Mercury(II) Trifluoroacetate1

(CF3CO2)2Hg

[13257-51-7]  · C4F6HgO4  · Mercury(II) Trifluoroacetate  · (MW 426.63)

(oxymercuration3 and aminomercuration17 of alkenes; polyene cyclization;21 allylic oxidation;32 macrolide synthesis37)

Alternate Name: mercuric trifluoroacetate.

Physical Data: mp 171-173 °C.

Solubility: sol THF, DME, dioxane; insol hexane.

Preparative Methods: can be prepared from Mercury(II) Oxide in Trifluoroacetic Acid2-4 or generated in situ from Mercury(II) Acetate and CF3CO2H.8b

Form Supplied in: white, hygroscopic crystals.

Handling, Storage, and Precautions: acute poison; is easily absorbed through the skin and is corrosive; exposure to all mercury compounds is to be strictly avoided. Releases toxic Hg fumes when heated to decomposition. Protect from light. Use in a fume hood.

Oxymercuration.3-5

The reactivity of mercury(II) trifluoroacetate is similar to that of Mercury(II) Acetate but exhibits higher electrophilicity.3 Thus, for instance, while cholesterol and other alkenes with a trisubstituted double bond are normally reluctant to react with (AcO)2Hg (unless neighboring group participation boosts the reaction), (CF3CO2)2Hg reacts readily with cholesteryl acetate (eq 1).6 Even a,b-unsaturated esters react with (CF3CO2)2Hg; the corresponding adducts are readily transformed into a-bromo derivatives (eq 2).7 Generally, in the presence of Cl2 or Br2, the HgX group in the original adduct is replaced by halogen.1,5

Increased selectivity in oxymercuration has been observed when (CF3CO2)2Hg and other HgII salts were generated in situ from HgO and RCO2H in an inert solvent (CH2Cl2, hexane, or aq THF) on sonication.4

Whereas oxymercuration normally occurs with anti stereoselectivity,3,4 products of syn addition have been observed for norbornene derivatives.8

Intramolecular oxymercuration9 has been extensively utilized for the construction of various oxygen heterocycles, usually with high stereoselectivity (eq 3).10 Both isolated9,10 and conjugated11 double bonds react, and 5-exo-trig, 5-endo-trig, and 6-exo-trig reactions have all been observed.9-11 Similarly, cyclic peroxides are formed from the unsaturated peroxides.12

The relatively stable trichloroacetals derived from allylic alcohols can be utilized as intermediates to accomplish stereo- and regiocontrolled mercuration which gives cis-1,2-diols (eq 4).13 However, this method has the following limitations: (1) the original hydroxy group must be equatorial, (2) the double bond cannot be trisubstituted, (3) the reaction is not regioselective for acyclic allylic alcohols, and (4) the reaction is unsuccessful with homoallylic alcohols.13

Hydroxy allenes seem to be particularly suitable substrates as the sp2-bonded mercury can be transmetalated with palladium salts, which expands the versatility of this methodology (eq 5).14 Alkynic alcohols also produce vinylmercurials (with some preference for (Z) isomers)15 which can be further elaborated, e.g. by conversion into vinyl halides on reaction with sources of electrophilic halogen (N-Iodosuccinimide).16

Aminomercuration.

Intramolecular aminomercuration followed by demercuration is a useful method of construction of a variety of nitrogen heterocycles such as aza sugars.17 Intramolecular amidomercuration, employing (CF3CO2)2Hg, has also been reported.18 It has been shown that the steric course of mercuration can be controlled by a neighboring nitrogen group (eq 6).19

Azidomercuration of an alkenic double bond can be accomplished in quantitative yield (in the case of a monosubstituted double bond) by the reaction carried out with (CF3CO2)2Hg and Sodium Azide (3:1) in THF-H2O (1:1) at 60 °C for 48 h.20 This method has been utilized for syntheses of amino sugars. By contrast, (AcO)2Hg gives only 20% in the same case.20

Polyene Cyclization.

