Triplet sensitized photochemistry

The cation radical VCB rearrangement has been found, at least in several cases, to occur intramolecularly, rather than by dissociation/recombination [86]. An especially interesting case is the rearrangement of the CB dimers of 1,3-cyclohexadiene generated by triplet sensitized photochemistry. In the presence of the usual aminium salt catalyst (3+ ), these CB dimers are quite stable, but if the more powerful hex-abromo aminium salt, 4+, is used, these individual dimers rearrange to the DA dimers via a stereospecific VCB rearrangement (Scheme 46). 

In summary, our results on the triplet-sensitized photochemistry of a series of 2-aza-1,4-dienes have uncovered the first examples of 2-ADPM rearrangements yielding cyclopropyl imines. In the case of azadiene 1, the N-vinylaziridine 3 was also obtained in the first example of a di-Jt-methane rearrangement... 

While di-i-butyl (34) and dicumyl hyponitrites (35) have proved convenient sources of Tbutoxy and cumyloxy radicals respectively in the laboratory,71 72 115"117 the utilization of hyponitrites as initiators of polymerization has been limited by difficulties in synthesis and commercial availability. Dialkyl hyponitrites (16) show only weak absorption at A>290 ntn and their photochemistry is largely a neglected area. The triplet sensitized decomposition of these materials has been investigated by Mendenhall et a .11 s... 

Since the photochemistry of many compounds that have been used as triplet sensitizers has been well studied, we will not attempt to cover these reactions in detail. Unless the investigator is unaware of them, common photochemical processes such as the Norrish Type II cleavage are not ordinarily a complication and as will be mentioned later, they can actually serve as mechanistic probes. A discussion of the mechanisms of triplet energy transfer1,3,9 is beyond the scope of this review as are other specific reactions which have been recently covered elsewhere. 

The stilbenes have played a crucial role in the development of modern photochemistry. Direct or triplet sensitized irradiation of trans-stilbene (t-1) in dilute solution results in isomerization to cis-stilbene (c-1) as the exclusive uni-molecular photochemical reaction (1-3). Direct irradiation of c-1 results in isomerization to both t-1 and trans-4a,4b-di-hydrophenanthrene (2), which revert to c-1 both thermally and photochemically and can be trapped by oxidants such as iodine or oxygen to yield phenanthrene (3) (4-6). Triplet sensitized irradiation of c-1 yields only t-1. These unimolecular isomerization pathways are summarized in eq. 1. 

While solid matrices have been employed successfully, they may be less than ideal for controlled mechanistic studies. A more appropriate technique for controlled doublet photochemistry appears to be two-photon excitation in solution. In this experiment, the first photon is used to initiate radical ion formation, whereas the second photon, appropriately delayed to coincide with the maximum concentration of the radical cation so generated and tuned to its absorption maximum, serves to excite these intermediates. However, we hasten to add that the benefits of this technique have yet to be demonstrated. The photoinduced rearrangement of radical cations very likely will benefit substantially from a mismatch between (quartet vs. doublet) potential surfaces, much as triplet sensitized isomer-izations can be ascribed to mismatches between triplet and ground state surfaces. 

Armesto, D., Ortiz, M.J., and Agarrabeitia, A.R. (2004) Novel di-Jt-methane rearrangements promoted by photoelectron transfer and triplet sensitization, in CRC Handbook of Organic Photochemistry and Photobiology, 2nd edn (eds W. M. Horspool and... 

The photochemistry of 1,3-dienes can be highly dependent on the diene structure and reaction conditions. Important variables include the ground state conformation [22,23], the reaction concentration, the use (or not) and properties of a triplet sensitizer [14] or an electron acceptor [18], and solvent polarity. The simplest dienes also often yield the most complex chemistry. For example, 1,3-butadiene 3 undergoes unimolecular isomerization in dilute solution to give only cyclobutene 4 and bicyclobutane 5 (Sch. 2), and polymerization in concentrated solution [24]. At intermediate... 

