Thioenols and thioamides substituted by two β-EWGs. Comparison with analogous amides and enols

DOI: 10.1002/poc.1359

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Journal of Physical Organic Chemistry p. 483 - 491 (2008)

Update date:2022-07-30

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Authors:

Basheer, Ahmad

Rappoport, Zvi

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Article abstract of DOI:10.1002/poc.1359

Condensation of organic isothiocyanates with active methylene compounds gave nine thioamides RNHCSCHYY' or their isomeric thioenols RNHC(SH) = CYY' for substrates in which Y and Y' are electron-withdrawing groups (EWG). These included derivatives of Meldrum's acid (MA) which showed 100% thioenol in all solvents. For other compounds the percentages of thioenol in CDCI₃ when R = Ph are 100% when Y = CN and Y'= CO₂Me or Y' = CO ₂CH₂CCI₃, 6% when Y = Y'= CO₂CH ₂CF₃, and 0% when Y = Y' = CO₂Me. The chemical shift of SH (highest values 12.0-16.0 ppm) served as a probe for the thioenol structures and also for the extent of hydrogen bonding to the SH group. In contrast to simple ketones and thioketones in which thioenolization is favored over enolization by factors as large as 10⁶, for intramolecular competition K_Thioenol/K_Enol ratios are much lower than for systems not substituted by β-EWGs. X-ray crystallography of the 5-anilido-MA derivative shows a hydrogen-bonded thioenol structure. δ(OH), δ(NH), K_Enol, and crystallographic data for analogous thioenol and enol systems are compared. Copyright

Full text of DOI:10.1002/poc.1359

Research Article

Received: 10 January 2008,

Revised: 13 February 2008,

Accepted: 18 February 2008,

Published online in Wiley InterScience: 2008

(www.interscience.wiley.com) DOI 10.1002/poc.1359

Thioenols and thioamides substituted by two

b-EWGs. Comparison with analogous amides

and enols

Ahmad Basheer^aand Zvi Rappoport^a*

Condensation of organic isothiocyanates with active methylene compounds gave nine thioamides RNHCSCHYY( or

their isomeric thioenols RNHC(SH) ¼ CYY( for substrates in which Y and Y( are electron-withdrawing groups (EWG).

These included derivatives of Meldrum’s acid (MA) which showed 100% thioenol in all solvents. For other compounds

the percentages of thioenol in CDCl₃when R ¼ Ph are 100% when Y ¼CN and Y( ¼CO₂Me or Y( ¼CO₂CH₂CCl₃, 6%

when Y ¼ Y( ¼CO₂CH₂CF₃, and 0% when Y ¼ Y( ¼CO₂Me. The chemical shift of SH (highest values 12.0–16.0 ppm)

served as a probe for the thioenol structures and also for the extent of hydrogen bonding to the SH group. In

contrast to simple ketones and thioketones in which thioenolization is favored over enolization by factors as large

as 10⁶, for intramolecular competition K_Thioenol/K_Enolratios are much lower than for systems not substituted by

b-EWGs. X-ray crystallography of the 5-anilido-MA derivative shows a hydrogen-bonded thioenol structure. d(OH),

d(NH), K_Enol, and crystallographic data for analogous thioenol and enol systems are compared. Copyright ß 2008 John

Wiley & Sons, Ltd.

Supplementary electronic material for this paper is available in Wiley InterScience at http://www.mrw.interscience.wiley.com/

suppmat/0894-3230/suppmat/

Keywords: enolization; thioenolization

INTRODUCTION

Enols of carboxylic acid amides (1) are relatively unstable

compared with the amides (2).

In the last decade we have succeeded in stabilizing these enols

and other enols of carboxylic acid derivatives by substituting

the C_bcarbon atoms with electron-withdrawing groups (EWGs) Y

and Y⁰capable of delocalizing negative charge by resonance.^[1–13]

This is due to the higher stabilization of the delocalized

zwitterionic contributing structure 1b of the enol than of the

amide contributing structure 2b by Y,Y⁰(Eqn (1)). The latter is

(a) of K_Enolfor 3 with K_Enolfor 5, which compares the inﬂuence

usually regarded as the reason for the low extent of enolization of

the amides. As a part of our studies of enolization of carboxylic

acid amides we recently studied the enolization of cyano

malonamides 3^[12]to their enols 4a or 4b and compared the

extent of enolization in terms of the equilibrium constant

—

of a nonenolizing C O versus C S on the extent of enolization

—

of two different analogous systems, and (b) of K_Enolfor 5 versus

K_Thioenolfor 5 which compares competing enolization and

thioenolization.

K_Enol¼ [enol]/[amide] to that of cyano monothiocarbonylmalo-

namides 5.^[13]For the compounds with structure 5 both

enolization to the enol–thioamides 6 with K_Enol¼ [6]/[Ami-

de–thioamide], and thioenolization to the amide–thioenols 7

tautomers with K_Thioenol¼ [7]/[5] were observed. Two compari-

sons were made:

Institute of Chemistry and the Lise Meitner Minerva Center for Computational

Quantum Chemistry, The Hebrew University, Jerusalem 91904, Israel.

E-mail: zr@vms.huji.ac.il

a A. Basheer, Z. Rappoport

Institute of Chemistry, The Hebrew University, Jerusalem 91904, Israel

J. Phys. Org. Chem. 2008, 21 483–491

A. BASHEER AND Z. RAPPOPORT

The observed results are that K_Enol(3) > K_Enol(5), so that the

combination of Y,Y⁰¼CN, CSNRR⁰is less efﬁcient in promoting

enolization than CN, CONRR⁰.^[12,13]Comparison of K_Enol(5) with

gave nine thioamides (10a-i)/ thioenols (11a-i) or their mixture

(Eqn (2)), depending on Y, Y⁰, R, and the solvent. Compounds

11a,^[23]11d^[24], and 11i^[25]are known.

