Substituent effects in the gas-phase pyrolysis of 4-(N/-arylamino)-1-butyl acetate and 5-(N/-arylamino)-1-pentyl acetate

Article abstract of DOI:10.1002/1099-1395(200011)13:11<675::AID-POC298>3.0.CO;2-5

A kinetic study of the gas-phase pyrolysis of 4- and 5-(N-arylamino)-1-butyl and -1-pentyl acetate was mace. Each ester was pyrolysed at five or six different temperatures and over a temperature range not less than 50 °C. These reactions were homogeneous and unimolecular. The reactivities of these compounds were compared with those of the parent compounds 4- and 5-(N-phenyl)-1-butyl and -1-pentyl acetate and with each other. The products of the decomposition reaction of these compounds suggest an anchimeric assistance of the amino substituen for a trans elimination, indicating that an ion-pair type of mechanism may be operating during the process of elimination. Copyright

Full text of DOI:10.1002/1099-1395(200011)13:11<675::AID-POC298>3.0.CO;2-5

JOURNAL OF PHYSICAL ORGANIC CHEMISTRY
J. Phys. Org. Chem. 2000; 13: 675–678
Substituent effects in the gas-phase pyrolysis of
4-(N-arylamino)-1-butyl acetate and 5-(N-arylamino)-
1-pentyl acetate
Nouria A. Al-Awadi,1* Mohamed H. Elnagdi,1 Kamini Kaul1 and Gabriel Chuchani2
1Department of Chemistry, University of Kuwait, P.O. Box 5969 Safat, 13060 Kuwait
2Centro de Quimica, Instituto Venezolano de Investigaciones Cienti®cas (IVIC), Apartado 21827, Caracas 1020-A, Venezuela
Received 20 January 2000; revised 15 April 2000; accepted 22 May 2000
ABSTRACT: A kinetic study of the gas-phase pyrolysis of 4- and 5-(N-arylamino)-1-butyl and -1-pentyl acetate was
made. Each ester was pyrolysed at five or six different temperatures and over a temperature range not less than 50°C.
These reactions were homogeneous and unimolecular. The reactivities of these compounds were compared with those
of the parent compounds 4- and 5-(N-phenyl)-1-butyl and -1-pentyl acetate and with each other. The products of the
decomposition reaction of these compounds suggest an anchimeric assistance of the amino substituent for a trans
elimination, indicating that an ion-pair type of mechanism may be operating during the process of elimination.
Copyright 2000 John Wiley & Sons, Ltd.
KEYWORDS: gas-phase pyrolysis; 4-(N-arylamino)-1-butyl acetate; 5-(N-arylamino)-1-pentyl acetate; substituent
effects
INTRODUCTION
acetate, which was found to give N-methylpyrrolidine
and methyl acetate.4 This result suggested an anchimeric
assistance provided by the (CH3)2N group and the
mechanism was explained in terms of a modest intimate
ion-pair type of intermediate, where the acetoxy leaving
group proceeds by intramolecular solvation to take up a
CH3 group from (CH3)2N to give the products.
Esters containing non-vinylic b-hydrogen atoms in the
alkyl group undergo elimination to an alkene and a
carboxylic acid, this takes place most readily if the
hydrogen atom and acyloxy group are cis to each other:1
This was followed by our recent work5 which
examined the extent to which the presence of a phenyl
group at the nitrogen atom may affect the participation of
the amino group in the gas-phase elimination of 4-(N-
methyl-N-phenylamino)-1-butyl acetate and 4-(N-pheny-
lamino)-1-butyl acetate [Ph(CH3)N(CH2)4OAc and
PhNH(CH2)4OAc, respectively]. Also, the comparative
influence of the N-atom on neighboring group participa-
tion was made clear from the kinetic results, which
suggested that the lower the nucleophilicity of the amino
substituent the less is its anchimeric assistance in the
stabilization of the polarized C—O bond in the five-
membered transition state (Scheme 1).
