Configurations of anions of β-ketoaldehydes in solutions studied by i.r. spectroscopy

Spectrochimica Acta,Vol. 36A,pp.621-625 0 PergamonPress Ltd., 1980.tintedinGreatBritain

Configurations

of anions of p-ketoaldehydes studied by ix. spectroscopy J.

Department

in solutions

TERPKGSKI

of Organic Chemistry, Institute of Organic Chemistry Polytechnical University, 00661 Warsaw, Poland (Received 29 November

and Technology,

1979)

Abstract-The

ix. spectra of the Li, Na and K salts of P-ketoaldehydes, RCOCH,CHO (R i-Pr and t-Bu) in DMSO, methanol and water were investigated. Assignments of bands to modes of particular configurations of the enolate system were given. Influence of the cation the Z,Zz$ E,E/Z,E equilibrium and of the substituent R on the E,E s Z,E equilibrium was Shifts of the enolate bands in protic solvents were observed and hydrogen-bond accounted for this phenomenon. INTRODUCTION

Our previous NMR investigations as well as those done by other authors [l, 21 have revealed alkali metal salts of /3-ketoaldehydes to exist in solution in the undissociated chelated form with the anion assuming the Z,Z-configuration and/or in the unchelated one with the anion assuming Z,E or E,Econfigurations, respectively. Equilibrium between the chelated form (characterized by a low coupling constant value of 4.1 Hz) and the unchelated one (characterized by a high coupling constant value of 10.4 Hz) has been shown to depend both on the cation and the solvent. However, our recent conductance studies on DMSO solutions [3] have revealed that the unchelated fraction of the salt observed by NMR spectroscopy is not fully dissociated. Investigations of i.r. spectra of alkali metal salts of /3-ketoaldehydes in solution were undertaken here in the hope of extending information gained by NMR and conductance studies on the equilibria between particular species of the salt being present in the solution. Moreover it was expected to clear up some correlations between the anion configuration and its vibrational spectrum since controversy still exists in this field [4-61. EXPERIMENTAL

Preparation of the Li, Na and K salts of pketoaldehydes, RCOCH,CHO, where R was Me, Et, i-Pr and t-Bu, respectively, was described previously [l]. Infrared spectra of 0.2 M solutions of salts in DMSO and DMSO-d, were recorded on a Perkin-Elmer 577 spectrometer between 1700 and 1000 cm-‘. A cell thickness of 0.067 mm and windows made from AsS were used. In similar conditions, spectra of solutions in methanol and methanol-d, as well as in dimethoxyethane were recorded. Spectra of water and heavy water solutions were taken in the form of capillary films between windows made from KRS-5 and Irtran. The band positions being calibrated against polystyrene are expected to be correct within *2cm-‘. RESULTS AND DISCUSSION

Since the previous conductance studies [3] and most of the NMR ones [l] on the Li, Na and K %(A) 3617

A

= Me, Et, stretching radius on observed. formation

salts of four /3-ketoaldehydes, RCOCH,CHO ‘[where R was Me (l), Et (2), i-Pr (3) and t-Bu (4), respectively] have been done on DMSO solutions, the i.r. spectra were more thoroughly investigated in the same solvent. Influence

of the cation

The typical influence of the cation on the behaviour of the salt in DMSO solution is best illustrated in Fig. 1, in which the spectra of the Li, Na and K salts of 4 (R= t-Bu) are compared. For the Li salt, which at the concentration being investigated remains over 90% chelated [l], the spectrum essentially corresponds to the Z,Zconfiguration of the anion. It is characterized by three strong bands. The first of them appears at 1615 cm-‘; the second one, being split into a doublet (probably because of Fermi resonance with the 6 CHI as mode), is seen at 1478 and 1467 cm-‘; a third one appears at 1365 cm ‘. Lithium salts of the remaining compounds l-3 exhibit bands at almost identical positions but do not show any splitting of the second band (see Table 1). Obviously these bands correspond to three of the four fundamental stretching modes of the enolate system. In agreement with the assignment given by JUNGE and Musso[7], and by ERNSTBRUNNER [6], for acetylacetonates, the band at highest frequency is assigned here to the symmetric mode (v~, 1) of mainly C-O stretch character. The second band corresponds to the antisymmetric mode (v,,, 2) and, as shown by isotopic substitution [7], consists mainly in stretching the C-C-C system. The third to the antisymmetband at 1365 cm-’ corresponds ric mode (v,,, 3) which mainly involves C-O stretching. The position of the band which could correspond to the fourth and symmetric stretch of the system (v,, 4) is not clear since several weak-tomedium bands appear within the expected region (e.g. 1255, 1195, 1155 cm-‘, respectively) and this question is left without an answer at present. In the spectrum of the Na salt, about half of which still remains in the chelated form [l, 31, a 621

622

J.

