Received 19th November 2012,Accepted 16th January 2013
DOI: 10.1039/c3ob27244c
www.rsc.org/obc
A new family of bis-ureidic receptors forpyrophosphate optical sensing†
Claudia Caltagirone,*a Carla Bazzicalupi,b Francesco Isaia,a Mark E. Light,c
Vito Lippolis,a Riccardo Montis,a Sergio Murgia,a Martina Olivaria andGiacomo Piccia
A new family of bis-ureidic receptors (L1–L6) has been synthesised. The binding properties of L1–L6
towards different anions (acetate, benzoate, glutarate, malonate, dihydrogen phosphate, hydrogen pyro-
phosphate, triphosphate, AMP and ADP) have been studied by means of 1H-NMR, UV-Vis and
fluorescence spectroscopies and a remarkable affinity for HPpi3− has been observed in the case L3 (in
DMSO-d6 and DMSO-d6–5% H2O) which also acts as a fluorimetric chemosensor, even to the naked eye,
for this anion. Theoretical calculations helped us explain the binding properties observed.
Introduction
Anion recognition and sensing has recently received consider-able attention because of the central role played by anions inbiological, industrial, and environmental processes.1
Colorimetric chemosensors for anions are of particularinterest because they can allow naked eye recognition andsensing, while fluorescent chemosensors offer many advan-tages over other types of chemosensors in terms of sensitivity,response time, and cost. Among anions, pyrophosphate (Ppi,this acronym with omitted charges will be used throughoutthis paper when referring to pyrophosphate independently ofits protonation degree) is a biologically important targetbecause it is the product of ATP hydrolysis under cellular con-ditions.2 Moreover, the detection of pyrophosphate hasbecome important in cancer research as telomerase (a bio-marker for cancer diagnosis) activity is measured by evaluatingthe amount of Ppi generated in the PCR amplification of thetelomerase elongation product.3 Furthermore, the high level ofPpi in synovial fluids is correlated to calcium pyrophosphatedehydrate disease (CPDD), a rheumatologic disorder.4 Forthese reasons the detection and discrimination of
pyrophosphate, especially by means of fluorescent chemo-sensors, has attracted the attention of chemists over the last 20years.5 One of the first examples has been reported by Czarnikin 1994.6 Czarnik’s system is based on an anthracene deriva-tive bearing polyamine pendant arms and it is able to bind Ppiin water at pH 7 and to discriminate pyrophosphate over phos-phate. Different strategies can be used to design chemosensorsfor anions (and Ppi, in particular) involving, for example,excimer formation7 and the displacement approach.8 In par-ticular, different systems containing metal ion complexes,9 orguanidinium10 or imidazolium11 based platforms have beensynthesised and studied. Also neutral receptors containing pyr-roles able to recognise pyrophosphate both in aqueous12 andnon-aqueous environments13 have been successfully devel-oped. However, fewer examples of urea/thiourea containingreceptors for pyrophosphate recognition have been reported inthe literature so far.14 In this context we decided to synthesiseand study a new family of bis-ureidic receptors based on 1,3-bis(aminomethyl)benzene and 2,6-bis(aminomethyl)pyridine.In 1995 Umezawa and co-workers15 reported on bis-ureas andthioureas obtained by the reaction of 1,3-bis(aminomethyl)-benzene with butyl isocyanate and butyl isothiocyanate whichshowed a remarkable affinity for H2PO4
− (Ka = 820 M−1 for thethiourea derivative) in DMSO-d6. The same group also studiedthe application of the phenyl-substituted thiourea as an iono-phore for an anion selective electrode with remarkable sulfateselectivity.16 More recently Sellergren and co-workers studiedthe recognition of tyrosine phosphorylated peptides by thevinylphenyl urea derivative of 1,3-bis(aminomethyl)benzene.17
However, to the best of our knowledge, examples of colori-metric and fluorimetric chemosensors based on 1,3-bis-(aminomethyl)benzene or 2,6-bis(aminomethyl)pyridine plat-forms have not been reported so far.
†Electronic supplementary information (ESI) available: Additional informationas noted in the text including synthetic details for the preparation of L1 and L2.CCDC 911916. For ESI and crystallographic data in CIF or other electronicformat see DOI: 10.1039/c3ob27244c
aDipartimento di Scienze Chimiche e Geologiche, Università degli Studi di Cagliari,
S.S. 554 Bivio per Sestu, Monserrato (CA), 09042, Italy.
