Alkene electronically excited states

One of the most characteristic types of ground-state reaction for alkenes is electrophilic addition, often involving a proton acid as addend or catalyst. In the excited state similar reactions can occur, with water, alcohols or carboxylic acids as commonly encountered addends. However, there is a variety of photochemical mechanisms according to the conditions or substrate used. In a few instances it is proposed that the electronically excited state is attacked directly by a proton from aqueous acid, for example when styrenes are converted to l-arylethanols (2.47 the rate constant for such attack is estimated to be eleven to fourteen orders of magnitude greater than that for attack on the ground state, and the orientation of addition is that expected on the basis of relativecarbonium ion stabilities (Markowni-kov addition). 

There are only a few examples of singlet photoreaction between alkenes that are not tethered or constrained in close proximity. In these cases, calculations have suggested the presence of exciplex [6] and/or diradical intermediates [7]. The short lifetime of the typical alkene singlet excited state (on the order of 10-20 ns) [8] limits the chance for productive collisions. On the other hand, photochemistry between electron rich and electron poor alkenes such as tetracyanoethylene and methoxy substituted alkenes, provides evidence for an electron transfer process [9]. These matched pairs benefit from ground state attraction and a resulting preorientation that enhances the alkene orbital overlap. Alternatively, electron transfer pathways have been accessed by employing electron transfer sensitizers (see Sch. 2), DCA is dicyanoanthracene) [10]. 

It occurred to us some time ago that since electronically excited states of functional groups such as ketone carbonyls and alkenes undergo reactions very similar to those of alkoxy and alkyl radicals, it might be possible to observe reactions of these excited states with three-coordinate phosphorus. Of particular interest would be attack at phosphorus in an intramolecular context as shown conceptually in the equations of Scheme II. In fact we observed the first example of a photorearrangement that may well proceed by such a mechanism over twenty years ago (10). 

For a more complete discussion of the electronically excited states of planar and twisted ethylene and alkenes, four rather than the three different electronic states shown in Figure 5 have to be considered (Table 18) (74,75). 

For reviews of the photobehavior of alkenes in the gas phase, see (a) Collin, G. J., Ring contraction of cyclic olefins chemical processes specific to electronically excited states /. Photochem., 38, 205, 1987 (b) Collin, G.J., Photochemistry of simple olefins chemistry of electronic excited states or hot ground state Adv. Photochem., 14, 135, 1988. 

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 different electron distribution in the excited state also may lead to other types of reactions. As an example, alkenes and polyenes display a low intermole-cular reactivity, but undergo extremely fast rearrangements, since the tt bonding character dramatically diminishes in the excited state. Thus, free rotation becomes feasible and, where appropriate, electrocyclic and sigmatropic processes take place (Figure 3.3). 

Alkenes, alkynes, and arenes become stronger bases in the singlet excited state. As a result, photoprotonation can occur under much more weakly acidic conditions than required in the ground state. Excited styrenes and phenylacetylenes, for example, are protonated by the solvent in 2,2,2-trifluoroethanol (TEE) and 1,1,1,3,3,3-hexafluoroisopropanol (HFIP), giving rise to phenethyl and a-arylvinylcations that can be observed by LFP (see Eq. 15). In a similar manner, benzenium ions can be observed by photoprotonation of electron-rich aromatics in FIFIP. " Equation 16 provides an example where the orientation of the protonation is different in the excited state from that of the ground state. 

Of course, a close stmctural relationship between radical cations and parent molecules is not likely to hold generally, but it is a fair approximation for alternant hydrocarbons. Deviations have been noted some stilbene radical cations have higher-lying excited states without precedent in the PE spectrum of the parent for radical cations of cross-conjugated systems (e.g., 1) already the first excited state is without such precedent. These states have been called non-Koop-manns states. Alkenes also feature major differences between parent and radical cation electronic structures. 

The cycloaddition of an enone with electron-rich alkenes also proceeds with remarkable regioselectivity. The rationalization is that in the excited state, polarization of the enone double bond is opposite in direction as compared with the ground state [190], In other words, photochemical excitation induces contrapolarization. With this consideration the head-to-head dimerization generally observed is reasonable as it involves one molecule each in the ground and excited states. 

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