CHEMISTRY

GIL'M KAMAI, E. V. KUZNETSOV and R. K. VALETDINOV

ON CYANO-SUBSTITUTED DIALKYL PHOSPHITES

(Presented by Academician B. A. Arbuzov, June 6, 1957)

Acid cyano-substituted esters of phosphorous acid have not yet been described in the literature. There is no doubt that the introduction of a cyano group into the molecule of a dialkyl phosphite should sharply change its properties. To this end we studied the reactions of equimolecular amounts of certain α-cyanohydrins with phosphorus trichloride. In doing so we established that this reaction proceeds with the formation of a mixture of products, namely: α-cyanoalkyl chloroanhydrides, di-α-cyanoalkylphosphorous acids, and tri-α-cyanoalkyl phosphites. The formation of these substances may be explained on the basis of the following series of consecutively occurring reactions:

[
\begin{aligned}
&\underset{\mathrm{CN}}{\overset{\mathrm{R'}}{\mathrm{R}-\mathrm{C}-\mathrm{OH}}}
+ \mathrm{PCl}_3
\rightleftarrows
\underset{\mathrm{CN}}{\overset{\mathrm{R'}}{\mathrm{R}-\mathrm{C}-\mathrm{OPCl}_2}}
+ \mathrm{HCl}
\end{aligned}
]

[
2\underset{\mathrm{CN}}{\overset{\mathrm{R'}}{\mathrm{R}-\mathrm{C}-\mathrm{OPCl}_2}}
\rightleftarrows
\left(
\underset{\mathrm{CN}}{\overset{\mathrm{R'}}{\mathrm{R}-\mathrm{C}-\mathrm{O}}}
\right)_3
\mathrm{PCl}
+ \mathrm{PCl}_3
]

[
2
\left(
\underset{\mathrm{CN}}{\overset{\mathrm{R'}}{\mathrm{R}-\mathrm{C}-\mathrm{O}}}
\right)_2
\mathrm{PCl}
\rightleftarrows
\left(
\underset{\mathrm{CN}}{\overset{\mathrm{R'}}{\mathrm{R}-\mathrm{C}-\mathrm{O}}}
\right)_3
\mathrm{P}
+
\underset{\mathrm{CN}}{\overset{\mathrm{R'}}{\mathrm{R}-\mathrm{C}-\mathrm{OPCl}_2}}
]

[
\left(
\underset{\mathrm{CN}}{\overset{\mathrm{R'}}{\mathrm{R}-\mathrm{C}-\mathrm{O}}}
\right)_2
\mathrm{PCl}
+
\underset{\mathrm{CN}}{\overset{\mathrm{R'}}{\mathrm{R}-\mathrm{C}-\mathrm{OPCl}_2}}
\rightleftarrows
\left(
\underset{\mathrm{CN}}{\overset{\mathrm{R'}}{\mathrm{R}-\mathrm{C}-\mathrm{O}}}
\right)_3
\mathrm{P}
+ \mathrm{PCl}_3
]

The most interesting feature in this complex reaction scheme is that the tri-α-cyanoalkyl phosphite formed does not undergo conversion into di-α-cyanoalkylphosphorous acid by the generally known Arbuzov rearrangement, even under such severe conditions as high temperature, a high concentration of reagents, and unbound hydrogen chloride (*). As a result of repeated fractional distillations of the reaction mixture from the products of interaction of α-cyanohydrins and phosphorus trichloride, we succeeded in isolating the following α-cyano-substituted phosphites and their chloroanhydrides (Table 1).

The isolated chloroanhydrides of α-cyanoalkyl- and di-α-cyanoalkylphosphorous acids are colorless liquids that fume in moist air.

We next studied the reaction of hydrolysis of the chloroanhydrides of di-α-cyanoalkylphosphorous acids under various conditions. As a result of the experiments carried out, we established that hydrolysis of the chloroanhydrides exactly

Table 1

with the calculated amount of water in an ether medium and in the presence of pyridine proceeds with formation of acidic cyano-substituted esters of phosphorous acid according to the scheme:

[
\left(
\begin{array}{c}
\mathrm{R'}\[-2mm]
\mathrm{R}-\mathrm{C}-\mathrm{O}\[-1mm]
\vert\[-1mm]
\mathrm{CN}
\end{array}
\right){2}\mathrm{PCl}
+\mathrm{HOH}+\mathrm{C}}\mathrm{H{5}\mathrm{N}
\rightarrow
\left(
\begin{array}{c}
\mathrm{R'}\[-2mm]
\mathrm{R}-\mathrm{C}-\mathrm{O}\[-1mm]
\vert\[-1mm]
\mathrm{CN}
\end{array}
\right)}\mathrm{POH
+\mathrm{C}{5}\mathrm{H}.}\mathrm{N}\cdot\mathrm{HCl
]

In this way the following di-(\alpha)-cyanoalkylphosphorous acids, summarized in Table 2, were obtained. The di-(\alpha)-cyanoalkylphosphorous acids isolated by us are colorless liquids with a faint odor. In contrast to ordinary dialkylphosphorous acids, their dicyano-substituted analogues behave as derivatives of trivalent phosphorus.

Table 2

When an equimolecular amount of di-(\alpha)-cyanoalkyl phosphite is mixed with cuprous chloride, characteristic heating is observed. On further heating to (115^\circ), the cuprous chloride dissolves completely, and a glassy, noncrystallizing mass is formed.

