Photodimerizations are observed not only for olefins, but also for aromatic compounds, allenes, and acetylenes. The photodimerization of anthracene,
which may be considered to be a ground-state-forbidden [,4, + ,4,1 cycload- dition, was in fact described as early as in 1867 (Fritsche, 1867).
Example 7.10:
Much information about the detailed mechanism of anthracene dimerization was gained in the study of intramolecular photoreactions of linked anthracenes such as a,w-bis(9-anthryl)alkanes (66). I t was shown that luminescence and cycloaddition are competing pathways for the deactivation of excimers. In compounds with sterically demanding substituents R and R' that impair the cycloaddition reaction, the radiative deactivation is enhanced (H.-D. Becker,
1982).
Photocyclization of bis(9-anthryl)methane (67) and the corresponding photo- cycloreversion were shown by picosecond laser spectroscopy to have a com- mon intermediate whose electronic structure is different in polar or nonpolar solvents as judged by different absorption spectra in various solvents (Manring et al.. 1985).
a,af-Disubstituted bis-9-anthrylmethyl ethers (68) are characterized in their meso form by mirror-plane symmetry (a) and perfectly overlapping anthracene moieties (68a). The corresponding racemic diastereomers (68b) assume a con- formation having a twofold axis of symmetry (CJ. Both the meso and racemic diastereomers cyclomerize. However, whereas the meso compounds are vir- tuall y nonfluorescent , the racemic diastereomers deactivate radiatively both from the locally excited state and from the excimer state. Thus, in the emitting excimer state of linked anthracenes the aromatic moieties overlap only par- tially. Apparently, the formation of luminescent excimers from bichromophoric aromatic compounds is associated with perfectly overlapping n systems only when intramolecular cycloaddition is an inefficient process (H.-D. Becker,
1982).
The [2 + 21 cyclodimerization of benzene has been studied theoretically (Engelke et al., 1984) and used practically in the synthesis of polycyclic hy- drocarbons such as 69 (Fessner et al., 1983). This represents a first step in the synthesis of the undecacyclic hydrocarbon pagodane (70), which in turn can be isomerized to give dodecahedrane (Fessner et al., 1987).
Mixed photocycloadditions of anthracene and conjugated polyenes yield products that correspond to a concerted reaction path, as well as others that are Woodward-Hoffmann-forbidden and presumably result from noncon- certed reactions. For example, the reaction of singlet-excited anthracene with 1,3-cyclohexadiene yields small quantities of the [,4, + ,2,] product 72 in addition to the allowed [,4, + ,4,] product 71.
The yield of 71 increases with increasing polarity of either the solvent or the substituent X in the 9-position of anthracene. This result has been ex- plained by invoking a stabilization of the exciplex, which should reduce the barrier between the exciplex minimum and the pericyclic minimum. The ob- servation that in the presence of methyl iodide 72 becomes the major product for X = H, due to the heavy-atom effect, is compatible with the obvious assumption that the multistep process is a triplet reaction. However, if X =
CN, no methyl iodide heavy-atom effect is observed (N. C. Yang et al., 1979, 1981).
420 ORGANIC PHOTOCHEMIS'I'KY Addition of an alkene to benzene can occur in three distinct ways indi- cated in Scheme 17: ortho, meta, or para.
In general, dienophiles produce the ortho product in addition to small amounts of the para product. Maleic anhydride gives the 1:2 adduct 73 (Gil- bert, 1980). Maleimides undergo analogous reactions. Both classes of com- pounds form CT complexes with benzene and its derivatives, and the reac- tion can be initiated by irradiation into the CT band (Bryce-Smith, 1973).
Alkylethylenes, on the other hand, produce the meta adducts.
The selectivity may be explained by a consideration of the possible inter- actions of the degenerate benzene HOMO, &, +,, and LUMO, $,., &,. (cf.
