Proton transfer reactions from dimethyl (4-nitrophenyl)malonate to N-bases in acetonitrile

Journal of

MOLECULAR STRUCTURE ELSEVIER

Journal of MolecularStructure384 (1996) 127-133

Proton transfer reactions from dimethyl (4-nitrophenyl)malonate to N-bases in acetonitrile G r z e g o r z S c h r o e d e r a, B o g u m i l B r z e z i n s k i a, A r n o l d J a r c z e w s k i a, E u g e n i u s z G r e c h b, Piotr Milart c aFaculty of Chemistry, Adam Mickiewicz University, Grunwaldzka 6, 60-780 Poznah, Poland blnstitute of Fundamental Chemistry, Technical University of Szczecin, 71-065 Szczecin, Poland CFaculty of Chemistry, Jagiellonian University, R. Ingardena 3, 30-060 Krak6w, Poland

Received28 March 1996; accepted29 April 1996

Abstract

Deprotonations of dimethyl (4-nitrophenyl)malonate (C-acid) by 7-methyl-l,5,7-triazabicyclo[4.4.0]dec-5-ene (MTBD), 1,5,7-triazabicyclo[4.4.0]dec-5-ene (TBD) and 1,8-bis(dimethylamino)naphthalene (DMAN) in acetonitrile were studied by kinetic as well as b-TIR and ~H NMR spectroscopic methods. In the 1:1 mixture of C-acid with DMAN no proton transfer was found. The N-bases with guanidine-like character deprotonate C-acid in the acetonitrile solution quantitatively. The mechanisms of proton transfer reactions are discussed. Keywords: Dimethyl (4-nitrophenyl)malonate; Strong N-base; Proton transfer reaction; Kinetics; ~H NMR and FFIR

spectroscopy

I. Introduction

The proton transfer in acid-base catalysis is an important chemical step and depends on many different factors[I-3]. The proton transfer may be slow or fast, depending on the systems in which the proton is transferred. The most decisive factor is the type of acid involved. In the case of proton transfer between oxygen and nitrogen atoms the rate is very often diffusion-controlled, if the process is thermodynamically favourable. However, a number of factors can make the proton transfer reaction slow. 1. Whether the process involves neutral molecules or species with like charges [4]. 2. Large steric factors of the reacting species, in particular the reacting base [5].

3. Strong intramolecular hydrogen bond in the substrate (e.g. protonated proton sponges, etc.) [6-9]. 4. Whether the electron distribution must undergo major rearrangement during proton transfer (e.g. acetylacetone, carbonic acids, etc.) [1,2]. 5. Solvation effects accompanying of the proton transfer reaction [10]. 6. Whether the proton is transferred from the C - H bond. The parameters influencing the proton transfer reaction from C - H acids have been studied for many years [11-19]. The most investigated C-acids were compounds involved nitro or cyano groups. Recently we have started to study a relatively new class of C-acid including ester groups [20-22]. In this paper the influence of N-bases with

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128

G. Schroeder et al./Journal of Molecular Structure 384 (1996) 127-133

guanidine-like character on the rate constants and the mechanism of the proton transfer reaction from dimethyl (4-nitrophenyl)malonate (C-acid) to these N-bases in acetonitrile is studied.

2. Experimental 7 - Methyl - 1,5,7 - triazabicyclo [4.4.0]dec - 5 -ene (MTBD) and 1,5,7-triazabicyclo[4.4.0]-dec-5-ene (TBD) were used as commercial products (Aldrich and Merck). Dimethyl (4-nitrophenyl)malonate was prepared according to the procedure described previously, m.p. 125-127 °C [23].

