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Predominance of the triketo tautomer in acyldipivaloylmethanes in solution and the solid state
Journal of Molecular Structure 1063 (2014) 123–130
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Predominance of the triketo tautomer in acyldipivaloylmethanes in solution and the solid state Vladimir Stilinovic´ a,⇑, Tomislav Portada b, Branko Kaitner a a b
Chemistry Department, Faculty of Science, University of Zagreb, Horvatovac 102a, HR-10000 Zagreb, Croatia - Boškovic´ Institute, P.O.B. 180, 10002 Zagreb, Croatia Laboratory for Supramolecular and Nucleoside Chemistry, Ruder
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Five 1,3,30 -triketones derived from
dipivaloylmethane have been prepared. Both in the solid and in solution no enol tautomers were detected for any of the compounds. Slow isotopic exchange in CD3OD solution indicates presence of a minute amount of the enol. The predominance of the triketo tautomer is attributed to steric hindrance in enol molecules.
a r t i c l e
i n f o
Article history: Received 8 November 2013 Received in revised form 24 January 2014 Accepted 24 January 2014 Available online 29 January 2014 Keywords: Triketones Keto–enol tautomerism Crystal structure NMR IR
a b s t r a c t A series of five acyldipivaloylmethanes was prepared and studied with respect to keto–enol tautomerism. In the solid state all the compounds exist as triketo tautomers with the triketo group of approximate C3 symmetry. MNR and IR spectroscopy were employed to study the compounds in a variety of solvents. No diketoenol tautomers were detected in any of the solutions. However, a slow deuteration was noticed in the CD3OD solution of acetyldipivaloylmethane which indicates presence of a minute amount of the enol form of this compound. The predominance of the triketo tautomer in all the compounds was explained by the destabilisation of the enol due to steric repulsions of the bulky tert-butyl substituents. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction 1,3-Dicarbonyl compounds have attracted much interest, particularly as ligands in coordination chemistry [1]. Although studied for more than a century (acetylacetone was the first chelating ligand employed by Werner [2]), research of diketonate complexes continues to date [3]. Of particular interest in inorganic and structural chemistry have been diketone derivatives with additional donor atoms, such as carbonyl groups, which can act ⇑ Corresponding author. Tel.: +385 1 4606371; fax: +385 1 4606341. E-mail address: [email protected] (V. Stilinovic´). http://dx.doi.org/10.1016/j.molstruc.2014.01.061 0022-2860/Ó 2014 Elsevier B.V. All rights reserved.
as bridging ligands and employed in the design and synthesis polynuclear coordination compounds [4] and metaloorganic frameworks [5]. In the majority of coordination compounds the diketone is bound to the metal atom as an enolate anion. Therefore, the ability of a diketone to coordinate a metal ion is in close connection to the keto–enol tautomerism which these compounds exhibit. 1,3-Dicarbonyl compounds usually exist as an equilibrium mixture of the diketo and keto–enol tautomeric forms in the solution and melt [6,7]. The equilibrium constant is greatly effected by the terminal substituents, as well as the substituent in the a position [8]. Generally, electron withdrawing a substituents will cause an increase
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in the amount of the keto–enol [9], while bulky substituents make the diketo form more favourable [10]. 1,3-Diketones have shown to be excellent model compounds for the study of keto–enol tautomerism for two main reasons. The concentrations of both tautomers are usually sufficiently high to be detectable, and the interconversion of the tautomers is sufficiently slow to allow the two forms can be separately detected by NMR spectroscopy. Also, a number of other methods for the determination of enol content (and thus the determination of the tautomerisation constant) have been devised and include the use of micellar solutions [11], as well as kinetic measurements of the halogenation [12] and nitrosation [10,13] reactions of the enol. The majority of studies has concentrated on simple 1,3-diketones, although there have been also studies of polyketones, in particularly 1,3,5-triketones which were found to exhibit interesting and complex keto– enol and enol–enol tautomerism [14]. A different type of triketones are triacylmethanes or 1,3,30 -triketones (Scheme 1) which are afforded by acylation of 1,3-diketones in the a position [15–17]. Such triketones can coordinate metal ions as enolate chelating ligands, leaving the third carbonyl in the a position free for bonding to another metal ion. Although this appears to make 1,3,30 -triketones ideal ligands for preparation of coordination polymers, only few coordination compounds of 1,3,30 -triketones have been reported to date [18]. Also, the studies of keto–enol tautomerism in these compounds are quite scarce; although keto–enol tautomerism of triacetylmethane has been studied to some extent [19–22], tautomerism in other triacylmethanes has received very little attention. This is rather surprising, not only since a triacylmethane (dibenzoylacetylmethane) was one of first compounds in which keto–enol tautomerism has been observed [23], but also since these compounds can provide very good models for the study of tautomerism – an asymmetric triacylmethane (R – R0 – R00 ) can exist in as many as seven different tautomeric forms, one triketone and six diketoenols. In triacetylmethane enol–enol tautomerism was detected, together with keto–enol, although in this case there is no net change of the molecular structure, due to its symmetry [24]. Attempted studies of keto–enol tautomerism in triacylmethanes other than triacetylmethane have given somewhat perplexing results. An early paper by Nonhebel reports a spectroscopic study of a number of triacylmethanes with an account of keto–enol tautomerism [25]. It was found that triacylmethanes with pivaloyl groups exist solely as triketo tautomers. This was explained by proposing that such triacylmethanes adopt a conformation with the tert-butyl group of the pivaloyl placed between the carbonyl groups of other two acyl substituents (the conformation which will afterwards be referred to as ‘C1 conformer’ on account of the symmetry of the triketo group) thus rendering proton transfer stericaly impossible. Recent X-ray diffraction studies [15–17,26] have shown that the triketones adopt a conformation with triketo group of approximate C3 symmetry, with all three carbonyl groups on the same side of the molecule, disproving the above proposed explanation. Also, the reported absence of any enolic forms seems to be at odds with the fact that triacetylmethane was found to be predominantly present as diketoenol Refs. [19–22]. We have
Scheme 1. A general molecular diagram of a triacylmethane in its triketo and one of diketoenol tautomers.
Scheme 2. General molecular diagram of the prepared compounds.
therefore set out to study the keto–enol tautomerism in aliphatic dipivaloylacylmethanes by synthesising a series of compounds (I–V, Scheme 2) and studying them both in solution and in the solid state. 2. Experimental section 2.1. Materials Dipivaloylmethane, acyl chlorides, and elemental sodium were purchased from commercial suppliers (Fluka, Sigma–Aldrich, Kemika) and were used without further purification. All solvents were purified and dried according to standard laboratory procedures. Reaction yields were not optimised. 2.2. Synthesis To a solution of 1.00 mL (897 mg, 4.87 mmol) of dipivaloylmethane (2,2,6,6-tetrametyl-3,5-heptanedione) in 20 mL of isooctane (2,2,4-trimethylpentane) approximately 50 mg of elemental sodium was added. The reaction mixture was refluxed for 30 min without stirring (to prevent scattering of melted sodium into small drops) and than allowed to cool to room temperature. The excess of sodium was removed with tweezers, magnetic stirrer was added and acyl chloride (4.87 mmol) was added with stirring. The white precipitate of sodium chloride formed immediately. The reaction mixture was refluxed and stirred for another 60 min, cooled to room temperature and filtrated through a Celite pad. The filtrate was evaporated on rotatory evaporator. The crude product was crystallised from isopropanol. 2.3. Methods X-ray data collection was performed at room temperature on an Oxford Xcalibur 3 CCD areadetector diffractometer [27]. The crystallographic data are given in Table 1. The structures were solved by direct methods using the SHELXS-97 program and refined by full-matrix least-squares procedures using SHELXL-97 [28]. All non-hydrogen atoms were refined anisotropicaly. All hydrogen atoms were placed on calculated positions and refined as riding entities. The molecular graphics were prepared using ORTEP-3 [29] and CCDC-Mercury [30]. Ambient and raised temperature 1H and 13C NMR spectra were recorded on the Bruker Advance 300 MHz spectrometer, 1H operating at 300.13 MHz and 13C at 75.475 MHz. Low temperature 1 H and 13C NMR spectra were recorded on the Bruker Advance 600 MHz spectrometer, 1H operating at 600.14 MHz and 13C at 150.92 MHz. Proton chemical shifts were referenced to tetramethylsylane (TMS) as an internal standard, whereas 13C chemical shifts were referenced to the solvent signal as follows: d13C(dmsod6) = 39.5 ppm, d13C(CD3OD) = 49.0 ppm, d13C(C6D6) = 128.1 ppm and d13C(CDCl3) = 77.2 ppm [31]. Sample concentrations were in the 8–15 mg mL1 (25–65 mmol L1) range.
