Chemistry

O. Ya. NEILAND and Academician of the Academy of Sciences of the Latvian SSR G. Ya. VANAG

IODONIUM DERIVATIVES OF 5-PHENYLCYCLOHEXANEDIONE-1,3 AND THEIR TRANSFORMATIONS

In the study of the reactions of β-diketones with phenyliodoso compounds it was found that 5-phenylcyclohexanedione-1,3 reacts analogously to dimedone (¹), forming a new compound—enolatebetainephenyl-(5-phenylcyclohexanedione-1,3-yl-2)-iodonium (I). In what follows, for convenience, the compound is called phenyliodone. This communication is devoted to the study of the properties and structural features of phenyliodone, its alkylation and cleavage products.

Phenyliodone is a white substance, soluble in chloroform and benzene with an intense yellow coloration. The substance is not characterized by great stability and, under the action of various reagents, undergoes cleavage. Phenyliodone is especially sensitive to the action of sunlight. Cleavage may take place, as is typical for unsymmetrical iodonium compounds, with rupture of the iodine—carbon bond in two directions. Thus, in the presence of hydrochloric acid, phenyliodone is smoothly cleaved to iodobenzene and 2-chloro-5-phenylcyclohexanedione-1,3 (II), while in pyridine solution under the influence of temperature the cleavage products are both iodobenzene and the phenyl ester of 2-iodo-5-phenylcyclohexanedione-1,3 (III). In the presence of silver nitrate and pyridine, cleavage occurs only at the iodine—phenyl bond, and the silver salt of 2-iodo-5-phenylcyclohexanedione-1,3 (IV) is formed.

\[ \begin{array}{cccc} & & \mathrm{C_6H_5} & \\ & & | & \\ \text{(II)}\quad \begin{array}{c} \mathrm{C_6H_5}\\ |\\ \text{cyclohexenone ring with } \mathrm{Cl},\ \mathrm{OH} \end{array} & \xleftarrow{\mathrm{HCl}} & \begin{array}{c} \mathrm{C_6H_5}\\ |\\ \text{phenyliodone (I)} \end{array} & \xrightarrow[\text{pyridine}]{t^\circ} \begin{array}{c} \mathrm{C_6H_5}\\ |\\ \text{ring with } \mathrm{OC_6H_5},\ \mathrm{I}\\ \text{(III)} \end{array} \\[2em] & \xleftarrow{(\mathrm{C_2H_5})_3\mathrm{O}^{\oplus}\mathrm{BF}_4^{\ominus}} & \begin{array}{c} \text{ring with } \mathrm{OC_2H_5},\ \mathrm{C_6H_5I}^{\oplus}\ \mathrm{BF}_4^{\ominus}\\ \text{(V)} \end{array} & \xrightarrow[\mathrm{AgNO_3}]{\text{pyridine}} \begin{array}{c} \text{silver salt}\\ \text{(IV)} \end{array} \\[2em] & \begin{array}{c} \xrightarrow{\text{pyridine}}\\ \text{pyridinium derivative (VI)} \end{array} & \longrightarrow & \begin{array}{c} \text{pyridinium betaine (VII)} \end{array} \end{array} \]

With strong acylating and alkylating agents (benzoyl chloride, triethyloxonium fluoroborate) phenyliodone is not cleaved, but forms

acyl- or alkyl derivatives. On interaction with triethyloxonium fluoroborate, phenyliodone forms phenyl-(O-ethyl-5-phenylcyclohexanedione-1,3-yl-2)-iodonium fluoroborate (V), a white, poorly stable substance. In the presence of pyridine this iodonium salt is readily cleaved into iodobenzene and N-(O-ethyl-5-phenylcyclohexanedione-1,3-yl-2)-pyridinium fluoroborate (VI). The latter compound is readily saponified and forms the corresponding pyridinium enolate betaine (VII).

