SPATIAL STRUCTURE OF SOME DERIVATIVES OF 2′-FORMYLBIPHENYL-2-CARBOXYLIC ACID

A. G. PECHENKIN, I. A. KHALFINA, V. E. CHISTYAKOV

(Presented by the scientific seminar of the Chemical-Technological Faculty)

It is known that reactions proceeding as a result of nucleophilic attack directed at carbonyl or carboxyl functional groups proceed readily in cases where there are no shielding substituents that hinder access of the reagent to the carbon atom.

The dependence of the ability to form esters on the structure of molecules has been studied in the aromatic series in sufficient detail. Thus, Meyer established, on a large body of experimental material, that one substituent in the o-position to the carboxyl group already decreases the reaction rate.

In this respect, biphenyl derivatives have not been studied at all.

In order to search for fungicidal agents, we synthesized a series of complex alkyl esters and their nitrogen-containing derivatives of 2′-formylbiphenyl-2-carboxylic acid of the general formula:

\[ \begin{aligned} &\text{biphenyl derivative with substituents } R_2=\mathrm{C(H)} \text{ and } \mathrm{COOR_1},\\ &R_1=\mathrm{CH_3},\ \mathrm{C_2H_5},\ n\text{-}\mathrm{C_3H_7},\ i\text{-}\mathrm{C_3H_7},\ n\text{-}\mathrm{C_4H_9},\ i\text{-}\mathrm{C_4H_9},\ i\text{-}\mathrm{C_5H_{11}},\\ &R_2=\mathrm{N{-}OH},\ =\mathrm{N{-}NHC_6H_5},\ =\mathrm{N{-}NH{-}CO{-}NH_2},\ \mathrm{N{-}NH{-}CS{-}NH_2}. \end{aligned} \]

It seemed of interest to determine whether steric hindrances occur in this series of synthesized biphenyl derivatives.

For this purpose we compared the ability of 2-biphenylcarboxylic acid and 2′-formylbiphenyl-2-carboxylic acid to form esters.

For a comparative evaluation of product yields, the experiments were carried out under identical conditions. Their results are given in Table 1.

Table 1

As is evident from the table presented, high yields in all cases were obtained for 2-biphenylcarboxylic acid. On the basis of the structure of these acids, the decrease in the yields of the esters of 2′-formylbiphenyl-2-carboxylic acid in comparison with 2-biphenylcarboxylic acid can be explained by the shielding effect of the aldehyde group on the carboxyl of the aldehyde acid.

Steric hindrances affect not only the chemical properties of compounds, but also certain physical properties.

It is known that compounds possessing a conjugated system are more polarizable and therefore have higher molecular refractions than compounds that do not possess a conjugated system. In the case where conjugation is diminished because of spatial effects, the molecular exaltation also decreases [2]. Thus, disruption of conjugation of a functional group with the biphenyl nucleus can be detected from a decrease in molecular exaltations.

Table 2 shows the decrease in the molecular exaltations of the biphenyl derivatives obtained, taking their structure into account.

Table 2

As is known, biphenyl belongs to compounds for which a deviation from additive refraction is characteristic, since in its molecule there is a considerable mutual influence of bonds—the conjugation effect. According to the data of R. Kremann [3], the magnitude of the exaltation for biphenyl reaches \(+0,51\ \text{cm}^3\). Upon introduction of a polar carboxyl group into the biphenyl molecule, one might have expected an increase in exaltation. In our experiments, however, \(\Delta R\) for 2-biphenylcarboxylic acid is equal to \(+0,792\ \text{cm}^3\). Upon introduction of a polar aldehyde group into the molecule of this acid, molecular exaltation is no longer observed, but rather molecular depression. Moreover, with an increase in the size of the functional groups located in positions 2,2′ relative to one another, an increase in molecular depression occurs. In accordance with the existing view, this phenomenon is explained by the steric hindrances present in the derivatives of 2′-formylbiphenyl-2-carboxylic acid synthesized by us, which are a consequence of the nonplanar structure of these molecules.

One of the most important characteristics of chemical compounds is the infrared spectrum. We studied the IR absorption spectra of the synthesized esters and their nitrogen-containing derivatives of 2′-formylbiphenyl-2-carboxylic acid.

As a result of the interpretation carried out for the spectra obtained, absorption bands were found in the region \(1720—1700\ \text{cm}^{-1}\), which should

...attributed to vibrations of the carbonyl group of esters formed by an aromatic carboxylic acid and aliphatic alcohols.

It is known that, in the case of $\pi$-electron interaction of a carbonyl group with substituents, a shift occurs in the frequency of its stretching vibration. When this interaction is disturbed, the frequency of the stretching vibration of the carbonyl group is not shifted, or is shifted only slightly. As the study of the IR spectra of derivatives of 2′-formyldiphenyl-2-carboxylic acid has shown, upon introduction of an electron-acceptor aldehyde group, as well as nitrogen-containing residues, into the 2′ position of the methyl ester molecule of 2′-formyldiphenyl-2-carboxylic acid, the frequency of the carbonyl vibration is almost equal to the carbonyl vibration frequency of the unsubstituted compound (Table 3).

Table 3

The introduction of nitrogen-containing residues into the molecules of complex alkyl esters of 2′-formyldiphenyl-2-carboxylic acid likewise does not lead to a noticeable decrease in the frequency of carbonyl absorption in the IR spectra, since the transmission of mutual influence is hindered owing to the non-coplanarity of the functional groups with the diphenyl nucleus, and also of the phenyl nuclei themselves.

Consequently, the nonplanar structure of the synthesized derivatives of 2′-formyldiphenyl-2-carboxylic acid, the consequence of which is a decrease in conjugation, is confirmed by the results both of IR spectroscopy and of refractometry.

REFERENCES

1. V. Meyer, Ber., 41, 4505, 1908.
2. Spatial effects in organic chemistry, 1960.
3. R. Kremann, M. Pestemer. Dependence between physical properties and chemical structure, 1939.