Paraquat electron acceptors

We have also examined the behavior of copolymers of o-tolyl vinyl ketone and methyl vinyl ketone (CoMT). In this case the light is absorbed exclusively at the aromatic carbonyl chromophore and the reaction proceeds from this site, while the methyl vinyl ketone moieties provide a relatively constant environment but prevent energy migration along the chain. The values of Tg and Tip in benzene have been included in Table II. These copolymers axe also soluble in some polar solvents for example, we have used a mixture of acetonitrile acetone methanol (30 30 Uo, referred to as AAM). This mixture is also a good solvent for the electron acceptor paraquat (PQ++) which has been shown to be good biradical trap in a number of other systems (9.). 

The herbicidal activity of the bipyridyliums depends on their redox properties. Their abilities as one-electron acceptors of the right redox potential (-350 mV for diquat and -450 mV for paraquat) allow them to siphon electrons out of the photosynthetic electron-transport system, competing with the natural acceptors. The radical anion produced is then reoxidized by oxygen, generating the real toxicant, hydrogen peroxide, which damages plant cells. Structure-activity relationships in this series have been reviewed (60MI10701). 

Two-fold Menschutkin reactions on 4,4 -bipyridine provide a route to compounds containing a redox-active paraquat unit [15]. Thus, reacting 54 with 6-to-syloxyhexyl thioacetate afforded 55 (Scheme 10.17). Perylene tetracarboxylic dii-mide presents another electron acceptor that has been functionalized with thiol end-groups. Thus, heating a mixture of the dianhydride 56 and 4-aminothiophe-nol (57) gave the product 58 (Scheme 10.18) [16],... 

The first example of a covalently linked metal-organic dyad is reported by Meyer and co-workers in a communication that appeared in 1978 [80]. This manuscript describe the synthesis, electrochemistry, and photophysics of 8 (Scheme 4), which consists of a pair of /V-methyl-4,4 -bipyridinium (monoquat) electron acceptors coordinated directly to the (bpy)2Run chromophore. This complex is an inner-sphere analog of the Ru(bpy) +/paraquat (3) system which has received significant attention in studies of bimolecular photoinduced ET. Complex 8 displays electrochemical features typical of both the Ru-bpy unit and the monoquat electron acceptor. The photophysical results indicate that in fluid solution the lowest excited state is based on a Ru — monoquat MLCT transition. 

Many xenobiotics, including a wide variety of quinones and nitro compounds, will accept an electron from almost any redox flavoenzyme. The microsomal reduction of nitroaromatic compounds, quinones, quinone-imines, some azoaromatic compounds, paraquat, and tetrazolium salts is catalyzed by NADPH-cytochrome P-450 reductase [44], One-electron transfer to these electron acceptors has been proved to be obligatory in the case of quinone and nitro compounds, and is probably obligatory in other cases as well. Therefore, a reduction of an aromatic compound by NADPH-cytochrome P-450 reductase can probably be assumed to form a free radical metabolite. In contrast, free radical formation by reductive dehalogenation is totally cytochrome P-450-dependent, with the reductase being inactive. 

Four donor acceptor [2]catenanes possessing cyclobis(paraquat-p-phenylene), as the 7i-electron acceptor, and 1,5-dioxynaphthalene-containing macrocyclic polyethers, as the 7i-electron donor moieties, have been constructed employing Cu+-catalyzed Huisgen 1,3-dipolar cycloaddition and Cu+2-mediated Eglinton coupling condition in the final step <07JA8236>. Desymmetrized [2]catenanes were synthesized by means of a template synthesis of pseudorotaxanes between 7i-rich crown ethers and a 7i-deficient pyromellitic subunit,... 

The most striking and nnexpected reactions of HO are those that prodnce anion radicals when hydroxide is added to solutions of aromatic ketones, quinones, paraquats, and strong electron acceptors such as tetracyanoethylene. If the primary reaction is a SET reaction, a radical pair will be produced (equation 181). 

