Di-tert-alkyl nitroxyl radicals. Synthesis, physical properties and applications as inhibitors of vinyl polmerization at elevated temperatures

Di-ferr-alkyl Nitroxyl Radicals. Synthesis, Physical Properties and Applications as

Inhibitors of Vinyl Polmerization at Elevated Temperatures

George Sosnovsky3, Mikolaj Jawdosiuk3, and J. Michael Clumpnerb

a Department of Chemistry, University of Wisconsin-Milwaukee, P. O. Box 413, Milwaukee,

WI 53201

b Nova Molecular Technologies, Inc., Janesville, WI 53545, USA

Reprint requests to Prof. G. Sosnovsky

Z. Naturforsch. 55b, 109-126 (2000); received July 26, 1999

Hydrogenation of Alkenes and Alkynes, Di-ter/-alkyl Amines, Di-rerr-alkyl Nitroxyl

(Aminoxyl) Radicals, Polymerization Inhibitors

Stable di-rm-alkylnitroxyl radicals, terf-butyl-fert-pentylnitroxyl (4a), di-rerf-pentylnitroxyl

(4b) and rm-octyl-tert-pentylnitroxyl (4c), the homologs of di-rm-butylnitroxyl (1), were

synthesized from tert-alkyl amines 7a-c via the 3-rm-alkylamino-3-methyl-l-butynes 8a-c. Oxi-

dation of 8a,b with hydrogen peroxide lead to relatively unstable N-rm-alkyl-N-(l,l-dimethyl-

prop-2-ynyl)nitroxyl radicals 15a,b. The thermal stability, vapor pressure data, ultraviolet,

visible and electron paramagnetic resonance spectra of 4a-c were recorded. The radicals were

explored as potential inhibitors of unwanted alkene polymerization reactions at elevated

temperatures, in comparison with the aliphatic di-terr-butyl nitroxyl (1), the alicyclic nitroxyl

radicals 2,2,6,6-tetramethylpiperidin-l-oxyl (2) and 4-hydroxy-2,2,6,6-tetramethyl-piperidin-

1-oxyl (3), some commercial polymerization inhibitors, such as diethyl-hydroxylamine (Penn-

stop, 16), ammonium salt of N-hydroxy-N-nitrosobenzenamine (Cupferron, 17 ), bis(2,2,6,6-

tetramethyl-4-piperidyl) sebacate (Tinuvin 770, 18), and the well-known spin traps 2-methyl-

2-nitrosopropane (19) and rerr-butylhydroxylamine (20).

Introduction

The nitrogen-oxygen bond in the nitroxyl mole-

cules is essentially a one-and-a-half bond. Accord-

ing to semi-empirical calculations [27] using the

SUHF INDO method, 73% of the spin density is

localized on the oxygen atom. The stability of nit-

roxyl radicals is mainly determined by the extent

they can undergo disproportionation reactions.

Thus, the dialkyl nitroxyls, which have either pri-

mary or secondary alkyl groups at the adjacent po-

sitions to the nitrogen atom, are rather unstable

[19] and can undergo disproportionation [19] to

give hydroxylamines and nitrones (Scheme 1).

In contrast, the inherent high stability of steri-

cally hindered nitroxyls with tertiary carbon atoms

at the adjacent position to the nitrogen atom is

attributable to the absence of such a mechanism

for a degradation. For example, the following sta-

ble nitroxyl radicals, the aliphatic acyclic di-tert-

butyl nitroxyl radical (DTBN, 1) and the heterocy-

Organic nitroxyl radicals (aminoxyls, also called

by a misnomer nitroxides) have been known since

the beginning of this century [1-3]. This esoteric

class of compounds received little attention until

the early sixties when the aliphatic di-rerf-butylnit-

roxyl radical [4-7] (1) and the series of stable het-

eroalicyclic nitroxyl radicals with piperidine, pyr-

roline and pyrrolidine rings [1-3] became readily

available. Since that time nitroxyl radicals have

been extensively explored in organic chemistry,

medicinal chemistry, biology and industry. Thus, in

addition to several monographs, [1-3,8-16] there

have been published more than 2 0 0 review articles

covering almost every possible theoretical and

practical aspect related to these subjects [17-26],

Nitroxyl radicals can be represented by two res-

onance structures A o B or simply depicted by

the structure C.

R3C- j^ C R3

FUC *®CR'

R3C^N' CR3

N

i•

O

00

9

A

B

D

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G. Sosnovsky et al. ■Di-fm-alkyl Nitroxyl Radicals

1 1 0

©

2R2HC-N-CR3 ------

►

R2H C - N- C R 3

+

R2C=N-CR3

O©

o

------------------------------- OH

Scheme 1

compound (1 ) can be prepared from 2 -methyl-2 -

nitropropane by the reduction either with sodium

metal in 1,2-dimethoxyethane [4-7] or with tert-

butylmagnesium chloride in ether [41], or by the

reaction of terf-butylmagnesium chloride and N-

nitrosodiphenylamine [42]. These methods are un-

suitable for economical large-scale preparations,

including industrial production.

The preparation of the nitroxyl 2 was first re-

ported [43] in 1962. The synthesis of the parent

2 ,2 ,6 ,6 -tetramethylpiperidine from 2 ,2 ,6 ,6 -tetra-

methyl-4-piperidone (triacetonamine) using the

Wolff-Kishner reaction is also not amenable to

large-scale industrial synthesis. In contrast, the

synthesis of 3 can be economically accomplished

from triacetonamine on an industrial scale.

clic 2,2,6,6 -tetramethylpiperidin-l-oxyl (TEMPO,

2 ) are stabilized by two adjacent tert-alkyl groups.

An equally stable but more hydrophilic radical

than 1 and 2 is the 4-hydroxy-2,2,6,6-tetramethyl-

piperidin-l-oxyl (3).

C H 3 C H 3

H3C-

;C

C 'C H ,

H3C

u

o

CH3

J

R = H

R = OH

These compounds are commercially available,

and can be stored indefinitely at room temper-

ature. In certain applications, such as inhibition of

polymerization at elevated temperatures [28-32]

and catalytic autoxidation of hydrocarbons [33-

35], the thermal stability of nitroxyl radicals is crit-

ical. According to literature data [33] nitroxyls 1

and 2 should have a certain degree of stability at

elevated temperatures. Thus, DTBN (1) can be

distilled without decomposition at 74-75 °C/34

torr [4], however, the half-life time of 1 in tert-

butylbenzene at 130 °C, both in oxygen and in an

argon atmosphere, was found [33] to be only

13.4 minutes. In the case of TEMPO (2), no de-

tectable decomposition was observed [33] at

130 °C for at least 200 minutes.

The di-rm-butyl nitroxyl radical (1) was first de-

scribed in 1961 [4,5] and then in greater detail in

1964 [6,7], Since that time 1 has been extensively

studied in various areas, e.g. stabilization of chlo-

roprene against polymerization [36,37], oxidative

degradation [37], stabilization of acrylic acid [38],

stabilization of anaerobic sealant compositions

containing polymerizable acrylate ester monomers

[39], the thermal stability [33,34], photolysis [40],

catalytic inhibition of hydrocarbon autoxidation

[33,34], and inhibition of vinyl polymerization

[28-32,36-39], to mention only a few areas. This

While the number of literature references to 1

and 2 exceeds several hundred for each com-

pound, the homologs of 1 , such as tert-bntyl-tert-

pentylnitroxyl [3,44] (4a) and di-terf-pentyl-nit-

roxyl (4b) [3] radicals have been mentioned very

briefly, and subsequently forgotten for more than

twenty-five years. Recently, a few di-tert-alkyl nit-

roxyl radicals enriched with deuterium and car-

bon-12 isotopes, such as perdeuterated DTBN,

12C-perdeuterated DTBN and higher homologs,

containing 9, 10 and 11 carbons, were reported

[45]. In order to better understand this mysterious

lack of interest it was decided to develop a general

synthetic method for these compounds, and to in-

vestigate their properties, in particular, their po-

tential as inhibitors of vinyl polymerizations. Al-

though over the years, the inhibition of vinyl

polymerizations at ambient temperatures has been

largely resolved [46-48] and the nitroxyl radicals

were shown [36-39] to be effective inhibitors, pre-

vention of polymerizations at elevated temper-

atures [28-32] remains a challenging task. In the

present study, an attempt was made to explore the

potential for applications in this particular area of

five aliphatic and two heterocyclic nitroxyl radicals

as compared to several industrial inhibitors.

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G. Sosnovsky et al. • Di-tert-alkyl Nitroxyl Radicals

1 1 1

Results and Discussion

ranged between 37-39% and were not further op-

timized. Much better results with 50-60% yields

The DTBN (1) homologs 4a-c can be synthe- were achieved when the reaction was carried out

sized from the corresponding amines 9a-c which at 20-25 °C for several days up to two weeks using

are obtained by the condensation of 2 -chloro-2 - an excess of rm-alkylamine as hydrogen chloride

methyl-l-butyne (6) with ter/-alkylamines 7a-c, fol- scavenger, as described in a patent [53]. The high-

lowed by the catalytic hydrogenation of the inter- est reproducible yields of 8a were obtained with

mediate acetylenic amines 8a-c to give 9a-c copper bronze-cuprous chloride as the catalyst.

