Electron 1,4-dimethoxynaphthalene

Electron Dimethoxynaphthalene anion Dicyano ethylene Rigid aliphatic 0.425 45... 

Sulfonamides are very difficult to hydrolyze. However, a photoactivated reductive method for desulfonylation has been developed.240 Sodium borohydride is used in conjunction with 1,2- or 1,4-dimethoxybenzene or 1,5-dimethoxynaphthalene. The photoexcited aromatic serves as an electron donor toward the sulfonyl group, which then fragments to give the deprotected amine. The NaBH4 reduces the radical cation and the sulfonyl radical. 

In the same way, the displacement of the unpaired electron over the whole molecules was observed for cation-radicals from Scheme 1.4d and 1.4e, in which 1,4-dimethoxynaphthalene units are syn- or anti-annealed to [2.2]paracyclophane (Wartini et al. 1998a, 1998b). In another study, the electron transfer between 1,4-dimethoxybenzene and 7,7-dicyanobenzoquinone methide moieties... 

Interestingly, the second reduction of the two bipyridinium units splits in the case of catenane 144+ (Fig. 13.15) obtained by interlocking cyclophane 124+ with a symmetric macrocycle containing two electron donor dimethoxynaphthalene (DMN)... 

Figure 4.12 Examples of bichromophoric molecules used for the study of intramolecular electron transfer, (a) Dimethoxynaphthalene electron donors dicyanoethylene electron acceptor with rigid spacers (b) porphyrin donor and quinone acceptors separated by flexible spacers...

The radical anions of electron-deficient aromatic compounds and aromatic hydrocarbons, which are generated by photoinduced electron transfer, can be proto-nated by protic solvents or by the radical cations of amines to produce their neutral radicals [Eq. (7)]. Dispropotionation of the radicals yields the reduction products. The radical anions of 1,1-diphenylethene in the presence of 1,4-dimethoxynaphthalene as an electron donor is also protonated by protic solvents to give Markownikoff adducts [402,403] (Scheme 118). 

A new radical chain group transfer reaction which does not involve tin reagents has been reported. The reaction proceeds by a photosensitized electron transfer reductive activation of PhSeSiR.3 using 1,5-dimethoxynaphthalene as the sensitizer [95ACIE2669]. In contrast to the tellurium transfer described above, the selenium transfer reaction gave higher diastereoselectivity (4 1 vs 2 1). 

Molecule 42 retains the benzoquinone moiety, but the naphthoquinone has ten converted to a dimethoxynaphthalene species which is not a good electron... 

Figure 2 shows the structures of the bifunctional model systems l(n), (n=4,6,8,10,12), investigatedThe synthesis of these systems and a discussion of their electrochemical properties as well as of their electronic absorption and emission spectra have been given elsewhere (Oevering et al., 1987). These molecules contain a 1, 4-dimethoxynaphthalene unit as the photoexcitable electron donor (E./2 =+1.1 V vs. see in acetonitrile) and a 1,1-dicyano-vinyl moiety as a moderately powerful electron acceptor (Ev=-. l V) separated by an array of at least n C-C sigma-bonds. The centre-to-centre separation (Rc) and the edge-to-... 

As reported earlier (Oevering et al., 1987 Warman et al., 1986) intramolecular electron transfer following photoexcitation occurs for all values of n and in a variety of solvents. This electron transfer results in quenching of the typical dimethoxynaphthalene fluorescence for l(n) as compared to that for the isolated donor 2. Determination of the rate constant (k) of this photoinduced charge-separation was achieved in a variety of solvents (Oevering et al., 1987) by comparison of the lifetime of the residual donor fluorescence in l(n) for n= 8, 10, 12 with that of the reference system 2 via eq.(2) ... 

As already pointed out in the case of rotaxanes, mechanical movements can also be induced in catenanes by chemical, electrochemical, and photochemical stimulation. Catenanes 164+ and 174+ (Fig. 19) are examples of systems in which the conformational motion can be controlled electrochemically [82, 83], They are made of a symmetric electron acceptor, tetracationic cyclophane, and a desymmetrized ring comprising two different electron donor units, namely a tetrathiafulvalene (TTF) and a dimethoxybenzene (DOB) (I64 1) or a dimethoxynaphthalene (DON) (174+) unit. Because the TTF moiety is a better electron donor than the dioxyarene units, as witnessed by the potentials values for their oxidation, the thermodynamically stable conformation of these catenanes is that in which the symmetric cyclophane encircles the TTF unit of the desymmetrized macrocycle (Fig. 19a, state 0). 

