DECEMBER
1961
MERCAPTOETHYLATION.
[COMMUNICATION NO. 2173
FROM THE
5111
IT
KODAK RESEAXCH rABORATORIES, EASTNAN ICODAK CO.]
Mercaptoethylation. 11. Preparation of 2-Mercaptoethyl Carbamates and Oligoethylene Sulfides D.
n. REYNOLDS, D. r,.
FIELDS,
AND
D. L. JOHNSON
Received May 16, lDBl
A series of 2-mercaptoethyl carbamates have been prepared in non-polar solvents by the reaction of primary and secondary amines with ethylene monothiolcarbonate. In the presence of an ionizing solvent, ethylene monothiolcarbonate and amine were found to react to produce low-molecular-weight polyethylene sulfides of the general formulas, %N( CHICHIS),H and RzNCOZ(CHdXXaS),H. The Zmercaptoethyl carbamates were further Characterized by oxidation to their respective disulfides.
s In a previous paper,' the preparation of ethylene (NarCOd /\ monothiolcarbonate (I) was described. Its uses as _I__) CHz-CHt COz A a convenient precursor to ethylene sulfide (11), XI RaNII and more recently as a mercaptoethylating agent SNCHzCHZSH f COz A Toluene, for primary and secondary amines (III)* were I11 RiNH demonstrated. It was noted in this latter investi>RzNCOzCHzCHA3H gation that attempts to prepare a bis(2-merDioxane, R.T. Iv captoethy1)amine by the reaction of two molar RINH &N( CH&HzS)nH equivalents of I with one equivalent of primary ____) v HtO, R.T. amine, in several instances, yielded a 2-mercaptoRnNCOz( CRPH$).H VI ethyl carbamate (IV) instead of the desired bismercaptoethylated amine. As a continuation of the study of the chemistry of ethylene monothiol- a slight molar excess of ethylene monothiolcarboiicarbonate, we shall describe here the preparation ate, and dioxane to stand a t room temperature for of a number of these 2-mercaptoethyl carbamates a reaction period dependent upon the nucleophiland some int,eresting low-molecular-weight poly- icity of the amine. The product was isolated either ethylene sulfides initiated by amine (V) and by by distillation or by crystallization after removal of solvent and unchanged starting materials. It carbamate (VI) groups. should be pointed out that while a general procedure The 2-mereaptoethyl carbamates were prepared by the reaction of a primary or secondary amine is proposed for the preparation of these carbamates, with I under mild, non-polar conditions unfavor- more vigorous conditions, and thus shorter reaction able to niercaptoethylation side reactions. On periods, may often be successfully employed for substituting a polar solvent such as water or ethanol specific cases. Table I demonstrates the generality of this for dioxane, the reaction media used in this carhamate synthesis resulted in the production of the preparation. Most of the carbamates were further oligomer^"^ V and VI to the complete exclusion characterized by oxidation to their respective of IV. The only previously reported 2-mercapto- disulfides, as listed in Table 11. The oxidation of ethyl carbamates were prepared by the uncatalyzed the his-2-mercaptoethyl carbamates of piperazine reactions of phenyl, p-methoxyphenyl and l-naph- and ethylenediamine led to interesting polymeric As the disulfides which mill be reported in a subsequent thy1 isocyanates with 2-mer~aptoethanol.~ work described in this paper was being completed, paper. Ais can be noted in Table I, in several cases the Smith and F r i e d r i ~ hreported ~~ their observations on the base-catalyzed polymerization of 2-mercapto- rates of ring opening of I by amine were quite slow, thus permitting some observation on the relative ethyl carbanilates to oligomers of structure VI. 2-Mercaptoethyl carbamates. The experimental nucleophilicities of the amines toward ethylene conditions used in preparing the 2-mercaptoethyl monothiolcarbonate. When the yields of representacarbamates (IV) were to allow a mixture of amine, tive examples of 2-mereaptoethyl carbamates are plotted against reaction time (Fig. 1) these varia(1) D. D Reynolds, J . Am. Chem.Soc., 79,4951 (1957). tions in reaction rates become more apparent. (2) D. I). Reynolds, M. K. Massad, D. L. Fields, and D. L. JohnIion, J.Org. Chem., 26,5109 (1961),Part I of this The orders of reactivity, Le., nucleophilicity, of the amines under these experimental conditions would series. (3) This is a convenient name to be used to refer to low- appear to be as given here. molecular-weight polyethylene sulfides (oligoethylene sulIt is evident that the rates of reaction are greatly fides) of D. P. ten or less. influenced by the steric requirements of the attack(4) (a) G . M. Bennett and E. M. mhincop, J . Chem. SQC.,119, 1861 (1921).(b) J. F. Smith and E. C. Friedrich, ing amine as well RS its basicity (see p K , values, J. Am. Chem. Soc., 81, 161 (1959). Table I). The decrease iu rates observed for iso-
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5112
VOL.
