Cascade Reactions of α-Phenylcinnamic Acid to Polycyclic Compounds
Promoted by High Valent Transition Metal Halides
Niccolò Bartalucci,a,b Dr. Lorenzo Biancalana,a,b Dr. Marco Bortoluzzi,b,c Prof. Guido Pampaloni,a,b Luca Giordano,a Prof. Stefano Zacchini,*,b,d and Prof. Fabio Marchetti*,a,b
a Università di Pisa, Dipartimento di Chimica e Chimica Industriale, Via Moruzzi 13, I-56124 Pisa.
b CIRCC, via Celso Ulpiani 27, I-70126 Bari, Italy.
c Università Ca’ Foscari Venezia, Dipartimento di Scienze Molecolari e Nanosistemi, Via Torino
155, I-30170 Mestre (VE), Italy.
d Università di Bologna, Dipartimento di Chimica Industriale “Toso Montanari”, Viale
Risorgimento 4, I-40136 Bologna, Italy.
* To whom correspondence should be addressed.
E-mail: fabio.marchetti1974@unipi.it; stefano.zacchini@unibo.it Webpage: www.dcci.unipi.it/~fabmar/
Abstract. α-Phenylcinnamic acid converts into polycyclic derivatives by interaction with high
valent transition metal halides in chloroform at reflux temperature. Different products, featured by local structural differences depending on the nature of the metal halide system (i.e. WCl6, PCl5/NbF5 or PCl5/NbCl5), are easily isolable after treatment with water. In particular, WCl6 configures as an efficient multitasking agent, working as 1) chlorinator of the carboxylic acid function, 2) catalytic precursor for intra- and intermolecular rearrangements, and 3) catalytic precursor for Baeyer Villiger lactonization in water at ambient temperature exploiting air as oxidant. The products have been purified by alumina chromatography, and identified by analytical and spectroscopic (IR,
NMR) techniques, and by single crystal X-ray diffraction in three cases. Their formation probably involves a common reaction pathway which has been rationalized by DFT calculations.
Introduction
High valent transition metal halides, the metal being in an oxidation state higher than +4, have emerged as valuable alternatives to the use of precious elements in organic synthesis. They combine a strong Lewis acidic character [1] with a high oxidation potential,[2] and thus are able to promote a multitude of transformations, including the dehydrogenative coupling of arenes [3] and other C-C bond forming processes,[4] the hydrofunctionalization of alkenes,[5] the polymerization of alkynes [6] and various cycloaddition reactions.[7]
In particular, WCl6 exhibits relatively weak W-Cl bonds,[8] allowing the stoichiometric modification of simple molecules through unconventional pathways. Examples include the transformation of imines into aza-2-allenium and nitriulium cations,[9] the former being generated via unusual N2 elimination, and of α-diimines into quinoxalinium salts.[10] Also niobium pentahalides have been found to provide unique reactivity patterns to organic species,[Error: Reference source not foundc,Error: Reference source not found,11] and their chemistry has witnessed a significant advance in the last years,[Error: Reference source not found,12] encouraged by their cheapness and the substantial non toxicity of the element niobium.[13] Compared to WCl6, NbX5 (X = F, Cl) are more inert to reduction processes, and comprise relatively strong niobium-halide bonds,[Error: Reference source not found] thus disfavouring halide/oxygen exchange reactions.[14] Therefore, NbF5 forms coordination adducts upon interaction with carboxylic acids,[15] while NbCl5 manifests the typical reactivity of low to middle valent transition metal chlorides,[16] affording metal carboxylates through HCl release.[17] Conversely, the chemistry of WCl6 (and the related group 6 chloride MoCl5) with simple carboxylic acids (RCO2H) somehow resembles that of PCl5,[18] and Cl/O exchange is usually working to give the corresponding acyl chlorides RC(O)Cl, possibly accompanied by side reactions.[19] In this setting, the interaction of
high valent metal halides with carboxylic acids (and their acyl chloride counterparts) with an additional, adjacent function has been sparingly explored heretofore.