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Table 1 Crystallographic data for II and III.
Formula Mr Colour/habit T (K) Crystal system Space group a (Å) b (Å) c (Å) b (°) V (Å3) Z qcalc (g cm3) Radiation l (mm1) h, k, l range
Reflections unique Reflections observed [I > 2r(I)] R1(obs) wR2(obs)
II
III
C14H24O3 240.33 Colourless/prism 295(2) Monoclinic Cc 8.9766(15) 17.142(2) 10.286(2) 106.746(16) 1515.7(4) 4 1.053 Mo Ka 0.072 11 < h < 11 21 < k < 8 12 < l < 13 1656 1430 0.0721 0.1915
C15H26O3 254.36 Colourless/prism 295(2) Monoclinic Cc 11.3511(19) 17.270(3) 9.1988(16) 117.137(17) 1604.8(5) 4 1.053 Mo Ka 0.071 14 < h < 14 22 < k < 21 11 < l < 11 1749 977 0.0603 0.1703
Fig. 1. Molecular structure of II with the atom labelling scheme. Thermal ellipsoids are drawn at the 50% probability level and hydrogen atoms are presented as small spheres of arbitrary radii.
IR spectra were recorded on a FT ABB Bomem MB 102 IR spectrometer with CsI optics and DTGS detector. The spectra of compounds in solutions were obtained by subtraction of corresponding solvent spectra from the raw spectra of solutions of the corresponding compound.
3. Results and discussion Crystal structures of I [17], IV [15,16] and V [26] have been reported previously and all have shown to be present as triketo tautomers in the solid state, with the triketo group of approximate C3 symmetry in I, and V, and exact C3 symmetry in IV, due to the symmetry of the crystal packing. We have therefore undergone to study the crystal structures of II and III in order to compare them with the structures of the other compounds. Both compounds crystallize in the monoclinic system in space group Cc, with four molecules per unit cell. Molecular structure of compound II with the atom labelling scheme is given in Fig. 1. All three CAO distances are of similar lengths: 1.210(5) Å (C2AO1), 1.209(5) Å (C7AO2), and 1.213(5) Å (C12AO3), which is in agreement with average for aliphatic ketone C@O bond length [32]. Also the bond angles are consistent with those expected for the carbonyl group, and the CAC bond lengths of the central triketo group of 1.542(6) Å (C1AC2), 1.541(5) Å (C1AC7), and 1.533(5) Å (C1AC12) are characteristic for single CAC bonds. It is therefore evident that compound II in the solid state exists as triketo tautomer. The triketo group is of approximate C3 symmetry with the oxygen atom of each carbonyl group approaching the carbon atom of a neighbouring carbonyl forming close intramolecular non-bonded contacts of 2.62 Å (O1 C12), 2.74 Å (O2 C2) and 2.76 Å (O3 C7). There are no significant intermolecular contacts, the only noticeable connection between molecules being C1AH1 O3 weak hydrogen bond of 3.65 Å between the methine group and a carbonyl oxygen of a neighbouring molecule. The molecular geometry of III is quite similar to that of II (Fig. 2). The lengths of CAO bonds of 1.212(5) Å (C2AO1), 1.207(4) Å (C7AO2), and 1.204(5) Å (C12AO3) are again in agreement with those expected for the C@O bond in aliphatic ketones, and the CAC bond lengths of the central triketo group of 1.512(5) Å (C1AC2), 1.518(5) Å (C1AC7), and 1.526(5) Å
Fig. 2. Molecular structure of III with the atom labelling scheme. Thermal ellipsoids are drawn at the 50% probability level and hydrogen atoms are presented as small spheres of arbitrary radii. Only the major component of the disordered butyryl group is shown.
(C1AC12) are characteristic for single CAC bonds. The structure shows some disorder, as the terminal methyl of the butyryl group is disordered over two positions with occupancies 0.55:0.45. As in II, the triketo group in III is of approximate C3 symmetry with C O non bonding contacts between the carbonyl groups of 2.72 Å (O1 C12), 2.70 Å (O2 C2) and 2.72 Å (O3 C7). In spite of the similarity of molecular structures and the fact that they crystallise in the same space group, II and III are not isostructural. In the crystal structure of II molecules form layers parallel to the c axis so that the axes of the molecular triketo groups are approximately perpendicular to the layer plane. The conformations of all the molecules within one such layer are of the same chirality (Fig. 3a). These layers then stack along the c axis so that the chirality of molecular conformation alternates along this axis, each layer of P-conformers being neighboured by two layers o M-conformers, and vice versa. In the crystal structure of III, equivalent
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126
Fig. 3. Layers of molecules in (a) II, viewed along the c axis and (b) III, viewed approximately along the [1 0 1] direction.
Table 2 Relevant signals in 1H MNR and Compound
13
C NMR spectra for assignation of the tautomeric form for compounds I–V in C6D6, CDCl3, CD3OD and dmso-d6 solutions.
Solvent
all-keto form
Keto–enol form
d(a-H) (ppm)
d(C@O) (ppm)
d(COH) (ppm)
d(COH) (ppm)
17.03 16.17 16.20 (bs) 16.31
201.7 201.7 202.5 201.1
Diketone
C6D6 CDCl3 CD3OD dmso-d6
3.32 3.74 3.87(s), 3.84(t) 3.88
Not Not Not Not
I
C6D6 CDCl3 CD3OD dmso-d6
5.34 5.64 6.03 6.19
198.6, 199.1, 201.3, 199.9,
204.9 205.1 208.3 206.8
Not Not Not Not
detected detected detected detected
Not Not Not Not
detected detected detected detected
II
C6D6 CDCl3 CD3OD dmso-d6
5.36 5.65 6.04 6.18
201.3, 201.6, 203.9, 202.4,
205.2 205.5 208.6 207.0
Not Not Not Not
detected detected detected detected
Not Not Not Not
detected detected detected detected
III
C6D6 CDCl3 CD3OD dmso-d6
5.38 5.64 6.03 6.17
200.5, 200.8, 203.0, 201.6,
205.0 205.3 208.4 206.8
Not Not Not Not
detected detected detected detected
Not Not Not Not
detected detected detected detected
IV
C6D6 CDCl3 CD3OD dmso-d6
5.60 5.92 6.18 6.30
206.6 207.2 209.5 208.4
Not Not Not Not
detected detected detected detected
Not Not Not Not
detected detected detected detected
V
C6D6 CDCl3 CD3OD dmso-d6
5.44 5.64 6.03 6.20
200.8, 201.2, 203.3, 201.8,
Not Not Not Not
detected detected detected detected
Not Not Not Not
detected detected detected detected
layers exist; however, here they are parallel to the crystallographic a axis. Because of this the layers are affected by the c glide plane symmetry and are no longer homochiral as in II, but rather comprise of molecules of both M and P conformations which alternate along the b axis (Fig. 3b). Having determined that the geometry of the triketo group of II and III in the solid state is equivalent to that reported in other compounds (I, IV and V), i.e. that all the compounds exist solely as triketo tautomers in the solid state, we underwent to study the tautomeric behaviour of the compounds in the solution by means of NMR spectroscopy. In order to acquire a more complete image, four solvents of different polarity were used, namely C6D6, CDCl3, CD3OD and dmso-d6. In addition to triacylmethanes I–V, the parent diketone (R@H) was also studied. As expected, in 1H NMR spectra of the diketone in all four solvents signals were observed corresponding to both keto–enol and diketo tautomer (Table 2). The signals of keto–enol form include a singlet at 16–17 ppm corresponding to the hydroxyl, and a singlet at 4.8–5.9 ppm corresponding to the methyne proton. In aprotic solvents the hydroxyl signal is sharp and clear, whereas in CD3OD solution it is weak, very broad and tends to disappear over
detected detected detected detected
205.1 205.4 208.4 206.9
time in the background noise due to the hydron1 exchange. The presence of diketo form is best evident from a sharp singlet of low intensity at 3.3–3.9 ppm which corresponds to the methylene proton(s). In the CD3OD solution in the near vicinity of the methylene singlet there is an additional low intensity triplet with approximate 1:1:1 splitting pattern, most probably corresponding to proton attached on mono-deuterated methylene group (CHD) generated by exchange of a methylene proton with a deuteron from solvent. In all four solvents the keto–enol form of diketone is clearly more abundant than the diketo form. By integrating signals corresponding to one and another form it is possible to estimate the fraction of the diketo form in tautomeric mixtures, which amounts to 2% in C6D6 and CDCl3, 8% in CD3OD, and as much as 20% in dmso-d6.
