GB2156344A - Liquid phase preparation of optionally 2-substituted-2-oxazolines by cyclodehydration - Google Patents

Abstract

2-R-2-Oxazolines, wherein R is H or optionally inertly substituted hydrocarbyl, are prepared by contacting in liquid phase a N-( beta -hydroxyalkyl) R-substituted amide with a catalytic amount of an inorganic zinc salt. Alternatively the carboxylic acid/amine precursors for the starting amide may be used.

Description

SPECIFICATION Liquid phase preparation of 2-H-2-oxazolines and 2-substituted-2-oxazolines with an inorganic zinc salt The 2-H-2-oxazolines form a known class of compounds having several members. 2-H-2-oxazoline is the first member of this series and is the "simplest" molecule. It corresponds to formula /.

The 2-H-2-oxazolines and particularly / above are generally quite different from the corresponding 2substituted-2-oxazolines. This is illustrated by the fact that many conventional processes for preparing 2substituted-2-oxazolines are not particularly satisfactory for the preparation of the corresponding 2-H-2- oxazolines.

For example, many 2-substituted-2-oxazolines are prepared by dehydrochlorinating a ss-chloroalkyl carboxamide with aqueous alkali. However, this process reportedly produced I in only very low yields and was accompanied by decomposition of the desired product. (H. Wenker, J. Am. Chem. Soc., 60, 2152 (1938)).

Another common technique for preparing oxazolines is the cyclodehydration of N-(,-hydroxy-al- kyl)carboxamides over various catalysts. Litt et al. (USP 3,681,329) claim that 2-H-2-oxazolines could be prepared by contracting the appropriate carboxamides with compounds of manganese, cobalt, molybdenum, tungsten and the rare earth metals. Unfortunately, there is no experimental data in Litt el al. which would substantiate this allegation. Hess teaches in Canadian Patent 536,594 and British Patent 536,594 and British Patent 758,972 that I can be prepared by a cyclodehydration of N-(ss-hydroxyethyl) formamide in the presence of a dehydrating agent (specifically, diatomaceous earth, sulfuric acid, aluminum oxide and iron oxide). The yields were higher than the yield reported by Wenker but were still commercially unsatisfactory.

Eisenbraun (USP 3,312,714) teaches the preparation of 2-oxazolines from N-(2-hydroxyethyl)-amides using a solid inorganic borate or inorganic salt of a boric acid. 2-Oxazoline yields of from about 63 to 77 percent are reported.

More recently, Ito et al. (J.Am. Chem. Soc., 95: 13, 4447 (1973)) reported that 2-H-2-oxazolines and oxazines could be prepared by reacting isonitrile with amino alcohols in the presence of a small amount of silver cyanide. Ito et al. report that I was thus produced in 67 percent yield. Isonitrile and silver cyanide appear to be very unique in this particular reaction.

The chemistry of oxazolines has been reviewed by Wiley et al., Chemical Reviews, Vol. 44, 447-476 (1949), Seeliger et al., Angew. Chem. International Edition, Vol. 5, No. 10, 875-888 (1966), and by Frump, Chemical Reviews, 1971, Vol. 71, No. 5, 483-505.

It has now been discovered that 2-H-2-oxalines may be prepared by reacting, by contacting in liquid phase, a N-(ss-hydroxyalkyl)formamide with a small but catalytic amount of an inorganic zinc salt. Similarly, 2-substituted -2-oxazolines are prepared by contacting N-(2-hydroxyalkyl) carboxamides with a small but catalytic amount of an inorganic zinc salt. The reaction is normally conducted at an elevated temperature and preferably under reduced pressure.

Accordingly, the present invention provides a cyclodehydration process for making a 2-R-2-oxazoline wherein R is hydrogen, hydrocarbyl or inertly-substituted hydrocarbyl, which comprises reacting, by contacting in liquid phase, (a) an optionally inertly-substituted N-(2-hydroxyalkyl) -carboxamide or carboxylic acid/amine precursor of N- (2-hydroxyalkyl)carboxamide, with (b) a catalytic amount of an inorganic zinc salt.

The present invention includes the use of the products of the above process as synthetic intermediates, i.e. as intermediates in chemical synthesis. For instance the 2-methyl groups of 2-methyl substituted oxazolines is activated and may be alkylated by alkyl halides.

This leads to a useful synthesis of carboxylic acids. Oxazolines are also useful as intermediates in the synthesis of oxazolines which are valuable in the synthesis of aromatic amino and keto acids.