REYNOLDS, FIELDS, AND JOHNSON
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26
DECEMBER
1961
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5113
MERCAPTOETHYLATION. I1
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Time ( H o l J r S )
Fig. 1. Variations of yields of 2-mereaptoethyl carbamates with reaction time.
higher mercaptoethylated carbamates reported by Smith and F r i e d r i ~ h . ~ ~ In contrast, the reaction of tert-butylamine under identical reaction conditions also led to an oligomer, but of structure V (nz7). The infrared spectra showed practically no carbonyl absorption. The proposed structure was again supported by elemental analysis and iodometric titration. Using the same methods of structure assignment, amines of intermediate reactivity such as isopropylamine and diethylamine were expected, and indeed were found to lead to mixtures of V and VI. Although varying the molar ratio of reactants did have an influence on the degree of polymerization of the oligomer, it did not appear to alter the general structure assigned. DISCUSSION
To explain the experimental facts thus far observed for the reaction of an amine with ethylene monothiolcarbonate, a reaction sequence is suggested in Chart I. From this postulated mechanism one might expect at least five (111, IV, V, propyl-, di-n-butyl-, and tert-butylamines as com- VI, and X) different types of isolable products. pared with n-butylamine can best be attributed to Indeed, four (111, IV, V, and VI) of the five posa steric effect. In comparing amines with similar sibilities have been obtained. Therefore, considersteric requirements such as piperidine and morpho- ing the complexity of this reaction, it is not surline, piperidine was found to be the faster reactant. prising that a careful choice of reaction conditions This would be predicted on the basis of their rela- was found necessary in order to obtain either 2tive basicities. N o reaction occurred with the weakly mercaptoethyl carbamates (IV) or 2-aminoethanebasic aniline or with the sterically bulky tert- thiols (111) in satisfactory yields. The 2-mercaptoethyl carbamates were isolated under such mild butylamine . Oligoethylene suljides. The nonpolar, room- conditions that Step 2 was not in play. In temperature reaction conditions used in preparing this regard it was earlier found2 that, in a the 2-mercaptoethyl carbamates (IV) were found few instances, 2-mercaptoethyl carbamates were necessary owing to their instability, especially in produced by refluxing in toluene a two-fold excess polar solvents and at elevated temperatures, to of I with the very reactive amines such as n-butylcatalytic amounts of organic or inorganic base. amine and allylamine. Evidently this rather high Attempted preparations of IV conducted in more reaction temperature was only possible because polar solvents such as water or ethanol resulted in these amines were rapidly converted to their no isolable IV, but instead yielded oligomers of carbamates (Step l), thus eliminating the base structure V and/or VI. The chain lengths of oli- necessary to catalyze the decomposition of the gomers derived from a large variety of amines were carbamates to ethylene sulfide (Step 2). found to vary between n = 2-10, depending on The previously described synthesis of 2-aminothe nucleophilicity and concentration of the amine, ethanethiols2 was effected in refluxing toluene by the solvent, and the reaction temperature. Whether the reaction of amine with I in the presence of the structure of the oligomer is a multi-mercapto- excess amine. The excess amine and higher reethylated amine (V), a multi-mercaptoethylated action temperature promoted the breakdown carbamate (VI), or a mixture of the two, seems (Step 2) of IV to ethylene sulfide while the use of dependent only on the nucleophilicity of the non-polar toluene and excess amine usually favored starting amine. the mercaptoethylation of amine over mercaptan, Thus, when n-butylamine and I were refluxed although products resulting from mercaptoethylatogether in a methanol-water solvent, a white tion of mercaptan always occurred to some extent. solid shortly began to precipitate from solution. The emphasized importance of solvent polarity This material was assigned the structure VI arises from the competitive reaction between the (n"4) based on elemental analysis, iodometric amine and the relatively weakly nucleophilic titration and infrared spectroscopy. It exhibited a thiois formed during the reaction for the ethylene strong carbonyl band, at 5.92 p , and a weak--SH sulfide. An amine is highly competitive only in band, a t 3.92 y, in agreement with spectral data of nonpolar, nonionizing solvents where the strongly
5114
REYNOLDS, FIELDS, AND JOHNSON
nucleophilic mercaptide ion is not favored. I n solvents conducive to mercaptide-ion formation, mercaptoethylation of mercaptan, Le., oligomer formation, is the predominant, if not exclusive, course of reaction. By considering the reaction scheme of Chart I,
26
in which the initial attack of the amine (Step 1) is by far the slowest step in the over-all reaction. The carbamate IV (R2N- = (CH&CNH-) is always in very low concentration since it is decomposing to ethylene sulfide (11) (Step 2) at a much faster rate than it is being formed, Le.,
74
J
VOL.
fast
RzNCOzCHZCH2SH
IV -R2NH
t iR2NH
+
[ R ~ N C O ~ C H ~ C H ~ SR@ ~ ~ H ~ I VI1
I
Step 2
slow (-2RzNH. Cop)
R'SCOz(CHZCH2S)nH
(-RISK,RzNH, COB) VI1
+
[ R S C O ~ C H ~ C H ~ SR~ ~ N H 1 ~ IX
RzNCHzCHzSH
/
@
1 "
S R' H
- 'c
HO'
r31L
R~NCO~(CHZCHZS) zH XI
I
Where R S I i
= 111, I\',
ir,VI, X
CHART I
explanations and predictions of the structures of rate of Step 1