K_Thioenol(5) shows that K_Enolis mostly higher, although when R is

an aromatic group, the extent of enolization and thioenolization

is approximately similar, and the latter is sometimes even slightly

higher.^[13]

The MA derivatives 11a–c were prepared with the expec-

tation that similarly to the MA activated enols of amides, the

enolization will not take place on the ester carbonyls,^[1,7]but on

—

the 5-C S group. Indeed, as shown in Fig. 1 this the case and

—

This is not the case for enols and thioenols which are not

activated by b-EWGs. On comparing such ‘simple’ systems (i.e.,

substituted only by alkyl or aryl groups) the thioenolization is

more extensive by several orders of magnitude.^[14–20]This is also

the case for enols of carboxylic acid esters and thioesters,^[21,22]as

discussed below.

11a is the ﬁrst example of an X-ray structure of a noncyclic

thioenol of a thioamide activated by two b-EWGs. 11a is also

exclusively the thioenol in all solvents (as shown below). The

solid state parameters of 11a are compared with those of the

analogous enol 9a, R ¼ Ph^[1]in Table 1. The C—S single bond

—

length is 1.739(2) A, compared with 1.693 A for a double C S

—

In the present paper we compare a few examples of the

previously investigated amides 8/enols 9 activated by b-EWGs

and the corresponding thioamides 10/thioenols 11. Systems 8/9

include ‘formal’ (‘formal’ is used to emphasize that the amides

appear frequently in mixtures with their enols or thioenol

tautomers) amides substituted by Meldrum’s acid (MA) moiety

(e.g., 8a,^[1]for which the structure of the enol 9a is shown) or

by combination of a CN group with an ester group such

as CO₂CH₃(8b)^[2]or CO₂CH₂CCl₃(8d)^[9]or a combination of two

ester groups, for example, CO₂CH(CF₃)₂and CO₂CH₂CF₃.^[6]

Two CO₂CH₂CF₃groups (8d/9d) generate a relatively stable enol

(K_Enol¼ 6.7),^[6]whereas two CO₂Me groups (8e/9e) give a low

bond of thioamides,^[26]and the ‘enolic’ double bond of

1.425(3) A, is only slightly shorter than the other C_a—C_bbonds.

—

The C O bond lengths are longer than those in normal esters

—

and the slightly longer bond (1.229(3) A) of the C O cis to the

SH indicates a stronger C

—

^{. . .}H—O hydrogen bond than of

—

. . .

—

O H—N bond (with C O bond length of 1.220(3) A).

—

the C

The S1—H bond length and the hydrogen bonded O2ꢀꢀꢀH

distance are 1.22(4) A and 1.76(4) A, respectively, with S1ꢀꢀꢀO2

non-bonding distance of 2.9026(18) A and SHO angle of 152(3)8.

Another intramolecular hydrogen bond is N1—HꢀꢀꢀO1 with

N—H, OꢀꢀꢀH, and NꢀꢀꢀO length and distances of 0.84(3), 1.90(3),

and 2.604(2) A, respectively. The doubly hydrogen bonded

_Enolof 0.1.^[1]For the thioamides 10/thioenols 11 we determined

system with S1—HꢀꢀꢀO2 and N1—HꢀꢀꢀO1 moieties can thus serve

as a model for an activated thioenol. For other data refer to

Supplementary data.

the percentage of thioenol as a function of the solvent, the

thioenols’ NMR chemical shifts and their structures.

Structure in solution: NMR spectra, conﬁguration,

and K_Thioenolvalues

RESULTS

Reactions of ﬁve active methylene compounds CH₂YY⁰12 where

Y,Y⁰are two ester groups or a cyano and an ester group, with

organic isothiocyanates (13) in the presence of dry Et₃N or Na

Dissolution of substrates 10/11 in different solvents establishes

immediately an equilibrium between the amide and thioenol

species, which did not change with time, that is, the equilibration

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J. Phys. Org. Chem. 2008, 21 483–491

ꢀꢁEWGS-SUBSTITUTED THIOENOLS AND -AMIDES

is that of the E-thioenol (Scheme 1), with the SH group weakly

associated with the solvent. The higher values in DMSO-d₆are

due not to the d(SH), but to the proton formed on ionization of

the thioenol, as found for enols in this high dielectric constant

solvent.^[5]The NH signals for all the enols appear at

11.26–12.97 ppm, with the highest values for the MA derivatives.

Their small range is consistent with the presence of

N—H^{. . .}O hydrogen bonds in all of them and an E-conﬁguration

in 11d–f.

Indeed, in b-thioxoesters HSC(R¹) ¼ CHCO₂R²the intramole-

—

cular S—HꢀꢀꢀO

hydrogen-bonded Z-isomer displays d(SH) at

5.68–8.19 ppm, whereas for the E-isomer the d(SH) is at ca. d

3.80 ppm.^[32]

Figure 1. ORTEP drawing of 11a

Table 3 displays the product distributions for all the 10/11

isomers in several solvents and the derived K_Thioenolvalues. For

comparison, the previously obtained K_Enolvalues of the

analogous 8/9 isomers^[1,2,6,7]are given. For ‘formal^’thioamide

systems 10a–i, complete thioenolization was obtained for the MA

and the cyanoester derivatives 11a–f were obtained, similarly to

the analogous enol derivatives 8/9a,b.^[1,2]For the triﬂuoroethyl-

malonic esters, the percentages of thioenol in CDCl₃are 13% for

11h and 6% for 11g, compared with 87% for the analogous enol

9d.^[6]No thioenolization was obtained for the dimethyl malonate

system 11i in CDCl₃, whereas the analogous amide enolized in

5–10%.^[1]Consequently, enolization is more extensive than

thioenolization for these systems.

