Hydrogen Bonds, Tautomers, and Conformation in 2 -Hydroxyphenyl 2 -hydroxy- 2 -( b -naphthyl)vinyl Ketone

Single crystal X-ray diffraction and IR spectroscopy have been used to study the conformation of 2 -hydroxyphenyl 2 -hydroxy- 2 -( a -naphthyl)vinyl ketone in solid state. It was found that one of the two possible enol tautomeric forms is stabilized in the crystal. The 1 -hydroxy- 3 -oxo- 1 , 3 -propenylene moiety, O——C—CH——C—OH, shows a strong intramolecular H bond with a deﬁnite character of reasonance-assisted hydrogen bond in spite of being in competition with ring intermolecular hydrogen bonds. The comparison of the present results with solution NMR data indicates that the molecular geometry in solid state and in solution are similar.

These compounds can exist in solution in three tautomeric species with two interdependent equilibria (see Scheme 1).However, NMR spectroscopy is only able to detect the first equilibrium involving the diketo and the averaged enolic structures.This slower equilibrium was studied in various analog compounds, where the naphthyl residue of the title compound is replaced by phenyl Scheme 1. Keto (I) and enol (II / III) structures arising from tau- tomeric equilibria of 1. [7][8][9] or methyl [10,11].The results of several X-ray and neutron diffraction studies performed on similar 1,3- dicarbonyl compounds [12, 13] would indicate that the enol forms are the ones more frequently found in the crystalline state [14].
The presence of a hydrogen-bonded heteroconjugated propene and of an ortho-hydroxyphenyl joined fragments as found in 2-hydroxyphenyl 2-hydroxy-2-(a- naphthyl)vinyl ketone (1) has a significant influence on both keto-enol and enol-enol equilibria.This is due to the capability of the phenolic OH group to participate in a hydrogen bond with the adjacent O atom.Several derivatives of 1 have shown that in solution a keto-enol equilibrium (I vs. II / III) shifted toward the enol forms (between 80-95%), [15,16] whereas the enol form II was the one that predominates in the rapid equilibrium II / III [17].This is consistent with the results of some crystal structure studies [18].Other examples reported in the literature showed the keto-enol form type II to be the preferred one [19][20][21][22], although the keto form has also been observed in the crystalline state [20].
The constant interest in the structural chemistry of compounds containing the 1-hydroxy-3-oxo-1,3- propenylene moiety, O --C -CH --C -OH, originates in their ability to form strong intra-and intermolecular hydrogen bonds that induce extended p delocalization.A relationship between the O• • •O distance shortening, the increment of p delocalization, the decrease of the IR n(OH) stretching frequencies, up to ca. 2560 cm − 1 , and the shift of the enolic O-H[ 1 H]NMR signal has been proposed by Bertolasi et al. [23][24][25] for these systems.The relationship between delocalization and O-H• • •O distance shortening has been explained for both intraand intermolecular bonds, by means of a semiempirical model called resonance-assisted hydrogen bond (RAHB) [24, 26].However, the factors that govern the stabilization of one of the tautomers and the relative influence of intra-and intermolecular H bonds in the stabilization have not been fully disclosed yet.Therefore, as part of a more general study on the subject, in this paper we show the results of the single crystal X-ray diffraction study of 1 along with their correlation with IR spectroscopy data in solid state and previously reported [ 1 H] and [ 13 C] NMR spectral data in solution [15].EXPERIMENTAL 2-Hydroxyphenyl 2-hydroxy-2-(a-naphthyl)vinyl ketone 1 was obtained from 2-acetylphenyl a-naphthoate, through a Baker-Venkataraman rearrangement by potassium hydroxide in molar ratio 1 : 3 (ester : KOH) at 60 8 C [15,16].The IR (KBr) spectrum was measured at 20 8 C with a Perkin Elmer model 1600 instrument.
The crystal structure was determined by X-ray single-crystal analysis.Suitable crystals were obtained by slow evaporation from a saturated solution of chloroform.Data were taken on an automatic four-circle CAD-4 diffractometer.For data collection and cell refinement, the CAD-4 software [27] was used.The code MoLEN [28] was employed for data reduction.The structure was solved by direct methods using the program SHELXS86 [29] and refined by full-matrix least-squares based on F 2 's, for all reflections except for 460 with very negative F 2 , using SHELXL93 [30].The hydrogen atoms were located from a Fourier synthesis.Hydrogen atoms were refined riding on the bound atom; their thermal param- eters were fixed to 1.2 times the equivalent isotropic displacement of the corresponding anisotropic atom to which they were attached.
The structure analysis was performed with PLA-TON [31] and PARST [32] and the molecular graphics were done with ORTEP [33].Table I shows crystal data and data collection details.It should be noted that weighted R factors, R w , and all goodnesses-of-fit S are based on F 2 , conventional R factors are based on F, with F set to zero for negative F 2 .The observed criterion of F 2 > 2j (F 2 ) is used only for calculating some magnitudes like the R factor and is not relevant to the choice of reflections for refinement.R factors based on F 2 are statistically about twice as large as those based on F and R factors based on all data will be even larger.The atomic coordinates are listed in Table II.An ORTEP drawing of the molecular is displayed in Fig. 1.4  a U eq is defined as one-third of the trace of the orthogonalized U ij tensor.

