Steric encumberance

J. G. Verkade has synthesized a series of bicyclic triamino phosphines that are potent a-donors to palladium, yet can be modified to incorporate a variety of steric encumberance.46 It was found that ligand L13 could promote the catalytic amination of both aryl chlorides and bromides with secondary cyclic amines and anilines. L13 was not as effective for aliphatic secondary acyclic amines and primary aliphatic amines. The scope was improved with ligand L14, but as with other third generation ligands, primary aliphatic amines were prone to giving de-halogenated arene. 

Bis(isopinocampheyl)borane adopts a conformation which minimizes steric interactions. This conformation can be represented schematically as in F and G, where the S, M, and L substituents are, respectively, the 3—H, 4—CH2, and 2—CHCH3 groups of the carbocyclic structure. The steric environment at boron in this conformation is such that Z-alkenes encounter less steric encumberance in transition state G than in F. 

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