(Scheme 2).

The reaction time was shortened from two weeks

to a few hours. This catalytic variant was particu-

This route leading to hindered secondary

amines was described in the literature [49], how- larly important in the case of the lipophilic tert-

ever, the experimental details were very sketchy.

An efficient conversion of 2-methyl-3-butyn-2-

octylamine 7c. Thus, the mixture of 6 and 7c, kept

at 23-25 °C for two months, gave 29% of 8c.

ol (5) to 6 was reported in the literature [50], At When a two-phase mixture containing 6, 7c and

temperatures below 5 °C, this procedure was used water was kept at 15-18 °C for four weeks, the

[51] to prepare 6 in kilogram quantities with a

yield exceeding 90%.

conversion was lower than 20%. An addition of

copper powder and copper (I) iodide to this reac-

The preparation of sterically hindered amines tion mixture instantly induced an exothermic reac-

similar to 9a-c, and the challenges encountered en tion, which increased the temperature inside the

route were discussed in a paper [52] dealing with flask to 75 °C, and brought the reaction to comple-

highly branched secondary amines. In the present tion. The optimization of the reaction conditions

study initial attempts in condensation of 6 with an resulted in a 60-80% yield for 8a-c. This method

was amenable to the preparation of kilogram

batch sizes of 8a-c.

The catalytic hydrogenation of C,C triple bonds

to the corresponding alkanes is, in general, a use-

excess of aqueous 7a were equally erratic. The ad-

dition of either sodium carbonate, potassium car-

bonate, or sodium hydroxide as the base resulted

in low yields of 10-40%. Because of limited sup-

plies and a high cost of te/Y-pentylamine (7b), the ful transformation. However, in several cases de-

condensation with 6 was conducted in the pres- scribed in the literature [54], this reaction can be

ence of an aqueous sodium carbonate solution as accompanied by highly undesirable hydrogeno-

hydrogen chloride scavenger. The yields of 8b

lysis reactions, which destroy the expected pro-

CH3

H O C -C -C I

? H3

HCI

HC-C-C-OH -----

CH3

*

C H 3

6

5

? h3

:

H 3C

c h 3

H3C

C H 3

R C NH2 + 6

-----

►

R -C -N -C -O C H — ^

R -C -N -C -C H 2CH3

CH3------------------------------H3C H CH3

"

H3C H c h 3

7a-c

8a-c

9a-c

a R = CH3

b R = CH3CH2

1

C0PPer bronze + Cu2Cl2

c

r = ( c h 3)3c c h 2

" various cata|ysts

Scheme 2

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G. Sosnovsky et al. ■Di-rm-alkyl Nitroxyl Radicals

1 1 2

ducts. Thus, the attempted hydrogenation of 1,4- of W-2 grade Raney nickel. The hydrogenations in

acetylenic glycols at low pressure in the presence the presence of more reactive grades of Raney

of palladium, platinum and Raney nickel catalysts nickel, such as W-4 and W-6 , resulted [52] in a

resulted in extensive hydrogenolysis [55]. An addi- more extensive hydrogenolysis. In this case, how-

tion of a small amount of either potassium or so- ever, the hydrogenolysis reaction could be sup-

dium hydroxide reduced the hydrogenolysis to a pressed [52] by the addition of an excess of potas-

negligible level [55]. Hydrogenation of 4-dieth- sium hydroxide to the W-2 catalyst. On the basis

of these examples, it is evident that the ordinary

ylaminohex-2-en-5-yne (10) in methanol in the

presence of an unspecified platinum catalyst re- hydrogenation reactions are by no means straight-

sulted [56] in an almost quantitative fission of the forward and predictable.

diethylamino group attached to a carbon atom po-

The preparation of hindered amines 9a and 9b

sitioned between two unsaturated centers by the hydrogenation of appropriate acetylenic

(Scheme 3). A lesser degree of fission of the car- compounds 8a and 8b has been reported in the

bon-nitrogen bond occurred in a cyclohexane solu- literature [3,49,52]. Thus, terr-butyl-/er/-pentylam-

tion.

Extensive hydrogenolysis also took place [57]

ine (8a) was obtained [49] in a 42% yield by the

hydrogenation of 3-te/Y-butylamino-3-methyl-l-

during a low pressure hydrogenation of 3-diethyl- butyne (8a) in the presence of a mildly active Ra-

amino-3-methyl-l-pentyne (11) in methanol in the ney nickel in ethanol, however, it was stated [49]

presence of Raney nickel to give a mixture of di- that the amount and activity of the nickel catalyst

are critical. A very rapid hydrogenation, especially

small yield of 3-diethylamino-3-methylpentane with the nickel catalyst, resulted [49] in excessive

(12) (Scheme 4). hydrogenolysis which was probably caused by a

Some saturated amines can be obtained [58] by rapid rise in temperature. Di-terf-pentylamine (9b)

ethylamine, 3-methylpentane, and only a very

was obtained [3] in a 52% yield by the hydrogena-

tion of 3-methyl-3-fcrr-pentylamino-l-butyne (8b),

the hydrogenation of the hydrochloride salts of

acetylenic amines in the presence of platinum ox-

ide. Although the products were isolated and posi- and in an 80% yield [52] by the hydrogenation of

N-bis(l,l-dimethyl-2-propynyl)amine (13) in abso-

tively identified [58], no yields were reported. The

hydrogenation of highly branched propargyl lute ethanol using a W-2 Raney nickel catalyst

amines and their hydrochloride salts in the pres- which was deactivated with potassium hydroxide.

ence of either of platinum or palladium catalysts The definition of mildly “active Raney nickel” [49]

is quite vague since there are, at least, seven

grades of Raney nickel, known as W-l through W-

in a variety of solvents gave [52] almost exclusively

the hydrogenolysis products. Somewhat better re-

sults of about 20% yields were reported [52] by 7 catalysts [59-64], These various grades of cata-

using low-pressure hydrogenation in the presence lysts are not commercially available and must be

H2/Pt

CH3OH

(CH3CH2)2NH

H3C-CH-CH-CH-C=CH

N(CH2CH3)2

10

Scheme 3

CH3

.

Ql_l3

h o c - c - c h 2c h 3

N(CH2CH3)2

11

c z

‘

o h

~

(ch 3c h 2)2nh

+

c h 3c h 2- c h - c h 2c h 3

CH3

+

c h 3c h 2- c - c h 2c h 3

3

N(CH2CH3)2

12

i = H2/Raney nickel

Scheme 4

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113

G. Sosnovsky et al. ■Di-rert-alkyl Nitroxyl Radicals

prepared each time under strictly controlled con- ter 225 minutes almost the entire alkyne 8a was

ditions.

converted to a mixture of the hydrogenation pro-

ducts 9a and 14a, rm-butylamine and a mixture

of unidentified hydrogenolysis products (Table I).

About 30% of the initial alkyne 8a underwent hy-

drogenolysis, as indicated by the amount of tert-

butylamine found in the reaction mixture. The ad-

dition of the second molecule of hydrogen gas was

much slower, and was accompanied by a much less

H 3C

C H 3

HC=C-C-N-C~C=CH

H3C H C H 3

13

In the present study, the hydrogenation experi- pronounced hydrogenolysis. The hydrogenolysis

ments with a commercial grade of Raney nickel, and cleavage of the C-N bond occurred at the un-

which was described by the supplier as analogous saturated side of the molecule to produce the tert-

to either Raney 28 or W-2, failed to produce any butylamine. The identity of the components was

significant amounts of the secondary amines. Thus,

established by GC/MS.

the hydrogenation of 8a in methanol produced

The reaction with the same Raney nickel cata-

only a small quantity of 9a. Instead, the reaction lyst failed in /7-pentane. Thus, the hydrogen gas

mixture contained mainly terr-butylamine, and

uptake was extremely slow, and after 6 days, with

other unidentified low-molecular-weight hydro- about 25% of conversion, the experiment was ter-

genolysis products. The hydrogenation of 8a in minated. The reaction in diethyl ether was some-

hexane in the presence of a 10% Pd/C catalyst re- what faster, however, the hydrogenation ceased at

sulted in the complete hydrogenolysis. Much bet- the intermediate stage of the alkene 14a with

ter results were obtained with a W-2 type Raney about a 60% yield.

nickel catalyst which was prepared from a com-

mercial aluminum-nickel alloy according to the lit- be amenable for the use of commercially available

erature procedure [62], and then partially deacti-

In an attempt to develop a method, which would

catalysts with predictable results, various combina-

vated by aging for several weeks under deionized tions of catalysts and solvents were tested. Thus,

water. In order to better understand the processes the 5% Ru/C catalyst in methanol was ineffective.

which occur during the hydrogenation reaction of

The 5% Ru/alumina catalyst in methanol had a

8a, the composition of the reaction mixture was higher activity, however, the conversion after 48

periodically analyzed by gas liquid chromatogra- hours was still incomplete, and in spite of a slow

phy and correlated with the volume of hydrogen reaction, the extent of hydrogenolysis was high.

gas consumption during this process (Table I).

