B-Centered Reactivity of Persistent P-Stabilized Boryl Radicals

Abstract

A new P-stabilized boryl radical [iPr2P­(naph)­BMes]• 2a was obtained by reduction of the corresponding phosphino-bromoborane 1a with Na­(Hg). The persistent radical 2a has been characterized by EPR, and its structure has been thoroughly studied by DFT. The corresponding Gomberg-type dimer has been analyzed by NMR and XRD, and the Gibbs free energy associated with the dimerization process has been evaluated by VT EPR. The replacement of the Ph substituents at phosphorus for iPr groups has a slight but noticeable impact: it increases the spin density at boron and favors the radical over its Gomberg-type dimer. An original cross-coupling product between 2a and the trityl radical Ph3C• has also been authenticated crystallographically. The P-stabilized boryl radicals 2a,b are readily trapped by TEMPO to give the corresponding B–O adducts 3a,b (naphthyl-bridged phosphine-boranes without P → B interaction). The reaction of 2a,b with Ph3CCl substantiates their ability to participate in halogen transfer reactions.

Lewis pairing and frustration of group 13/15 elements geometrically enforced by (ace)naphthalene, biphenylene and (thio)xanthene backbones

“…Reduction of 83 and 84 with 1.3 equivalent of a 1% Na(Hg) amalgam in toluene gave the corresponding boryl radicals 85 and 86 in moderate to high yields (Scheme 26). 47,48 The X-band EPR spectrum of 85 is very similar to those of 81 and 82, with a well resolved signal centred at giso = 2.0026. The EPR spectrum of 86 also shows a complex and well resolved signal centred (giso = 2.0024) but its hyperfine structure somewhat differs from those of 81, 82 and 85.…”

“…This encouraging result spurred the development of the chemical reduction of phosphinebromoboranes 61 and 62 with 1.2 equivalents of 1% Na(Hg) amalgam in THF (Scheme 25). 47,48 Xband EPR analysis of the ensuing species 81 and 82 revealed complex but well resolved signals centred at giso = 2.0026. No change in the spectra was observed over days at room temperature, demonstrating the persistent character of these boryl radicals.…”

Frustrated Lewis Pair based on a peri‐Substituted Biphenylene Scaffold

“…The exploitation of frustrated Lewis pairs (FLPs), for the capture and activation of small unreactive molecules, such as dihydrogen and carbon dioxide, marks a paradigm change in the field of catalysis. Single component FLPs (and related regular Lewis pairs) based on phosphinoboranes mounted on rigid peri ‐substituted aromatic scaffolds, such as naphthalene, acenaphthene, dimethylxanthene, and dibenzofurane (Scheme ), may offer two potential advantages over conventional two component FLP systems: (i) Firstly, the rigid scaffolds allow tuning of the donor acceptor distances, which for high catalytic activity needs to be in the range between approximately 3 Å and 5 Å . (ii) Secondly, the entropy penalty associated with the capture of small molecules should be considerably smaller in single component FLPs than in two component FLPs , .…”

The Pincer Platform Beyond Classical Coordination Patterns

“…ligand‐stabilized boryl radicals (LR 2 B), are known. [ 66–73 ] Neutral boron compounds with an occupied p z ‐orbital and two stabilizing ligands can be isolated, if σ‐donating/π‐accepting substituents like cyclic alkylaminocarbenes (CAACs) or carbon monoxide are used. [ 74–76 ] The pronounced π‐back‐bonding in these compounds allows for the stabilization of an occupied p z ‐orbital and results in an overall trigonal planar geometry.…”