The N-(p-hydroxyalkyl)carboxamides used in the instant process are a known class of compounds which can be represented by the formula

wherein R is hydrogen, a hydrocarbon or an inertly- substituted hydrocarbon group and R1-R4 are each separately hydrogen or inert organic radicals. By "inert" is meant inert in the instant process. When R is hydrogen, the carboxamide reactant is properly called an N-(ss- hydroxyalkyl)formamide.

The formamide compounds are typically prepared by reacting a lower alkyl formate (e.g., methyl formate) or formic acid with an ethanolamine of the formula Ill shown below. The carboxamides are typically prepared by reacting a carboxylic acid (RC(O)OH) or a lower alkyl ester of the carboxylic acid with an ethanolamine of the formula Ill:

wherein R1-R4 have the aforesaid meanings. The formic acid/amine precursor (or salt) which is formed initially in these reactions can be used in the instant process in place of the formamide or carboxamide.

When such formic acid/amine salts or carboxylic acid/amine salts are used, the formamide or carboxamide is generated in situ. In formulas II and III, R3 and R4 are each preferably hydrogen and R1 and R2 are each preferably hydrogen, lower alkyl (C1-C6), hydroxymethyl or alkanoyloxymethyl (alkyl-C(O)-O-O-CH2)groups of up to about 17 carbon atoms. More preferably, R1-R4 are each hydrogen. N-(p-hydroxyethyl)- formamide is the most preferred reactant for use in the instant process which leads to the preparation of 2-H- 2-oxazoline.Other suitable N-(ss-hydroxyalkyl) formamides include compounds of formula // having the following values for Rl-R4 TABLE I R1 EE2 R3 R4 Ch H H H CH3 CH3 H H CH2OH CH2OH H H C4H9 H H H CH5 H C6H H CH3 H CH3 H C6H4CH3 H CH3 H C,7H35C(O)OCH2 H H H In formula // R is preferably hydrogen, alkyl of from 1 to 17 carbon atoms or phenyl and is more preferably hydrogen, methyl, ethyl or phenyl and is most preferably methyl or ethyl.Examples of suitable N-(ss- hydroxyalkyl) carboxamides include compounds of formula II having the following values for R and R, R4: TABLE II R R, R2 R3 R4 CH3 H H H H CH3 C4H9 H H H CH3 C2H5 C2H H H C2H5 H H H H C2H5 CH2OH C2OH H H C2H5 CH2O(O)C-C,7H35 H H H CH7 C,H H C,H5 H C; ;7H.5 CH3 H H H C9 H19 CH3 CH3 H C11H23 CH3 H CH3 H C,7H35 H H H H CH9 H H H H C6H4CH3 C6H H CH3 H C6H5CH2 H H CH3 CH3 CH3(CH2)7CH=CH(CH2)7 H H H H cyclohexyl H H H H The catalysts in the instant cyclodehydration reaction are inorganic zinc salts which are soluble in the formamide reactant, carboxamide reactant or liquid reaction medium. The term "soluble is not meant to imply that the zinc salts are soluble or miscible in all proportions with formamide, carboxamide or liquid reaction medium but instead has at least a minimum effective solubility (e.g., about 100 parts per million or more) at reaction temperatures. The zinc saits are used in the process in small, that is, catalytic, amount.Normally, the zinc salts are charged in amounts of from 0.005 to 0.4 mole of zinc salt per mole of formamide reactant but greater or lesser quantities of the zinc salts can be used, it desired. Zinc salts derived from strong inorganic mineral acids (sulfuric, nitric, hydrochloric, hydrobromic, hydroiodic, phosphoric, etc.) and other strong acids (hypophosphorous acid, sulfonic acid, etc.) are preferred for this invention. The strength of the acids described is a measure of the concentration of the hydrogen ion that results from ionization in water. The approximate pH values of the preferred acids are in a range of from about 0.1 to about 2.4. Weaker inorganic acids such as hydrogen sulfide, arsenious acid, boric acid, etc.

may be useful in the preparation of the zinc salts but are not as efficient (i.e., result in lower yields) as the stronger acids. Approximate pH values of the acids suitable for this invention are based on measurements made at 25"C and reported in the CRC Handbook of Chemistry and Physics, 58th edition, CRC Press, 1977-1978, page D-135. Suitable such salts include, for example, zinc sulfate, bisulfate, chloride, bromide and iodide. Zinc chloride and zinc sulfate are the preferred catalysts.

