Another photocycloaddition reaction that has been known for a long time is the Paterno-Buchi reaction, which involves the formation of oxetanes through the addition of an excited carbonyl compound to olefins:
As far as the addition of aromatic carbonyl compounds is concerned, only the triplet state is reactive, and hydrogen abstraction occurs as a side reac- tion. Another competing reaction path involves energy transfer (cf. Section 7.1.2); efficient oxetane formation is therefore observed only when the trip- let energy of the carbonyl compound is not high enough for sensitized triplet excitation of the olefin. As is to be expected from the loose geometry of a triplet biradical intermediate, the reaction is not stereospecific; the same mixture of oxetanes 76 and 77 is produced at low conversion by irradiating cis- or trans-2-butene with benzophenone (Turro et al., 1972a; Carless,
1973):
That the reaction is partly regiospecific, as indicated in Scheme 22, has been attributed to the differing stabilities of the biradical intermediates (Yang et al., 1964).
Scheme 22
The mechanism of the photochemical oxetane formation is summarized in Figure 7.33. The 3(n,n*) excited state of the ketone produced by light absorption and subsequent intersystem crossing (ISC) attacks the olefin to form a triplet 1,Cbiradical (k,,). Intersystem crossing (kist) to the singlet, either directly or via a contact ion pair (kip), leads to ring closure (kc) to form the oxetane, or to /3 cleavage to re-form the reagents (k8). The contact ion
I
Figure 7.33. Mechanism of the photochemical oxetane formation: k,, k,,, and kc are rate constants of triplet biradical formation, intersystem crossing, and cycliza- tion; kip and k, correspond to the formation of a contact ion pair and solvent-sepa- rated ion pairs; and kS and k,, are rate constants of the back reaction of the singlet biradical and back-electron transfer, respectively (by permission from Buschmann et al., 1991).
pair may also dissociate to a solvent-separated ion pair (k,) or return to the reactants by back-electron transfer (k,,). The triplet biradical intermediate was identified by picosecond spectroscopy (Freilich and Peters, 1985). A mechanism involving the formation of polar exciplexes in the first step, as well as electron-transfer processes, has been invoked in the interpretation of biacetyl emission quenching by electron-rich alkenes (Mattay et al.,
1984b; Cersdorf et al., 1987).
Diastereomeric oxetanes are formed from chiral carbonyl compounds such as menthyl phenylglyoxylate (78) (Buschmann et al., 1989).
The general kinetic scheme (Fig. 7.34) displays two stages of diastereo- selection: (1) a preferred formation of that of the two diastereomeric 1,4-
0I:CANIC PHOTOCHEMISTRY
Figure 7.34. A simplified kinetic scheme for the diastereoselective oxetane forma- tion in the Paterno-Bilchi reaction (k, and k; as well as k , and k; include intersystem crossing steps) (by permission from Buschmann et al., 1989).
biradicals (k, > k;) which leads to the majority oxetane product, and (2) a preferred /3 cleavage of the biradical that leads t o the minority oxetane prod- uct (k; > &,), o r a preferred ring closure of the biradical, which leads to the majority oxetane product (k, > k;). Upon continued irradiation, the Pcleav- age products reenter the reaction cycle, establishing a "photon-driven selec- tion pump."
The Arrhenius plot for the diastereoselectivity (cf. Section 6.1.5.2) con- tains two linear regions, one with a positive and one with a negative slope, changing into each other at the "inversion temperature," Tin,. Each region has a different dominant selection step. At temperatures above Tin", selection is driven primarily by enthalpy, a s expected for an early transition state in the bond cleavage in the energy-rich biradical intermediate. Below Ti,,, se- lection is driven primarily by entropy, a s expected for an early transition state in the bond-forming step in the formation of the biradical from the energy-rich reactants (Buschmann et al., 1989).
Example 7.11:
An interesting example of diastereoselectivity is provided by the photocy- cloaddition of aromatic aldehydes to electron-rich cyclic olefins such as 2.3- dihydrofuran (79):
rn e ndo
Figure 7.35. Preferred conformations for intersystem crossing during the diastereoselective oxetane formation (adapted from Griesbeck and Stadtmiiller,
1991).
