Malonic anhydrides

Wentrup and co-workers investigated the flash vacuum pyrolysis of isopropylidene (l-methyIpyrrolidin-2-ylidene)malonate (1261) (86CC369) (Scheme 51). When the pyrolysis was carried out at 450°C (10-4 torr, contact time 10 3 sec), and the product was condensed on a cold finger at -196°C, (pyrrolidinylidene)malonic anhydride (1262) could be identified. Malonic anhydride (1262) in chloroform solution at -20°C lost carbon dioxide to give methyleneketene (1263), which was reacted with a few drops of water or methanol to yield acrylates (1264). Flash vacuum pyrolysis of 1261 at higher temperature (800°C) gave pyrrolopyrrolone (1265). The products were characterized by IR and 13C-NMR data. 

Dimethylbisthiomalonic acid (120), when treated with an acylated enol, produced intermediate 121 thietanediones 122 and 123 then were formed by elimination of thioketone and acetic acid (see Eq. 12). Similarly, cyclobutane-l,l-bisthiolcarboxylic acid gave 123, a previously unknown sulfur analog of malonic anhydride, (Eq. 12). Thermolysis of 120 also results in the formation of the four-membered heterocycles in addition to carbonyl sulfide, hydrogen sulfide, and thiocarbonic acid. ... 

Ozonolysis of diketene affords formaldehyde [50-00-0] and the very un-stable malonic anhydride [15159-48-5] (114), which readily decomposes to C02 and ketene, but can be converted at low temperature to malonic acid monoesters or Meldrum s acid (115) (see Malonic acid AND DERIVATIVES). 

Malonic anhydride cannot be made directly from malonic acid because attempted dehydration of the acid leads to the exotic molecule carbon suboxide C3O2. 

The thermal decomposition of disubstituted malonic anhydrides gives ketoketenes, R,C = C = 0. A similar synthesis of an aldoketene, RHC=C=0, from a monosubstituted malonic anhydride has never been accomplished. 

The anhydrides can be prepared by the action of acetic anhydride on the corresponding malonic acid in the presence of a small amount of sulfuric acid, followed by neutralization of the mineral acid with powdered barium carbonate and evaporation to dryness in a high vacuum. The residual malonic anhydride is then heated to the decomposition point at a low pressure, and the ketene is collected in a cold receiver. This procedure has been applied to the synthesis of low-molecular-weight dialkylketenes (R is methyl, ethyl, f2-propyl, or isopropyl) in 50-80% yields. ... 

Because malonic anhydrides are difficultly accessible, Staudinger recommended the decomposition of mixed anhydrides as a generally applicable method these mixed anhydrides can be obtained by the action of, e.g., di-phenylketene on disubstituted malonic acids in ethereal solution.25... 

A unique preparation of ketenes is by thermal decomposition of malonic anhydrides, which form ketenes and carbon dioxide (Scheme 7.47). Ketene, methylketene, and dimethylketene are formed by this methodology. Surprisingly, the monomethyl malonic anhydride is the most reactive and the dimethyl is the slowest. 

Scheme 7.47 Formation of ketenes by thermal decomposition of malonic anhydrides.

The peroxy compounds have high energy contents. The weakness of the 0—0 bond (35 kcal/mole) (53) is a reflection of the unfavorable O (hard) and O (soft) combination. Diacyl peroxides, for example, react vigorously with soft donors such as phosphines (54, 55). This deoxygenation process has been applied to the synthesis of the highly strained malonic anhydrides (56). 

Several malcmic acids have been used as acylating reagents in the presence of polyphosphoric acid (PPA) as both catalyst and solvent [31]. Probably the reactive intermediates are malonic anhydrides. Decarboxylation following the acylation gives the desired products (Scheme 11) [31]. 

A method of synthesis of ketoketenes which is closely related to the pyrolysis described in the preceding section consists in the thermal decomposition of disubstituted malonic anhydrides, of either the simple or mixed types. 

Monosubstituted malonic anhydrides have not yielded aldoketenes, but malonic acid itself jdelds carbon suboxide when heated with phos-pho] pentoxide. ... 

The disubstituted malonic anhydrides can be prepared from the corresponding malonic acids and acetic anhydride in the presence of a little sulfuric acid, with neutralization of the mineral acid by treatment with barium carbonate and removal of acetic acid and acetic anhydride by distillation. The r dual malonic anhydride is decomposed by heating under low pressure. This method appears to have been used only for dimethylketene (80% yield), diethylketene (55% yield), methylethyl-ketene (65% yield), and dipropyl- and diisopropyl-ketenes (50% yields). 

Dimethylketene has beep obtained in unspecified yield by heating the monochloride of dimethylmalonic acid it is probable that the malonic anhydride was formed as an intermediate. This method failed in attempted applications to diethylketene, and carbon suboxide. Low yields of carbon suboxide have been obtained by tteating malonyl chloride with lead, silver, or zinc oxide or with silver oxalate or malonate, and by treating silver malonate with cinnamoyl chloride malonic anhydrides or mixed anhydrides may be intermediates. 

Ebtokbtenbs Pbepabed fbom Malonic Acid Derivatives A. From Simple Malonic Anhydrides... 

---