Thus, an interesting directionality is observed in the cleavage of the iodine—carbon bond in the iodonium derivatives studied, depending on structural features and on the reagent, as has already been noted for analogous dimedone derivatives ($^2$). Since it is assumed that, in iodonium compounds, the bond with the lower electron density is cleaved ($^3$), then in phenyliodone in nonacidic media, naturally, the iodine—phenyl bond is characterized by a lower electron density than the iodine—β-diketone-residue bond. This is clearly seen from the cleavage reaction of phenyliodone in the presence of pyridine and silver nitrate. Under these conditions phenyliodone undoubtedly exists in the enolate-anion form, from which it follows that at the carbon atom of the active methylene group in the enolate-anion form the electron density is increased in comparison with the nonionized form. For example, in the presence of hydrochloric acid phenyliodone exists in the nonionized form, and cleavage occurs at the iodine—β-diketone-residue bond. The same applies to the cleavage of alkyl products of phenyliodone. Consequently, the enolate-anion system is characterized by great mobility of electronic charge.

Curious conclusions are obtained from the absorption spectra of phenyliodone (I) and pyridinium enolate betaine (VII). The normal enolate-anion system of 5-phenylcyclohexanedione-1,3 (for example, in the sodium salt) is characterized in the infrared region by very intense absorption in the region 1510—1530 cm$^{-1}$. In the UV spectra there is observed a maximum characteristic of enolate anions of cyclohexanediones-1,3 at about 282 mµ. The iodine atom causes a considerable decrease in the frequency of the enolate-anion system, since the silver salt of 2-iodo-5-phenylcyclohexanedione-1,3 (IV) absorbs strongly in the region 1470—1490 cm$^{-1}$.

But phenyliodone (I) in chloroform solution absorbs intensely at 1564 cm$^{-1}$, and pyridinium enolate betaine (VII) at 1546 cm$^{-1}$. Absorption maxima in the UV region in methanol solutions are found at 261 mµ for phenyliodone and at 252 mµ for pyridinium enolate betaine (VII). Such a phenomenon is also observed for analogous dimedone derivatives ($^4$).

Such large shifts of the characteristic maxima of the enolate-anion system of enolate betaines, in comparison with the normal system, can be explained only by a change in the electronic structure and in the multiplicity of the bonds. It is natural to suppose that, under the influence of a positively charged substituent, the electron density at the carbon atom of the active methylene group increases and thereby the character of the carbon—carbon and carbon—oxygen bonds in the system changes

\[ \left(\mathrm{O}^{\cdots}\!-\!\mathrm{C}^{\cdots}\!-\!\mathrm{C}^{\cdots}\!-\!\mathrm{C}^{\cdots}\!-\!\mathrm{O}\right)^{-}\!\bullet \]

For representing the correct structure of enolate anions, there is still no satisfactory method. The leveling of the charge on both oxygen atoms is well represented, as is usually accepted, by denoting noninteger bonds with dotted lines (for example, IV) ($^5$). It is only necessary, in symmetrical systems, to keep in mind the equivalence of both carbonyl groups. But the point is that, under the influence of various structural factors, as is shown in our examples, the electron density at the oxygen and carbon atoms can change, and the fractions of charges localized on individual atoms are still difficult to estimate. Therefore—

…there arises a natural difficulty in correctly representing the structure of enol betaines. The method (VIII) used up to now is insufficiently accurate.

\[ \begin{array}{cc} \mathrm{O{=}C{-}C(A){=}C{\!\overset{\delta\ominus}{-}O^{\oplus}}} & \mathrm{(VIII)} \qquad\qquad \mathrm{\delta\ominus O{\cdots}C{\cdots}C(A){\cdots}C{\cdots}O\delta\ominus} & \mathrm{(IX)} \end{array} \]

The most correct method would be (IX), which will be used in our subsequent papers, taking into account that the fractions of the charges \(\delta\) and \(\delta'\) are still difficult to estimate, and \(2\delta+\delta'=1\).

Riga Polytechnic Institute
Received
15 X 1959

CITED LITERATURE

1. E. Gudrinietse, O. Neiland, G. Vanag, ZhOKh, 27, 2737 (1957).
2. O. Ya. Neiland, G. Ya. Vanag, DAN, 130, No. 1 (1960).
3. O. A. Reutov, O. A. Ptitsyna, N. B. Statskina, DAN, 122, 1032 (1958).
4. O. Ya. Neiland, G. Ya. Vanag, DAN, 129, No. 2 (1959).
5. T. I. Temnikova, A Course in the Theoretical Foundations of Organic Chemistry, Leningrad, 1959, p. 487.