P-dimethylaminobenzoate

The need to consider the effect of zwitter-ion formation applied also to Me2N. For aqueous solutions there appear to be no data relevant to obtaining a precise value of am, but van de Graaf and colleagues have examined p-dimethylaminobenzoic acid236. They obtained values of pK and pK2 (water, 25 °C) of 2.568 and 4.996, respectively, and calculated a value of pK of 4.90 for the ionization of the neutral acid. (The ratio of zwitter-ion to neutral acid is 19.4 80.6.) From this pK value and 4.21 for the pKtl value of benzoic acid, ap for Me2N is —0.69. 

In most of these studies, hydroxyl has been the typical resident functional group, which is derivatized with appropriate acids containing chromophores suitable for exciton coupling. The ideal chromophore would have a very intense UV-visible transition, located in a convenient spectral window, and with the orientation of its electric transition moment being well-defined relative to alcohol R—OH bond. One of the most successful has been p-dimethylaminobenzoate, which has an intense (s ca 30, 000) transition in an... 

TABLE 9. Conformational structures (a) and Newman projection diagrams (b) of (IS,25)- and (IR,2ft)-cyclohexane diol. Bis-p-dimethylaminobenzoate derivatives (c), bisignate CD Cotton effect data and torsion angles (d)14... 

Fig. 2.4. Absorption A, fluorescence F, and degree of polarization of fluorescence P of DMABN and DMABEE (p-dimethylaminobenzoic acid ethylester) in n-hexane at room temperature, c = 5 x 10 M, and ethyl alcohol at 140 K, c = 1 x 10 4Af, respectively. Arrows indicate excitation wavenumber for measurements of P.6 Subscripts A and B refer to final and initial emission, respectively.

By implication, the H-bonding within the dimer seems to be of some importance. The accuracy of this predic-tion was tested by mixing 3,5-dinitrobenzoic acid with p-dimethylaminobenzoic acid to see if a 1 1 crystalline material formed. In this case, H-bonding between the nitro and amino groups is precluded by the methylation of the amino groups. Apparently, a stoichiometric mixed solid does form (as evidenced by an unmistakable change in color to red), although the structure has not yet been determined. 

Applications of the Exciton Chirality Rule have become numerous during the past ten years and claim an extraordinarily high degree of success. In most applications of exciton chirality to the determination of absolute configuration, the molecule under study is deriva-tized with a suitable chromophore, e.g.,p-dimethylaminobenzoate, and its circular dichroism spectra are measured and analyzed. The success of this method has led to a search for new and better chromophores, especially those which absorb intensely in the visible spectrum. [39,40] These derivatives can be detected visually, for convenience in laboratory manipulations, and their molecular weights tend to be substantially higher than the underiv-... 

Figure 30. Reorientation of the p-dimethylaminobenzoate long wavdength electric transition dipole ( --- ) following rotations about the (a) C—C=0, (c) O—C=0 and (b) R—O...

Table XI. Conformational structures (a and b) and Newman projection diagrams (c and d) of (15,25) and (lR,2R)-cyclohexane diol (a) and their bis-p-dimethylaminobenzoate derivatives (b). CD data for the bisignate Cotton effects of the latter are shown below.

Figure 31. CD (upper) and UV (lower) spectra of (15,25)-cyclohexanediol bis-p-dimethylaminobenzoate (1) and mono-p-dimethylaminobenzoate (2) in CH3OH solvent at 22°C.

Orientation, proximity and chromophore are paramount considerations in the Exciton Chirality Rule. Extrachromophoric considerations are relatively unimportant. Thus the CD spectra of bis-p-dimethylaminobenzoates of 5a-cholestan-2a,3/3-diol and (2R,2R)-cyclohex-anediol (both diequatorial diols of the same absolute configuration) are essentially irip.ntir.al (Figure 32). Other steroid diols, whether with vicinal hydroxyls or very distant hydroxyls, give bisignate CE s originating from exciton coupling and with a signed order consistent with the Exciton Chirality Rule (Table XII). 

Table XII. (Also continued on next page) Exciton chirality of steroid diol bis-p-dimethylaminobenzoates"... 

