Tautomerism of substituted trichloroacetamidines

Spectrochimica Acta, 1964,Vol. 20,pp. 1555to 1560.PergamonReds Ltd. Printedin NorthernIreland

Tautomerism of substituted trichloroacetamidines A. G. MORITZ Defence Standards Laboratories, Maribyrnong, Victoria, Australia (Received

16 December

1963)

triAbstract-A comparison of the N-H stretching frequencies of various N-substituted chloroacetamidines shows that in all cases these compounds exist as a mixture of amino and N-alkyl substituted derivatives exist principally in the imino form whereas imino tautomers. contrary to literature claims, N-aryl substituted compounds are shown to exist predominantly as the amino tautomer. These results are confirmed unambiguously from the spectra of the partially deuterated derivatives. INTRODUCTION

and TAURINS [I] examined the infra-red spectra of a number of N-mono and N,N-disubstituted trichloroacetamidines and concluded that both the N-alkyl and N-aryl substituted compounds exist only in the imino form (I) in non-polar solvents. However, re-examination of their data for the N-H stretching frequencies suggested that the N-aryl derivatives could be reassigned on the basis that they existed as the amino tautomer (II) NH NH,

GRIVAS

ccl,-c

/ \

+ CC&-c

/ \

NHR

NR

(II)

(I)

To confirm this reassignment, two N,N-disubstituted trichloroacetamidines have been examined as model compounds for the imino N-H stretching frequency and the parent compound has also been examined as a model compound for both the imino and primary amino group N-H stretching frequencies. As additional evidence, partially deuterated derivatives of trichloroacetamidine and two N-aryl berivatives have been examined for unequivocal identification [2] of the NH, stretching frequencies. EXPERIMENTAL

Muterials The substituted trichloroacetamidines were prepared according to the method of GRIVAS and TAURINS [3] and had physical constants in accordance with the published values. Trichloroacetamidine was prepared by the addition of [l] J. C. GRIVAS and A. TAURINS, Can. J. Chem. 37, 795 (1959). [2] A. G. MORITZ, &ectrochim. Acta 18, 671 (1962). [3] J. C. GRIVAS and A. TAURINS, Can. J. Chem. 36, 771 (1958). 1555

1556

A.

G.

MORITZ

trichloroacetonitrile to liquid ammonia and recrystallized from light petroleum 40-60°C). N,iV-diphenylguanidine was a commercial sample recrystallized.

(b.p.

Infru-red spectra The N-H &retching frequenci& were measured with a Uhicam SP 700 double beam spectrometer equipped with a Mervyn N.P.L. 7500 l/in. grating used in the first order, Ge filter and PbS detector. The effective slit width employed was 3 N 4 cm-l at 3500 cm-l. The spectra were calibrated against ammonia gas [4] which was run after each compound was examined. Reported values are averages from at least three measurements and were reproducible to better than 1 cm-l. Compounds were examined as dilute solutions in carbon tetrachloride in 30 mm cells. Carbon tetrachloride was washed with water and distilled twice from phosphorus pentoxide. The results are given in Table 1. Table

1. K-H

stretching

Substituted

NH,

t,richloroacetamidine

VU

frequencies

(cm’)

NHI)

in dilute

CC14 solution

NH,

trans

cis

V8

3463 -

3416 3417 3417

YC=NH

Z’NH __~

NOIE ,V-methyl N-ethyl

3520 3506 3506

N-bmzyl

3505

N-phenyl x-p-to1y1 S,N-dimethyl AV,N-diethyl

3509 3508 -

S+V-diphenylguanidine

3499

3485 -

3418 3470 3470

3443 3441

3402 3400

3471 { 3458 3467 1 3451 3464 3440 3441 -.

3451

3405

{ 3427 3444

3351 3353 3353 3351 3343 3344 3361 3366

-

ASSIGNMENTS

Imino N-H

stretching frequencies

Very few imino N-H stretching frequencies have been reported for compounds examined in non-polar solvents under high resolution. As with N-H stretching frequencies of aromatic amines, imino N-H stretching frequencies appear to be sensitive to electronic effects of adjacent substituents. For example, MASON [5] finds 3278 cm-l for l-ethyl-l : 4-dihydro-4-iminopyridine and 3325 cm-l for l-ethylN,N-dialkyl trichloroacetamidines show a single 1: 2-dihydro-2-iminopyridine. absorption band at even higher frequencies ( ~3360 cm-l in Ccl,, Table 1). It is therefore to be expected that imino stretching frequencies obtained for N-alkyl and X-aryl substituted trichloroacetamidines will show variations from the frequencies obtained for N,N-dialkyl derivatives and this is observed (Table 1). The shifts are not large enough however to leave any doubt as to the correct assignment to the imino group. [4]

Tables of Wavenumbers for the Calibration (1961).

