Bartók, Mihály

Molnár, Árpád

English

Molnár Árpád

Bartók Mihály

University of Szeged

C H E M I S T R Y O F D I O L S A N D C Y C L I C E T H E R S , X L V * • S T E R I C A L L Y - H I N D E R E D H Y D R O G E N - T R A N S F E R R E A C T I O N S I N T H E T R A N S F O R M A T I O N S O F 2 - S U B S T T T U T E D 1 , 3 - P R O P A N E D I O L S O N A C O P P E R C A T A L Y S T By Á. MOLNÁR and M. B A R T Ó K Institute of Organic Chemistry, József Attila University, Szeged (Received 28th September 1979) During the transformations of 2-;i-butyl-, 2-isopropyl-, 2-/e«-butyl-, 2-cyclohexyl- and 2' phenyl-l,3-propanediol (I—V) on a copper catalyst, unsaturated aldehyde (and unsaturated al cohol) is also formed besides the saturated compounds. In the case of V, the formation of the un-saturated compounds is the characteristic reaction. The product distribution can be interpreted' in terms of the steric and electronic properties of the substituents. In the course of our earlier investigations in connection with the mechanisms of the metal-catalyzed reactions of 1,3-diols, it was established that in the presence of copper catalysts the 1,3-diols are transformed to oxo compounds containing the same number of carbon atoms, or are cleaved to give two molecules of oxo compound (Fig. 1) [1, 2]. " The final step of the dehydration process occurs via a hydrogen-transfer reac-tion between the a, ^-unsaturated oxo compounds formed and the starting diol. Our results relating to diols containing a cyclohexane skeleton [3] showed that R-CH-CHj-CH,— I L I -H2 OH OH R - < H + C H r < H A • R - Ç H 4 H 2 - C ^ R - C H = C H - < ^ R-CH2-CH2-<° OH • R-C-CHj-CH, —R-C-CH-CH2 • R-Ç-CH2-CH3 S , ¿H ^ & 0 2'i R-C-CH3+CH, Il J II 1 o 0 Fig. 1. Reaction pathways of 1,3-diols on copper catalysts. Pathway 1 and 1': dehydration Pathway 2 and 2': dealdolization * Part XLIV: M. Bartok, Á. Molnár: Acta Chim. (Buda pest) 100, 203 (1979). 162 Á. MOLNÁR and M. BARTÓK with certain compounds [2-(l-hydroxyethyl)-l-cyclohexanol and 2-(l-hydroxybutyl)-1-cyclohexanol] this step is hindered, and thus the intermediate unsaturated oxo compounds also appear among the products. In our recent investigations, some new examples of this hindered hydrogen-transfer have been found. The present paper describes results obtained during the transformations of some 2-substituted 1,3-propanediols (I—V). R R = «-butyl I | isopropyl II CH2—CH—CH2 /erf-butyl i n | | cyclohexyl IV OH OH P 1 1 ^ 1 v Our preliminary studies with 2-isopropyl-l,3-propanediol (II) on three diffe-rent copper catalysts (Cu/Al, Cu/SiOa, and Cu) demonstrated the otherwise ex-pected fact that the ratio of the saturated and unsaturated compounds depends on the nature of the catalyst (Table I). Accordingly, since our aim was to study how Table I Product distribution of 2-isopropyl-l,3-propanediol (II) on copper catalysts (conversion 100%) Catalyst Cu/Al Cu/SiOj Cu Temperature (C°) 200 225 210 3-methylbutanal 8 15 19 2-isopropylpropenal 27 5 30 2,3-dimethylbutanal 37 50 21 3-methyl-l-butanol 6 13 13 2,3-dimethyl-l-butanol 12 14 6 2-isopropyl-2-propen-1 -ol 5 — 8 unidentified 5 3 3 the properties of the substituent in position 2 (steric and electronic effects) influence the occurrence of the hydrogen-transfer, we carried out examinations on only a single catalyst. For this purpose the Cu catalyst prepared by reduction from CuO was selected, for, as indicated by the data in Table 1, this displayed the lowest activity in the hydrogen-transfer. Experimental Diols I, II and IV were prepared in a known way, by malonic ester synthesis and subsequent reduction with Li[AlH4], (I: b.p. 102—107 °C/5 torr; b.p. [4]: 105—108 °C/5 torr. II: b.p. 110—115 °C/5 torr; b.p. [5]: 114—116 °C/6 torr. IV: m.p. 93.5 °C; m.p. [5]: 94 °C.) In the preparation of III, the procedure described in [6] was followed, and the diol was then obtained by reduction (m.p. 59—61 °C; m.p. [5]: 61 °C). V was prepared by Li[AlH4] reduction of diethylphenylmalonate, •obtained by the method described in [7] (m.p. 44—48 °C; m.p. [8]: 48.5—49 °C). CHEMISTRY O F DIOLS and CYCLIC ETHERS, XLV. 163 The experimental technique and the preparation of the Cu catalyst were re-ported earlier [2]. Products were identified with the aid of a GC technique and IR and NMR spectroscopy. The aldehydes were also identified via precipitation of (2,4-dinitrophenyl)-hydrazones. The experimental results are summarized in Tables I—V. Table II Transformation of 2-n-butyl-l,¡-propanediol (I) on Cu catalyst at 250 °C (conversion 100%) Products mol % hexanal 22 2-methylhexanal 38 1-hexanol 16 2-methyl-l -hexanol 22 unidentified 2 Table III Transformation of 2-tert-butyl-l,¡-propanediol (01) on Cu catalyst at 250 °C (conversion 100%) Products mol % 3,3-dimethylbutanal 