Mercury rearrangement reactions

Cvetanovic, R. J., and L. C. Doyle Molecular rearrangement in the mercury-photosensitized reaction of butene-1. J. chem. Physics 37, 543 (1962). 

Sometimes the reaction conditions used in this reaction are too harsh since heating is involved and rearrangement reactions can occur. A milder method that gives better results is to treat the alkene with mercuric acetate [Hg(OAc)2] then sodium borohydride. The reaction involves electrophilic addition of the mercury reagent to form an intermediate mercuronium ion. This reacts with water to give an organomercury intermediate. Reduction with sodium... 

For example, reaction of propiophenone and benzonitrile with mercury(ll) tosylate affords 2,4-diphenyl-5-methyloxazole 196 (Scheme 1.52). Mechanistically, the authors proposed initial formation of a di(a-arylalkanoyl)Hg(ll) species 192 from reaction of the enol tautomer of propiophenone with mercury(II) tosylate. Reaction of 192 with benzonitrile generates 193, which collapses to the oxirane 194 with loss of mercury. Rearrangement of 194 and loss of... 

Some natural products outside the carbohydrate field - particularly those with highly hydroxylated cyclohexane ring components - have been subject to synthetic studies which are dependent on the mercury-based rearrangement reaction the alkaloid (+)-lycoricidine (30) [12] and the cycloheptane-based (+)-calystegine 62(31) which stimulates growth of nitrogen-fixing Rhizobia [29] are examples, and compound 32 which offers novel access to the anthracyclinone components of anthracyclin anti-cancer compounds, has been produced by cycloaddition of a naphthalene-based o-xylylene to a 2,3-unsaturated hex-4-uloside followed by carbocyclization of the product by use of the mercuration procedure [30]. Studies on HMG-CoA reductase inhibitors like compactin have afforded the tetra-carbon-substituted 33 made from a hex-5-enopyranoside with deoxy-branch chains at C-2, C-3 and C-4 [31]. 

Oxymercuration-demercuration gives the product that would result from direct hydration of an alkene. However, the reactions occur with a higher yield than the direct hydration reaction because the competing reverse reaction, dehydration, does not occur. Because most of the positive charge in the mercurinium ion is on the mercury atom, the mercurinium ion has little carbocation character, and rearrangement reactions do not occur. 

The reaction of alkenyl mercurials with alkenes forms 7r-allylpalladium intermediates by the rearrangement of Pd via the elimination of H—Pd—Cl and its reverse readdition. Further transformations such as trapping with nucleophiles or elimination form conjugated dienes[379]. The 7r-allylpalladium intermediate 418 formed from 3-butenoic acid reacts intramolecularly with carboxylic acid to yield the 7-vinyl-7-laCtone 4I9[380], The /i,7-titisaturated amide 421 is obtained by the reaction of 4-vinyl-2-azetidinone (420) with an organomercur-ial. Similarly homoallylic alcohols are obtained from vinylic oxetanes[381]. 

Only a trace of the corresponding cubane 167 is formed on irradiation of the tricy-clooctadiene 168 in pentane at ambient temperatures using a 125-watt mercury arc lamp. The principal product 169 is the result of rearrangement within a biradical intermediate79. A review of the synthetic approaches to cubane and to its reactions has been published77. The diene 170 photochemically converts on irradiation in pentane solution at 254 nm to yield a photostationary mixture of the cubane 171, the starting material 170 and the isomeric diene 17280. Other additions of this type have been used for synthesis of the propellaprismane 173, essentially a heavily substituted cubane, by the intramolecular (2 + 2)-photocycloaddition of the diene 17481. 

Demercuration of organomercury compounds is a critical step in synthetic procedures, which involve mercuration-initiated cyclization reactions [e.g. 41], Many of the standard procedures for demercuration result in rearrangement or ring cleavage of the system, but reductive carbon-mercury cleavage (e.g. Scheme 11.4) with an excess of the quaternary ammonium borohydride is effective under phase-transfer conditions [e.g. 42,43]. 

All ECi adsorption coupled mechanisms have been verified by experiments with azobenzene/hydrazobenzene redox couple at a hanging mercury drop electrode [86,128,130]. As mentioned in Sect. 2.5.3, azobenzene undergoes a two-electron and two-proton chemically reversible reduction to hydrazobenzene (reaction 2.202). In an acidic medium, hydrazobenzene rearranges to electrochemically inactive benzidine, through a chemically irreversible follow-up chemical reaction (reaction 2.203). The rate of benzidine rearrangement is controlled by the proton concentration in the electrolyte solution. Both azobenzene and hydrazobenzene, and probably benzidine, adsorb strongly on the mercury electrode surface. 

The metal ion-water exchange process must be important in areas other than those of simple metal complex formation. For example, the discharge of nickel ion at a mercury cathode is probably controlled, not by diffusion, but by rearrangement of the water coordination shell. The estimated rates and heat of activation for this agree with the idea that this, in turn, is related to the water exchange process (66). Then too, the dimerization rate of metal hydroxy species may be controlled by water exchange. The reaction... 

The procedure described illustrates a new general synthetic method for the preparation of (E)-3-allyloxyacrylic acids and their conversion to a-unsubstituted y,6-unsaturated aldehydes by subsequent Claisen rearrangement-decarboxylation. Such aldehydes are traditionally prepared by Claisen rearrangements of allyl vinyl ethers. Allyl vinyl ethers are typically prepared by either mercury-catalyzed vinyl ether exchange with allylic alcohols or acid-catalyzed vinylation of allylic alcohols with acetals. The basic conditions required for alkoxide addition to the betaine to produce carboxyvinyl allyl ethers, as described in this report, nicely complements these two methods. In addition, this Claisen rearrangement is an experimentally very simple procedure, since sealed tube and other high pressure vessels are not required. The allyloxyacrylic acids are heated neat (in most cases a small amount of hydroquinone is added) and, by adjusting the pressure at which the reaction is performed, the aldehyde products distill from the reaction mixture in analytically pure form. 

Trimethylsilyl-protected 1-isopropenylcyclobutanols were rearranged to cyclohexanones via a two-step reaction sequence.114 The vinylcyclobutanes 1 were first treated with mercury(II) trifluoroacetate. The crude products were then converted into chlorides (which can be isolated) and finally reduced with tributyltin hydride to give 2. Sometimes small amounts of five-mem-bered ring compounds, which are intermediates of this two-step ring enlargement, are formed as side products. 

In the presence of proton and/or Lewis acid and strong nucleophiles bicyclo[3.2.0]heptan-6-ones are converted to 3-substituted cycloheptanones (Table 15). Bicyclo[3.2.0]heptan-6-ones rearrange to give 3-iodocycloheptanones on treatment with iodotrimethylsilane. Zinc(II) iodide or mercury(II) halides as catalysts enhance the rate and the selectivity of the reaction.31 If a second, enolizable carbonyl group is present, an intramolecular alkylation may follow the ring enlargement under these reaction conditions.32 Consecutive treatment with tributyltin hydride/ 2,2 -azobisisobutyronitrile affords reduced, iodo-free cycloheptanones, whilst treatment with l,8-diazabicyclo[5.4.0]undecene yields cycloheptenones.33 Similarly, benzenethiol adds to the central bond of bicyclo[3.2.0]heptan-6-ones in the presence of zinc(II) chloride and hydrochloric acid under anhydrous conditions to form 3-(phenylsulfanyl)cycloheptanones.34... 

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