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. 2019 Jun 18;10(30):7177-7182.
doi: 10.1039/c9sc02463h. eCollection 2019 Aug 14.

Tetragonal phosphorus(v) cations as tunable and robust catalytic Lewis acids

James C Gilhula  1 , Alexander T Radosevich  1
Affiliations

Tetragonal phosphorus(v) cations as tunable and robust catalytic Lewis acids

James C Gilhula et al. Chem Sci. .

Abstract

The synthesis and catalytic reactivity of a class of water-tolerant cationic phosphorus-based Lewis acids is reported. Corrole-based phosphorus(v) cations of the type [ArP(cor)][B(C6F5)4] (Ar = C6H5, 3,5-(CF3)2C6H3; cor = 5,10,15-(C6H5)3corrolato3-, 5,10,15-(C6F5)3corrolato3-) were synthesized and characterized by NMR and X-ray diffraction. The visible electronic absorption spectra of these cationic phosphacorroles depend strongly on the coordination environment at phosphorus, and their Lewis acidities are quantified by spectrophotometric titrations. DFT analyses establish that the character of the P-acceptor orbital comprises P-N antibonding interactions in the basal plane of the phosphacorrole. Consequently, the cationic phosphacorroles display unprecedented stability to water and alcohols while remaining highly active and robust Lewis acid catalysts for carbonyl hydrosilylation, Csp3 -H bond functionalization, and carbohydrate deoxygenation reactions.

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Figures

Scheme 1
Scheme 1. (top) Electrophilic phosphonium cations and their hydrolytic decomposition to phosphine oxides. (bottom) Notional tetragonal electrophilic phosphonium cations that enhance robustness but preserve Lewis acidity.
Fig. 1
Fig. 1. (a) Synthetic path to phosphacorroles 1+–4+. (i) PhPCl4 or 3,5-(CF3)2C6H3PCl4, Et3N, PhMe, Δ, 1 h; [Bu4N][BH(OAc)3], PhMe, RT, overnight (ii) [Ph3C][B(C6F5)4], CH2Cl2, RT, 5 min. (b) X-ray structure of 1+. Counterions and hydrogen atoms are omitted for clarity. Thermal ellipsoids are rendered at the 50% probability level. Selected bond lengths [Å], angles [°], and dihedrals [°]: P(1)–Navg 1.804(6), P(1)–C(1) 1.819(3), N(1)–P(1)–N(3) 158.1(1), N(2)–P(1)–N(4) 152.9(1), N(1)–N(2)–N(3)–N(4) –3.38(1). (c) X-ray structure of 4+. Counterions and hydrogen atoms are omitted for clarity. Thermal ellipsoids are rendered at the 50% probability level. Selected bond lengths [Å], angles [°], and dihedrals [°]: P(1)–Navg 1.792(3), P(1)–C(1) 1.817(2), N(1)–P(1)–N(3) 158.21(7), N(2)–P(1)–N(4) 155.48(2), N(1)–N(2)–N(3)–N(4) 1.90(1).
Fig. 2
Fig. 2. (top) Q-band region of 4+ upon titration with 0 to 2 equiv. of (n-octyl)3PO in CH2Cl2. (bottom) Binding isotherms for (n-octyl)3PO with 4+ (red, circle), 3+ (blue, square), 2+ (green, triangle), and (inset) 1+ (black, diamond).
Fig. 3
Fig. 3. Kohn–Sham orbital LUMO+3 for 4+ (top: in perspective; bottom: side-on view) computed at the B3LYP/def2-TZVP//B3LYP/def2-SVP level.
Scheme 2
Scheme 2. Reaction of water with 3+ and formation of putative water adduct 3+·OH2. Deprotonation by MgSO4 gives product 3·OH, which is reactivated to 3+ by TMS–OTf.
Scheme 3
Scheme 3. (a) Reaction was performed on a 1.0 mmol scale. Isolated yield is reported. (b) Reaction was performed with 0.1 mmol of 13C6-glucose. Quantitative 13C NMR yields are reported. The ratio of products is 27 : 22 : 14 : 4 n-hexane : 3-methylpentane : 2-methylpentane : hexenes.
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