TERPII;ISKI

P

%T

: I

:,;

1

1600 1500 1400 1300

1600 1500 1400 1300 12c NC!

Li

Fig.

series

of

observed

new

1. Comparison

bands

at similar

of the

appears

double-bond region of i.r. spectra of the pivaloylacetaldehyde, 4, in DMSO solution.

besides

positions

as in the

bands

being

spectrum

of

the Li salt. The bands observed at 1620, 1480, 1475 and 1368 cm-’ correspond to the chelated Z,Z-configuration, while the new bands observed at 1608, 1520, 1508, 1220 and 1210cm-’ correspond to the open form of the anion. A similar situation is observed in the spectra of Na salts of the remaining compounds l-3 (see Table 2). In the spectrum of the K salt, this sequence of bands corresponding to the open form of the anion becomes predominant. This confirms that the K salt remains almost exclusively in the unchelated form in DMSO solution. However, the pattern of bands observed in this region of the spectra of the K salts of compounds l-4 clearly depends on the substituent R (this is, of course, also true in case of the Na salts as far as the unchelated form is concerned). Influence

of the substituent

R

In Fig. 2, we compare the spectra of the K salts of compounds l-4 in the region 16X-1450cm~‘. In the spectrum of the methyl derivative, 1, one medium and three strong bands may be discerned in this region. They appear at 1605, 1587, 1553 As the size of the and 1527cm ‘, respectively.

Table

1, R=Me

2, R=Et

1621~s 148Ovs 1474sh 1361~s

1625sh 1621~s 1478vs 1473sh 1360vs 1255~

1200w 115Sm

taken

in

DME,

I

Li. Na

and

K salts

ri

k

of

ri

ii ZE

J-3

However, steric repulsions between the substituent R and the p-substituent in the E,E-configuration are growing up with the increasing size of R and destabilize this configuration in favour of the Z,E one. The observed stepwise shift of the equilibrium between the E,E- and Z.E-configurations under the influence of the size of the substituent remains

100 cm- ’ region of i.r. spectra of P-ketoaldehydes

3, R= i-Pr

4, R= r-Bu

1625sh 1620~s 1479vs 1473sh 1366~s 1255w 1195w 1 ISSW 112Om

16lYsh 1615~s 1478vs 1467~s 1365~s 1255m 1lYSw 11ssw 112Om

these bands

1

substituent is gradually increased from Me to t-Bu, the intensities of the two internal bands decrease, while simultaneously those of the external bands increase. These modifications find a clear explanation when one assumes that they reflect changes in equilibria between particular configurations of the unchelated anion. It can adopt an E,E- or a Z,Econfiguration [I. 21. The E,E-configuration is energetically the most favourable one because of both mesomeric and electrostatic effects and is adopted. for example, by the malonaldehyde anion (R= H) [8].

1. Band positions (cm- ‘) in the 1700-I salts (chelated Z,Z-configuration)

* In spectra

,

1600 1500 1400 1300 12’ 0 cm-’ K

appear

of Li

Assignment

z,z; z,z;

U,(C”o) V,,,((‘-c”c)

z,z;

V,,(c=o)*

at the similar

positions.

/

Configurations Table

of anions of P-ketoaldehydes

2. Band positions (cm-‘) in the 1700-1200cm-’ Na salts of P-ketoaldehydes, RCOCH,CHO,

1,R=Me 162.5s 1605sh 1587s 1577sh 1563sh 1553vs 1527s 1518sh 1507sh 1470s

2, R=Et

3, R= i-Pr

4, R= t-Bu

1625s 1608s 1584m 1577sh 1562sh 15.54s 1528~s 1518sh 1506sh 1476s

1620~s 1608vs

1625s 1610s 1587s 1577sh 1562sh 1549vs 1527~s

1360s

1508sh 1478~s 1473sh 1360s

1366s

1200s

1200s

1210m 1200m

q

cm-’

Fig.