The cyclic nature of reaction (1), first proposed by
Hurd and Blunck,2 is now recognized as a symmetry-
allowed 1,5-hydrogen shift, of which many examples are
known. This mechanism was shown to be consistent with
the cis character of the elimination reaction, which has
been demonstrated by a number of experiments. The
most elegant of these is the pyrolysis of threo- and
erythro-2-deutero-1,2-diphenylethyl acetate.3
An unusual homogeneous, unimolecular trans elim-
ination of an ester in the gas phase has been described in
the pyrolytic kinetics of 4-(N,N-dimethylamino)-1-butyl
*Correspondence to: N. Al-Awadi, Department of Chemistry,
University of Kuwait, P.O. Box 5969 Safat, 13060 Kuwait.
Contract/grant sponsor: University of Kuwait; Contract/grant num-
ber: SC 098.
Contract/grant sponsor: CONICIT (Consejo Nacional de Investiga-
ciones Cientificas y Tecnologicas); Contract/grant number: S1-
970000S.
Scheme 1
Copyright 2000 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2000; 13: 675–678

676
N. A. AL-AWADI ET AL.
The above is in agreement with our recent findings on
RESULTS AND DISCUSSION
the gas-phase pyrolysis of 5-(N-phenylamino)-1-pentyl
acetate, which took place via a six-membered transition
state to produce N-phenylpiperidine (Scheme 2).
The kinetic data and Arrhenius log A and Ea values for
the present series of compounds are given in Table 1.
Each of the substrates gave excellent and reproducible
first-order kinetics. The homogeneity of the reaction was
tested by carrying out several runs in a reaction tube with
a surface-to-volume ratio of 6.0 relative to that of the
normal reaction tube. The results show that the change in
the rate constant was within experimental error. Hence an
increase in the surface area of more than 50% caused a
negligible increase in the reaction rate constant. The
pyrolysis reaction of butyl and pentyl esters proceeds
according to the previously suggested pathways (Scheme
1) to produce N-arylpyrrolidine and N-arylpiperidine,
respectively, which were perfectly characterized by gas
chromatography–mass spectrometry (GC–MS) and NMR
spectroscopy.
Scheme 2
In order to learn more about this interesting neighbor-
ing group participation in the gas-phase pyrolysis of
amino acetates, the effects of different substituents on
the reaction rate were evaluated. We therefore prepared
the following substrates and measured their rates of
gas-phase pyrolysis: ArNH(CH2)4OAc and ArNH(CH2)5
OAc, where Ar = 4-MeOC6H4, 4-MeC6H4 and 4-ClC6H4.
The present and reported kinetic data in Table 1
together with the relative rate constants in Table 2 are
discussed on the basis of the elimination pathway
represented in Scheme 1. The kinetic data and product
analysis reveal the following features of this pyrolysis
reaction.
Table 1. Kinetic data for the gas-phase pyrolysis of ArNH(CH2)nOAc
1
1
n
Ar
T(K)
104k(s
1.74
4.82
8.41
11.10
24.90
1.84
2.92
6.75
9.91
19.30
0.66
1.80
4.19
9.30
)
Log [A(s 1)]
11.95 Æ 0.05
Ea (kJ mol )
5
4-MeOC6H4
558.6
574.6
583.7
588.8
602.8
580.9
589.7
605.3
611.4
625.0
571.4
586.3
599.8
614.0
628.2
537.2
543.4
549.1
558.9
573.0
536.1
540.1
546.4
567.7
572.3
578.5
568.1
585.7
588.8
592.6
606.0
167.95 Æ 0.58
161.90 Æ 2.54
184.51 Æ 1.90
171.43 Æ 0.75
140.78 Æ 0.18
5
5
4
4
4-MeC6H4
4-ClC6H4
10.82 Æ 0.05
12.69 Æ 0.17
13.05 Æ 0.07
10.15 Æ 0.16
22.70
2.38
3.73
4-MeOC6H4
4-MeC6H4
5.52
10.80
26.30
2.69
3.43
5.16
15.40
20.70
27.40
2.54
7.32
8.57
4
4-ClC6H4
11.08 Æ 0.23
159.47 Æ 2.61
10.60
20.90
Copyright 2000 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2000; 13: 675–678

PYROLYSIS OF 4- AND 5-(N-ARYLAMINO)-1-BUTYL AND -1-PENTYL ACETATES
Table 2. Rate constants at 600 K and relative rates of
thermal gas-phase elimination reactions of
XC6H4(CH2)nOAc
677
EXPERIMENTAL
Synthesis. A solution of 4-bromobutyl acetate or 5-
bromopentyl acetate (0.05 mol) and the substituted
aniline (0.05 mol) in 20 ml of toluene was refluxed for
9–12 h. The reaction mixture was acidified with 10%
hydrochloric acid. The toluene layer was separated and
the aqueous layer was basified by slow addition of solid
sodium hydrogencarbonate. It was then repeatedly
extracted with diethyl ether, dried over sodium sulfate
and concentrated in vacuo, followed by distillation under
reduced pressure to give the following compounds.
