Alkenes excited states

II alkene photochemical fragmentation. The singlet, by contrast, undergoes chemistry similar to that of a benzyl cation. Examples of hydrogen abstraction by singlet alkene excited states are also well documented 12,13). 

ITie discussion in Chapter 11 described photochemical reactions of al-kenes in terms of orbital symmetry, but the possible intermediacy of multiple, independent alkene excited states complicates the analysis of photochemical reactions in terms of the Woodward-Hoffmann rules. For example, photochemical ring opening of cyclobutenes appears to be nonstereospecific. Irradiation of the cyclobutene 31 with 193 nm UV radiation produced not only the allowed as,cfs,cis-l,3,5-cyclodecatriene 32, but also the cis,trans,cis isomer 33 and the cis,cis,trans isomer 34. Among the possible explanations for the forbidden products are ... 

The complementary relationship between thermal and photochemical reactions can be illustrated by considering some of the same reaction types discussed in Chapter 11 and applying orbital symmetry considerations to the photochemical mode of reaction. The case of [2ti + 2ti] cycloaddition of two alkenes can serve as an example. This reaction was classified as a forbidden thermal reaction (Section 11.3) The correlation diagram for cycloaddition of two ethylene molecules (Fig. 13.2) shows that the ground-state molecules would lead to an excited state of cyclobutane and that the cycloaddition would therefore involve a prohibitive thermal activation energy. 

The bicyclic product is formed by coupling of the two radical sites, while the alkene results from an intramolecular hydrogen-atom transfer. These reactions can be sensitized by aromatic ketones and quenched by typical triplet quenchers and are therefore believed to proceed via triplet excited states. 

The diradical is believed to be preceded on the reaction path by a complex of the alkene with excited-state ketone. This reaction, particularly its stereochemistry and regioselec-... 

The isomerization of alkenes is believed to take place via an excited state in which the two sp carbons are twisted 90° with respect to one another. This state is referred to as the p (perpendicular) state. This geometry is believed to be the minimum-energy geometry for both the singlet and triplet excited states. 

Aromatic compounds such as toluene, xylene, and phenol can photosensitize cis-trans interconversion of simple alkenes. This is a case in which the sensitization process must be somewhat endothermic because of the energy relationships between the excited states of the alkene and the sensitizers. The photostationary state obtained under these conditions favors the less strained of the alkene isomers. The explanation for this effect can be summarized with reference to Fig. 13.12. Isomerization takes place through a twisted triplet state. This state is achieved by a combination of energy transfer Irom the sensitizer and thermal activation. Because the Z isomer is somewhat higher in energy, its requirement for activation to the excited state is somewhat less than for the E isomer. If it is also assumed that the excited state forms the Z- and -isomers with equal ease, the rate of... 

These reactions are believed to proceed through a complex of the alkene with a singlet excited state of the aromatic compound (an exciplex). The alkene and aromatic ring are presumed to be oriented in such a manner that the alkene n system reacts with p orbitals on 1,3-carbons of the aromatic. The structure of the excited-state species has been probed in more detail using CAS-SCF ab initio calculations. ... 

Two different alkenes can be brought to reaction to give a [2 -I- 2] cycloaddition product. If one of the reactants is an o, /3-unsaturated ketone 11, this will be easier to bring to an excited state than an ordinary alkene or an enol ether e.g. 12. Consequently the excited carbonyl compound reacts with the ground state enol ether. By a competing reaction pathway, the Patemo-Buchi reaction of the 0, /3-unsaturated ketone may lead to formation of an oxetane, which however shall not be taken into account here ... 

The irradiation is usually carried out with light of the near UV region, in order to activate only ihc n n transition of the carbonyl function," thus generating excited carbonyl species. Depending on the substrate, it can be a singlet or triplet excited state. With aromatic carbonyl compounds, the reactive species are usually in a Ti-state, while with aliphatic carbonyl compounds the reactive species are in a Si-state. An excited carbonyl species reacts with a ground state alkene molecule to form an exciplex, from which in turn diradical species can be formed—e.g. 4 and 5 in the following example ... 

In contrast with the thermal process, photochemical [2 + 2] cycloadditions me observed. Irradiation of an alkene with UV light excites an electron from i /, the ground-slate HOMO, to which becomes the excited-slate HOMO. Interaction between the excited-state HOMO of one alkene and the LUMO of the second alkene allows a photochemical [2 + 2j cycloaddition reaction to occur by a suprafacial pathway (Figure 30.10b). 

The isomerization takes place because the excited states, both 5i and T, of many alkenes have a perpendicular instead of a planar geometry (p. 311), so cis-trans isomerism disappears upon excitation. When the excited molecule drops back to the So state, either isomer can be formed. A useful example is the photochemical conversion of c/s-cyclooctene to the much less stable trans isomer." Another interesting example of this isomerization involves azo crown ethers. The crown ether (5), in which the N=N bond is anti, preferentially binds NH4, Li, and Na, but the syn isomer preferentially binds and Rb (see p. 105). Thus, ions can be selectively put in or taken out of solution merely by turning a light source on or off." ... 

