Backbone-Modified Bisdiazaphospholanes for Regioselective Rhodium-Catalyzed Hydroformylation of Alkenes

A series of tetraaryl
bisdiazaphospholane (BDP) ligands were prepared
varying the phosphine bridge, backbone, and substituents in the 2-
and 5-positions of the diazaphospholane ring. The parent acylhydrazine
backbone was transformed to an alkylhydrazine via a borane reduction
procedure. These reduced ligands contained an all sp3 hybridized
ring mimicking the all sp3 phospholane of (R,R)-Ph-BPE, a highly selective ligand in asymmetric hydroformylation.
The reduced bisdiazaphospholane (red-BDP) ligands were shown crystallographically
to have an increased C–N–N–C torsion anglethis
puckering resembles the structure of (R,R)-Ph-BPE
and has a dramatic influence on regioselectivity in rhodium catalyzed
hydroformylation. The red-BDPs demonstrated up to a 5-fold increase
in selectivity for the branched aldehyde compared to the acylhydrazine
parent ligands. This work demonstrates a facile procedure for increased
branched selectivity from the highly active and accessible class of
BDP ligands in hydroformylation.