The hydrogenation of 8a in methanol in a Parr

The hydrogenolysis of 8a to give some /m-butyl- shaker in the presence of a Raney cobalt catalyst

amine (TBA) ensued at the very onset of the pro- resulted in a 65% yield of 9a. Comparable results

cess. The intermediate alkene 14a (Scheme 5) were obtained in the temperature-controlled hy-

seemed to be more resistant to hydrogenolysis. Af-

drogenation of 8a using a 10% Pd/C catalyst in

Table I. Catalytic hydrogenation of 3-ferr-butylamino-3-methyl-l-butyne (8a) in ethanol in the presence of W-2

Raney nickel.

Hydrogen

uptake“

Time

[min]

Composition of reaction mixture11

[% of theory]

Alkvne 8a

Alkene 14a

tert- Butyl amine

Alkane 9a

weight %

molar%

weight %

molar%

weight %

molar%

weight%

40

60

150

225

720

25.0

33.0

55.0

86.0

105.0

68.0

59.6

31.3

0.7

65.6

56.5

28.6

0.6

23.0

29.3

49.6

51.6

7.5

21.8

27.4

44.7

43.3

6.2

3.3

4.3

7.6

26.2

67.0

3.1

4.0

6.7

21.3

54.0

5.2

6.8

11.5

21.5

25.6

9.5

12.2

20.1

34.5

40.0

0.0

a Volume of hydrogen gas consumed compared to the theoretical volume required to complete the conversion of

8a to 9a; b determined by GC.

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114

G. Sosnovsky et al. • Di-rm-alkyl Nitroxyl Radicals

tert-butyl methyl ether. In this case, the hydrogen-

olysis was suppressed to such an extent that in

large-scale experiments the reproducible yields of

9a were in the range of 60-72%. The hydrogena-

tion in a Parr hydrogenator, especially in a larger

5% Pd/C catalyst. When the process was termi-

nated after the first stage, relatively pure alkenes

14a-c, contaminated with small admixtures (less

than 3%) of 9a-c, were isolated.

Large-scale experiments in 300 ml and 4000 ml

2

1 glass vessel, generally gave lower yields which autoclaves equipped with cooling coils allowed the

were caused by a lack of internal and external maintenance of the reaction temperatures below

cooling. Thus, a series of hydrogenation experi- 30-35 °C, and resulted in higher yields of 65-80%

ments with 8c in tert-butyl methyl ether in the

presence of a 5% Pd/C catalyst resulted in low

10-32% yields of ter/-octyl-terf-pentylamine (9c),

and in high yields of 70-90% of the hydrogeno-

lysis product, i.e. terf-octylamine (7c). A moderate

improvement was accomplished by dividing the

hydrogenation process into two stages. The first

stage of the hydrogenation was carried out in tert-

butyl methyl ether in the presence of a Lindlar

catalyst, i.e. 5% palladium on calcium carbonate

poisoned with lead. Thus, the alkynes 8a-c were

of amines 9a-c.

The routine oxidation of 9a,b with a 30% aque-

ous hydrogen peroxide solution in methanol in the

presence of sodium tungstate gave [3] high yields

of nitroxyl radicals 4a and 4b, respectively, how-

ever, in the case of the lipophilic 9c, the oxidation

of the heterogenous reaction mixture failed to

produce the desired radical 4c. The oxidation of

9c with tert-butyl hydroperoxide in 1,2-dichloro-

ethane at 20-25 °C in the presence of either

Mo(CO

) 6 or M0 O3 catalyst [65] gave 4c in a

selectively converted to 14a-c (Scheme 5), then the 25% yield.

catalyst was filtered off and the second stage of

hydrogenation was conducted in the presence of a

The isolated 4c had a much lower thermal sta-

bility than those of compounds 1, 4a and 4b, and

the distillation even at a reduced pressure at 72-

75 °C/0.5 torr resulted in a partial decomposition.

All three radicals 4a-c are dark red-orange li-

quids with a strong terpene-like odor. Their color

is attributable to the absorbance in the visible

range at 464-465 nm. The ultraviolet and visible

spectra in n-heptane have two maxima, one at

464-465 nm in the visible region (e = 7.5-8) and

the other, a much stronger one, at 247-249 nm.

The electron paramagnetic resonance spectra

(EPR) spectra of all di-tert-alkylnitroxyl radicals 1

H 3C

C H 3

8a-c

R - C - N- C - C H =C H 2

H3C

H

C H 3

a R = CH3

b R = CH3CH2

c R = (CH3)3CCH2

14a-c

/' = Lindlar catalyst

Scheme 5

H O

H3?

9H3

9a,b

— 2 2

R -C -N -C -C H 2CH3

Na2W 04

H 3C

o

CH3

;H3

a R = CH3

4a,b

b R = CH3CH2

C H 3 H3C

C H 3

9c

►

H3C -C -C H 2-C -N -C -C H 2CH3

/;■

1

I

I• I

c h 3 h3c o c h 3

4c

c R = (CH3)3CCH2

= (CH3)3COOH

// = Mo(CO)6 or Mo0 3 catalysts

i

Scheme

6

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G. Sosnovsky et al. • Di-rm-alkyl Nitroxyl Radicals

115

and 4a-c consist of three lines of equal intensity Table III. Vapor pressure of nitroxyl radicals at various

with the spin density located on a 14N nitrogen temperatures.

atom (S=l). The hyperfine coupling constants of

2

1

4a

3

di-/er/-alkylnitroxyl radicals 1 and 4a-c ranged be-

tween 14.75 to 15.00 Gauss (Table II).

The branched acetylenic amines 8 a,b also can be

oxidized [6 6 ] to the corresponding nitroxyl radi-

cals 15 a,b (Scheme 7, Table II).

t [°C] p [torr] t[°C] p [torr]

P [torr]

0.10

0.18

0.76

2.4

6.5

15.5

34

t[°C]

20

38

66

93

121

149

163

t[°C]

20

38

66

93

121

149

177

204

232

0

10

20

30

0.13

0.30

0.63

1.3

0

38

66

93

0.1

1.2

19

0.18

0.64

4.0

18

65

210

58

40

50

60

70

2.5

4.7

8.1

121

149

177

204

150

365

780

650

Lj n

H3C

CH3

i

14

85

290

ä

.

rioUo

i

80

23

8a,b

R -C -N -C -C =C H

i

i• i

o c h 3

90

39

Na2W 04

100

110

120

130

140

150

60

93

145

210

305

440

h3c

a R = CH3

,

b R = CH3CH2

15a’b

Scheme 7

Table II. UV-Vis spectra and EPR hyperfine coupling

constants of di-rm-alkylnitroxyl radicals.

Radical

(UV) nm Amax (Vis) nm £a

E PR b aN (Gauss)

15.00

Amax

249

1

2

4a

4b

4c

15a

466

471

464

465

437

8.5

10.6

8.0

7.5

247

249

247

240

15.00

14.75

15.25

15.00

25.45

a In n-heptane; b in cyclohexane 10-3 m; 3 lines of

equal intensity.

The stability of these compounds under normal

conditions is limited to several weeks for 15a and

a few days for 15b. Compound 15a can be stored

for a prolonged period of time in a freezer at -

30 °C, otherwise, the decomposition occurs slowly

even at 3-5 °C.

The temperature dependence of vapor pressure

for 1 and 4a was measured with an isoteniscope,

and compared with that of 2 and 3. The available

quantities of 4b and 4c were too small for the iso-

teniscope measurements. The numerical results

from the Table III were plotted as shown in Fig-

ure 1 .

The thermal stability of di-terf-butyl nitroxyl (1)

was found to be somewhat limited. The reported

[33] half-life at 130 °C was only 13.4 minutes.

Heating of 1 in terf-butylbenzene for 50 minutes at

130 °C, both in oxygen and in an argon atmo-

sphere resulted in a 82% yield of 2 -methyl-2 -nitro-

sopropane [33-35] as one of the decomposition

products. The thermal decomposition of the nit-

roxyl containing an acetylenic function, i.e. 15a

°C

Fig. 1. The temperature dependence of vapor pressures

of nitroxyl radicals DTBN (1), 2,2,6,6-tetramethylpiperi-

din-l-oxyl (2), 4-hydroxy-2,2,6,6-tetramethylpiperidin-l-

nitroxyl (3) and rm-butyl-rm-pentylnitroxyl (4a).

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G. Sosnovsky et al. ■Di-rm-alkyl Nitroxyl Radicals

gave at 80 °C in 5 minutes a mixture of products a green, and then into a blue color. In all cases, a

[67], while the decomposition of the alkene ana- new maximum at 680-684 nm developed, which

log, i.e. N-fm-butyl-N-(l,l-dimethylprop-2-enyl)- is indicative of the presence of aliphatic nitroso

nitroxyl occurred readily at 25 °C [6 8 ], The ther- compounds (Table IV).

mal stability of 1 and 4a-c was investigated at

On the basis of the results presented in Table IV,

98 °C and 128 °C, in boiling «-heptane and n-oc- it is evident that the thermal stability decreases

tane, respectively. The decomposition was moni- with the increasing molecular weight. Such a ten-

tored by measuring the decrease in absorbance of

the nitroxyl moiety at 464-465 nm (Table II). The classes of compounds, such as branched azoal-

dency is also characteristic [69-73] of some other

characteristic orange-red color of the nitroxyl

compounds gradually faded and changed first into

kanes, peroxides, highly branched hydrocarbons,

and free radicals obtained by thermolysis thereof.