Formation of cycloadducts can be completely quenched by conducting the experiment in a nucleophilic solvent. This intercepts radical cations so rapidly that they cannot react with the olefins to yield adducts. In Scheme 54 the regiochemistry of solvent addition to I-phenylcyclohexene is seen to depend on the oxidizability or reducibiiity of the electron-transfer sensitizer. With ]-cyanonaphthalene the radical cation of the olefin is generated, and nucleophilic capture then occurs at position 2 to afford the more stable radical. Electron transfer from excited 1,4-dimethoxynaphthalene, however, generates a radical anion. Its protonation in position 2 gives a radical that is oxidized by back electron transfer to the sensitizer radical before being attacked by the nucleophilic solvent in position 1. Thus, by judicious choice of the electron-transfer sensitizer, it is possible to direct the photochemical addition in either a Markovnikov (157) or anti-Markovnikov (158) fashion (Maroulis and Arnold, 1979). 

When Nu is electron donating the product is as a rule more easily oxidized than the starting material, resulting in further oxidation under the reaction conditions and, frequently, complex reaction mixtures. The anodic methoxylation of naphthalene, which results in 1-methoxy-, 1,2-dimethoxy-, and 1,4-dimethoxynaphthalene, approximately in a 1 2 1 ratio, serves as an illustration of this problem [67]. However, in other cases, a single major product is obtained after a sequence of reactions, such as the oxidation of mesitylene in MeCN-diluted H2SO4 to 2,4,6-trimethyl-4-hydroxycyclohexa-2,5-dien-l-one in a substitution-elimination reaction [68] or the oxidation of anthracene in MeOH to 9,9,10,l0-tetramethoxy-9,10-dihydroanthracene in a substitution-addition reaction [Eq. (28)] [69]. 

It is surprising that the rate of photodriven electron transfer in 17 is as great as it is. It was noted above that simple electron transfer theories predict an exponential dependence of electron transfer rates on donor-acceptor separation. Calculations based on an estimate of the donor-acceptor distance in 17 and the quantitative dependence of electron transfer on distance found for other porphyrin-quinone systems [27, 62-64] suggest that the quantum yield of formation of C-P -QA(OMe)2-Qr should be near zero. It seems likely, then, that the dimethoxynaphthalene 7t-electron system and perhaps the bicyclic bridge are playing some role in the electron transfer process. 

Other Elements - Though most of the focus on photoinitiated SET processes remains on the mesolytic reactions of radical cations, organoselenium radical anions, produced by SET sensitisation by 1,5-dimethoxynaphthalene, show synthetic promise as intermediates leading to unimolecular group transfer radical sequences. Thus, irradiation of (525) in the presence of 1,5-dimethoxynaphthalene and ascorbic acid as sacrificial electron donor in aqueous acetonitrile... 

Deprotection of the amino function of (188) can be brought about by irradiation using X >300 nm in aqueous acetonitrile solution. The reaction is a SET process and depends on the presence of 1,5-dimethoxynaphthalene as the electron donor. The photodissociation of the ammonium borate salts (189) and (190) has been reported.Irradiation brings about homolysis of the N-C... 

It turns out that electron donor solvent molecules tend to neutralize the substituent effect (the variation of Hammett a constants with solvent is not known it is likely that both q and a are affected by the reaction medium). The presence of electron donors in the reaction medium (acenaphthene, pyrene, phenanthrene, 1-methyl, 2,6-dimethyl, 2,3,6-trimethyl, 1-methoxy, 2-methoxy and 1,7-dimethoxynaphthalene) brings about a considerable decrease of the reaction rate (Table III). If the complex still reacts it conserves only at most 10% of the reactivity of uncomplexed substrate. Surprizingly benzoates and the corresponding cinnamates are inhibited in a very similar fashion. 

Figure 9.17 Distance-dependence of in some donor-acceptor molecules. Plots of log( et) edge-to-edge distances in a variety of linked donor-acceptor systems, (a) Forward(D) and reverse (x) electron-transfer rate constants for zinc porphyrin-anthraquinone compounds in butyronitrile. (b) Forward ( ) and reverse (x) electron-transfer rate constants for dimethoxynaphthalene-dicyanoethylene compounds in benzene, compared with the forward rate constants for three anthracene-dimethylaniline systems (V) and four analogous pyrene-dimethylaniline molecules (-I-), all in acetonitrile. From J.S. Connolly and J.R. Bolton, in Ref. [21,e, p. 322].

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