Intermediate, biradical exciplex

The Patemo-Buchi reaction is one of the more predictable photocycloaddition reactions. Regiocontrol of the photoproduced oxetane is a function of the stepwise addition of the carbonyl chromophore to the alkene [30]. In the case of electron-rich alkenes, excitation of the carbonyl group produces a triplet species that adds to the alkene. The product regioselectivity is a result of addition that generates the most stable biradical, and the triplet lifetime of the intermediate biradical allows for substantial stereoselectivity prior to closing (see Scheme 2). Electron poor alkenes are more likely to undergo cycloaddition with carbonyl groups directly from an exciplex [31]. 

Styrene derivatives are commonly used addends in the photocycloaddition studies of 1,4-quinones. With Z- and -anethole, 1,4-benzoquinone (BQ), 1,4-naphthoquinone (NQ), and 9,10-anthraquinone in acetonitrile solvent yield spiro-oxetanes in which the trans-isomer (e.g., 4 from naphthoquinone) predominates. The process has been studied in detail by CIDNP techniques from which it is deduced that product formation proceeds from triplet radical ion pairs to the triplet biradical, and that there is no significant contribution from direct conversion of exciplex intermediates into the biradicals. Spiro-oxetane formation between simple alkenes and BQ generally has low regioselectivity but this is markedly improved with alkylidene cyclohexanes (Figure 87.3) such that the major isomer can be used as a new access to useful synthetic building blocks. For the BQ/homobenzvalene 5 system, however, where the difference in stability between the intermediate biradicals can be expected to be considerably less, the selectivity ratio for the spiro-oxetanes 6 and 7 is reduced to 3 1, respectively, and the addition to NQ yields only the cyclobutane derivative 8. Quadricyclane and norbornadiene undergo the same photocycloaddition reaction to BQ, affording the oxolane 9 and the spiro-oxetane 10. Evidence from CIDNP... 

The primary interaction of singlet oxygen, produced by energy transfer from the excited sensitizer, with the diene can give rise to an exciplet that then collapses to peroxide, to a 1,4-biradical or to a 1,4-zwitterion alternatively, the adduct is the result of a concerted action without the involvement of an intermediate. Detailed kinetic Diels-Alder investigations of singlet oxygen and furans indicate that the reactions proceed concertedly but are asynchronous with the involvement of an exciplex as the primary reaction intermediate [63]. 

Many mechanisms had been proposed in the past to rationalize this selectivity (tri-oxanes, perepoxide, exciplex, dipolar or biradical intermediates) however, it is now generally accepted that the reaction proceeds through an intermediate exciplex which has the structural requirements of a perepoxide. This assumption is supported by (a) the lack of stereoselectivity in the reactions with chiral oxazolines and tiglic acid esters (b) the comparison of the diastereoselectivity of dialkyl substituted acrylic esters with structurally similar non-functionalized aUtenes (c) the intermolecular isotope effects in the photooxygenation of methyl tiglate and (d) the solvent effects on regioselectivity. ... 

The following subsections are devoted to various mechanistic aspects of the ortho photocycloaddition. The possible role of ground-state complexes will be discussed and, subsequently, the intermediate species that are formed or may be formed upon photoexcitation will be treated the reactive excited state, exciplexes, and zwitterions, biradicals, and ion pairs. Empirical rules, aimed at predicting under what circumstances ortho photocycloaddition (or other modes of addition) may occur, will be discussed next and, finally, the results of theoretical considerations and calculations will be reviewed. 

Excellent regioselectivity and stereoselectivity has been achieved in each photocycloaddition mode [45 48], Regiochemistry and stereochemistry in the meta process is decided by the orientation of the addends in the exciplex and by stabilization of biradical intermediates having a change transfer (CT) character (6) by the substituents on the arene. Intermolecular meta cycloaddition of arenes with cycloalkenes proceeds with endo selectivity (7) (Scheme 5). In the ortho-process, selectivities can be controlled mainly by the substituents on the reactants. 