1 Hydron is a general name for all H+ species regardless of the isotopic composition [33]. Generally, one should always refer H+ as a ’hydron’, (consequently ’hydronation’, ’dehydronation’, etc.) reserving ’proton’ specifically for 1H+. While one could perhaps argue that insisting on such aberration from the customary (although, strictly speaking, incorrect) terminology is unnecessarily pedantic, in this case the application of the term ’hydron’ is quite necessary, as it refers both to the leaving proton and the incoming deuteron.
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The 13C spectra of the diketone are consistent with the dominance of keto–enol form. However, appearance of additional signals near to tert-butyl methyl signal clearly signifies the presence of the diketo tautomer. In dmso-d6 an additional weak signal at 39.0 ppm, almost completely shadowed by the more intense solvent signal, is visible in 13C NMR spectrum of the diketone. That signal corresponds to methylene carbon of the diketo form. Unlike the spectra of the diketone, in all (both 1H and 13C) spectra of triketones, the observed values of chemical shifts, signal intensities, and multiplicities are in full accordance with the values expected for the corresponding triketo tautomers [24]. In no spectrum there is any trace of a signal which would correspond to a hydroxyl proton. Additionally, if there were any significant amount of an enolic tautomer present in any solution, it would be observable by appearance of additional signals in the near vicinity of the signals of the dominant triketo tautomer. Since no such signals were detected in the measured spectra, it appears that in all of the investigated solutions of all the triketones (I–V) the predominant tautomeric form is the triketo one. Although the NMR spectra indicate that only the triketo tautomer is present in all the studied solutions, this cannot be accepted as an evidence of absolute absence of enolic forms. The reason for this is twofold. On the one hand, if the dominant form represents more than about 95%, the presence of signals corresponding to minor components might be overlooked. On the other, the signal corresponding to the enolic hydrogen can appear at large chemical shifts (up to 18 ppm) [24] and thus there is possibility that it falls beyond the range of our measurements. Therefore, in order to ascertain whether there are really no enolic tautomers present, the compounds were investigated by (more sensitive) FT-IR spectroscopy, both in KBr pellets and in solutions. For the FT-IR investigation of solutions, two solvents of different polarity were chosen, benzene and chloroform. It was expected that carbonyl groups from all-keto form would show signals at 1700–1750 cm1 characteristic for saturated aliphatic ketones, while carbonyl group from keto–enol form would be expected at somewhat lower wavenumbers (about 1600 cm1), characteristic for C@O stretching in an enolic ring with an intramolecular hydrogen bond [34]. In the spectra of the diketone, both in a KBr pellet and in solutions, there is only one signal in the carbonyl region, which corresponds to the dominant keto–enol form (Table 3). By contrast, in the spectra of triketones in KBr pellet there are two dominant sharp signals in the all-keto carbonyl region at approx. 1700 and 1735 cm1, the first signal being additionally split in two very close bands. Both signals correspond to stretching vibrations of two nonequivalent carbonyl groups from all-keto form. With an exception of benzene solution of compound I, in both chloroform and benzene solutions of triketones, the first signal is not split, although the main signal shows a weakly pronounced shoulder. Otherwise, all the spectra of I–V are very similar to one another. A C@O stretching band at about 1600 cm1 which would correspond to an enolic carbonyl was not noticed in any spectrum of I–V either in the solid or in solutions, indicating an absence of the diketo–enol forms. While multiple C@O stretching bands are expected for asymmetric compounds, they are quite surprising for the symmetrical triketone IV and might indicate the presence of a C1 conformer with the tert-butyl group of one pivaloyl placed between the carbonyl groups of other two – a conformer where the carbonyl groups would be in inequivalent surroundings. In contrast to this result, the NMR spectra are in full agreement with the high symmetry of the molecule (two signals in 1H and four in 13C MNR spectrum). As this might be the result of fast interconversion of conformers, we have also performed a measurement at lower temperatures, to ascertain whether a splitting of the signals might
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Table 3 Relevant signals in IR spectra for assignation of the tautomeric form for compounds I– V measured as solids in KBr pellets and as benzene and chloroform solutions. Compound
Conditions (solvent)
~(C@O) all-keto form m (cm1)
Keto–enol form m~(C@O) (cm1)
Diketone
KBr Benzene Chloroform
Not detected Not detected Not detected
1608 1603 1598
I
KBr Benzene Chloroform
1692, 1701, 1722 1693, 1709, 1734 1707, 1734
Not detected Not detected Not detected
II
KBr Benzene Chloroform
1692, 1699, 1726 1703, 1735 1702, 1735
Not detected Not detected Not detected
III
KBr Benzene Chloroform
1691, 1706, 1730 1703, 1733 1701, 1732
Not detected Not detected Not detected
IV
KBr Benzene Chloroform
1691, 1709, 1721 1699, 1730 1697, 1729
Not detected Not detected Not detected
V
KBr Benzene Chloroform
1694, 1705, 1730 1703, 1732 1700, 1732
Not detected Not detected Not detected
appear due to the blocking of the conformation change, thus proving the above supposition. However, in CDCl3 solution of triketone IV even at 50 °C no splitting of signals was noticed, leaving the question of the conformational equilibrium unresolved. The apparent presence of the C1 conformer in solution seems to support the Nonhebel’s explanation of the lack of the diketoenol tautomer in acyldipivaloylmethanes. However, a closer inspection of the optimised geometry of the C1 conformer (Fig. 4) does not quite reveal how this molecular geometry might render the proton transfer impossible. Quite the contrary, the inversion of one pivaloyl group leads to a close intramolecular contact between its carbonyl oxygen and the methine hydrogen (ca. 2.4 Å), which could lead to an easier proton transfer than in the C3 conformer. The only possible explanation would be that the rotation of the protonated pivaloyl is slow due to the sterical hindrance and thus makes the triketone kinetically inert. However, this cannot be reconciled with the NMR spectra which indicate C3 symmetry, which is either statistical (rapid transformation of C1 to C3 conformer) or absolute (no C1 conformer is present at all). Thus, the possible presence of the C1 conformation of acyldipivaloylmethanes (in solution) does not appear to be a convincing reason for the extreme predominance of the triketo tautomers in this group of compounds. There are two possible reasons for the (apparent) absence of the enol tautomer. The enols might be significantly less stable than the triketones (which would lead to small amounts of enols in the equilibrium mixture). Alternatively, the transition from triketone to diketoenol might occur at a very slow rate, which would cause that the keto–enol equilibrium would not be attained immediately upon dissolution of the (crystalline) triketone. In order to test the latter supposition, a solution of I in CD3OD was prepared and left at room temperature for fourteen days. I was chosen as the compound most probable to exhibit a change over time (having the smallest R substituent), and the solvent as one which is most likely to catalyse the tautomerisation if it in fact occurs. Additionally, as in the case of the enol hydrogen is rather acidic, over a prolonged period hydron exchange with the solvent should occur, and even in the case no detectable amount of enol is present at any time; its occurrence could be proven by changes in the NMR spectra due to the gradual replacement of hydrogen with deuterium. After seven days of standing on room temperature, the NMR spectra of solution have shown no any significant changes – there were
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Fig. 4. Geometry of the putative C1 conformer of IV optimised by semiempirical PM3 method.
neither new signals nor additional splitting of existing ones. However, ten days after starting the experiment, minute changes started to appear both in the 1H and in the 13C NMR spectrum. In the 1H NMR spectrum the intensity of the signal corresponding to methyne proton decreased, and two new singlets appeared, one in the near vicinity of the acetyl singlet (a signal at 2.02 ppm appeared near the original at 2.16 ppm) and the other near to the
t-butyl methyl singlet (1.17 ppm near 1.15 ppm). In the 13C NMR spectrum two new singlets appeared; in the near vicinity of the carbonyl signal (208.2 near to the original 208.3) and t-butyl methyl signal (27.7 ppm near 27.0 ppm). Also, the acetyl methyl signal at 29.9 split in two very close signals. A similar effect was observed when a freshly prepared solution of I in CD3OD was kept at the boiling point of methanol, when the new signals, equivalent to those in the first experiment, have appeared after only 24 h, and their intensities further increased over the next two days. There are three reasonable causes of the observed changes in the spectra of I; the new signals could appear due to (1) formation of an enol tautomer (extremely slow equilibration), (2) degradation of the compound or (3) isotopic exchange. As the newly appeared signals did not seem to be consistent with the either an enol tautomer or a ketal, it was proposed that the changes in the spectrum have appeared due to the replacement of the methyne hydrogen atom with deuterium. To test this hypothesis an additional experiment was performed where I was dissolved in nondeuterated methanol and the solution kept at the boiling point of methanol identically as the CD3OD solution. After three days the solvent was evaporated and the remaining solid dissolved in CDCl3. The spectra were found to be identical to the original spectra of I with no trace of additional signals which had appeared in the CD3OD solution. This showed quite unequivocally that I did not undergo a (thermal) degradation, or a transformation into another compound. It also could not be reconciled with the supposed slow
Fig. 5. The geometry of diketones and keto–enols based on the structural data deposited with the CSD: (a) Torsion angles between the CAO bonds vs. the distance between the two oxygen atoms showing a distinct grupation of keto–enols as opposed to diketones. (b) Distribution of the torsion angles between the CAO bonds in keto–enols.
