Research in the Chemistry of Carbazole Derivatives

XIV. Reduction of 3,6-Diacetyl-9-methyl- and 3,6-Diacetyl-9-ethylcarbazoles with Aluminum Isopropoxide

V. P. Lopatinskii, E. E. Sirotkina

(Presented by the scientific seminar of the organic chemistry section)

In the course of studying the properties of certain 3,6-diacetyl-9-alkylcarbazoles obtained by us earlier [1], we attempted to reduce their carbonyl groups to alcohol groups, with the intention of subsequently obtaining the corresponding 3,6-divinyl-9-alkylcarbazoles by dehydration of the carbinols. For this purpose we chose the Meerwein–Ponndorf method [2], since it usually gives good yields of reduction products, is simple to carry out, and gives almost no by-products. We knew of one attempt to reduce, by the Meerwein–Ponndorf method, 3,6-diacetyl-9-ethyl-, 3,6-diacetyl-9-allyl-, and 3,6-diacetyl-9-benzylcarbazoles [3], which, in the authors’ opinion, led to the corresponding carbinols. However, verification of this assertion using 3,6-diacetyl-9-ethylcarbazole as an example, as well as experiments on the reduction of 3,6-diacetyl-9-methylcarbazole, showed it to be erroneous. Reduction of 3,6-diacetyl-9-methyl- and 3,6-diacetyl-9-ethylcarbazoles with aluminum isopropoxide in isopropyl alcohol led us not to the carbinols, but to their diisopropyl ethers, as in the case of certain 3-acetyl-9-alkylcarbazoles studied by us earlier [4]. The structure of the products as diisopropyl ethers of 3,6-di(α-hydroxyethyl)-9-methyl- and 3,6-di(α-hydroxyethyl)-9-ethylcarbazoles was confirmed by the results of elemental analyses and determinations of molecular weight, as well as by the results of analyses of the products of their cleavage. Like the isopropyl ethers of 3-α-hydroxyethyl-9-alkylcarbazoles [4], the diisopropyl ethers do not change upon prolonged heating with caustic alkalis, while on heating with KHSO₄ or Al₂O₃ in xylene they are cleaved with liberation of isopropyl alcohol and a polymer of the corresponding 3,6-divinyl-9-alkylcarbazole. The results obtained, as well as the experiments with 3-acetyl-9-alkylcarbazoles [4], indicate the anomalous behavior of acetyl derivatives of 9-alkylcarbazoles under the usual conditions of reduction by the Meerwein–Ponndorf method [2]. It should be noted that we undertook attempts to reduce 3,6-diacetyl-9-alkylcarbazoles with aluminum isopropoxide in aromatic solu—

vents. In the case of 3,6-diacetyl-9-ethylcarbazole we succeeded in obtaining 3,6-divinyl-9-ethylcarbazole in low yield in the form of an oily liquid with a boiling point of 195–198°C (2–3 mm Hg), which could be polymerized by known methods. The greater part of the reaction product was a polymer of 3,6-divinyl-9-ethylcarbazole.

The results of this part of the study will be set forth in greater detail in our next communication.

Experimental Part

The starting 3,6-diacetyl-9-methyl- and 3,6-diacetyl-9-ethylcarbazoles were obtained by the procedure developed by us earlier [1], and had melting points of 197–198°C and 182–182.5°C, respectively. Isopropyl alcohol of “purified” grade was freed from acetone. Aluminum isopropylate was prepared by a known procedure [2].

Diisopropyl Ether of 3,6-di(α-hydroxyethyl)-9-methylcarbazole

Into a 300 ml round-bottom flask were placed 26.5 g (0.1 mole) of 3,6-diacetyl-9-methylcarbazole, 61.2 g (0.3 mole) of aluminum isopropylate, and 400 ml of absolute isopropyl alcohol. The flask was connected to a dephlegmator and a straight condenser and heated on a boiling water bath. The distillation of acetone begins at once and continues for 3 hours. When the test for acetone in the distillate is negative (reaction with 2,4-dinitrophenylhydrazine [2]), the reduction is completed; the contents of the flask are cooled to 5°C and treated at this temperature with a 15% aqueous solution of caustic soda (200 ml). The separated oily layer is separated and distilled. After distillation of the isopropyl alcohol, the residue is distilled in vacuum. At 210–215°C (2–3 mm Hg), 26.5 g of a colorless oil is distilled off (yield 75.0% of theory), which crystallizes on standing. After recrystallization from methanol, a colorless crystalline substance was obtained with a melting point of 97.5–98°C, readily soluble in hot aromatic hydrocarbons, methanol, and ethanol.

