B-Centered Reactivity of Persistent P-Stabilized Boryl Radicals

A new P-stabilized boryl radical [iPr(2)P(naph)BMes](center dot) 2a was obtained by reduction of the corresponding phosphino-bromoborane la 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(center dot) 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.