Fig. 6. (a) The expected geometry of the diketoenol tautomer of IV. (b) Torsion angles between the CAO bonds of the keto–enol ring in the diketoenol tautomers of tripivaloylmethane (IV), acetyldipivaloylmethane (3 tautomers), diacetylpivaloylmethane (3 tautomers) and triacetylmethane as obtained from molecular geometries optimised by the PM3 semiempirical method. The presence of t-butyl groups leads to increase of the angle, i.e. the reduction of planarity of the keto–enol ring.
V. Stilinovic´ et al. / Journal of Molecular Structure 1063 (2014) 123–130
interconversion of the tautomers, as in this case the isolation of both forms should be expected. The observed isotopic exchange therefore indicates that a minute presence of enol tautomer is indeed present in the methanol solution of I, albeit in insufficient amount to be observed directly in the spectrum. However, when equivalent experiments were performed with IV, no isotopic exchange whatsoever was noticed. This would indicate that in I the triketo tautomer is considerably more stable then any of the possible enols, while in IV, this difference in stabilities is greater still, as not even by this indirect method any trace of the enol could be detected.2 This raises the question of the reason of such pronounced difference in relative stabilities of the two tautomers. An important factor in the stabilisation of enols in b-dicarbonyl compounds is the intramolecular resonance assisted hydrogen bond between the enol hydroxyl and the b-carbonyl groups [35]. Therefore it is crucial for the keto–enol hydrogen bonded ring to be as closely planar as possible, in order for the resonance effects to be possible. The deviation from planarity can easily be ascertained by the torsion angle defined by the two carbonyl groups (O@C C@O torsion, h), which is expected to be 0° for a perfectly planar enol with the intramolecular hydrogen bond. This can be seen by inspecting the values of h for simple 1,3-diketones in the solid state according to structures deposited with the CSD [36] (Fig. 5): the data show a clear distinction between enols and diketones, with enols having shorter O O distances (as the oxygen atoms are hydrogen bonded), and with h rarely exceeding 5° (only in two cases where additional hydrogen bonding involving one or both the oxygen atoms is present). In contrast, h in diketo tautomers is much more variable. Therefore if there is a steric hindrance preventing h to achieve values close to 0° (i.e. preventing the planarity of the keto–enol hydrogen bonded ring), this will greatly destabilise the enol (preventing the resonance stabilisation), while it should not have as pronounced effect on the ketone. To test whether the pivaloyl groups might indeed have such an effect, a series of models of putative diketoenol tautomers was constructed and their geometries optimised by semiempirical methods (AM1 and PM3). The optimised geometries have demonstrated that the sterically demanding pivaloyl groups indeed lead to an increase of h, i.e. render the molecule less planar. The largest effect is in the diketoenol form of IV (Fig. 6), where h is expected about 27°, much larger than found in isolated enols. In other studied compounds three different diketoenols are possible, and there the value of h is dependant also of the position of the bulkiest substituents. This is best demonstrated by I, where there is the largest difference between substituents (methyl vs. tert-butyl), where the expected h varies from 14°, when the two pivaloyl groups form the hydrogen bonded ring to 6°. The values of h for II, III and V were estimated to be above those for I and below those for IV – in all cases above the values found in enols. For further comparison, the geometries of three diketoenol tautomers of diacetylpivaloylmethane, and one of triacetylmethane were also optimised. All three tautomers of diacetylpivaloylmethane have h between 2.5° and 1°, and triacetylmethane 1°, which is in accordance with the expected values for enols, and in agreement with the fact that triacetylmethane was found to be mostly present in diketoenol form [16–19]. The same computational method also gave the value of h in the parent dipivaloylmethane of 0.9° which is in good agreement with one observed in the crystal structure (0.3°) [37], where it is present as enol, which is also the predominant form in the solution. 2 It should be noted that this is based on a finite time in which the experiment was carried out – it is not inconceivable that if the boiling of IV in CD3OD were continued for a substantially longer time (months or years), isotopic exchange would become noticeable.
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4. Conclusion All the studied acyldipivaloylmethanes are present as triketo tautomers in the solid state. No diketoenol tautomer was noticeable even in solution of any of the compounds, regardless of the solvent used or temperature at which measurements were performed. This behaviour is quite contrary to that of the parent diketone, which is predominantly present as ketoenol under the same conditions, and is also known to crystallise as such. Apparently the addition of an additional acyl group in the a-position of dipivaloylmethane renders the diketoenol form unstable as, due to steric repulsions, the keto–enol ring of the diketoenol tautomer would deviate from planarity and thus could not be sufficiently stabilised by resonance, as it is the case in dipivaloylmethane and also less sterically hindered triacetylmethane. This conclusion is also in accord with the fact that the existence of a minute amount of an enol tautomer, as well as the tautomeric equilibrium, are indicated by very slow isotopic exchange noticed in CD3OD solution of I, which is sterically least hindered, while no isotopic exchange was detected in the CD3OD solution of much more hindered IV. Acknowledgements The authors would like to thank the Ministry of Science, Education and Sports of the Republic of Croatia, for financial support of this work through Grants 119-1193079-3069 and 098-09829042912. Appendix A. Supplementary material Supplementary crystallographic data for this paper can be obtained via www.ccdc.cam.ac.uk/conts/retrieving.html, (or from the Cambridge Cystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033; or deposit@ccdc. cam.ac.uk). CCDC 968873 and 968874 contain the supplementary crystallographic data for this paper. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.molstruc.2014.01. 061. References [1] (a) R.C. Mehrotra, Pure Appl. Chem. 60 (1988) 1349; (b) R.C. Mehrotra, R. Bohra, D.P. Gaur, Metal b-Diketonates and Allied Derivatives, Academic Press, London, 1978; (c) K.C. Joshi, V.N. Pathak, Coord. Chem. Rev. 22 (1977) 37; (d) D.P. Gibson, Coord. Chem. Rev. 4 (1969) 225; (e) D.P. Graddon, Coord. Chem. Rev. 4 (1969) 1. [2] A. Werner, Chem. Ber. 34 (1901) 2584. [3] (a) J. Wu, D.J. MacDonald, R. Clerac, I.-R. Jeon, M. Jennings, A.J. Lough, J. Britten, C. Robertson, P.A. Dube, K.E. Preuss, Inorg. Chem. 51 (2012) 3827; (b) A. Beziau, S.A. Baudron, M.W. Hosseini, Dalton Trans. 41 (2012) 7227; (c) Jingya Li, Hongfeng Li, Pengfei Yan, Peng Chen, Guangfeng Hou, Guangming Li, Inorg. Chem. 51 (2012) 5050; (d) M.A.G. Berg, M.K. Ritchie, J.S. Merola, Polyhedron 38 (2012) 126; (e) A. Zaim, H. Nozary, L. Guenee, C. Besnard, J.-F. Lemonnier, S. Petoud, C. Piguet, Chem. – Eur. J. 18 (2012) 7155; (f) A.D. Burrows, M.F. Mahon, C.L. Renouf, C. Richardson, A.J. Warren, J.E. Warren, Dalton Trans. 41 (2012) 4153; (g) L. Carlucci, G. Ciani, D.M. Proserpio, M. Visconti, CrystEngComm 13 (2011) 5891; (h) J. Yuasa, T. Ohno, K. Miyata, H. Tsumatori, Y. Hasegawa, T. Kawai, J. Am. Chem. Soc. 133 (2011) 9892; (i) V. Stilinovic´, K. Uzˇarevic´, I. Cvrtila, B. Kaitner, CrystEngComm 14 (2012) 7493; (j) R. Horikoshi, Y. Funasako, T. Yajima, T. Mochida, Y. Kobayashi, H. Kageyama, Polyhedron 50 (2013) 66; (k) I. Cvrtila, V. Stilinovic´, B. Kaitner, Struct. Chem. 23 (2012) 587; (l) I. Cvrtila, V. Stilinovic´, B. Kaitner, CrystEngComm 15 (2013) 6585; (m) V. Stilinovic´, B. Kaitner, J. Coord. Chem. 62 (2009) 2698; (n) V. Stilinovic´, D.-K. Bucˇar, I. Halasz, E. Meštrovic´, New J. Chem. 37 (2013) 619.