Numerous photodimerization studies of 1,3-cyclohexadiene 36 have been reported (Sch. 9). Thermal cycloaddition yields a 4 1 mixture of endo/ exo [4+2] adducts 37 and 38 in modest yield. Irradiation of the diene in cyclohexane near its 2max of 254 nm yields very little dimer, but irradiation at 313 nm leads to a mixture of dimers, favoring the [2+2] adducts 39 [37]. The use of y-radiation produces similar mixtures [38,39]. A triplet sensitizer leads to largely the [2+2] adducts plus exo 38 and little of the endo [4+2] isomer 37 [40]. When the photochemistry is conducted in the presence of the electron acceptors anthracene 41, LiC104-42 or pyrylium 43, only [4+2]... 

Attempts to sensitize the rearrangement of [79a] to [80a] with xanthone (ET = 74 kcal/mole) under conditions where the sensitizer absorbed essentially all of the incident radiation resulted in no observable chemical change (35). To determine if the cyclobutanone was receiving triplet energy from the sensitizer, the direct and sensitized photochemistry of syn- and anti-2-sec-butylidenecyclobutanones [81] and [82] were investigated (35). Under direct irradiation (313 nm), the isomeric acetals [83] and [84] were produced quantitatively ( = 0.1-0.2) yet under sensitized conditions (e.g., with xanthone, acetophenone, benzophenone, or triphenylene),... 

From this state, ring strain facilitated predissociation to a "biradical-like" transition state [135] or vibrational relaxation (k ) to S may occur. It is also conceivable that transition state [135] could be produced directly from S °. Alternatively, molecules in the S ° state could intersystem cross (kST) to the triplet manifold (T ). For 2-alkylidenecyclobutanones, reactivity is manifested in isomerization about the exocyclic carbon-carbon double bond, while for the saturated cyclobutanone derivatives studied, definitive evidence for solution-phase reactivity is not available. If analogy is again made to the vapor-phase photochemistry of cyclobutanone [21], reactivity could conceivably result in decarbonylated products. Indeed, preliminary evidence has been obtained from sensitization experiments employing m-xylene as triplet sensitizer that decarbonylation of a saturated cyclobutanone is enhanced by selective population of its state (35). ... 

The 3-oxa-di- i -methane rearrangement is far less common. As a symmetrical system, the product should be a vinyl epoxide (equation 23). So far, no 3-oxa-di-T -methane products have been observed from the divinyl ethers depicted in the equation. It was shown that the solution photochemistry of (21a) derives from its singlet i , i -state and parallels that of the unsubstituted divinyl ether (21d) and of furan in the gas phase. All products could be rationalized in terms of the initial formation of a singlet vinyl-vi-nyloxy radical pair (equation 24). Triplet sensitization brings about cis-trans isomerization and consequent deactivation. Had the 3-oxa-di- ir-methane product, i.e. the vinyloxirane (22), been formed, it would have decomposed via Griffin fragmentation as confirmed by irradiation of an authentic sample. 

Scheffer and co-workers have carried out extensive research on the use of oiganic salts for, e.g asymmetric synthesis (Schemes 36-38), heavy atom effects (Scheme 39), and triplet sensitization (Scheme 40) [43a,64-73]. Chiral crystals of prochiral oiganic acids or bases were prepared by salt formation with optically active amines or acids, respectively, and the solid-state photochemistry of the resultant salts led to asymmetric induction in prochiral acids or bases. These are illustrated for the solid-state di-7C-methane photoreaction of 9,10-ethenoanthra-cene (dibenzobarrelene) derivatives (Schemes 36 and 37) [65-67] and for the Norrish type II photocylization of phenyl ketones (Scheme 38) [68-71]. Nonab-soibing, optically active amines or acids must be used as a chiral handle. Solution photolysis gave photoproducts with no optical activity. 

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