4-Amino-N-(p-methoxyphenyl)butyl acetate, solidi-
fied after solvent evaporation and crystallized from
benzene–light petroleum in 55% yield, m.p. 66°C. Anal.
Calcd for C13H19NO3 (237): C, 65.82; H, 8.01; N, 5.91.
Found: C, 66.15, H, 7.96; N, 6.01%. 1H NMR (CDCl3): ꢀ
1.76–1.82 (s, 4H, 2CH2), 2.04 (s, 3H, CH3), 2.92–3.22 (t,
2H, CH2), 3.74 (s, 3H, CH3), 3.94–4.21 (t, 2H, CH2);
n = 4
n = 5
1
1
X
104 k (s
)
krel
104 k (s
)
krel
4-MeO
4-Me
4-Cl
H
133.4
78.9
15.6
460.0
272.1
53.8
1.0
21.3
5.25
4.22
0.08
280.3
69.1
55.5
1.0
0.29
1. The overall reactivities are the result of (i) the
electronic synergism involving the availability of the
lone pair of electrons on the nitrogen atom (Scheme
2), which in turn effects the polarity of bond (a) (see
Scheme 3) and (ii) the stabilizing influences asso-
ciated with the product formation.
2. The rate constants of the thermal gas-phase elimina-
tion reactions (Table 2) reveal large differences in the
relative reactivities with the pentyl esters, which are
less reactive than their butyl equivalents, e.g. 5-[N-(4-
methylphenylamino)]-1-pentyl acetate is 15 times less
reactive than its butyl counterpart. This rate enhance-
ment of butyl over pentyl esters is attributed to the fact
that the five-membered structure is more favored than
the six-membered structure in the neighboring group
participation.6
3. Reactivity is increased by the mesomerically electron-
donating 4-methoxy substituent. This is because the
methoxy substituent will enhance the nucleophilicity
of the nitrogen atom, which will lead to a large
anchimeric assistance in the stabilization of the
polarized Ca—O bond in the transition state (Scheme
3). This argument is well justified when the methoxy
group is replaced by a chloro substituent.
‡
6.54–6.82 (dd, 4H, ArH). MS: m/z 237 (M ), 176 (M
‡
CH3CO2H2), 136 [100%, (CH3OC6H4NHCH2) ], 123
(CH3OC6H4NH2).
4-Amino-N-(p-methylphenyl)butyl acetate, solidified
after solvent evaporation and crystallized from diethyl
ether–light petroleum in 60% yield, m.p. 69–70°C. Anal.
Calcd for C13H19NO2 (221): C, 70.58; H, 8.59; N, 6.33.
Found: C, 70.46, H, 8.45; N, 6.23%. 1H NMR (DMSO): ꢀ
1.48–1.75 (m, 4H, 2CH2), 2.01 (s, 3H, CH3), 2.88–3.11 (t,
2H, CH2), 3.65 (s, 3H, Ar-CH3), 3.91–4.16 (t, 2H, CH2),
‡
6.43–6.80 (q, 4H, ArH). MS: m/z 221 (M ), 160 (M-
‡
CH3CO2H2), 120 [100%, (CH3C6H4NHCH2) ], 106
‡
[(CH3C6H4NH) ].
4-Amino-N-(p-chlorophenyl)butyl acetate, b.p. 168–
170°C (1 mbar), solidifies while distilling to a pale
yellow semi-solid in 62% yield, m.p. 59–60°C. Anal.
