Organic &Biomolecular Chemistry

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Cite this: DOI: 10.1039/c3ob40467f

Received 7th March 2013,Accepted 12th March 2013

DOI: 10.1039/c3ob40467f

www.rsc.org/obc

Towards a dynamic covalent molecular switch:substituent effects in chalcone/flavanone isomerism†

Jesse Mai, Ermal Hoxha, Caitlin E. Morton, Brian M. Muller and Marc J. Adler*

Chalcone/flavanone interconversion occurs facilely under

aqueous alkaline conditions making it a promising scaffold for the

development of a covalent molecular switch. In this study, a

single methoxy substituent is shown to have a significant impact

on the equilibrium dynamics of this reaction; this impact is depen-

dent on the site of substitution.

Molecules capable of predictable, reversible conformationalchanges have become increasingly desirable targets for syn-thetic organic chemists.1 Such molecules have been applied inthe fields of chemical sensing2 and neurobiology,3 amongothers. The preponderance of these compounds rely on eitherfast-equilibrating spontaneously dynamic non-covalent inter-actions or external stimuli, such as light.4

These so-called ‘molecular switches’ have also been used asquantitative tools to study a variety of non-covalent inter-actions.5 Notably, Wilcox’s torsion balance5c is a scaffoldcapable of quantifying edge-to-face pi stacking interactions, alesser-understood yet important contributor to protein folding.More recently, Hamilton has exploited an elegant diphenyl-acetylene-based switching system to explore several differentphenomena.5g–i

While several compounds have been developed for suchpurposes, to our knowledge none have taken advantage of theblossoming field of dynamic covalent chemistry6 to constructa scaffold that could establish the required binary equilibrium.This paper seeks to establish the hydroxychalcone (1A)/flava-none (2A) (Scheme 1) scaffold as a system of precisely suchcharacter, predictably respondent to changing pH.7 This isindeed a binary system, as the otherwise rotatable sigma bondin the chalcone state is highly favored towards the depicted

conformation due to its involvement in a tight hydrogen bondwith the proton of the phenol.8

The interconversion of hydroxychalcone and flavanone canbe considered to occur via the equilibrium depicted inScheme 2.7a–d At pH below ∼10, the energetic minimum of thesystem lies at the flavanone 2, as evidenced by the slow butcomplete conversion of hydroxychalcones to flavanones inacidic aqueous solutions. Above pH ∼13, the uncyclizedchalconate 3 predominates. In the interim pH range however,rapid reversible interconversion occurs between two or more ofthe states shown.

This intermediate-pH-range phenomenon was previouslystudied by Cisak and Mielczarek for the purpose of exploringthe kinetics and dynamics of the interconversion. Theirstudies suggested that the chalcone and flavanone werepresent in a 1 : 1 ratio at pH 11.8, with lower pH (down to 10)favoring the flavanone and higher pH (up to 13) shifting thebalance increasingly towards the chalcone.7e

As a first step towards developing molecular switches basedon this scaffold, we were interested in exploring if and how theincorporation of substituents on either aromatic ring wouldinfluence the equilibrium in a predictable manner. The

Scheme 1 Hydroxychalcone/flavanone isomerism.

Scheme 2 Mechanistic insight into the interconversion.

†Electronic supplementary information (ESI) available: General procedure forthe synthesis of 1A–1E, characterization data (1H, 13C, IR, and HRMS) for com-pounds 1A–1E, detailed procedures for UV/Vis studies, and absorbance plots forthe raw UV/Vis data. See DOI: 10.1039/c3ob40467f

Department of Chemistry & Biochemistry, Northern Illinois University,

1425 W. Lincoln Hwy., DeKalb, IL 60115, USA. E-mail: mjadler@niu.edu;

Fax: +1 815 753 4802; Tel: +1 815 753 9236

This journal is © The Royal Society of Chemistry 2013 Org. Biomol. Chem.

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figurative equivalence point reported by Cisak and Mielczarekprovided a straightforward manner with which the effect ofthese substituents could be gauged.

