Domján, Attila

Dorkó, Éva

Kovács, Benjámin

Kótai, Bianka

Soós, Tibor

Ghent University Academic Bibliography

The idea of Frustrated Lewis pair (FLP) chemistry is based on the notion that the combinations ofLewis acids and bases which are sterically prevented from forming classical Lewis acid–baseadducts (Fig. 1) have Lewis acidity and basicity available for interaction with a third molecule. Thisconcept has been extended to provide homogeneous, metal-free catalysis including reactions likehydrogenation, hydroamination, and CO2 reduction [1,2]. Moreover, a certain group of triarylboranes substituted with F or Cl in each orto position (Fig. 2, the roman numbers refer to the totalnumber of fluorines), showed to be somewhat water tolerant as well, thus applicable e. g. forreductive amination [3] in which water is a byproduct. Since these boranes have at least oneassymetrically substituted ring, that means a multifarious environment for the accessing Lewis basewhat directly leads to the possibility for the complex to manifest multifarious conformation. (In ourwork piperidine was chosen as model amine). The steric and electronic relations always depend onthe pathway of adduction. Accordingly, since the Gibbs free energies for complexation are differentin each case we face a peculiar situation: multiple Lewis acid strengths can be derived for a singleLewis acid within the same reaction. Therefore, our main goal was to identify the conformers of theabove mentioned triaryl borane – piperidine adducts. Besides the theoretical delicacy (whatemerged from merely the definiton of Lewis acidity) the proof of the concept would bring us closerto influence the stereoselectivity of a reaction by designing the steric and electronic relationsproperly [4]. In addition, if these conformations are at dynamic equilibrium, the multiple acidstrengths of the Lewis acid are even oscillating in time. That would indirectly reflect the”frustrated” feature of the complex, thus may support the relative catalytic effectiveness of thetriaryl boranes in question. Herein the results for the borane II – piperidine system will be presentedin details, but a comparison with the adducts of boranes I and III will be given in the Conclusion.Introduction1 NMR and Structure Analysis Research Group, Department of Organic and Macromolecular Chemistry, Ghent University2 Hungarian Academy of Sciences, Research Centre for Natural Sciences, Institute of Organic Chemistry, Budapest, HungaryB. Kovács1, É. Dorkó2, B. Kótai2, A. Domján2, T. Soós2Is it possible that a Lewis acid has revolving acid strengths?”frustrated” stateclassical dative complex U0 rTable 1: the comparison of the theoretical (Xt) and observed (Xo) normalized Boltzmann factors (298 K) confirmed the assignmentΔEfMethods Theoretical: The experiments were preceded by Monte Carlo conformational search which was followed by ab initio (level oftheory: B3LYP-D3/6-311G(d,p)) geometry optimization. The respective calculated Gibbs free energies of adduction (ΔGa)characterized the stability of each complex conformation. Theoretical normalized Boltzmann factors could be derived fromthese values which – after their comparison with the observed ones – validated the assignment (Table 1). Another useful toolwas the calculation of chemical shielding properties (level of theory: B3LYP-D3/6-311+G(2d,p)). Experimental: For theassignment 19F spectra were recorded, and 19F DOSY and 19F-1H HOESY experiments were also carried out, while 2DEXSY measurements provided information about dynamical exchange phenomena.Resultspiperidine’saliphaticprotons (N)H andaryl protonsF2 (19F)/ppmF2 (19F)/ppmF1 (19F)/ppm--F1 (1H)/ppm2D 19F EXSY (303 K, mixing time: 800 ms)19F-1H HOESY (273 K, mixing time: 500 ms)Form ΔGa /kcal.mol–1 Xt Xo1 2,5 0,54 0,532 2,8 0,32 0,303 3,3 0,14 0,172d 3p 1d 1p3u 2u4”frustrated” state interconversion of adduct conformersFClBHNCdpdpuu2134ConclusionReferences: [1] Stephan, D. W., Acc. Chem. Res., 2015, 48, 306–316; [2] T. A. Rokob et al., Angew. Chem., Int. Ed. 2008, 47, 2435–2438; [3] É. Dorkó et al., unpublished; [4] Stephan, D. W., J. Am. Chem. Soc., 2015, 137, 10018–10032The