[21908-53-2] · HgO · Mercury(II) Oxide · (MW 216.59)
Alternate Name: mercuric oxide.
Physical Data: mp 500 °C (dec); d 11.14 g cm-3.
Solubility: sol dilute HCl, HNO3; insol water, ethanol.
Form Supplied in: commercially available in both yellow and red crystalline forms.
Handling, Storage, and Precautions: highly toxic; oxidizer; protect from light.
Mercury(II) oxide promotes the Wittig-thio-Claisen rearrangement sequence which converts ketones into unsaturated aldehydes (eq 1).2 The reaction proceeds via formation of an allyl vinyl sulfide followed by rearrangement to form the a-allyl aldehyde.
A mixture of mercury(II) oxide/35% aqueous Tetrafluoroboric Acid/THF can be used for the hydrolysis of 1,3-benzodithioles (eq 2), 1,3-benzoxathioles (eq 3), 1,3-oxathiolanes (eq 4), 1,3-dithiolanes (eq 5), and 1,3-dithianes (eq 6).3 These reaction conditions offer an alternative to those reported by Vedejs and Fuchs19 which use a mixture of mercury(II) oxide/Boron Trifluoride Etherate/THF.
The use of mercury(II) oxide/50% aqueous tetrafluoroboric acid/THF and an alcohol is a fast and mild procedure for the hydrolysis of a-hydroxy thioorthoesters to the corresponding ester (eq 7).4
Primary and secondary unbranched aliphatic alcohols are readily converted into the corresponding tetrahydrofuran derivatives using mercury(II) oxide and Iodine in carbon tetrachloride (eqs 8 and 9).5
The inherent instability of b-substituted polynitroaliphatic alcohols in acidic and basic media precludes the use of traditional vinyl ether synthesis. The use of mercury(II) oxide/Trifluoroacetic Acid in refluxing dichloromethane permits a one-step, high-yielding synthesis of b-substituted polynitroalkyl ethers (eq 10).6 The use of mercury(II) oxide alone produces the product in 20-30% yield, whereas the addition of TFA as cocatalyst substantially increases the yield of the desired product.
Oxidation of allylbenzene using mercury(II) oxide/tetrafluoroboric acid and alcohols in THF forms exclusively trans-cinnamyl ethers (eq 11).7
The mercuration of alkenes using mercury(II) oxide and tetrafluoroboric acid in the presence of excess amine leads to the formation of 1,2-diamines (eq 12).8 Presumably, the reaction takes place via formation of a b-aminomercury(II) tetrafluoroborate which undergoes nucleophilic attack of an amine.
A one-pot procedure for hydroxy(alkoxy)phenylamination of alkenes has been developed using mercury(II) oxide and tetrafluoroboric acid (eq 13).9 The reaction proceeds through the formation of the aminomercurial from the alkene followed by reaction with water or alcohol. The ratio of products depends on the substitution pattern on the alkene. For terminal alkenes, the rearranged isomer predominates and cyclic alkenes afford trans products (eq 14).9
A variety of g-methylenebutyrolactones have been successfully synthesized by cyclization of the appropriate alkynecarboxylic acid with mercury(II) oxide as catalyst (eqs 15 and 16).10 In some instances the reaction must be run neat; in other cases, solvents such as chloroform, acetone, benzene, dioxane and even refluxing DMF are necessary.
In a similar approach, mercury(II) oxide has also been used to catalyze the cyclization of g-allenic acids into g-ethylenic d-lactones (eq 17).11
Alkanes react with a mixture of Bromine and mercury(II) oxide to afford the corresponding alkyl bromide in good yield (eqs 18 and 19).12 This combination of reagents is more reactive than bromine or N-Bromosuccinimide. The mechanism has been postulated to proceed through the formation of bromine monoxide in situ as the brominating agent.12 Primary and secondary alkyl bromides are readily formed, but tertiary bromides and benzylic bromides are rather problematic.
Alkyl and aryl halides can also be synthesized using the Cristol-Firth modification of the Hunsdiecker reaction (eqs 20 and 21).13,14 Even bridgehead carboxylic acids can be transformed into the corresponding bromide using mercury(II) oxide and bromine. Aromatic acids form insoluble mercury(II) salts which normally lead to lower yields, but it has been found that irradiation of the reaction with a 100 W bulb affords excellent yields.14 Interestingly, irradiation of reactions of aliphatic acids does not improve the yield and, in some cases, even produces lower yields.
Decarboxylation of bridgehead carboxylic acids has been accomplished in two steps. The bromide is formed first followed by reduction with Tri-n-butylstannane. The deuterated analog can be made using tributyltin deuteride (eq 22).15
In most cases, mercury(II) oxide can be used instead of Mercury(II) Trifluoroacetate to oxidize phenols and hydroquinones (eqs 23 and 24).16 These conditions are attractive in that they do not require the use of an acid scavenger to neutralize the trifluoroacetic acid that is formed during the course of the reaction.
Aliphatic and aromatic hydrazones are oxidized with mercury(II) oxide to afford the corresponding diazo compound (eqs 25-27).17 Reaction of the monohydrazones derived from a-diketones give a-diazo ketones, which, upon heating, form ketenes (eq 28).17 Treatment of the bis-hydrazones of a-diketones with mercury(II) oxide at higher temperatures affords alkynes (eq 29).17 Seven- and eight-membered cycloalkynes as well as even larger cycloalkynes have been synthesized using this methodology (eq 30).17
A mixture of mercury(II) oxide and Sulfuric Acid effects hydration of alkynes to form the corresponding methyl ketone. This protocol was utilized in the synthesis of some daunomycinone analogs (eq 31).18
Ellen M. Leahy
Affymax Research Institute, Palo Alto, CA, USA