There are limited data for the solvent effect on the

thioenolization (Table 3), since the reaction is complete for

several systems in all solvents. Where data are available K_Thioenol

decreases in the order CDCl₃> CD₃CN > DMSO-d₆as found

earlier for enol systems^{[1–3,6–9,11–13]}and this is attributed to the

higher polarity of the amide than of the hydrogen bonded enol.

is instantaneous under our conditions. The ¹H and ¹³C NMR

spectra were recorded for all systems investigated. Selected

¹H and ¹³C NMR spectral parameters are given in Table 2.

The complete NMR data are given in Tables S1 and S2 of the

Supplementary data. The SH chemical shifts (in CDCl₃or CCl₄) are

at a lower ﬁeld than in many aliphatic and alicyclic thioenols

[15]

(2.13–2.85 ppm),^[27–31]in Ph C C(SH)Ph (3.28 ppm),

or in

—

polyﬂuorinated thioenols (3.6–4.1 ppm).^[31]In our systems the

d(SH) values for the MA derivatives 11a–c are the highest at

12.02–16.04 ppm (12.02–14.17 ppm in CDCl₃; d ¼ 9.81 for 11b in

DMSO-d₆is an exception). These values are ascribed to strong

—

C hydrogen bonds. Only for the cyano ester 11d–f

—

S—H^{. . .}

conﬁgurational isomers are possible since in 11g–i the two

b-ester groups are identical and there are both S—H^{. . .}O and

N—H^{. . .}O hydrogen bonds to their cis-CO₂R groups (Scheme 1).

The hexaﬂuoromalonic ester derivatives 11g and 11h in which

the hydrogen bonds are weaker display moderate d(SH) values at

4.95–7.40 ppm. Since the geometry does not allow

intramolecular CN^{. . .}H—S hydrogen bonds, the d(SH) values are

lower (4.61–4.83 ppm) in the cyanoesters 11d–f. The

N—H^{. . .}O hydrogen bonds are stronger than S—H^{. . .}O bonds,

and in 11d–f the former is the only hydrogen bond formed. The

d(SH) have the lowest values of 4.61–4.83 ppm and the structure

DISCUSSION

—

Enolization on C O versus thioenolization on C

—

The relative ease of enolization versus thioenolization was

previously compared. Qualitatively, on comparing structurally

Table 1. Comparison of selected X-ray data for Meldrum’s acid derivatives 9a (R ¼ Ph)^[1]and 11a (R ¼ Ph) at room temperature

Lengths (A)

Degree (8)

Bond

11a

Angle

11a

C(1)–C(2)

C(1)–N(1)

C(1)–S(1)

C(1)–O(1)

C(2)–C(3)

C(2)–C(4)

C(3)–O(1)

C(4)–O(2)

S(1)–H(S1)

O(1)–H(O1)

N(1)–H(N1)

O(2)ꢀꢀꢀH(S1)

O(2)ꢀꢀꢀH(O1)

O(1)ꢀꢀꢀH(N1)

1.425 (3)

1.324 (3)

1.739 (2)

—

1.445 (3)

1.436 (3)

1.220 (3)

1.229(3)

1.22 (4)

—

1.426 (5)

1.333 (4)

—

1.300 (4)

1.414 (5)

1.420 (5)

1.221 (4)

1.240 (4)

—

1.125

0.942

—

1.385

1.939

S(1)–C(1)–N(1)

O(1)–C(1)–N(1)

S(1)–C(1)–C(2)

O(1)–C(1)–C(2)

N(1)–C(1)–C(2)

C(1)–C(2)–C(3)

C(1)–C(2)–C(4)

C(2)–C(3)–O(1)

C(2)–C(4)–O(2)

C(1)–N(1)–C(8)

S(1)–HꢀꢀꢀO(2)

O(1)–HꢀꢀꢀO(2)

N(1)–HꢀꢀꢀO(1)

—

115.27 (17)

—

123.85 (16)

—

118.4 (4)

—

120.1 (3)

121.5 (3)

122.4 (3)

117.2 (3)

126.1 (3)

116.9 (3)

130.3 (3)

—

120.88 (19)

118.74 (18)

122.60 (19)

126.52 (19)

127.1 (2)

125.44 (19)

152 (3)

—

141 (3)

—

0.84 (3)

1.76 (4)

—

158.24

138.54

—

1.90 (3)

J. Phys. Org. Chem. 2008, 21 483–491

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A. BASHEER AND Z. RAPPOPORT

—

Table 2. Selected H (CH, SH, NH) and C (CH, C , C S, C ) chemical shifts for systems 10/11 in several solvents at room

—

temperature

—

S or C

—

Compd.

Solvent

Species

CH or C_b

10a/11a

CDCl₃

Thioenol

Thioamide

Thioenol

Thioamide

Thioenol

Thioamide

Thioenol

Thioamide

—

5.31

—

5.25

5.49

—

5.54

—

5.50

5.68

5.05

5.15

12.02

13.72

12.79

12.67

13.16

13.17

12.97

9.81

14.17

16.04

14.32

12.43

4.71

4.61

4.69

8.76

4.83

11.19

—

4.95

—

6.75

—

7.40

—

12.93

12.97

12.25

11.52

11.44

13.09

11.72

12.86

11.33

11.31

11.26

11.31

11.68

11.91

11.73

12.26

11.48

11.83

10.33

11.95

10.26

10.53

12.24

11.12

11.59

10.47

12.24

10.68

11.90

83.11

83.26

83.48

84.11

83.44

83.50

83.90

84.53

81.85

82.29

82.07

82.40

72.55

72.62

72.38

76.20

71.88

72.32

178.71

179.98

179.23

179.59

179.79

180.99

179.81

181.41

177.99

180.63

178.50

178.56

169.40

170.72

169.62

179.83

171.04

THF-d₈

CD₃CN

DMSO-d₆

CDCl₃

THF-d₈

CD₃CN

DMSO-d₆

CDCl₃

THF-d₈

CD₃CN

DMSO-d₆

CDCl₃

10b/11b

10c/11c

1-Np

i-Pr

10d/11d

10e/11e

1-Np

CD₃CN

DMSO-d₆

CDCl₃

DMSO-d₆

CDCl₃

10f/11f

Not observed

185.01 (d, J ¼ 5.7 Hz)