O'Donnell et al. with final distillation o v e r P 2 0 5 (81.5-82.0 °C)[24]. The kinetic runs were carried out using a stoppedflow spectrophotometer (Applied Photophysics) with the cell block thermostated to +- 0.1 °C. The kinetic runs were completed under pseudo-first-order conditions with the base concentration in large excess. The observed rate constants were calculated from the traces of absorbance vs. time. The observed rate constant kobs depends on the base concentration and is given by the equation: ]Cobs = k[B] + k_ [BH + ]Kd 1 for a two - step

mechanism

2.1. Spectroscopic measurements

or

Spectroscopic grade acetonitrile and [2H3]acetonitrile were dried over 3 ,~ molecular sieves. The 1:1 complexes of C-acid with N-bases were obtained by mixing of equimolar amounts of 0.1 mol dm-3 acetonitrile solutions of the C-acid with the corresponding N-base. The concentration of the solutions was 0.1 mol dm -3. The 1:1 complex of MTBD with HAuC14 was prepared by mixing of 0.1 mol dm-3 ethanol solutions of N-base and HAuC14. The solvent was evaporated under reduced pressure at room temperature. The residue was dissolved in acetonitrile to obtain the concentration of solutions equal to 0.1 mol dm -3. The 1H NMR spectra were recorded in [2H3]acetonitrile (0.1 mol dm -3) at 20 °C with a Bruker DPX spectrometer at 400 MHz, using TMS as internal standard. The spectral width was 40 000 Hz, the number of transients - 250, the acquisition time 1.998 s and the pulse width, 8.0/xs. [2H3]acetonitrile was used as the solvent and internal lock. The IR spectra of the samples were taken with a FTIR spectrophotometer (Bruker IFS 113 v) using a cell with Si windows and a wedge-shaped layer to avoid interference (sample thickness, 0.176 mm; detector, DTGS; resolution, 2 cm-1). The temperature of the samples was 293 K. All preparations and transfers of the solutions were carried out in a carefully dried glove box.

kobs=k[B]+k_[BH +] for one step of proton

2.2. Kinetic measurements Acetonitrile was purified by the method of

transfer mechanism where k is the rate constant for the forward proton transfer reaction, k. the rate constant for the reverse proton transfer reaction, [B] the initial base concentration, [BH +] the concentration of the cation, and Kd the dissociation constant of the ion pair. Rate constants for the forward (k) reaction were calculated by linear least-squares fit of the variation of kobs VS. base concentration. The activation parameters were calculated by a linear least-squares fit of In k vs. 1/T.

O•ctOCH3 O2N~!'"'"H o//CXocH3 C-acid

CH3XN/ CH3CH3 ~N~CH3 HI DMAN

TBD

Scheme l.

I

CH3 MTBD

G. Schroeder et al./Journal of Molecular Structure 384 (1996) 127-133

3. Results and discussion MTBD (pKa -- 24.7 [211), TBD (pKa- 24.97 [25]) and 1,8-bis(dimethylamino)naphthalene (DMAN) (pK~ = 17.28 [26]) were used for the deprotonation of dimethyl (4-nitrophenyl)malonate (C-acid) in acetonitrile. Scheme 1.

3.1. Spectroscopic measurements The IR spectra of C-acid and its 1:1 mixture with DMAN are given in Fig. 1. For comparison the spectrum of DMAN is also shown. In the spectrum of an acetonitrile solution of DMAN two intense so-called Bohlmann bands are observed at 2780 and 2831 cm -1. In the 1:1 mixture of DMAN with C-acid the integrated intensity of these Bohlmann bands is almost the same as in the spectrum of DMAN, which proves that no proton transfer from C-acid to DMAN occurs and no hydrogen bonds between these two molecules exist in this mixture because previously it was observed that these bands vanished completely with protonation of DMAN [27-30]. The same independent information can be drawn from the very intense bands of u(C = O) vibrations of COOCH3 groups as well as p~s(NO2) and v~(NO2) vibrations. The bands v(C = O) vibrations of COOCH3 groups are recorded at 1738 and 1760 cm -1 as well as the v~(NO2) and