In the framework of our effort in the development of the chemistry of homoleptic halides of groups 5 and 6 elements, herein we report on the domino reactions of a commercial α,β-alkenyl carboxylic acid (i.e., α-phenylcinnamic acid), directed by stoichiometric amounts of WCl6 or PCl5/NbX5 (X = F, Cl), affording polycyclic compounds and revealing some convenience with respect to classical synthetic protocols.
Results and discussion
We were interested in the unexplored interaction of niobium pentafluoride with carboxylic acid chlorides. In general, the sluggishness of the organic reactant to cleave the Nb-F-Nb bridges in the tetranuclear structure of NbF5 [Error: Reference source not founde-f,20] disfavours the formation of coordination compounds. Nevertheless, when an equimolar mixture of NbF5 and α-phenylcinnamoyl chloride, cleanly obtained from cis-PhCH=CPhC(=O)OH by means of PCl5 (Scheme 1a, see SI for details), was heated in chloroform at reflux temperature, progressive conversion of the organic substrate occurred over 24 hours. Then, hydrolytic treatment of the mixture upon air contact allowed to separate insoluble metal derivatives and to recover an organic phase. The crystalline compound 1 was finally isolated in 47% yield after chromatographic purification (Scheme 1b). X-ray quality crystals of 1 were collected from a dichloromethane/hexane mixture, at 30 °C. The molecular structure of 1 is shown in Figure 1, with relevant bonding parameters reported in the caption. Compound 1 is a chiral molecule which crystallizes as a racemic mixture (SSSS and RRRR) in the centrosymmetric space group P21/c. The IR spectrum of 1 (solid state) includes a broad, intense absorption centred at 1713 cm, accounting for the ketonic moieties. The NMR spectra of 1 consist of a single set of resonances, presumably corresponding to the mixtures of enantiomers observed in the solid state, thus indicating that the formation of this compound is substantially diastereoselective.
Scheme 1. Formation of α-phenylcinnamoyl chloride and subsequent polycyclization reactions. * Pathway f
Figure 1. Molecular structure of 1 with key atoms labelled. Displacement ellipsoids are at the 30% probability
level. Main bond distances (Å) and angles (°): C(1)-O(1) 1.217(3), C(12)-O(2) 1.210(3), C(1)-C(2) 1.466(4), C(1)-C(5) 1.546(4), C(4)-C(5) 1.559(3),C(4)-C(9) 1.534(3), C(8)-C(9) 1.540(3), C(8)-C(12) 1.532(3), C(11)-C(12) 1.476(4), C(1)-C(5)-C(6) 103.7(2), C(4)-C(5)-C(6) 110.2(2).
It is noteworthy that cis-PhCH=CPhC(=O)Cl was completely stable under the conditions employed for the formation of 1, i.e. in the presence of POCl3 (derived from PCl5) but in the absence of NbF5 (Scheme 1c). Due to the observation that 1 forms from cis-PhCH=CPhC(=O)Cl also using a stoichiometric amount of AlCl3,[21] the Lewis acidic metal species must play a crucial role along the reaction pathway leading to 1. We carried out a preliminary DFT study in order to trace a reasonable pathway (Scheme 2). The calculations indicate that the intramolecular Friedel-Crafts cyclization of α-phenylcinnamoyl chloride (compound A in Scheme 2) to 2-phenyl-indenone (B) via HCl release is a feasible reaction (G = 19.1 kcal mol). Successive Diels-Alder dimerization can take place in head to head mode, affording C.[Error: Reference source not found,22] A possible transition state for this annulation step was preliminary determined without the presence of metal fragments (TS, imaginary frequency i497 cm-1), and the Gibbs activation energy is estimated around 34 kcal mol-1. IRC calculation supports the fact that the obtained saddle point is related to the Diels-Alder cyclization. The introduction of NbF5 as model metal fragment coordinated to the carbonyl oxygen
atom of B caused a meaningful lowering of the energy barrier for annulation, i.e. 28.1 kcal mol-1 when NbF5 is coordinated to the diene and 24.1 kcal mol-1 if the interaction occurs with the dienophile (see inset of Scheme 2; transition states are indicated as TS-NbF5-diene and TS-NbF5
-dienophile). DFT calculations therefore support the idea of the non-innocent role of oxophilic metal
fragments in one of the key steps of the process. Hydrogen transfer is expected to follow to regenerate the aromaticity of one arene ring, and this last step affording 1 represents the driving force of the entire mechanism (GC4 = 26.5 kcal mol). The experimental and simulated structures of 1 are in good agreement, the RMSD being 0.229 Å.
