Cycloaddition reactions electron transfer mechanism

We emphasize that the critical ion pair stilbene+, CA in the two photoactivation methodologies (i.e., charge-transfer activation as well as chloranil activation) is the same, and the different multiplicities of the ion pairs control only the timescale of reaction sequences.14 Moreover, based on the detailed kinetic analysis of the time-resolved absorption spectra and the effect of solvent polarity (and added salt) on photochemical efficiencies for the oxetane formation, it is readily concluded that the initially formed ion pair undergoes a slow coupling (kc - 108 s-1). Thus competition to form solvent-separated ion pairs as well as back electron transfer limits the quantum yields of oxetane production. Such ion-pair dynamics are readily modulated by choosing a solvent of low polarity for the efficient production of oxetane. Also note that a similar electron-transfer mechanism was demonstrated for the cycloaddition of a variety of diarylacetylenes with a quinone via the [D, A] complex56 (Scheme 12). 

The thermal Diels-Alder reactions of anthracene with electron-poor olefinic acceptors such as tetracyanoethylene, maleic anhydride, maleimides, etc. have been studied extensively. It is noteworthy that these reactions are often accelerated in the presence of light. Since photoinduced [4 + 2] cycloadditions are symmetry-forbidden according to the Woodward-Hoffman rules, an electron-transfer mechanism has been suggested to reconcile experiment and theory.212 For example, photocycloaddition of anthracene to maleic anhydride and various maleimides occurs in high yield (> 90%) under conditions in which the thermal reaction is completely suppressed (equation 75). 

For instance, Kochi and co-workers [89,90] reported the photochemical coupling of various stilbenes and chloranil by specific charge-transfer activation of the precursor donor-acceptor complex (EDA) to form rrans-oxetanes selectively. The primary reaction intermediate is the singlet radical ion pair as revealed by time-resolved spectroscopy and thus establishing the electron-transfer pathway for this typical Paterno-Biichi reaction. This radical ion pair either collapses to a 1,4-biradical species or yields the original EDA complex after back-electron transfer. Because the alternative cycloaddition via specific activation of the carbonyl compound yields the same oxetane regioisomers in identical molar ratios, it can be concluded that a common electron-transfer mechanism is applicable (Scheme 53) [89,90]. 

The involvement of styryl as a diene partner has also been demonstrated in the photosensitized electron transfer [4 + 2] intramolecular cycloaddition of l,l,8,8-tetraphenyl-l-7-octadiene (Scheme 9.29) [43]. This reaction is promoted by 1,4-dicyanobencene (DCB), and a single electron-transfer mechanism has been suggested. 

Almost all of the reactions of metals can be classified into just a few typical reactions, and the reactions that metals promote in organic chemistry are simple combinations of these typical reactions. If you learn these typical reactions, you will have no trouble drawing metal-mediated mechanisms. The typical reactions of metal complexes are ligand addition/ligand dissociation/ligand substitution, oxidative addition/reductive elimination, insertion/j8-elimination, a-insertion/ a-elimination, cr-bond metathesis (including transmetallations and abstraction reactions), [2 + 2] cycloaddition, and electron transfer. 

Electron Transfer Processes - A single electron-transfer mechanism is involved in the cycloaddition of alkenes, such as 2-methylpropene, to 1,2-dicyanonaphthalene. Reaction of the alkene radical cation with the radical anion of the sensitiser results in the products shown in the Scheme 1. Incorporation of solvent to give (55) occurs as one of the main products in addition to what are essentially photo-NOCAS products (56). 

Similar electron transfer-induced oxygenation and [3+2] cycloaddition reactions occur when the EDA complexes of 77a-c and TCNE are stirred under oxygen in the dark at ambient temperature. This result suggests that an electron transfer mechanism is also operable in the thermal [3+2] cycloaddition of 77 and TCNE. ... 

Electrophilic substitution and other reactions of naphthalenes (alkylation, acylation, condensation and migration in acidic ionic liquids have been reported. Anthracene undergoes photochemical [4+4] cycloaddition reactions - in acidic chloroaluminate(III) ionic liquids. One interesting study ineluded a one-pot synthesis of anthraquinone from benzene giving a 94% yield. In general a much wider range of redox products are formed than occur in conventional solvents the strong Bronsted acidity of die ionic liquid induces protonation of anthracene, by residual traces of HCl, to form an anthracenium species which couples readily via photochemically driven electron transfer mechanisms. 

