Abstract Generated abstract
The study examines isomerization of the cyclohexyl cation generated during nitrous acid deamination of tritium labeled cyclohexylamine, with emphasis on distinguishing possible hydride shifts. Cyclohexylamine labeled in the alpha positions was synthesized from cyclohexanone via tritiation, oxime formation, and lithium aluminum hydride reduction, and the resulting deamination products, cyclohexanol and cyclohexene, were analyzed through oxidative degradation and conversion to dibenzylidene derivatives for radioactivity measurements. The data indicate an overall cyclohexyl cation isomerization of 4.8 ± 0.3 percent, attributed mainly to a 1,2-hydride shift of about 3.5 percent, with a smaller 1,3-hydride shift of about 1.3 percent. The limited extent of rearrangement is interpreted as reflecting the short lifetime of the carbocation intermediate.
Full Text
66.062.22
CHEMISTRY
Yu. G. Bundel’, V. A. Savin, M. N. Ryabtsev,
Academician O. A. Reutov
ISOMERIZATION OF THE CYCLOHEXYL-2,6-\(\mathrm{H}_2^3\) CATION UNDER THE CONDITIONS OF THE DEAMINATION REACTION
Earlier we noted that the cyclohexyl cation formed under the conditions of the deamination reaction of cyclohexyl-2,6-\(\mathrm{H}_2^2\)-amine under the action of nitrous acid undergoes isomerization caused by migration of the hydride ion \((^1)\).
In the present work, using as an example the deamination reaction of cyclohexyl-2,6-\(\mathrm{H}_2^2\)-amine under the action of nitrous acid, the type of isomerization has been determined. For this purpose cyclohexylamine labeled with tritium in the \(\alpha\)-position was synthesized:
\[ \mathrm{cyclohexanone} \ \xrightarrow{\mathrm{T_2O}}\ \mathrm{(T)\!-\!cyclohexanone\!-\!(T)} \ \xrightarrow{\mathrm{NH_2OH}}\ \mathrm{(T)\!-\!cyclohexanone\ oxime\!-\!(T)} \ \xrightarrow{\mathrm{LiAlH_4}}\ \mathrm{(T)\!-\!cyclohexylamine\!-\!(T)} \]
The products of the deamination reaction of cyclohexyl-2,6-\(\mathrm{H}_2^3\)-amine are cyclohexene and cyclohexanol.
\[ \mathrm{(T)\!-\!cyclohexylamine\ perchlorate\!-\!(T)} \ \xrightarrow{\mathrm{HNO_2}}\ \mathrm{T\!-\!cyclohexanol} \ +\ \mathrm{T\!-\!cyclohexene} \]
Carrying out the degradation of cyclohexanol-\(\mathrm{H}^3\) according to the following scheme:
\[ \mathrm{T\!-\!cyclohexanol} \ \xrightarrow[\mathrm{H^+}]{\mathrm{CrO_3}}\ \mathrm{T\!-\!cyclohexanone} \ \xrightarrow[\mathrm{OH^-}]{\mathrm{KMnO_4}}\ \mathrm{T\!-\!dicarboxylic\ acid} \ \xrightarrow[\mathrm{BaO}]{t^\circ}\ \mathrm{T\!-\!cyclopentanone} \]
\[ \mathrm{T\!-\!cyclohexanone} \longrightarrow \mathrm{T\!-\!2,6\text{-}bis(benzylidene)cyclohexanone} \]
\[ \mathrm{T\!-\!cyclopentanone} \longrightarrow \mathrm{T\!-\!2,5\text{-}bis(benzylidene)cyclopentanone} \]
we established that the degree of isomerization of the cyclohexyl cation is \(4.8 \pm 0.3\%\), and that the isomerization is mainly caused by a 1,2-hydride shift, amounting to \(3.5\%\). The 1,3-hydride shift in this case amounted to \(1.3\%\). The degree of isomerization caused by the 1,3-hydride shift \((d)\) and by the 1,2-hydride shift \((e)\) is found, respectively, from the following formulas:
\[ d = 2\frac{(2b-a)}{c}\cdot 100\%; \qquad e = 2\left(\frac{a}{c}\cdot 100-\frac{d}{2}\right)\%. \]
where \(a\) is the activity of dibenzalcyclohexanone in imp/min·mm hydrogen, \(b\) is the activity of dibenzalcyclopentanone in imp/min·mm hydrogen, and \(c\) is the activity of the isomerized alcohol in imp/min·mm hydrogen. The transition state for each of the types of hydride shifts may be represented as follows:
1,2-hydride
shift
1,3-hydride
shift
Apparently, the small percentage of isomerization of the cyclohexyl cation is explained by its short lifetime. Indeed, from the literature it is known that the deamination of cis-2-H\(_1^2\)-cyclohexylamine proceeds with retention of configuration, i.e., the lifetime of the carbocation is less than the time required to establish conformational equilibrium \((^2)\).
Table 1
| Activity of cyclohexanol, imp/min·mm H\(_2\) | Activity of cyclohexanol, imp/min·mm H\(_2\) | Activity of DB\(_5\), imp/min·mm H\(_2\) | Activity of DB\(_{c5}\), imp/min·mm H\(_2\) | Activity of DB\(_{c5}\), imp/min·mm H\(_2\) |
|---|---|---|---|---|
| measured | reduced | measured | reduced | |
| 45 316 | 30 208 | 700 | 512 | 455 |
| 41 379 | 27 582 | 684 | 476 | 422 |
Notes. 1. DB\(_5\) and DB\(_{c5}\) are the activities of dibenzalcyclopentanone and dibenzalcyclohexanone, respectively.
2. The reduced activity is the activity recalculated for 18 hydrogen atoms.
The results of measuring the activity of the isomerization products are presented in Table 1.
Moscow State University
named after M. V. Lomonosov
Received
18 V 1965
CITED LITERATURE
\(^1\) Yu. G. Bundel’, V. A. Savid, O. A. Reutov, DAN, 165, No. 5 (1965).
\(^2\) A. Streitwieser, C. E. Coverdel, J. Am. Chem. Soc., 81, 4278 (1959).