Scheme 2. Relative Gibbs free energies of proposed intermediates for the conversion of -phenylcinnamoyl chloride to 1. Effects of NbF5 in the annulation step are shown in the inset. C-PCM/B97X calculations, chloroform as continuous medium.
Considering the powerful chlorinating ability of WCl6,[Error: Reference source not found,23] we reckoned that the direct reaction of cis-PhCH=CPhC(=O)OH with WCl6 could efficiently reproduce the formation of
1, via initial, in situ Cl/O transfer between the metal and the organic substrate. The use of WCl6 appeared convenient also in that one equivalent of WCl6, in principle, could serve the chlorination of two equivalents of acid, the former converting into WO2Cl2.[24] The 2:1 molar reaction of cis-PhCH=CPhC(=O)OH with WCl6 was performed in the same conditions as those employed for NbF5/cis-PhCH=CPhC(=O)Cl. The subsequent work-up under air cleanly afforded the coumarin compound 2 in 51% yield, and a less amount of 3 (Scheme 1d), and the identity of these products was unambiguously checked by single crystal X-ray diffraction analyses. Surprisingly, no traces of
1 were found. A similar result was obtained by allowing WCl6 to react with cis-PhCH=CPhC(=O)OH in 1:1 molar ratio. On the other hand, the reaction of cis-PhCH=CPhC(=O)Cl with NbCl5 revealed poorly selective, affording a mixture of products among which 2 and 4 were identified (ESI-MS, 1H NMR), see Scheme 1e. Compound 4 (phenylcinnamalone) and substituted derivatives were obtained in the past from cis-PhCH=CPhC(=O)Cl only under solventless, harsh conditions (T ≥ 160 °C).[25,26]
The structure of 2 (Figure S1) matches that previously published;[Error: Reference source not found,27] a view of the structure of the unprecedented compound 3 is shown in Figure 2, with relevant bonding parameters given in the caption.
Figure 2. Molecular structure of 3 with key atoms labelled. Displacement ellipsoids are at the 30% probability
level. Main bond distances (Å) and angles (°): C(2)-O(1) 1.192(5), C(18)-O(2) 1.213(5), C(1)-Cl(1) 1.847(4), C(1)-C(2) 1.552(6), C(2)-C(3) 1.471(6), C(10)-C(11) 1.556(5), C(11)-C(12) 1.538(5), C(11)-C(18) 1.548(5), C(12)-C(11)-C(18) 106.3(3), C(12)-C(11)-C(10) 110.1(3).
Analogously to 1, 2 and 3 comprise two and four, respectively, chiral centres, and crystallise in the centrosymmetric space groups P21/c and P21/n. As a consequence, their crystals contain 1:1 mixtures of enantiomers (in 2: RS and SR; in 3: SSRR and RRSS). The IR spectra of 2-3 (in the solid state) clearly show intense bands accounting for the carbonyl groups, i.e. at 1712 cm1 (2) and 1732 and 1713 cm1 (3). Moreover, the non aromatic, alkenyl moiety in 2 manifests itself by a medium absorption at 1646 cm1. In the 1H spectrum of 2, the unique non aromatic proton resonates as a singlet at 4.95 ppm. Conversely, the structure of 3 comprises two adjacent Csp3-bound hydrogens, resonating as two doublets in the 1H NMR spectrum (δ = 4.47 and 4.36 ppm, 3JHH = 6.7 Hz).