The impact of the pKa of the phenolic proton HA

(Scheme 2) has often been recognized as a key factor in thisequilibration; in the slow, irreversible conversion of chalconeto flavanone at low pH, it dictates the pace of conversion of 1to 3 and ultimately to the low energy isomer 2. However, atelevated pH, the pKa of HB could play a vital role, as it keepsthe gate for the reverse reaction of 2 to 3 as the pH isincreased. In general, it is our conjecture that factors thatlower the pKa of HA and/or lower the pKa of HB shift the inflec-tion point towards lower pH while contributions that raiseeither pKa will shift the equivalence point to a higher pH.

To investigate the effect of substitution on this scaffold, fivechalcones were synthesized and studied. In addition to theparent compound 1A, the 4′- (1B), 5′- (1C), 3- (1D), and 4-methoxy (1E) derivatives were prepared via a simple aldolcondensation of the appropriate hydroxyacetophenone andbenzaldehyde (Scheme 3).

Inspired by the method of Cisak and Mielczarek, the chal-cones were placed in 5% ethanol solutions at a variety of pHsbetween 10 and 13. It is worthy to note that in their originalstudy, it was demonstrated that the same results are acquiredwhether one begins with the chalcone or the flavanone; forsynthetic ease, we began all trials with the chalcone. After anequilibration period they were acidified and subsequently ana-lyzed by UV/Vis spectroscopy. Each chalcone isomer absorbsstrongly at its respective λmax and thus its presence is easy toobserve. When the intensity of the absorbance is plotted as afunction of pH, the data can be fitted to a sigmoidal curvewith a high correlation (r2 > 0.99). The inflection point of theresulting curve corresponds to the point at which the solutionis 50% chalcone (and consequently 50% flavanone).

Fig. 1 shows the normalized absorbance data and sigmoidalfitting of the UV/Vis data for compounds 1A–E and the pH atwhich each of these systems display a 1 : 1 ratio of chalcone toflavanone. The full dynamic pH range was captured for all syn-thesized compounds. These results clearly demonstrate thatthe balance of this system is susceptible to electronic altera-tion of either of the aromatic moieties.

In order to verify the validity of these measurements, a stan-dardization analysis was performed for the parent compound 1A(Fig. 2). In this experiment, known amounts of 1A and 2A wereplaced in pre-acidified (to prevent rapid equilibration) buffer

solutions of various original pHs. The results of this investi-gation show that not only is there a linear correlation betweenpercent chalcone and absorbance, but that the absorbance at50% chalcone (0.364) is nearly identical to the absorbance pre-dicted by the sigmoidal curve at precisely the midpoint pH.

It is beyond the scope of this study to quantitatively analyzethe perturbation of the putative equivalence pH due to substi-tution. However it is worthwhile to endeavor a qualitativeexploration of the factors that contribute to the position-dependent impact of the methoxy substituent on this complexequilibrium.

The methoxy substituent in 1B serves directly as an induc-tive electron-withdrawing group to the phenol, thus stabilizingthe chalconate (3) and presumably lowering the pKa of HA.

Fig. 1 Normalized UV/Vis absorbance data for compounds 1A–1E. The puta-tive equivalence point pH for each compound is noted in parentheses in the key.

Fig. 2 Method verification study of 1A. Each trial was performed with a pre-acidified buffer solution of the indicated pH to demonstrate that salt contentdid not affect absorbance.

Scheme 3 Synthesis of the studied chalcones.

Communication Organic & Biomolecular Chemistry

Org. Biomol. Chem. This journal is © The Royal Society of Chemistry 2013

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At the same time, its position relative to the carbonyl increasesthe strength of the hydrogen bond between the carbonyl andthe phenolic proton HA via conjugatively increasing the elec-tron density at the carbonyl carbon, thus potentially raisingthe pKa of HA. The effect on the pKa of proton HB, however, ismuch more straightforward to predict: adding electron densityto the carbonyl should result in an elevation of this value.

When the methoxy group is shifted one carbon over (1C),the opposite is expected. By increasing the electron density onthe arylic carbon bearing the hydroxy group (C1′), the oxanionis expected to be destabilized and thus the pKa of HA elevated.By virtue of the methoxy’s meta orientation to the carbonyl,the carbonyl-phenol hydrogen bond should be weakened, andthus result in an effect directly opposed to the first. Thisinductive withdrawal of electron density from the carbonyl in 2should serve to lower the pKa of HB.