188.75 (d, J ¼ 6.5 Hz)

187.69 (d, J ¼ 5.8 Hz)

174.01

10g/11g

65.89 (d, J ¼ 141.8 Hz)

CD₃CN

CDCl₃

65.63 (d, J ¼ 134.2 Hz)

65.36 (d, J ¼ 141.2 Hz)

87.56

10h/11h

1-Np

THF-d₈

65.57 (d, J ¼ 135.4 Hz)

190.41 (d, J ¼ 6.7 Hz)

CD₃CN

DMSO-d₆

CDCl₃

65.04 (d, J ¼ 135.2 Hz)

64.63 (d, J ¼ 131.5 Hz)

66.74 (d, J ¼ 142 Hz)

66.53 (d, J ¼ 143.8 Hz)

191.61 (d, J ¼ 6.9 Hz)

191.72 (d, J ¼ 7.1 Hz)

186.80 (d, J ¼ 5.5 Hz)

190.32 (d, J ¼ 6.8 Hz)

10i/11i

DMSO-d₆

^aThe percentage of thioenol is low, not enabling to record its ¹³C signals.

^bThe compound ionizes to the thioenolate ion in both CD₃CN and DMSO-d₆.

^cIonizes in DMSO-d₆.

Scheme 1. Conﬁgurations of and hydrogen bonding in compounds 11

—

analogous systems with one enolization site R RCH—C X (X

—

recently a few quantitative data were obtained for such

—

O, S), thioenolization is much more facile than enolization. Simple

thioenols are observable species^[14]whereas the corresponding

enols are less stable than their carbonyl tautomers.^[33]Only

competition. The triarylethenethiols, 14, X S, Ar ¼ Ph, p-An

—

do not isomerize to the thioketones at 60 8C in hexane after 2

weeks and K_Thioenolof ꢂ 100 in hexane was estimated.^[15]In

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ꢀꢁEWGS-SUBSTITUTED THIOENOLS AND -AMIDES

Table 3. Effect of Y, Y⁰, R and the solvent on K_Thioenolfor 10/11 systems, compared with K_Enolfor the 8/9 systems

Compd.

Solvent

(%) TA^a

(%) TE^b

K_Thioenol

Compd.

(%) Enol^c

K_Enol

10a/11a

CDCl₃

100

ꢂ50

0.18

8a/9a

100

—

100

—

100

ꢂ50

THF-d₈

CD₃CN

DMSO-d₆

CDCl₃

THF-d₈

CD₃CN

DMSO-d₆

CDCl₃

THF-d₈

CD₃CN

DMSO-d₆

CDCl₃

DMSO-d₆

CDCl₃

DMSO-d₆

CDCl₃

CD₃CN

DMSO-d₆

CDCl₃

CD₃CN

DMSO-d₆

CDCl₃

THF-d₈

CD₃CN

DMSO-d₆

CDCl₃

ꢂ50

—

10b/11b

10c/11c

1-Np

—

i-Pr

8c/9c

ꢂ50

ꢄ0.02^b

10d/11d

10e/11e

10f/11f

8d/9d

100

ꢂ50

ꢄ0.02

1-Np

—

100

—

<10

—

8f/9f

ꢂ50

ꢄ0.02^b

10g/11g

10h/11h

100

0.07

8g/9g

6.7

0.05

ꢄ0.02

—

ꢄ0.02

0.15

ꢄ0.02

1-Np

—

ꢄ0.1

10i/11i

8i/9i

DMSO-d₆

ꢄ0.02

^aThioamide.

^bThioenol.

^cFrom previous work on systems 8/9.

^dThe anion was obtained.

BDE is ca. 80 kcal/mol,^[39]much lower than the value quoted

above. Two approaches can circumvent this problem. First,^[40]

since the DH_f(PhX)ꢁDH_f(vinyl-X) difference is almost indepen-

dent of X, the hypothetical reaction 5 (using experimental

—

contrast, K_Enolfor Ph C C(Ph)OH at 295 K is ca. 0.06 in DMSO,

—

the best enol stabilizing solvent.^[16]It is estimated that K_Thioenol

K_Enolfor 14, Ar ¼ Ph is ꢂ 10⁶.^[16]A similar ratio was extrapolated

[17]

—

for the PhC(R) C(XH)CH Ph, X ¼ S, O systems.

Table 4 gives

these and all the other ratios known to us for the thionoester/

—

DH_f’s),^[41]where thiocamphor and camphor represent C S and

—

ester 15,^[21]the mesityl derivatives 16,^[18]the benzo[b]-2,3-

O species, is useful. The exothermic DH of 8.3 ꢃ 1.2 kcal/mol,

—

dihydrothiophene-2-thione system 17,^[22]and the estimated

ﬁts the values derived from the bond energies.

[20]

—

value for the MeC( X)OR system.

—

Thiocamphor þ PhOH ! Camphor þ PhSH

The much higher extent of thioenolization than of enolization

is ascribed^[34]to differences in bond energies. A rough estimate

which is based on identical C—C and C—H bond energies in the

DH_fðkcal=molÞ ꢁ6:0

¼ ꢁ8:3

ꢁ23:9

ꢁ63:9

þ26:6

ð5Þ

—

ketone and thioketone or the C C bond energies in the enol

—

and thioenol and disregard the effect of the C C bond on the

—

O—H and S—H bonds is based on the following bond energies in

The second approach is based on theoretical calculations.

