of 35 but that release from E-34 took place in a concerted reaction to form a chlorine atom and 36. This claim was further supported by the photoreactivity of 38 (Scheme 25). Photolysis of 38 in methanol resulted in products 39, 40, 41, and 42, and Bergmark suggested that the formation of 42 came from a photo-Favorksii rearrangement, which supports the notion that products 39 2 were formed via a carbocation intermediate. 

The much studied photochemistry of aryldisilanes carried out in earlier years has been reviewed51,52. Cleavage of the silicon-silicon bond of the disilyl moiety is always involved, but various other reactions have been observed depending on the structure of the disilane and the conditions employed. Thus cleavage to a pair of silyl radicals, path a of Scheme 15, is normally observed, and their subsequent disproportionation to a silene and silane, path b, is often observed. There is evidence that the formation of this latter pair of compounds may also occur by a concerted process directly from the photoex-cited aryldisilane (path c). Probably the most common photoreaction is a 1,3-silyl shift onto the aromatic ring to form a silatriene, 105, path d, which may proceed via radical recombination52. A very minor process, observed occasionally, is the extrusion of a silylene from the molecule (path e), as shown in Scheme 15. 

The relative linearity of a Stern-Volmer plot of 1/< > versus 1/[A] for the photoreaction of cyclohexenone supports this mechanism and suggests that only one excited state is involved (30). The [2+2] addition of the triplet to a ground state molecule is not concerted, and there is a large solvent polarity effect on the regioselectivity for the head-to-head (HH) versus head-to-tail (HT) photodimers, with the more polar head-to-head isomer being favored in polar media. The polarity effect is attributed to the large difference in the dipole moments of the transition states leading to the products. 

The oxazines (325) are formed as primary products in the photoreactions of tetrahydro[3,4-d]isoxazoles (324), but themselves undergo cyclization to give the furooxazines (326) on further irradiation (Scheme 23). This reaction (325)-< (326) seems to be a concerted process involving migration of OR. This process seems to be similar to the classical d-re-methane rearrangement <88CCC3171, 88ZC22). 

In an earlier report Mazzocchi and his coworkers reported that the photo-reaction of A) methylnaphthalimide (325) with phenyIcyclopropane involved the production of a radical cation/radical anion pair. The product from the reaction was the cyclic ether (326). - A study of the mechanism of this reaction using suitably deuteriated compounds has demonstrated that the reaction is not concerted and takes place via the biradical (327). - Other systems related to these have been studied. In the present paper the photoreactivity of the naphthalimide (328) with alkenes in methanol was examined. Thus, with 1-methylstyrene cycloaddition occurs to the naphthalene moiety to afford the adducts (329) and (330). The mechanism proposed for the addition involves an electron transfer process whereby the radical cation of the styrene is trapped by methanol as the radical (331). This adds to the radical anion (332) ultimately to afford the observed products. Several examples of the reaction were descr ibed. 

Pericyclic minima and funnels that can be easily predicted by means of correlation diagrams are of great importance in concerted pericyclic photoreactions. However, there may be additional minima and barriers on the excited-state surfaces that affect or even determine the course of the photoreaction. 

Phenomenologically, we can distinguish the following photochemical reactions photoisomerization, photocyclization (photoaddition), photocleavage, hydrogen abstraction, photo-concerted reaction, etc. For a photochemical reaction to occur, efficient absorption of ultraviolet or visible light is necessary, thus the photoreactive molecules should contain in their structure one of the bond types listed in Table 1.13. The characteristics of the typical photochemical reactions of C = C and C = O groups are summarized as follows. 

A group of papers dealing with the intramolecular photodimerization reaction of a,a>-bis(9-anthryl)alkanes (185 n = 2—10) fails to produce agreement about the detailed mechanism of the reaction. Measurements of quantum yields for fluorescence, photoreaction, and intramolecular deactivation as a function of temperature are said to provide no support for a biradical intermediate, but rather to support a concerted mechanism. In reply, the proposers of the biradical mechanism reinterpret these data and find them consistent with their mechanism. A third research group reports results in fluid solution at room tempera-ture " their concern is more with the question of excimer involvement in the mechanism, and they report that in many of the systems unidentified photoproducts are formed via excimers that do not lead to the normal 9,9 -linked photodimer. The internal photodimer from (185 n = 3) has been studied in a matrix at 10 and photodissociation is shown to lead to two different modifications of (185 n = 3) with different reactivities. A geometrical constraint on the intramolecular photoreactions of 9,9 -linked bisanthracenes is demonstrated by the failure of the di-substituted ethylenes (186) to give internal dimers. 

A novel intra-ion-pair electron(i) transfer cleavage of the —C-B— bond is reported by Chatterjee et al. [98] from the photoreaction of triphenyl alkyl borate salts of cyanine dye (110) to generate a carbon-centered radical (113) whose utility for initiating a free radical polymerization reaction has been demonstrated (Scheme 26). An intramolecular concerted bond cleavage/coupling process is described [99] from phenyl anthracene sulphoniiun salt derivatives as a part of designing a novel Photoacid system. 

All four participants, 1,2, 5, and 6, in the photoisomerization process absorb light, albeit with differing intensities in the region between 254 and 350 nm (vide infra), and six different photoreactions with different quantum yields happen in concert. Taking account of this fact, it is not surprising to learn that the most diverse efforts have been vmdertaken to raise the chemical yield of 7. 

To gain a more indepth insight into the primary photochemical process, Ce(IV) photoreactions have been studied in the presence of various organic substrates using EPR spectroscopy of frozen solutions at 77 Thus, photolysis of 4M HCIO solutions of 0.1 M Ce(ClO ) and 0.1 M RCO2H produces the corresponding alkyl radical R. The primary photochemical event is a concerted oxidative decarboxylation ... 

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