Meldrum's acid

Meldrum's acid or 2,2-dimethyl-1,3-dioxane-4,6-dione is an organic compound with formula C
6H
8O
4. Its molecule has a heterocyclic core with four carbon and two oxygen atoms; the formula can also be written as [−O−(C(CH
3)
2)−O−(C=O)−(CH
2)−(C=O)−].

Meldrum's acid
Names
IUPAC name
2,2-Dimethyl-1,3-dioxane-4,6-dione
Other names
Isopropylidene malonate
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.016.358
UNII
Properties
C6H8O4
Molar mass 144.126 g·mol−1
Melting point 94 to 95 °C (201 to 203 °F; 367 to 368 K) (decomposes)[1]
Acidity (pKa) 4.97
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N verify (what is YN ?)
Infobox references

It is a crystalline colorless solid, sparingly soluble in water. It decomposes on heating with release of carbon dioxide and acetone.[2][3]

Properties

Acidity

The compound can easily lose a hydrogen ion from the methylene element CH
2 in the ring (carbon 5); which creates a double bond between it and one of the adjacent carbons (number 4 or 6), and a negative charge in the corresponding oxygen. The resulting anion [C
6H
7O
4]− is stabilized by resonance between the two alternatives, so that the double bond is delocalized and each oxygen in the carbonyls has a formal charge of −1/2.

The ionization constant pKa is 4.97; which makes it behave as a monobasic acid even though it contains no carboxylic acid groups.[2] In this and other properties, the compound resembles dimedone and barbituric acid. However, while dimedone exists in solution predominantly as the mono-enol tautomer, Meldrum's acid is almost entirely as the diketone form.[2]

The unusually high acidity of this compound was long considered anomalous—it is 8 orders of magnitude more acidic than the closely related compound dimethyl malonate. In 2004, Ohwada and coworkers determined that the energy-minimizing conformation structure of the compound places the alpha proton's σCH orbital in the proper geometry to align with the π*CO, so that the ground state poses unusually strong destabilization of the C-H bond.[4]

Preparation

Original synthesis

The compound was first made by Meldrum by a condensation reaction of acetone with malonic acid in acetic anhydride and sulfuric acid.[3]

Alternative syntheses

As an alternative to its original preparation, Meldrum's acid can be synthesized from malonic acid, isopropenyl acetate (an enol derivative of acetone), and catalytic sulfuric acid.

A third route is the reaction of carbon suboxide C
3O
2 with acetone in the presence of oxalic acid.[2]

Uses

Like malonic acid and its ester derivatives, and other 1,3-dicarbonyl compounds, Meldrum's acid can and serve as a reactant for a variety of nucleophilic reactions.

Alkylation and acylation

The acidity of carbon 5 (between the two carbonyl groups) allows simple alkylation and acylation of Meldrum's acid at this position. For example, deprotonation and reaction with a simple alkyl halide (R−Cl) attaches the alkyl group (R−) at that position:

The analogous reaction with an acyl chloride (R−(C=O)−Cl) attaches the acyl (R−(C=O)−) instead:

These two reactions allow Meldrum's acid to serve as a starting scaffold for the synthesis of many different structures with various functional groups. The alkylated products can be further manipulated to produce various amide and ester compounds. Heating the acyl product in the presence of an alcohol leads to ester exchange and decarboxylation in a process similar to the malonic ester synthesis. The reactive nature of the cyclic-diester allows good reactivity even for alcohols as hindered as t-butanol.[5] Ketoesters of this type are useful in the Knorr pyrrole synthesis.

Synthesis of ketenes

At temperatures greater than 200 °C[6] Meldrum's acid undergoes a pericyclic reaction that releases acetone and carbon dioxide and produces a highly reactive ketene compound:[7]

These ketenes can be isolated using flash vacuum pyrolysis (FVP). Ketenes are highly electrophilic and can undergo addition reaction with other chemicals. With this approach it is possible to form new C–C bonds, rings, amides, esters, and acids:

                     

Alternately, the pyrolysis can be performed in solution, to obtain the same results without isolating the ketene, in a one-pot reaction. The ability to form such diverse products makes Meldrum's acid a very useful reagent for synthetic chemists.[8][9][10]


History
Incorrect structure.

