Synthesis, Structure, and Solution Studies of Lithiated Allylic Phosphines and Phosphine Oxides

This study reports a new series of 12 α-lithiated allylic phosphines and phosphine oxides. By incorporating Lewis base donors including diethyl ether (Et2O), tetrahydrofuran (THF), N,N,N′,N′,-tetramethylethylenediamine (TMEDA), and N,N,N′,N′,N″,-pentamethyl­diethylenetriamine (PMDETA), nine complexes were structurally characterized by single-crystal X-ray crystallography. This includes novel dilithiated allylic phosphine 4 [PhP­{CHCHCH2Li­(TMEDA)}2] and a rare hemisolvated lithiated phosphine oxide 6 [{Ph2P­(O)­CHC­(Me)­CH2Li}2(TMEDA)]. Interestingly, in the solid state, P­(III) complexes take advantage of Li−π interactions to the newly formed delocalized system, in comparison to P­(V) complexes where the oxophillic nature of the lithium atom dominates. All 12 complexes were fully characterized in the solution state by multinuclear NMR spectroscopy. DFT calculations on isomers of monomeric lithiated complex 3 [Ph2PCHC­(Me)­CH2Li­(PMDETA)] described the low energy barrier between transition steps of the subtle delocalization of the allylic chain.