May 5, 1961
EQUILIBRIA OF BIS-(ACETYLACETONE)-ETHYLENEDIIMINE
[CONTRIBUTION FROM THE
DEPARTMENT OF CHEMISTRY, HARVARD UNIVERSITY,
38,
CAMBRIDGE
2099
MASSACHUSETTS]
A Proton Resonance Study of Bis-(acety1acetone)-ethylenediimine and Related Schiff
Bases B Y GERALD 0. DUDEK AND RICHARD H. HOLM RECEIVED NOVEMBER 7, 1960 The tautomeric equilibria of bis-(acety1acetone)-ethylenediimiue and related Schiff bases have been studied by proton magnetic resonance. The spectra of the compounds examined indicate t h a t in common solvents except acetone they exist almost entirely as chelated hydrogen-bonded tautomers. A significant portion of these tautomers is present in t h e ketamine form. The solvent dependence of these equilibria is small. The effect of aromatic solvents on the proton spectrum is large. The aromatic diamagnetic shifts are, however, explicable in terms of the geometry of the system and an association between the carbonyl of the base and the protons of the aromatic. The use of these diamagnetic shifts for structural inferences in n.m.r. studies is briefly discussed.
Introduction Condensation products of acetylacetone (2,4pentanedione) and related P-diketones with monoand diamines have been the subjects of a variety of physical studies in recent years. Martell and coworkers have examined the infrared’sS and ultraviolet2S8spectra and obtained dipole r n o m e n t ~of~ + ~ a number of these Schiff bases derived from diamines while Holtzclaw, et al.,6 have studied the /CH3 CH3 i infrared spectra of several analogous bases ob/,c-0, &( O-U. /CH tained from monoamines. Studies of these comHC, pounds assume additional importance in view of C=rJ‘ p‘,= ;c, the ability of the Schiff bases to coordinate divalent metal ions.2v6J CH? ‘CH~-CH~ CH?. Paramount in the consideration of these Schiff I1 bases are the questions of the positions of the ketoCH3, ,CH? enol or amineimine equilibrium and the nature ,c=q .o=c of the hydrogen bond in the six-membered chelate H gk ‘CH HC ring. / 4 Selecting bis-(acety1acetone)-ethylenediimine as CHzCHz the prototype of the compounds to be considered CH3 CHs in this work, the forms 1-111 may be present in I11 tautomeric equilibrium as shown. In an attempt to answer the foregoing questions The distinction between I1 and I11 is not large as a slight displacement of the hydrogen nucleus we report here the results of an investigation of would interconvert the two forms. Nevertheless, the proton resonance spectra of a number of bisthe two forms should be distinguishable entities (acety1acetone)-diimines in a variety of solvents. provided the potential barrier separating the two It has been concluded from infrared evidence that sites adjacent to the oxygen and nitrogen is suf- these compounds are largely tautomerized into both I1 and I11 with appreciable amounts of each ficiently great. Clearly some of the same considerations apply existing in the solid and solution.1,2 Similar when dealing with p-diketones, j3-ketoesters and spectra have been obtained for acetylacetonerelated dicarbonyl compounds. Of the physical monoimines.6 In addition Cromwell, et aLJ9have methods applied to the study of the equilibria of inferred from an infrared study of the carbonyl these compounds, proton magnetic resonance has region that the a$-unsaturated-@-amino ketones been conspicuous in its ability to assess keto-enol examined by them have the chelated ketamine ratios and to reflect the electronic nature of the structure protons involved in the hydrogen bond of the enolCH3 chelate.8 %=o.
-v
v\ f-”,
(1) K.Ueno and A. E. Martell, J . P h p . Chcm., 69, 008 (1055). (2) A. E. Martell, R. L. Belford and M. Calvh. J . Xnorg. Nuclcdt Chem., 6, 170 (1858). (3) K. Ueno and A. E. MarteU, J . Phys. Chem., 61, 267 (1017). (4) P. J. McCarthy and A. E. Martell, J . Am. Chcm. Soc., 78, 284 (1966). (5) R. J. Hovey and A. E. Martell, ibid., 81. 384 (1080). (6) H. F.Holtzclaw, Jr., J. P. Collrnan and R. M . Alire, ibid., 80, 1100 (1958). (7) P. J. McCarthy, R. J. Hovey, K. Ueno and A. E. Martell, ibid., 47, 6820 (1055). (8) For a summary and references t o work in this area up t o 1068, see J. A. Poplr, W. G. Schneider, € J.I. Bernstein, “High-resolution Nuclear Magnetic Resonance,” McGraw-Hill Book Co.. New York, N. Y.,1959, pp. 433-441.
