Phosphorane chemical shifts

The meta- and para-isomers of the phosphoranes (FC6H4)3P(OEt)2 have been prepared from the corresponding phosphines and diethyl peroxide. From their F chemical shifts it was concluded that the groups... 

The first phosphorane (74) with a a"phosphorus-silicon bond has been prepared (albeit in low, 20%,yield) by the reaction of (72) with bis(trimethylsilyl)magnesium (73)99. It has an unusually high phosphorus chemical shift but no 2L ray data are, as yet, available. 

Treatment of all four diasteromers of 72 with sodium or potassium bases yielded stilbenes with high stereoselectivity (Scheme 15) <2003T255>. Two of the diastereomers gave rise to some retro-aldol products. In two cases, hexacoordinate species were identified by the upheld 31P chemical shift (5—112 ppm). It was noted that phosphoranes that ring-closed to form hexacoordinate tricyclic species were the ones that did not undergo the retro-aldol reaction to produce aldehydes. The ring closure was disfavored for intermediates with steric repulsion between trifluoromethyl and phenyl groups. 

Up to this point, the criterion used to ascertain the presence of an equilibrium, i.e. the simultaneous presence of two well defined entities, was mainly based on 1H, 13C or 31P NMR determinations at different temperatures or in various solvents. However, it turns out that the chemical shift in 31P NMR spectra (usually negative with respect to the H3P04 reference) is often inadequate to permit a choice among various possibilities, namely a true phosphorane, a phosphonium salt or an equilibrium between these two forms. 

Tertiary phosphines (67), give dipolar ions (68), and not phosphoranes in this reaction, presumably because the decrease in the number of electronegative oxygens attached to the phosphorus decreases its tendency to become pentacovalent.115-117 The S1P n.m.r. chemical shift in (68) is S = —10-9 ppm. 

In a particularly useful study, Pandolfo et al. have reported the solid state C and CP/MAS NMR spectra of ylides (152) and (153) and their complexes (154)-(156). For example, ketenylidene(triphenyl)phosphorane (152) has a solid state P chemical shift of 4.5 ppm (compared with a CDCh solution value of 5.45 ppm), which shifts to lower frequency in complexes (154) (27.7 ppm) and (155)... 

The assignment of the trans arrangement in sarracinic acid was derived from a study of chemical shift in NMR spectra. Edwards et al. (142) have now synthesized both forms and confirm the structures assigned. Acet-oxypyruvic acid was allowed to react with ethylidenetriphenyl-phosphorane to give a mixture of the cis and trans forms, LXVa and LXVIa, which on hydrolysis yielded sarracinic acid (LXVb) and its cis... 

X-Ray analysis has shown that although the chemical shift of the phosphorane (79) is + 66.9p.p.m. and the geometry around the phosphorus is approximately trigonal bipyramidal, the PO bond length is 2.14 A. Contributions to the structure by the dipolar species (80) and... 

Six coordinate spiroperphosphoranides have P chemical shifts in the range —57 to — 181 ppm upheld of the corresponding five coordinate phosphoranes but towards the low field region of the range ((5p —57 to —440) for all six coordinate compounds , for example (80 m = 0,... 

Moving to three-coordinate phosphorus, the effect of rings on chemical shifts of tertiary phosphites is much less definite than those described above for oxy-phosphoranes. There is some evidence that both six-member and five-member rings produce shielding, with the former exhibiting the... 

The importance of equilibria in the chemistry of pentacoordinated phosphorus arises from the possibility of passing from a given structure to another, even if the latter has a totally different molecular geometry and distribution of electrons, by a mere shift of hybridization. This has important consequences on the chemical properties of phosphoranes and on the possibilities of interconversion of isomers by bond rupture and recombination processes akin to regular true stereomutations, i.e. without bond cleavage. 

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