Extensive calculations^[34]show that the differences in the

—

kcal/mol: C—S, 61 S—H, 82, C S, 115; C—O, 88, O—H, 110, C

enolization energies for CH CH X, X ¼ S, O depend on the

—

level of theory, but thioenolization (DH ¼ ꢁ5.5–8 kcal/mol) is

always favored over enolization (DH ¼ 5–7 kcal/mol). For

O, 177.^[35–38]From these values the thioenolization equilibrium is

7 kcal/mol more favored than the enolization, in agreement with

the K_Thioenol/K_Enolratios of 10⁴:10⁶in Table 4.

—

(CH ) C X, the DDH value is 8.8 kcal/mol at B3LYP/6-31G . In

—

3 2

A drawback of this approach is that the O—H and S—H bond

energies are those for saturated and not for vinylic systems.

—

less extensive calculations the DDH values were 8–11 kcal/mol.^[42]

For systems substituted by two strong EWGs the K_Thioenol/K_Enol

ratios differ from those in the ﬁrst seven entries of Table 4.

Indeed, for compounds like 14, Ar ¼ Ph, p-An, X O, the O—H

—

J. Phys. Org. Chem. 2008, 21 483–491

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A. BASHEER AND Z. RAPPOPORT

Table 4. Collected literature pK_Enoland pK_Thioenolvalues and K_Enol/K_Thioenolratios for various systems (X ¼O, S)

System

—

Solvent

pK_Enol

pK_Thioenol

K_Thioenol/K_Enol

Reference

1.22^a

ꢄꢁ2^b

5.80

0.94

13.2^g

ꢂ10^6b

[15, 16]

[21]

—

Ph C C(Ar)XH, 14 Ar Ph

Hexane, DMSO-d₆

—

FlC( X)OMe, 15

H₂O

10⁴

—

MesC( X)Me, 16

6.92

18.5^f

3.62

—

10⁶

[18, 19]

[20]

[22]

—

MeC( X)OR

—

ꢂ10⁵

, 17

ꢁ1.3

ꢂ10⁵

PhC(R) C(XH)CH R, R ¼ H, Ph

—

ꢂ10⁶

[16]

—

(RR CH) C X, R ¼ R ¼ Me, R ¼ H, R ¼ i-Pr

—

ꢂ10⁶

—

RNHC( X)CH(CN)CONMe R ¼ p-An, Ph 3/5

CDCl₃

THF-d₈

CD₃CN

CDCl₃

ꢁ0.10 0.60

0.95 0.95

0.72 0.72

0.12 ꢁ0.03

1.06 0.72

ꢁ1.7 ꢁ1.7

0.91

ꢁ0.57 ꢁ0.60

1.15 1.10

ꢁ0.57 ꢁ0.44

ꢁ0.68 ꢁ0.68

ꢁ0.32 ꢁ0.32

ꢁ0.38 ꢁ0.54

ꢁ0.49

0.06 0.07

1.57 1.39

0.05 0.07

0.16 0.19

0.04 0.09

ꢄ0.008 ꢄ 0.07

0.04

[13]

—

RNHC( X)CH(CN)CONHMe R ¼ p-An, Ph 3/5

—

RNHC( X)CH(CN)CONHMe R ¼ p-An, Ph 3/5

—

RNHC( X)CH(CN)CONHMe R ¼ p-An, Ph 3/5

—

RNHC( X)CH(CN)CONHMe R ¼ i-Pr, t-Bu 3/5

—

RNHC( X)CH(CN)CONH R ¼ i-Pr, t-Bu 3/5

—

i- PrNHC( X)CH(CN)CONHPr-i 3/5

—

^aEstimated in hexane.

^bIn DMSO-d₆.

^cFl ¼ Fluorenyl.

^dValue not given, the ratio is estimated in Reference [21].

^eMes ¼ Mesityl.

^fR ¼ OMe.

^gR ¼ SEt.

^hData estimated in Reference [16] from literature data.

Comparing the K_Thioenolvalues of the cyano monothiocarbonyl-

malonamides 5 with the K_Enolvalues for cyano malonamides 3

especially in elongation of C(1)—C(2), caused by the two EWGs

(cf. structure 1b). Since bond lengths are correlated with their

strengths, we believe that the calculations mentioned above,

which are approximately applicable to the unactivated systems,

should not necessarily apply to systems activated by EWGs. We

give substituent and solvent-dependent

K_Thioenol(5)/K_Enol(3)

ratios. As shown in the last seven entries of Table 4 the ratios

are mostly < 0.2 and only two of them are 1.4–1.6.

—

Considering the approximate agreement between the values

based on the extensive calculations and the bond energy

calculations and the ﬁrst entries of Table 4, it could be expected

that the comparisons in Table 3 will give similar results. The

strong effect of the EWGs on both the amides and the thioamides

caused a ‘complete’ enolization (within the limit of detection

of < 1–2% of one of the species by our NMR method), for 10a–c/

11a–c and 8a–b/9a–b, whereas 8c/9c in DMSO-d₆gave the

enolate anion. However, disregarding other data for the 8/9

systems in DMSO-d₆since enolate ion may also be formed,

note however that the C C bond elongation is the same in both

—

the thioenol 11a and the enol 9a (Table 1).