129

J's(NO2) band at 1530 and 1351 cm -1, respectively. The positions and the intensities of these bands are the same in both C-acid and its 1:1 mixture of C - H acid with DMAN spectra. In Fig. 2 the spectra of C-acid and its 1:1 mixture with MTBD as well as the complex of MTBD with H A u C I 4 are shown. The spectrum of the 1:1 complex of MTBD with HmuCl4 shows one band of v(N-H) + vibration at 3377 cm -1 and two intense bands of v(C-N) vibrations at 1625 and 1600 cm -1. These bands are very characteristic of the protonated MTBD because in the case of acetonitrile solution of the non-protonated MTBD these three bands were not observed [31,32]. In the spectrum of 1:1 mixture of C-acid with MTBD the three bands at 3377, 1625 and at 1600 cm -1 are observed, indicating a proton transfer reaction in this mixture. Furthermore, the v(C = O) vibrations of ester groups as well as the v~(NO2) and vs(NO2) vibrations, observed in the spectrum of free C-acid, vanished completely, and instead of the last two bands new intense broad bands 1270 and 1170 cm -1 are observed. These bands can be ascribed to the v~s(NO2) and ~,s(NO2) vibrations, respectively. Previously such intense bands in this region were observed for nitro compounds in which the nitro groups were coordinated to a metal and for NO~ salts [33,34]. All these results demonstrate that the proton abstraction from C-acid by MTBD is

O

:"

[--

O

"4000

3500

3000

2500

2000

1500

1000

WAVENUMBER [cm-1] Fig. 1. FrlR spectra in acetonitrile solutions of: ( - - - ) C-acid, (

) its mixture with DMAN, and ( - • - ) DMAN.

500

G. Schroederet al./Journalof Molecular Structure384 (1996) 127-133

130

o

! ~i',~

ii

! "i ,1

;I'W i I II

II h ,!! I1~ "'~I a

0 "4000

:3'500

3000

~,

l\ ~

r/

2500

~ i.'/

x\

2000

•

I

!

~

,-.

!

/'~/ \I

i ~

000

~',-'~

r I "v ~,

ii l! ,~ !

1500

t

"~i

"-'x.

"

500

-'~x'-

\r :

,. ).., i

"~ "1

e.,

i

r~i

I

l ~

l

I:\ .lll ir b

t~. i I !i~.i l

J

8o0

7oo

6oo

,soo

4oo

WAVENUMBER Fig. 2. F r I R spectra in acetonitrile solutions of ( - - - ) C-acid, ( in the regions: (a) 4000-400 cm -l, (b) 1800-1000 cm -1.

1300

1200

I'( 000

1100

[ c m -1]

) its mixture with MTBD, and ( - • - ) 1:1 complex of MTBD with a m u c l 4

quantitative and that the negative charge in carbanion is strongly delocalized in the molecule. Very similar information about the proton transfer occurring in the acetonitrile solution of a 1:1 mixture of C-acid with MTBD is obtained from the analysis of the 1H NMR signals of the C-acid. The 1H NMR chemical shifts of the C-acid protons are summarised in Table 1. The most acidic proton 7-H is found in the

spectrum of the C-acid as a singlet at 4.96 ppm. This signal is no longer present in the spectrum of the 1:1 mixture of C-acid with MTBD, and instead a broadened signal of protonated MTBD at 4.02 ppm is found, indicating a 100% proton transfer in this mixture. The signal of 2,6-H protons in the spectrum of C-acid is found as a doublet at 8.21 ppm, i.e. at a much

Table l 1 H NMR chemical shifts (ppm) and coupling constants (Hz) of C-acid and its mixture with MTBD

Compound

NH ~/dTBD)

2,6-H

3,5-H

7-H

COOCH3

J(2,3)

C-acid C-acid + MTBD

4.02

8.21d 7.77d

7.36d 7.50d

4.96s -

3.72s 3.52s

8.8 9.2

s, singlet; d, doublet.

G. Schroeder et al./Journal of Molecular Structure 384 (1996) 127-133

lower field than the signal of 3,5-H protons. This shift is due to the deshielding effect of the NO 2 group in the ortho position. In the tH NMR spectrum of the mixture the signal of 2,6-H protons shifts to the higher field, indicating an increase of the negative charge on the NO2 group (NO~). Very similar behaviour is observed for the signal of the COOCH3 protons, which shifts from 3.72 ppm to 3.52 ppm.

131

O~,C/OCH3 O2N

C'"'"H

h

+

O:'OCH

R'

O,~,./(X~ H3 C

O,

.