[j]

S. F. MASON, J. Chem. Sot.

1281

(1959).

of Infra-red

Spectrometers,

Butterworths,

London

Tautomerism

of substituted trichloroacetamidines

1557

Trichloroacetamidine Trichloroacetamidine in dilute solution in carbon tetrachloride shows three bands in the N--H stretching region at 3520, 3416 and 3351 cm-l (Fig. la). By comparison with N,N-dialkyl trichlorbacetamidines the 3351 cm-l band is assigned to the imino group and the other two bands are therefore the asymmetric and symmetric NH, stretching bands. This is confirmed by partial deuteration in which it is observed that the three N-H stretching bands decrease in intensity and two new bands appear at 3485 and 3463 cm--l (Fig. 1A). The mean of these two bands (3473 cm-l) is close to the mean of the 3520 and 3416 crh-l bands (3468 cm-l) and therefore unambiguously [2] shows that the two high frequency bands of trichloroacetamidine are v, and v,, NH,. N-aryl trichloroacetamidines GRIVAS and TAURINS [l] observed only two bands for N-phenyl trichloroacetamidine at 3520 and 3415 cm-l (in C&Cl,) and assigned them to secondary amino and imino N-H stretching frequencies respect’ively. Under higher resolution (Fig. IB), this compound shows two strong N-H stretching bands at 3509 and 3402 cm-l and two weaker bands at 3440 and 3343 cm-l, the position and relative intensity of which were independent of concentration for dilute solutions in Ccl,. After partial deuteration (Fig. lB), two bands appear at 3470 and 3443 cm-l, the mean of which (3456 cm-l) is close to the mean of the 3402 and 3509 cm-l bands (3455 cm-l). The two strong bands are therefore the symmetric and asymmetric NH, stretching frequencies. The low intensity bands at 3440 and 3343 cm-l are typical for a secondary amino group [6] and imino group (Table 1) respectively. N-p-tolyl trichloroacetamidine showed a similar behaviour to the S-phenyl derivative. N-alkyl trichloroacetamidines The spectrum of N-methyl trichloroacetamidine (Fig. 1C) is distinctly different, from the N-phenyl derivative. In this case two strong bands appear at 3471 and 3353 cm-l with two weaker bands at 3507 and 3417 cm-l. The strong band at 3353 cm-l is typical for an imino group and the two weaker bands are in a similar position to the bands of the primary amino group in trichloroacetamidine and the n’-phenyl derivative. This leaves the 3471 cm-l band to be assigned to the secondary amino group. The assignment is confirmed by partial deuteration as no new bands appear near the mean of 3471 and 3353 em-l. This eliminates the possibility that they are due to a primary amino group. The failure to observe bands due to the NHD group presumably arises from overlap with the intense 3471 cm-l band. Similar spectra were observed for the N-ethyl and X-benzyl derivatives with the exception that an additional shoulder was observed on the secondary amino group band. Diphenylguanidine Diphenylguanidine, which is analogous to the 6-aryl amidines, can also exist in either the imino or amino forms (see also below). In dilute solution in carbon tetrachloride, two strong bands appear at 3499 and 3405 cm-l typical for a primary amino group and two weaker bands at 3444 and 3427 cm-l. After partial deuteration, [6] R. A. RUSSELL and H. W. THOMPSON,J. Chem.80~.

483 (1958).

1658

A. G. MORITZ

a broad band at 3451 cm-l appeared near the mean of the 3499 and 3405 cm-l bands (3452 cm-l). The two strong bands therefore arise from an NH, group. The frequencies of the two weaker bands are too high to belong to an imino group and therefore presumably belong to a secondary amino group.

Fig. 1. The infra-red spectra in dilute WI, solution of (A) trichloroacetamidine (---) and partially deuterated (---), (B)N-phenyl-trichloroacetamidine(--) and partially deuterated (--) and (C) N-methyl-trichloroacetamidine.

Partially deuterated compounds The two bands arising from the NHD group in partially deuterated trichloroacetamidine and the N-aryl derivatives canbe assigned to cis and trans isomers of the NHD group (III and IV; R=H or aryl). NR

NR ccl,-c

// \

CCI,-C N-D I H

(III) trans

d \

N-H I!