Acyclic isoprenoids are stereospecifically cyclized on reaction with (CF3CO2)2Hg to afford polycyclic structures (eq 7).21 An aromatic ring can be used in place of the nucleophilic double bond in a similar type of cyclization.22

Cyclopropane Ring Opening.

Mercury(II) trifluoroacetate readily opens up cyclopropane rings.23 The reaction is usually faster than that with (AcO)2Hg and slower than for Mercury(II) Nitrate.

Mercuration of Aromatics.

Being a good electrophile, (CF3CO2)2Hg and other HgII salts also attack aromatic rings.24,25 Combined with the subsequent reaction with iodine, this method has long served to accomplish iodination in cases where the arene is inert to direct iodination. Polyhaloaromatics can also be synthesized in this way.26

[3,3]-Sigmatropic Rearrangements.

Oxy-Cope rearrangement can be initiated at rt by (CF3CO2)2Hg (1 equiv); the intermediate a-mercurio ketone is then demercurated with Sodium Borohydride.27 The reaction can be carried out with a catalytic amount of HgII and an excess of lithium triflate or trifluoroacetate.28 The Claisen rearrangement of allylic carbamates can be best achieved via a suprafacial [3,3]-migration mediated by (CF3CO2)2Hg (eq 8); the reaction is thermodynamically controlled.29 Using a carbamate derived from an optically active amine, formation one of the diastereoisomers may be preferred; however, this method seems to have a limited value due to the low de.30

Allylic Oxidation (Treibs Reaction31).32

More effective than the acetate, (CF3CO2)2Hg facilitates allylic oxidation33,34 of alkenes which are generally reluctant to be oxymercurated (namely those with trisubstituted double bonds). Thus, cholesterol and its esters are converted, after hydrolysis, to the 3b,6b-dihydroxy-4-ene at 0 °C in 40-70% yields.33

Hydrolysis of Vinyl Chlorides.

The original Wichterle reaction35 relied on the hydrolysis of vinyl chlorides with H2SO4. The improved version utilizing (CF3CO2)2Hg allows the control of regioselectivity by the solvent.36 Thus, hydrolysis in MeNO2, CH2Cl2, or AcOH gives only 1,5-diketone (to be cyclized) in up to 97% yield, whereas hydrolysis in MeOH gives mainly the 1,4-diketone (eq 9).36

Synthesis of Macrolides.

t-Butyl thioesters are converted to esters on reaction with (CF3CO2)2Hg or (MeSO3)2Hg. This method is general for synthesis of macrolides (eq 10) but cannot be used for substrates containing electron-rich centers such as double bonds (which undergo oxymercuration); other less electrophilic cations can be employed in those instances (Cu+, Cu2+, or Ag+).37

Miscellaneous.

Like HgO or TlIII, mercury(II) trifluoroacetate oxidizes phenols and hydroquinones to quinones in the presence of an acid scavenger.38 Monohydrazones of a-diketones are oxidized with (CF3CO2)2Hg to give a-diazo ketones.2 Enol esters, obtained by the reaction of 4-dimethylamino-3-butyn-2-one with o-hydroxy carboxylic acids, react with (CF3CO2)2Hg or 10-Camphorsulfonic Acid to give macrocyclic lactones.39 1,3-Dioxolanium salts are converted to a-mercurio derivatives of carboxylic acids on reaction with (CF3CO2)2Hg or (AcO)2Hg.40 TMS derivatives of tertiary vinyl alcohols undergo Wagner-Meerwein migration on treatment with (CF3CO2)2Hg; this can be used as a ring-expansion methodology in cyclic systems.41

Related Reagents.

t-Butyldimethylsilyl Hydroperoxide-Mercury(II) Trifluoroacetate; Mercury(II) Acetate.


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Pavel Ko&cbreve;ovský

University of Leicester, UK



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