Phosphorus is an important element in naturally occurring as well as in man-made products. Typical e — Organic Chemistry Chemistry Question
The Nature of Phosphorus
Phosphorus is an important element in naturally occurring as well as in man-made products. Typical examples are phospholipids, nucleic acids and ligands for efficient catalysis. Furthermore, 31P-NMR spectra can provide valuable information of P containing products. A characteristic feature of 31P-NMR spectra (interaction with protons is removed by decoupling) is the rather large chemical shift differences for structurally related structures. (Note: Optical antipodes = enantiomers do not show a difference in NMR-spectra.)
The dialkyl phosphate A is derived from racemic butan-2-oland dialkyl phosphate B from enantiomerically (optically) pure (S)-butan-2-ol. The 1H-decoupled 31P-spectrum of A is shown in the figure.
The spectrum of B shows only one peak and that has the same chemical shift as one of the signal of A.
Note: A dialkylphosphite has the formula (RO)2P-OH.
Compound C is a dialkyl phosphate derived from methanol.
Compound D is a dialkyl phosphate derived from propan-2-ol.
Compound E is a dialkyl phosphate derived from racemic 1-phenylethanol.
Draw the spatial structures and the corresponding Fischer projections of the stereoisomers that can account for the spectrum of A (Figure 1).
Draw the spatial structures and the Fischer projections of compound B.
How many signals are present in the 31P-NMR spectra of the compound C, D and E? If there is more than one signal, then indicate also the relative peak areas.
Model Answer
C: (CH3O)2P-OH one signal
D: [(CH3)2CHO]-P-OH one signal
E: (Ph-(CH3)CH2O)2P-OH three signals as in 10-1 ratio 1:2:1
(meso-I : RR + SS : meso-II)
Sketch the 1H-NMR spectrum of A (assume that there is no overlap of signals). Show the relative peak heights also for the splitting pattern of the signals.)
William S. Knowles (Nobel prize 2001) used a rhodium catalyst containing the phosphorus-ligand F for the synthesis of L(-) DOPA which is an important anti-Parkinson drug.
Draw the spatial structures of all possible steroisomers of compound F and indicate which of them would be suitable in an asymmetric synthesis of either L(-) or D(+) DOPA. (Use spatial structures as shown for F.)
Model Answer
It should be noted that is extra information which was not part of the question. The essence is that the non-chiral meso isomer is recognized.
In compound F phosphorus is pyramidal. This is:
Absolutely essential for compound F to serve as a chiral ligand.
Not essential at all.
Only true when there is no pyramidal inversion.
Only true at very high temperatures.
More than one correct answer is possible.
Model Answer
Option 1 and option 3; P is the asymmetric centre
Phosphorous compounds are pyramidal and they are configurationally very stable (no inversion).
Later the ligand G was developed for the same purpose.
Draw the spatial structures of all possible stereoisomers of ligand G and indicate which of them would be suitable in an asymmetric synthesis of either L(-) or D(+) DOPA. (Use spatial structures as in the figure above.)
In ligand G phosphorus is pyramidal. This is:
Absolutely essential for compound G to serve as a chiral ligand.
Not essential.
Only true when there is a pyramidal inversion.
Only true at very high temperature.
More than one correct answer is possible.
Model Answer
Option 2.
How many signals will be present in the 31P-NMR spectrum of ligand G? If there is more than one signal then indicate also the relative peak areas.
Model Answer
One signal, substituents have the same chirality (R). No splitting.