This mechanistic sequence (Sch. 4) wherein the triplet excited enone adds to the alkene, either via an exciplex intermediate or directly, to afford triplet 1,4-biradicals, which (after undergoing intersystem crossing) either cyclize to product(s) or revert to ground state reactants, is confirmed by both semi-empirical and ab-initio calculations [21-24], The origin of regioselectivity is supposed to stem from the primary binding step, the enone triplet being considered as a (nucleophilic) alkyl radical at C(3) linked to an (electrophilic) ot-acyl radical at C(2) [25], Thus additions of C(2) to the less substituted terminus of electron rich alkenes and of C(3) to the least substituted terminus of electron deficient alkenes should occur preferentially [26],... 

Both N-N and N-C bond fission occurs on irradiation of the hydrazone derivatives (191). The photodegradation of the phenylhydrazone and the hydrazone of benzil have also been described. a-Ketoiminyl radicals are formed on irradiation of oximino ketones at low temperature. A study of the photochemical decomposition of sulfamic esters and their use as initiators of cross-linking of a melamine resin have been described. The bispyridinyl radical (192) is formed by one electron reduction of the corresponding pyridinium salts. The irradiation of this biradical at 77 K results in C-N bond fission with the formation of benzene-1,3-diyl. The predominant products from the irradiation (X,> 340 nm) of (193) in methanol were identified as A -hydroxy-2-pyridone and (194) from the fission of the C-O bond. Other products were 2-pyridone, (195) and (196) that arise from O-N bond fission. The reaction is to some extent substituent dependent and a detailed analysis of the reaction systems has identified an intramolecular exciplex as the key intermediate in the C-O bond heterolysis. 

A frequently encountered case is depicted in Scheme 3.4, where the singlet state of A, Si(A), reacts with a substrate B, for example an alkene yielding a cycloaddition product C. In most cases the substrate B will be in large excess, so that cB can be treated as a constant. Scheme 3.4 includes an intermediate A- B that may represent an exciplex (Section 5.2) or a biradical intermediate (Section 5.4.4), which can proceed either to product C, A Hkr or... 

Intersystem crossing from the excited singlet state of simple alkenes is inefficient. Triplet state cycloadditions, therefore, are usually achievable via triplet sensitization rather than direct irradiation.701 702 Such a process often involves exciplexes, formed between electron-poor and electron-rich alkenes,703 or 1,4-biradical intermediates (Scheme 6.45).704 Thus the tendency to achieve loose geometries in such species then favours a nonconcerted (stepwise) pathway, in which rotation about the central C—C bond occurs, eventually leading to loss of reaction stereospecifity. In general, the cycloaddition... 

The photocycloaddition mechanism, and consequently the reaction selectivity, may vary considerably depending on the structure of the initial material and reaction conditions. In general, an excited arene and a ground-state alkene may react with initial polarization to form an exciplex.802 In [2 + 2] photocycloaddition reactions, biradical intermediates are often involved (Scheme 6.80a), although excitation of a ground-state charge-transfer (CT) complex (Section 2.2.3) has also been discussed in some cases, such as the [2 + 2] photocycloaddition of benzene with maleic anhydride (Scheme 6.80b).817 Here a zwitterion intermediate 194 collapses to the adduct 195 only in the absence of an acid. 

Cycloaddition reactions of triplet excited 1,4-quinones to ground-state alkenes occur either via a triplet exciplex intermediate, which collapses to a triplet biradical,1000 or via separated radical ion intermediacy.990 The existence of biradical intermediates has been proven by measurements of chemically induced dynamic nuclear polarization (CIDNP) (Special Topic 5.3), for example in the reaction of 1,4-benzoquinone (313) with norbomadiene (314) yielding two products, the spiro-oxetane 315 and the spiro-oxolane 316 (Scheme 6.139).1001 Interestingly, quadricyclane (317) provides the same reaction as norbomadiene. 

This process, formally related to the Diels Alder reaction, may also proceed by various mechanisms (Scheme 6.257)1421 1443 similar to those of [2 + 2] cycloaddition (Scheme 6.251), such as a concerted process or formation of charge-transfer (exciplex, 532), biradical (533), zwitterion (534) or perepoxide (535) intermediates. A concerted pathway1444 and exciplex1445 intermediacy was proposed to be involved in most cases. The [4 + 2] photooxygenation may be accompanied by other related processes (e.g. [2 + 2]). 

