Ph₂P(BH₃)Li: From Ditopicity to Dual Reactivity

Abstract

A multinuclear NMR study shows that the deprotonation of diphenylphosphine-borane by n-BuLi in THF leads to a disolvated lithium phosphido-borane Ph(2)P(BH(3))Li of which Li(+) is connected to the hydrides on the boron and two THF molecules rather than to the phosphorus. This entity behaves as both a phosphination and a reducing agent, depending on the kinetic or thermodynamic control imposed to the reaction medium. Density functional theory computations show that H(2)P(BH(3))Li exhibits a ditopic character (the lithium cation can be in the vicinity of the hydride or of the phosphorus). It explains its dual reactivity (H- or P-addition), both routes going through somewhat similar six-membered transition states with low activation barriers.

Synthesis and Identification of Aryl and Alkyl Gem‐Dilithium Phosphido‐Boranes: A Boost to the Chemistry of Phosphandiides

“…The structural investigations of the two dilithiated phosphido‐borane Li 2 [RP⋅BH 3 ] freshly prepared above were conducted on the basis of the lithium NMR data with the aim of locating the two lithium cations. Note that, in the following, 7 Li NMR is used by default, but we also resorted to 6 Li NMR (deprotonating the phosphine‐boranes with 6 Li‐labeled n BuLi) to either strengthen the conclusion derived from the observations by means of 7 Li NMR or reduce the possibility of peak overlapping…”

“…Previous work on the structure of a secondary lithium phosphido‐borane (Li[Ph 2 P⋅BH 3 ]) showed that the lithium atom is exclusively next to the borane group. This has led to substantiating the dual reactivity observed for such species.…”

“…The fact that this Li signal corresponds to a singlet regardless of the temperature and the isotope suggests that Li 2 [PhP⋅BH 3 ] would not comprise P−Li bonds. Nevertheless, the 6 Li‐ 1 H HOESY spectrum recorded at −40 °C (Figure ) shows that the Li signal strongly correlates with the BH 3 group, a result similar to our previous observations on secondary phosphido‐boranes and more modestly, but in a non‐negligible way, also with the ortho protons of the phenyl appendage. If these results allow us to discard the option of a separated ion pair, two possible dispositions of the lithium cations with respect to the [PhP⋅BH 3 ] moiety are to be considered to fit with the absence of P‐Li scalar coupling: i) the two Li atoms are equivalent, being in interaction with both the phenyl and the BH 3 groups (Figure , left); ii) one Li interacts with the phenyl and the other with the BH 3 , and they undergo a rapid exchange process (Figure , right).…”

A Combined 1H/6Li NMR DOSY Strategy Finally Uncovers the Structure of Isopropyllithium in THF

“…A 1 H DOSY experiment,, associated with a diffusion coefficient–formula weight correlation analysis in the presence of three internal references (squalene, triphenylbenzene and hexamethyldisilane), was next undertaken. Unfortunately, the two proton signals at −1.00 ppm (monomer) and −1.07 ppm (oligomer 2 or 3 ) are too close to give two different diffusion coefficient values (pages 15S–16S in Supporting Information).…”

Spectroscopic Advances in Structural Lithium Chemistry: Diffusion‐Ordered Spectroscopy and Solid‐State NMR

“…In 2011, Oulyadi et al [45] proposed the first structural study in solution by multi-NMR ( 1 H, 6 Li, 11 B, 13 C) of lithium phosphido-borane (Scheme 4.2), performed in relation to a mechanistic study on the coupling reaction between the secondary borane phosphines and aryl halides. The data collected by multi-NMR analyses ( 1 H, 6 Li, 11 B, 13 C) are in favor of a preferential interaction of lithium with hydrides rather than with phosphorus.…”