Intramolecular hydrogen bonds are absent in the systems

unactivated by EWGs, but S—H^{. . .}O and O—H^{. . .}O bonds are

present in the b-ester activated systems. The importance of the

O—H^{. . .}O versus O—H^{. . .}S and O^{. . .}H—S hydrogen bonds is

demonstrated in the thioenol/enol equilibrium in the

2—SH-substituted tropone system 18a. The equilibrium with

the enol 2-OH-tropothione 18b (Eqn (6)) was reported to favor

18b in solution^[44]and in the solid.^[45]This indicates that in a

direct enol/thioenol competition the enol with the

O—H^{. . .}S hydrogen bond is favored over the thioenol with an

S—H^{. . .}O hydrogen bond.

_Thioenol/K_Enolfor 8g/9g–10g/11g is ca. 0.01 in CDCl₃, still small

in CD₃CN and is ca. 0.2 for 8i/9i–10i/11i. Consequently, for the

limited data available the values of < 1 are lower than those in the

top entries of Table 4. They roughly resemble those at the bottom

of Table 4 for systems activated by two EWGs.

The signiﬁcant differences in K_Thioenol/K_Enolratios for com-

pounds substituted and unsubstitted by EWGs may be due either

to (i) the fact that our systems are thioenols of carboxylic acid

amides, (ii) to the presence of the two EWGs, or (iii) to the

presence or absence of hydrogen bonds of different strengths.

Since the ratios for carboxylic esters and thionoesters lead to the

high K_Thioenol/K_Enolratios characteristic of ‘simple’ ketones and

—

thioketones shown in Table 4, so that, for example, Ph CHC(

—

S)OR undergoes complete thioenolization whereas the oxygen

analog does not enolize,^[43]explanation (i) is excluded.

Explanation (ii) seems to contribute to the different ratios. The

data of Table 1 show an extensive change in bond lengths,

However, structure 18a is stabilized by O—H^{. . .}O hydrogen

bonds with [Ph₂C(OH)CꢅꢅC]₂in

2:1 clathrate,^[46]with

O^{. . .}O distances of 2.824 A. Hence, an O—H^{. . .}O bond is preferred

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J. Phys. Org. Chem. 2008, 21 483–491

ꢀꢁEWGS-SUBSTITUTED THIOENOLS AND -AMIDES

over an O^{. . .}H—S bond. This should decrease the K_Thioenol/K_Enol

ratios. We conclude that explanations (ii) and (iii) account

qualitatively for the K_Thioenol/K_Enolratios.

model for activated thioenols. Comparison of the solid state

structures of MA-substituted thioenol and enol show a stronger

hydrogen bond in the enol than in the thioenol.

Comparison of the solid state structures of Meldrum’s acid

derivatives of the enol 9a, R ¼ Ph and the thioenol 11a

EXPERIMENTAL

General methods

The C(1)—C(2) bond lengths of 1.426 (5) A in 9a, R ¼ Ph^[1]and the

Melting points, ¹H and ¹³C NMR and IR spectra were measured as

described previously.^[50]All the commercial precursors and

solvents were purchased from Aldrich.

almost identical 1.425 (3) A in 11a (Table 1) are longer than a

—

normal C C double bond, but shorter than a single C—C bond

—

(see below). The C(4)—O(2) bond of 1.229 (4) A for 11a is

0.011 A shorter than for 9a, R ¼ Ph^[1], and this is ascribed to a

weaker OꢀꢀꢀH—S than OꢀꢀꢀH—O hydrogen bond. The correspond-

Reaction of Meldrum’s acid with phenyl, 1-naphthyl, and isopropyl

isothiocyanates to form 11a–c

ing C(3)—O(1) bond difference is just 0.001 A, indicating a similar

OꢀꢀꢀH—N hydrogen bonding for both derivatives. In both of them

the C(4)—O(2) bond is longer than the C(3)—O(1) bond due to

—

To a mixture of MA (0.72 g, 5 mmol) and triethylamine (1.5 ml,

10 mmol) in dry DMF (5 ml), 1-naphthyl isothiocyanate (926 mg,

5 mmol) was added. The mixture was stirred at room temperature

(RT) for 4 h, poured into an ice-cold 6% HCl solution (100 ml) and

the yellow solid formed was ﬁltered and washed with cold water,

giving 1.45 g (88%) of the thioenol of 5-(1-naphthylamino-

thiocarbonyl)-2,2-dimethyl-1,3-dioxane-4,6-dione 11b, mp 150–

151 8C (dec). Anal. Calcd for C₁₇H₁₅NO₄S: C, 62.01; H, 4.56; N, 4.26.

—

stronger C OꢀꢀꢀH—O and C OꢀꢀꢀH—S than C OꢀꢀꢀH—N

—

hydrogen bonds. As expected, the O—H and OꢀꢀꢀH lengths are

shorter than the respective S—H and SꢀꢀꢀH lengths and

angle < OHS is ꢆOHO, while the <NHN angles are closer

(Table 1). The C(2)C(4)O(2) angle for the thio derivative is 127.18,

—

10.28 larger than for the oxo derivative with the stronger C

OꢀꢀꢀH—O hydrogen bond, while the C(2)C(3)O(1) angles are

almost identical (126.528 for 11a and 126.18 for 9a, R ¼ Ph),

because of the similar N—HꢀꢀꢀN bonds. Most of the angles around

the C(1)—C(2) bond are close to 1208 (Table 1). We conclude that

both derivatives are enolic and that the hydrogen bond in the

thioenol is weaker than in the enol.