3.2. Kinetic studies

o/ Proton transfer reactions between dimethyl (4nitrophenyl)-malonate, and TBD or MTBD in acetonitrile solutions give a coloured, very stable carbanion (Xm~x = 550 nm), which is characteristic of free ions as demonstrated by F r l R and tH NMR studies (Scheme 2). Kinetic parameters of the proton transfer reactions from dimethyl (4-nitrophenyl)malonate to 7BD and M7BD in acetonitrile are collected in Tables 2 and 3, respectively. The difference of the pKa values between TBD and MTBD is reflected by the difference of the rate

--X.=J

C: 3

o./'C\ocH \',.

I

I

R

H

Scheme 2.

constants (k). The rate constants at 25 °C for the proton transfer reactions with TBD and MTBD are 335 × 103 and 6.61 x 103 M -1 s -I, respectively. The same trend is observed for the potential energy barrier, which is higher for the reaction between dimethyl (4-nitrophenyl)malonate and MTBD.

Table 2 Kinetic parameters ( ___ standard deviation) for the reaction of dimethyl (4-nitrophenyl)maionate with TBD in acetonitrile Temp.

kobs (S -1)

10-3k

Int

AH #

- AS#

AG #

(°C)

Base concentration

(M -a s -1)

(s -a)

(El mo1-1)

(J tool -l deg -a)

(kJ tool-I)

172 --- 3 214 ± 4 254 ± 3 335+7

14.4 _+ 7.7 11.3 + 8.2 40.8 ± 6.3 3 5 . 8 ± 3.3 1 4 . 6 7 _ 1.72

90±6

41.53 + 1.72

0 5 15 25

0.0005

0.0015

0.0025

0.0035

(M)

(M)

(M)

(M)

102 112 166 201

± ± ± -

1 4 4 4

274 ± 338 ± 420 + 540±

8 4 6 6

435 551 682 875

--- 4 ± 3 --- 4 ±8

621 --- 2 754 ± 4 924 - 8 1205--_ 9

Table 3 Kinetic parameters ( +_ standard deviation) for the reaction of dimethyl (4-nitrophenyl)malonate with MTBD in acetonitrile Temp.

kobs (s -t)

10-3k

Int

AH #

(°C)

Base concentration

(M -1 s -1)

(s -1)

(kJ mo1-1)

5 15 25 35 45

0.0005

0.0015

0.0025

0.0035

(M)

(M)

(M)

(M)

2.32 +- 0.01 3.84 --- 0.01 4.61 - 0.01 7.83 ± 0.01 11.64 ± 0.01

5.83 ± 0.02 8.25 ± 0.01 11.57 +_ 0.06 17.34 _-. 0.07 25.00 ±_ 0.03

9.24 ___ 0.01 13.50 ___ 0.02 17.84 ___ 0.05 26.43 + 0.04 38.74 ± 0.02

13.42 18.32 24.51 36.18 52.11

± ± ±+

0.02 3.67 0.04 4.87 0.06 6.61 0.03 9.41 0.0813.51

--- 0.12 0.36 _+ 0.11 1.24 --- 0.09 1.42 -+ 0.08 3.11 ± 0.014.84

- AS #

AG #

(J mo1-1 deg -x) (El mo1-1)

±- 0.28 --- 0.25 ± 0.20 21.51 ± 1.27 99 - 3 ± 0.19 ± 0.12

51.11 + 1.27

132

G. Schroeder et aL/Journal of Molecular Structure 384 (1996) 127-133

.CH3

o'.

......

C~

) ...........

o', . -C3

(5

........... ,C~O O \

0

\ OH3

a

/

b

CH3

Fig. 3. Structures of transition states for the reactions of C-acid with (a) TBD and (b) MTBD.

The values of AG * depend on the N-base molecules used. For the studied reactions the value of AG * for TBD is smaller by about 10 kJ mo1-1. This difference can be explained by the difference of the pKa values of the N-bases, the hindrance of the methyl group in the case of MTBD as well as by the formation of the additional C = O..-H-N hydrogen bond in the transition state of the reaction between TBD and C-acid (Fig. 3). Large, negative values of entropy of activation (AS*) for the two studied reactions are observed, indicating a considerable ordering of the solvation in the transition state.

Acknowledgements The authors are grateful for financial support from KBN Grant No. 2P303 121 07.

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