The compound is named after Andrew Norman Meldrum who reported its synthesis in 1908.[3] He misidentified its structure as a β-lactone of β-hydroxyisopropylmalonic acid; the correct structure, the bislactone of 1,3-dioxane was reported in 1948.[11]

References
  1. "Meldrum's Acid". The Merck Index. 14th. edition. Merck Research Laboratories. 2006. p. 1005. ISBN 978-0-911910-00-1.
  2. McNab, Hamish (1978). "Meldrum's acid". Chemical Society Reviews. 7: 345–358. doi:10.1039/CS9780700345.
  3. Norman Meldrum, Andrew (1908). "A β-lactonic acid from acetone and malonic acid". Journal of the Chemical Society, Transactions. 93: 598–601. doi:10.1039/CT9089300598.
  4. Nakamura, Satoshi; Hirao, Hajime; Ohwada, Tomohiko (2004). "Rationale for the Acidity of Meldrum's Acid. Consistent Relation of C−H Acidities to the Properties of Localized Reactive Orbital". J. Org. Chem. 69 (13): 4309–4316. doi:10.1021/jo049456f. PMID 15202884.
  5. Oikawa, Yuji; Sugano, Kiyoshi; Yonemitsu, Osamu (1978). "Meldrum's acid in organic synthesis. 2. A general and versatile synthesis of β-keto esters". J. Org. Chem. 43 (10): 2087–2088. doi:10.1021/jo00404a066.
  6. Gaber, Abd El-Aal M.; McNab, Hamish (2001). "Synthetic Applications of the Pyrolysis of Meldrum's Acid Derivatives". Synthesis. 2001 (14): 2059–2074. doi:10.1055/s-2001-18057.
  7. Dumas, Aaron M.; Fillion, Eric (2009). "Meldrum's Acids and 5-Alkylidene Meldrum's Acids in Catalytic Carbon–Carbon Bond-Forming Processes". Acc. Chem. Res. 43 (3): 440–454. doi:10.1021/ar900229z.
  8. Oikawa, Yuji; Hirasawa, Hitoshi; Yonemitsu, Osamu (1978). "Meldrum's acid in organic synthesis. 1. A convenient one-pot synthesis of ethyl indolepropionates". Tetrahedron Letters. 19 (20): 1759–1762. doi:10.1016/0040-4039(78)80037-9.
  9. Lipson, Victoria V.; Gorobets, Nikolay Yu. (2009). "One hundred years of Meldrum's acid: Advances in the synthesis of pyridine and pyrimidine derivatives". Mol. Divers. 13 (4): 399–419. doi:10.1007/s11030-009-9136-x. PMID 19381852.
  10. Bonifácio, Vasco D. B. (2004). "Meldrum's Acid". Synlett. 2004 (9): 1649–1650. doi:10.1055/s-2004-829539.
  11. Davidson, David; Bernhard, Sidney A. (1948). "The Structure of Meldrum's Supposed β-Lactonic Acid". Journal of the American Chemical Society. 70 (10): 3426–3428. doi:10.1021/ja01190a060. PMID 18891879.

Further reading
  • Gerencsér, János; Dormán, György; Darvas, Ferenc (2006). "Meldrum's Acid in Multicomponent Reactions: Applications to Combinatorial and Diversity-Oriented Synthesis". QSAR & Combinatorial Science. 25 (5–6): 439–448. doi:10.1002/qsar.200540212.
  • Ivanov, Andrey S. (2008). "Meldrum's acid and related compounds in the synthesis of natural products and analogs". Chem. Soc. Rev. 37 (4): 789–811. doi:10.1039/B716020H.
  • Kidd, Hamish (October 29, 2008). "Meldrum's Acid". Chemistry World: 35–36.
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