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Articles Reduction of Representative Organic Functional Groups with Gallane-Trimethylamine

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Articles Reduction of Representative Organic Functional Groups with Gallane-Trimethylamine 274 Bull. Korean Chem. Soc. 1997, Vol. 18, No. 3 Jung Hoon Choi et al. Articles Reduction of Representative Organic Functional Groups with Gallane-Trimethylamine Jung Hoon Choi*, Young Joo Oh, Min Jung Kim, Book Kee Hwang, and Dae Jin Baek* Department of Chemistry, Hanyang University, Seoul 133-791, Korea ^Department of chemistry, Hanseo University, 360, Daegokri, Haemif Seosan, Chung-Nam 352-820, Korea Received January 30, 1996 The rates and stoichiometry of the reaction of gallane-trimethylamine with selected organic compounds con­ taining representative functional groups were examined in tetrahydrofuran solution under standardized con­ ditions (THF, 0 °C). And its reducing characteristics were compared with those of aluminum hydride- triethylamine(AHTEA). The rate of hydrogen evolution from active hydrogen compounds varied considerably with the nature of the functional group and the structure of the hydrocarbon moiety. Alcohols, phenol, amines, thiols evolved hydrogen rapidly and quantitatively. Aldehydes and ketones were reduced moderately to the cor­ responding alcohols. Cinnamaldehyde was reduced to cinnamyl alcohol, which means that the conjugated dou­ ble bond was not attacked by gallane-trimethylamine. Carboxylic acids, esters, and lactones were stable to the reagent under standard conditions. Acid chlorides also were rapidly reduced to the corresponding alcohols. Epoxides and halides were inert to the reagent. Caproamide and nitrile were stable to the reagent, whereas ben­ zamide was rapidly reduced to benzylamine. Nitropropane, nitrobenzene and azoxybenzene were stable to the reagent, whereas azobenzene was reduced to 1,2-diphenylhydrazine. Oximes and pyridine N-oxide were reduc­ ed rapidly. Di-n-butyl disulfide and dimethyl sulfoxide were reduced only slowly, but diphenyl disulfide was reduced rapidly. Finally, sulfones and sulfonic acids were inert to the reagent under the reaction. Introduction under standardized conditions (THF, 0 °C). And we also compared the reducing properties of this reagent to those of In 1939, H. C. Brown used diborane for the reduction of the aluminum hydridetriethylamine (AHTEA).28 organic compounds containing a carbonyl group, which was the first application of boron hydrides for the reduction of Results and Discussion organic functional groups.1 The discovery of lithium alu­ minum hydride2 and lithium borohydride3 brought about a Gallane-trimethylamine was prepared by the reaction of revolutionary change in the procedures utilized for the reduc­ lithium gallium hydride with trimethylammonium chloride tion of functional groups in organic chemistry. Of these two in diethyl ether as reported by Shrink and Shriver.31,32 reagents, lithium borohydride4 is a relatively mild reducing agent, practically specific to the carbonyl group in al­ (C2H5)O LiGaH4+[(CH3)3NH]Cl 一一 - GaH3 N(CH3)3+LiCl+H2 dehydes, ketones, and esters. On the other hand, lithium alu­ minum hydride5 is an exceedingly powerful reagent that at­ The general procedure for reduction was to add 1 mmol tacks almost all reducible groups. Therefore, it has been stu­ of the organic compound to 1 mmol of gallane-tri­ died for so many reducing agents to control the reducing methylamine in THF to give 8 mL of solution at 0 °C. The abilities.6~28 But, the previous studies were restricted to alu­ hydrogen evolved on adding the compound to the reagent minium hydride and borohydride species. was noted. After the desired reaction time, the solution was In 1947, Lithium gallium hydride is synthesized and re­ hydrolyzed and the hydrogen evolved was noted. The hy­ ported by Finholt, Bond, Schlesinger.2 And in 1977, dride utilized for reduction by the compound was calculated Schrink and Schriver reported modified method of synthesis from hydrogen evolution. In this way, it was possible to ar­ of lithium gallium hydride in good yields and purification.29 rive at a value for the number of hydrides utilized for the Recently we reported that reducing power of lithium gal­ reduction. lium hydride showed generally more powerful than borohy­ In some cases the hydride-to-compound ratio of 4:1 was dride, but less than aluminium hydride.30 On the other hand, inadequate to achieve complete reduction. In such cases the the reducing characters of gallane, which is a Lewis acidic hydride concentration was maintained constant, but the con­ type hydride, has not been studied. It has been reported gal- centration of compound reduced to give a higher ratio. lane itself is unstable, although gallane-trimethylamine com­ Reaction of Alcohols, Phenol, Amines, and plex is stable.31,32 We therefore decided to examine the Thiols. The alcohols, phenol, and thiols examined all li­ reducing characteristics of gallane-trimethylamine complex berated hydrogen instantly and quantitatively. On the other systematically toward the standard organic functionalities hand, n-hexylamine and diethyl amine liberated only 1 Reduction of Organic Functional Groups with Gallane-Trimethylamine Bull. Korean Chem. Soc. 1997, Vol. 18, No. 3 275 Table 1. Reaction of Gallane-Trimethylamine with Represen­ Table 3. Reaction of Trimethylamine-Gallane with Represen­ tative Alcohols in Tetrahydrofuran at 0 °C tative Aldehydes in Tetrahydrofuran at 0 °C Time Hydrogen Hydride Hydride used Time Hydrogen Hydride Hydride used Compound Compound*1 (hr) evolved*' usedh,c for reduction"' (hr) evolved"， used”。 for reduction* 1-Hexanol 0.5 0.99 1.00 0.01 Caproaldehyde 0.5 0.00 0.64 0.64 1.0 0.99 1.01 0.02 1.0 0.00 0.92 0.92 3.0 0.99 1.00 0.01 3.0 0.00 1.02 1.02 Benzyl alcohol 0.5 0.99 0.99 0.00 6.0 0.00 1.02 1.02 1.0 0.99 1.02 0.03 Benzaldehyde 0.5 0.03 0.60 0.57 3.0 0.99 1.00 0.01 1.0 0.03 0.89 0.86 3-Hexanol 0.5 1.02 1.03 0.01 3.0 0.03 1.02 0.99 1.0 1.02 1.04 0.02 6.0 0.03 1.02 0.99 3.0 1.02 1.04 0.02 Cinnamaldehyde 0.5 0.03 0.91 0.88 3-Ethyl-3-pentanol 0.5 0.85 0.87 0.02 1.0 0.05 1.08 1.03 1.0 0.91 0.94 0.03 3.0 0.05 1.08 1.03 3.0 0.97 0.99 0.02 24.0 0.05 1.12 1.07 6.0 1.01 1.02 0.01 See conesponding footnotes in Table 1. Phenol 0.5 0.95 0.98 0.03 Table 4. Reaction of Gallane-Trimethylamine with Represen­ 1.0 0.98 1.01 0.03 tative Ketones in Tetrahydrofuran at 0 °C 3.0 0.98 0.99 0.01 Time Hydrogen Hydride Hydride used Compound"1 a 1 M of compound to 1.33 mmol of Ga니 3 NMe3 (4 mmol of (hr) evolved'* used"，for reduction" hydride) in 8 mL of solution 0.125 M in compound and 0.5 M 2-Heptanone in hydride. "M of compound. cHydrogen evolved from blank 0.5 0.04 0.51 0.47 0.84 minus the hydrogen evolved on hydrolysis of the reaction 1.0 0.04 0.88 0.04 0.97 mixture after the indicated reaction period. 3.0 1.01 6.0 0.04 1.02 0.98 Table 2. Reaction of Gallane-Trimethylamine with Other Active Norcamphor 0.5 0.03 0.76 0.73 Hydrogen Compounds in Tetrahydrofuran at 0 °C 1.0 0.03 0.87 0.84 3.0 0.03 1.03 1.00 Time Hydrogen Hydride Hydride used Compound" 6.0 0.03 1.03 1.00 (hr) evolved"' used”。 for reduction" Acetophenone 0.5 0.02 0.67 0.65 n-Hexylamine 0.5 0.53 0.55 0.02 1.0 0.02 0.86 0.83 1.0 0.69 0.70 0.01 3.0 0.02 1.03 1.01 3.0 0.88 0.90 0.02 6.0 0.02 1.03 1.01 6.0 1.01 1.02 0.01 Benzophenone 0.5 0.02 0.74 0.72 12.0 1.03 1.05 0.02 1.0 0.02 0.89 0.87 Diethylamine 0.5 0.85 0.87 0.02 3.0 0.02 1.03 1.01 1.0 0.94 0.95 0.01 6.0 0.02 1.05 1.03 3.0 0.99 1.01 0.02 6.0 1.02 1.03 0.01 Cyclohexanone 0.5 0.02 1.03 1.01 12.0 1.03 1.05 0.02 1.0 0.02 1.04 1.02 1-Hexanethiol 0.5 0.95 0.98 0.03 3.0 0.02 1.04 1.02 1.0 0.95 0.96 0.01 Methyl vinyl ketone 0.5 0.06 0.87 0.81 3.0 0.95 0.96 0.01 1.0 0.06 0.98 0.92 3.0 0.06 1.05 0.99 Benzenethiol 0.5 1.05 1.05 0.00 6.0 0.06 1.05 0.99 1.0 1.05 1.06 0.01 3.0 1.05 1.06 0.01 2-Cyclohexene-l-one 0.5 0.02 0.42 0.40 1.0 0.02 0.75 0.73 See conesponding footnotes in Table 1. 3.0 0.02 0.93 0.91 6.0 0.02 1.06 1.04 equiv of hydrogen, and no more hydrogen evolution was ob­ 12.0 0.02 1.07 1.05 served for 24 h. The results are summarized in Tables 1 See corresponding footnotes in Table 1. and 2. In the reaction with a quantitative amount of aluminium hydride-triethylamine (AHTEA) complex, all of the alcohols, dehydes and ketones examined consumed one equivalent of phenol and thiols examined liberate hydrogen instantly and hydride, indicating a moderate reduction rate to the cor­ quantitatively at room temperature.
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