Acidity of cubanes

Ritchie, J. P. Bachrach, S. M. Comparison of the calculated acidity of cubane with that of other strained and unstrained hydrocarbons, 7. Am. Chem. Soc. 1990, 112, 6514-6517. 

Calculated Acidity of Cubane with That of Other Strained and Unstrained Hydrocarbons. 

Rg. 27.The structure of the tetrakis(2-fluoro-2,2-dinitroethyl)ester of the 1,2,4,7-tetracaiboxylic acid of cubane. 

R. J. Butcher, A. Bashir-Hashemi, R. Gilardi, Network hydrogen bonding the crystal and molecular structure of cubane-l,35,7-tetracarboxylic acid dihydrate , J. Chem. Cryst., 1997, 27, 99-107. 

Dinitrocubane (28) has been synthesized by Eaton and co-workers via two routes both starting from cubane-l,4-dicarboxylic acid (25). The first of these routes uses diphenylphos-phoryl azide in the presence of a base and tert-butyl alcohol to effect direct conversion of the carboxylic acid (25) to the tert-butylcarbamate (26). Hydrolysis of (26) with mineral acid, followed by direct oxidation of the diamine (27) with m-CPBA, yields 1,4-diiutrocubane (28). Initial attempts to convert cubane-l,4-dicarboxylic acid (25) to 1,4-diaminocubane (27) via a Curtins rearrangement of the corresponding diacylazide (29) were abandoned due to the extremely explosive nature of the latter. However, subsequent experiments showed that treatment of the acid chloride of cubane-l,4-dicarboxylic acid with trimethylsilyl azide allows the formation of the diisocyanate (30) without prior isolation of the dangerous diacylazide (29) from solution. Oxidation of the diisocyanate (30) to 1,4-dinitrocubane (28) was achieved with dimethyldioxirane in wet acetone. Dimethyldioxirane is also reported to oxidize both the diamine (27) and its hydrochloride salt to 1,4-dinitrocubane (28) in excellent yield. ... 

Eaton and co-workers also reported the synthesis of 1,3,5-trinitrocubane and 1,3,5,7-tetranitrocubane (39) ° The required tri- and tetra-substituted cubane precursors were initially prepared via stepwise substitution of the cubane core using amide functionality to permit ort/jo-lithiation of adjacent positions. The synthesis of precursors like cubane-1,3,5,7-tetracarboxylic acid was long and inefficient by this method and required the synthesis of toxic organomercury intermediates. Bashir-Hashemi reported an ingenious route to cubane-1,3,5,7-tetracarboxylic acid chloride (35) involving photochemical chlorocarbonylation of cubane carboxylic acid chloride (34) with a mercury lamp and excess oxalyl chloride. Under optimum conditions this reaction is reported to give a 70 8 22 isomeric mixture of 35 36 37... 

The bond dissociation energy (BDE) and acidity constant (p ) of cubane (Cub—H) have been determined experimentally by Eaton and co-workers (Hare, M. Emrick, T. Eaton, P. E. Kass, S. R, J. Am. Chem. Soc., 1997, 119, 237-238). The BDE is unusually high for a tertiary C—H bond, 427 kJ/mol, about 25 kJ/mol higher than in isobutane. The C—H bond is also quite acidic, comparable to the acidity of the N—H bond in ammonia, pK = 36, indicating an unusual stability for the anion... 

Each P-cluster is actually a joined pair of cubane-type clusters, one Fe4S4 and one Fe4S3 with two bridging cysteine -SH groups and one iron atom bonded to three sulfide sulfur atoms (Fig. 24-3).17/23 The FeMo-coenzyme can be released from the MoFe-protein by acid denaturation followed by extraction with dimethylformamide.24 While homocitrate was identified as a component of the isolated coenzyme, the three-dimensional structure of FeMo-co was deduced from X-ray crystallography of the intact molybdenum-iron protein.14/17/18... 

Other acids (often as esters) have been found in fermented molasses. Usually these substances are products of bacteriological action and they are not normal constituents of unfermented molasses. Bauer" oil from the yeast fermentation of Cuban blackstrap consists chiefly of the ethyl esters of capric, lauric, myristic and palmitic acids.122 The fat from the scums of hot-room Louisiana molasses contained hexanoic (caproic) and octanoic (caprylic) acids.10 The occurrence of such volatile acids as propionic,128 butyric128 124 and valeric acids124 requires more adequate establishment. 

The concentration of fats and related substances in molasses is low analytical values depend on the extracting solvent.126 These tenaciously retained materials can be removed by fractionation of blackstrap on fuller s earth clay.70 Chromatography on a calcium silicate of the fat fraction of Cuban molasses led to the isolation of melissyl alcohol, a phytosterol fraction, chlorophyll a and a fat fraction containing a glyceride of linoleic acid.70 Stigmasterol and syringic acid are reported as ether-extractable constituents of molasses.127... 

The acidity of 1,3,5,7-tetranitrocubane, which features nitro groups on alternate comers of cubane, has been measured (p/C, 21) and reactions of the corresponding o-nitro anion have been explored.183... 

The double Favorski-like contraction of dibromodiketone 193 (KOH, H2O H+) which yielded p-cubanedicarboxylic acid (194) is another example of this class of ring contraction reaction. This reaction was used by Eaton and Cole (68) in their synthesis of cubane. 

The last example is the acidity of the ethynyl-expanded cubane 3. Inspired by the synthesis of the methoxy-substituted buta-l,3-diynediyl expanded cubane 4, Bachrach examined some of the properties of its smaller homolog 3. Since the s-character of the C-H bonds of 3 should be large and its conjugate base 3cb can potentially delocalize the anion into three neighboring alkynyl units, 3 might be quite acidic. 

Baker, F.W., Parish, R.C. and Stock, L.M. (1967). Dissociation Constants of Bicyclo[2.2.2]oct-2-ene-l-carboxylic Acids, Dibenzobicyclo[2.2.2]octa-2,5-diene-l-carboxylic Acids, and Cubane-carboxylic Acids. J.Am. ChemSoc., 89,5677-5685. 

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