Calcd for C12H16CINO2 (241.5): C, 59.62; H, 6.63; N,
1
5.79. Found: C, 59.47, H, 6.79; N, 5.79%. H NMR
(DMSO): ꢀ 1.45–1.76 (m, 4H, 2CH2), 1.99 (s, 3H, CH3),
2.90–3.08 (t, 2H, CH2), 3.95–4.11 (t, 2H, CH2), 5.66
(br s, 1H, NH), 6.49–6.60 (dd, 2H, ArH), 7.01–7.12 (dd,
‡
2H, ArH). MS: m/z 241.1, 243 (M ), 207 [(M-Cl) ‡ 1],
180, 182 (M-CH3CO2H2), 140, 142 [100% (ClC6H4
‡
NHCH2) ], 127, 129 (ClC6H4NH2).
Scheme 3. Effective participation of the lone pair of
electrons in the gas-phase pyrolysis of 4-[N-(4-methoxy-
phenylamino]-1-butyl acetate
5-Amino-N-(p-methoxyphenyl)pentyl acetate, dis-
tilled at b.p. 198°C (1.3 mbar) in 50% yield. Anal. Calcd
for C14H21NO3 (251): C, 66.93; H, 8.36; N, 5.57. Found:
C, 66.80, H, 8.30, N, 5.62%. 1H NMR (CDCl3): ꢀ 1.4–1.6
(m, 6H, 3CH2), 2.04 (s, 3H, CH3), 2.85 (s, 1H, NH,
exchanges with D2O), 3.10–3.20 (t, 2H, CH2), 3.73 (s,
3H, CH3), 3.98–4.09 (t, 2H, CH2), 6.48–6.84 (m, 4H,
4. The most reactive compound in this series is 4-
MeOC6H4NH(CH2)4OAc. This is to be expected as a
consequence of the electronic effect of the methoxy
group on the availability of the lone pair of electrons
on the nitrogen atom and accordingly on intramol-
ecular solvation of the acetoxy leaving group, together
with the effective participation of the neighboring
group in the formation of a five-membered structure
(N-p-methoxyphenylpyrrolidine) over a six-mem-
bered structure (N-p-methoxyphenylpiperidine).
‡
ArH). MS: m/z 251 (M) , 192 (M OCOCH3), 176
[(M CH3CO2H) CH3], 136 [100%, (CH3OC6H4
‡
NHCH2) ].
5-Amino-N-(p-methylphenyl)pentyl acetate, distilled
at 180°C (1.5 mbar) in 51% yield. Anal. Calcd for
C14H21NO2 (235): C, 71.48; H, 8.93; N, 5.95. Found: C,
71.21, H, 8.72, N, 6.26%. 1H NMR (CDCl3): ꢀ 1.22–1.78
Copyright 2000 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2000; 13: 675–678

678
N. A. AL-AWADI ET AL.
(m, 6H, 3CH2), 2.03 (s, 3H, CH3), 2.90–3.07 (t, 2H,
CH2), 3.73 (s, 3H, CH3), 3.96–4.09 (t, 2H, CH2), 4.64 (s,
1H, NH, exchanges with D2O), 6.48–6.88 (m, 4H, ArH).
A sample (0.2 ml) of the standard solution was placed
in the reaction tube (9 ml) and the contents were sealed
under vacuum. The tube was placed swiftly in the
pyrolyzer which had been preheated to a threshold
temperature that together with a convenient length of
reaction time would allow only 10–20% pyrolysis. The
contents of the reaction tube were then analyzed using
HPLC. For each substrate, the rate coefficient represents
an average from triplicate runs with a rate agreement to
within Æ2%. Rates were measured at regular intervals of
1–10°C over a 46–50°C temperature range and up to
ꢀ95% reaction.
‡
MS: m/z 235 (M ), 174 (M CH3CO2H2), 120 [100%,
‡
‡
(CH3C6H4NHCH2) ], 106 [(CH3C6H4NH )].
