Enantioselectivity in an Asymmetric Heck Reaction
Click the structures and reaction black arrows in sequence to view the 3D models and animations respectively
The enantioselectivity of this reaction is bound by the most stable conformations available. Here we have focused on the trans-syn-Re conformation. The enantioselectivity of the reaction is greatly increased by increasing the size of the R group on the oxazoline ring which in this case is an isopropyl group.
The trans conformation is more stable than the cis conformation due to ligand - ligand interactions being reduced as well as activating the Pd-Ph double bond. This is due to the trans mode positioning the phenyl group trans to the phosphorous atom rather than the nitrogen. As phosphorous has a higher trans influence it weakens the Pd-Ph bond. This makes it more suceptible to the carbopalladation step.
The Re conformation has a distorted square planar geometry with palladium. Along with this the restricted seven membered ring pushes the axial phenyl group closer to the palladium centre creating a more crowded plane above the palladium, enhancing the enantioselectivity of this reaction.
Animations based upon Wen-Qiong Wu, Qian Peng, Da-Xuan Donng, Xue-Longe Hou, and Yun-Dong Wu J.AM.Chem.Soc. 2008, 130, 9717-9725