Canadian Journal of Chemistry Published by T H E NATIONAL RESEARCH COUNCIL 01.. CANADA

VOLUME 47 MARCH 1, 1969 NUMBER 5

Metal ion initiated halogenation reactions of N-haloaminesl

DENNIS D. TANNER AND MELVYN W. MOSHER~ Department of Chemistry, University of Alberta, Edmortton, Alberta

Received July 5 , 1968

The metal ion catalyzed chlorination of hydrocarbon substrates by N-chloroamines and imides in acid media proceeds by a free radical chain mechanism. Reactions of four different N-chloro halogenating reagents with cuprous chloride, ferrous sulfate, and ferrous chloride initiators have been investigated. Contrary to the previously reported observations no experimental differences were observed in the halogenation reactions of individual N-halo compounds or with the different initiators. The chain carrying species in all cases was shown to be the chlorine atom and the radical chain process showed identical reactivity to that of photo-chlorination by molecular chlorine. Canadian Journal of Chemistry, 47, 715 (1969)

Introduction In recent years considerable attention has been

given to the radical chain reactions of N-halo- amines, amides, and imides. Divalent nitrogen species have been proposed as reactive inter- mediates in the free radical addition reactions of these positive halogen compounds with olefins (1-14) as well as in their halogenation reactions involvillg hydrogen abstraction.

The reactions leading to abstraction by nitrogen radicals fall into two classifications, those involv- ing intramolecular abstraction and those involv- ing intermolecular abstraction. In the case of the intramolecular reactions (i.e. to give 6 halogena- tion) a further division can be noted; whether they proceed by a chain sequence with or without acid catalysis. The acid-catalyzed reactions com- prise the reactions of N-haloamines and are usually referred to under the general heading of the Hofmann-Eoffler-Freytag Reaction (15, 16). The intramolecular reactions (i.e. to give 6 halogenation) proceeding under neutral con- ditions are the reactions of N-haloimides (17) and N-haloamides (18, 19) and can be generally referred to as "Barton type" reactions for these classes of compounds.

'Presented in part at the 51st national meeting of the Chemical Institute of Canada, Organic Chemistry Section, June 1968.

ZUniversity of Alberta Postdoctoral Fellow (1967- 1969).

Since it has been established that the inter- molecular free radical halogenation reactions of N-haloamides and N-haloimides proceed under neutral conditions by a process involving halogen atom abstraction in the chain propagating step (20-22), only examples of acid-catalyzed inter- molecular chain halogenation reactions can claim to involve nitrogen radical abstraction. The photo-initiated reaction of N-iodosuccinimide in trifluoroacetic acid to abstract hydrogen from tri- fluoroacetic acid (23), and the metal ion initiated, acid-catalyzed chain halogenation reactions of N- haloamines (24-28), which have been extensively studied by Minisci and co-workers, remain to our knowledge the only examples of chain reactions involving intermolecular hydrogen transfer by divalent nitrogen.

The metal ion initiated halogenation reactions of N-haloamines, as reported, seemed a unique, synthetically useful, and theoretically interesting reaction. Minisci has proposed the mechanism of the reaction to involve hydrogen abstraction by a protonated aminium radical (see Scheme 1) (24-28).

The relationship between the reactivity (is. , the ability of the aminium radical to differentiate between different hydrogens in the same mole- cule) of the nitrogen radical and the structure of the starting N-chloro- and N-bromoamine has been studied (24, 25, 28). The authors reported varying distributions of products in the chlorjna-

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716 CANADIAN JOURNAL OF CHEMISTRY VOL. 47, 1969

H H tion was seen to change, reaction 4, Table I. An R2NCI + Mn+ + R2N. + MCln+ 0 0 analogous test of the stability of the brominated

esters showed similar results. H H

R2N. + RH + RzNH + R'. 0

TABLE I

Comparison of the isomer distribution for the halo- H H genation of methyl butyrate* at 25"

