Influence of the Solvent Polarity on the Stereoselectivity of the Uncatalyzed [4+2] Cycloaddition of Cyclopentadiene to N,N′-Fumaroyldi[(2R)-borane-10,2-sultam]

A correlation between the solvent polarity and the logarithm of the diastereoisomer ratio (dr) was found for the uncatalyzed 4+2] cycloaddition of cyclopentadiene to N,N′-fumaroyldi(2R)-bornane-10,2-sultam]. Using the Abboud-Abraham-Kamlet-Taft parameters, predictive values for this method, allowed an optimum diastereoisomeric excess (de) of 96% (EtOH, −78°). A similar diastereoselectivity was achieved using 5M LiClO₄/Et₂O or H₂O/β-cyclodextrin, and the influence of supercritical CO₂ is also reported. Selective cycloadditions of apolar diene in polar solvents are entropically favored by the greater dipole moment of the N-enoylcamphorsultam syn-s-cis conformers and of the C(α)-re transition states. Implications on the stereochemical course of the reaction are discussed.