Project:  Hypervalent P

1.  Phosphinidene (R–P) Generation:  Within NCN Pincer Ligands

Our first paper in which we tried to isolate a phosphinidene within an NCN pincer resulted in the discovery of an aromatic azaphosphole "bell-clapper". Dynamic scaffolds of this nature are rare, but were observed by J.C Martin (in the 1970s and 80s) during his investigations to isolate molecules featuring pentavalent carbons!  Check it out!

   

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Our preliminary communication is reported here:

Hyvl, J.; Yoshida, W.Y.; Rheingold, A.L.; Hughes, R.P.; Cain, M.F.  Chem. Eur. J.  2016, 22, 17562-17565.  DOI:  10.1002/chem.201604415

 

Related chemistry reported in a special issue in Polyhedron is reported here:

Hyvl, J.; Yoshida, W.Y.; Moore, C.E.; Rheingold, A.L.; Cain, M.F.  Polyhedron  2018, 143, 99-104.

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If you cannot access these articles, email me and I will provide a PDF.

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"Offshoot" and Ongoing Project:  New Pnictogen-Containing Heterocycles.  Azaphospholes and -arsoles.  

We discovered that if a P(I)/As(I) center was generated within a simpler NC chelate (rather than an NCN pincer) that a similar redistribution of electron density occurred, resulting in the synthesis of new PN and AsN heterocycles without the tethered imine donor.  These molecules are 10 pi-electron species that have demonstrated some interesting reactivity.   

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A full article on the synthesis of static aromatics, "bell-clappers", and hypervalent Group 15 derivatives,

new benzoazaphospholes and arsoles, and detailed calculations on their aromaticity can be found here:

Kremláček, V.; Hyvl, J.; Yoshida, W.Y.; Růžička, A.; Rheingold, A.L.; Turek, J.; Hughes, R.P.; Dostál, L.; Cain, M.F.

 Organometallics  2018, 37, 2481-2490.  DOI: 10.1021/acs.organomet.8b00290

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If you cannot access this article, email me and I will provide a PDF.

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Functionalized (Dearomatized) Benzazaphospholes: Transmetalating Agents

Sequential treatment of benzazaphospholes with HCl followed by a nucleophile affords new 6 pi-electron species with an exocyclic P–X bond that is elongated due to hyperconjugation with the adjacent N atom (see the papers cited below for a detailed explanation to why).  This renders normally inert P–C bonds more labile, which we exploited to generate new transmetalating agents for Pd-catalyzed cross-coupling.  If the functionalized benzazaphosphole is a P–CCPh derivative, we demonstrated that the alkynyl unit could be transferred to Pd(II) in a stoichiometric fashion and ultimately, used in catalysis for a "Cu-free" Sonogashira-type reaction.  

 

Currently (1/2021 and beyond), we are addressing the limitations associated with the alkynylation reaction, while simultaneously synthesizing new P–functionalized (X = CF3, etc) heterocycles.  

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A full article on these new P-Alkynyl Functionalized Benzazaphosphole as Transmetalating Agents can be found here:

Zhou, D.Y.; Miura-Akagi, P.M.; McCarty, S.M.; Guiles, C.H.; O'Donnell, T.J.; Yoshida, W.Y.; Krause, C.E.; Rheingold, A.L.;

Hughes, R.P.; Cain, M.F. Dalton Trans. 2021, 50, 599-611.  DOI: 10.1039/D0DT01367F

 

A critical review on benzazaphospholes from MFC can be found here:

Cain, M.F. Comments on Inorganic Chemistry 2020, 40, 25-51.  DOI:  10.1080/02603594.2019.1701447

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Functionalized (Dearomatized) Benzazaphospholes: P-Halogenated Derivatives & Phosphenium Ions

P-Halogenated benzazaphospholes should serve as direct precursors to phosphenium ions with small HOMO-LUMO gaps, potentially promoting small molecule activation at phosphorus.  

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Before exploring the phosphenium chemistry, we synthesized and fully characterized all of the P-halogenated derivatives by NMR spectroscopy and X-ray crystallography. Exposure of the heavier analogues (P–Br and P–I) to Pt(0) centers resulted in the formation of unusual Pt(I)-Pt(I) dimers, a process resembling bimetallic oxidative addition.  Our recent paper in Organometallics (2023, DOI: 10.1021/acs.organomet.3c00070) describes our findings from both an experimental and computational perspective.  

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2.  Phosphinidene (R–P) Generation: Within OCO Pincer Ligands 

Switching from NCN to OCO pincers with sp3-hybridized benzylic arms (to prevent the formation of aromatic P-heterocycles) was expected to strengthen the 3-center, 4-electron O–P–O bond of the targeted 10–P–3 species, BUT unexpected and interesting reactivity was observed.  Subjecting an O-Mes substituted pincer to the lithiation/phosphination/reduction protocol afforded to an (RP)3 species (R = OCO pincer), a cyclotriphosphane, while use of a modified Martin Ligand led to cyclization and dealkylation.  Currently (Summer 2021), new OCO pincers not likely to form these "byproducts" are being developed.  

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These results were disclosed here:

Chinen, B.L.; Hyvl, J.; Brayton, D.F.; Riek, M.M.; Yoshida, W.Y.; Chapp, T.W.; Rheingold, A.L.; Cain, M.F. RSC Advances, 2021, 11, 28602-28613.

DOI: 10.1039/d1ra05926b  

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