Malonaldehyde

FIGURE 7.8 The integrand I(z) for the second pre-exponential factor Si in the case of malon-aldehyde. The curve with sharp peaks corresponds to the interpolated global potential energy surface from Reference [166], The smooth curve is obtained without global interpolation of the ab initio data. (Taken from Reference [122] with permission.) 

81 kcal/mol at the CCSD(T)/(aug-)cc-pVTZ level of theory the effect of triple-f level of basis sets decreases the barrier height. Computations at the level of CCSD(T)/ (aug-)cc-pVTZ are, however, very much time consuming and in the following calculations the best level of theory employed is CCSD(T)/(aug-)cc-pVDZ. 

FIGURE 7.9 Instanton trajectory in the malonaldehyde obtained by the direct ab initio calculations without global interpolation (a) MP2/cc-pVDZ, (b) QCISD/(aug-)cc-pVDZ, and (c) CCSD(T)/(aug-)cc-pVDZ. (Taken from Reference [122] with permission.) 

Finally, we demonstrate a strong effect of out-of-plane vibrational motions on the tunneling splitting [183]. In order to do this, first we have to define 15 internal coordinates to describe in-plane motions and construct the corresponding Hamiltonian. Following the same method as that explained in Section 6.3.1, we introduce Cartesian coordinates in the body-fixed (BF) frame of reference. 

The three constraints given by Equations (7.45) and (7.46) define the BF system. The independent internal coordinates q ik = 1,2. 15) are chosen to be 14 Cartesian coordinates of seven atoms, Oi, C2, H3, C4, Ce, H7, and Hg, andxig (x coordinate of Og). The kinetic metric defined as 

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With Lewis acids as catalysts, compounds containing more than one alkoxy group on a carbon atom add across vinyl ether double bonds. Acetals give 3-alkoxyacetals since the products are also acetals, they can react further with excess vinyl ether to give oligomers (228—230). Orthoformic esters give diacetals of malonaldehyde (231). With Lewis acids and mercuric salts as catalysts, vinyl ethers add in similar fashion to give acetals of 3-butenal (232,233). 

One of the most thoroughly studied examples of intramolecular tunneling is isomerization of malonaldehyde involving the transfer of an H atom in an OH O fragment. 

Fig. 38, Contour plot, MEP and instanton trajectory for isomerization of malonaldehyde (6.4). The instanton is drawn for large but finite in the limit = oo it emanates from the potential minimum.

Unlike in the case of the gas-phase measurements, no tunneling has been detected in the IR spectra of the malonaldehyde molecule in the noble matrices at 15-30K [Firth et al. 1989], The lack of tunneling is caused by detuning of the potential as a result of weak antisymmetric coupling to the environment. 

The anions of malonaldehyde [106, 107] and of organophosphonates [108, 109, 110] are fluorinated in good yields to provide interesting fluorinated intermediates The At-fluoro compound B in Table 3a is also effective in the fluonnation of phosphonate anions [109] (equations 60 and 61). 

The addition of phenylisocyanate to aldehyde-derived enamines resulted in the formation of aminobutyrolactams (438,439). As aminal derivatives these produets can be hydrolyzed to the linear aldehyde amides and thus furnish a route to derivatives of the synthetically valuable malonaldehyde-acid system. With this class of reactions, a second acylation on nitrogen becomes possible and the six-membered cyclization products have been reported (440). Closely related to the reactions of enamines with isocyanates is the condensation of cyclohexanone with urea in base (441). 

Treatment of nitrolic acid 34, which is in turn available from hydroximino malonaldehyde dioxime (N2O4 in CH2CI2, 20 min, 10-15°C), with TFA gives furoxan-4-carboxylic acid in 35% yield (93CHE952, 93KGS1117) (Scheme 39). 

In the past, periodate titrations have been of limited value for establishing the structure of quercitols or cyclohexanetetrols. The former show overoxidation, because of the fact that malonaldehyde is formed, and this compound undergoes further oxidation. Some isomers of the tetrols... 

In theory, periodate oxidation could have given a clear-cut answer as to the composition of the isomeric mixture of deoxy ribose phosphates. The 4-phosphate (73), devoid of vicinal diol groups, should be resistant to periodate the 3-phosphate (74) should reduce one and only one molar equivalent of the oxidant and yield one molar equivalent of both formaldehyde and the phosphorylated dialdehyde (75), whereas the 5-phosphate (76) could be expected to reduce one molar equivalent of periodate relatively rapidly, followed by a slower overoxidation reaction owing to the oxidation of malonaldehyde, formed as a result of the glycol cleavage. 

Despite the above-mentioned short-comings, this approach to the estimation of those deoxy sugars which yield malonaldehyde when oxidized with periodate, seemed promising, since, as has been seen (58,59), the dye is formed quantitatively in the reaction of malonaldehyde with 2-thiobarbituric acid also, more recently, its constitution (49,57) and molar extinction coefficient (36) have been established. Thus, if conditions could be found in which malonaldehyde, while being formed quantitatively from the deoxy sugars, would be stable, an ideal method, independent of standard compounds, would be available for the quantitative determination of all of these sugars. 