5-Amino-N-(p-chlorophenyl)pentyl acetate, distilled at
210°C (0.5 mbar) in 55% yield. Anal. Calcd for
C13H18NO2Cl (255.5): C, 61.05; H, 7.05; N, 5.48. Found:
C, 61.09, H, 6.80, N, 5.66%. 1H NMR (DMSO): ꢀ 1.42–
1.78 (m, 6H, 3CH2), 2.02 (s, 3H, CH3), 2.85–3.37 (t, 2H,
CH2), 3.85–4.06 (t, 2H, CH2), 6.51–6.62 (d, 2H, ArH),
‡
7.04–7.15 (d, 2H, ArH). MS: m/z 255, 257 (M ), 194, 196
[(M CH3CO2H2)], 140, 142 [100%, (ClC6H4
Rates were reproduced using tubes packed with glass
helices in order to increase the reactor surface up to about
ninefold. This precaution was to test for reactor surface
effects. In addition, rates were measured in the presence
and absence of free-radical scavengers (e.g. cyclohexene)
to confirm the non-involvement of free radical pathways.
The rate coefficients were calculated using the
expression kt = ln(a0/a) for first-order reactions. The
Arrhenius parameters were obtained from a plot of log k
vs 1/T (K). The elimination rate constant at T is given by
‡
NHCH2) ], 127, 129 (ClC6H4NH2).
Instrumentation. The pyrolyzer was a CDS custom-
made unit. It consisted of an insulated aluminum block to
ensure a low temperature gradient and resistance to high
temperature, a Pyrex reaction vessel to fit in a groove
along the cylindrical axis of the block, a platinum
resistance thermometer and a thermocouple connected to
a microprocessor. The temperature of the block was
controlled by means of a Eurotherm 093 precision
temperature regulator. The temperature setting was
achieved with a digital switch which allowed temperature
read-out accurate to Æ0.5°C. The actual temperature of
the reactor groove was measured by means of a Comark
microprocessor thermometer. Comparative quantitative
analyses of reaction mixtures were carried out using a
high-performance liquid chromatographic (HPLC) sys-
tem equipped with a UV–visible detector (Bio-Rad,
Shimadzu SPD-10AV). The HPLC accessories included
an LC-8 column (25 cm  4.6 mm i.d.). Samples were
injected with a Supelco 5 mm precision syringe.
1
1
log k ˆ log‰Aꢁs †Š EaꢁkJ mol †=4:574TꢁK†
Product analysis. The products analyzed were for
pyrolyses conducted at temperatures commensurate with
those of the kinetic measurements. No secondary
decomposition was observed for any of the compounds
under study. In both the kinetic runs and product analysis,
the initial conversion of 10–20% and reaction extent of
ꢀ95% pyrolysis were maintained.
Characterization of the substrates and the constituent
fragments of the pyrolysates included elemental analysis,
GC-MS, Fourier transform (FT) IR and 1H NMR
spectroscopic analysis. The GC-MS instrument included
a Finigan MAT INCOS XL quadruple mass spectro-
meter. FT-IR spectra were measured with a Perkin-Elmer
Model 2000 spectrometer and H NMR spectra with a
Bruker AC-80 spectrometer. The elemental analyzer was
a LECO CHNS-932 unit.
Acknowledgements
The support of the University of Kuwait through research
grant SC 098 and the facilities of ANALAB and SAF are
gratefully acknowledged. The support of CONICIT
(Consejo Nacional de Investigaciones Cientificas y
Tecnologicas) of Venezuela, Project No. S1-970000S,
is also appreciated.
1
Kinetic runs and data analysis. Dilute (ppm) standard
solutions of the substrates in a suitable solvent (e.g.
acetonitrile) were prepared and an internal standard (e.g.
chlorobenzene) was added to each solution. Care was
taken to ensure that both the solvents and the internal
standards were not pyrolyzable under the conditions of
the reaction. Further, the standard solutions used in the
kinetic runs always comprised a mixture in which the
substrate gave a peak one third higher than that of the
internal standard in the analytical HPLC trace. Prepara-
tion of a standard solution allowed several kinetic runs to
be performed on the same reaction mixture.
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6. Capon B, McManus SP. Neighboring Group Participation, vol. I.
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Copyright 2000 John Wiley & Sons, Ltd.
J. Phys. Org. Chem. 2000; 13: 675–678