Synthesis of 1-adamantyl alkyl ethers by intermolecular dehydration of 1-adamantanol with alcohols catalyzed by copper compounds

Adamantyl alkyl ethers were synthesized in high yields by intermolecular dehydration of 1-adamantanol with alcohols in the presence of copper-containing catalysts


Introduction
1-Adamantyl alkyl ethers are of considerable interest as additives that increase the oxidative stability and viscosity of lubricating oils, hydraulic and transmission fluids. 1,2hotolysis of mixtures of 1-haloadamantanes with alcohols in various solvents is the most widely used method for preparing 1-adamantyl alkyl ethers.4][5][6] Another approach for the synthesis of 1-adamantyl methyl ether in 65-90% yield involves the reactions of 1-chloro-or 1-iodoadamantanes with methanol in the presence of silver perchlorate. 7The reaction is partially reversible; therefore the precipitated AgCl is immediately filtered off.The excess of chloride ions is precipitated by adding silver perchlorate in acetone.

Results and Discussion
The key regularities of the synthesis of 1-adamantyl alkyl ethers were studied in relation to the reaction of 1-adamantanol 1 with methanol in the presence of the following series of coppercontaining catalysts: CuBr2, CuCl2 From the data presented in Table 1, it can be seen that the highest yield of 1-adamantyl methyl ether 2 of 85% is achieved when the intermolecular dehydration of 1-adamantanol 1 with methanol is carried out under the following conditions: 220 о С, 1 h, and a 1 : 100 : 1000 ratio of the catalyst, compound 1 and methanol (Table 1, entry 2).The reaction is general.Under the optimal conditions, it can be performed for a broad range of alcohols ROH, where R = С2Н5, n-C3H7, n-C4H9, n-C5H11, n-C8H17, cyclo-C5H9, cyclo-C6H11 and C6H5CH2 (Table 2).
It is worth noting that the use of a 10-fold excess of high-boiling alcohols such as n-octanol, cyclopentanol, cyclohexanol or benzyl alcohol is undesirable, because it complicates the isolation of the desired adamantyl alkyl ethers by necessitating the removal of unreacted alcohols and homoethers (dioctyl, dicyclopentyl, dicyclohexyl or dibenzyl) from the reaction mixture.
In order to increase the conversion of alcohols (ROH) and decrease the yield of side homoethers (ROR), the intermolecular dehydration of 1-adamantanol 1 with n-octanol, cyclopentanol, cyclohexanol and benzyl alcohol was carried out at an equimolar reactant ratio, which increased the yield of the target 1-adamantyl alkyl ethers and also increased the conversion of the alcohols (ROH) (Table 3).

Conclusions
It was found that copper compounds CuBr2, CuCl2•2H2O and Cu(PhCO2)2•2H2O are effective catalysts of the synthesis of 1-adamantyl alkyl ethers by intermolecular dehydration of 1adamantanol and alcohols.In the presence of these catalysts, we performed the synthesis of the known 1-adamantyl methyl, ethyl, propyl and benzyl ethers and the previously unknown 1adamantyl butyl, octyl cyclopentyl, and cyclohexyl ethers.

Experimental Section
General. 1 Н and 13 С NMR spectra were measured on a Bruker Avance-400 spectrometer (400.13 and 100.62 MHz, respectively) in CDCl3, the chemical shifts are referred to TMS.Mass spectra were run on a Shimadzu GCMS-QP2010Plus GC/MS spectrometer (an SPB-5 capillary column, 30 m × 0.25 mm, helium as a carrier gas, temperature programming from 40 to 300 o C at 8 °C/min, evaporation temperature 280 о C, temperature of the ion source 200 о C, ionization energy 70 eV).Chromatographic analysis was carried out on a Shimadzu GC-9A, GC-2014 instrument [2 m × 3 mm column, silicone SE-30 (5%) on Chromaton N-AW-HMDS as the stationary phase, temperature programming from 50 to 270 о С at 8 °C/min, helium as the carrier gas (47 mL/min)].
The elemental composition of the samples was determined on a Carlo Erba 1106 elemental analyzer.

General procedure for the synthesis of compounds 2, 3a-h
In an argon flow, the 10-mL glass ampoule was charged with CuBr2 (0.013 mmol), 1adamantanol 1 (1.3 mmol), alcohol (13 mmol of methanol, ethanol, 1-propanol, 1-butanol, 1-pentanol or 1.3 mmol of 1-octanol, cyclopentanol, cyclohexanol, benzyl alcohol).The sealed ampoule and appropriate alcohol (2 mL) to counterbalance the internal pressure of the reactant mixture when heated were placed in a 17-mL stainless-steel micro autoclave, the autoclave was sealed and heated at 220 о С for 1 h.After completion of the reaction, the autoclave was cooled to room temperature, the ampoule was opened, and the reaction mixture was filtered through a paper filter.The solvent was distilled off.The adamantyl alkyl ether was separated from the unreacted 1-adamantanol 1 by decantation in hexane.Alternatively, the mixture was separated by column chromatography on silica gel using a 1 : 1 hexane : benzene mixture for elution (l = 21cm, d = 1.2 cm).Detailed NMR spectra are given in the Supplementary Data file.1-Adamantyl methyl ether (2). 10

Table 1 .
). Synthesis of 1-adamantyl methyl ether 2 by the reaction of 1-adamantanol 1 with methanol in the presence of copper-containing catalysts (T = 220 ºC unless otherwise stated)

Table 3 .
The reaction of 1-adamantanol with an equivalent amount of high-boiling alcohol

Table 3 .
Continued a Yields are given relative to converted 1 b Yields are given relative to converted alcohol (ROH)