Two different pathways have been proposed to explain the over-oxidation reaction of malonaldehyde. Huebner and his collaborators (32) based their conclusion on the observed behavior of digitoxose and suggested that malonaldehyde (7) was oxidized by three molar equivalents of periodate with the concomitant formation of three molar equivalents of formic acid ... 

In neither case were the final products obtained in stoichiometric amounts. The behavior of crystalline malonaldehyde (33) has not been investigated, probably because it is extremely unstable. 

Thus, if triose reductone is, in fact, the first intermediate in the periodate oxidation of malonaldehyde, the total consumption of periodate per mole of malonaldehyde should be four molar equivalents two moles of formic acid and one mole of carbon dioxide should be formed, in accordance with the sequence proposed by Fleury and his collaborators (22). As in the case of the periodate oxidation of malonic acid (32) the rate determining step should be the hydroxylation step. 

However, when we oxidized malonaldehyde (56) in the conditions just described for triose reductone, although formic acid and carbon dioxide were produced in high yields, the periodate consumption was erratic. Similar results were obtained with deoxy sugars. This discrepancy may be caused by the incomplete enolization of the first intermediate, hydroxy malonaldehyde —i.e. tartronic dialdehyde (5,22,32), to triose reductone, or may concern the hydroxylation step itself. 

It is, however, more likely that the discrepancies observed in the periodate oxidation of malonaldehyde concern mainly the hydroxylation step. In the mechanism proposed (5) for this reaction, it is the enol form of malonaldehyde which is hydroxylated. However, titrations of a solution of malonaldehyde, prepared by hydrolysis of an aqueous solution (33) of carefully distilled 1, 3, 3-tri-ethoxypropene (46, 47), both with strong base and with iodine, indicate that only about 80% of the enol form is present in the equilibrium solution. On the other hand, the thio-barbituric acid test (58, 59) gave consistently higher values for the malonaldehyde content of the solution. The fact that only about 80% of the enol form is present in the equilibrium solution is all the more important as it can be shown (56) by titration with strong base that the enolization is slow, and moreover does not seem to go to completion. 

However, in view of the above observations, it seems that malonaldehyde solutions, obtained by the hydrolysis of the appropriate acetals, contain two different forms of malonaldehyde one, which can be titrated with base and with iodine and which reacts with periodate according... 

Attempts were made to estimate the amount of the second form by studying the weak absorption band of malonaldehyde solutions at 350 mju. This band has been attributed to nonconjugated carbonyl absorption—i.e. to the dialdehydo form of malonaldehyde (40, 48). 

When 1, 3, 3-triethoxypropene was hydrolyzed with IN sulfuric acid, a solution of malonaldehyde whose optical density was perfectly stable at 350 m/x for at least one week was obtained. If the solution was made alkaline, the optical density at the same wavelength increased by a small value and then remained virtually constant for at least one week (56). It was also observed that in these solutions the extinction coefficient at 350 m/x was very low (observed 8.3, 61.5 and 69, for solutions of pH 0.4, 7.15 and 9.4 respectively) compared with previously reported values which varied from 200 ( 40) to 1000 ( 48). On the other hand, the absorption of solutions having a pH of 3 to 5, increased considerably with time (at pH 4.75, the extinction coefficient of malonaldehyde at 350 m/x was initially about 40 after four weeks a value of about 930 was recorded and the optical density of the solution was still increasing). This increase in absorption was accompanied by a marked decrease in the malonaldehyde content of the solution, as measured by the thiobarbituric acid method. As a corollary, it was found that aqueous solutions of malonaldehyde, prepared by autocatalyzed hydrolysis (33) of the same acetal and which had a pH of about 3.5, showed, at the completion of the hydrolysis, considerably higher extinction coefficient values at 350 m/x than did those malonaldehyde solutions which were prepared by hydrolysis with IN acid and subsequently adjusted to pH 4. It appears, therefore, that at pH values at which most of the periodate oxidations are carried out, malonaldehyde is unstable and undergoes a chemical reaction, the nature of which is not, as yet, known. 

These observations provide at least one explanation for the fact that variable results are obtained when malonaldehyde is oxidized with periodate. They also explain why widely differing values for the extinction coefficient of malonaldehyde at 350 m/x have been reported and make it unlikely that the absorption band at this wavelength is caused by the dialdehydo form of malonaldehyde. 

In view of these results, periodate titrations of malonaldehyde were carried out at several pH values other than 4. However, in no instances were stoichiometric amounts of periodate reduced the deoxy sugars gave similar results. 

Substrate Moles 10 u re- malonaldehyde formed Approx. Time for... 

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