The synthetic pathways affording 2-4 appear strictly correlated to each other, following the formation of 1. More precisely, 4 is the result of dehydrogenation of 1, presumably occurring via O2 activation during the aerobic hydrolytic treatment (vide infra).[Error: Reference source not found] Subsequent HCl capture (from hydrolysis, see below) by the resulting alkene function [28] leads to 3. Supported by previous experimental and theoretical studies,[Error: Reference source not found,29] H2O2 produced in the O2-driven
dehydrogenation step is likely to be responsible for the lactonization process affording 2. In order to give insight into this hypothesis, the hydrolytic treatment of the reaction mixture cis-PhCH=CPhC(=O)OH/WCl6 was repeated in anaerobic conditions, i.e. using deaerated water under a nitrogen atmosphere (Scheme 1f). This experiment led us to isolate, after work up, compound 1, in admixture with side-products not including 2-4. This outcome indicates that WCl6 is able to convert α-phenylcinnamic acid into 2 through a two-pots cascade process, proceeding with initial Cl/O exchange between the metal chloride and two equivalents of acid, followed by the formation of 1 presumably via the sequence displayed in Scheme 2. Subsequent lactonization of 1 appears to exploit air as external oxygen source (Baeyer-Villiger oxidation), and to be promoted by tungsten species generated during the hydrolytic treatment. Two points deserve to be remarked: 1) niobium systems in aqueous medium do not exhibit an appreciable oxidation potential in the present case (see Scheme 1, compare paths b and e with path d); 2) the air-oxidation step is effective at ambient temperature (Scheme 1d), otherwise it required, for a series of substituted derivatives of 1, drastic conditions (T = 120 °C) when In(III) halides were used as catalysts.[Error: Reference source not found] Attempts to unambiguously identify aqueous W-compounds were not successful. However, it is well documented that W(VI) chloride complexes generally release all their ligands in water, the chlorides being eliminated as HCl,[30] then polynuclear tungstates are expected to form at acidic pH values.[31] The activity of these species seems sensitive to the pH of the medium: when the hydrolysis of the
cis-PhCH=CPhC(=O)OH/WCl6 reaction mixture was carried out with a diluted NaHCO3 aqueous
solution, a complicated mixture of organic products different from 2 was finally separated.
We studied also the reactions of WCl6 with differently substituted α,β-alkenyl carboxylic acids, using experimental conditions matching those employed for α-phenylcinnamic acid. However, these reactions stopped at the Cl/O exchange step, affording the corresponding carboxylic acid chlorides (see SI for details).
We have reported the synthesis of polycyclic compounds by domino reactions of commercially available α-phenylcinnamic acid (or its acyl chloride counterpart) triggered by a series of high valent transition metal halide systems, the choice of the latter leading to some structural diversity. In particular, WCl6 works as an efficient multitasking agent and permits the convenient, two-pots transformation of two equivalents of acid into compound 2, whose structure recalls that of several bioactive products, being the fusion between chromen-6-one and C-nor-D-homo-steroid skeletons. [32]
The synthesis of 2 requires a final Baeyer Villiger oxidation step, which is noticeably achieved exploiting air as oxygen source, in aqueous medium at ambient temperature, presumably assisted by tungstate derivatives. It must be mentioned that the Baeyer Villiger reaction is a key process for synthetic chemistry, and the sustainable conditions reported here may represent an interesting development.[33] Although the growing of the molecular complexity of α-phenylcinnamic acid is rather unique in the context of α,β-alkenyl carboxylic acids, our results might be reasonably extended to phenyl-substituted derivatives,[Error: Reference source not found] and confirm the strong potential of high valent transition metal halides in promoting valuable and convenient synthetic transformations. Further exploration of this field of chemistry is desirable and possibly beneficial to the knowledge and the advance of organic synthesis.
Supporting Information. Experimental details; Figures S1-S16: Views of DFT calculated
intermediates along the synthetic route to 1; X-Ray molecular structure of 2; NMR spectra. Cartesian coordinates of all DFT-optimized compounds are collected in a separated .xyz file. CCDC reference numbers 1843861 (1), 1843859 (2) [Error: Reference source not found] and 1843860 (3) contain the supplementary crystallographic data for the X-ray studies reported in this paper. These data can be obtained free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12, Union Road, CambridgeCB2 1EZ, UK; fax: (internat.) +44-1223/336-033; e-mail: deposit@ccdc.cam.ac.uk).
Acknowledgements.
The M.I.U.R. (Fondo di Finanziamento delle Attività Base di Ricerca, FFABR) is gratefully acknowledgegd for financial support.
Keywords: Baeyer Villiger oxidation; domino reactions; high valent transition metal halides;
α-phenylcinnamic acid; tungsten hexachloride.
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