We prefer that the inorganic zinc salt is a salt of a strong inorganic acid having an approximate pH value of less than 5.

The instant cyclodehydration reaction may be conducted neat or in solution with a suitable inert solvent. By "inert" is meant inert in the process. Suitable such solvents include, for example, chlorinated hydrocarbon solvents, aromatic hydrocarbons, cycloaliphatic hydrocarbons, and aliphatic hydrocarbons and the like. We prefer, however, to conduct the reaction neat (i.e., that is without any solvent added).

The reaction temperature must, obviously, be sufficient to promote the cyclodehydration reaction and is normally selected in the range of from about 140"C to about 270"C. Preferred reaction rates have been observed at temperatures of from about 1600C to about 220"C. The instant cyclodehydration reaction is also preferably conducted under reduced pressure. This facilitates product recovery in that frequently a reaction temperature may be chosen which is above the boiling point of the 2-H-2-oxazoline or 2-substituted-2-oxazoline product and below the boiling point of the N-(B-hydroxyalkyl) formamide or N-(ss-hy- droxyalkyl)carboxamide. In this manner, the 2-H-2-oxazoline or 2-substituted-2-oxazoline can be removed from the reaction mixture as a volatile gas essentially as it is formed.This is very desirable since the instant cyclodehydration reaction is a reversible process and by removing the product the reaction is forced to completion by substantially reducing the reverse reaction. Water normally codistills with the 2 H-2-oxazoline or 2-substituted-2-oxazoline product.

The instant process may be conducted in a batch process or by a continuous process. In the continuous process, of course, the N-(p-hydrnxyalkyl) formamide or carboxamide reactant is metered into the reaction vessel at essentially the same rate as the oxazoline product and water are removed.

The following examples will further illustrate the invention.

Example 1 - Preparation of 2-H-2-Oxazoline Zinc chloride (5 g) and N-(p-hydroxyethyl)- formamide (20-30 g) were charged to a reaction vessel equipped with a heating means, a stirring means, an addition funnel, a distillation head, condenser and receiver. The pressure over the reaction mixture was lowered to 50 mm Hg (6.7 kPa) using a water aspirator and the mixture heated to a pot temperature of approximately 175 C. When the reaction began, as evidence by the appearance of an overhead distillate, the dropwise addition of more formamide reactant was started.The formamide feed was stooped when the catalysts began to be deactivated, as evidenced by the formation of a very dark material in the bottom of the reaction vessel and a rapidly rising head temperature.The water-white overheads were continuously collected and when the reaction was complete, the overheads were extracted with chloroform to remove the 2-H-2-oxazoline from the water which codistilled. Distillation of the chloroform extracts at elevated temperature and atmospheric pressure using a 15-plate Oldershaw column gave the pure 2-H-2-oxazoline boiling at 98"C. The oxazoline product was produced in 70.9 percent yield in the process.

Example 2 2-H-2-Oxazoline was produced in 75.6 percent yield, based on formamide charged, following the procedure described in Example 1 except using the amine/acid salt formed by blending equimolar amounts of formic acid with ethanolamine in place of the N-(ss-hydroxyethyl) formamide.

Example 3 2-H-2-Oxazoline was produced in 56.1 percent yield, based on formamide charged, following the procedure described in Example 1 except using ZnSO4.7H2O as the catalyst.

Other 2-H-2-oxazolines can be similarly prepared using zinc chloride as the catalyst and other N-(ss- hydroxyalkyl) formamides as the reactants or by using other combinations of catalysts and formamide reactants as set forth above.

Example 4 - Preparation of 2-Substituted 2-Oxazoline Zinc oxide (10.0 g) and 95.4 percent pure N-(ss-hydroxyethyl)propionamide (20.0 g) were charged to a reaction vessel equipped with a stirring means, a metering pump, and a 5-plate Oldershaw distillation column with a take-off head. The pressure over the reaction mixture was adjusted to 50 mm Hg (6.7 kPa) and the reaction mixture heated to 200 C. The reaction mixture was held at 200"C and 95.4 percent pure N-(,8-hydroxyethyl) propionamide (290 g) was pumped in at approximately 0.9 g/min to the system.As the propionamide was added to the reaction mixture, a water-white distillate was collected overhead through the distillation apparatus at a head temperature of 40"C-45"C. After the addition of the propionamide was complete, the pot was heated to 220"C to drive off the last amounts of 2-ethyl-2-oxazoline.