2. Comparison of the double-bond spectra of the K salts of compounds

in accord

with

conformational

changes

region in DMSO

region 1-4. observed

of i.r.

by

i.r. spectroscopy in series of vinyl ketones [9, 101 and p-substituted vinyl ketones [ll, 121. This stepwise change of the E,E+Z,E equilibrium contradicts the view expressed by ESAKOV ef al. [2] who on the basis of NMR spectra assumed 1 alone to exist in two configurations (E,E and Z,E) while the remaining compounds 2-4 were postulated to be in the Z,E-configuration only. Considering the relative intensity changes, the internal pair of bands (for 1: 1587 and 1553 cm -‘) is assigned, therefore, to the E,E-configuration of the anion, while the external pair, observed for 1 at is assigned to the Z,E one. 1605 and 1527cm-‘, These bands correspond to the two stretching modes of the E,E and Z,E enolate systems. Moreover, there are two regions of lower frequency in which changes in band intensities are observed

623 of i.r. spectra solution

of

Configuration of anion zz ZE E,E

E,E Z,E

152ovs 1508~s 148Ovs 1475vs 1368s 1355m 1320m 1220m 1210m

.zz zz

ZE E,E

together with changes in configurational equilibria. The first region lies between 1350 and 1310 cm-’ and the second one near 1200cm-‘. We assign bands appearing in these two regions to the two remaining stretching modes of the two enolate configurations, viz. E,E and Z,E (see Table 3). The assignment given here remains in accord with those proposed previously for similar systems [6, 81 as far as the first three stretches are concerned, but differs in the assignment of the fourth one. For the E,Econfiguration of malonaldehyde anion, GEORGE and MANSELL [8] have assigned a band at 1022 cm-’ to this mode, while, for the acetylanion in acetone water (presumably Z,Econfiguration), ERNSTBRUNNER [6] attributed it to the 1188 cm-’ band. However, it should be emphasized that a strong polarized line was observed [5] at 124Ocm-’ in the Raman spectrum of the acetylacetone anion. Although it was assigned to an overtone, it could fit equally well with our assignment of the fourth symmetric mode. Influence

of the solvent

Spectra taken in different solvents have been often compared without any real knowledge of the configuration actually taken by the anion. Therefore, we extended our studies to other highly polar protic solvents like methanol and water. Spectra recorded in DME, in which only the Li and Na salts are sufficiently soluble exist entirely in the chelated Z,Z-configuration and reveal no change of band positions compared to the DMSO solution. Because of strong solvent absorption, this could only be observed on the first and the third band of the enolate system (compare Table 1). On the contrary, methanol and water, in which the anion of the ketoaldehyde exists in the open form (E,E or Z,E-configuration), show a distinct influence on band positions. However, as may be

Table

3. Band

positions

(cm-‘)

in

the

1, R=Me

DMSO

MeOH

Hz0

i.r. spectra R-C-CH-CH-0) &

of

MeOH

the

K salts of in solutions

HsO

DMSO

MeOH

l595sh

1585s

1576”s

1570s

l605m

1595sh

1SXOsh 1607s

1595s

1580s

1572%

15x5s

1577%

1570x

1575”s

1570”s

1562sh

1563ah 1553”s

154ovs*

1527”s

152&h*

1507sh

15Oish

153&a*

1532vs*

1525”s

1505v9*

150Ssh

1508ah 146lhn

1562sh 1555~s

1525sh

1345s

1525”s

1528~s

1500~s

1490~s

1507sh

141ow

139nw

1380m

139ow

1365111

1365m

1340m

1345”s

1370s

129ow

1250~

1270m

1285”s

1295s

1335s

1478”s

1478”s

lSO8vs

1505sh

1472”s

Y, CC=O)

E,E:

u,(CxO)

,475~

1470sh

14SSsh

1428s

1450sh

1420~

1423s

14201”

1370s

1380~

1385m

1382m

E.E;

ua,(C-C-C)

Z,E;

v.,(C-C=CJ

L

; CHx

S,: CH,

1345vs

1344sh 1320s

1322~s

1325”s

1323~s

E,E;

v,,(C-0)

Z,E:

v.,(C=OI

1275~

126011,

1250~

1250sh

1210s

1220s

1235111 1232”s

1225vs

1200s

Z,E:

“=tC-C=C)

1210s

1222m

E.E:

v,(C=C-C)

1220s

1200s

113ow

1135w

vents

1522”s

Z,E;

1310s

1170w

seen

1570sh

1310m

116Om

‘Broad

1578”s

S, ; =C-H

1310sh 126Om

1218m

1577%

1595”s 158Osh

1315s

121Osh 1200s

1603”s

1425m

1320”s

1320m

128Ow

DzO

1367111

1330m 1320m

Hz0

1560sh

146Xw

1385w 1345”s

1560sh 1530”s

1435m

1350s 1340”s

,548”s

1435m 1430m

143ow

153ovs*

Assig”liWts

MeOH

,578sh

157Rsh

1577sh

DMSO

anions,

1620sh

158Ssh

1584s

(unchelated

4, R = r-B”

Hz0

1624sh

1605m

P-ketoaldehydes

3, R = i-Pr

2, R=I3

DMSO

DzO

lh25sh

1335m

TERPI~~SK~

J.

