Furan mercury derivatives

The main application of mercury derivatives has been to the synthesis of heterocyclic arsenicals. Thus diphenylene oxide-1-arsonic acid and pyridyl-3-arsonic acid have been isolated following oxidation of the initial reaction products obtained from arsenic trichloride and the appropriate heterocyclic mercury chloride 114,115). Furan and thiophene derivatives have been similarly prepared 116,117,118). 

Mercury(II) acetate tends to mercurate all the free nuclear positions in pyrrole, furan and thiophene to give derivatives of type (74). The acetoxymercuration of thiophene has been estimated to proceed ca. 10 times faster than that of benzene. Mercuration of rings with deactivating substituents such as ethoxycarbonyl and nitro is still possible with this reagent, as shown by the formation of compounds (75) and (76). Mercury(II) chloride is a milder mercurating agent, as illustrated by the chloromercuration of thiophene to give either the 2- or 2,5-disubstituted product (Scheme 25). 

Studies are restricted to syntheses of Group 2B - organometallics and mainly to mercurials. Early studies of reactions of mercuric salts with furan- and thiophencarboxylic acids or their salts have been thoroughly reviewed (7-4). More recently several tetrafluoropyridyl derivatives have been prepared by decarboxylation [Eqs. (78) (87), (79) (88), or (80),... 

Under the catalysis of mercury(II) oxide and p-toluenesulfonic acid, allenic /8-keto esters 43 and 45 afforded the furan derivatives 44 and 46 [27]. 

N-Substituted pyrroles, furans and thiophenes can be 2-lithiated, and these lithio derivatives are important synthetic intermediates (Section 3.3.3.8). 2-Mercuri and 2-palladio derivatives are also important (Sections 3.3.3.8.8 and 3.3.3.8.9). 

Isocyanide-mercury(II) chloride complexes when reacted with acetylacetone in the presence of triethylamine yield furan derivatives (290) (75CPB2842). The aminofuranones (291)... 

Several examples of intramolecular solvomercuration of arylalkynes have been reported (equations 81 to 83).200 The mercury moiety can be removed by protiodemetallation or converted to a variety of other functional groups. The use of a methyl ether as the nucleophilic functionality is noteworthy, as is the change in the regioselectivity (equation 82 versus equation 83). A bromocyclization related to equation (83) using a phenylethynylbenzoate ester also gives an isocoumarin derivative via 6-endo cyclization.201 Palladium-catalyzed cyclizations of 3,y-alkynic ketones or 2-methyl-3-alkyn-l-ols to form substituted furans are discussed in Chapter 1.4, Section 3.1.3 in this volume. 

The reaction of aryllead triacetates is not limited to substituted phenyllead derivatives. It has also been extended to the use of heteroaryllead compounds, derived from furan and thiophene.Due to their relative instability and moisture-sensitivity, they are best prepared by metal exchange, such as mercury-lead or tin-lead, and used in situ in the reactions of carbon nucleophiles. 

Intramolecnlar alkenylation at a furan a- or P-position by an alkyne occurs, with the formation of bicycUc derivatives, when promoted by mercury(II) acetate (or Hg(OAc)(OTf), generated in situ from mercuric acetate and scandinm triflate). In the case of closure onto a p-position, a spirocyclic intermediate from preferred attack at the a-position, may be involved, as shown. 

Treatment of the quaternary ammonium salt PhC=CCH2NMe2CH2COPh Br" with sodium hydroxide gives, inter alia, the betaine (98). Controlled-potential electrolysis of 7-nitro-ketones on a mercury cathode yields 1-pyrroline 1-oxides, pyrrolines, or pyrrolidines, according to conditions thus the oxide (99) is obtained from the compound 02NCMe2CHPhCH2C0Ph. The pyrrolinone (101) results from autoxidation of the furan derivative (100). Irradiation of the amide (102) leads to the pyrrolidinone (103) by an unprecedented [1,6] shift of hydrogen. " ... 

Multiple substitution of reactive aromatic compounds may occur. Thiophene gives a 2, 5-disubstituted and furan a 2,3,4, 5-tetrasubstituted derivative with mercury (II) acetate in boiling ethanol. The preferential mercuration of thiophene has been used for its removal from commercial benzene. Mercuration of ferrocene (p. 284) also occurs readily. 

The photolysis of the hypoiodites generated from the cyclobutanols, derived from the [2-l-2]-photo-addition of 2-hydroxynaphthoquinone with alkenes (A) and of 2-aminonaphthoquinone with vinyl arenes (B) in the presence of mercury(II) oxide and iodine induced regioselective P-fragmentations of the cyclobutanoxyl radicals to give a 2,3-dihydronaphtho[2,3-h]furan-4,9-dione and its [ l,2-h]furan-4,5-dione isomer (Scheme 76). 

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