RzN-Cl + R'. -+ R'CI + RzN- -

0 0 0 Scheme 1 0 CH30C-CH2-CH2-CH,

1.f R2NHCl/Fe+ + - - 86.4 13.6 0

tion of chlorocyclohexane with several N-chloro- 2 R2NHCl/Fe+ + - - 77 6 23 4 amines (25). Furthermore they claim that by 3 c12/cC14 1 6 1 2 7 6 0 9 2 4 3

4 2,HzS04 - \ arying the metal ion used to initiate the reaction

2 1 64 3 33.6

"Values are averages of the results of three or more Independent (2712 variations in the product com~ositiolls were eyper,ments. Duplicate analyses ,%ere obtained for each experiment;

obtailled for a number of organic halogenations. the ~nd~vidual experimental values agreed to wlthln i5%. ?Taken from ref. 27.

A proper selection of N-haloamine, initiator, and substrate would allow one to selectively The mechanism proposed for halogenation by substitute chlorine or bromine at a desired posi- N-haloamines was based on the comparisoll of tion in the substrate molec~~le. 111 order to realize the in distributions reported the full synthetic usefulness of this reaction a for halogellation reactions between molecular more detailed investigation of these halogena- halogen and the N-chloroamine, and that N- tioils has been undertakes. bromodimethylamine and N - chlorodimethyl-

It was apparent that an incon- amine gave the same isomer distributions (26). sistency in the proposed mechanism for these The obvious instability of the brominated and reactions existed. The variability of the product chlorinated esters in sulfuric acid - acetic acid distribution with the initiator used argues for a mixtures casts some doubt on the reliability of more complicated sequence of reactions than had these mechanistic conclusions. been proposed, since no account had been taken The halogenation from the &lorina- of the of the Illeta' ioll in the product tion in acid media of l-ch]orobutane or l..chloro- determining abstraction reaction. propane were found to be stable for the length of

time required for the completion of the reaction. Results and Discussion These substrates were used to determine the

The ferrous sulfate initiated chlorination of relative reactivity of the different hydrogens in the methyl butyrate with N-chlorodiethylamine in reactants towards the abstracting radical in the sulfuric acid - acetic acid solvent has been halogenation process. reported to give the distribution of products seen I t was found for N-chlorodiethylamine that in Table I, reaction 1 (27). Cornparison of this catalytic amounts of cuprous chloride (2 %) were product distribution with that found for the sufficient to cause a 96 % conversion of the active photo-initiated chlorination of the ester in halogen to halogenated 1-chloropropane in 7 h, carbon tetrachloride with molecular chlorine is while under identical conditions in the absence of seen in Table I, reaction 3. When reaction 1 was cuprous ions no halogellation could be detected. repeated the product distribution shown in The catalytic effect of trace amounts of cuprous reaction 2 was obtained. ion was convincing evidence for a radical chain

It was found, however, that when the mixture process. of chlorinated esters obtained in reaction 3 was The photo-chlorination of 1-chloropropane placed in the sulfuric acid - acetic acid solvent, (0.1 M ) with molecular chlorine (1.4 x 1 O P 4 M ) allowed to stand for the same length of time, and in the sulfuric acid - acetic acid mixture gave a temperature as was reported for the N-chloro- quantitative yield of the three chloril~ated 1- amine halogenation, after re-isolation of the chloropropanes; in the presence of cuprous resultant mixture and re-analysis by gas-liquid chloride (5 x M) identical results were partition chromatography, the product distribu- obtained. The cuprous chloride (8.4 x lop4 M )

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TANNER AND MOSHER: METAL ION INITIATED HALOGENATION 717

initiated chlorination of I-chloropropane (0.1 M) with N-chlorodiethylamine (3.5 x M) in the acid media gave a 96 % yield of the chlorinated substrate. The high yield of chlorides in the copper-catalyzed chlorinations by both molecular chlorine and the chloroamine was good evidence for the lack of copper-catalyzed oxidation of the intermediate radicals (Schen~e 2) (29-32).

R ~ N H + R'H -> R2NH2 + R ' . @ 0

The production of carbonium ion intermedi- ates in concentrated sulfuric acid-acetic acid was incoilsistent with the formation of high yields of chlorination products in this media. The possibility of the inclusion of an oxidation- reduction process in the mechanistic scheme for halogenation, therefore, seems unlikely.