E-mail: [email protected]; Fax: +(39) 0706754456; Tel: +(39) 0706754452bDipartimento di Chimica “Ugo Schiff”, Università degli Studi di Firenze, Via della
Lastruccia 3, Sesto Fiorentino (FI), 50019, ItalycSchool of Chemistry, University of Southampton, Southampton, SO17 1BJ, UK.
We designed and successfully synthesised L1–L6 using thereaction scheme shown in Scheme 1. In the case of L1–L3 thecommercially available 1,3-bis(aminomethyl)benzene reactedwith the suitable isocyanate (phenyl isocyanate, 2-nitrophenylisocyanate, and 1-naphthyl isocyanate) in dichloromethane atroom temperature under nitrogen to give the three receptorsin 70–80% yield.
In the case of L4–L6 we first synthesised the 2,6-bis(amino-methyl)pyridine hydrochloridric salt by a Delepine reaction of2,6-bis(bromomethyl)pyridine obtained from 2,6-bis(hydroxy-methyl)pyridine by bromination with HBr and PBr3.
18 1,3-Bis-(aminomethyl)pyridine hydrochloridric salt was reacted withthe suitable isocyanate (phenyl isocyanate, 2-nitrophenyl iso-cyanate, and 1-naphthyl isocyanate) in refluxing dichloro-methane in the presence of triethylamine to obtain L4–L6 in40–50% yield (see ESI†).
In this way we built up a family of ligands comprising twovirtually “optically innocent” receptors (L1 and L4), two
potential colorimetric sensors (L2 and L5), and two potentialfluorimetric sensors (L3 and L6).
Firstly, we performed anion-binding studies by means of1H-NMR titrations in DMSO-d6. The EQNMR program19 wasused to calculate stability constants from the 1H-NMR titrationcurves obtained (see ESI†) by fitting the data to a 1 : 1 bindingmodel as shown in Table 1.
As shown in Table 1 the six receptors showed a medium–
low affinity for all the anions considered except for dihydrogenphosphate (in the case of L4) and hydrogen pyrophosphate inthe case of L1 and L3.
In particular, L4–L6 immediately deprotonated (both ureidicNHs) after the addition of 0.3 equivalents of HPpi3−. L2, on theother hand, underwent a partial deprotonation of the ureidicNHs adjacent to the nitro group (in green in Fig. 1) in the pres-ence of 0.3 equivalents of the anion. The signals of the otherNHs (red) and of the phenyl CH (orange) progressively shifteddownfield until one equivalent of the HPpi3− anion wasadded. Beyond this point also the signal of the ureidic protonsadjacent to the aromatic ring disappeared (Fig. 1).
In the case of L1 it was possible to follow the downfieldshift of the two NHs signals (green and red in Fig. 2) and ofthe phenyl CH signal (orange) up to the addition of two
Scheme 1 Synthesis of L1–L6. Fig. 1 1H-NMR stack plot of a DMSO-d6 solution of L2 (0.005 M) upon additionof tetrabutylammonium hydrogen pyrophosphate (0.075 M) in DMSO-d6.
Table 1 Equilibrium constants (Ka/M−1) for the reactions of L1–L6 with the tetrabutylammonium salts of the anion considered in DMSO-d6 at 300 K. All errors are
equivalents of the anion. A further excess of anion caused thedisappearance of the two NHs signals (Fig. 2).
L3 showed different behaviour compared to L1 and L2.‡ Infact, after addition of 0.3 equivalents of HPpi3− we observedthe broadening and the downfield shift of the peak attributedto the NHs adjacent to the naphthylic group (green in Fig. 3);however this signal could be followed until the end of the titra-tion experiment (over five equivalents of anion added). Thesignal of the other ureidic NHs (red in Fig. 3) also shiftedupon addition of the anion without broadening. Differently towhat was observed with L1 and L2, in the case of L3 we couldnot observe a shift of the phenyl CH singlet. Instead, a doubletattributed by 1H-COSY to the naphthylic CH pointing towardsthe pseudo-cavity of the receptor (blue in Fig. 3) considerablyshifted downfield upon addition of the anion.
We also performed titrations of L3 in the presence of the tri-phosphate anion (P3O10
5−), AMP− and ADP− (as their sodiumsalts) (Table 2).