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Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc
Predominance of the triketo tautomer in acyldipivaloylmethanes in solution and the solid state Vladimir Stilinovic´ a,⇑, Tomislav Portada b, Branko Kaitner a a b
Chemistry Department, Faculty of Science, University of Zagreb, Horvatovac 102a, HR-10000 Zagreb, Croatia - Boškovic´ Institute, P.O.B. 180, 10002 Zagreb, Croatia Laboratory for Supramolecular and Nucleoside Chemistry, Ruder
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
Five 1,3,30 -triketones derived from
dipivaloylmethane have been prepared. Both in the solid and in solution no enol tautomers were detected for any of the compounds. Slow isotopic exchange in CD3OD solution indicates presence of a minute amount of the enol. The predominance of the triketo tautomer is attributed to steric hindrance in enol molecules.
a r t i c l e
i n f o
Article history: Received 8 November 2013 Received in revised form 24 January 2014 Accepted 24 January 2014 Available online 29 January 2014 Keywords: Triketones Keto–enol tautomerism Crystal structure NMR IR
a b s t r a c t A series of five acyldipivaloylmethanes was prepared and studied with respect to keto–enol tautomerism. In the solid state all the compounds exist as triketo tautomers with the triketo group of approximate C3 symmetry. MNR and IR spectroscopy were employed to study the compounds in a variety of solvents. No diketoenol tautomers were detected in any of the solutions. However, a slow deuteration was noticed in the CD3OD solution of acetyldipivaloylmethane which indicates presence of a minute amount of the enol form of this compound. The predominance of the triketo tautomer in all the compounds was explained by the destabilisation of the enol due to steric repulsions of the bulky tert-butyl substituents. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction 1,3-Dicarbonyl compounds have attracted much interest, particularly as ligands in coordination chemistry [1]. Although studied for more than a century (acetylacetone was the first chelating ligand employed by Werner [2]), research of diketonate complexes continues to date [3]. Of particular interest in inorganic and structural chemistry have been diketone derivatives with additional donor atoms, such as carbonyl groups, which can act ⇑ Corresponding author. Tel.: +385 1 4606371; fax: +385 1 4606341. E-mail address: [email protected] (V. Stilinovic´). http://dx.doi.org/10.1016/j.molstruc.2014.01.061 0022-2860/Ó 2014 Elsevier B.V. All rights reserved.
as bridging ligands and employed in the design and synthesis polynuclear coordination compounds [4] and metaloorganic frameworks [5]. In the majority of coordination compounds the diketone is bound to the metal atom as an enolate anion. Therefore, the ability of a diketone to coordinate a metal ion is in close connection to the keto–enol tautomerism which these compounds exhibit. 1,3-Dicarbonyl compounds usually exist as an equilibrium mixture of the diketo and keto–enol tautomeric forms in the solution and melt [6,7]. The equilibrium constant is greatly effected by the terminal substituents, as well as the substituent in the a position [8]. Generally, electron withdrawing a substituents will cause an increase
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in the amount of the keto–enol [9], while bulky substituents make the diketo form more favourable [10]. 1,3-Diketones have shown to be excellent model compounds for the study of keto–enol tautomerism for two main reasons. The concentrations of both tautomers are usually sufficiently high to be detectable, and the interconversion of the tautomers is sufficiently slow to allow the two forms can be separately detected by NMR spectroscopy. Also, a number of other methods for the determination of enol content (and thus the determination of the tautomerisation constant) have been devised and include the use of micellar solutions [11], as well as kinetic measurements of the halogenation [12] and nitrosation [10,13] reactions of the enol. The majority of studies has concentrated on simple 1,3-diketones, although there have been also studies of polyketones, in particularly 1,3,5-triketones which were found to exhibit interesting and complex keto– enol and enol–enol tautomerism [14]. A different type of triketones are triacylmethanes or 1,3,30 -triketones (Scheme 1) which are afforded by acylation of 1,3-diketones in the a position [15–17]. Such triketones can coordinate metal ions as enolate chelating ligands, leaving the third carbonyl in the a position free for bonding to another metal ion. Although this appears to make 1,3,30 -triketones ideal ligands for preparation of coordination polymers, only few coordination compounds of 1,3,30 -triketones have been reported to date [18]. Also, the studies of keto–enol tautomerism in these compounds are quite scarce; although keto–enol tautomerism of triacetylmethane has been studied to some extent [19–22], tautomerism in other triacylmethanes has received very little attention. This is rather surprising, not only since a triacylmethane (dibenzoylacetylmethane) was one of first compounds in which keto–enol tautomerism has been observed [23], but also since these compounds can provide very good models for the study of tautomerism – an asymmetric triacylmethane (R – R0 – R00 ) can exist in as many as seven different tautomeric forms, one triketone and six diketoenols. In triacetylmethane enol–enol tautomerism was detected, together with keto–enol, although in this case there is no net change of the molecular structure, due to its symmetry [24]. Attempted studies of keto–enol tautomerism in triacylmethanes other than triacetylmethane have given somewhat perplexing results. An early paper by Nonhebel reports a spectroscopic study of a number of triacylmethanes with an account of keto–enol tautomerism [25]. It was found that triacylmethanes with pivaloyl groups exist solely as triketo tautomers. This was explained by proposing that such triacylmethanes adopt a conformation with the tert-butyl group of the pivaloyl placed between the carbonyl groups of other two acyl substituents (the conformation which will afterwards be referred to as ‘C1 conformer’ on account of the symmetry of the triketo group) thus rendering proton transfer stericaly impossible. Recent X-ray diffraction studies [15–17,26] have shown that the triketones adopt a conformation with triketo group of approximate C3 symmetry, with all three carbonyl groups on the same side of the molecule, disproving the above proposed explanation. Also, the reported absence of any enolic forms seems to be at odds with the fact that triacetylmethane was found to be predominantly present as diketoenol Refs. [19–22]. We have
Scheme 1. A general molecular diagram of a triacylmethane in its triketo and one of diketoenol tautomers.
Scheme 2. General molecular diagram of the prepared compounds.
therefore set out to study the keto–enol tautomerism in aliphatic dipivaloylacylmethanes by synthesising a series of compounds (I–V, Scheme 2) and studying them both in solution and in the solid state. 2. Experimental section 2.1. Materials Dipivaloylmethane, acyl chlorides, and elemental sodium were purchased from commercial suppliers (Fluka, Sigma–Aldrich, Kemika) and were used without further purification. All solvents were purified and dried according to standard laboratory procedures. Reaction yields were not optimised. 2.2. Synthesis To a solution of 1.00 mL (897 mg, 4.87 mmol) of dipivaloylmethane (2,2,6,6-tetrametyl-3,5-heptanedione) in 20 mL of isooctane (2,2,4-trimethylpentane) approximately 50 mg of elemental sodium was added. The reaction mixture was refluxed for 30 min without stirring (to prevent scattering of melted sodium into small drops) and than allowed to cool to room temperature. The excess of sodium was removed with tweezers, magnetic stirrer was added and acyl chloride (4.87 mmol) was added with stirring. The white precipitate of sodium chloride formed immediately. The reaction mixture was refluxed and stirred for another 60 min, cooled to room temperature and filtrated through a Celite pad. The filtrate was evaporated on rotatory evaporator. The crude product was crystallised from isopropanol. 2.3. Methods X-ray data collection was performed at room temperature on an Oxford Xcalibur 3 CCD areadetector diffractometer [27]. The crystallographic data are given in Table 1. The structures were solved by direct methods using the SHELXS-97 program and refined by full-matrix least-squares procedures using SHELXL-97 [28]. All non-hydrogen atoms were refined anisotropicaly. All hydrogen atoms were placed on calculated positions and refined as riding entities. The molecular graphics were prepared using ORTEP-3 [29] and CCDC-Mercury [30]. Ambient and raised temperature 1H and 13C NMR spectra were recorded on the Bruker Advance 300 MHz spectrometer, 1H operating at 300.13 MHz and 13C at 75.475 MHz. Low temperature 1 H and 13C NMR spectra were recorded on the Bruker Advance 600 MHz spectrometer, 1H operating at 600.14 MHz and 13C at 150.92 MHz. Proton chemical shifts were referenced to tetramethylsylane (TMS) as an internal standard, whereas 13C chemical shifts were referenced to the solvent signal as follows: d13C(dmsod6) = 39.5 ppm, d13C(CD3OD) = 49.0 ppm, d13C(C6D6) = 128.1 ppm and d13C(CDCl3) = 77.2 ppm [31]. Sample concentrations were in the 8–15 mg mL1 (25–65 mmol L1) range.