The overhead distillate temperature reached a maximum of 41"C during this post-heating step. A total of 294.8 g of water-white distillate was thus obtained overhead leaving 21.7 g of a tan, wet paste remaining in the pot. Analysis of the distillate overheads by gas chromatography using an internal standard and also a Karl Fischer water titration showed the material to be 2-ethyl-2-oxazoline, water and very minor amounts of unreacted propionamide and 2-methyl-2-oxazoline. The impurities in the propionamide reactant were: water (approximately 1 percent); monoethanolamide (approximately 2-3 percent); and the amidoester of propionic acid and monoethanolamine (approximately 1 percent).

The oxazoline was produced in 83.0 percent yield, based on the pure N-(ss-hydroxyethyl)-propionamide charged to the system. The amount of water produced according to analysis was 93.0 percent of theory.

The 2-ethyl-2- oxazoline can be easily separated from the mixture by selective extraction using dimethyl benzene followed by distillation.

Examples 5-8 Using substantially the same technique as described in Example 4, except that the catalyst was varied, the reactions summarized in Table A were conducted with the indicated results.

TABLE A > h91 1 0f47 < Example Catalyst Percent Yield 5 Zn(OH)2 86.3 6 ZnCI2 94.5 7 Znl2 91.5 8 ZnSO4.7H2O 91.9

Claims (16)

1. A cyclodehydration process for making a 2-R-2-oxazoline wherein R is hydrogen, hydrocarbyl or intertly-substituted hydrocarbyl, which comprises reacting, by contacting in liquid phase, (a) an optionally inertly-substituted N-(2-hydroxyalkyl) -carboxamide or carboxylic acid/amine precursor of N- (2-hydroxyalkyl)carboxamide, with (b) a catalytic amount of an inorganic zinc salt.

2. A process as claimed in Claim 1 wherein the inorganic zinc salt is a salt of a strong inorganic acid having an approximate pH value (as hereinbefore defined) of less than 5 based on a measurement made in an aqueous solution at 25"C.

3. A process as claimed in Claim 1 wherein the inorganic zinc salt is an inorganic zinc salt of sulfuric, nitric, hydrochloric, hydrobromic, hydroiodic, phosphoric, hypophosphorous, or sulfonic acid.

4. A process as claimed in Claim 3 wherein the inorganic zinc salt is zinc chloride or zinc sulfate.

5. A process as claimed in any one of the preceding claims wherein the catalyst is charged in an amount of from about 0.005 to 0.4 mole of inorganic zinc salt per mole of carboxamide reactant.

6. A process as claimed in any one of the preceding claims wherein the N-(2-hydroxyalkyl) carboxamide corresponds to the formula O # R-C-N H-CR1R2-CR3R4-OH, wherein R is hydrogen, alkyl of 1 to 17 carbon atoms or phenyl, Rl and R2 are each independtly hydrogen, lower alkyl, hydroxymethyl or alkanoyloxymethyl groups of up to about 17 carbon atoms, and R3 and R4 are each hydrogen.

7. A process as claimed in Claim 6 wherein R1 and R2 are each hydrogen.

8. A process as claimed in Claim 7 wherein R is hydrogen, methyl, ethyl or phenyl.

9. A process as claimed in Claim 8 wherein R is methyl or ethyl.

10. A process as claimed in any one of the preceding claims wherein the process is conducted under conditions of temperature and pressure such that the oxazoline product is removed from the reaction mixture as a volatile gas essentially as it is formed.

11. A process as claimed in any one of the preceding claims which is carried out at a temperature of from 140"C to 170 C.

12. A process as claimed in any one of the preceding claims wherein the N-(2-hydroxyalkyl)-carbox- amide is prepared by reacting a carboxylic acid RCOOH or lower alkyl ester thereof with an optionally inertly substituted ethanolamine.

13. A cyclodehydration process for making 2-R-2 oxazoline substantially as hereinbefore described, other than one of the prior art processes described hereinbefore

14. A cyclodehydration process for making 2-R-2 oxazoline substantially as herein before described in any one of the Examples.

15. A 2-R-2-oxazoline whenever prepared by a process as claimed in any one of the preceding claims

16. The use of a 2-R-2-oxazoline as claimed in Claim 15 as a synthetic intermediate.