The product distributions for chlorination of the methyl esters reported by Miilisci et al. (27) were compared to the distributions found by Singh and Tedder (33) for the chlorination of these same esters by molecular chlorine in carbon tetrachloride solvent. Since dramatic solvent effects have been noted for chlorinations of hydrocarbons with molecular chlorine (34-36),

The variation in the product distribution for the two substrates in the two different solvents was small. As a check on the analytical procedure (i.e. isolation and analysis), and to rule out the possibility of any larger effects with other sub- strates, the comparisons of the isomer distribu- tions and relative reactivities for the halogena- tions studied were made with the results obtained from photo-initiated halogenations run in the acid solvent.

-

Attempts to obtain hydrogen selectivities for the abstracting radical in the N-chloroamine halogenatiolls proved unsuccessf~~l. When halo- genatioils of isobutane (1 "13" abstraction ratios) were attempted in the acid media isobutjll chloride was obtained and only traces of t-butyl chloride could be detected by g.1.p.c. analysis. Similar results were obtained in the chlorination of 2,3-dimethylbutane. These results illustrated the instability of the tertiary chlorides to the acid media.

Hydrogen abstraction from negatively sub- stituted hydrocarbons has been shown to be sub- stantially influenced by polar interactions be- tween the abstracting radical and the electron withdrawing substitue~it (37-39). Aminium rad- icals generated from chloroamines derived from amines having a wide range of basicities would be expected to show a varying range of polar inter- actions with substituted hydrocarbons. The cuprous ion initiated halogenation patterns for the chlorination of 1-chloropropane with AT- chlorodiethylamine, N,N-dichloropropylamine, N-chloropiperidine, and N-chlorosuccinimide are shown in Table 111.

the photo-initiated chlorination of l-chloro- TABLE 111 butane and l -ch loro~ro~ane was carried Out

Comparison of the isomer distribution for I-chloro- carbon tetrachloride and in the sulfuric acid - propane with various halogenation agents* acetic acid solvent. The halogenation patterns for " the two substrates in these solvents are compared Cl-CH2-CH2-CH,

in Table 11. N,N-Dichloropropyla~nine 16.9 51.6 31.5 N-Chloropiperidine 16.8 51.7 31.4 N-Chlorodiethylamine 16.6 51.4 32.0

TABLE I1 N-Chlorosuccinimide 16.9 51.6 31.3 Conlparison of the isomer distribution for I-chloro- C12/hv

propane and I-chlorobutane with solvent* N-Bromodiethylan~ine Br2/hv

39.2 60.8 Trace 40.4 59.6 Trace

C1-CH2-CH2-CH3 *Values are averages of the results of 3 or more independent experi-

C12/CC14 11.8 57.6 30.7 ments and duplicate analyses were obtained for each experiment. The individual experimental values agreed to within i 1 %.

C12,H2S04/HOAc 16.0 54.1 29.9 CI-CHz-CH2-CH2-CH3

C12/CC14 10.5 27.6 45.3 13.7 It can be seen that within experimental error Cl,,H,SO,/HOAc 5 . 4 20.9 52.0 21.6 the N-chloroamines, N-chlorosuccinimide, and

*values are averages of the results of 3 or more independent the photo-initiated chlorination reactions with experiments and duplicate analyses were obtained for each experiment. The individual experimental values agreed to within i 5 % . molecular chlorine gave identical results.

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718 CANADIAN JOURNAL OF CHEMISTRY. VOL. 47, 1969

The photo-initiated bromination of l-chloro- propane with molecular bromine and its cuprous ion initiated bromination with N-bromodiethyl- amine gave the same distribution of brominated products. The isomer distributions for the brom- illation and chlorination reactions are clearly different and in the case of the two N-halo- diethylamines must proceed through two inter- mediates which differ in their selectivities (see Table 111).

