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. 2021 Aug 25;11(46):28602-28613.
doi: 10.1039/d1ra05926b. eCollection 2021 Aug 23.

Trimerization and cyclization of reactive P-functionalities confined within OCO pincers

Beatrice L Chinen  1 , Jakub Hyvl  1 , Daniel F Brayton  1 , Matthew M Riek  1 , Wesley Y Yoshida  1 , Timothy W Chapp  2 , Arnold L Rheingold  3 , Matthew F Cain  1
Affiliations

Trimerization and cyclization of reactive P-functionalities confined within OCO pincers

Beatrice L Chinen et al. RSC Adv. .

Abstract

In order to stabilize a 10-P-3 species with C 2v symmetry and two lone pairs on the central phosphorus atom, a specialized ligand is required. Using an NCN pincer, previous efforts to enforce this planarized geometry at P resulted in the formation of a C s-symmetric, 10π-electron benzazaphosphole that existed as a dynamic "bell-clapper" in solution. Here, OCO pincers 1 and 2 were synthesized, operating under the hypothesis that the more electron-withdrawing oxygen donors would better stabilize the 3-center, 4-electron O-P-O bond of the 10-P-3 target and the sp3-hybridized benzylic carbon atoms would prevent the formation of aromatic P-heterocycles. However, subjecting 1 to a metalation/phosphination/reduction sequence afforded cyclotriphosphane 3, resulting from trimerization of the P(i) center unbound by its oxygen donors. Pincer 2 featuring four benzylic CF3 groups was expected to strengthen the O-P-O bond of the target, but after metal-halogen exchange and quenching with PCl3, unexpected cyclization with loss of CH3Cl was observed to give monochlorinated 5. Treatment of 5 with (p-CH3)C6H4MgBr generated crystalline P-(p-Tol) derivative 6, which was characterized by NMR spectroscopy, elemental analysis, and X-ray crystallography. The complex 19F NMR spectra of 5 and 6 observed experimentally, were reproduced by simulations with MestreNova.

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Conflict of interest statement

There are no conflicts to declare.

Figures

Fig. 1
Fig. 1. Structures of geometrically distorted A and B with contributing resonance structures B′ and B′′.
Fig. 2
Fig. 2. Simplified frontier orbital comparison between B′′ and an Ir(PCP) pincer fragment.
Scheme 1
Scheme 1. Synthesis of a benzazaphosphole “bell-clapper” with 10–Bi–3 species C shown in the inset.
Chart 1
Chart 1. The targeted 10–P–3 species and brominated OCO pincers 1 and 2.
Scheme 2
Scheme 2. Synthesis of 1.
Fig. 3
Fig. 3. X-ray crystal structure of 1. All bond lengths (Å) and angles (deg) can be found in the ESI.
Scheme 3
Scheme 3. Synthesis of 3 and a selected view of its 31P{1H} NMR spectrum (top) and related (RP)n oligomers (bottom).
Fig. 4
Fig. 4. X-ray crystal structure of 3. Selected bond lengths (Å) and angles (deg): P1–P2 = 2.194(2), P2–P3 = 2.217(2), P1–P3 = 2.237(2), P1–C1 = 1.839(5), P2–C27 = 1.853(5), P3–C53 = 1.850(5), P3–P2–P1 = 60.95(7), P1–P3–P2 = 59.02(7), P3–P1–P2 = 60.03(7).
Scheme 4
Scheme 4. Synthesis of 2.
Fig. 5
Fig. 5. X-ray crystal structure of 2. All bond lengths (Å) and angles (deg) can be found in the ESI.
Scheme 5
Scheme 5. Intramolecular cyclization of 4 to 5 and synthesis of crystalline derivative 6.
Fig. 6
Fig. 6. Experimental (red) and simulated (blue) 19F NMR signals at −68.9237 (A), −70.4093 (B), −73.3518 (C) and −76.2722 (D) for 5 at 282 MHz.
Fig. 7
Fig. 7. Experimental (red) and simulated (blue) 19F NMR signals at −69.4819 and −69.6176 (A), −73.9876 (B), and −76.5368 (C) for 6 at 282 MHz.
Fig. 8
Fig. 8. A selected view of the 13C{1H} NMR spectrum (C6D6) of 6 displaying the quaternary carbons.
Fig. 9
Fig. 9. X-ray crystal structure of 6. All bond lengths (Å) and angles (deg) can be found in the ESI. A picture of one of the large blocks is shown in the inset.
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