/”“\
/
R-C
-R
1 / b T R ,
CH3
IV
No previous proton resonance studies of Schiff bases appear to have been done except for the very recent work of Reeves, Allen and Str@mme,lo who report chemical shifts for protons in the hy(0) N. H.Crornwell,F. A. Miller, A. R. Johnson, R. L. Frank and D. J. Wallace, J . Am. Chcm. Sac... 71., 3337 (1949). .~ (10) L. W. Reeves, E. A. Allen and E. 0. Str$rnme, Can. J . Chcm., I
88, 1249 (1060).
2100
GERALD 0. DUDEKAND RICHARD H. HOLM
I
1 1
I
I
11.0
H-
-
4
1
solvent
I 4.35
4 83
'
3 . 4 0 I3.14
2.42
7
1 9 8 1.41
I
I ye7
Oppm
o ppm
1.18
SOIY.
1-01. s3
Solvents and Solutions.-Common solvents were cointnercial reagent grade. Chloroform was treated wit11 Woelm alumina to remove alcohol. Spectra in benzene dried with phosphorus pentoxide were identical with those using the undried solvent. Hexadeuterioacetone was A-ew England Nuclenr Co. material of 9970 isotopic purity. Tetramethylsilane (Xnderson Chemical Co.) was purified by distillation from concentrated sulfuric acid. Pyrrole was distilled and used immediately to prepare the necessary solutions, The required solution was prepared and a sample sealed in vacuo in a n n.m.r. tube and stored in liquid nitrogen until used. With the exception of studies involving hexadeuterioacctone as the solvent, the solutions were made up to a concentration of 200 mg. of base (-0.2 AC) in 5.00 ml. of a solvent which contained 0.15 ml. (37, v./v.) of tetramethylsilane as a n internal standard. I n all cases the concentration of the solute was only slightly less than its solubility limit in carbon tetrachloride. T o assist in sidebanding these dilute solutions, the concentration of the internal standard was slightly larger than usual. When hexadeuterioacetone was used as the solvent the solutions were prepared on a weight basis in the sample tube. Concentrations were -0.4 -11 so t h a t the methyl signals of the base were clearly defined relative to the residual protiuni in the solvent. All samples were degassed, sealed, and stored in liquicl nitrogen until used. Spectra.-Spectra were obtained 011 a Varian V-4300 spectrometer operating a t 60.000 Me. The line positions were determined by the sideband method with the audio oscillator being monitored by a frequency counter. T h e line positions, with reference t o tetramethylsilane as the internal standard,'s are accurate t o =k 0.2 C.P.S. or 0.003 p.p.m. except where the band width precluded such accuracy. The chemical shifts 6 (in p.p.m.) are defined as p - pLr/p0wherepc,and are the resonant reference frequency and the fixed radio frequency, respectively.
Fig. 1.-Proton spectra ( a t 60.000 M c , ) of: X, bis(acety1acetone)-eth!lenediiniine and B bis-(acetylacetone). propylenediitnine (in B, the low field signal a t 10.8 p.p.in. is omitted).