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Table 1 Crystallographic data for II and III.
Formula Mr Colour/habit T (K) Crystal system Space group a (Å) b (Å) c (Å) b (°) V (Å3) Z qcalc (g cm3) Radiation l (mm1) h, k, l range
Reflections unique Reflections observed [I > 2r(I)] R1(obs) wR2(obs)
II
III
C14H24O3 240.33 Colourless/prism 295(2) Monoclinic Cc 8.9766(15) 17.142(2) 10.286(2) 106.746(16) 1515.7(4) 4 1.053 Mo Ka 0.072 11 < h < 11 21 < k < 8 12 < l < 13 1656 1430 0.0721 0.1915
C15H26O3 254.36 Colourless/prism 295(2) Monoclinic Cc 11.3511(19) 17.270(3) 9.1988(16) 117.137(17) 1604.8(5) 4 1.053 Mo Ka 0.071 14 < h < 14 22 < k < 21 11 < l < 11 1749 977 0.0603 0.1703
Fig. 1. Molecular structure of II with the atom labelling scheme. Thermal ellipsoids are drawn at the 50% probability level and hydrogen atoms are presented as small spheres of arbitrary radii.
IR spectra were recorded on a FT ABB Bomem MB 102 IR spectrometer with CsI optics and DTGS detector. The spectra of compounds in solutions were obtained by subtraction of corresponding solvent spectra from the raw spectra of solutions of the corresponding compound.
3. Results and discussion Crystal structures of I [17], IV [15,16] and V [26] have been reported previously and all have shown to be present as triketo tautomers in the solid state, with the triketo group of approximate C3 symmetry in I, and V, and exact C3 symmetry in IV, due to the symmetry of the crystal packing. We have therefore undergone to study the crystal structures of II and III in order to compare them with the structures of the other compounds. Both compounds crystallize in the monoclinic system in space group Cc, with four molecules per unit cell. Molecular structure of compound II with the atom labelling scheme is given in Fig. 1. All three CAO distances are of similar lengths: 1.210(5) Å (C2AO1), 1.209(5) Å (C7AO2), and 1.213(5) Å (C12AO3), which is in agreement with average for aliphatic ketone C@O bond length [32]. Also the bond angles are consistent with those expected for the carbonyl group, and the CAC bond lengths of the central triketo group of 1.542(6) Å (C1AC2), 1.541(5) Å (C1AC7), and 1.533(5) Å (C1AC12) are characteristic for single CAC bonds. It is therefore evident that compound II in the solid state exists as triketo tautomer. The triketo group is of approximate C3 symmetry with the oxygen atom of each carbonyl group approaching the carbon atom of a neighbouring carbonyl forming close intramolecular non-bonded contacts of 2.62 Å (O1 C12), 2.74 Å (O2 C2) and 2.76 Å (O3 C7). There are no significant intermolecular contacts, the only noticeable connection between molecules being C1AH1 O3 weak hydrogen bond of 3.65 Å between the methine group and a carbonyl oxygen of a neighbouring molecule. The molecular geometry of III is quite similar to that of II (Fig. 2). The lengths of CAO bonds of 1.212(5) Å (C2AO1), 1.207(4) Å (C7AO2), and 1.204(5) Å (C12AO3) are again in agreement with those expected for the C@O bond in aliphatic ketones, and the CAC bond lengths of the central triketo group of 1.512(5) Å (C1AC2), 1.518(5) Å (C1AC7), and 1.526(5) Å
Fig. 2. Molecular structure of III with the atom labelling scheme. Thermal ellipsoids are drawn at the 50% probability level and hydrogen atoms are presented as small spheres of arbitrary radii. Only the major component of the disordered butyryl group is shown.
(C1AC12) are characteristic for single CAC bonds. The structure shows some disorder, as the terminal methyl of the butyryl group is disordered over two positions with occupancies 0.55:0.45. As in II, the triketo group in III is of approximate C3 symmetry with C O non bonding contacts between the carbonyl groups of 2.72 Å (O1 C12), 2.70 Å (O2 C2) and 2.72 Å (O3 C7). In spite of the similarity of molecular structures and the fact that they crystallise in the same space group, II and III are not isostructural. In the crystal structure of II molecules form layers parallel to the c axis so that the axes of the molecular triketo groups are approximately perpendicular to the layer plane. The conformations of all the molecules within one such layer are of the same chirality (Fig. 3a). These layers then stack along the c axis so that the chirality of molecular conformation alternates along this axis, each layer of P-conformers being neighboured by two layers o M-conformers, and vice versa. In the crystal structure of III, equivalent
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126
Fig. 3. Layers of molecules in (a) II, viewed along the c axis and (b) III, viewed approximately along the [1 0 1] direction.
Table 2 Relevant signals in 1H MNR and Compound
13
C NMR spectra for assignation of the tautomeric form for compounds I–V in C6D6, CDCl3, CD3OD and dmso-d6 solutions.
Solvent
all-keto form
Keto–enol form
d(a-H) (ppm)
d(C@O) (ppm)
d(COH) (ppm)
d(COH) (ppm)
17.03 16.17 16.20 (bs) 16.31
201.7 201.7 202.5 201.1
Diketone
C6D6 CDCl3 CD3OD dmso-d6
3.32 3.74 3.87(s), 3.84(t) 3.88
Not Not Not Not
I
C6D6 CDCl3 CD3OD dmso-d6
5.34 5.64 6.03 6.19
198.6, 199.1, 201.3, 199.9,
204.9 205.1 208.3 206.8
Not Not Not Not
detected detected detected detected
Not Not Not Not
detected detected detected detected
II
C6D6 CDCl3 CD3OD dmso-d6
5.36 5.65 6.04 6.18
201.3, 201.6, 203.9, 202.4,
205.2 205.5 208.6 207.0
Not Not Not Not
detected detected detected detected
Not Not Not Not
detected detected detected detected
III
C6D6 CDCl3 CD3OD dmso-d6
5.38 5.64 6.03 6.17
200.5, 200.8, 203.0, 201.6,
205.0 205.3 208.4 206.8
Not Not Not Not
detected detected detected detected
Not Not Not Not
detected detected detected detected
IV
C6D6 CDCl3 CD3OD dmso-d6
5.60 5.92 6.18 6.30
206.6 207.2 209.5 208.4
Not Not Not Not
detected detected detected detected
Not Not Not Not
detected detected detected detected
V
C6D6 CDCl3 CD3OD dmso-d6
5.44 5.64 6.03 6.20
200.8, 201.2, 203.3, 201.8,
Not Not Not Not
detected detected detected detected
Not Not Not Not
detected detected detected detected
layers exist; however, here they are parallel to the crystallographic a axis. Because of this the layers are affected by the c glide plane symmetry and are no longer homochiral as in II, but rather comprise of molecules of both M and P conformations which alternate along the b axis (Fig. 3b). Having determined that the geometry of the triketo group of II and III in the solid state is equivalent to that reported in other compounds (I, IV and V), i.e. that all the compounds exist solely as triketo tautomers in the solid state, we underwent to study the tautomeric behaviour of the compounds in the solution by means of NMR spectroscopy. In order to acquire a more complete image, four solvents of different polarity were used, namely C6D6, CDCl3, CD3OD and dmso-d6. In addition to triacylmethanes I–V, the parent diketone (R@H) was also studied. As expected, in 1H NMR spectra of the diketone in all four solvents signals were observed corresponding to both keto–enol and diketo tautomer (Table 2). The signals of keto–enol form include a singlet at 16–17 ppm corresponding to the hydroxyl, and a singlet at 4.8–5.9 ppm corresponding to the methyne proton. In aprotic solvents the hydroxyl signal is sharp and clear, whereas in CD3OD solution it is weak, very broad and tends to disappear over
detected detected detected detected
205.1 205.4 208.4 206.9
time in the background noise due to the hydron1 exchange. The presence of diketo form is best evident from a sharp singlet of low intensity at 3.3–3.9 ppm which corresponds to the methylene proton(s). In the CD3OD solution in the near vicinity of the methylene singlet there is an additional low intensity triplet with approximate 1:1:1 splitting pattern, most probably corresponding to proton attached on mono-deuterated methylene group (CHD) generated by exchange of a methylene proton with a deuteron from solvent. In all four solvents the keto–enol form of diketone is clearly more abundant than the diketo form. By integrating signals corresponding to one and another form it is possible to estimate the fraction of the diketo form in tautomeric mixtures, which amounts to 2% in C6D6 and CDCl3, 8% in CD3OD, and as much as 20% in dmso-d6.