RESULTS
The IR spectrum of 1 is displayed as a very broad and weak band without a definite maximum, that extends from ca. 2200 cm − 1 to approximately 350 cm − 1 where it ends as a shoulder of medium intensity peak of the aromatic C-H stretching band.Two very strong intensity peaks at 1600.0 and 1582.1 cm − 1 and a medium one at 1486.0 cm − 1 were also worthy of note.
The resolution of the molecular structure showed that in the solid state the "symmetric" enol tautomeric form II is stabilized (see Fig. 1).The intramolecular bond lengths and angles, along with selected torsional angles and nonbonded intra-and intermolecular interactions are presented in Table III.The analysis of the molecular geometry evidenced that the o-hydroxybenzoylmethylenecarbonyl moiety is not completely planar, although it can be interpreted as an extended conjugated system.According to Cremer and Pople [34], the naphthyl group is also not planar, with a total puckering amplitude of Q c 0.0944(53) A ˚.It presents a mean plane rotation out of the mean plane of the o-hydroxybenzoylmethylenecarbonyl portion by an angle of 45.06(6) 8 .The weighted average ring bond distance in the phenyl ring is 1.385(6)A ˚.The C11-C61 and C21-C31 bond length, 1.403(5) and 1.408(6)A ˚, respectively, are larger than the mean.The observed geometry of the benzene fragment was analyzed following the ideas of Norrestam and Schepper [35] and Domenicano and Murray-Rust [36].The use of the angular parameters given by those authors to model the influence of the substituents allows prediction of benzene angle values that are coincident, within two standard deviations, with the experimental bond angles.However, the C11-C21 bond length value, 1.382(5)A ˚, is smaller than expected, according to Walsh's rule [37].The enol system is nonsymmetric in terms of bond lengths.This can be seen when comparing C1-O1, 1.337(4) A ˚, and C21-O3, 1.354(5) A ˚.The structure tends to the enol form (II) shown in the scheme, according to the C2-C3 and C1-C2 bond length values, 1.421(5) and 1.358(5)A ˚, respectively.The strength of the intramolecular hydrogen bonds can be assessed from the observed short distances between adjacent oxygen atoms: O1• • •O2, 2.546(4) A ˚, and O2• • •O3, 2.566(4) A ˚. Bond lengths in the fragment, which could be involved in RAHB, were compared with the expected mean values.The latter were taken from Bürgi and Dunitz [38].These mean values are coincident within one standard deviation with the geometry defined by Gilli et al [24] as unperturbed by hydrogen-bond formation.The comparison showed that the O1-C1 and O2-C3 bond length values, 1.337(4) and 1.268(4)A ˚, are, respectively, shorter and larger than the mean values, 1.353 and 1.222A ˚.In addition, the C2-C3 bond length, 1.421(5)A ˚, is shorter than the mean, 1.464A ˚, and the C1-C2, 1.358(5) A ˚, is larger than the mean, 1.349  degree of steric conflict.The examination of the nonbonded intermolecular distances indicated that molecules related by the symmetry operation − x, 1− y, 1− z are con-nected by four hydrogen bonds, involving the hydrogen and oxygen atoms of the enol, the phenolic hydrogen, and the carbonyl oxygen.Thus, centrosymmetric dimers are formed with an extended delocalization.The analysis of the molecular packing, shown in Fig. 2, shows that the planes of the o-hydroxybenzoylmethylenecarbonyl fragments related by the inversion center at 0.5, 0.5, 0.5 are at a perpendicular distance of 3.37A ˚.This favors the interactions between the p clouds of the carbonyl and phenyl moieties.The naphthyl groups, related by the symmetry operation − x, − y, 1− z, are stacked.The planes of these groups are at a distance of 3.55 A ˚.The closest distance between naphthyl ring centers, 3.78 A ˚, is found between neighbor six-member rings bound to C1.Therefore, the packing can be described as (010) layers of naphthyl groups, centered in y c 0, intercalated with layers of o-hydroxybenzoylmethylenecarbonyl moieties, centered in y c 0.5.