Only with AMP− we observed significant interactions (Ka =369 M−1) while with triphosphate and ADP− the changes in the1H-NMR spectrum of L3 were negligible. The affinity of L3 forpyrophosphate was also tested in a more competitive solventmixture, i.e. DMSO-d6–5% H2O: under these experimental con-ditions the stability constant calculated for the formation of
the 1 : 1 adduct of L3 with HPpi3− was 570 M−1. This value ishowever higher than those calculated for the formation con-stants of the 1 : 1 adducts of L3 with the other anions con-sidered under the same experimental conditions.
This interesting behaviour of L3 could be explained consid-ering the important contribution given by the naphthylic CHin assisting the interaction of the receptor with HPpi3−. Anadditional proof of the intervention of this proton in anionbinding was brought by the X-ray crystal structure obtained forthe 1 : 2 L6/BzO− adduct obtained from L6 in the presence ofan excess of tetrabutylammonium benzoate by slow evapor-ation of ethyl acetate (Fig. 4). In contrast to what was observedin solution (see Table 1) a complex with a 1 : 2 stoichiometrywas isolated in the solid state where each anion interacts withthe receptor via two hydrogen bonds (N1–H901⋯O3 2.816(4);N2–H902⋯O4 2.809(4); N4–H904⋯O5 2.831(4); N5–H905⋯O62.760(4) Å) and via a weaker interaction with the naphthylicCH (C28–H28⋯O6 3.164 and C2–H2⋯O3 3.136 Å).
In order to better understand the interactions betweenpyrophosphate and L3, theoretical calculations were carriedout. Molecular modelling investigations on the adductsformed by L1 and L3 with HPpi3−, and by L3 with H2PO4
− andP3O10
5− in a 1 : 1 ligand-to-anion molar ratio have been per-formed by means of an empirical force field method(AMBER3),20 evaluating the atomic partial charges at the PM3
Fig. 2 1H-NMR stack plot of a DMSO-d6 solution of L1 (0.005 M) upon additionof tetrabutylammonium hydrogen pyrophosphate (0.075 M) in DMSO-d6.
Fig. 3 1H-NMR stack plot of a DMSO-d6 solution of L3 (0.005 M) upon additionof tetrabutylammonium hydrogen pyrophosphate (0.075 M) in DMSO-d6.
Table 2 Equilibrium constants (Ka/M−1) for the reactions of L3 with tetrabutyl-
ammonium salts of hydrogen pyrophosphate, dihydrogen phosphate, acetateand benzoate and the sodium salt of AMP in DMSO-d6 and DMSO-d6–0.5%H2O at 300 K. All errors are estimated to be ≤14% (see ESI)
‡Crystal data were collected on a Rigaku AFC12 equipped with a Saturn724+detector mounted at the window of an FR-E+ molybdenum rotating anode gen-erator Mo-Kα radiation (λ = 0.71075). Standard solution and refinement pro-cedures were followed, with the exception that the Squeeze algorithm17 wasapplied to model unresolvable disordered solvent. Crystal data for L1:C18H14N4O; M = 302.33, T = 100(2) K, hexagonal, space group R3̄, a = 32.989(7) Å,c = 7.098(2) Å, V = 6689(3) Å3, ρc = 1.351 g cm−3, μ = 0.088 mm−1, Z = 18, reflec-tions collected: 2613, independent reflections: 2613 (Rint = 0.0000), final R
semi-empirical level of theory21 and using an implicit simu-lation of the solvent environment (ε = 47 Fm−1). The potentialenergy surface of all the systems has been explored by means ofsimulated annealing (T = 600 K, equilibration time = 10 ps, runtime = 10 ps and cooling time = 10 ps, time step = 1.0 fs). Foreach studied system, 80 conformations have been sampled.
Two main results can be derived from our molecular model-ling investigations. First of all, the binding between the L3
ligand and the studied substrates is not only due to H-bondinteractions. Actually, the number of H-bonds established bythe receptors grows with the number of acceptor groups of theanions and with their charge, so that the triphosphate aniongives rise to the same or even higher number of H-bonds withrespect to the HPpi3− anion, which forms the most stableadduct with both L1 and L3 (see Fig. 5).
On the other hand, differently from P3O105− and H2PO4
−,only the HPpi3− ion is capable of fitting into the pseudo-cavitydefined by the lateral arms of the receptor. As a consequence,entropic effects due to desolvation upon host–guest inter-actions are supposed to play an important role in determiningthe observed stability trend.
The second significant result is relative to the comparisonof the low energy conformers calculated for the HPpi3−/L1
Fig. 4 X-ray crystal structure of the [L6(BzO)2]2− adduct showing the main
interactions with the numbering scheme adopted. Hydrogen atoms exceptthose involved in hydrogen bonds and tetrabutylammonium counter ions havebeen omitted for clarity.
