Meta arene-alkene photocycloaddition

Arene-alkene photocycloaddition reactions have been reviewed in detail. This review includes a tabular survey, synthetic applications and exhaustive bibliography. Many of the reactions considered in this review were however performed to determine answers to mechanistic questions, therefore they are not necessarily optimized and chemical yields are not indicated. Tables 2 and 3 bring together, from the references cited in the review , those intermolecular photochemical meta cycloadditions which are principally synthetic methods. More concise general treatments of this reaction have since been published, as have other papers mainly concerned with its mechanistic aspects. """ ... 

In spite of these deviations, the ionization potential rule still has considerable predictive value, although it may be a little too restricted to cover all the addends [116,184], A slight modification of the rule was introduced by Gilbert et al. [12] who proposed that it might be more meaningful to relate the relative effi-ciences of the meta and ortho photocycloaddition with the difference in ionization potential only within a series of structurally very similar alkenes. It was also recognized that ionization potentials relate to properties of the ground state of the reactants rather than of the excited state [185], Nevertheless, the IP rule retains its predictive value in a series in which various arenes are irradiated in the presence of the same alkene [133], Ethyl vinyl ether and benzonitrile (IP = 10.02 eV) yield... 

This arene-alkene intramolecular meta photocycloaddition was shown to provide the basis for an effective and general approach to polyquinane natural products, such as (+ )-cedrene or ( + )-isocomene. ... 

Wender, P. A. and Dore, T. M., A formal synthesis of crinipellin B based on the arene-alkene meta-photocycloaddition reaction. Tetrahedron Lett., 39, 8589, 1998. 

Several reviews have been appeared about the synthetic utility and mechanistic details of photocycloadditions to aromatic rings [18,20-23], In this subsection, we will deal with mainly the examples of last two decades. Photocycloaddition of alkenes to benzene rings proceeds at ortho, meta, and para positions as described earlier. It has been suggested that alkenes and arenes having either electron-releasing or electron-accepting substituents favor the ortho process, whereas relatively simple alkenes and arenes undergo meta addition [24,44],... 

Chappell, D. and Russell, A.T. (2006) From a-cedrene to crinipellin B and onward 25 years of the alkene-arene meta-photocycloaddition reaction in natural product synthesis. Organic tg Biomolecular Chemistry, 4 (24), 4409-4430. 

The most characteristic photochemical reaction of aromatic compounds is their cydoaddition with alkenes. The intramolecular reaction is suitable for the synthesis of complex structures, such as those depicted in Scheme 9.49, where [3+2]-photocycloaddition leads to structures which resemble natural products (aphidico-line and stemoclinone). An interaction of the arene singlet excited state with the alkene ground state gives rise to the meta adduct [83, 84]. 

Boyd, J.W., Greaves, N., Kettle, J., Russell, A.T., and Steed, J.W. (2005) Alkene-Arene meta photocycloadditions with a four-carbon-atom tetheu efficient approach toward the polycyclic ring systems of aphidicolin and stemodinone. Angewandte Chemie, International Edition, 44, 944—946. 

While the meta photocycloaddition occurs at the n,n singlet state of the arene [4], different cases must be distinguished for the ortho cycloaddition depending on the structure of the substrates [5] (a) excitation of a ground state charge-transfer complex (b) excitation of the alkene (or alkyne) reaction partner (c) excitation of the arene partner and reaction at the singlet state and (d) excitation of the arene followed by intersystem crossing and reaction with the alkene at the triplet state. 

At the singlet excited state, ortho and meta photocycloadditions are often competitive processes and physicochemical investigations were carried out to rationalize the modes of cycloaddition of arenes with alkenes. In the context of the study of photochemical electron transfer reactions, it has been proposed that the difference of the redox potentials of the reaction partners might play an important role in this competition [10]. Such a discussion involves the intervention of an exciplex as intermediate. The Rehm-Weller equation [11] was used to quantify the relationship. When an electron transfer process is strongly endergonic (AG>1.5eV), the meta cycloaddition should be favored. When such a process is less endergonic (1 < AG< 1.5 eY), the ortho addition dominate [12]. This means that the... 

The photochemical transformations of vinylcyclopropanes are of mechanistic interest, but because of the high energy of the reaction intermediates, especially in the triplet manifold, practical applications are scarce. The regio- and stereo-selective aspects are, however, similar to those of the thermal processes. Additional methods that provide for the cyclopentene rearrangement involve the degenerate photochemical rearrangements observed by Wender during meta-photocycloaddition of arenes and alkenes and utilized extensively in the synthesis of triquinane sesquiterpenes. -" -" (See Section 8.1.9 or ref. 41 for a recent summary.)... 

Table 3. meta Photocycloadditions of Arenes with Other Alkenes... 

Some years ago Cornelisse reported that deuteration of alkyl benzenes results in a deuterium isotope effect upon the quantum yield of the meta photocycloaddition reaction with alkenes. In a new report the same group has published an analysis describing how the observed isotope effect upon the reaction quantvun yield can be ascribed to a kinetic deuterium isotope effect on the excited state reaction and distinguished from an effect upon the unimolecular photophysical modes of decay of the excited state. In addition, it is reported that when the quantum yield of meta photocycloaddition of cyclopentene to alkyl benzenes is measured using a mixture of deuterated and non-deuterated benzenes, the quantum yield is arene concentration dependent.The authors argue that this arises from competition between cycloaddition and the formation of mixed excimers between deuterated and non-deuterated alkyl benzenes which dissociate to yield excited deuterated alkyl benzene and ground state non-deuterated alkyl benzene preferentially. 

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