1 Hydron is a general name for all H+ species regardless of the isotopic composition [33]. Generally, one should always refer H+ as a ’hydron’, (consequently ’hydronation’, ’dehydronation’, etc.) reserving ’proton’ specifically for 1H+. While one could perhaps argue that insisting on such aberration from the customary (although, strictly speaking, incorrect) terminology is unnecessarily pedantic, in this case the application of the term ’hydron’ is quite necessary, as it refers both to the leaving proton and the incoming deuteron.
V. Stilinovic´ et al. / Journal of Molecular Structure 1063 (2014) 123–130
The 13C spectra of the diketone are consistent with the dominance of keto–enol form. However, appearance of additional signals near to tert-butyl methyl signal clearly signifies the presence of the diketo tautomer. In dmso-d6 an additional weak signal at 39.0 ppm, almost completely shadowed by the more intense solvent signal, is visible in 13C NMR spectrum of the diketone. That signal corresponds to methylene carbon of the diketo form. Unlike the spectra of the diketone, in all (both 1H and 13C) spectra of triketones, the observed values of chemical shifts, signal intensities, and multiplicities are in full accordance with the values expected for the corresponding triketo tautomers [24]. In no spectrum there is any trace of a signal which would correspond to a hydroxyl proton. Additionally, if there were any significant amount of an enolic tautomer present in any solution, it would be observable by appearance of additional signals in the near vicinity of the signals of the dominant triketo tautomer. Since no such signals were detected in the measured spectra, it appears that in all of the investigated solutions of all the triketones (I–V) the predominant tautomeric form is the triketo one. Although the NMR spectra indicate that only the triketo tautomer is present in all the studied solutions, this cannot be accepted as an evidence of absolute absence of enolic forms. The reason for this is twofold. On the one hand, if the dominant form represents more than about 95%, the presence of signals corresponding to minor components might be overlooked. On the other, the signal corresponding to the enolic hydrogen can appear at large chemical shifts (up to 18 ppm) [24] and thus there is possibility that it falls beyond the range of our measurements. Therefore, in order to ascertain whether there are really no enolic tautomers present, the compounds were investigated by (more sensitive) FT-IR spectroscopy, both in KBr pellets and in solutions. For the FT-IR investigation of solutions, two solvents of different polarity were chosen, benzene and chloroform. It was expected that carbonyl groups from all-keto form would show signals at 1700–1750 cm1 characteristic for saturated aliphatic ketones, while carbonyl group from keto–enol form would be expected at somewhat lower wavenumbers (about 1600 cm1), characteristic for C@O stretching in an enolic ring with an intramolecular hydrogen bond [34]. In the spectra of the diketone, both in a KBr pellet and in solutions, there is only one signal in the carbonyl region, which corresponds to the dominant keto–enol form (Table 3). By contrast, in the spectra of triketones in KBr pellet there are two dominant sharp signals in the all-keto carbonyl region at approx. 1700 and 1735 cm1, the first signal being additionally split in two very close bands. Both signals correspond to stretching vibrations of two nonequivalent carbonyl groups from all-keto form. With an exception of benzene solution of compound I, in both chloroform and benzene solutions of triketones, the first signal is not split, although the main signal shows a weakly pronounced shoulder. Otherwise, all the spectra of I–V are very similar to one another. A C@O stretching band at about 1600 cm1 which would correspond to an enolic carbonyl was not noticed in any spectrum of I–V either in the solid or in solutions, indicating an absence of the diketo–enol forms. While multiple C@O stretching bands are expected for asymmetric compounds, they are quite surprising for the symmetrical triketone IV and might indicate the presence of a C1 conformer with the tert-butyl group of one pivaloyl placed between the carbonyl groups of other two – a conformer where the carbonyl groups would be in inequivalent surroundings. In contrast to this result, the NMR spectra are in full agreement with the high symmetry of the molecule (two signals in 1H and four in 13C MNR spectrum). As this might be the result of fast interconversion of conformers, we have also performed a measurement at lower temperatures, to ascertain whether a splitting of the signals might
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Table 3 Relevant signals in IR spectra for assignation of the tautomeric form for compounds I– V measured as solids in KBr pellets and as benzene and chloroform solutions. Compound
Conditions (solvent)
~(C@O) all-keto form m (cm1)
Keto–enol form m~(C@O) (cm1)
Diketone
KBr Benzene Chloroform
Not detected Not detected Not detected
1608 1603 1598
I
KBr Benzene Chloroform
1692, 1701, 1722 1693, 1709, 1734 1707, 1734
Not detected Not detected Not detected
II
KBr Benzene Chloroform
1692, 1699, 1726 1703, 1735 1702, 1735
Not detected Not detected Not detected
III
KBr Benzene Chloroform
1691, 1706, 1730 1703, 1733 1701, 1732
Not detected Not detected Not detected
IV
KBr Benzene Chloroform
1691, 1709, 1721 1699, 1730 1697, 1729
Not detected Not detected Not detected
V
KBr Benzene Chloroform
1694, 1705, 1730 1703, 1732 1700, 1732
Not detected Not detected Not detected
appear due to the blocking of the conformation change, thus proving the above supposition. However, in CDCl3 solution of triketone IV even at 50 °C no splitting of signals was noticed, leaving the question of the conformational equilibrium unresolved. The apparent presence of the C1 conformer in solution seems to support the Nonhebel’s explanation of the lack of the diketoenol tautomer in acyldipivaloylmethanes. However, a closer inspection of the optimised geometry of the C1 conformer (Fig. 4) does not quite reveal how this molecular geometry might render the proton transfer impossible. Quite the contrary, the inversion of one pivaloyl group leads to a close intramolecular contact between its carbonyl oxygen and the methine hydrogen (ca. 2.4 Å), which could lead to an easier proton transfer than in the C3 conformer. The only possible explanation would be that the rotation of the protonated pivaloyl is slow due to the sterical hindrance and thus makes the triketone kinetically inert. However, this cannot be reconciled with the NMR spectra which indicate C3 symmetry, which is either statistical (rapid transformation of C1 to C3 conformer) or absolute (no C1 conformer is present at all). Thus, the possible presence of the C1 conformation of acyldipivaloylmethanes (in solution) does not appear to be a convincing reason for the extreme predominance of the triketo tautomers in this group of compounds. There are two possible reasons for the (apparent) absence of the enol tautomer. The enols might be significantly less stable than the triketones (which would lead to small amounts of enols in the equilibrium mixture). Alternatively, the transition from triketone to diketoenol might occur at a very slow rate, which would cause that the keto–enol equilibrium would not be attained immediately upon dissolution of the (crystalline) triketone. In order to test the latter supposition, a solution of I in CD3OD was prepared and left at room temperature for fourteen days. I was chosen as the compound most probable to exhibit a change over time (having the smallest R substituent), and the solvent as one which is most likely to catalyse the tautomerisation if it in fact occurs. Additionally, as in the case of the enol hydrogen is rather acidic, over a prolonged period hydron exchange with the solvent should occur, and even in the case no detectable amount of enol is present at any time; its occurrence could be proven by changes in the NMR spectra due to the gradual replacement of hydrogen with deuterium. After seven days of standing on room temperature, the NMR spectra of solution have shown no any significant changes – there were
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Fig. 4. Geometry of the putative C1 conformer of IV optimised by semiempirical PM3 method.
neither new signals nor additional splitting of existing ones. However, ten days after starting the experiment, minute changes started to appear both in the 1H and in the 13C NMR spectrum. In the 1H NMR spectrum the intensity of the signal corresponding to methyne proton decreased, and two new singlets appeared, one in the near vicinity of the acetyl singlet (a signal at 2.02 ppm appeared near the original at 2.16 ppm) and the other near to the
t-butyl methyl singlet (1.17 ppm near 1.15 ppm). In the 13C NMR spectrum two new singlets appeared; in the near vicinity of the carbonyl signal (208.2 near to the original 208.3) and t-butyl methyl signal (27.7 ppm near 27.0 ppm). Also, the acetyl methyl signal at 29.9 split in two very close signals. A similar effect was observed when a freshly prepared solution of I in CD3OD was kept at the boiling point of methanol, when the new signals, equivalent to those in the first experiment, have appeared after only 24 h, and their intensities further increased over the next two days. There are three reasonable causes of the observed changes in the spectra of I; the new signals could appear due to (1) formation of an enol tautomer (extremely slow equilibration), (2) degradation of the compound or (3) isotopic exchange. As the newly appeared signals did not seem to be consistent with the either an enol tautomer or a ketal, it was proposed that the changes in the spectrum have appeared due to the replacement of the methyne hydrogen atom with deuterium. To test this hypothesis an additional experiment was performed where I was dissolved in nondeuterated methanol and the solution kept at the boiling point of methanol identically as the CD3OD solution. After three days the solvent was evaporated and the remaining solid dissolved in CDCl3. The spectra were found to be identical to the original spectra of I with no trace of additional signals which had appeared in the CD3OD solution. This showed quite unequivocally that I did not undergo a (thermal) degradation, or a transformation into another compound. It also could not be reconciled with the supposed slow
Fig. 5. The geometry of diketones and keto–enols based on the structural data deposited with the CSD: (a) Torsion angles between the CAO bonds vs. the distance between the two oxygen atoms showing a distinct grupation of keto–enols as opposed to diketones. (b) Distribution of the torsion angles between the CAO bonds in keto–enols.