The considerations are slightly different when the substi-tuent is moved to the other aromatic ring. In 1D, the methoxyis inductively electron withdrawing to the carbonyl, whichresults in a weakening of the hydrogen bond in 1 and also amore effective stabilization of the anion in 3. Its impact on thepKa of HB is simply to serve as a remote electron-withdrawinggroup, which will lower the expected pKa.

For 1E, the analysis of the pKa of HA is the inverse: themethoxy is now electron-donating, thus increasing thestrength of the hydrogen bond in 1 and decreasing the abilityof the carbonyl to attenuate the charge in 3. But as conjugativeeffects on the aromatic ring bearing the methoxy group in 1Ecannot affect the pKa of HB, we are left to assume that the electro-negativity of the methoxy oxygen will result again in areduction of the pKa of HB.

With this analysis in mind, the observed changes in pH atthe equivalence point can be used to qualitatively weight theposition-dependent impact of the methoxy group.

When the substituent resides on the acetophenone-derivedring, the determining factor seems to be the effect of the sub-stituent on the oxanion in compound 3. Thus the electron-withdrawing capacity of the methoxy (relative to the phenol) in1B lowers the pH at which the inflection point occurs, and theelectron-donation (again, relative to the phenol) by themethoxy in 1C raises this value.

However it appears as though a different effect takes pre-cedence when the substituent lies on the ring originating fromthe benzaldehyde. The equivalence point of both 1D and 1Edisplay shifts towards lower pH when compared to the parentcompound. While these two compounds are expected to affectthe pKa of HA differently, they are both expected to lower thepKa of HB; it would seem, in this case, that the effect on HB

predominates.These findings are encouraging for the utility of this

scaffold as a dynamic covalent molecular switch. The keyelement in the development of such a tool is the ability to biasthe system in one direction or another; in this case, the incor-poration of a mere single substituent (methoxy) at different

locations on the scaffold has resulted in a significant variance(0.75) of the equivalent point pH.

Further studies are already underway investigating variousother substituents and also the possibility of synergistic effectswhen both rings are substituted. At the very least, however,this study indicates that this scaffold, with its inherentcovalent dynamic equilibrium, is a promising platform for thedevelopment of a novel class of molecular switch.

Acknowledgements

This paper is dedicated to Prof. Andrew D. Hamilton on theoccasion of his 60th birthday. The authors would like to thankDr Heike N. Hofstetter (NIU) for technical consultation andhelpful conversation.

Notes and references

1 Molecular Switches, ed. B. L. Feringa and W. R. Browne,Wiley-VCH, Weinheim, Germany, 2nd edn, 2011.

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4 A.-M. Stadler and J. Ramírez, Top. Curr. Chem., 2012, 322,261–290.

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6 (a) S. J. Rowan, S. J. Cantrill, G. R. Cousins, J. K. Sanders andJ. F. Stoddart, Angew. Chem., Int. Ed., 2002, 41(6), 898–952;(b) J.-M. Lehn, Chem. Soc. Rev., 2007, 36, 151–160.

7 (a) J. J. P. Furlong and N. S. Nudelman, J. Chem. Soc., PerkinTrans. 2, 1985, 633–639; (b) C. O. Miles and L. Main,J. Chem. Soc., Perkin Trans. 2, 1989, 1623–1632;(c) N. S. Nudelman and J. J. P. Furlong, J. Phys. Org. Chem.,1991, 4, 263–270; (d) R. G. Button and P. J. Taylor, J. Chem.Soc., Perkin Trans. 2, 1992, 1571–1580; (e) A. Cisak andC. Mielczarek, J. Chem. Soc., Perkin Trans. 2, 1992, 1603–1607.

8 A. M. S. Silva, H. R. Tavares, A. I. N. R. A. Barros andJ. A. S. Cavaleiro, Spectrosc. Lett., 1997, 30(8), 1655–1667.

Organic & Biomolecular Chemistry Communication

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