DISCUSSION
The inspection of the IR spectrum of 1 showed two important features: (1) The absence of the unconjugated C --O stretching frequency at its typical wavenumber values, indicating a great preponderance of the enol forms II / III; (2) the appearance and position of the peaks, described above, which are consistent with the coexistence of the 1-hydroxy-3-oxo-1,3-propen- ylene moiety, O --C -CH --C -OH, and a very strong intramolecular hydrogen bond [39][40][41].The peaks found in the range 1600-1480 cm − 1 can be ascribed to the associated conjugate carbonyl and the vinyl double bond of the enol form.Furthermore, the broadness and low intensity at ca. 3000-2200 cm − 1 are characteristic of an OH stretching band arising from this kind of intramolecular hydrogen bond.The lack of the band at ca. 3200 cm − 1 suggests that the phenolic OH is also involved in a strong hydrogen bond, but its stretching frequency cannot be determined from the present data.Taking into account the similarity of both O• • •O distances and of the proved correlation between the O• • •O distance and the n OH stretching frequencies, when strong O-H• • •O hydrogen bonds are at play [23, 24], we can assume that the stretching frequencies of both O-H groups should be alike.At this stage, it is unclear whether the lack of a distinct O-H stretching peak is due to the superposition of the contributions of both OH fragments or the aromatic C-H stretching overlaps the phenolic OH frequency.
The existing [ 1 H]NMR spectral data [15] provided evidence that the molecular characteristics of 1 in solution are similar to the solid state ones.The keto-enol equilibrium is strongly shifted, containing ca. 95% of the enol forms (II / III), as was measured from the inte- gral [ 1 H]NMR curve, whereas weak signals, due to the minor diketo tautomer I as, e.g., phenolic OH, 11.93 and methylene, 4.72 ppm have been assigned.Evidence of the nature and force of the hydrogen bonding for the major tautomer is provided by the enolic OH and phenolic OH proton resonance values at 15.61 and 12.09 ppm, respectively.It is interesting to notice that the chemical shifts observed for the phenolic OH in both tautomers I and II / III are 11.93 and 12.09 ppm, respectively.Look- ing at the possible structures of 1 (see Scheme 1), an opposite chemical shift sequence should be expected: The phenolic OH in the diketo tautomer I suffers the deshielding of O2, with a net C --O character, whereas the same effect would be decreased in the fast equilibrium tautomers II / III where O2 presents an averaged carbonyl-enol character.
The above-mentioned results suggest that the fast equilibrium in solution would be strongly shifted toward the type II tautomer.The analysis of [ 13 C]NMR, spectral data of 1 and analog compounds [17] has also shown the predominance of the tautomeric form II over III in the fast equilibrium.If the RAHB model were in force (Scheme 2), the apparent contradiction would be explained.A deshielding on the hydrogen atom bonded to O3 due to a partial negative charge at O2, arising from the IIa structure, would be at play, which would cause tautomers II / III to present a larger deshielding of the phenolic OH than tautomer I, as was found.
The X-ray data results showed that the nonbonded, hydrogen bond type, intramolecular interactions induce a quasi-planarity of the o-hydroxybenzoylmethylenecarbonyl moiety.The strength of the intramolecular bonds involving the OH and enol groups can be assessed from the geometry of the interaction shown in Table III.In our case, the O• • •O distance value, 2.567(4) A ˚, would indicate that the system is not fully delocalized.
