B‑Centered Reactivity of Persistent P‑Stabilized Boryl Radicals

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.通过钠汞齐（Na(Hg)）还原相应的膦基溴代硼烷1a，成功得到一种新型P-稳定硼基自由基[iPr₂P(naph)BMes]• 2a。该持久性自由基2a已通过电子顺磁共振波谱（Electron Paramagnetic Resonance，EPR）进行表征，并通过密度泛函理论（Density Functional Theory，DFT）对其结构开展了深入研究。其对应的贡伯格型二聚体（Gomberg-type dimer）已通过核磁共振波谱（Nuclear Magnetic Resonance，NMR）与X射线衍射（X-ray Diffraction，XRD）完成分析，且二聚反应相关的吉布斯自由能已通过变温电子顺磁共振波谱（Variable Temperature Electron Paramagnetic Resonance，VT EPR）进行评估。将磷原子上的苯基取代基替换为异丙基（iPr）后，会产生轻微但可观测的影响：其可提升硼原子位点的自旋密度，并使自由基相较于其贡伯格型二聚体更为稳定。一种由2a与三苯甲基自由基（Trityl Radical）Ph₃C•生成的新型交叉偶联产物，也已通过晶体学手段得以确证。P-稳定硼基自由基2a、2b可被四甲基哌啶氮氧化物（2,2,6,6-Tetramethylpiperidinyl-1-oxyl，TEMPO）快速捕获，生成相应的B-O键加合物3a、3b（即不含P→B配位作用的萘桥联膦硼烷）。2a、2b与三苯基氯甲烷（Ph₃CCl）的反应证实了其可参与卤素转移反应的能力。