Fig. 6. (a) The expected geometry of the diketoenol tautomer of IV. (b) Torsion angles between the CAO bonds of the keto–enol ring in the diketoenol tautomers of tripivaloylmethane (IV), acetyldipivaloylmethane (3 tautomers), diacetylpivaloylmethane (3 tautomers) and triacetylmethane as obtained from molecular geometries optimised by the PM3 semiempirical method. The presence of t-butyl groups leads to increase of the angle, i.e. the reduction of planarity of the keto–enol ring.
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interconversion of the tautomers, as in this case the isolation of both forms should be expected. The observed isotopic exchange therefore indicates that a minute presence of enol tautomer is indeed present in the methanol solution of I, albeit in insufficient amount to be observed directly in the spectrum. However, when equivalent experiments were performed with IV, no isotopic exchange whatsoever was noticed. This would indicate that in I the triketo tautomer is considerably more stable then any of the possible enols, while in IV, this difference in stabilities is greater still, as not even by this indirect method any trace of the enol could be detected.2 This raises the question of the reason of such pronounced difference in relative stabilities of the two tautomers. An important factor in the stabilisation of enols in b-dicarbonyl compounds is the intramolecular resonance assisted hydrogen bond between the enol hydroxyl and the b-carbonyl groups [35]. Therefore it is crucial for the keto–enol hydrogen bonded ring to be as closely planar as possible, in order for the resonance effects to be possible. The deviation from planarity can easily be ascertained by the torsion angle defined by the two carbonyl groups (O@C C@O torsion, h), which is expected to be 0° for a perfectly planar enol with the intramolecular hydrogen bond. This can be seen by inspecting the values of h for simple 1,3-diketones in the solid state according to structures deposited with the CSD [36] (Fig. 5): the data show a clear distinction between enols and diketones, with enols having shorter O O distances (as the oxygen atoms are hydrogen bonded), and with h rarely exceeding 5° (only in two cases where additional hydrogen bonding involving one or both the oxygen atoms is present). In contrast, h in diketo tautomers is much more variable. Therefore if there is a steric hindrance preventing h to achieve values close to 0° (i.e. preventing the planarity of the keto–enol hydrogen bonded ring), this will greatly destabilise the enol (preventing the resonance stabilisation), while it should not have as pronounced effect on the ketone. To test whether the pivaloyl groups might indeed have such an effect, a series of models of putative diketoenol tautomers was constructed and their geometries optimised by semiempirical methods (AM1 and PM3). The optimised geometries have demonstrated that the sterically demanding pivaloyl groups indeed lead to an increase of h, i.e. render the molecule less planar. The largest effect is in the diketoenol form of IV (Fig. 6), where h is expected about 27°, much larger than found in isolated enols. In other studied compounds three different diketoenols are possible, and there the value of h is dependant also of the position of the bulkiest substituents. This is best demonstrated by I, where there is the largest difference between substituents (methyl vs. tert-butyl), where the expected h varies from 14°, when the two pivaloyl groups form the hydrogen bonded ring to 6°. The values of h for II, III and V were estimated to be above those for I and below those for IV – in all cases above the values found in enols. For further comparison, the geometries of three diketoenol tautomers of diacetylpivaloylmethane, and one of triacetylmethane were also optimised. All three tautomers of diacetylpivaloylmethane have h between 2.5° and 1°, and triacetylmethane 1°, which is in accordance with the expected values for enols, and in agreement with the fact that triacetylmethane was found to be mostly present in diketoenol form [16–19]. The same computational method also gave the value of h in the parent dipivaloylmethane of 0.9° which is in good agreement with one observed in the crystal structure (0.3°) [37], where it is present as enol, which is also the predominant form in the solution. 2 It should be noted that this is based on a finite time in which the experiment was carried out – it is not inconceivable that if the boiling of IV in CD3OD were continued for a substantially longer time (months or years), isotopic exchange would become noticeable.
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4. Conclusion All the studied acyldipivaloylmethanes are present as triketo tautomers in the solid state. No diketoenol tautomer was noticeable even in solution of any of the compounds, regardless of the solvent used or temperature at which measurements were performed. This behaviour is quite contrary to that of the parent diketone, which is predominantly present as ketoenol under the same conditions, and is also known to crystallise as such. Apparently the addition of an additional acyl group in the a-position of dipivaloylmethane renders the diketoenol form unstable as, due to steric repulsions, the keto–enol ring of the diketoenol tautomer would deviate from planarity and thus could not be sufficiently stabilised by resonance, as it is the case in dipivaloylmethane and also less sterically hindered triacetylmethane. This conclusion is also in accord with the fact that the existence of a minute amount of an enol tautomer, as well as the tautomeric equilibrium, are indicated by very slow isotopic exchange noticed in CD3OD solution of I, which is sterically least hindered, while no isotopic exchange was detected in the CD3OD solution of much more hindered IV. Acknowledgements The authors would like to thank the Ministry of Science, Education and Sports of the Republic of Croatia, for financial support of this work through Grants 119-1193079-3069 and 098-09829042912. Appendix A. Supplementary material Supplementary crystallographic data for this paper can be obtained via www.ccdc.cam.ac.uk/conts/retrieving.html, (or from the Cambridge Cystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033; or deposit@ccdc. cam.ac.uk). CCDC 968873 and 968874 contain the supplementary crystallographic data for this paper. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.molstruc.2014.01. 061. References [1] (a) R.C. Mehrotra, Pure Appl. Chem. 60 (1988) 1349; (b) R.C. Mehrotra, R. Bohra, D.P. Gaur, Metal b-Diketonates and Allied Derivatives, Academic Press, London, 1978; (c) K.C. Joshi, V.N. Pathak, Coord. Chem. Rev. 22 (1977) 37; (d) D.P. Gibson, Coord. Chem. Rev. 4 (1969) 225; (e) D.P. Graddon, Coord. Chem. Rev. 4 (1969) 1. [2] A. Werner, Chem. Ber. 34 (1901) 2584. [3] (a) J. Wu, D.J. MacDonald, R. Clerac, I.-R. Jeon, M. Jennings, A.J. Lough, J. Britten, C. Robertson, P.A. Dube, K.E. Preuss, Inorg. Chem. 51 (2012) 3827; (b) A. Beziau, S.A. Baudron, M.W. Hosseini, Dalton Trans. 41 (2012) 7227; (c) Jingya Li, Hongfeng Li, Pengfei Yan, Peng Chen, Guangfeng Hou, Guangming Li, Inorg. Chem. 51 (2012) 5050; (d) M.A.G. Berg, M.K. Ritchie, J.S. Merola, Polyhedron 38 (2012) 126; (e) A. Zaim, H. Nozary, L. Guenee, C. Besnard, J.-F. Lemonnier, S. Petoud, C. Piguet, Chem. – Eur. J. 18 (2012) 7155; (f) A.D. Burrows, M.F. Mahon, C.L. Renouf, C. Richardson, A.J. Warren, J.E. Warren, Dalton Trans. 41 (2012) 4153; (g) L. Carlucci, G. Ciani, D.M. Proserpio, M. Visconti, CrystEngComm 13 (2011) 5891; (h) J. Yuasa, T. Ohno, K. Miyata, H. Tsumatori, Y. Hasegawa, T. Kawai, J. Am. Chem. Soc. 133 (2011) 9892; (i) V. Stilinovic´, K. Uzˇarevic´, I. Cvrtila, B. Kaitner, CrystEngComm 14 (2012) 7493; (j) R. Horikoshi, Y. Funasako, T. Yajima, T. Mochida, Y. Kobayashi, H. Kageyama, Polyhedron 50 (2013) 66; (k) I. Cvrtila, V. Stilinovic´, B. Kaitner, Struct. Chem. 23 (2012) 587; (l) I. Cvrtila, V. Stilinovic´, B. Kaitner, CrystEngComm 15 (2013) 6585; (m) V. Stilinovic´, B. Kaitner, J. Coord. Chem. 62 (2009) 2698; (n) V. Stilinovic´, D.-K. Bucˇar, I. Halasz, E. Meštrovic´, New J. Chem. 37 (2013) 619.
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