Dimethyl Dithiocarbamates as Intermediates in the Preparation of

10 Dialkylhydroxybenzyl-N,N-Dimethyl Dithiocarbamates as Intermediates in the

Downloaded by UNIV OF SYDNEY on May 3, 2015 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0085.ch010

Preparation of Phenolic Polymer Stabilizers FRANCIS X . O'SHEA UniRoyal Chemical, Naugatuck, Conn. 06771

3,5-Dialkyl-4-hydroxybenzyl-N,N-dimethyldithiocarbamates, readily prepared by reaction of 2,6-dialkylphenols with formaldehyde, dimethylamine, and carbon disulfide, serve as useful intermediates in preparing substituted, sterically hindered phenols which are effective polymer stabilizers. These intermediates react with nucleophiles—alcohols, mercaptans, and amines—in the presence of sodium hydroxide to replace the dithiocarbamate group. The reaction apparently proceeds by generation of a transient 2,6-dialkylquinonemethide intermediate and its subsequent reaction with the nucleophile. 2-Hydroxy-3,5-dialkylbenzyl-N,N-dimethyl dithiocarbamates and 2-hydroxy-3,5-dialkylbenzyl 2-benzothiazolyl sulfides, prepared from 2,4-dialkylphenols, undergo similar nucleophilic reactions to give isomeric compounds through 4,6-dialkylquinone methide intermediates. The products derived from reaction of these latter intermediates with dimercaptans are particularly effective polymer stabilizers.

C t e r i c a l l y hindered phenols are used widely today as effective non^ discoloring, nonstaining antioxidants for hydrocarbon systems, par ticularly rubbers and plastics. These compounds may be grouped i n two broad classifications—monocyclic phenols represented by a compound such as 2,6-di-ferf-butyl-4-methylphenol ( I ) and polycyclic phenols such as 2,2'-methylenebis(4-methyl-6-ieri-butylphenol) ( I I ) . (Throughout this chapter, X represents terf-butyl). 126 In Stabilization of Polymers and Stabilizer Processes; Platzer, N.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

10.

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127

Phenolic Polymer Stabilizers OH

OH

OH

II

I


CH

3

3

Monocyclic phenols generally afford excellent nondiscoloring proper ties and good activity against aging at ambient temperature. These com pounds are volatile, however, and generally provide poor protection at elevated temperatures. The polycyclic phenols, being somewhat more effective and relatively nonvolatile, afford good protection at both ambient and elevated tem peratures. This improvement in activity, however, is generally accom panied by a sacrifice in color. For methylenebisphenols the discoloration may be caused by oxidation products of the antioxidant in which a highly conjugated system is developed. For example, Kharasch and Joshi (7) found that a purple color developed when oxygen was bubbled through an alkaline alcoholic solution of 4,4'-methylenebis ( 2,6-di-ter£-butylphenol), III, a commercial antioxidant. They showed that this color was caused by the anion, V , of a quinonemethide type oxidation product, IV, of the phenol.

III

IV

V They synthesized I V independently and found it to be bright yellow. W e have found that rubber samples containing 4,4'-methylenebis ( 2,6-diferi-butylphenol ) tend to turn yellow on aging and that, in some cases, films laid from high p H latexes have turned purple on heat aging. It is also known that 2,2 -methylenebis ( 4-methyl-6-terf-butylphenol ) II, tends to give a pink discoloration in certain latex film systems. Once again, the possibility of oxidation of this material and subsequent ioniza tion to produce the ion V I may account for this discoloration. /

In Stabilization of Polymers and Stabilizer Processes; Platzer, N.; Advances in Chemistry; American Chemical Society: Washington, DC, 1968.