Found: C, 62.20; H, 4.53; N, 4.50. H NMR (CDCl₃, 298 K, 400 K) d:

1.81 (6H, s, Me), 7.51–7.60 (3H, m, Ar-H), 7.83–7.95 (4H, m, Ar-H),

11.44 (1H, s, NH), 13.16 (1H, s, SH). ¹³C NMR (CDCl₃) d: 26.39 (q,

J ¼ 128.7 Hz, Me), 83.44 (s, C_b), 103.40 (m, J ¼ 4.8 Hz, CMe₂),

121.89, 125.08, 125.13, 126.98, 127.54, 128.31, 129.00, 129.48,

—

In the up to August 2007 version of CSD we found X-ray

—

structures of only 5 non-aromatic compounds with the C

—

C—S—H moiety derived from thioketones. These include (i) a

clathrate of two molecules of 2-monothiotropone associate

hydrogen bonded to 1,1,6,6-tetraphenylhaxa-2,4-diyne-1,6-diol

132.32, 134.27, 165.65(C O), 167.59(C O), 179.79 (C ).

—

The thioenol of 5-(1-phenylaminothiocarbonyl)-2,2-dimethyl-

1,3-dioxane-4,6-dione 11a, mp 113–114 8C, was similarly

obtained in 2.21 g (79%) from MA (1.44 g, 10 mmol) and phenyl

isothiocyanate (1.2 ml, 10 mmol). The thioenol of 5-(1-isopro-

pylaminothiocarbonyl)-2,2-dimethyl-1,3-dioxane-4,6-dione 11c,

mp 100–102 8C, was obtained in only 13% yield (0.33 g) from

MA (1.44 g, 10 mmol) and isopropyl isothiocyanate (1.08 ml,

10 mmol). Spectral and analytical data are given in Tables S1–S3

in the Supplementary data.

with a C C—S bond length of 1.384 A and a S—H^{. . .}O hydrogen

—

bond;^[46](ii) dithiotropone with C C—S bond length of 1.389 A

and an unsymmetrical S—H^{. . .}S hydrogen bond;^[47](iii)

(3,4,5-triphenyl-3,4-dihydro-2H-thiopyran-2-ylidene)methanethiol

—

[48]

—

with a C C—S bond length of 1.338 A;

(iv) N-phenyl-2,

—

C—C(SH)

—

[49]

3-dithiomaleimide,

dithioenol having

—

C(SH)— moiety, with a C C—S bond length of 1.327(8) A;

—

The thioenols (11e) of methyl (1-naphthylaminothiocarbonyl)

cyanoacetate (10e), and (11d) of methyl

1-phenylaminothiocarbonyl) cyanoacetate (10d)

(v) triphenylethenethiol, with a C C—S bond length of

—

[15]

1.356(5) A.

—

Consequently, all the C C bond lengths in these cases

—

are < 1.389 A, compared with the bond length of 1.425 A for 11a.

To a stirred mixture of methyl cyanoacetate (0.25 g, 2.5 mmol) and

dry Et₃N (0.75 ml, 5 mmol) in dry DMF (3 ml), 1-naphthyl

isothiocyanate (463 mg, 2.5 mmol) was added and the mixture

was stirred for 24 h at RT giving a white solid. The solid was

ﬁltered and washed with dry ether, giving 0.68 g (71%) of the

—

This difference reﬂects the effect of conjugation of the C

—

bond with additional C C bonds, phenyl group, or a single

—

EWGs in systems (i)–(v), compared with the much stronger

delocalizing ability of the two b-EWGs in 11a which is reﬂected in

contributing structure 1b. 11a is therefore a record holder of the

ammonium salt of 11e [C₁₅H₁₁N₂O₂S]^ꢁEt₃N^þH. H NMR (DMSO-

—

C bond length among the known thioenols.

d₆, 298 K, 400 Hz) d: 1.16 (9H, t, J ¼ 7.3 Hz, CH₃CH₂N), 3.08 (6H, q,

J ¼ 7.3 Hz, CH₃CH₂N), 3.62 (3H, s, OMe), 7.46 (1H, t, J ¼ 7.7 Hz,

Ar-H), 7.49–7.56 (2H, m, Ar-H), 7.69 (1H, d, J ¼ 8.5 Hz, Ar-H), 7.92

(1H, d, J ¼ 7.3 Hz, Ar-H), 7.98 (1H, d, J ¼ 8.1 Hz, Ar-H), 8.25 (1H, d,

J ¼ 7.3 Hz, Ar-H), 8.82 (1H, br s, [Et₃NH]), 12.25 (1H, s, NH). A

solution of the salt in DMF (5 ml) was added dropwise during

10 min to a stirred ice-cold 6% HCl solution (5.0 ml). The pure

white precipitate formed was ﬁltered, washed with cold water

(100 ml) and dried to give 0.43 g (60%) of 10e, mp 139–140 8C.

Anal. Calcd for C₁₅H₁₂N₂O₂S: C, 63.38; H, 4.23; N, 9.86. Found: C,

63.47; H, 4.26; N, 10.09. ¹H NMR (CDCl₃, 298 K, 400 K) d: 3.87 (3H, s,

OMe), 4.61 (1H, s, SH), 7.49 (1H, d, J ¼ 7.3 Hz, Ar-H), 7.53 (1H, t,

J ¼ 8.0 Hz, Ar-H), 7.56–7.64 (2H, m, Ar-H), 7.87–7.96 (3H, m, Ar-H),

The d(SH) values and the relative strengths of the OꢀꢀꢀꢀH—O,

OꢀꢀꢀꢀH—S, and OꢀꢀꢀꢀH—N hydrogen bonds served as probes for

the thioenol structures. Both intermolecular and intramolecular

competition between enolization and thioenolization in thioa-

mides substituted by two b-EWGs shows that enolization is more

extensive than thioenolization, in contrast to simple ketones and

thioketones in which thioenolization is favored over enolization

by large factors.