Table IV. Thermal decomposition of nitroxyl radicals.

Radical

Time

Absorbance

[A]

A e

[nm]

Absorbance

[A]

Xd

[nm]

la

0

466

465

464

462

449

-

464

468

473

474

464

465

-

465

464

476

514

480

464

-

0.50

0.46

0.29

0.19

0.00

0.27

0.29

0.28

0.23

0.00

0.36

0.34

0.31

0.07

0.60

0.00

0.27

0.13

0.02

0.21

0.15

0.08

0.07

0.00

0.42

0.22

0.06

0.03

0.90

0.92

0.85

0.88

0.83

5 h 45 min

12 h 35 min

22 h 45 min

26 h

30 h

0

1 h

4 h

12 h

16 h

0

1 h

3 h

6 h

14 h 30 min

0

5 min

0

1 h

2 h

0

30 min

1 h

4aa

4ba

0.23

0.44

680

684

0.29

4c»

lb

4ab

1 h 10 min

0

30 min

1 h

1 h 15 min

0

44 h

0

48 h

0

464

60 min

0

4bb

464

463

464

468

478

471

0.55

464

-

683

684

0.21

0.37

0.40

2 b

3b

2 C

30 min

0.41

0.72

0.38

0.11

3C

-

30 min

60 min

a In boiling n-hexane at 98±2 °C; b in boiling n-octane at 128± 2 °C; c in boiling 1,2,4-trimethylbenzene at 167±3 °C:

d nitroxyl moiety; e nitroso moiety.

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G. Sosnovsky et al. ■Di-tert-alkyl Nitroxyl Radicals

117

The cyclic nitroxyl radicals 2 and 3, which were

used for comparison, were stable in boiling n-oc-

tane for two days. After 44 and 48 hours the absor-

bance at 465 nm remained unchanged, then the ex-

periments were terminated.

A

considerable

decomposition was observed only at a higher tem-

perature when n-octane was replaced with 1,2,4-

trimethylbenzene. Thus, the half-life of TEMPO

(2) at 167 °C was only one hour.

Nitroxyl radicals have been recognized [28—

32,36-39] as effective polymerization inhibitors

and mentioned in a comprehensive review article

[74], in several publications [28,29,75,76], and in

recent [30-32] and relatively old patents [36-39].

The substantial vapor pressure, volatility and satis-

factory stability up to 100 °C, and in some cases

much higher, are the prerequisites for radicals

which could be employed as inhibitors of unde-

sired polymerization at elevated temperatures. Po-

lymerization inhibitors are needed in industry to

prevent unwanted polymerizations, which are

started by random free radical initiators. Such ran-

dom initiators can be spontaneously produced

either by autoxidative processes or by thermal

H

/

(CH3)3CNO

(H3C)3C-N

OH

19

20

fragmentation of molecules. Undesired and unex- gress could be monitored by measuring changes in

pected polymerization that may occur either dur- the refractive indices. Thus, for example, styrene

ing the synthesis or purification of various vinyl at 75 °C in the absence of an initiator failed to

monomers, results in great economical losses. It polymerize during several hours, whereas, in the

presence of either benzoyl peroxide or AIBN, the

was intriguing to evaluate the efficacy of aliphatic

di-tert-alkyl nitroxyl radicals 1, 4a-c, and 15a as po- polymerization commenced immediately, and in

tential inhibitors of such unwanted vinyl polymer- 75 minutes the sample attained a viscosity level

ization in comparison with two heterocyclic nitrox- that prevented further measurements of the re-

yls with different lipophilicity, namely 2 and 3, and fractive index. In contrast, an addition of the in-

commercial inhibitors, such as diethylhydroxyl- hibitor appreciably delayed the beginning of the

amine (16, commercial Pennstop PF2697), ammo- polymerization reaction. After the cessation of the

nium salt of N-hydroxy-N-nitrosoaniline (17, inhibition period, during which time the refractive

Cupferron), and bis(2,2,6,6-tetramethyl-4-piperi- index remained constant, the polymerization of

dyl) sebacate (18, Tinuvin770). In addition, 2- styrene commenced and proceeded at about the

methyl-2-nitrosopropane (19) and tert-butylhy- same rate as that without inhibitor, and reached

droxylamine (20) were tested (Tables V-VIII).

the same level of completion in 75 minutes. The

Several vinyl monomers, i.e. styrene, acrylonit- polymerization of the reference samples of the vi-

ryle, butyl acrylate, methyl acrylate, and vinyl ace- nyl monomers without inhibitors proceeded much

tate were subjected to polymerization at 70-75 °C, faster, i.e. within 2 to 10 minutes. The refractive

indices of mixtures containing the inhibitor re-

initiated either with benzoyl peroxide or azoisobu-

tyronitrile (AIBN). These upper temperature lim- mained constant during the entire inhibition

its were imposed by the boiling points of the cho- period, then a vigorous polymerization com-

sen monomers used in the study, e.g. acrylonitryle menced, causing a sudden formation of the respec-

boiling at 77 °C, and methyl acrylate boiling at tive polymer. In some cases, when the polymeriza-

80 °C. For those monomers, which undergo poly- tion failed to begin within several hours of heating,

merizations at a moderate rate, the reaction pro- the experiments were terminated. The results pre-

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118

G. Sosnovsky et al. ■Di-rm-alkyl Nitroxyl Radicals

Table V. Retardation of polymerization of styrene at 75 °C using 10 ml of the monomer and 0.35 g of benzoyl perox-

ide.

a

b

c

f

d

e

g

„20

time.

[min]

An

time,

[min]

An

time

[min

An

time,

[min]

An

time,

[min]

An

time

[min

An

time.

[min]

"D

n D

no

«D

1.5453

* 2d°

]

-

_

-

0

7

15

47

63

74

thick

1.5456

1.5508 0.0052

1.5562 0.0106 27 1.5466

1.5578 0.0162 50 1.5438

1.5678 0.0222

1.5718 0.0262 97 1.5440

polymer

0

1.5420

10 1.5420

1.5439

35 1.5439

1.5418

100 1.5420

150 1.5433

0

1.5430

1.5450

10 1.5446

0

20

1.5430

1.5422

0

-

60 1.5422

1.5424

105 1.5420

12 1.5448

29 1.5442

52 1.5450

-

1.5412

80

40 1.5422

25

70

1.5397

60 1.5445 0.0015 45

80 70 1.5405

160 1.5443 0.00

78 1.5438

81

1.5450

1.5458 0.0028

-

1.5397

115 1.5400

0.0080 140 1.5415

1.5465 0.0036 210 1.5503 0.0(

240 1.5482 0.0043

95 1.5483 0.0030 100

112 1.5522 0.0069

127 1.5556 0.0103 140

0.0033 90

210

1.5463

-

240

265

0.00

115 1.5440

129 1.5440

144 1.5462 0.0012 140 1.5586

1.5520

1.5538 0.01

120 1.5478 0.0048

-

1.5500 0.0061

1.5558 0.0119 300 1.5565 0.01

260

300

1.5510

-

200

0.0133 170 1.5538 0.0108

1.5415

1.5422

160 1.5504 0.0054 158 1.5623 0.0170 200

0.0126 247

1.5556

-

a No inhibitor; b 0.1 ml 1; c 0. 1 ml of 4a: J 0.1 m

1 ml of

100 mg of TEMPO (2); g 100 mg of 4-OH

4c; f

of 4b; e 0.

TEMPO (3).

Note: Cupferron (17, 100 mg) and diethylhydroxylamine (PennstopPF2697, 16, 0.1 ml) inhibited polymerization of

styrene for 8 h. In the samples containing 0.1 ml of 9a or 0.1 ml of 9b the polymerization started immediately and

proceeded at the same rate as in sample a. The refractive index of the sample without initiator and inhibitor

remained constant for 240 min, nd1= 1.5442.

Table VI. Retardation of polymerization of /7-butyl acrylate at 70 °C using 10 ml of the monomer and 0.35 g of

benzoyl peroxide.

a

b

c

d

f

e

g

time

«d

time

«0

«d

time

«o

time

fto

time

n&

time

«d

[min]

0

40

60

120

190

250

340

385

420

505

1.4145

1.4148

1.4141

1.4140

1.4145

1.4135

1.4137

1.4141

1.4143

1.4188

1.4175

1.4185

1.4190

10

35

1.4200

1.4205

10

30

60

130

200

1.4178

1.4182

1.4185

1.4188

1.4213

10

40

50

1.4178

1.4182

1.4187

12

22

1.4185

1.4180

10

20

1.4200

1.4203

1.4205

7

35

60

110

140

180

45 polymerization

30 polymerization 35

45 polymerization

70 polymerization

210 polymerization

240

1.4194

300

1.4178

No polymerization

a No initiator; b 0.1 ml of 1; c 0.1 ml of 4a; d 0.1 ml of 4b; e 0.1 ml of 4c; f 0.1 ml of Pennstop PF2697 (16); g0.35 g of

Tinuvin 770 (18). A reference sample containing 0.35 g of benzoyl peroxide and no inhibitor polymerized at 70 °C

instantenously. A sample containing 0.35 g of benzoyl peroxide and 100 mg of TEMPO (2) failed to polymerize

during 450 min of heating at 75 ‘

5C.