128

STABILIZATION OF POLYMERS A N D STABILIZER PROCESSES


VI Several years ago we set out to synthesize new nondiscoloring phe nolic antioxidants with the objective of combining the excellent color of monocyclic phenols with the activity of polycyclic phenols. W e hoped to accomplish this by preparing polycyclic phenols in which the linkages between rings were designed so as to prevent the development of con jugation between the rings. Initially, we had two secondary objectives: (1) to investigate the use of recently available 2,6-dialkylphenols as starting materials, and (2) to investigate the effect of sulfur atoms on antioxidant activity by incorporating sulfur in the linkages. The first series of compounds to be developed in our program were bis (3,5-dialkyl-4-hydroxybenzyl) sulfides, V I I . Compounds of this type were prepared by a unique reaction of 2,6-dialkylphenols with formalde hyde and sodium sulfide which we have reported (13, 18). OH

A t that time we postulated a mechanism involving methylolation of the phenol followed by dehydration to produce a transient quinonemethide intermediate, VIII. Nucleophilic attack on such an intermediate by Ο

CH

2

VIII


10.

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129

Phenolic Polymer Stabilizers


sulfide ion would, in two steps, generate the product. The existence of quinonemethide intermediates is well substantiated in the literature (3, 5, 9). The reaction of nucleophiles with compounds capable of generat ing quinonemethide intermediates has also been described by Gardner (4, 10) and others ( I , 2, 6, 9, 23, 24). When this reaction was carried out on 2,6-di-ter£-butylphenol, bis( 3,5-di-fer^butyl-4-hydroxybenzyl ) sulfide was produced in 90% yield. However, 2-methyl-6-tert-butylphenol gave only a 64% yield of the ex pected sulfide compound, IX, along with a 26% yield of 4,4'-methylenebis(2-methyl-6-terf-butyl phenol), X .

The by-product in this reaction apparently arises from reaction of the quinonemethide intermediate with the as yet unreacted parent phenol. Since initial evaluations showed bis ( 3-methyl-4-hydroxy-5-£ert-butylbenzyl) sulfide, IX, to be a promising antioxidant, we sought an improved method for its synthesis. A n obvious way to avoid methylenebisphenol formation would be to generate the quinonemethide in the absence of any unreacted parent phenol. Therefore, a suitable intermediate was sought. The preparation of chloromethyl and hydroxymethyl intermediates was ruled out as being difficult to achieve in good yields by the methods then known. The use of Mannich base methiodides (4) was considered uneconomical. Our answer came in the form of an intermediate which had been prepared for another purpose. Turner (25) has shown that 2,6-di-tert-butylphenol and 2-methyl-6ierf-butylphenol will react with formaldehyde and a dialkylafftine in excess carbon disulfide to produce 3,5-dialkyl-4-hydroxybenzyl-N,Ndialkyldithiocarbamates, X I . OH

OH XI CH SCNR R 2

3

4

W e have also applied this reaction to 2,6-dialkylphenols including less hindered phenols such as 2,6-dimethylphenol. For all compounds studied


130


the reaction takes place under very mild conditions—e.g., in refluxing methanol. The yields of such compounds are generally in excess of 90%. Kulka (8) has reported that benzyl-N,N-dimethyldithiocarbamate can be hydrolyzed with alkali to benzyl mercaptan. S /7-Λ II //_\VCH SCN(CH )


2

3

KOH 2

/ΓΛ ^_\VCH SH

+

2

When we attempted to hydrolyze 3,5-di-terf-butyl-4-hydroxybenzyl-N,Ndimethyldithiocarbamate with sodium hydroxide in methanol, the reaction produced 3,5-di-£eri-butyl-4-hydroxybenzyl methyl ether, X I I , instead of the corresponding benzyl mercaptan. OH

OH

It is reasonable to postulate the formation of the product through the quinonemethide intermediate. OH NaOH

S II NaSCN(CH ) 3

CH SCN(CH ) 2

3

2

OH 2

+

"0* CH

θ

CH OH 3

2

Since these dithiocarbamates appeared to be suitable compounds for generating quinonemethide intermediates, we investigated their reaction with sodium sulfide. It was found that 3,5-di-teri-butyl-4-hydroxybenzylΝ,Ν-dimethyldithiocarbamate reacts with sodium sulfide to produce bis (3,5-di-ieri-butyl-4-hydroxybenzyl) sulfide, XIII, in essentially quanti tative yield (20).


10.