Furthermore, it was found that cuprous chloride, ferrous sulfate, and ferrous chloride initiation of the four N-chloro compounds stud- ied gave isomer distributions for the halogenation of 1-chloropropane which were indistinguishable experimentally from the results obtained for the photo-initiated chlorination of that substrate with molecular chlorine in solvent sulf~iric acid (see Table IV).

TABLE IV Comparison of the isomer distribution of I-chloropropane with various initiators*

C1-CH,-CH,-CH, FeSO, 15.6 53.9 30.5 F ~ C I ~ - 15.9 55.2 29.7 CuCl 16.9 51.4 32.0 hv (without acid) 17.6 51.0 31.2 Clzhv 17.2 50.7 32.2

*Values are averages of the results of 3 or more independent experiments and duplicate analyses were ob- tained for each experiment. The individual experimental values agreed to within i4%.

The results clearly show, at least with the substrate investigated, that neither the N-chloro reagent involved nor the initiator used shows any effect on the isomer distribution for halogenation. The evidence indicates that the mechanisms for the reactions studied were all radical chain reac- tions involving atomic chlorine as the abstracting species. The radical chain proposed for the chlorination reactions is identical with the reac- tion sequence established in neutral media for halogenation by N-bromo- and N-chlorosuc- cinimide (20-22,40), (see Scheme 3).

0 0 HCI + R'zNHCI -+ Clz + R'ZNHz

Clz + R . -+ RCl f C1.

Scheme 3

The chlorine atom chain mechanism was con- sidered by the earlier workers in their studies, but was rejected on the basis of their previously discussed results. They furthermore rejected the possibility of the operation of the halogen atom chain, since they reported that in sulfuric acid - acetic acid media anhydrous hydrogen chloride did not react with the N-chloroamines they studied (27). Contrary to their findings we found that when excess anhydrous hydrogen chloride was stirred with an acid solution of the N-chloro- amines, an immediate reaction took place with the liberation of a nearly quantitative yield of molecular chlorine. The observed reaction be- tween hydrogen chloride and AT-chloroamines in acid media is in agreement with the observation of Komori et al. (41) in the same systems.

Further evidence for the chlorine atom chain was obtained by the competitive halogenation of a series of substituted and unsubstituted hydro- carbons. The relative rates of the cuprous chloride initiated reactions of cyclohexane to the various substrates with the four N-halo com- pounds are compared with the photo-initiated reaction with molecular chlorine. The results are listed in Table V

TABLE V

Relative reactivities of hydrocarbons toward N-chlorodi- ethylamine and molecular chlorineT

- - --

Relative reactivity/molecule~

Cyclohexane 1 .OO 1 .OO Cyclopentane 0.841.0.07f 0.83+0.04 2,3-Dimethylbutane 0.83+0.10f 0 .89k0.06 Norbornane 0.6810.03 0.741.0.08 2,2,3,3-Tetramethylbutane 0.59 + 0.03 0.65 + 0.01 1 -Chlorobutane 0.36+0.03$ 0.36+0.08

*Values are averages of the results of 3 or more illdependent experiments and duplicate analyses were obta~ned for each.

+The experimental errors nere those reported for.?'-chlorodiethyla- mine.

$Identical results were obtained with A'-chlorosuccinimide and N,N-dichloropropylamine.

The use of relative reactivities as a probe for the investigation of the intermolecular selec- tivities of a specific radical has the advantage that the ratio of rate constants is obtained from the determination of the relative consumption of the two substrates, and that this value is not affected by the subsequent fate of the inter- mediates involved. Within experimental error the

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TAKNER AND MOSEIER: METAL ION INITIATED HALOGENATION 719

relative rates for the cuprous chloride initiated chloride. The solution was degassed by the freeze-thaw

halogenation reactions of N-chlorodiethylamine, mahod and then irradiated In sealed Pyrex ampoules until the chlorine color had disappeared. The reaction

N-chlorosuccinimide, and N,N-dich lOrO~ro~~l - m,xture ass analyzed upon a 10 ft x + in, DEGS g,l.p.c. amine were found to be identical with the column. Three of the four chlorination product peaks relative rates determined for the photo-initiated were identified by their identical retention times on chlorination reactions of lnolecular chlorine. 2 different g.1.p.c. colun~ns with commercially available