Table VII. Retardation of polymerization of vinyl monomers in the presence of benzoyl peroxide initiator and

various inhibitors.

Inhibitor

Quantity

Methyl acrylate at 75 °C

time [min]

Vinyl acetate at 70 °C

time [min]

Acrylonitrile at 70 °C

time [min]

1

2

0.1 ml

0.1 g

180

60

320

180

130

385

3

0.1 g

150

450

360

240

300

>480

360

4a

4b

4c

15a

16

19

20

0.1 ml

o.i g

0.1 ml

0.1 g

400

330

395

510

345

180

>570

360

480

55

260

90

>510

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G. Sosnovsky et al. • Di-rert-alkyl Nitroxyl Radicals

119

Experimental Part

Table VIII. Retardation of polymerization of vinyl mo-

nomers in the presence of AIBN initiator and various

inhibitors at 75 °C.

Materials. The following reagent-grade starting

materials of 97-99% purity were purchased from

commercial sources and used without further puri-

fication: /err-Butylamine (7a), /m-pentylamine

(7b), /e/Y-octylamine (7c), 2-methyl-3-butyn-2-ol

(5), 2-methyl-2-nitrosopropane dimer (19), tert-

butylhydroxylamine (2 0 ), styrene, methyl metha-

Acrylonitrile

Inhibitor time [min]

Methyl methacrylate

time [min)

Vinyl acetate

time [min]

1

2

145

180

150

50

16

>240

16

>360

190

>900

60

120

>900

80

>540

240

>540

50

220

>540

4a

15a

17

19

20

crylate,

2 ,2 ,6 ,6 -tetramethyl-piperidin-l-oxyl

300

(TEMPO, 2), 4-hydroxy-2,2,6,6-tetramethylpiperi-

din-l-oxyl (4-hydroxy-TEMPO, 3), molybdenum

(IV) oxide, molybdenum hexacarbonyl, copper

bronze, cuprous chloride, Raney nickel active cat-

alyst (analogous to Raney 28 or W-2, 50% slurry in

water), palladium on calcium carbonate poisoned

with lead (Lindlar catalyst), ammonium salt of N-

hydroxy-N-nitrosoaniline (Cupferron, 17), «-hep-

tane, n-octane, cyclohexane, hydrochloric acid,

tert-butyl methyl ether, tert-butyl hydroxyperox-

ide, and benzoyl peroxide were purchased from

Aldrich Chemical Co., Milwaukee, WI. Nickel-alu-

minum Raney nickel was obtained from Fluka

AG, Buchs, Switzerland. Bis(2,2,6,6-tetramethyl-4-

piperidyl) sebacate (Tinuvin 770, 18) was

purchased from Ciba-Geigy, Additives Division,

Hawthorne, New York. Palladium on powdered

charcoal (10%) was obtained from E. H.

Sargent & Co. Palladium 5% on carbon, type 37 -

from Johnson Matthey, West Depford, NJ, Ruthe-

nium on carbon (5%) - from Engelhard Corpora-

tion, Newark, NJ. Raney cobalt 2700 Catalyst in

water - from Grace Davidson, Chattanooga, TN.

Acrylonitrile, methyl methacrylate and butyl acry-

late were obtained from Janssen Chimica, Geel,

Belgium. Vinyl acetate was purchased from

Matheson Coleman and Bell, Norwood, Cincin-

nati, Ohio. Diethylhydroxylamine (Pennstop PF

2697, 16) was donated by ATO Chem North

America, Woodbury, New Jersey. Sodium tung-

state was purchased from Fisher Scientific Co.,

Fair Lawn, New Jersey, and azoisobutyronitrile,

(AIBN, 2,2’-azobis-2-methylpropionitrile) was

purchased from Kodak, Rochester, NY. The di-

tert-butyl nitroxyl radical (DTBN) was prepared

according to the published procedure [4,6] from 2-

methyl-2-nitropropane. Nova Molecular Technol-

ogy, Janesville, 3-chloro-3-methyl-l-butyne (6 ) was

prepared from 2-methyl-3-butyn-2-ol (5) [50-51].

The N-terf-butyl- and N-rm-pentyl-N-(l,l-dime-

thylprop-2-ynyl)nitroxyl radicals, 15a and 15b,

respectively, were obtained by the oxidation of 8a

and 8b, in accordance with the literature pro-

cedure [6 6 ],

sented in Tables V-VIII were obtained with the

practical, arbitrarily chosen, nonstoichiometric

quantities of various inhibitors, i.e. 0 .1 g for the

solids and 0.1 ml for the liquids. Nevertheless, on

the basis of results obtained in the present study,

it can be concluded that all of the investigated rad-

icals can be employed in practical applications.

On the basis of the results of experiments shown

in Tables V, VII, VIII, the following conclusions

can be made:

1) There is no general correlation between the

decrease in the inhibition time and the decrease

in the concentration of the nitroxyl molecule.

2) There is no general correlation between the

decrease in the inhibition time and the decrease

in the thermal stability.

In fact, in the series of 1 and 4a-c, the least sta-

ble radical 4c at the lowest molar concentration

was found to be in all cases superior to the more

stable DTBN (1). These observations can be ex-

plained by an assumption that the thermal degra-

dations of nitroxyls during the experiment lead to

formation of the hindered nitroso compounds

which have been known as efficient spin traps

[77-80] and polymerization inhibitors [80-83]. To

support this assumption, a few experiments were

conducted using 2-methyl-2-nitroso-propane (19)

instead of the nitroxyl radicals (Tables VII and

VIII). It was confirmed that the admixture of 19

to vinyl monomers offers a remarkable protection

against unwanted polymerization of vinyl mono-

mers with inhibition times comparable to those

obtained with nitroxyls. Nevertheless, even in the

extreme case of 15a, which undergoes decomposi-

tion below 70 °C, the inhibition times were satis-

factory (Tables VII and VIII).

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G. Sosnovsky et al. • Di-te/7-alkvl Nitroxyl Radicals

Analytical procedures. Melting points were de- (CDC13, TMS); (3 = 31.75, 33.34, 47.77, 52.12,

termined on a Thomas-Hoover capillary melting

point apparatus. Model 6406-K, using a calibrated

69.91, 92.03. MS (Cl): m/z = 140 [M+ + 1],

A solution of 8a (0.5 g) in diethyl ether (7 ml)

thermometer. Mass spectra were recorded on a was slowly added into a 1 .0 msolution of hydrogen

Hewlett-Packard mass spectrometer. Model 5985

chloride in diethyl ether ( 1 0 ml) to precipitate the

GS either in the electron impact (El) or chemical corresponding hydrochloride salt, m.p. 2 2 0 -

ionization (Cl) modes. In the latter case methane 223 °C (dec.) (lit. [49] m.p. 221-223 °C).

was used as the reactant gas. 'H NMR spectra

C9H 18C1N (175.70)

were recorded on a 250 MHz Bruker NMR spec-

trometer, Model WM-250, using CDC13 as a

solvent and TMS as the reference. Microanalyses

Calcd C 61.52 H 10.33 N 7.97%,

Found C 61.60 H 10.48 N8.16%.

were performed on a Perkin-Elmer elemental ana-

lyzer. Model 240C. Capillary gas chromatography

was carried out on a Hewlett-Packard gas chroma-

Procedure B. terf-Butylamine (7a, 106.6 g,

tograph, Model 5890, using a 15 m DB-1 column. 153 ml, 1.46 mol) and water (55 ml) were com-

UV-Vis spectra were recorded on a Nicolet spec- bined in a 1000 ml 3-neck flask equipped with a

trophotometer, in quartz cuvettes either in n-hep- stirrer, pressure equalizing funnel and thermome-

tane or n-octane. EPR spectra were recorded ter. Since the temperature of the mixture sponta-

using 10~3 M cyclohexane solutions on a Varian neously rose to about 55 °C, it was lowered to

El 15 spectrometer with a E102 microwave bridge

in 5 mm glass tubes. The vapor pressures of nit- Catalytic quantities of cuprous chloride (0.1 g) and

roxyl radicals were obtained at Phoenix Chemical

about 30 °C by external cooling with cold water.

copper bronze (0.05 g) were added to the stirred

Laboratory, Inc., Chicago, IL by the static deter- mixture, followed by a dropwise addition of 3-

minations of vapor pressure using an Isoteniscope chloro-3-methyl-l-butyne-l (6 , 60 g, 0.58 mol) for

ASTM D2879.