The papers on simple alkenes [151] and enol ethers [152] discussed above brought the possibility of exciplex intermediacy to the attention of singlet oxygen chemists and since then several papers concerned with reactions involving allylic hydroperoxide, dioxetane and/or endoperoxide formation have invoked their participation [157-165], some as the sole intermediate, some as precursors to perepoxides, open zwitterions or biradicals. The molecules shown in structures 43-51, and relatives thereof, have been... 

These two mechanisms, the reversible biradical intermediate and the intermediate exciplex [25] have both been useful for analysis of the regioselectivity and stereoselectivity observed in [2 -I- 2] photocycloaddition between enones and alkenes. Hoffman et al. [26], for example, describes his stereoselective photocycloadditions as arising from a biradical from the triplet excited enone which may or may not involve exciplex formation.. Ground state trans-cycloalkenones have also been proposed as the reactive intermediates which lead to [2 + 2] cycloadducts [27]. The distance between the reacting partners is clearly an issue. If the n systems are not sufficiently close, then photocycloaddition will not occur [28]. 

There are few experimental examples of singlet photoreaction between alkenes. Calculations have suggested the presence of exciplex [39] and/or biradical intermediates [40], In general, regiocontrol and stereocontrol of the singlet alkene -I- alkene reaction have not been impressive. 

The trans stereoselectivity may easily be rationalized as a result of sterics, although direct irradiation of the 1-phenylcyclohexene gives a similar ratio of trans cis stereoselectivity. The singlet reaction presumably does not involve a biradical intermediate of significant lifetime. Thus, the trans selectivity may be enhanced by sterics in the case of the triplet reaction. The preference for trans isomer may be inherent to the approach of the alkenes or, perhaps, it is a function of selective reversibility of biradical intermediates (see Scheme 1). Caldwell has published a formula for predicting the potential for photocycloaddition of alkenes and arenes in the singlet excited state [42], His analysis implicates an exciplex. 

Caldwell et al. have also reported a stereoselective [2 + 2] photocycloaddition where the major product was the most thermodynamically stable. Scheme 54 shows that the cyclobutane product with the two aryl groups trans to each other predominates, but it is not the exclusive product. The proposed intermediates include a 1,2-biradical, where the p-orbitals are perpendicular to each other, and a 1,4-biradical intermediate which has time to assiune the most stable conformation before closing. The 1,2-biradical intermediate is supported by rate studies and quenching data, but these studies are not conclusive [41a]. In addition, the possibility of involvement of an exciplex prior to cycloaddition cannot be ruled out based on the studies Caldwell et al. have reported. 

These intramolecular addition reactions are remarkable in that they have no intermolecular counterpart. In fact, A/,W-dialky-lamides and tetraalkyl ureas fail to quench styrene fluorescence. However, photoaddition of some 1,1-diarylethylenes and tetra-methylurea has been reported. The intramolecular reactions are proposed to occur via weakly bound nonfluorescent singlet exciplex intermediates, which undergo a-C-H transfer to yield the biradical precursors of the observed products. A triplet mechanism was excluded based on the failure of sensitization by xanthone or quenching by 1,3-pentadiene. The involvement of charge transfer is consistent with the requirement of polar solvents for these reactions. The quantum yields for adduct formation from 19 and 25 are much higher than those of their p-methoxy derivatives, in which the styrene is a much weaker electron acceptor. ... 

The photochemical behavior of the orr/to-(aminoalkyl)slil-benes 92—94 is also dependent upon the polymethylene linker (Scheme II). Irradiation of 93 and 94 results in formation of the benzazepines 96 and 97. These adducts are presumably formed by regioselective N—H transfer to the proximal end of the stilbene double bond in 93 and the distal end in 94, in both cases resulting in the formation of a 1,7-biradical intermediate. Since intramolecular stilbene-amine addition reactions are non-regioselective, the regioselectivily of N-H transfer must be subject to exciplex conformational control. The biradical inter-... 

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