Analyses: found, %: C—78.5; 78.28. H—8.73; 8.78. N—3.9; 3.94. Molecular weight—354.8. C₂₃H₃₁NO₂. Calculated, %: C—78.18; H—8.78; N—3.96. Molecular weight—353.5.

On heating the substance with KHSO₄ or with Al₂O₃ in xylene to 140°C, a liquid with a boiling point of 79–80.5°C and \(n = 1.4771\), identified as isopropyl alcohol, was isolated, along with a solid glassy polymer, identified as a polymer of 3,6-divinyl-9-methylcarbazole (analyses: found, %: N—5.90; 5.85. (C₁₇H₁₅N). Calculated, %: N—6.01).

Diisopropyl Ether of 3,6-di(α-hydroxyethyl)-9-ethylcarbazole

It was obtained by the procedure described above from 55.8 g (0.2 mole) of 3,6-diacetyl-9-ethylcarbazole, 161.6 g (0.4 mole) of aluminum isopropylate, and 450 ml of absolute isopropyl alcohol. The reduction was completed after 4.5 hours. Distillation in vacuum (222–225°C at 2–3 mm Hg) gave 58.7 g of a colorless oil (yield 80.4% of theory), which solidified on standing. After recrystallization from ethanol, a colorless crystalline substance was obtained with a melting point of 112.5–113°C, readily soluble in hot aromatic hydrocarbons, methanol, and ethanol.

Analyses: found, %: C — 78.53; 78.43. H — 8.98; 9.1. N — 3.90; 3.90. Molecular weight — 366.4. $\mathrm{C}_{24}\mathrm{H}_{33}\mathrm{NO}_2$. Calculated, %: — C — 78.47; H — 8.98; N — 3.91. Molecular weight — 367.5. On heating the substance with $\mathrm{KHSO}_4$ or with $\mathrm{Al}_2\mathrm{O}_3$ in xylene to 140°C, isopropyl alcohol was isolated (boiling point 80–81°C, $n_{20}=1.4775$) and a solid glassy polymer of 3,6-divinyl-9-ethylcarbazole (found, %: N — 5.38; 5.42. $(\mathrm{C}_{18}\mathrm{H}_{17}\mathrm{N})_n$. Calculated, %: N — 5.67).

Conclusions

1. It has been established that, upon reduction of 3,6-diacetyl-9-methyl- and 3,6-diacetyl-9-ethylcarbazoles with aluminum isopropylate in isopropyl alcohol, diisopropyl ethers of 3,6-di($\alpha$-oxyethyl)-9-methyl- and 3,6-di($\alpha$-oxyethyl)-9-ethylcarbazoles are formed in yields of 75 and 80% of theory.

2. It has been shown that, on heating with potassium bisulfate or with active aluminum oxide, the diisopropyl ethers decompose with liberation of isopropyl alcohol and polymers of 3,6-divinyl-9-methyl- and 3,6-divinyl-9-ethylcarbazoles.

References

1. V. P. Lopatinskii, E. E. Sirotkina, M. M. Anosova, T. V. Sonina. Izvestiya Tomskogo politekhnicheskogo instituta, 120, 1963.

2. Organic Reactions. Vol. 2, Izd. inostrannoi literatury, 1950.

3. S. Kawai et al. J. Chem. Soc. Jap., 73, 103, 1952.

4. V. P. Lopatinskii, E. E. Sirotkina. Izvestiya Tomskogo politekhnicheskogo instituta, 120, 1963.