(IV) cis

Tautomerism

of substituted

trichloroacetamidines

1559

Similar doubling of the N-H stretching band in NHD groups has been observed in monodeuterated derivatives of intramolecularly hydrogen bonded primary aromatic amines [2, 71, primary amides [S, 91 and sterically hindered amines [lo]. The shift in frequency between the cis and trans forms of the NHD group (22 cm-l) is similar to that observed for primary amides and the higher frequency band is tentatively assigned to the isomer with the N-H bond trans to the C-N bond (III) by analogy with amides [8, 9, 111. Conformations

of amino and imino tautomers

For N-substituted trichloroacetamidines, two conformations are possible for both the amino (V and VI) and imino forms (VII and VIII) as well as additional conformational isomerism about the C=NH bond in the case of the imino tautomers. ccl,

ccl, \

C=N

in

\

+

/

/

NH,

\

NH, (V)

//

“C-N/

+ \

(VW

R

(VI)

C-N NH

C-N

R

NH

//

‘1

H

(VIII)

It is noteworthy that the N-H stretching frequencies of the secondary amino group in the imino forms of N-methyl and N-phenyl trichloroacetamidines (3471 and 3440 cm-l respectively) are appreciably different. By analogy with the N-H stretching frequencies of the monodeuterated N-phenyl trichloroacetamidine (amino form, trans NHD, 3470 cm-l; cis NHD, 3443 cm-l), this would at first suggest that N-methyl trichloroacetamidine exists in the imino tautomer as the trans isomer (VII) whereas the imino form of N-phenyl trichloroacetamidine exists as the cis isomer (VIII). However, the cis isomer for the N-phenyl derivative is unlikely for steric reasons. Moreover, the shift in frequency is closely paralleled by the structurally similar N-methyl and N-phenyl acetamides (tra.ns, 3472 and 3445 cm-l respectively) [ 111 both of which exist predominantly in the trans conformation. The shift in frequency can thus be reasonably accounted for by the electron attracting effect of the phenyl group. The additional band for the secondary amino group in N-ethyl- and N-benzyl-trichloroacetamidines may possibly be due to the cis and [7]

[8] [9] [lo] [ll]

A. A. A. A. R.

G. G. G. N. A.

MORITZ, Spectrochim. Acta 16, 1176 (1960). MORITZ, Nature 195, 800 (1962). MORITZ, to be published. HAMBLY and B. V. O’GRADY, Chem. and Iw_i. 86 (1963). RUSSELL and H. TV. THOMPSON, Spectrochim. Acta 8, 138 (1956).

A. G. MORITZ

1560

trans forms (VII) and (VIII) although additional possibilities of rotational isomerism can occur for these derivatives about the second C-N bond. The latter explanation wouldbesupportedbythefailuretoobservesimilardoublingin t,heN-methylderivative. C,H,-N

0\ \

/

/

\

C-N NH,

/

‘\

H

C&H,-N C-N

/

H

Apart from the bands due to the primary amino group, N,N’-diphenylguanidine also shows two weak bands at 3444 and 3427 cm- l. As either band is too high to be attributed to an amino group and no other bands are observed, they have been assigned to the secondary amino group, corresponding to the rotational isomers (IX) and (X). In this case, the difference in steric interaction between the two forms would not be large as compared with the trichloroacetamidines. The conformation adopted for the amino form of trichloroacetamidines (V) or (VI) cannot be unequivocally determined from the N-H stretching frequencies, although by analogy with the imino form, the predominant isomer is expected to be (VI). DISCUSSION The comparison of the spectra of trichloroacetamidine with the N-alkyl and N-aryl derivatives, together with the partially deuterated compounds shows COW elusively that whereas the N-alkyl derivatives exist predominantly as the imino tautomer, the N-aryl derivatives are principally in the amino form. The conclusions of Grivas and Taurins that both the N-alkyl and IN-aryl derivatives exist exclusively in the imino form, cannot therefore be supported. It is not apparent why the N-alkyl derivatives exist predominantly in the imino form. For the N-aryl compounds, a change in the equilibrium to favour the amino tautomer would be anticipated as in this case it is possible for the aromatic ring t,o be conjugated with the C-N bond. Although accurate assessment of the equilibrium constants would be difficult and involve a number of assumptions, a comparison of the intensity ratios of the N-alkyl and N-aryl compounds suggests that there is an This energy difference of ~3 Kcal/mole between the two series of compounds. value is within the range of resonance energies derived from heats of combustion for a double bond conjugated with an aromatic ring. The results obtained here are similar to those obtained by PREVORSEK [I;] u-ho showed that certain N-alkyl amidines exist, predominantly but not exclusively in the imino form. His results differ however in that no bands could be detected for the imino form in the case of the N-sryl derivat,ives. Acknou&xlgenzent-The author wishes to t~hank the Chief Scientist, Australian Defence Sri for permission t,o entific Service, Department of Supply, Melbourne, Victoria, Andralia, publish this paper. [12] D. C. PREVORSEX, J. Whys. Chena. 66,

769 (1962).