10 min. In spite of the external cooling with cold

3-tert-Butylamino-3-methyl-1-butyne (8a). Pro- water the temperature in the flask rose to about

cedure A. 3-Chloro-3-methyl-l-butyne (6 , 225 g, 55 °C. The reaction mixture was stirred for an ad-

2.2 mol), /er/-butylamine (7a, 392 g, 536 ml, 5.36 ditional 15 min, then transferred to a separatory

mol) and water ( 2 0 0 ml) were mixed in a 2 1

Erlenmeyer flask to form a homogeneous solu-

funnel. The dark top layer (about 100 ml) was sep-

arated, dried with anhydrous sodium carbonate,

tion, whereby the temperature spontaneously filtered, and flash-distilled at 50-70 °C/70-80 torr

rose to about 35 °C. The resulting reaction mix- to give 50 g (0.36 mol, 62%) of 8a with a 95%

ture was kept for two weeks at 20-25 °C. During purity. The product was redistilled at 135-136 °C/

this time the mixture separated into two layers. 743 ±3 torr to the 98.5% purity by GC.

The mixture was diluted with water (200 ml), the

Procedure C. A 6 -fold scale-up was carried out

organic top layer was separated, and the aqueous in a 3 1 flask, placed in a cooling bath, with the

layer was extracted with rerr-butyl methyl ether following quantities of the starting materials: 7a

(2 x 50 ml). The combined organic layers were (534 g, 767 ml, 7.3 mol), water (300 ml), copper (I)

dried with anhydrous sodium carbonate, and chloride (0.7 g), copper bronze (0.2 g), and

6

then with potassium hydroxide pellets. The (90%, technical grade, 362 g , equivalent to 3.2 mol

solvent and other volatile components were re- of pure 6 ). Compound 6 was added to the reaction

moved by distillation between 55 and 135 °C at mixture over a period of 40 min. The temperature

743±3 torr and the remaining liquid was distilled of the reaction mixture was maintained at 30-

at 50-70 °C/70-80 torr in order to remove the 35 °C, but never to exceed 45 °C. The isolation and

product from the unsaturated by-product. It is purification procedures of the reaction mixture, as

important to perform this distillation at this described in B, gave 380 g (8 6 %) of 8a. The initial

point at a reduced pressure, otherwise a vigorous purification of crude product can also be accom-

decomposition of the brown residue in the final plished by steam distillation. In this case, an effi-

stages of the distillation can occur, resulting in cient cooling must be provided to trap the tert-

a contamination of the distillate. The final distil- butylamine. The distillate separates into two

lation at 135-136 °C/743±3 torr (lit. [49] 135— phases. The aqueous phase contains an excess of

136 °C), gave 185 g (60%) of 8a as a colorless

/‘m-butylamine. which can be recovered by adding

product of 99% purity by GC, d->0 = 0.787, n$ = sodium hydroxide. The organic phase of the distil-

1.4322. ‘H NMR (CDC13, TMS): d = 1.27 (s, 9H, late can be dried with potassium hydroxide and

f-Bu), 1.43 (s. 6 H. 2Me), 2.28 (s, 1H). ,3C NMR distilled. This procedure can be conveniently em-

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G. Sosnovsky et al. ■Di-rm-alkyl Nitroxyl Radicals

121

ployed for the recovery of most of the tertiary layer was dried with potassium hydroxide pellets

amine, which was used as HC1 scavenger.

3-Methyl-3-tert-pentylamino-l-butyne (8 b). Pro-

cedure A. The compound was prepared by an anal- mostly 7c, the second fraction (26 g), collected at

(5 g), decanted and distilled. The first fraction

(3.5 g) collected at 35-45 °C/90 torr contained

ogous procedure as described for 8 a. Reaction of 80-85 °C/85 torr contained a 3:2 mixture of 7c and

8 c, and the third fraction (36 g), collected at 95-

rm-pentylamine (7b, 50 g, 0.574 mol) and 3-

chloro-3-methyl-l-butyne (6 , 45 g, 50 ml, 0.44 mol) 97 °C/25 torr, was the expected product (8 c). The

for 4 weeks gave 27 g (39%) of 8 b of 97% purity second fraction was redistilled and the product

by GC, b.p. 155-158 °C (lit. [3,66] 52-53 °C/15 was collected at 206-209 °C/749 torr to give an

torr),

= 1,4432 (lit. [3] n$ = 1.4430). MS (El): additional quantity (8.1 g) of 8 c. The combined

product (44.3 g, 0.23 mol) was obtained in a 46%

m/z = 153 [M+, 1.4], 138 [M+- CH3, 3], 124 (18),

83 (9), 72 (29), 71 (14), 6 8 (26), 67 (12), 58 (100). yield. MS (El): m/z = 180 ([M+ -CH3] 2.4), 124

'H NMR (CDC13, TMS): d = 0.87 (t, 3H), 1.22 (s, (54), 108 (11), 6 8 (18), 67 (23), 65 (14), 58 (100).

6 H, 2Me), 1.43 (s, 6 H), 1.53 (s, 2H), 2.26 (s, 1H). 'H NMR (CDCI3, TMS): Ö = 1.02 (s, 9H. t-Bu),

In a similar experiment, with the identical quanti-

ties of the starting materials, the recovery of the 2.27 (s, 1H). 13C NMR (CDC13, TMS): d = 30.66,

1.34 (s, 6 H, 2Me), 1.41 (s, 6 H, 2Me), 1.53 (s, 2H),

product and the yield were improved to 50% by 31.80, 31.98, 33.79, 47.42, 56.26, 56.95, 70.14, 91.99.

repeated distillation of the extracts under reduced

pressure at 76-80 °C/20 torr (lit. [3,66] b.p. 52-

53 °C/15 torr).

Procedure B. A three-neck 5 1 flask equipped

with a stirrer, 1 liter addition funnel and thermo-

meter was placed in an ice/water bath. The flask

was filled with terf-octylamine (7c, 1450 g, 11.2

Procedure B. terf-Pentylamine (7b, 1271 g, 14.58

mol), water (580 ml), cuprous chloride (2 g), and mol), water (450 ml), copper bronze (0.3 g) and

copper bronze (1 g) were combined with stirring in cuprous chloride (1 g) and the addition funnel

a 3 1flask. The temperature of the mixture rose im- with 3-chloro-3-methyl-l-butyne (6 , 500 g, 4.9

mediately to 41 °C. The flask was cooled until the mol). The first 250 ml portion of 6 was added

temperature reached 25 °C. 3-Chloro-3-methyl-l- rapidly for 7 minutes. The initial reaction temper-

butyne (⁶, 684 g, 6.67 mol) was added dropwise ature (22 °C) rose during 15 min to 38 °C and then

over a period of one h. The exothermic reaction was subsided. The remaining

6

was added during

controlled by the rate of addition and external cool- 40 min period while maintaining the temperature

ing, maintaining the reaction mixture below 30- in the flask between 25-38 °C, then the reaction

35 °C. After the addition the reaction mixture was mixture was stirred for 30 min. The total reaction

time was 90 min. The reaction mixture was trans-

stirred for an additional 2 h at 25-35 °C. The reac-

tion mixture was transferred to a separatory funnel ferred to a separatory funnel. The top organic

and the bottom aqueous layer containing the hy- layer (450 g) contained mainly 81% 8 c by GC,

drochloride salts was removed, neutralized with a 13% of 7c and several minor products. The aque-

50% aqueous sodium hydroxide solution and ex- ous layer treated with a 50% sodium hydroxide

tracted with tert-butyl methyl ether (3 x 500 ml). solution formed an organic layer containing 61%

The combined organic layers were dried with solid of 7c and 31% of 8 c. The combined organic layers

sodium hydroxide ( 1 0 0 g), then the solvent and the were dried overnight with sodium hydroxide pel-

excess of rm-pentylarnine were removed at the pre- lets (50 g), and then distilled to give 7c (970 g, 7.5

vailing barometric pressure. The distillation of the mol), collected at 40-45 °C/15 torr, an interim

remaining liquid at 52-54 °C/15 torr gave 700 g fraction (63 g) and the product 8 c (576 g, 2.95 mol,

(6 8 %) of 8 b with a 99% purity by GC.

3-Methyl-3-tert-octylamino-l-butyne (8 c). Pro-

cedure A. A heterogenous mixture of tert-octylam-

60%), collected at 100-102 °C/25 torr, then at 55-

57 °C/0.25 torr. The flask residue (120 g) contained

some amount of 8 c and other unsaturated pro-

ine (7c, 129 g, 1 mol), 3-chloro-3-methyl-l-butyne ducts, which tended to polymerize when over-

(6 , 51 g, 0.5 mol) and water (100 ml) was left in a heated. The product was redistilled at 103-107 °C/

closed flask at 23-25 °C for three weeks. Then, 30-35 torr to give 561 g (59%) of ⁸c with a 97%

the mixture was diluted with water ( 1 0 0 ml), and

the organic layer was separated. The aqueous

purity, «d = 1.4500, d2o = 0.8153.

3-tert-Butylamino-3-methyl-l-butene (14a).