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131


2 CH SCN(CH ) 2

3

+


XIII

2

S II 2 NaSCN(CH ) 3

2

This reaction sequence was applied to 2-methyl-6-£erf-butylphenol, and it was found that bis ( 3-methyl-4-hydroxy-5-ierf-butylbenzyl ) sulfide can be produced i n an over-all yield greater than 90% i n pilot-plant scale reactions. Since the reaction mother liquor, after removal of the product, contains sodium dimethyldithiocarbamate, it can be recycled and neu tralized, thus regenerating carbon disulfide and dimethylamine for reuse in preparing the intermediate. In essence, therefore, the over-all reaction consumes only 2,6-dialkylphenol, formaldehyde, and sodium sulfide. The scope of this reaction sequence was demonstrated by preparing bis (3,5dialkyl-4-hydroxybenzyl) sulfides from a number of phenols ranging from 2,6-dimethylphenol to 2,6-di-tert-butylphenol (20). The ease with which the dithiocarbamate intermediates can be pre pared prompted further study of their utilization in preparing potential antioxidants by reaction with other nucleophiles. In addition to their reaction with alcohols to produce 3,5-dialkyl-4-hydroxybenzyl alkyl ethers (22) we found that the intermediates react with secondary amines to give 3,5-dialkyl-4-hydroxybenzylamines, X I V , and with primary amines to give bis (3,5-dialkyl-4-hydroxybenzyl) amines, X V , (19).


132


Although the compounds in which R " is alkyl can be prepared more readily by the simple Mannich reaction, this sequence is useful for nitro gen compounds such as aromatic amines, hydrazines, and hydroxylamine which do not ordinarily enter into the Mannich reaction. These inter mediates react cleanly with mercaptans (21) and with dimercaptans (17) to produce compounds of the type X V I and X V I I .


OH

OH

R'

XVII

R'

The latter compounds, para-linked bisphenols derived from dimercaptans, have proved to be of particular interest since they exhibit excellent anti oxidant activity along with very good nondiscoloring properties. For example, Table I shows the results of a screening test in cis-polyisoprene in which films of the rubber containing 1 % of the antioxidants were deposited from solution on salt plates and aged at 130°C. in a circulating air oven. Stability of the films is determined by the time to development of a carbonyl band in the infrared spectra. The results show a significant advantage in activity for l,2-bis(3,5-di-feri-butyl-4-hydroxybenzylthio)ethane over 4,4'-methylenebis ( 2,6-di-tert-butylphenol ) and 2,6-di-f erf butyl-4-methylphenol. Of the three antioxidants 2,6-di-ier/-butyl-4-methylphenol was the least effective discoloring agent after one-hour oven aging at 130°C. and Table I.

Aging of ris-Polyisoprene

Antioxidant 2,6-Di-ierf-butyl-4-methylphenol 4,4'-Methylenebis ( 2,6-di-f erfbutylphenol 1,2-Bis ( 3,5-di-terf-butyl-4hydroxybenzylthio ) ethane

Time to Carbonyl Development, hours 0.5 4 9



10.

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133

8-hour sunlamp aging of bulk samples of the rubber. 4,4'-Methylenebis( 2,6-di-terf-butylphenol ) was the most effective discoloring agent, impart ing a yellow color in both tests. l,2-Bis(3,5-di-ieri-butyl-4-hydroxybenzylthio) ethane was intermediate in discoloration properties, being sig nificantly better than the methylenebis compound. Since compounds of the type X V I I have shown comparable activity in a number of systems including cis-polybutadiene, styrene-butadiene rubber, and ethylene-propylene rubber, they have some commercial promise, and development work on these compounds is continuing. Nevertheless, they are not completely nondiscoloring, and in certain applications, particularly carboxylated styrene-butadiene latex films, yel low discoloration caused by the antioxidant is a serious drawback. W e therefore turned our attention to ortho-linked compounds derived from 2,4-dialky lphenols. Unfortunately, Mannich bases derived from 2,4-dialkylphenols do not react readily with carbon disulfide. Under forcing conditions—e.g., in a high boiling solvent with slow addition of carbon disulfide or under pressure in an autoclave—dithiocarbamates are formed but in poorer conversions than from 2,6-dialkylphenols. OH >O^CH N(CH ) 2

1

,

3

1

CH

f,

OH 2

+CS

125-135°C. 2

^k^CH SCN(CH ) 2

1

3

2

11

6 HRS.

3

The poor conversion may be accounted for, at least in part, by the apparent reversibility of this reaction. This was demonstrated by heating 2-hydroxy-3-ierf-butyl-5-methylbenzyl-N,N-dimethyl dithiocarbamate in diethylene glycol dimethyl ether whereupon the Mannich base X V I I I was formed. OH

,,

OH

S

CH SCN (CH ) 2

8

2

130°C. 4 HRS.