Fig. 5 Lowest energy conformers for L3 in adduct with H2PO4− (top), HPpi3
−
(middle) and P3O105− (bottom).
Fig. 6 Conformers found for L1 in the adduct with HPpi3− with their relativeabundances.
adduct with those calculated for the HPpi3−/L3 adduct. Actu-ally, in all the conformations calculated for HPpi3−/L1 (Fig. 6)the phenyl groups are oriented in such a way that no inter-actions are observed between the aromatic protons and theoxygen of pyrophosphate, while in the case of the HPpi3−/L3
adduct a CH⋯O interaction is formed in all the low energyconformations calculated (Fig. 7).
A UV-Vis spectroscopy study was performed in DMSO inorder to evaluate the potential application of L2 and L5, bothcontaining two nitrophenyl units, as colorimetric sensors foranion recognition. In DMSO L2 as a free receptor showed anintense band in the UV region at 277 nm (ε = 11 760 mol−1
cm−1 L−1) and one less intense in the visible region at380 nm (ε = 7260 mol−1 cm−1 L−1). No significant changes inthe UV-Vis spectrum of L2 were observed upon addition ofacetate, benzoate, glutarate, malonate and dihydrogen phos-phate. Only in the case of HPpi3− we observed some
remarkable changes. As shown in Fig. 8a upon addition ofincreasing amounts of the anion we observed the disappear-ance of the band at 277 nm and the decrease of the band at380 nm with a concomitant hypsochromic shift of 25 nm, thatcould be attributed either to the partial deprotonationobserved in the 1H-NMR titration or to an interaction viahydrogen bonding. In fact, as has been recently reported byGunnlaugsson and co-workers,22 high concentrations (asthose employed during 1H-NMR titrations) could lead to adeprotonation while under more diluted conditions aneffective anion binding could be observed. Receptor L5 showedonly one absorption band at 345 nm (ε = 4460 mol−1
cm−1 L−1). The addition of HPpi3− caused the formation oftwo new bands, one intense at 284 nm (ε = 25 580 mol−1 cm−1
L−1) and the other at 490 nm (ε = 5220 mol−1 cm−1 L−1), asshown in Fig. 8b. This latter could be probably attributed tothe full deprotonation of the receptor.23 In both cases theinteraction with HPpi3− could be detected by the naked eyewith a colour change from yellow (free receptors) to red-orange
Fig. 7 Conformers found for L3 in the adduct with HPpi3− with their relativeabundances.
Fig. 8 Changes in the fluorescence spectra of L2 (a) (1.50 × 10−4 M) and L5 (b)(8.5 × 10−5 M) upon addition of increasing amounts of (TBA)3HPpi (2.5 × 10−2
M) in DMSO. Inset: colour change of L2 (a) and L5 (b) (both 0.005 M) uponaddition of five anion equivalents (0.075 M) in DMSO. From left to right: L2 (a)or L5 (b), HPpi3−, AcO−, BzO−, H2PO4
(receptors in the presence of 1 equivalent of HPpi3−) as shownin the inset in Fig. 8a and 8b for L2 and L5, respectively.
The behaviour of L3 and L6 towards the chosen set ofanions was studied by spectrofluorimetric titrations in DMSO.L3 shows an emission band at 380 nm (Φ = 0.46) when excitedat 345 nm. This band can be attributed to the emission of asingle fragment urea substituted naphthalene according to thedata reported in the literature.24 No significant changes wereobserved in the presence of all anions except hydrogenpyrophosphate.
Indeed, as shown in Fig. 9a in the case of L3 uponaddition of increasing amounts of HPpi3− a quenching ofthe emission band of the receptor was accompanied by theformation of a new band at 500 nm. This new bandcould probably originate from the intramolecular interactionin the excited state of the two naphthalene moieties.This interaction could be favoured by the presence of the co-ordinated anion as also suggested by theoretical calculations(see Fig. 7).
Competitive studies confirmed the selectivity of L3 forHPpi3− (see ESI†).
Similar behaviour was observed in the case of L6. Thisreceptor when excited at 345 nm showed an emission band at375 nm (Φ = 0.56). Upon addition of HPPi3− we observed thedecrease of the band relative to the free receptor and the for-mation of a band at 495 nm. Also in this case, although the1H-NMR suggested a deprotonation event, the changes in thefluorescence spectrum of L6 could be attributed to the bindingevent (Fig. 9b).
