Atomic Proximity Due to Molecular Congestion:  Rational Design of Bis(phosphite) Ligands by Restriction of Molecular Motion¹

Abstract

The synthesis and conformational analysis of the sterically congested bis(phosphite) ligand {2-{1-{3,5-(t)Bu(2)-2-[2,2'-CHCH(3)(4,6-(t)Bu(2)C(6)H(2)O)(2)PO]C(6)H(2)}Et}-4,6-(t)Bu(2)C(6)H(2)O}(PhO)(2)P (5) are reported. X-ray crystallographic, dynamic (31)P{(1)H} NMR, NOE, DNOE, CP-MAS (31)P NMR, and calculational studies of 5 as well as the structurally related bis(phosphites) 1 and 6 suggest that the conformational freedom of the molecule is severely restricted because of geometric restraints due to steric congestion. A through-space mechanism of coupling is suggested to explain the observed eight-bond P-P J coupling of 27.5 Hz in the (31)P{(1)H} NMR spectrum of 5, which is a result of the proximity of the two phosphorus atoms in 5. The results of this study support the contention that the restriction of molecular motion by steric congestion can be used to rationally design a ligand favoring a particular disposition of phosphorus atoms.

Steric Consequences on the Conformation of Medium-Sized Rings: Solution NMR, Solid-State Crystallographic, ab Initio Molecular Orbital Calculations, and Molecular Mechanics Studies on Substituted Eight-Membered Organosilicon Ring Systems1

“…These steric interactions can be relieved in both 1 and 2 by a change in conformational preference from the BC to the TB family of conformations. These conclusions are supported by the observation that the solid-state conformations of analogous P(III)-containing 12 H -dibenzo[ d , g ][1,3,2]dioxaphosphocins are BC with the sterically undemanding lone pair of electron on phosphorus occupying a pseudoaxial position in the cleft between the 4- and 8- tert -butyl substituents. ,,

2 Pseudoaxial and pseudoequatorial positions in the BC conformation, and endocyclic and exocyclic positions in the family of TB conformations.

3 ORTEP of 7a MAJOR .

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Phosphate-, Phosphite-, Ylide-, and Phosphonate-Terminated Dendrimers

“…These experiments confirm the existence of the 7 J PP coupling constant in compound 14-[G 1 ] . Very few coupling constants through so many bonds have been reported in the literature, most of them concerning through-space couplings, which are unlikely for compound 14-[G 1 ] for steric reasons. However, a phosphorus−phosphorus coupling constant over seven bonds has already been measured for a compound whose structure is closely related to that of 14-[G 1 ] : the analysis of the outer 13 C satellites in the 31 P NMR spectrum of (EtO) 2 P(O)CH 2 C 6 H 4 CH 2 P(O)(OEt) 2 gave 7 J PP = 7.8 Hz, a value attributed to a large Π-electron contribution .…”

Synthesis and Conformation of Chiral Eight-Membered 12H-Dibenzo[d,g][1,3,2]dioxaphosphocins1

“…Very few X-ray crystal structures of 2 H -dibenzo[ d,g ][1,3,2]dioxaphosphocins containing tricoordinate P(III) are known. BC conformations have been found in the solid state with a pseudoequatorial substituent on phosphorus, , although caution must be exercised in comparing solid-state and solution conformations. Lattice energy and the resultant crystal-packing effects in the solid state can render the solid-state conformation different from that in solution.…”