* ^ ! ^ C H Ϊ

2

N (CH ) 3

2

XVIII

Despite this difficulty, dithiocarbamate intermediates were prepared from a variety of 2,4-dialkylphenols. These compounds are described in Table II. It was demonstrated ( 20 ) that these intermediates react with sodium sulfide in a manner analogous to the dithiocarbamates derived from 2,6-


134


Table II.

Dithiocarbamate Intermediates

OH R

^J-CH SCN(CH ) 2

3

R'

2

R

—CH —CH —C(CH ) —CH -C(CH ) -C(CH ) —CH —C H —CH —C(CH ) -C(CH ) —CH —C (CH ) φ —CH " Based on intermediate Mannich base. 2,2,4,4-Tetramethylbutyl. 3

3

1 7

3


% Yield"

127-128 97-98 208-209 111-112 114-116 109-111 131-133

22 55 50 48 47 82 34

3

3

8

m.p., °C. 3

3

8

8

6

8

3

2

C

3

8

8

8

3

8

c

2

c

b

dialkylphenols. For example, 2-hydroxy-3-ieri-butyl-5-methylbenzyl-2V,Ndimethyldithiocarbamate was converted to bis ( 2-hydroxy-3-ferf-butyl-5methylbenzyl) sulfide, X I X , in 9 3 % yield. OH CH SCN(CH ) 2

3

OH CH SCH *

2

2

+ Na S

2

2

XIX Another useful class of intermediates i n this series was found to be the 2-hydroxy-3,5-dialkylbenzyl 2-benzothiazolyl sulfides, some of which are described in Table III. John B i l l of our laboratories had previously found that 2-terf-butyl-p-cresol w i l l react with formaldehyde and mercaptobenzothiazole under acid conditions to produce 2-hydroxy-3-£erfbutyl-5-methylbenzyl 2-benzothiazolyl sulfide, X X , i n 70% yield. OH + CHoO + CH

C-SH

Η —

Φ

3

M

CH S 2

C

XX


10.

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135

Prepared from 2,4-Dialkylphenols


Calculated

Found

c

H

Ν

S

c

H

Ν

S

56.43 60.56 63.67 64.54 66.81 60.56 66.81

6.71 7.79 8.61 8.84 7.01 7.79 7.01

5.49 4.71 4.13 3.96 3.90 4.71 3.90

25.11 21.56 18.89 18.14 17.84 21.56 17.84

56.65 60.70 63.87 64.62 66.82 60.70 66.69

6.84 7.70 8.51 8.75 7.01 7.71 7.29

5.72 4.60 3.95 3.88 3.80 4.82 3.70

24.94 21.68 19.22 18.15 18.04 21.53 17.77

c

α,α-Dimethylbenzyl.

Since this compound contains a sequence of atoms quite similar to the dithiocarbamate, we postulated that it could react i n the same way. This was demonstrated (14) by the reaction of X X with sodium sulfide to produce bis ( 2-hydroxy-3-feri-butyl-5-methylbenzyl ) sulfide in 94% yield. OH ' CH SC ,

2

Κ

IJ

2

^ S ^ ^

+Na S 2

+ 2 NaSC •

3D

After isolating the product, the mercaptobenzothiazole can be recovered in such reactions by acidification of the mother liquor. Both of these types of intermediates react with nucleophiles in reac tions which presumably proceed through an o-quinonemethide inter mediate of the type X X I . Ο

XXI


136


Table III.