-,-he initiation reaction for the production of compounds. The chloromethyl butyrate was assigned by elimination and by comparison of this substitution pattern

chlorine atoms by the interactioll of the appro- with that obtained from the reaction of molecular ~ r i a t e metal ion and an N-halo compound can chlorine. ;rise by one of two paths; either (1) the reduction of the ili-chloroammoniurn salt to the neutral arnine and a chlorine atom, or (2) the production of an arninium radical and a chloride ion. The chloride ion can subsequently react with the N-halo compound to produce molecular chlorine (see Scheme 4). No direct evidence is available for a choice lietween either of these initiation reactions.

Conclusion The metal ion catalyzed chlorination of hydro-

carbon substrates by N-chloroamines and imides proceeds by a free radical chain mechanism. The chain carrying species in these halogenations is the chlorine atom, and the selectivity and re- activity of the reagents are unaffected by the structure of the N-halogenating agent or the metal ion catalyst used to initiate the reaction. No evidence is found for metal ion promoted oxidation-reduction reactions of the inter- mediates involved.

Experimental Materials

The method of Coleman (42) was used to prepare the N-haloamines. These were used without further purifica- tion. Iodometric titration showed a purity of 90-102%. Phillips research grade cyclohexane, 2,3-dimethylbutane, cyclopentane, n-butane, and i-butane were used without further purification. All other substrates were com- mercially available; their purity was checked by gas-liquid partition chromotography and they were distilled when necessary.

Methyl butyrate was chlorinated by thc method of Minisci et al. (27,28) using N-chlorodiethylamine andiron sulfate. The reaction products were isolated and analyzed on a 10 ft x & in. DEGS g.1.p.c. column.

Effect of Sulfuric Acid - Acetic Acid upon Chlorirzated Methyl Butyrate, I-Chloropropane, and I-Chlorobutane

The mixture obtained in the chlorination of methvl butyrate with molecular chlorine in carbon tetrachloride was subjected to the reaction reported for the N-chloro- alnine chlorination and the resultant mixture was re- isolated using this isolation procedure (24) and reanalyzed.

When this procedure was repeated for the mono- chlorinated 1-chloropropanes and 1-chlorobutanes the reanalyzed isomer distributions were found to remain unchanged. The monochlorinated 1-chloropropanes and I-chlorobutanes were identified and quantitated by the method previously reported by this laboratory (39).

Bromination of I-C/zloropropane Bromination products of 1-chloropropane were

analyzed on a 25 ft x Bin. 5 % Polar g.1.p.c. column. The products were identified by comparison with the previously reported bromination of I-chloro-propane with n~olecular bromine (33).

Catalytic Effect of Cuprous Chloride The formation of the three dichlorinated l-chloro-

propanes was observed from the reaction of N-chloro- diethylamine (2.9 x M), 1-chloropropane (0.1 M), and cuprous chloride (5.2 x lo-' M ) . The mixture was degassed and shaken for 7 h, l,4-dichlorobutanewas added as an external standard and the organic layer analyzed as above. Molar amounts of the chlorination products were calculated from calibration plots of area ratios vs. mole ratios for 1,4-dichlorobutane vs. the authentic samples. The yield of the three dichlorinated 1-chloropropanes was found to be 96 %. Photo-initiated Chlorination of I-Chloropropane and

I-Chlorobutane with Molecular Chlorine in Sulfuric AcidlAcetic Acid

Heterogeneous mixtures of a solution of l-chloro- propane (0.1 M) and chlorine (1.4 x M ) and 10 ml of 85/15 sulfuric acidlacetic acid solution were placed in Pyrex ampoules, degassed and sealed. The mixtures were continuously shaken and irradiated at room temperature for 15 min. The yield of the three dichlorinated l-chloro- propanes was determined to be 98% based upon 1,4- dichlorobutane as an external standard.

