“…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 .…”
The conformation of the eight-membered membered
12H-dibenzo[d,g][1,3,2]dioxasilocin
ring system was
investigated both in the solid-state by X-ray crystallography and in
solution by NOE experiments. Ab initio
Hartree−Fock calculations were performed to locate all stationary points for the
unsubstituted
12H-dibenzo[d,g][1,3,2]dioxasilocin ring system. The MM2* force field was parametrized to
reproduce our ab initio results, and these data
were compared to the experimental data. The transition states for
conformational interchange were identified. The
pseudoequatorial preference for a methyl substituent on either the
C(12) carbon atom or silicon is greater than the
difference in energy between a boat-chair and twist-boat conformation
in 2,4,8,10-tetra-tert-butyl-substituted
12H-dibenzo[d,g][1,3,2]dioxasilocins.
The conformations observed in the solid-state X-ray crystal
structures of 12H-dibenzo[d,g][1,3,2]dioxasilocins
are sensitive to crystal-packing forces and may be different from that
in solution.
“…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 .…”
The conformation of the eight-membered membered
12H-dibenzo[d,g][1,3,2]dioxasilocin
ring system was
investigated both in the solid-state by X-ray crystallography and in
solution by NOE experiments. Ab initio
Hartree−Fock calculations were performed to locate all stationary points for the
unsubstituted
12H-dibenzo[d,g][1,3,2]dioxasilocin ring system. The MM2* force field was parametrized to
reproduce our ab initio results, and these data
were compared to the experimental data. The transition states for
conformational interchange were identified. The
pseudoequatorial preference for a methyl substituent on either the
C(12) carbon atom or silicon is greater than the
difference in energy between a boat-chair and twist-boat conformation
in 2,4,8,10-tetra-tert-butyl-substituted
12H-dibenzo[d,g][1,3,2]dioxasilocins.
The conformations observed in the solid-state X-ray crystal
structures of 12H-dibenzo[d,g][1,3,2]dioxasilocins
are sensitive to crystal-packing forces and may be different from that
in solution.
“…Flexible solid-state conformations, however, can be strongly influenced by crystal-packing forces, and considerable differences between solid-state and solution conformations may exist . The only P(III)-containing 12 H -dibenzo[ d,g ][1,3,2]dioxaphosphocin reported for which X-ray crystallographic data exist assumes the BC conformation in the solid state with the substituent of phosphorus positioned pseudoequatorially . The pseudoequatorial placement of the substituent on phosphorus is particularly favored by the 4,8-di- tert- butyl substitution because this aptitude allows the sterically undemanding lone pair of electrons to be placed pseudoaxially in a cavity between the tert -butyl groups.…”
Section: Resultsmentioning
confidence: 99%
“…The solution to the structure of 5 provides a rare example of an X-ray crystal structure of a 12 H -dibenzo[ d,g ][1,3,2]dioxaphosphocin ring containing tricoordinate P(III) ) .…”
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.
“…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.…”
The
synthesis and characterization of several chiral derivatives of the
12H-dibenzo[d,g][1,3,2]dioxaphosphocin
ring system are described. The conformation of the eight-membered
dioxaphosphocin ring is a boat−chair in both solution and the solid
state. A variable-temperature
13P{1H} NMR study of a Pt(II)
complex is described.
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