Gilli et al. [42] found two parameters that can be used to establish the degree of conjugation: the antisymmetrical stretching coordinate Q c d1 − d4 + d3 − d2 and l c (1 + Q / 3.32) / 2. In the title com- pound, these parameters take the values 0.13 A ˚and 0.48, respectively.The Q value denotes that the structure presents an enol-keto form, while the l value shows that the delocalization is important, although not complete.These results confirm the validity of the model assumed in the [ 1 H]NMR data interpretation.The obtained molecular parameters, compared with those reported by Cunningham et al. [20] for 1-(2-hydroxy-4methoxyphenyl)-3-phenylpropane-1,3-dione (2), denote a lesser degree of alternancy in the bond order at the fragment O2 --C3-C2 --C1-O1-H1A.Other structural features are similar.In compound 2 the enolization has been interpreted as not completed, because although the substituted phenyl ring is coplanar with the enol ring, the other phenyl ring is twisted 14 8 out of the p system.This twist has been ascribed to repulsion between the olefinic H and its ortho H.When the methoxy substituent is moved to the other ring, a planarization of the molecule was observed, the average torsion angle in (Z)-3(2-hydroxyphenyl)-1-(4-methoxyphenyl)-3-oxoprop-1-en-1-ol is 1.8(2) 8 [21].This increased planarity is attributed to the methoxy group-extending conjugation.Complete enolization is found when the methoxy substituent is removed [22], the largest distance from the least-squares plane passing through each of the two crystallographically inequivalent molecules of 1- (2-hydroxyphenyl)-3-phenyl-1,3-propanedione does not exceed 0.085 A ˚.In the title compound, the torsional angle C2-C1-C12-C22, 43.5(6) 8 , is more pronounced than in the above-mentioned compounds and would indicate an even lesser degree of enolization than in 2. The observed rotation of the naphthyl out of that plane has relieved the otherwise conflicting interaction between the hydrogens H2 and H22.The relative positions of the naphthyl and o-hydroxybenzoylmethylenecarbonyl fragments induce a H22• • •H2 distance of 2.453 A ˚.In previous works on a-naphthyl derivatives, which included the title compound, the analysis of the [ 1 H], [ 13 C] [17], and [ 17 O]NMR [43] chemical shifts indicated a considerable molecular nonplanarity in solution.This feature, which has not been observed in the b-naphthyl analogs [17, 43], was assigned to steric interference in coincidence with our interpretation.Yet, in the title compound it can not be disregarded that the extent of the rotation is helped by an attractive-interaction between O1 and H82.This last hypothesis is supported by the C-H• • •O geometry described in Table III, c.The analysis of the hydrogen bonds relative strength, estimated from their geometry, would indicate that the intramolecular hydrogen bonds are stronger than the intermolecular ones.However, the intermolecular hydrogen bonds, as well as the intramolecular ones, induce pseudo six-membered planar rings.These intermolecular hydrogen bond rings, which build the dimers, seem to extend the conjugation to both o-hydroxybenzoylmethylenecarbonyl moieties supporting more efficient molecular packing.

CONCLUSIONS
The results presented above showed that in the solid state the stabilized tautomeric form is the "symmetric" enol.The comparison of the IR and single crystal XRD data with solution NMR results indicates that, in solid state and in solution, the geometry adopted by the molecule of 1 aids the formation of two strong hydrogen bonds with a RAHB character.The intramolecular delocalization is important, but not complete, because of steric conflicts.Intermolecular hydrogen bonds originate centrosymmetric dimers where delocalization involving both o-hydroxybenzoylmethylenecarbonyl groups of the dimer seems to be at play.This extended delocalization would help the stabilization of the three-dimensional packing of the molecules.

Table I .
Crystal Data and Structure Refinement for 2-Hydroxybenzoyl a-Naphthoyl Methane

Table III .
A ˚.The intramolecular hydrogen bonds geometry is depicted in Table IIIc.A rather short nonbonded intramolecular H• • •H distance, H2• • •H61 c 2.014 A ˚is also observed.It suggests some a.Intramolecular bond lengths (A ˚) and angles ( 8 ) a a esd's in parentheses.b− x, − y + 1, − z + 1.cNo esd's are given because the H82 atom position has not been refined.