Results and Discussion Because the spectrum of acetylacetone in carbon tetrachloride solution with tetramethylsilane as a n internal reference is useful for comparison drogen bonded rings of several bis- (salicylalde- purposes, it was measured and the results are listed in Table 11. The agreement with previous hyde)-diimines. investigators is good.8 However, in this solvent Experimental the compound is primarily in the enolic form so Preparation of Compounds.l1 The condensation products that a t the concentrations necessary for equivaof acetylacetone and amines were synthesized according t o lence to the much less soluble diimines, the signals previously published methods from freshly distilled acetylacetone and the corresponding diamine. from the small amount of keto form ( ~ 5 7 were ~ ) ni.p., not discernible. This Base Ref. work Lit. The spectrum of bis-(acety1acetone)-ethyleneBis- (acety1acetone)-ethylenediiminediimine(-&en) in carbon tetrachloride solution is (hen)" 7 111 111-111.,j pictured in Fig. 1 and the exact band positions are Bis-(acety1acetone)-propylenediimine 7 80-90 91 Ris-!acetylacetone)-trimeth~lenediiniineb 2 61 51 listed in Table I. The two bands near 1.SS p.p.m. Bis-(acetylacetone) -tetramethylenediare due to the slightly non-equivalent methyl groups imine 12 10:;-104 10.5 .5-10G.5 in the acetylacetone moiety. The group a t 3.46 p.Ris-(benzoy1acetone)-ethyienediimine 7 181 150 ; p.m. is the ethylene bridge; its peculiar shape will Acetylacetone anil-(-l-anilino-8-pentene ?-one) 13 47-?8 4s be discussed later. The signal a t 4.S1 p.p.m. Diethyl-(eth~lene-p-aminocrotouate:c 2 ... corresponds to the vinylic hydrogen (at 3.30 p.p.m. a Deuterated form was prepared by re-crystallization from in acetylacetone) and, coupled with the very broad D,O. * 5 S calcd., 11.78; found, 11.46. Deuterated absorption (-30 C.P.S. wide) a t 10.9 p.p.m. ascribform was prepared bl- dissolving 0.5 g. in the minimuin able only to a hydrogen-bonded proton, indicates volume of hot dioxane and then adding D?O slowly until the that in this solvent the diirnine is primarily in the solution became cloudy. The solution then was cooled and filtered. The procedure m a s repeated oiice and this sarnple hydrogen-bonded chelate form I1 or 111. The used in the spectral work. absence of a methylene band near 3.07 p . p m Spectra of 4-ainino-3-pente1~e-2-one and 4-amino-4-(@- (see following discussion of the spectrum in acepyridyl)-3-butene-2-one were obtained from E. Hand of this tone) is further confirmation of the tautomerized department .I4 chelate character of the base. ____-The other diimines studied which differ only in (11) Throughout this paper the compounds studied will be referred to as diimines; however, t h i s terminology is not intended t o imply the nature of the bridge joining the nitrogens structure. have the anticipated spectra (cf. Fig. 1 and Table (12) R. J. Hovey, J. J. O'Connell and A . E. Martell, J . A m . Chem. OC.
Soc., 81, 3189 (19593. (13) E . Roberts and E. E. Turner J . Chem. Soc., 1832 (1927).
(14) For further details see E. Hand, Thesis, Harvard Univ.. 1961. (15) G 1- D. Tiers. J . Phys C h e w . , 6 2 , 1151 (1958).
May
2101
EQUILIBRIA OF BIS-(ACETITLACETONE)-ETHYLENELXIMINE
5, 1961
PROTON RESONANCE DATAFOR BIS-(ACETYLACETONE) SCHIFFBASESIN Methyls Compound Solvent Bis-(acety1acetone)-ethylene- CCh diimine CeHa B = -CHs-CHtCHCla 1 CDnCOCDs CSI Pyridine Pyrrole CcHsNOz CsHaBr Eis-(acety1acetone)-propyl- CClc ene diimine C6Ha 1 CDCOCDa B = -CH-CHzZ
RELATIVETO TETRAMETHYLSILANE
P.P.M.
B
1.80 1.39 1.91 1.58
2.03 1.83 1.97 1.94
Hydrogens a ba 4.83 10.9 4.83 11.0 4.99 10.9 4.95 10.8 4.81 10.9 5.02 11.2 4.81 10.7 4.95 11.1 4.82 10.9
1.89 1.47,1.51 1.93
1.84 1.97 1.87
4.79 4.81 4.95
11.1 3.37 M 1 1 . 1 2.53 X I 1 0 . 0 3.40 M
Unresolved Unresolved
2.02
4.83 4.8E
10.9 3.33,3.43 11.1 2.68.2.59
Under methyl 1.05 M
4.82 4 S8
10.9 3.32,3.25 11.1 2 . 5 6 , 2 . 4 0
1 71 M 1.01 I\I
a 1 85 1.41 1.91 1.87
b
1.90 1.98
2.00 1.91
1.82
l b
Comments
3
2
3.39,3.49 2.42,2.53 3.37, 3 . 4 8 3.50 3.29,3.39 3,18,3.28 2.38,2.48 3.37,3.48 2.79,2,89
B
collapsed t o single broad signal, new band a t 3.07 (acetone) Me’s only slightly split (CSd
1.24,1.35 0.59,0.70 1.18.1.29
New band a t 3.14
I
CHI 3 Bis- (acety1acetone)-trimethylenediimine B = -CHzCHzCHr
CCl4 CsHa
1.42
1 2 BI s- (acetylacetone)-tetramet hylenediimine B = CHzCHCHnCHr
cc14 CaHa
1.42
1 a
1.90
1.87 2.04
1 is a quartet, Me’s only ( < l C.P.S.) slightly split (CC11)
Me’s only slightly split (