The solid state structure of the 5-anilido-MA derivative 11a is

the ﬁrst X-ray structure of a noncyclic thioenol of a thioamide

activated by two b-EWGs and the doubly hydrogen bonded

system with S1—HꢀꢀꢀO2 and N1—HꢀꢀꢀO1 moieties can serve as a

J. Phys. Org. Chem. 2008, 21 483–491

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A. BASHEER AND Z. RAPPOPORT

11.91 (1H, s, NH). ¹³C NMR: 51.95 (q, J ¼ 147.9 Hz, Me), 72.62 (s, C_b),

127.16, 127.72, 128.51, 129.38, 129.53, 133.04, 134.26, 168.24,

170.72.

analytical data are given in Tables S1–S3 in the Supplementary

data.

The phenyl derivative 11d, mp 132–133 8C, was similarly

obtained in 53% yield, but no ammonium salt was obtained in the

ﬁrst step. Spectral and analytical data are given in Tables S1–S3 in

the Supplementary data.

SUPPLEMENTARY DATA

Tables S1 and S2 with complete ¹H and ¹³C NMR data and Table

S3 with analytical data are available online. The full crystal-

lographic data for compound 11a is given as a cif.

The thioenol (11f) of 2,2,2-trichloroethyl

(1-phenylaminothiocarbonyl) cyanoacetate (10f)

Acknowledgements

A mixture of 2,2,2-trichloroethyl cyanoacetate (0.54 g, 2.5 mmol),

dry Et₃N (0.75 ml, 5 mmol), and phenyl isothiocyanate (0.3 ml,

2.5 mmol) in DMF (5 ml) was stirred for 1 h at RT, and poured into

an ice-cold 6% HCl solution (50 ml). The oily solid obtained

solidiﬁed on cooling overnight in a refrigerator, giving 0.46 g

(52%) of 10f as a yellow solid, mp 130–131 8C. Anal. Calcd

for C₁₂H₉Cl₃N₂O₂S: C, 40.97; H, 2.56; N, 7.97. Found: C, 41.27; H,

2.64; N, 8.04. ¹H NMR (CDCl₃, 298 K, 400 K) d: 4.83 (1H, s, SH), 4.86

(2H, s, CH₂), 7.29 (2H, d, J ¼ 8.1 Hz, Ph-H), 7.40–7.51 (3H, m, Ph-H),

11.48 (1H, s, NH). Spectral and analytical data are given in Tables

S1–S3 in the Supplementary data.

We are indebted to the Israel Science Foundation for support and

to Dr. Shmuel Cohen for the X-ray structure determination.

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Bis(2,2,2-triﬂuoroethyl) (1-phenylaminothiocarbonyl)malonate

(10g) and bis(2,2,2-triﬂuoroethyl)

(1-naphthylaminothiocarbonyl)malonate (10h)

To a stirred mixture of bis(2,2,2-triﬂuoroethyl)malonate (1.34 g,

5 mmol) and dry triethylamine (1.5 ml, 10 mmol) in dry DMF

(10 ml), phenyl isothiocyanate (0.6 ml, 5 mmol) was added and

the stirring continued overnight at RT. The solution was added

dropwise with stirring into an ice-cold 6% HCl solution (100 ml)

and the yellow precipitate was ﬁltered, washed with cold water

and dried in air to give 0.89 g (68%) of 10g/11g, mp 75–76 8C.

Anal. Calcd for C₁₄H₁₁F₆NO₄S: C, 41.69; H, 2.73; N, 3.47. Found: C,

41.16; H, 3.12; N, 3.45.

¹H NMR (CDCl₃, 298 K, 400 K) Thioamide 10g (94%) d: 4.51–4.75

(4H, m, CH₂CF₃), 5.31 (1H, s, CH), 7.31 (t, J ¼ 7.6 Hz), 7.43 (t,

J ¼ 8.0 Hz), 7.70 (d, J ¼ 8.0 Hz), 10.33 (1H, s, NH). Thioenol 11g

(6%): 4.34–4.52 (0.25 H, m, CH₂CF₃), 4.95 (0.07 H, s, SH), the phenyl

signals overlap the 9g signals, 12.24 (0.07 H, s, NH).

The 1-naphthyl derivative 10h, mp 78–80 8C, was similarly

obtained in 79% yield. Spectral and analytical data are given in

Tables S1–S3 in the Supplementary data.

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2269.

Dimethyl (1-phenylaminothiocarbonyl)malonate (10i)

Dimethyl malonate (2.64 g, 20 mmol) in dry THF (20 ml) was

added dropwise with stirring during 10 min to a dispersion of Na

(0.48 g, 21 mmol) in dry THF (50 ml). On stirring overnight, all the

Na reacted. Phenyl isothiocyanate (2.4 g, 20 mmol) in dry THF

(30 ml) was added dropwise to the stirred mixture during 30 min

and the mixture was reﬂuxed for 3 h. The solvent was evaporated,

the remaining sodium salt was dissolved in DMF (5 ml) and the

solution was poured into ice-cold 6% HCl solution (100 ml) with

stirring. The oily product obtained was extracted with a 4:1

hexane/ether mixture (300 ml), washed with ice-cold water

(3 ꢇ 100 ml) and dried (Na₂SO₄). The solvents were evaporated

under reduced pressure, leaving 2.83 g (53%) of 10i as a red,

heavy oil. Anal. Calcd for C₁₂H₁₃NO₄S: C, 53.93; H, 4.87; N, 5.24.

Found: C, 53.49; H, 5.01; N, 5.54. H NMR (CDCl₃, 298 K, 400 K) d:

3.74 (6H, s, Me), 5.05 (1H, s, CH), 7.18 (1H, t, J ¼ 7.7 Hz), 7.31 (2H, t,

[35] J. March, Advanced Organic Chemistry, 4th edn. Wiley, New York,

1992, p. 24.

J ¼ 7.7 Hz), 7.70 (2H, d, J ¼ 8.0 Hz), 10.68 (1H, s, NH). Spectral and

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ꢀꢁEWGS-SUBSTITUTED THIOENOLS AND -AMIDES

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