A Phosphorus Analogue of p-Quinodimethane with a Planar P4 Ring: A Metal-Free Diphosphorus Source.

Para-quinodimethane (pQDM) is a fundamental structural component in many π-conjugated organic molecules and materials. The incorporation of phosphorus atom into π-conjugated frameworks offers unique opportunities for controlling the properties of derived species. A phosphorus analogue of p-quinodimethane (pQDM), (IPrC)2 P4 [5, IPr=C{N(Ar)CH2 }2 ; Ar=2,6-iPr2 C6 H3 ] featuring a planar P4 ring, was readily accessible by KC8 -reduction of (IPrC)(PCl2 )2 (2). Base-mediated C-H functionalization of IPrCH2 (1) with PCl3 afforded 2. The formation of 5 was expected to occur through a dimerization of the transient 3H-diphosphirene (IPrC)P2 (4), which was theoretically suggested to have an intermediate diradical character. Compound 5 underwent photo-induced ring-contraction reaction to form the singlet diradicaloid (IPrCP)2 VI and white phosphorus (P4 ). The formation of and VI and P4 suggested the formal diphosphorus (P2 ) elimination from 5. Indeed, photolysis of a mixture of 1,3-cyclohexadiene (CHD) and 5 led to the formation of P2 -entrapped product (CHD)2 P2 (6). The compound 5 represents the first organophosphorus species that functions as a P2 source.