Figure 7.35a shows the postulated geometry of favored intersystem crossing in the biradical derived from an unsubstituted cycloalkene; bond formation fol- lowing spin inversion is faster than conforrnational changes, and the endo ad- duct is formed from the less sterically hindered conformation. The exoselec- tivity observed in I-substituted cycloalkenes is explained by increasing gauche interactions with the Balkoxy group that favor the biradical conformations shown in Figure 7.35b (Griesbeck and Stadtmiiller, 1991).
The photoaddition of a furan and an aldehyde can serve as a photochem- ical version of a stereoselective aldol reaction, since the photoadduct can be viewed a s a protected aldol, a s indicated in Scheme 23 (Schreiber et al., 1983).
Scheme 23
For aliphatic ketones, the situation is complicated by less efficient inter- system crossing, thus permitting reaction of the '(n,n*) a s well a s the '(n,llr) state of the carbonyl compound, a s revealed by the use of triplet quenchers.
The S, reaction is more stereospecific, presumably because of the tight ge- ometry of the singlet biradical, and yields less cis-trans isomerization by a competing path.
Thus, the stereochemistry of cis-I-methoxy-I-butene (80) is partially re- tained when acetone singlets attack the olefin, but it is almost completely scrambled in the reaction of triplet acetone (Turro and Wriede, 1970):
The stereochemistry of the reaction can be accounted for by the conforma- tional dependence of .;pin-orhit cor~plinp elements dihc~~rred in Section 4.3.4.
428 ORGANIC PHOTOCHEMISTRY The situation is different with electron-poor olefins such as 1,2-dicyano- ethylene. Only the S, state of acetone forms oxetanes, and the reaction is highly stereospecific, as indicated in Scheme 24. The competing cis-trans isomerization of the olefin arises exclusively from the triplet state (Dalton et al., 1970).
+
Scheme 24 NCAcN CN NC
The specificity of this reaction has been used to chemically rirrate both the excited-singlet acetone and the triplet acetone produced through thermal decomposition of tetramethyl-1,2-dioxetane. (Cf. Section 7.6.4.) For this purpose the thermolysis was carried out in the presence of trans-1 ,Zdicy- anoethylene, and the quantities of singlet and triplet acetone formed were obtained from the yields of dioxetane and cis-1 ,2-dicyanoethylene, respec- tively (Turro and Lechtken, 1972).
The various reactions of excited carbonyl compounds with olefins may be rationalized on the basis of correlation diagrams. In principle, four dif- ferent pathways have to be discussed: the perpendicular and the parallel approaches (Figure 7.36) and the initial formation of a CO and a CC bond, yielding a C,C-biradical and C,O-biradical, respectively:
However, only three out of these four possibilities are realistic, since there is no carbonyl group orbital available for a perpendicular approach a ~ d for- mation of a CC bond. Formation of a C,O-biradical is therefore possible only through a parallel approach.
Perpendicular approach
Parallel approach
Figure 7.36. The perpendicular and the parallel approach for the interaction of an mn* excited ketone with a ground-state olefin.
Figure 7.37. Correlation diagram a) for the perpendicular approach of ketone and olefin, and for the parallel approach resulting in b) a C,C-biradical and c) a C,O- biradical.
From Figure 7.37a, the perpendicular approach leading to a C,C-biradical is seen to be electronically allowed. Since, however, the two p AOs of the unpaired electrons are oriented perpendicular to each other, a rotation of the CH, group of the ketone is required prior to cyclization both for the singlet biradical and for the triplet biradical, which assume similar geometries and therefore can each give an oxetane. Small rotations in the less sterically hindered direction to optimal geometries for ISC and subsequent reaction may lead to stereospecific oxetane formation from the triplet state. (Cf. Ex- ample 7.11 .)
Correlation diagrams for the two modes of parallel approach are shown in Figures 7.37b and 7.37~. If the CO bond is formed first, the (n,n*) excited reactant states correlate with highly excited (n,aGo) states of the products and a correlation-induced barrier results. Hence this reaction is electroni- cally forbidden (Figure 7.37b). If, however, the CC bond is formed first, the unpaired electron at the oxygen can be localized in a p A 0 with either a or n symmetry, that is, either in the fco MO or in the no orbital. Since the '.3(n,n*) reactant states correlate with the 'v3B,,., product states, no correla- tion-induced barrier is to be expected, and the reaction is likely to be exo- thermic (Figure 7.37~). In contrast to the perpendicular approach, the triplet biradical of the parallel approach will have a loose geometry and should result in cis-trans isomerization of the olefin.