Further experimental evidence of the binding event vs.deprotonation was provided by recording the emission spectraof a 1 : 1 solution of L3 and L6 and TBAOH in DMSO (see ESI†)in which we did not observe any emission at around 500 nm.
In conclusion we synthesised a new family of bis-ureidicanion receptors based on the two 1,3-bis(aminomethyl)-benzene and 2,6-bis(aminomethyl)pyridine platforms. Wehave demonstrated that by simply changing the substituentsin the pendant ureidic moieties both a colorimetric and afluorimetric selective sensing of hydrogen pyrophosphate,even to the naked eye, can be achieved in DMSO. In particular,in the case of the receptors bearing naphthyl group pendantarms, both binding and optical fluorimetric selectivity isachieved thanks to the uncommon interaction of an aromaticCH from the fluorophore with the HPpi3− guest.
We would like to thank MIUR (Ministero dell’Istruzionedell’Università e della Ricerca Scientifica) for financial support(Project PRIN 2009 – 2009Z9ASCA) and Regione Autonomadella Sardegna (Programma Master & Back) for funding a fel-lowship to Riccardo Montis.
Notes and references
1 (a) J. L. Sessler, P. A. Gale and W. S. Cho, Anion ReceptorChemistry, The Royal Society of Chemistry, Cambridge, UK,2006; (b) M. Wenzel, J. R. Hiscock and P. A. Gale, Chem.Soc. Rev., 2012, 41, 480; (c) P. A. Gale and T. Gunnlaugsson(guest editors), Supramolecular chemistry of anionicspecies themed issue, Chem. Soc. Rev., 2010, 39;(d) C. Caltagirone and P. A. Gale, Chem. Soc. Rev., 2009, 38,520; (e) P. A. Gale, S. E. Garcìa-Garrido and J. Garric, Chem.Soc. Rev., 2008, 37, 151; (f ) R. Martinez-Manez andF. Sancenon, Chem. Rev., 2003, 103, 4419;(g) H. Ait-Haddou, J. J. Lavigne and E. V. Anslyn, Acc. Chem.Res., 2001, 34, 963.
2 (a) H. N. Lee, Z. Xu, S. K. Kim, K. M. K. Swamy, Y. Kim,S.-J. Kim and J. Yoon, J. Am. Chem. Soc., 2007, 129, 3828;(b) C. P. Mathews and K. E. van Hold, Biochemistry, TheBenjamin/Cummings Publishing Co., Inc., Redwood City,CA, 1990.
3 S. Xu, M. He, H. Yu, X. Cai, X. Tan, B. Lu and B. Shu, AnalBiochem., 2001, 299, 188.
4 (a) M. Doherty, C. Becher, M. Regan, A. Jones andJ. Ledingham, Ann. Rheum. Dis., 1996, 66, 432;
Fig. 9 Changes in the fluorescence spectra of L3 (a) (1.50 × 10−4 M) and L6 (b)(8.5 × 10−5 M) upon addition of increasing amounts of (TBA)3HPpi (2.5 × 10−2
M) in DMSO. Inset: emission change of L3 (a) and L6 (b) (both 0.005 M) uponaddition of five anion equivalents (0.075 M) in DMSO. From left to right: L3 (a)or L6 (b), HPpi3−, AcO−, BzO−, H2PO4
(b) A. E. Timms, Y. Zhang, R. G. Russell and M. A. Brown,Rheumatology, 2002, 41, 725.
5 S. K. Kim, D. H. Lee, J.-I. Hong and J. Yoon, Acc. Chem.Res., 2009, 42, 23.
6 D. H. Vance and A. W. Czarnik, J. Am. Chem. Soc., 1994,116, 9397.
7 (a) H. K. Cho, D. H. Lee and J.-I. Hong, Chem. Commun.,2005, 1690; (b) H. N. Lee, Z. Xu, S. K. Kim, K. M. K. Swamy,Y. Kim, S.-J. Kim and J. Yoon, J. Am. Chem. Soc., 2007, 129,3828.
8 (a) L. Fabbrizzi, N. Marcotte, F. Stomeo and A. Taglietti,Angew. Chem., Int. Ed., 2002, 41, 3811;(b) M. J. McDonough, A. J. Reynolds, W. Y. G. Lee andK. A. Jolliffe, Chem. Commun., 2006, 2971; (c) D. H. Lee,S. Y. Kim and J.-I. Hong, Tetrahedron Lett., 2007, 48, 4477.
