Comparison of steric effects on 31P chemical shifts in some 7-norbornyl, 7-norbornenyl and cyclohexyl phosphorus compounds
✍ Scribed by Lory B. Littlefield; Louis D. Quin
- Publisher
- John Wiley and Sons
- Year
- 1979
- Tongue
- English
- Weight
- 356 KB
- Volume
- 12
- Category
- Article
- ISSN
- 0749-1581
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✦ Synopsis
Phosphorus-31 NMR spectra were obtained+ on both syn and anti isomers of norbornenes with the 7-position bearing CI,P-, Me,P-, Me,(S)P-and Me,P as substituents. Norbornanes with 7-C12P-and 7-MezP-were also studied. Just as is true for 13C shits for 7-Me in these bicydo[2.2.l]heptane derivatives, the 31P shifts all fall noticeably upfield of published values for comparable cyclohexane derivatives. Consistent also with the 13C effect, a slight shielding of "P in Me,P-(1.1 ppm) was noted when this group was syn to the double bond of the norbornene system, relative to the value for the anti compound, suggesting the sensitivity of "P to a diamagnetic effect from the double bond. However, the more space-demanding CI,P group when moved from the crowded 5nti to the less crowded syn position experienced a strong deshielding effect, as did the Me,(S)P-and Me3P-groups. The opposite is seen when these same groups are moved from the crowded axial to the less crowded equatorial position of cyclohexane, and a new structural influence appears to be operating in this system. A recent proposal that steric compression causes bond angles about phosphorus to increase and that this is the cause of the y-shielding effect is not supported by published NMR and X-ray structural data for cis-and trans-1-methyl->-tert-bntyl-4-phosphorinanol. Bond angles about phosphorus are virtually identical in these isomers, yet "P shifts differ by 6.9 ppm.