624

1220”s

1220sh

117ow 1110s

hand.

when

comparing

(Figs.

3 and

spectra 4) with

taken

those

in those

taken

sol-

in DMSO

(Fig. 2), band envelopes of particular compounds are very similar irrespective of the solvent. This means that both methanol and water have only a secondary influence on the E,E e Z,E equilibrium, although a slight shift towards the E,E form may exist. In order to gain insight in regions obscured by solvent absorption, spectra of compounds 1 and 4 were also taken in methanol-d, and deuterium oxide (see Figs. 5 and 6). The deuterated solvents showed no influence on the v,-v4 bands compared to their ‘H analogues. It is worthwhile mentioning this since in such solvents the a-hydrogen undergoes fast exchange with deuterium and we would have expected a frequency shift if the in-plane

%T

t

%T

I I

D

I

1400

I

I

1200

I

I

10010

cm-’ Fig. 4. Infrared

I

1600 Fig. 3. Infrared

spectra

I

1550

I

1500

of the K salts of compounds in methanol.

cm -’ l-4

spectra

of the K salts of compounds in H,O.

l-4

=C-H bending mode was involved. All the results obtained are compared in Table 3. The distinct shifts of bands accompanied often by some intensity changes are certainly due to hydrogen bond formation between solvent molecules and the anion. So the band at highest frequency shows a red shift of ca. 10 cm-’ when changing from

Configurations I

I

of anions

1

of P-ketoaldehydes

625

the first two bands of the Z,E-configuration are much stronger, while those of the remaining two bands are weaker than those of the corresponding bands of the E,E-configuration. CONCLUSIONS

1. Infrared spectroscopy appears to be a suitable method for investigating the influence of cations and of structural changes on the configurational equilibria of enolate anions. 2. DMSO seems to be a particularly good solvent to investigate the i.r. spectra of enolates. We demonstrated that in this solvent the increase of the alkali metal cation radius changes the chelated Z,Z-configuration into the unchelated E,Eand/or Z,E-configuration. We demonstrated also that the increase of steric hindrances created by substituent R in the

%T

Fig. 5. Infrared

spectra of the K salts of compounds 4 in methanol-d,.

R__C-_CH-CH==O&

1 and

DMSO to methanol. This shift increases up to 20-30 cm-’ when going to a water solution. A similar decrease in frequency of about 20-30 cm-’ is observed for the second enolate band. On the contrary, both the third and the fourth bands show blue shifts when going from DMSO to methanol and water. For the third band, this shift is, in general, of the order of +lO cm-’ and about +20 cm-’ for the fourth one. In all cases, shifts of

system shifts rights the E,E+Z,E equilibrium. 3. Particular anion configurations show characteristic, discrete sets of bands and these were assigned to particular stretching modes of the enolate system. 4. Protic solvents str’ongly shift the bands of the unchelated E,Eand Z,E-configurations of the enolate system; this was ascribed to hydrogen bond formation between anion and solvent molecules. REFERENCES J. TERP&KI,

Roczniki Chem. 47, 537 (1973). S. M. ESAKOV, A. A. PETROV and B. A. ERSHOV, J. Org. Chem. U.S.S.R. 9, 849 (1973); 11, 679

::3

(1975); 12,774 (1976). [31 J. TERPI~SKI and W.

%T

[41 [51

I :;3 181 [91 [lOI [Ill [121 Fig. 6. Infrared

spectra

of compounds

1 and 4 in D,O.

KEPYS, Polish J. Chem. 53, 1597 (1979). K. L. WIERZCHOWSKI and D. SHUGAR, Spectrochim. Acta 21, 943 (1965). W. 0. GEORGE and F. V. ROBINSON, J. Chem. Sot. A 1950 (1968). E. E. ERNSTBRUNNER,J. Chem. Sot. A 1558 (1970). H. JUNGE and H. Musso, Spectrochim. Acra 24A, 1219 (1968). W. 0. GEORGE and V. G. MANSELL, Spectrochim Acta 24A. 145 (1968). J. KOSSANYI, Bull. Sot. Chim. France 704 (1965). K. NOACK and R. N. JONES, Can. J. Chem. 39.2201, 2225 (1961). J. DABROWSKI and K. KAMIE~SKA-TRELA, Spectrochim. Acta 22, 221 (1966). J. D~BROWSKI and K. KAMIE~SKA-TRELA, Bull. Chem. Sot. Japan 39, 2565 (1966).