Sterically Congested Phosphite Ligands: Synthesis, crystallographic characterization, and observation of unprecedented eight‐Bond ³¹P,³¹P coupling in the ³¹P‐NMR spectra

Abstract

The synthesis and characterization of 2‐{1‐{3,5‐bis(1,1‐dimethylethyl)‐2‐{[2,4,8,10‐tetrakis(1,1‐dimethylethyl)dibenzo[d,f][1,3,2]dioxaphosphepin‐6‐yl]oxy}phenyl}ethyl}‐4,6‐bis(1,1‐dimethylethyl)phenyl diphenyl phosphite (6) is described. In the 31P‐NMR spectrum (1H‐decoupled) of 6, an unprecedented eight‐bond P,P coupling of J = 72.8 Hz is observed. In the X‐ray crystal structure of 6, an intramolecular P–P distance of 3.67 Å is found, which is within the sum of the van‐der‐Waals radii of the P‐atoms. The observed intramolecular P–P distance suggests that a through‐space coupling mechanism is operative. The solid‐state conformation of 6 is compared to the conformation obtained by semi‐empirical MO geometry optimizations (PM3 method). The calculated geometry suggests that the solid‐state structure is near a true energy minimum, but that crystal‐packing forces decrease the intramolecular P–P distance in the solid state. In the absence of crystal‐packing forces, however, the collisional and vibrational energy available in solution may lead to the population of states with a shortened intramolecular P–P distance in 6. The proximity of the P‐atoms in 6 is due to restricted conformational freedom resulting from steric congestion within the molecule. The free energy of activation (ΔG* = 10.2 and 10.8 kcal/mol for unequal populations of exchanging conformers) for ring inversion of the dibenzo[d,f][1,3,2]dioxaphosphepin ring in 6 is determined by variable‐temperature 31P‐NMR spectroscopy. Semi‐empirical MO calculation on model compounds suggest that the structure of the transition state for ring inversion has the two aryl rings and O‐atoms in a common plane, with the P‐atom lying above this plane.

Chiral Diphosphites Derived fromD-Glucose: New Ligands for the Asymmetric Catalytic Hydroformylation of Vinyl Arenes

“…Preparation of enantiomerically pure diols (13,15,18,20): Diols 13 and 18 were synthesized stereospecifically in two steps from 1,2-protected-3-Oacetylated furanoses 11 and 16, respectively (Scheme 1). Compounds 11 and 16 are easily prepared on a large scale by previously reported, highly effective methods from d-glucose.…”

Sterically Congested Ligands: Synthesis and Solution Conformation of Bisphosphite Ligand Precursors

“…Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms [ 1,3,2]dioxaphosphocin rings may adopt an unusual diequatorial placement in pentaoxypho~phoranes. '.~ 31P Chemical shift correlations of cyclic oxyphosphoranes containing these and other medium-sized ring systems were reported by Holmes and P r a k a~h a .~ Quite interestingly, in the 31P{1H} NMR spectrum of the ligand 1, unprecedented eight-bond P-P J coupling of 72.8 Hz is observed.2 Crystallographic and semi- empirical calculations supported a through-space coupling mechanism in which the close proximity of the two phosphorus atoms to one another is due to restricted conformational freedom because of steric congestion within the molecule.…”

Versatile Phosphite Ligands Based on Silsesquioxane Backbones

“…360 FTIR spectrometer. Dibenzo[d, f]-3,3',5,5'-tetratert-butyl-[1,3,2]dioxaphosphepin P-chloride [44] as well as the metal precursors PdCl 2 (cod), [45] and Mo(CO) 4 (pip) 2 [46] were prepared according to literature procedures. Also (c-C 5 H 9 ) 7 Si 8 O 13 Tl (A) and (c-C 5 H 9 ) 7 Si 7 O 9 (OH) 2 (OSiMePh 2 ) (B) were prepared according to literature procedures.…”