A

layer was extracted with tert-butyl methyl ether solution of 3-terr-butylamino-3-methyl-l-butyne

(3 x 25 ml), and the solvent was removed at 20-30 (8 a, 309 g, 2.22 mol) in tert-butyl methyl ether

torr to give 12 g of 7c. A 50% solution of sodium (500 ml) was placed in a 2 1glass Parr hydrogena-

hydroxide was added to the aqueous layer to give tion bottle and cooled to -20 °C. To the solution

an additional quantity of 7c (64 g). The organic

was added the Lindlar catalyst, i.e. 5% palladium

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G. Sosnovsky et al. ■Di-fe/7-alkyl Nitroxyl Radicals

122

torr; d20 = 0.8225, n$ = 1.4493. !H NMR (CDCh,

The flask was connected to the Parr shaker, evacu- TMS): (3 = 1.01 (s, 9H, /-Bu), 1.18 and 1.23 (s, 6 H.

on calcium carbonate, poisoned with lead (3 g).

ated to 30 torr, purged twice with nitrogen gas, 2Me), 1.44 (s, CH,). 4.84-4.98, (m. J = 10.8, 17.6,

0.04 Hz, 2H), 5.96-6.06 (d of d, J = 10.8, 17.6 Hz,

then twice with hydrogen gas. The flask was pres-

surized to 50 psi (3.4 atm) and the agitation 1H). 13C NMR (CDCL, TMS): <3 = 30.85, 31.62,

31.90, 32.08, 54.75, 56.37, 58.41, 109.36. 150.76.

tert-Butyl-tert-pentylamine (9a). Procedure A.

Hydrogenation using a 5% palladium on carbon

catalyst in tert-butyl methyl ether. A solution of 3-

started. Every 10 to 15 min, the reaction was in-

terrupted. and the solution was cooled to -10 °C.

Each time, the reaction mixture was analyzed by

GC. After 30 min the hydrogen gas uptake ceased,

although the reaction was incomplete. A new por- ferf-butylamino-3-methyl-l-butyne (8 a, 60 g, 0.43

tion of a fresh catalyst (2 g) was added, and the mol) in terf-butyl methyl ether (150 ml) was placed

procedure was repeated until the absorption of hy- in a 300 ml stainless steel autoclave equipped with

a sampling outlet, cooling coil and mechanical agi-

tator. To the solution sparged with a stream of ni-

trogen gas, was added the Johnson-Matthey 87L

5% Pd/C catalyst (0.5 g). The autoclave was

closed, and first purged with nitrogen gas, then

with hydrogen gas, and pressurized to 2 0 0 psig.

drogen gas stopped and the compound 8 a was ex-

hausted. The reaction mixture contained a sub-

stantial amount (17%) of rm-butylamine,

produced by the hydrogenolysis, and a small

amount (2.7%) of terf-butyl-terf-pentylamine. The

catalyst was removed by filtration through a Celite

filter aid on high-density filter paper, using a weak The temperature of the agitated mixture was

maintained between 17 and 25 °C by internal cool-

ing with cold water. The hydrogenation process

suction, and the solvent was removed at about 2 0 -

25 °C/100 -200 torr. A distillation at 82-83 °C/40

torr gave 200 g (64%) of colorless 14a with a 97% was monitored by GC, and was completed in 6 h.

The catalyst was removed by filtration through a

Celite filter aid on high-density filter paper, aided

with mild suction, and the reaction mixture was

distilled under atmospheric pressure. The first

fraction contained the solvent and the hydrogeno-

lysis products. Further distillation of the remaining

liquid at 143-145 °C/743±3 torr gave 45 g (73%)

of 9a with a 98% purity by GC, n$ = 1.4218 (lit

[49] ng = 1,4179).

purity by GC. An analytical sample was obtained

by a distillation at 141-143 °C/760 torr, d20 =

0.7743, /ig> = 1.4316. ]H NMR (CDC1,, TMS): (3 =

1.14 (s, 9H, t-Bu), 1.24 (s, 6 H, 2Me), 4.87-5.01, (m.

J = 10.8, 17.6, 0.04 Hz, 2H), and 5.95-6.04 (d of d,

./ = 10.8, 17.6 Hz, 1H). 13C NMR (CDCL, TMS):

6 = 30.55, 32.68. 52.05, 54.87, 109.74. 150.34.

3-Methyl-3-tert-pentylamino-l-butene (14b). A

solution of 3-methyl-3-pentylamino-l-butyne (8 b.

200 g, 1.3 mol) in tert-b\i\.y\ methyl ether (300 ml)

Procedure B. - The above procedure was scaled

was hydrogenated in the presence of the Lindlar up 16.7 times and carried out in a 3.8 1autoclave.

In this case, the hydrogenation of 8 a (1000 g, 7.2

mol) yielded 700 g (4.9 mol, 6 8 %) of 9a.

Procedure C. Hydrogenation using a Raney

nickel catalyst, a) Preparation of the catalyst.

Nickel-aluminum alloy (30 g, 50-50) was digested

in a 2 0 % aqueous sodium hydroxide solution

catalyst (1 g), as described for 14a. The catalyst

was removed by filtration, and the filtrate was

saved. The hydrogenation was repeated using

1 0 0 g and 2 0 0 g of 8 b. and the filtrates from all

three experiments were combined. The solvent

was removed and the residual liquid was distilled

at 53-56 °C/15 torr to give of 415 g (83%) of 14b (250 ml). The initial exothermic reaction required

external cooling. Subsequently, the mixture was

heated on a steam bath to 80 °C and kept at this

with a 100% purity by GC. d2o = 0.793,

1.4395. ’H NMR (CDC1,, TMS): (3 = 0.85 (t, 3H,

=

Me), 1.07 (s, 6 H. 2Me), 1.12 (s, 1H, NH), 1.24 (s, temperature for 6 h. The resulting slurry was di-

6 H, 2Me), 1.40 (q, 2H, CH?), 4.92-5.0 (m, 2H, luted with deionized water (500 ml) and the aque-

ous layer was carefully decanted. The nickel was

washed with deionized water (30 x 100-150 ml),

each time the decanted water was discarded. Then,

the slurry was washed several times with ethanol

(5 x 50 ml) and the catalyst was stored under etha-

nol in a closed flask. The flask was shaken vigor-

ously before each hydrogenation experiment, and

an aliquot of the resulting suspension (1 - 2 ml)

was transferred with a wide plastic pipet to the

CH2), 5.97-6.03 (m. 1H, CH).

3-Methyl-3-tert-octylamino-1-butene (14c). The

reaction was carried out in the same fashion as

in the case of 14a. A solution of 3-methyl-3-/err-

octylamino-l-butyne (8 c, 420 g, 2.14 mol) in tert-

butyl methyl ether (600 ml) was hydrogenated in

a 2 1 glass vessel in the presence of the Lindlar

catalyst ( 6 g), added in two portions. The reaction

mixture was distilled at 48-52 °C/0.2 torr to give

332 g (79%) of 14c with a 97% purity by GC. The hydrogenation flask.

analytical sample was distilled at 215-216 °C/760

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123

G. Sosnovsky et a i ■D i-/m -alkyl Nitroxyl R adicals

torr or 92-95 °C/60 torr); n $ 1.4301 (lit. [3]

Hydrogenation. The hydrogenation experi-

b)

1.4281). MS(EI): m /z (70 eV) = 157 [M+, weak],

128 [M+-C^H5, 6], 72 [isopentane, 31], 71 [t-pentyl,

11], 58 (100). 'H NMR (CDC13, TMS): 6 = 0.87 (t,

6H), 1.07 (s, 1H), 1.11 (s, 12H), 1.42 (q, 4H).

Procedure B. 3-M ethyl-3-/e/t-pentylamino-l-bu-

tene (14b,170 g, 1.08 mol) in rm-butyl methyl

ether (300 ml) was hydrogenated in a 2 1 Parr

shaker in the presence of a 5% Pd/C catalyst

(2.1 g) to give 127 g (71%) of 9b with a 98% purity

by GC.

tert-Octyl-tert-pentylamine (9c). A solution of 3-

m ethyl-3-rm-octylam ino-l-butyne (8c, 113 g, 0.55

mol) in rm -butyl methyl ether (150 ml) was hydro-

genated in a 300 ml glass flask overnight in a Parr

shaker (Parr Instrument Co. Moline, Illinois) in

the presence of a 5% Pd/C catalyst (1 g) at 57 psi

ments were carried out in a 3-neck 100 ml flask

placed in a water cooling bath. The central neck

was closed with

a

glass three-way stopcock

adapter. One arm of the stopcock was connected

to a cylinder with hydrogen gas. The second arm

was connected with a tygon tubing to a glass tube

going through a rubber stopper to the top of an

inverted 2 1 calibrated glass cylinder filled with

water. This cylinder was also connected through a

second shorter glass tube, inserted in the same

rubber stopper, to a pressure equalizing burette

filled with water. The flask, equipped with a small

magnetic stirring bar, was filled with methanol

(25 ml), compound 9a (5 g) and the Ra/Ni catalyst

(2 ml). The remaining two necks of the reaction

flask were closed with rubber septa allowing a

withdrawal of small aliquots of the reaction mix- (-3.9 atm). The catalyst was removed by filtration