With phenyl azide these acids react with liberation of nitrogen. Thus, for example, when 1.43 g of di-(\alpha)-cyanoisopropyl phosphite and 0.70 g of phenyl azide were allowed to react in ether solution, a weak evolution of nitrogen bubbles was observed. After four days, crystals in the form of long needles appeared. The crystals were then filtered off and dried. M.p. (87^\circ). On the basis of analysis for nitrogen and phosphorus, the substance has the following structure:

[
\left(
\begin{array}{c}
\mathrm{CH}{3}\backslash\[-1mm]
\mathrm{C}-\mathrm{O}\[-1mm]
\mathrm{CH}/\quad\vert\[-1mm]
\mathrm{CN}
\end{array}
\right){2}
\mathrm{P}
\begin{array}{c}
=\mathrm{NC}}\mathrm{H{5}\[-1mm]
\vert\[-1mm]
\mathrm{OH}
\end{array}
\rightarrow
\left(
\begin{array}{c}
\mathrm{CH}\backslash\[-1mm]
\mathrm{C}-\mathrm{O}\[-1mm]
\mathrm{CH}{3}/\quad\vert\[-1mm]
\mathrm{CN}
\end{array}
\right)
\mathrm{P}
\begin{array}{c}
\mathrm{NHC}{6}\mathrm{H}\[-1mm]
\Vert\[-1mm]
\mathrm{O}
\end{array}.
]

It should be noted here that di-(\alpha)-cyanoalkyl phosphites containing secondary radicals react with phenyl azide much more vigorously, with liberation of nitrogen.

Studying the structure of phosphorous acid and its esters, A. E. Arbuzov in 1950 came to the conclusion that all its neutral esters are built according to trivalent-

to phosphorus, while phosphorous acid itself and its acid esters contain pentavalent phosphorus (^{(2)}). A. E. Arbuzov even then expressed the idea of the possible existence of phosphorous acid and its acid esters in the form of tautomeric forms.

[
\begin{array}{ccccc}
& \mathrm{V} & & \mathrm{III} & \
\mathrm{RO}\backslash & & \mathrm{O} & & \mathrm{RO}\backslash \
& \mathrm{P} & \parallel & \rightleftarrows & & \mathrm{P}—\mathrm{OH}. \
\mathrm{RO}/ & & \mathrm{H} & & \mathrm{RO}/ \
& \mathrm{I} & & \mathrm{II} &
\end{array}
]

Structure I, in his opinion, is possessed by the free form of phosphorous acid. In solutions it can exist in the tautomeric form II (^{(3)}). Physicochemical investigations (^{(4–6)}) of recent years have brilliantly confirmed the conclusions, made more than half a century ago by A. E. Arbuzov, concerning the pentavalency of phosphorus in acid esters of phosphorous acid and the probability of their tautomerism.

The chemical properties of di-(\alpha)-cyanoalkyl phosphites indicate that, apparently, the tautomeric equilibrium is shifted toward the form of trivalent phosphorus. Thus, the position of the tautomeric equilibrium of the acid esters of phosphorous acid, as M. Kabachnik (^{(7)}) previously indicated, also depends on the nature of the radicals. The presence of di-(\alpha)-cyanoalkyl radicals in the acid esters of phosphorous acid studied by us promotes a shift of the tautomeric equilibrium toward the form of trivalent phosphorus.

Furthermore, we have established that di-(\alpha)-cyanoalkylphosphorous acids with secondary cyano-containing radicals also exhibit the properties of mixed esters of phosphorous acid. Upon heating they readily undergo intermolecular rearrangement, exchanging the hydroxyl group for the corresponding radical:

[
2\bigl(\mathrm{R—CHO}\bigr)_2\mathrm{POH}
\;\longrightarrow\;
\bigl(\mathrm{R—CHO}\bigr)_3\mathrm{P}
+
\mathrm{R—CHOPO_2H_2}.
]
[
\begin{array}{ccc}
\big| & \big| & \big| \
\mathrm{CN} & \mathrm{CN} & \mathrm{CN}
\end{array}
]

However, di-(\alpha)-cyanoisopropylphosphorous acid, containing a tertiary radical, practically does not exhibit this property.

Kazan Chemical-Technological Institute
named after S. M. Kirov

Received
3 VI 1957

CITED LITERATURE

1. E. V. Kuznetsov; R. K. Valetdinov, Trudy Kazansk. khim.-tekhnol. inst. im. S. M. Kirova, 21, 167 (1956).
2. A. E. Arbuzov, On the Structure of Phosphorous Acid and Its Derivatives, Selected Works, Publishing House of the Academy of Sciences of the USSR, 1952, p. 41.
3. A. E. Arbuzov, ibid., pp. 462–465.
4. A. E. Arbuzov, M. I. Batuev, V. S. Vinogradova, DAN, 54, 603 (1946).
5. A. E. Arbuzov, P. I. Rakov, Izv. AN SSSR, OKhN, 1950, 237.
6. A. E. Arbuzov, V. S. Vinogradova, Izv. AN SSSR, OKhN, 1947, 617.
7. M. I. Kabachnik, in Chemistry and Application of Organophosphorus Compounds (Proceedings of the First Conference), Izv. AN SSSR, 1957, p. 37.