Figure 34. (a) Dihydroxylation of benzo[a]pyrene. (b) Coupling constant and exciton chirality CD CEs of the bis-p-dimethylaminobenzoate. (c) (-) Exciton chirality and relative orientation of the transition moment vectors, hence the absolute configuration of the diol. 

Figure 37. (a) Formation of the ds,trans-diol of (-)-spiro[4.4]nonane-l,6-dione. (b) Observed CD of the bis-p-dimethylaminobenzoate of the cis,trans-diol. (c) Exciton chirality. 

In the aromatic series such a reaction is more often observed. For example, p- dimethylaminobenzoic acid can be nitrated to 2,4-dinitrodimethylaniline (Re-verdin [201]) ... 

Thermolysis of peroxide [29c] in benzene solution generates a chemiluminescent emission whose spectrum is identical to the fluorescence spectrum of photoexcited p-dimethylaminobenzoic acid under similar conditions. Thus the direct chemiluminescence is attributed to the formation of the singlet excited acid. The yield of directly generated excited acid is reported to be 0.24% (Dixon and Schuster, 1981). Since none of the other peroxybenzoates generate detectable direct chemiluminescence it was not possible to compare this yield to the other peroxides. However, by extrapolation it was concluded that the dimethylamino-substituted peroxide generates excited singlet products at least one thousand times more efficiently than does the peroxyacetate or any of the other peroxybenzoates examined. 

For the dimethylamino-substituted peroxyester [29c] a third type of behavior is observed. The corrected chemiluminescence intensity obtained is independent of the structure of the activator. This is just what is expected for simple indirect chemiluminescence where the activator is excited by energy transfer from some first-formed singlet state. As indicated above, the initial excited state in this system is p-dimethylaminobenzoic acid. Evidently, the electron donating p-dimethylamino-substituent renders the peroxybenzoate [29c] sufficiently difficult to reduce that the value of k2 is so small that the bimolecular path is never able to compete successfully with unimolecular decomposition. 

Other examples of ancillary ligands used to enhance analyte selectivity include amide-modified D03A (1,4,7,10-tetraazacyclododecane-l,4,7-trisacetate) complexed to Tb , which selectively binds the bidentate analytes p-dimethylaminobenzoic acid (DMABA) and SA (160-162). The binary complex of with EDTA can effectively detect SA, 4-aminosalicylic acid and 5-fluorosalicylic acid (163). [Tb(EDTA)] also has been used to detect catalysis of hydroxybenzoic acid (HBA) by hemin via formation of a ternary complex with the HBA oxidation product (164). Diaza-crown ethers have been utilized with and Eu to detect phthalate, benzoate,... 

A full report of the c.d. of steroidal diol bis-(p-dimethylaminobenzoates) confirms the reliability of theoretical calculations of the coupled Cotton effects of the remote ester groups (exciton chirality method). The c.d. curves show two maxima of opposite signs, separated by some 27 nm, and with intensity inversely proportional to the square of the interchromophore distance. The profile of the observed c.d. curve results from the superimposition of two component curves, each of asymmetric shape.A review of the uses of chiroptical techniques for structural and conformational studies includes examples of the assignment of stereochemistry to steroids and terpenoids, among a wide variety of natural products. The Octant Rule for carbonyl compounds and the rules applicable to other chromophoric systems are discussed critically. 

The CD exciton chirality method has been successfully applied to a variety of natural products to determine their ACs. This method enables one to deduce the AC of a chiral compound without any reference compound, and therefore, it is established as a nonempirical method. The principles of the CD exciton chirality method are explained using the steroidal bis(p-dimethylaminobenzoate) shown below as a model compound, where the nonempirical nature of this method is easily proved.8 11 1... 

Figure 8 Application of the CD exciton chirality method to cholest-5-ene-3/3,4/3-diol bis(p-dimethylaminobenzoate) 1 CD and UV spectra in EtOH. Redrawn from N. Harada K. Nakanishi, Circular Dichroic Spectroscopy - Exciton Coupling in Organic Stereochemistry, University Science Books Mill Valley, CA, and Oxford University Press Oxford, 1983.

Ethyl p-dimethylaminobenzoate Benzoic acid, 4-dimethylamino-, ethyl ester (10287-53-3), 76, 276... 

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