Extremely bulky triarylphosphines incorporating 2,6-diisopropylphenyl substituents; consideration of steric shielding and steric pressure

The geometry of 15 triarylphosphines which differ in the nature of the substituents at the 2 and 6 positions of one or more phenyl rings are compared. The sum of angles around phosphorus, ∑{∠CPC}, is used as the primary measure of steric bulk. The roles of substituents, which act as steric shields surrounding the phosphine lone pair and of those which generate steric pressure and cause flattening of the C3P pyramid are identified. Both crystallographic and computational (HF 6-31G(d)) structures are used to assess ∑{∠CPC}, the pyramidalization angle α, the average C–P distance, and the helical twist angle of the aryl rings β. All of these parameters confirm that ortho-2,6-diisopropyl substituted aryl groups generate the most sterically congested triarylphosphines. Comparison to Tolman cone angles are made where these are available.

In ortho substituted triarylphosphines, substituents in the exo position (Ex) act as steric shields surrounding the phosphorus lone pairs electrons, whereas those in the endo position (En) exert steric pressure which can be sufficient to flaten the C3P pyramid; when the two effects work together, the shielding of the phosphorus lone pair can be dramatically increased.Figure optionsDownload as PowerPoint slide