9 (a) S. Mizukami, T. Nagano, Y. Urano, A. Odani andK. Kikuchi, J. Am. Chem. Soc., 2002, 124, 3920;(b) D. H. Lee, J. H. Im, S. U. Son, Y. K. Chung andJ.-I. Hong, J. Am. Chem. Soc., 2003, 125, 7752; (c) D. H. Lee,S. Y. Kim and J.-I. Hong, Angew. Chem., Int. Ed., 2004, 43,4777; (d) J. H. Lee, J. Park, M. S. Lah, J. Chin and J.-I. Hong,Org. Lett., 2007, 9, 3729; (e) J. F. Zhang, S. Kim, J. H. Han,S.-J. Lee, T. Pradhan, Q. Y. Cao, S. J. Lee, C. Kang andJ. S. Kim, Org. Lett., 2011, 13, 5294.
10 (a) S. Nishizawa, Y. Kato and N. Teramae, J. Am. Chem. Soc.,1999, 121, 9463; (b) Y. Sun, C. Zhong, R. Gong and E. Fu,Org. Biomol. Chem., 2008, 6, 3044.
11 (a) J. Y. Kwon, N. J. Singh, H. Kim, S. K. Kim, K. S. Kim andJ. Yoon, J. Am. Chem. Soc., 2004, 126, 8892; (b) J. Yoon,S. K. Kim, N. J. Singh, J. W. Lee, Y. J. Yang, K. Chellappanand K. S. Kim, J. Org. Chem., 2004, 69, 581; (c) S. K. Kim,N. J. Singh, S. J. Kim, H. G. Kim, J. K. Kim, J. W. Lee,K. S. Kim and J. Yoon, Org. Lett., 2003, 5, 2083;(d) S. K. Kim, N. J. Singh, J. Kwon, I.-C. Hwang, S. J. Park,K. S. Kim and J. Yoon, Tetrahedron, 2006, 62, 6065.
12 (a) D. Aldakov, M. A. Palacios and P. Anzenbacher Jr.,Chem. Mater., 2005, 17, 5238; (b) D. Aldakov andP. Anzenbacher Jr., J. Am. Chem. Soc., 2004, 126, 4752;(c) D. Aldakov and P. Anzenbacher Jr., Chem. Commun.,2003, 1394.
13 P. Anzenbacher Jr., K. Jursìkovà and J. L. Sessler, J. Am.Chem. Soc., 2000, 122, 9350.
14 (a) J. Y. Kwon, Y. J. Yang, S. K. Kim, K.-H. Lee, J. S. Kim andJ. Yoon, J. Org. Chem., 2004, 69, 5155; (b) T. Gunnlaugsson,A. P. Davis, J. O. Brien and M. Glynn, Org. Biomol. Chem.,2005, 3, 48.
15 (a) S. Nishizawa, P. Bühlmann, M. Iwao and Y. Umezawa,Tetrahedron Lett., 1995, 36, 6483; (b) S. Nishizawa,P. Bühlmann, K. P. Xiao and Y. Umezawa, Anal. Chim. Acta,1998, 358, 35.
16 P. S. Corbin, S. C. Zimmermann, P. A. Thiessen,N. A. Hawryluk and T. J. Murray, J. Am. Chem. Soc., 2001,123, 10475.
17 M. Emgenbroich, C. Borrelli, S. Shinde, I. Lazraq, F. Vilela,A. J. Hall, J. Oxelbark, E. De Lorenzi, J. Courtois,A. Simanova, J. Verhage, K. Irgum, K. Karim andB. Sellergren, Chem.–Eur. J., 2008, 14, 9516.
18 C. Nolan and T. Gunnlaugsson, Tetrahedron Lett., 2008, 49,1993.
19 M. J. Hynes, J. Chem. Soc., Dalton Trans., 1993, 311.20 Hyperchem Release 7.51 for Windows MM System, Hypercube,
Inc., Gainesville, FL, 2002.21 (a) J. J. P. J. Stewart, Comput. Chem., 1989, 10, 209–220;
(b) J. J. P. J. Stewart, Comput. Chem., 1989, 10, 221–264.22 R. M. Duke, T. McCabe, W. Schmitt and T. Gunnlaugsson,
J. Org. Chem., 2012, 77, 3115.23 M. Boiocchi, L. Del Boca, D. Esteban-Gomez,
L. Fabbrizzi, M. Licchelli and E. Monzani, Chem.–Eur. J.,2005, 11, 3097.