C1zlorinatio~z of Methyl Butyrate Heterogeneous mixtures of a solution of l-chloro- Methyl butyrate (0.1 M ) in carbon tetrachloride was propane (0.1 M) and chlorine (1.4 x M ) and 10 ml

added to a solution of chlorine (0.04 M) in carbon tetra- of 85/15 sulfuric acidlacetic acid solution containing

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720 CANADIAN JOURNAL OF CHEMISTRY. VOL. 47, 1969

cuprous chloride (5 x M) were placed in Pyrex ampoules, degassed and sealed. The mixtures were con- tinuously shaken and irradiated at room temperature for 15 min. The yield of the three dichlorinated l-chloropro- panes was determined to be 96% based upon l,4-dichloro- butane as an external standard.

An identical procedure was carried for the photo- initiated chlorination and isolation of the halogenation products of 1-chlorobutane. The four dichlorinated bu- tanes were analyzed by the method previously reported (39).

Chlorination of I-Chloropropane by N-Chlorodiethylatnit~e To 1-chloropropane (0.1 M ) - and N-chlorodiethyl-

amine (3.5 x M ) in a Pyrex ampoule were added 10 n ~ l of sulfuric - acetic acid (85115) containing cuprous chloride (8.4 x M). The ampoules were degassed and shaken for 3 h. The yield of the three dichlorinated 1-chloropropanes was determined to be 96$/, using 1,4- dichlorobutane as an external standard.

Chlorination of I-Chloropvopane and I-Chlorobutarze with Molecular Chlorine in Carbon Tetraclzloride

The 1-chlorohydrocarbons were chlorinated and quantitated by the method previously reported (39).

Chlorination with N-Haloamines and Zmides To a mixture of the accurately weighed N-haloanline

or imide (0.126 M ) in 50 ml of sulfuric acidlacetic acid (85115) were added the hydrocarbon (1.9 M) and the finely powdered metal ion initiator (0.126 M). The reac- tion mixture was stirred rapidly at room temperature for 2 h and then poured on ice, and the organic layer was separated, washed with 5 % sodium bicarbonate solution, and dried over magnesiuin sulfate. The dried reaction mixture was then examined by gas-liquid partition chro- matography. The isolation method was shown to be suit- able for this analysis by subjecting known synthetic mix- tures to this procedure.

In an alternative method an accurately weighed portion of the N-haloamine or imide (0.126 M ) was added to the hydrocarbon (1.9 M ) in Pyrex ampoules. The solution was frozen and a solution of the metal initiator in sulfuric acidlacetic acid (85115) was added to give a metal ion concentration between 0.0126 and 0.063 M. The tubes were degassed by the freeze-thaw method and the sealed ampoules were continuously shaken for 1-4 h. The ampoules were opened and the organiclayer was examined by gas-liquid partition chromatography.

The values obtained by both of the above methods gave results within experimental error. The latter method uas used exclusively in the competition reactions.

Detection of Chlorine from N-Chloroatnitzes In a Pyrex ampoule was placed a standard solution

containing 2 x M of the N-chloroamine in sulfuric acidlacetic acid (85115). The contents were frozen and dry hydrogen chloride was condensed on the frozen solution. The ampoule was connected to a vacuum line and the system evacuated and allowed to warm. An instant reaction commenced with the evolution of a gas. The gas was fractionated through a series of traps and found to freeze at between -98 and - 105" as a yellow solid. The literature value reported for chlorine was - 101.5" (43). The material was identified as chlorine by

conlparison of its retention on a Polypak 1 (6 ft x +in.) g.1.p.c. column and by its mass spectrum. The yield of chlorine was approximated to be 100% by comparing peak areas obtained from known concentrations of chlorine with the areas of the gas obtained from the reaction.

Coinpetitive Chlorinafiotz of Hvdrocarbotzs The competitive chlorinations using molecular chlorine,

N-chloroamines, or imides werecarried out by the method previously reported (39). The relative reactivities were calculated by the method previously reported in this laboratory (44). The values obtained are reported in Table \'.

Acknowledgments The au thors wish to thank the National

Research Council of Canada and the University of Alberta for their generous support of this work.

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