Kochi and co-workers reported the photochemical addition of various stilbenes to chloroanil 53, which is controlled by the charge-transfer (CT) activation of the precursor electron-donor/acceptor (EDA) complex. The [2-1-2]-cycloaddition products 54 were established by an x-ray structure of the trans-oxetane formed selectively in high yields. Time-resolved (fs/ps) spectroscopy revealed that the (singlet) ion-radical pair is the primary reaction intermediate and established the electron-transfer pathway for this Patern6-BUchi transformation. The alternative pathway via direct electronic activation of the carbonyl component led to the same oxetane regioisomers in identical ratios. Thus, a common electron-transfer mechanism applies that involves quenching of the excited quinone acceptor by the stilbene donor to afford a triplet ion-radical intermediate, which appears on a nanosecond/microsecond time scale. The spin multiplicities of the critical ion-pair intermediates in the two photoactivation paths determine the time scale of the reaction sequences and also the efficiency of the relatively slow ion-pair collapse k = 10 s ) to the 1,4-biradical that ultimately leads to the oxetane product 54. 

Acetylchloride is a trapping agent that allows the reaction to go completion, transforming the product into a less oxidizable compound.The results of other reactions between indole (57) and substituted cyclohexa-1,3-dienes show that the photo-induced Diels-Alder reaction is almost completely regioselective. In the absence of 59 the cycloaddition did not occur the presence of [2+2] adducts was never detected. Experimental data support the mechanism illustrated in Scheme 4.14. The intermediate 57a, originated from bond formation between the indole cation radical and 58, undergoes a back-electron transfer to form the adduct 60 trapped by acetyl chloride. 

It is now well established that the cation radicals of unsaturated and strained hydrocarbons undergo a variety of isomerization (e.g., Scheme 18) and cycloaddition reactions with much faster rates than those of the corresponding neutral molecules [162-165]. A cation radical chain mechanism analogous to Scheme 17 was reported for one-way photoisomerization of cis-stilbene (c-S) to truws-stilbene (f-S) via photoinduced electron transfer, as shown in Scheme 18 [166], Once c-S + is formed, it is known to isomerize to t-S + [167,168]. The free energy change of electron transfer... 

Olefins are also susceptible to cycloaddition reactions (Fig 105) (150). In particular, some olefin-containing APIs can dimerize with another molecule of API to form a 2+2 cycloaddition product under photo conditions (151). A classic example of such a 2+2 cycloaddition catalyzed by UV radiation is that of the nucleoside thymidine (Fig. 106) (152,153). These reactions are proposed to go through more than one mechanism concerted, diradical, electron transfer, and radical ion pairs. 

Photoinduced electron transfer from the amine to the fullerene core leading to a radical ion pair is suggested to be the initial step in the reaction mechanism (Scheme 39). Formation of the bis-[6,6] closed adduct proceeds via [3 + 2] cycloaddition of the tertiary amine followed by a [2 + 2] cycloaddition of the vinyl group and the C6o double bond adjacent to the previously formed ring connection leading to a structure analogous to 1,2,3,4-C6oH4-... 

In the presence of two equivalents of silver fluoride, N-protected bis[(trimethylsi-lyl)methyl]amines lead also to azomethine ylids which can be trapped by dipolarophiles. The mechanism of the cycloaddition reaction involves sequential electron-Me3Si+-electron transfer process from the amine to silver fluoride, which forms silver metal, ruling out a fluoride-induced desilylation process. Although silver is recovered at the end of the reaction, a cheaper oxidizing reagent is still lacking.448,449... 

The electron transfer activates molecule and enables the cycloaddition process. The kinetics of these reactions is well documented much less information is on the nature of regio- and stereoselectivity. The stepwise mechanism was proposed for the electron-transfer-catalysed Diels-Alder reaction of indole and 1,3-cyclohexadiene catalysed by fn s(4-bromophenyl)aminium hexachloroantimonate in dichloromethane at 0°C in the presence of NaHC03 and CH3COCI (Scheme 3).64... 

Meanwhile, the basis of photoinduced electron transfer as well as the mechanisms of fundamental processes such as PET-sensitization, co-sensitization or medium and salt effects are widely understood [6], As a consequence the number of applications of PET reactions in organic synthesis has increased dramatically as noticeable from a series of summarizing reviews and book chapters [2,7]. Among these applications are for example cycloadditions [8], fragmentations [9], macrocyclizations [10], and... 

In an earlier report Mazzocchi and his coworkers reported that the photo-reaction of A) methylnaphthalimide (325) with phenyIcyclopropane involved the production of a radical cation/radical anion pair. The product from the reaction was the cyclic ether (326). - A study of the mechanism of this reaction using suitably deuteriated compounds has demonstrated that the reaction is not concerted and takes place via the biradical (327). - Other systems related to these have been studied. In the present paper the photoreactivity of the naphthalimide (328) with alkenes in methanol was examined. Thus, with 1-methylstyrene cycloaddition occurs to the naphthalene moiety to afford the adducts (329) and (330). The mechanism proposed for the addition involves an electron transfer process whereby the radical cation of the styrene is trapped by methanol as the radical (331). This adds to the radical anion (332) ultimately to afford the observed products. Several examples of the reaction were descr ibed. 

---