ture for gas chromatography. The flask was purged

several times with hydrogen gas, the burette was

filled with hydrogen gas and the magnetic stirrer

was started. Hydrogen uptake began immediately

as measured by the rising water level. The results

and the filtrate (223 g) was distilled. The first frac-

tion (130 ml), collected at 43-55 °C, contained, in

addition to rm -butyl methyl ether, the low-boiling

unidentified neutral hydrogenolysis products. The

second fraction, collected at 48-53 °C/28-32 torr,

are presented in Table I. The aliquots correspond- contained 56 g (0.43 mol, 79%) of 7c. The third

ing to the 55% and 86% of the theoretical hy- fraction, collected at 111-112 °C/30 torr, con-

drogen uptake were analyzed by GC/MS analysis

carried out in the electron impact (E l) mode. The

MS scans were taken every 2 seconds. The initial

tem perature was 50 °C and the program rate

20 °C/min. The identity of the major peaks was

confirmed by comparison of the retention times

(RT) of standards injected independently and the

fragmentation patterns. Thus, the RT of 36 sec cor-

responded to 7a, however, the parent peak (MW=

73) was missing. The most prominent peak 59 was

recorded for [M+ - CH 3 + H], The RT of 146-

150 sec concurred with 8a; the weak parent peak

at 139, and the fragmentation peaks at 124 [M+ -

CH 3], 68, and 58 were observed. The RT of 168

sec was assigned to 3-rm-butylarnino-3-methyl-l-

butene (MW=141) on the basis of a weak peak at

141, and the fragmentation peaks at 126, 70 and

58. The RT of 184 sec corresponded to 9a, how-

ever, the parent peak at 143 was absent. O ther

tained 26 g (0.11 mol, 20%) of 9c.

The recovered catalyst was used several times in

consecutive experiments in which the concentra-

tion of 8c was gradually decreased. Deactivation

of the catalyst and lower concentrations of 8c re-

sulted in increased yields of 9c . Thus, a reduced

quantity of 8c (82 g, 0.42 mol) in the same amount

of tert-butyl methyl ether (150 ml) was converted

to 9c (20 g, 0.1 mol, 24%) and 7c (39 g, 0.3 mol,

72%). Further dilution (e.g. 60 g, 0,31 mol of 8c)

with tm -butyl methyl ether (200 ml) caused some

additional improvement, i.e. 19.5 g (32%) of 9c. In

another experiment, using 81 g (100 ml, 0.42 mol)

of 8c in tert-butyl methyl ether (200 ml) 9c and 7c

were obtained in 35% and 60% yield, respectively.

d25 = 0.808, n $ = 1.4426, MS (El) (15 eV): m /z =

184 [M+-Me, 1], 129 (13), 114 (6), 72 (13), 71 (4),

59 (4), 58 (100), 57 (12); MS(CI): m /z = 198 (6.5),

184 (29), 170 (8), 129 (9), 128 (100), 89 (7), 88 (89),

fragmentation peaks were observed at 128 [M+ - 86 (10), 72 (8) 71 (12), 61 (10), 58 (18), 57 (52), 56

CH3], 114 [M+ - C2H 5], 99, 84, 72, and 58. (8), 55 (28). 'H NMR (CDC13, TMS): Ö = 0.87 (t,

Di-tert-pentylamine (9b). Procedure A. The cata- 3H, MeO, 1.02 (s, 9H, t-Bu), 1.12 (s, 6H, 2Me), 1.23

lytic hydrogenation of 8b (15.7 g, 0.11 mol) was

carried out in a Parr shaker in m ethanol (100 ml)

in the presence of Raney cobalt (2 g) for 10 h. The

initial hydrogen pressure decreased from 57 to 41

psi (2.8-4 atm). The catalyst was removed by

filtration, and the filtrate was distilled through a

short path (25 cm) column. The product 4 g (25%)

was collected at 155-160 °C (lit. [8] 93-95 °C/62

(s, 6H, 2Me), 1.39 (q, 2H CH?), 1.42 (s, 2H, CH 2).

13C NM R (CDC13, TMS): Ö = 9.72 (Me), 30.02,

31.99, 32.17 (t, 3Me in t-Bu), 38.26 (t, CH?, 52.88,

55.95, 59.25 (t, CH 2).

Large-scale hydrogenation experiments carried

out in 300 ml and 3.8 1autoclaves, in a fashion sim-

ilar to that described for 9a, gave 9c in 65-80%

yields.

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124

tert-Butyl-tert-pentylnitroxyl (4a). To a stirred

G. Sosnovsky et al. • Di-terf-alkyl Nitroxyl Radicals

diluted with rerf-butyl methyl ether (200 ml) to

precipitate the inorganic products. The solution

was filtered through a Celite filter aid, and the

solvent was removed in vacuo. The residue was

distilled at 72-75 °C/0.5 torr using a short path

distillation apparatus to give 1.7 g (40%) of 4c.

MS (El, 15 eV): m /z = 214 [M+, 6], 170 (8), 152

(10), 144 (11), 129 (11), 128 (12), 114 (22), 113

(100), 103 (10), 102 (13).

solution of terr-butyl-ter/-pentylamine (9a. 85 g,

0.59 mol) in methanol (400 ml) was added sodium

tungstate dihydrate (2 g) followed by one portion

of a 30% aqueous solution of hydrogen peroxide

(160 ml). The tem perature of the reaction mixture

rose to 31 °C as a result of exothermic reaction,

however, no external cooling was necessary. The

color of the solution changed gradually from a

faint yellow to dark orange. The reaction mixture

was stirred for 48 h without external cooling. The

solution was transferred to a separatory funnel,

and diluted with water (500 ml). The top brown

layer was separated, and the bottom aqueous layer

was extracted with tert-butyl methyl ether (3 x

50 ml). The combined organic layers were dried

with anhydrous magnesium sulfate and distilled at

60-63 °C/2 torr to give 82 g (87%) of 4a as a red

liquid with a strong terpene-like odor (lit. [3] b.p.

56-57 °C/5 torr). EPR (in cyclohexane): aN = 15

Gauss. UV-Vis (/7-octane): Amax = 247 (e = 8.0),

464 nm.

Thermal stability measurements

The therm al decomposition studies were carried

out by heating solutions of selected nitroxyl radi-

cals in either /i-heptane, n-octane, or in 1,2,4-tri-

methylbenzene in a 3-neck 250-ml flask equipped

with a magnetic stirring bar, therm om eter, and re-

flux condenser. One of the necks was closed with

a rubber septum. The flask was placed in a heating

mantle on top of a magnetic stirring plate, and

filled with a nitroxyl solution (100 ml). The agi-

tated solutions were heated to boiling until com-

plete decomposition occurred which was indicated

by the disappearance of the brown-red color and a

change into a bluish solution. Small samples were

removed periodically with a glass syringe and used

for measuring the UV-Vis spectra. After each

Di-tert-pentylnitroxyl (4b). To a solution of di-

terf-amylamine (9b, 3 g, 0.002 mol) in m ethanol

(16 ml) was added sodium tungstate dihydrate

(0.1 g) and a 30% aqueous hydrogen peroxide

solution (8 ml). Reaction for 3 days and work- analysis the samples were returned to the flask.

up as described for 4a gave 4b (2.5 g, 76% ),

b.p. 52-54 °C/0.35 torr (lit. [3] 76-77 °C/5 torr).

Polym erization experiments

Polymerization experiments were carried out in

25 ml glass tubes which were open to the air, and

filled with portions of vinyl monomers (10 ml). To

each sample was added an initiator and inhibitor

as shown in Tables V-VIII. The samples were

placed in test tube racks and heated in a water

bath, which was equipped with a thermostat. Be-

cause of the toxic, hazardous character of many

vinyl monomers, the experiments were conducted

in an exhaust hood. In those cases, when applica-

M S(EI) (70 eV): m /z = 172 [M+, 1.2], 102 (5),

88 (4), 83 (4), 74(15), 71 (100), 74 (15), 58 (9),

55 (15).

tert-Octyl-tert-pentylnitroxyl (4c). A solution of

tert-butyl hydroperoxide (4 ml, 40 mmol) in 1,2-

dichloroethane (50 ml) was added to a stirred

solution of /err-octyl-rm-pentylamine (9c, 4.0 g,

20 mmol) in 1,2-dichloroethane (50 ml) contain-

ing

(100 mg)

a

mixture of molybdenum (IV) oxide

and molybdenum hexacarbonyl

(100 mg), in a 3-neck 200 ml reaction flask equ- ble, small samples of the solutions were removed

with glass pipets and used for refractive index

measurements. A fter each measurement the

prisms in the refractom eter were carefully cleaned

with toluene and acetone. In other cases, the sam-

ples were heated until the action of the inhibitors

ceased and the samples polymerized.

ipped with a stirrer and reflux condenser. TTie

reaction mixture was brought to a gentle boiling.

A fter 30 min the reaction mixture became light

yellow in color. The reaction mixture was left at

23-25 °C for

3 weeks during which time it

turned brown in color. The reaction mixture was

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G. Sosnovsky et al. • D i-?m -alkyl N itroxyl Radicals

125

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Di-tert-alkyl nitroxyl radicals. Synthesis, physical properties and applications as inhibitors of vinyl polmerization at elevated temperatures

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