Isomerization of ( /Z) isomers is another important transformation. Isomerization of ( ) and (Z-) conjugated amides is effected photochemically " (photo-isomerization " ). There is a rather high energy barrier for the excited state required for (E/Z) isomerization. Isomerization of the C=C units in dienes is also induced photochemically. " Isomerization of cyclic alkenes is more difficult but cyclooctene is isomerized photochemically. " Conjugated aldehydes have been isomerized... 

Ordinary aldehydes and ketones can add to alkenes, under the influence of UV light, to give oxetanes. Quinones also react to give spirocyclic oxetanes. This reaction, called the Patemo-BUchi reaction,is similar to the photochemical dimerization of alkenes discussed at 15-61.In general, the mechanism consists of the addition of an excited state of the carbonyl compound to the ground state of the alkene. Both singlet (5i) and n,n triplet states have been shown to add to... 

Photocycloaddition of Alkenes and Dienes. Photochemical cycloadditions provide a method that is often complementary to thermal cycloadditions with regard to the types of compounds that can be prepared. The theoretical basis for this complementary relationship between thermal and photochemical modes of reaction lies in orbital symmetry relationships, as discussed in Chapter 10 of Part A. The reaction types permitted by photochemical excitation that are particularly useful for synthesis are [2 + 2] additions between two carbon-carbon double bonds and [2+2] additions of alkenes and carbonyl groups to form oxetanes. Photochemical cycloadditions are often not concerted processes because in many cases the reactive excited state is a triplet. The initial adduct is a triplet 1,4-diradical that must undergo spin inversion before product formation is complete. Stereospecificity is lost if the intermediate 1,4-diradical undergoes bond rotation faster than ring closure. 

The reaction is stereospecific for at least some aliphatic ketones but not for aromatic carbonyls.197 This result suggests that the reactive excited state is a singlet for aliphatics and a triplets for aromatics. With aromatic ketones, the regioselectivity of addition can usually be predicted on the basis of formation of the more stable of the two possible diradical intermediates obtained by bond formation between oxygen and the alkene.198... 

Fischer, M. Wan, P. m-Quinone methides from m-hydroxy-1,1-diaryl alkenes via excited-state (formal) intramolecular proton transfer mediated by a water trimer. J. Am. Chem. Soc. 1998, 120, 2680-2681. 

III. Rearrangements in the Excited State vs. Carbene-Alkene Complexes. .. 57... 

III. REARRANGEMENTS IN THE EXCITED STATE VS. CARBENE-ALKENE COMPLEXES... 

The second intermediate s identity has been debated since the mid-1980s. In 1984, Liu and Tomioka suggested that it was a carbene-alkenc complex (CAC).17 Similar complexes had been previously postulated to rationalize the negative activation energies observed in certain carbene-alkene addition reactions.11,30 A second intermediate is not limited to the CAC, however. In fact any other intermediate, in addition to the carbene, will satisfy the kinetic observations i.e., that a correlation of addn/rearr vs. [alkene] is curved, whereas the double reciprocal plot is linear.31 Proposed second intermediates include the CAC,17 an excited carbene,31 a diazo compound,23 or an excited diazirine.22,26 We will consider the last three proposals collectively below as rearrangements in the excited state (RIES). 

In the absence of direct evidence for CACs other second intermediates have been proposed to rationalize the curvature observed in correlations of addn/rearr vs. [alkene], A particularly viable candidate is an excited state (or species derived therefrom) of the carbene precursor, a suggestion that is quite apposite when the precursor is a photolytically decomposed diazirine. 

If Scheme 2 accurately represented the PhCH2CCl chemistry, curvature in the addn/rearr vs. [alkene] correlation would persist when the carbene was generated from 37. The absence of curvature in this case counts against Scheme 2 (and the CAC mechanism), but accords with the RIES mechanism, Scheme 3. Elimination of the diazirine precursor eliminates the diazirine excited state. From 37, both cyclopropane formation and 1,2-H rearrangement proceed from a single (carbene) intermediate, and addn/rearr vs. [alkene] is linear.25... 

Of all the carbenes considered in this chapter, benzylchlorocarbene (10a) has produced the most debate, from carbene-alkene complexes vs. excited state... 

Understand the importance and role of the lowest (n,n ) excited state of alkenes in the photochemistry of alkenes. 

Describe the structure of the vertical and nonvertical excited states of alkenes and show how the interconversion and deactivation of these states leads to stereochemical isomerisation. 

Absorption of a photon by an alkene produces a (tt,Jt ) vertical (Franck-Condon) excited state in which the geometry of the ground state from which it was formed is retained. Since the (it,it ) state has no net n bonding, there is little barrier to free rotation about the former double bond. Thus, relaxation takes place rapidly, giving a nonvertical (it,it ) state with a lower energy and different geometry to the vertical excited 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 ... 

The excited states of alkenes can undergo sensitised addition reactions ... 

Predict the products of the reactions of excited-state carbonyl compounds with alkenes (Paterno-Buchi reaction). 

We now come to the third principal reaction of the ketone (n,7t ) excited state addition to alkenes. 

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