The ability of electron-rich macromolecules 37 to act as donors to the electron-acceptor Paraquat 38 was investigated by means of H NMR and UV-Vis spectroscopies. A 1 1 mixture of 37 and 38 exhibited a broad CT band centered at 601 nm in addition to significant changes in the H-a and H-)3 chemical shifts in the Paraquat unit 38 <1999EJ03335>. 

Examples are the 1, l -dibenzyl-4, 4 -bipyridinium electron-acceptor dication threaded into the 1, 5-dinaphtho-38-crown-10 (Fig. 2a) [10], and the acyclic polyether containing a dioxybenzene electron-donor unit threaded into the electron-acceptor cyclobis(paraquat-p-phenylene) tetracationic cyclophane (Fig. 2b) [11]. Although in these cases a large contribution to the association driving force comes from the electron-donor/acceptor (charge-transfer, CT) interactions, hydrogen bonding can also play an important role, as clearly shown in the cases of pseudorotaxanes constituted by 4, 4 -bipyridinium [12a] or l,2-bis(pyridinium)ethane [12b] threads and crown ethers. 

The variety of electron transfer reactions described above for PQ2+ and PQ + obviously provide some (model compound) information related to the possible mechanism of action of PQ2+ in herbicidal applications. An even bigger and more applicable range of electron transfer processes involving PQ2+ and PQ + is provided by studies of the action of paraquat as a mediator (i.e. electron-transfer bridge), or terminal electron acceptor, in redox processes involving biologically active compounds (see for example Steckham and Kuwana, 1974 Krasnovsky, 1972). Whilst such studies properly reside outside the scope of this review, there arc two aspects of bipyridylium ion... 

A number of other herbicides interfere with photosynthesis in specific ways. Amitrole inhibits biosynthesis of chlorophyll and carotenoids. The affected plants present a bleached appearance before they die because of the loss of their characteristic pigments. Another herbicide, atrazine, inhibits the oxidation of water to hydrogen ion and oxygen. Still other herbicides interfere with electron transfer in the two photosystems. In photosystem II, diuron inhibits electron transfer to plastoquinone, whereas bigyridylium herbicides accept electrons by competing with the electron acceptors in photosystem I. The inhibitors active in photosystem I include diquat and paraquat. The latter substance attained some notoriety when it was used to interfere with an... 

The interaction of bipyridyl herbicides (paraquat and diquat) with photosynthesis is different from that of the electron transport inhibitors. These compounds, with highly negative redox potentials (paraquat E 446mV diquat Eq -349mV), interact in the vicinity of ferredoxin causing a diversion of electron flow from the ultimate electron acceptor NADP. This was clearly seen in paraquat-treated plant material as a progressive inhibition of carbon dioxide uptake (25)... 

Until recently, it was accepted that paraquat is reduced by the primary electron acceptor of PSI, which is also responsible for the reduction of NADP. Recent advances in this area, however, suggest that electron flow after PSI is branched (see Ref. 91). One branch is involved in NADP reduction whereas another branch is involved in paraquat- or ferredoxin-mediated O2 reduction. The branch for reduction of O2 offers a potential site for regulating paraquat toxicity since electron flow to paraquat could be inhibited without a block of NADP reduction. An unidentified inhibitor from hemolyzed rabbit sera was found to inhibit the paraquat- or ferredoxin-mediated O2 evolution without inhibiting NADP -dependent O2 evolution. Similar results were obtained with hemoglobin, which offered partial protection of pea chloroplasts against paraquat by inhibiting only the paraquat-mediated O2 reduction. ... 

A wide class of aiyl-based quaternary surfactants derives from heterocycles such as pyridine and quinoline. The Aralkyl pyridinium halides are easily synthesized from alkyl halides, and the paraquat family, based upon the 4, 4 -bipyridine species, provides many interesting surface active species widely studied in electron donor-acceptor processes. Cationic surfactants are not particularly useful as cleansing agents, but they play a widespread role as charge control (antistatic) agents in detergency and in many coating and thin film related products. 

Bipyridyliums with redox potentials in the range of -300 to -500 mV, such as diquat and paraquat, can accept electrons in competition with the acceptor of photosystem I (Figure 2, site 4) and have herbicidal activity. Interception of electron flow from photosystem I essentially shunts the electron transport chain. 

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