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

Abstract

AbstractPhosphorus‐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.