The conclusions from the correlation diagrams have been nicely con- firmed by early ab initio calculations for the carbon-xygen attack of for- maldehyde on ethylene (Salem, 1974) and by more recent calculations on the same model reaction considering both modes of attack. (Palmer et al., 1994).
The results for the carbon-oxygen attack are summarized schematically in Figure 7.38. The excited-state branch of the reaction path terminates in a conical intersection point at a CO distance of 177 pm before the biradical is fully formed (cf. Figure 7.37a). Thus the system can evolve back to the reac- tants or produce a transient C,C-biradical intermediate that is isolated by small barriers (< 3 kcallmol) to fragmentation (TS,) or to rotation and ring closure to oxetane (TS,). The singlet and triplet biradical minima are essen- tially coincident.
A schematic representation of the surfaces for the carbon-carbon attack is shown in Figure 7.39. The very flat region of the S,, surface (barriers of the order of I kcallmol) corresponds to the '.3B,., C.0-biradical. The '.3B,, biradical has a CC bond length of 156 pm and corresponds to a conical in- tersection geometry in the case of the singlet, and to a minimum in the case of the triplet. Thus for the singlet photochemistry the decay to So occurs close to the products, and the reaction appears to be concerted. Since, how- ever, the formation of the singlet biradical is also possible from the same funnel, a certain fraction of photoexcited reactant can evolve via a noncon- certed route.
Figure 7.38. Photocycloaddition of formaldehyde and ethylene. Schematic repre- sentation of the surfaces involved in the carbon-oxygen attack as a function of the C,O distance R, and the dihedral angle rp between the formaldehyde and ethylene fragments. )( and denote the parallel and perpendicular approach of the reactants, respectively; CI marks the conical intersection (by permission from Palmer et al., 1 994).
Figure 7.39. Photoaddition of formaldehyde and ethylene. Schematic representa- tion of the carbondxygen attack as a function of the C,C distance R,, and the di- hedral angle rp between the formaldehyde and ethylene fragments (by permission from Palmer et al., 1994).
The overall nature of the singlet reaction path is determined by whether the system returns to S,, through an early funnel along the path of carbon- oxygen attack to produce a C,C-biradical intermediate (Figure 7.38) or through a late funnel along the path of carbon-carbon attack to produce oxetane in a direct process (Figure 7.39). The stereochemistry and efficiency of the reaction will depend on which of these two excited-state reaction paths is followed.
Conclusions similar to those obtained from the correlation diagrams can be reached by means of either PMO theory (Herndon, 1974) or simple fron- tier orbital theory, as indicated in Figure 7.40. Orbital energies may be esti- mated from the assumption that the olefinic n MOs will be stabilized by electron-withdrawing substituents and destabilized by electron-releasing ones. The HOMO energy of an electron-rich olefin will then be comparable to that of the carbonyl n orbital, whereas the LUMO of electron-poor olefins will be similar in energy to the n* MO of the carbonyl group. Interactions between orbitals of comparable energy require a parallel approach in the case of electron-poor olefins and result in CC bond making and formation of a C,O-biradical. Perpendicular attack is required for electron-rich olefins and yields a C,C-biradical by CO bond formation.
ORGANIC PHOTOCHEMISTRY
Electron-pwr olef ins
Electron - rich olef ins
Figure 7.40. Frontier orbitals for oxetane formation. Interaction of one of the half- filled carbonyl orbitals with the LUMO of electron-poor olefins (left) and with the HOMO of electron-rich olefins (right).
The magnitude of the LCAO MO coefficients of the interacting orbitals permits a prediction of the expected oxetane regiochemistry. Both in the HOMO of a donor-substituted olefin and in the LUMO of an acceptor-sub- stituted olefin, the coefficient of the unsubstituted carbon atom is the larger one in absolute value. Therefore, electron-poor olefins regioselectively af- ford the oxetane with the substituted carbon next to the oxygen. (Cf. Barl- trop and Carless, 1972.) In contrast, an electron-rich olefin predominantly yields the oxetane with the unsubstituted carbon next to the oxygen. (Cf.
Scheme 22.)
Finally, Figure 7.40 permits the conclusion that oxetane formation can be considered to be either a nucleophilic attack by a ketone on an electron- poor olefin or an electrophilic attack by a ketone on an electron-rich olefin, corresponding to the predominant interaction of a half-filled carbonyl orbital either with the empty n* MO or with the doubly filled rc MO of the olefin.