Section 2.2.5 and Figure 2.33), with the ethylene n and n* MOs (Houk, 1982). In the ortho approach, the benzene MOs @a and &,. can interact with the ethylene MOs n and n*, while in the meta approach, and @a. can interact with nand d. Therefore, the configuration .,@, is predicted to be energetically favored along the ortho cycloaddition path, while the configu- ration .,@, is stabilized for the ortho and particularly the meta cycloaddi- tion path. The magnitude of the stabilization depends on orbital overlap and relative orbital energies. The former factor favors the ortho approach and the latter the meta approach of the reactants.
Altogether, one arrives at the prediction that for the addition of ethylene to the S, ('B,,) state of benzene, whose wave function can be written as
(a,,. - @,,,.)/fl, comparable amounts of ortho and meta adducts should be formed, while the meta addition should dominate whenever' the alkene MOs n and n* are close in energy to the benzene MOs +,, &, and @,., &..
Figure 7.32. Orbital energy diagram for the photocycloaddition of excited benzene to an olefin a) with an electron-donating substituent X and b) with an electron-with- drawing substituent Z (adapted from Houk, 1982).
The presence of substituents alters the situation in that charge transfer be- tween the reaction partners may become significant, and ortho addition should then be preferred over meta addition. For electron-rich alkenes or electron-poor arenes, the ethylene IT MO lies at much higher energy than the singly occupied bonding MO of the excited arene ($a for the ortho path), and thus charge transfer from the alkene to the arene will take place. For an electron-poor alkene or an electron-rich arene, charge transfer from the arene to the alkene will occur (Figure 7.32).
This analysis does not require a specification of the timing of the bond- forming events. This could correspond (1) to a fully concerted, synchronous pathway, (2) to the cyclization of benzene to a biradical referred to as pre- fulvene followed by addition of the olefin, or (3) to the bonding of the olefin to meta positions and subsequent cyclopropane formation, as indicated for the case of meta cycloaddition in Scheme 18.
Mechanism 2 of Scheme 18, which was first proposed in 1966 (Bryce- Smith et al., 1966), has been discarded because recent experimental evi- dence excludes the intermediacy of prefulvene (Bryce-Smith et al., 1986).
Mechanism 3 is presently favored. Experimental and theoretical results sup-
( ) I \ ,rU\1IC I'HO'I'OCHEMISI'KY
port the notion that the a bonds between the reactants are formed during the initial stage of the reaction, while the cyclopropane ring closure is not ex- pected until crossing to the ground-state energy surface (De Vaal et al., 1986;
van der Hart et al., 1987). A fully concerted process (mechanism 1 in Scheme 18) is considered unlikely. An argument against it, and in favor of mechanism 3, is provided by independent photochemical generation of the proposed biradical intermediate (Scheme 19), which yields the same product ratio as is obtained from the corresponding arene-alkene photocycloaddition at low conversions (Reedich and Sheridan, 1985).
Furthermore, the occurrence of an exciplex intermediate is assumed; an empirical correlation based on the AGE, values calculated from the Weller relation, Equation (5.28), has been established which allows the prediction of mode selectivity for a wide range of arene-alkene photocycloadditions (Mattay, 1987). For negative or very small values of AGE,, addition is pre- ferred over cycloaddition. This is the case for the reactions of certain elec- tron-rich alkenes with excited arenes. For positive values of AGE,, cycload-
dition is expected. Ortho cycloaddition is favored for AGE, values up to about 1.4-1.6 eV, and meta cycloaddition for more strongly positive values (Scheme 20).
Scheme 20
Regioselectivity in the meta cycloaddition to substituted benzenes has been assumed to depend on charge polarization in the biradical intermediate shown in Scheme 21 (van der Hart et al., 1987). The theoretical calculations mentioned earlier, however, point out that in the early stage of the reaction the excited state has appreciable polar character that disappears as the re- action proceeds; the biradical itself does not have any particular polarity.
Irradiation of mixtures of various acetylenes with benzene gives cyclooc- tatetraenes, presumably via an intermediate ortho adduct 74 (Bryce-Smith et al., 1970). For acetylenes with bulky substituents, the bicyclooctatriene intermediate is sufficiently stable for a subsequent intramolecular photocy- cloaddition to a tetracyclooctene (75) (Tinnemans and Neckers, 1977):
424 ORGANIC PHOTOCHEMISTRY
I