Racemic and Meso Diastereomers of a P‑Chirogenic Diboranyldiphosphinoethane

Diphosphine ligands have enjoyed great success as supporting
moieties
for transition elements, where modification of both primary and secondary
coordination spheres has been shown to elicit differences in chemical
reactivity. This work describes a new family of such ligands constructed
from the previously reported scaffold (±)-rac/meso-(t-Bu)ClP-CH2CH2-PCl(t-Bu) that bears bulky tert-butyl substituents. The reaction of this precursor with allyl magnesium
chloride and subsequent hydroboration using HBCy2 (Cy =
cyclohexyl) provide access to a diboranyldiphosphinoethanethe
first in a class of ligands having two electrophilic boranes in their
secondary coordination sphere. As a testing ground for coordination,
these motifs (and their allyl precursor) have been installed at Ni(0)
using [Ni(COD)2] (COD = 1,5-cyclooctadiene) in order to
determine methods for clean installation. Product stereochemical outcomes
are in some cases complicated and have been determined by nuclear
magnetic resonance spectroscopic and crystallographic means. Density
functional theory optimizations and accurate DLPNO-CCSD(T) energy
calculations were also performed to support structural assignment.