Addition to quinone

Electrophilic addition to quinones, eg, the reaction of 2-chloro-l,4-ben2oquinones with dia2onium salts, represents a marked contrast with acetoxylation in product distribution (58). Phenyldia2onium chloride (Ar = C H ) yields 8% 2,3-substitution [80632-59-3] 75% 2,5-substitution [39171-11-4] and 17% 2,6-substitution [80632-60-6]. Fory)-chlorophenyldia2onium chloride, the pattern is 28% 2,3-substitution [80632-61-7], 35%... 

Hydrogen bromide adds to acetylene to form vinyl bromide or ethyHdene bromide, depending on stoichiometry. The acid cleaves acycHc and cycHc ethers. It adds to the cyclopropane group by ring-opening. Additions to quinones afford bromohydroquinones. Hydrobromic acid and aldehydes can be used to introduce bromoalkyl groups into various molecules. For example, reaction with formaldehyde and an alcohol produces a bromomethyl ether. Bromomethylation of aromatic nuclei can be carried out with formaldehyde and hydrobromic acid (6). 

Dialkyl phosphite addition to quinone monoimine species, followed by elimination and rearrangement (Figure 6.7).23... 

Tandem 1,2- and 1,4-additions to quinones.1 The lithium alkoxide formed by 1,2-addition of an alkyllithium to a p-benzoquinone can react as a Michael acceptor with some nucleophiles in the presence of HMPT or DMPU (13, 122). The process involves lithium-metal exchange followed by intramolecular delivery... 

Michael additions to quinones. In the presence of TrC104, enol silyl ethers undergo 1,4-addition to benzoquinone to give adducts that cyclize to benzofurans.1 A similar reaction with diimidoquinones produces indole derivatives. 

Subsequently, Lee-Ruff and Wang (1991) conducted a similar study of 6-methylbenzo[a]pyrene, a methyl isomer whose carcinogenicity is approximately equal to that of BaP. It photooxidized 20 times faster than BaP and, in addition to quinones, formed as a major product a seco ketone analogous to XXXIII from BaP. Its formation (as with BaP) is ascribed to a ( -mediated mechanism. 

Nucleophilic Substitution Reactions. Many of the transformations realized through Michael additions to quinones can also be achieved using nucleophilic substitution chemistry. In some instances die stereoselectivity can be markedly improved in this fashion, e.g., in the reaction of benzenethiol with esters (R = CH3C=0> and ethers (RJ = CH,) of 1,4-naphthoquinones. 2-Bromo-5-acetyloxy-1,4-naphthoquinone, R1 — Hr, yields 75% of 2-thiophenyl-5-acetyloxy-1,4-naphdloquinone. R1 = SCr.Hv 3-Bronio-5-methoxy-l,4-naplitlioquinone, R2 = Br, yields 82% of 3-thiophenyl-5-methoxy-1,4-naphthoquinone R2 = SCr.IIs. 

In azide addition to quinones, the triazoline adducts are spontaneously oxidized to the triazoles.1-8 9,279-281,317,392 393 Potassium permanganate32,155,286,288 and nickel peroxide394 also effect triazoline oxidation. Permanganate oxidation of 1,5-substituted triazolines in a two-phase system using a phase-transfer catalyst provides a convenient route to the synthesis of 1,5-disubstituted triazoles (Scheme 118).395,396 Triazoline 4-carboxylic esters32,287,288 and a 4-carboxamide397 are converted to triazoles by potassium permanganate and nickel peroxide, respectively. 

Structure-reactivity studies on nucleophile addition to quinone methides 68... 

Our work on nucleophile addition to quinone methides is a direct extension of studies on the formation and reaction of ring-substituted benzyl carboca-tions,89,90,128 146 and has shown strong overlap with the interests of Kresge and coworkers. The main goal of this work has been to characterize the effect of the strongly electron-donating 4-0 substituent on the reactivity of the simple benzyl carbocation, with an emphasis on understanding the effect of this substituent on the complex structure reactivity relationships observed for nucleophile addition to benzylic carbocations. 

During my early years as an assistant professor at the University of Kentucky, I demonstrated the synthesis of a simple quinone methide as the product of the nucleophilic aromatic substitution reaction of water at a highly destabilized 4-methoxybenzyl carbocation. I was struck by the notion that the distinctive chemical reactivity of quinone methides is related to the striking combination of neutral nonaromatic and zwitterionic aromatic valence bond resonance structures that contribute to their hybrid resonance structures. This served as the starting point for the interpretation of the results of our studies on nucleophile addition to quinone methides. At the same time, many other talented chemists have worked to develop methods for the generation of quinone methides and applications for these compounds in organic syntheses and chemical biology. The chapter coauthored with Maria Toteva presents an overview of this work. 

Addition to quinone monoketals and quinol ethers.2 Complexation of quinone monoketals or quinol ethers with MAD permits 1,4-addition of organo-lithium and Grignard reagents. Highest yields obtain with aryl, vinyl, and acetylenic organometallics. 

In addition to quinone groups, and to some extent to pyrones (which undoubtedly confer basicity to carbons), the possibility of existence of an oxygen-free, positively charged basic site on the carbon surface was acknowledged in the elegant early studies by Rivin [22] ... 

In addition to quinone reduction and hydroquinone oxidation, electrode reactions of many organic compounds are also inner-sphere. In these charge transfer is accompanied by profound transformation of the organic molecules. Some reactions are complicated by reactant and/or product adsorption. Anodic oxidation of chlorpro-mazine [54], ascorbic acid [127], anthraquinone-2,6-disulfonate [128], amines [129], phenol, and isopropanol [130] have been investigated. The latter reaction can be used for purification of wastewater. The cyclic voltammogram for cathodic reduction of fullerene Cm in acetonitrile solution exhibits 5 current peaks corresponding to different redox steps [131]. 

Conjugate addition to quinone monoacetals.2 These substrates undergo only 1,2-addition with (CH3)2CuLi and most Grignard reagents, but 1,4-addition can be achieved by complexation of the acetal with 2 equiv. of 1. 

Addition to quinones. Ethyl diazoacetate adds to the more reactive double bond of 1,4-naphthoquinone in warm benzene without loss of nitrogen, possibly by 1,3-di -olar attack of the structure formulated. The products isolated, 3-carboethoxy-... 

One of the most versatile electron-accepting molecules is the quinonoid compound, and the redox reaction of the quinone-hydroquinone couple is one of the most thoroughly studied proton-coupled electron transfer systems of organic molecules. Quinones show the reversible two-step le reduction in aprotic organic solvents (Fig. 2). One-electron addition to quinone forms the semiquinone radical with five n electrons. The stability of the semiquinone form is affected by the existence of a minute amount of proton, which appears as the large shift of the reduction potentials in the positive direction. This implies that quinonoid compoimds are representative acceptor molecules of which redox properties are influenced by external perturbation, such as protonation and solvation (Fig. 2). They are employed in covalently and noncovalently linked donor-acceptor systems of particular interest in the study of proton-coupled electron transfer and photoin-duced electron transfer. ... 

Phenolic acetylenes (183) are prepared by lithium acetylide addition to quinones followed by reduction oxidation yields the corresponding phenoxyl radicals (184). Some of the spin density is shown by e.s.r. to reside on the acetylenic carbon and this is reflected in the nature of the dimeric product (185) isolated.i ... 

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