Comparison of steric effects on 31P chemical shifts in some 7-norbornyl, 7-norbornenyl and cyclohexyl phosphorus compounds

Phosphorus-31 NMR spectra were obtained on both syn and anti isomers of norbornenes with the 7-position bearing Cl₂P-, Me₂P-, Me₂(S)P- and Me₃P as substituents. Norbornanes with 7-Cl₂P- and 7-Me₂P- were also studied. Just as is true for ¹³C shifts for 7-Me in these bicyclo[2.2.1]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 ³¹P in Me₂P- (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 ³¹P to a diamagnetic effect from the double bond. However, the more space-demanding Cl₂P group when moved from the crowded anti to the less crowded syn position experienced a strong deshielding effect, as did the Me₂(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 γ-shielding effect is not supported by published NMR and X-ray structural data for cis- and trans-1-methyl-4-tert-butyl-4-phosphorinanol. Bond angles about phosphorus are virtually identical in these isomers, yet ³¹P shifts differ by 6.9 ppm.