32 2-ierr-butylpropenal 13 2,2,3-trimethylbutanal 14 3,3-dimethyl-l-butanol 34 2,3,3-trimethyl-l-butanol 2 2-?er/-butyl-2-propen-l-ol 2 unidentified 3 Table IV Transformation of 2-cychlohexyl-l,3-propanediol (IV) on Cu catalyst at 270 °C (conversion 100%) Products mol % cychlohexylacetaldehyde 24 2-cyclohexyI pro pena 1 26 2-cyclohexylpropanal 24 2-cyclohexylethanol 20 2-cyclohexyl-1 -propanol 3 2-cyclohexyl-2-propen-l-ol 2 unidentified 1 6 164 Á. MOLNÁR and M. BARTÓK Table V Transformation of 2-phenyl-l,3-propanediol (V) on Cu catalyst at 280 °C (conversion 100%) ' Products mol % phenylacetaldehyde 9 2-phenylpropenal 34 2-phenylpropanal 5 2-phenylethanol 21 2-pheny]-l-propanoI 3 2-phenyl-2-propen-1 -ol 22 unidentified 6 Discussion The experimental results in Tables I—V, and the comprehensive data in Table VI, lead to the following findings: Table VI Product distributions of the transformations of compounds I—V according to the reaction pathways Compound Percentage amounts of products with the same number of carbon atoms { as the diol' unsaturated saturated total I 60 60 38 II 38 27 65 32 III 15 16 31 66 IV 28 27 55 44 V 56 8 64 30 Percentage amounts of products obtained by dealdolization 1. The straight-chain substituent (the «-butyl group in compound I) does not exert an inhibitory effect on hydrogen-transfer (Table II). 2. The branched-chain substituents (isopropyl and re/7-butyl groups, II and III) impede hydrogen-transfer, and hence unsaturated aldehyde and (by reduction of this) unsaturated alcohol are also obtained in both cases (Tables I and III). In the case of III, the large bulk of the tert-butyl group not only affects this reaction^ but at the same time increases the probability that the intermediate hydroxy-oxo compound will undergo further conversion by the other reaction pathway, i.e. dealdolization (Fig. 1, pathway 2 and 2'). The large extent of this side-reaction means that the effect of the tert-butyl group on hydrogen-transfer can not be examin-ed, and accordingly can not be compared with that of the isopropyl group. 3. The cyclohexyl group influences hydrogen-transfer to a similar extent as do the branched-chain substituents (Table IV). 4. In the transformation of the diol (V) containing the phenyl group, strikingly large amounts of unsaturated compounds are formed (Table .V). In our view, such CHEMISTRY OF DIOLS and CYCLIC ETHERS, XLV. 165 a marked increase in the inhibition can be ascribed to electronic factors. The con-jugation effect between the phenyl group and the double-bond leads to such an electron-shift (e.g. the structure A in Fig. 2) that as a consequence the С—С double-bond becomes depleted in electrons. A similar situation results from the meso-merism between the C = C and C = 0 bonds (Fig. 2, B). The mechanism of the А В Fig. 2. Electron-shift effects occurring in 2-phenylpropenal hydrogen-transfer was elucidated by EADON and SHIEKH [9]. In the first step, hydro-gen is bound on the jS-carbon atom of the a, /¡-unsaturated oxo compound, and therefore the above effects probably induce this significant phenomenon by decreas-ing the rate of this reaction step. References [11 Bartók, M., A. Molnár: Kémiai Közlemények 45, 335 (1976). [2] Molnár, A., M. Bartók: Acta Chim. (Budapest) 89, 393 (1976). [3] Molnár, A., M. Bartók: React. Kinet. Catal. Lett. 3, 421 (1975). [4] Pichat, L., C. Baret. M. Audinot: Bull. soc. chim: France 1954, 92. [5] Bartók, M., B. Kozma, A. S. Gilde: Acta Phys. et Chem. Szeged 11, 39 (1965). [6] Eliel, E. L., R.'O. Hutchins, M. Knoeber: Ore. Synth. 50, 38 (1970). [7] Levene, P. A., G. M. Meyer: Org. Synth. Coll. Vol. 11, 288 (1943). [8] Adkins, H„ H. R. Billica: J. Amer. Chem. Soc. 70, 3125 (1948). [9] Eadon, G„ M. Y. Shiekh: J. Amer. Chem. Soc. 96, 2288 (1974). Х И М И Я ДИОЛОВ И ЦИКЛИЧЕСКИХ ЭФИРОВ, XLV. Пространственно затрудненный переход водорода при превращении 2-замещённых 1,3-пропандио.юв на медном катализаторе. ', А. Молнар и М. Барток При превращении 2-н-бутил-, 2-изопропил-, 2-трет-бутил-, 2-циклогексил-, 2-фенил-1,3-пропандиола (I—V) на медном катализаторе образуются ненасыщенный альдегид (и ненасы-щенный спирт), кроме насыщенных соединений. В случае диола V, образование ненасыщенных соединений является характерным. Результаты обяснъяются пространственными и электрон-ными факторами заместителей. 6« 

Chemistry of diols and cyclic ethers 45. : Sterically-hindered hydrogen-transfer reactions in the transformation of 2-substituted 1,3-propanediols on a copper-catalyst

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Chemistry of diols and cyclic ethers 45. : Sterically-hindered hydrogen-transfer reactions in the transformation of 2-substituted 1,3-propanediols on a copper-catalyst

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