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. 2020 Jun 25;11(27):7086-7091.
doi: 10.1039/d0sc02551h. eCollection 2020 Jul 21.

Pnictogen-bonding catalysis: brevetoxin-type polyether cyclizations

Andrea Gini  1 , Miguel Paraja  1 , Bartomeu Galmés  2 , Celine Besnard  1 , Amalia I Poblador-Bahamonde  1 , Naomi Sakai  1 , Antonio Frontera  2 , Stefan Matile  1
Affiliations

Pnictogen-bonding catalysis: brevetoxin-type polyether cyclizations

Andrea Gini et al. Chem Sci. .

Abstract

Pnictogen-bond donors are attractive for use in catalysis because of deep σ holes, high multivalency, rich hypervalency, and chiral binding pockets. We here report natural product inspired epoxide-opening polyether cyclizations catalyzed by fluoroarylated Sb(v) > Sb(iii) > Bi > Sn > Ge. The distinctive characteristic found for pnictogen-bonding catalysis is the breaking of the Baldwin rules, that is selective endo cyclization into the trans-fused ladder oligomers known from the brevetoxins. Moreover, tris(3,4,5-trifluorophenyl)stibines and their hypervalent stiborane catecholates afford different anti-Baldwin stereoselectivity. Lewis (SbCl3), Brønsted (AcOH) and π acids fail to provide similar access to these forbidden rings. Like hydrogen-bonding catalysis differs from Brønsted acid catalysis, pnictogen-bonding catalysis thus emerges as the supramolecular counterpart of covalent Lewis acid catalysis.

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Figures

Fig. 1
Fig. 1. (a) Region of interest in the periodic table. (b and c) General structure of tetrel and pnictogen-bond donors (D; DIII: trivalent; DV: pentavalent) interacting with their acceptors (A); blue circles, σ holes; red orbitals, lone pairs. (d) Pnictogen-bonding catalysis defined as a non-covalent counterpart of (e) Lewis acid catalysis (La), analogous to (f) hydrogen-bonding and (g) Brønsted acid catalysis (Ba, conjugate base: Bb–). S, substrate; P, product; etc.*: ligand (L) exchange, proton release from S upon addition to La, etc.
Fig. 2
Fig. 2. Catalyst synthesis and structures (a), with MEP and crystal structures (b and c); distances in Å; *: positive MEP maxima (kcal mol–1), corresponding to σ holes. Dashed circle: inaccessible σ hole. (i) nBuLi, Et2O, –78 °C to rt, 12 h, 45%; (ii) CH2Cl2, rt, 10 min, 78%.
Fig. 3
Fig. 3. Cyclization of (a) monomers 16–19 and (b) dimers 28 with representative 1H NMR spectrum of a product mixture generated for 28 with 1. (c) NMR fingerprints for 28 cyclized with AcOH, SbCl3, 1 and 2. (d) 1H NMR spectrum and (e) crystal structure of trans,trans (AA)-33.
Fig. 4
Fig. 4. (a) Cascade cyclization of 34 and 35 with pure B and A oligomers shown as extreme products. (b) 1H NMR and (c) GC fingerprints of 34 converted with AcOH, SbCl3, 1 and 2 (c, Baldwin products: tR 6.5–6.9 min, anti-Baldwin: tR > 6.9 min, substrates: tR < 4.0 min).
Fig. 5
Fig. 5. BP86-D3/def2-TZVP optimized intermediates with (a) 19 bound to 3, (b) 35 to 1, and (c) 34 to 2, with schematic drawings, distances in Å, polyepoxide foldamers in (b) carousel-like with parallel and (c) snake-like conformation with antiparallel epoxides.
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