R

R' -CH -C(CH ) —CH, —CH,

-C(CH ) -C(CH,)„ 3

3

3

—C H ° —0(ΟΗ ) φ 8

8

17

3


2-Hydroxy-3,5-dialkylbenzyl

2

&

8

m.p., °C.

% Yield

164-165 153-154 102-103 141-143

72 28 83 33

" 2,2,4,4-Tetramethylbutyl. For example, they react with alcohols (15, 22) to produce 2-hydroxy-3,5dialkylbenzyl alkyl ethers, X X I I . OH R

YV

OH NaOH

C I Î 2 Z

ψ

+ R

"°

H

R

2

ψ

m

R'

Z =

N^YCH OR" XXII R'

~ C JTjj SC

S

O

—

R

S C N

(CH ) 3

2

They react with secondary amines (11, 19) to produce hydroxybenzylamines of the type X X I I I . OH R V ^ V C H

TCjT

OH 2

Z

NaOH

+ R " N H

2

R N ^ S ^ C H ^ "

— •

ICjJ

XXIII

Reaction with primary amines proceeds differently from the 2,6-dialkylphenol case, generally stopping at the 1:1 adduct, X X I V . OH + R"NH

NaOH

+

Rv^^^v^CHgNHR" XXIV R'


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137

Phenolic Polymer Stabilizers OH

2-Benzothiazolyl Sulfides

κ


Calculated

Found

c

Η

Ν

S

C

H

N

S

66.43 68.53 69.13 71.07

6.16 7.06 7.31 5.72

4.08 3.63 3.51 3.45

18.67 16.63 16.05 15.81

66.65 68.54 69.10 71.36

6.14 6.93 7.28 5.66

4.06 3.83 3.79 3.44

18.56 16.68 16.24 15.85

h

α,α-Dimethylbenzyl.

This is probably because intramolecular hydrogen bonding with the phenolic hydroxyl in the 1:1 adduct reduces the nucleophilic character of the amine nitrogen sufficiently to render it unreactive in a second step. Reaction with mercaptans and dimercaptans (12, 16, 21) proceeds normally to produce the 2-hydroxybenzyl thioether compounds X X V and X X V I .

Of all the compounds prepared i n this work, these latter compounds, the ortho-linked compounds derived from dimercaptans, come the closest to realizing our objective of structures with the activity of a bisphenol and the nondiscoloring characteristics of a monocyclic phenol. For example, Table I V shows some results in cw-polybutadiene which dem onstrate that a compound of the type X X V I is superior as a heat stabilizer to both 2,6-di-ferf-butyl-4-methylphenol and 2,2'-methylenebis ( 4-methyl6-ter£-butylphenol ) in both activity and color. Screening results i n cis-


138


Table IV.

c/s-Polybutadiene Aged for 150 Hours at 100°C. % Gel

Mooney Viscosity*

Color Rating

2,6-Di-ter*-butyl-4methylphenol (0.9 parts)

60

60

2

2,2'-Methylenebis(4methyl-6-terf-butylphenol (0.5 parts)

33

50

3

nil

37

1

Antioxidant

β,β'-Bis ( 2-hydroxy-3-f ertbutyl-5-methylbenzylthio ) Downloaded by UNIV OF SYDNEY on May 3, 2015 | http://pubs.acs.org Publication Date: June 1, 1968 | doi: 10.1021/ba-1968-0085.ch010

diethyl ether (0.5 parts) β

Unaged Mooney viscosity was 33.

polyisoprene using the carbonyl development test previously described are shown in Table V and again demonstrate the superiority in activity of these new compounds. After one-hour oven aging at 130 °C. and 8-hour sunlamp aging of bulk samples of the rubber, ^,/S -bis(2-hydroxy3-ierf-butyl-5-methylbenzylthio) diethyl ether was found to be equivalent to 2,6-di-ferf-butyl-4-methylphenol in nondiscoloring characteristics and far superior to 2,2 -methylenebis(4-methyl-6-feri-butylphenol) which turned the rubber yellow-brown. These ortho-linked compounds have also shown considerable promise in emulsion and solution SBR and are particularly valuable i n carboxylated SBR latex films combining good activity with excellent color—i.e., no pinking. The results of resinification tests on films laid from cafboxylated SBR latex shown i n Table V I demonstrate that the polycyclic nature of these compounds contributes significantly to their stabilizing activity, monocyclic compounds of similar structure being markedly poorer in activity. Although having excellent color in most systems, the antioxidant activity of these compounds i n certain systems such as polypropylene and ethylene-propylene rubber has been only fair. Possibly intramolecular hydrogen bonding between the phenolic hydroxyl and the benzylic sulfur /

/

Table V .

