31P NMR shifts

Ring closure after rotation of the bonds of the open chain betaine can lead to stereoisomers (71JA4004). Two examples are shown in Scheme 28. 31P NMR shifts are useful for the identification of the different species. In the case of (178) and (179), prepared by the reaction of benzil with trisdimethylaminophosphine, two crystalline forms were isolated. They equilibrate in solution. 

Two particular structures and the starting compounds indicate a stronger 31p nmr shift for the A phosphorus to lower field in case of an E configuration. The J(AB) coupling is not diagnostic of an E or Z structure. 

With respect to the two structures known so far, the more positive 31p nmr shift pointed towards an E configuration. This seemed to be supported by a series of compounds prepared with differently substituted isocyanide dichlorides, showing the more space requiring groups to move opposite to the substituent on the methylene phosphorus. Moreover, because of easy cis-migra-tion, only the Z compounds seemed to be capable of forming diastereomeric modifications of the silatantomer. 

A remarkable effect was observed for salt 28, as no 31P NMR signals could be obtained between —100 and +30 °C. According to theoretical calculations of the 31P NMR shift values and an inversion barrier, this effect is related to molecular dynamics <2003AGE2778>. Dynamic NMR spectroscopic evidence was established for a rapid 1,3-migrational exchange of the phosphorus atoms A and B within the four-membered ring of triphospha bicyclopentene 31 <1998CC1537>. 

The interaction with cations lays the basis for the31P NMR spectroscopic method, allowing one to distinguish between extracellular and intracellular pools of PolyPs. Ethylenedi-aminetetraacetic acid (EDTA) can be used to complex the divalent cations bound to PolyPs and to produce a new 31P NMR shift. However, because the cell membrane is impermeabile to EDTA, only extracellular PolyPs is affected. This method was used successfully on a cell suspension of Propionibacterium sp. (Serafim et al., 2002). 

Conversions in pentane proceed in an nonuniform manner at high temperatures. In addition to unidentified products exhibiting different 31P NMR AX patterns, as well as the main product dichlorophosphane and another unknown substance, which exhibits a 31P NMR shift of + 301 ppm, one obtains a red compound, of which elemental analysis and the molecular mass point to the trimer phosphathioketene. The number of isomer compounds of this composition is limited by the 31P NMR spectrum. The A2X system, of which the X triplet is split into a double doublet if detected in solution in chloroform, indicates two acyclic PC double bonds and one phosphorus atom as a ring member (117). A possible explanation is given in Fig. 22. This could be in agreement with the addition of a monomer to the dimer, forming the six-membered ring compound with the proposed structure. 

The molecular structure of the compound was used to calculate its electronic structure in order to understand better the anomalous 31P NMR shifts described above. 

Figure 3. DFT-optimized geometry (B3LYP/6-311G ) for exo-6-Me-arachno-6,l-PCBgHi2 (lb) and comparisons of its experimentally-determined and GlAO-calculated (B3LYP/6-311G ) 1 B, 13C, and 31P NMR shifts and assignments...

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