Regioselective Metalation of a Tertiary Phosphine. Synthesis and Structural Characterization of an Unusual Tetrameric Lithium Phosphinomethanide

The tertiary phosphine MeP(C6H4-2-CH2NMe2)2 (3) is conveniently prepared by the reaction
of MePCl2 with Li(C6H4-2-CH2NMe2) in high yields. Treatment of 3 with n-BuLi in light
petroleum yields the tetrameric lithium phosphinomethanide complex [Li{CH2P(C6H4-2-CH2NMe2)2}]4·(PhMe)3 (5a) after recrystallization from toluene, whereas treatment of 3 with
t-BuLi under similar conditions yields the benzyllithium complex [MeP(C6H4-2-CH2NMe2){C6H4-2-CH(Li)NMe2}]2 (4). Complex 5a has been characterized by elemental analyses,
multielement NMR spectroscopy, and X-ray crystallography. Complex 5 can also be accessed
by moderate thermal treatment of 4 in toluene solution. The kinetics of this conversion were
followed by 31P NMR spectroscopy and show a first-order dependence upon the concentration
of 4 and are independent of the concentration of 3. Analysis of the temperature dependence
of the reaction over the range T = 308−338 K yielded ΔH⧧ = 68.1 ± 4.0 kJ mol-1 and ΔS⧧
= −124 ± 30 J K-1 mol-1 for this reaction, consistent with the reaction proceeding via a
concerted intramolecular H−Li exchange mechanism.