Polyisoprene Heat Aged at 130°C. Time for Carbonyl

Antioxidant, 1 % 2,6-Di-ierf-butyl-4-methylphenol 2,2'-Methylenebis ( 4-methyl-6-terfbutylphenol) β,β'-Bis ( 2-hydraxy-3-ter*-butyl-5methylbenzylthio) diethyl ether

Development, hours 0.5 3 10


10.

O'SHEA

Table VI

Carboxylated SBR Latex Films Aged at 2 7 0 ° F .

Antioxidant 2-Hydroxy-3-feri-butyl-5-methylbenzyl β-hydroxyethyl sulfide β,β'-Bis ( 2-hydroxy-3-ierf-butyl-5methylbenzylthio) diethyl ether


139


Time to Resinify, hours 44 587

atom affects hydrogen abstraction processes i n these systems. The gen erally poor level of activity found for othro-linked compounds containing benzylic oxygen or nitrogen atoms may be the result of the stronger hydrogen bonding expected. Therefore, although our original goal has been accomplished to a large extent i n this work, we believe that the optimum result has yet to be achieved. Perhaps further work on bisphenol linkages w i l l afford greater improvements in this area.

Literature Cited (1) Atkinson, R. O., J. Chem. Soc. 1954, 1329. (2) Fields, D. L., Miller, J. B., Reynolds, D. D., J. Org. Chem. 30, 3962 (1965). (3) Filar, L. J., Winstein, S., Tetrahedron Letters 25, 9 (1960). (4) Gardner, P. D., Rafsanjani, H. S., Rand, L., J. Am. Chem. Soc. 81, 3364 (1959). (5) Gardner, P. D., Sarrafizadeh, H., Brandon, R. & R. L., J. Am. Chem. Soc. 81, 5515 (1959). (6) Hellman, H., Pohlmann, J. L. W., Ann. 642, 40 (1961). (7) Kharasch, M. S., Joshi, B. S., J. Org. Chem. 22, 1435 (1957). (8) Kulka, M., Can. J. Chem. 34, 1093 (1956). (9) McClure, J. D., J. Org. Chem. 27, 2365 (1962). (10) Merijan, Α., Gardner, P. D., J. Org. Chem. 30, 3965 (1965). (11) O'Shea, F. X., U. S. Patent 3,219,701 (Nov. 23, 1965). (12) Ibid., 3,260,757 (July 12, 1966). (13) Ibid., 3,272,869 (Sept. 13, 1966). (14) Ibid., 3,281,473 (Oct. 25, 1966). (15) Ibid., 3,291,841 (Dec. 13, 1966). (16) Ibid., 3,299,147 (Jan. 17, 1967). (17) Ibid., 3,310,587 (March 21, 1967). (18) O'Shea, F. X., Root, F. B., "Abstracts of Papers," 140th Meeting, ACS, Sept. 1961, 49Q. (19) O'Shea, F. X., Root, F. B., U. S. Patent 3,219,700 (Nov. 23, 1965). (20) Ibid., 3,260,756 (July 12, 1966). (21) Ibid., 3,260,758 (July 12, 1966). (22) Ibid., 3,291,842 (Dec. 13, 1966). (23) Poppelsdorf, F., Holt, S. J., J. Chem. Soc. 1954, 4094. (24) Smith, L. I., Horner, Jr., J. W., J. Am. Chem. Soc. 60, 676 (1938). (25) Turner, J. M., U. S. Patent 3,117,947 (Jan. 14, 1964). RECEIVED November 30, 1966.


Dimethyl Dithiocarbamates as Intermediates in the Preparation of

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