revolving Lewis acidity of the chosen Lewis acid has been disclosed, since the theoretical prognostisations and experimental observationswere in great agreement. The ratio of the three borane II – piperidine adduct conformations suggests that the preferred pathway for the adductionis when three chlorines point towards the piperidine (since 1 is the most stable). Moreover, the ”frustrated” feature of the system was not merelypointed out indirectly by the exchange of the Lewis pair conformers, but also in a direct fashion, what never had been successful before usingNMR spectroscopy. The summarized investigations were performed with the borane III – piperidine and borane I – piperidine Lewis pairs aswell. Those adducts manifested four and three conformations respectively, and the ”frustrated” state for the former was also identifiable directly.On the timescales of the EXSY measurements (up to 1000 ms) no exchange phenomena could be discovered between the borane III – piperidinecomplex conformers. On the other hand, the borane I – piperidine Lewis pair is so floppy, that above 273 K the forms were still in fastexchange, so only their average was detectable. By these results we can say that for these complexes the level of ”frustration” is decreasing withthe number of the fluorines and that is perfectly reflected in their catalytic effectiveness (Fig. 2).Affiliation: Benjamin.Kovacs@UGent.beborane I borane II borane III Figure 2: the asymmetrically substituted boranes catalysed reductive amination though their effectiveness differed (up). The piperidine experiencemultifariuos environment when it is accessing towards these Lewis acids. Only the most stable borane conformations are depicted (down).: ? ? ? ? ? ? conversion:(80 oC, 24 h)test reaction [3]: benzylamine  benzaldehydeboraneI or II or III dibenzylamine +water20 bar H2toulene2% 10% 36%this work:adding Figure 1: Schematic potential curves illustrate the difference between the classical bonding and the ” frustrated” state [2]Results 2d3p1d4 ? 3u2u1pWe see that several peaks appeared in the 19F spectrum of the borane II – piperidine mixture, not only two. Each complexconformer (totally three, marked with arabic numbers) is represented by two peaks with matching intensities. The chemicalshifts (and partly the hyperfine structures) reflect the orientation of the respective fluorinated ring. If the fluorine is pointingupwards (”u”) to the H(N) of the piperidine, so forms hydrogen bond with it, a lower 19F chemical shift is experienced. If theadjacent chlorine does so (then the fluorine is pointing downwards: ”d”), or if the ring is remoted (perpendicular-like position:”p”) from the piperidine’s H(N), a higher chemical shift can be measured (check: figures in the red frame). The error of thecalculated 19F chemical shifts showed to be ring orientation dependent, and were consistently ~11 ppm for the ”d”, ~14 ppmfor the ”p” and ~18 ppm for the ”u” positions. (As that was the case for the borane I – piperidine and borane III – piperidinecomplex conformers as well.) There is a lone peak close to the position of the borane II’s (respective 19F spectrum recordedbeforehand), which gives weak correlations with the aliphatic protons of piperidine in the 19F-1H HOESY spectrum. Per theresults of the DOSY measurements (D4 < D1 ~ D2 ~ D3 < Dborane II) it likely represents the ”frustrated” state.19F spectrum (273 K)F–H(N) hydrogen bond: JHF= ~ 27 Hz F–F through space coupling: JFF= ~ 40 Hz CS of borane III = one FII = two FIII = three FResults After the assignment the 2D EXSY experiments disclosed exchange relations. In the summarizing schemewith the red frame the size of the black arrows between the identified structures qualitatively reflect therelative ratio of the exhange phenomena. The values were sort out by the stepwise raise of the mixingtime. The cross peaks indicating the same exchange were marked with the same color.

Is it possible that a Lewis acid has revolving acid strengths?

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Is it possible that a Lewis acid has revolving acid strengths?

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