Catalytic Ring-Closing Metathesis of Doubly Armed, Bridged Bicyclic Sulfones. Evaluation of Chain Length and Possible Intramolecular SO2 Group Ligation to the Ruthenium Carbenoid
Disubstituted bicyclic sulfones 3a−3d, which were prepared by the 2-fold alkylation of 1,6-dilithio-9-thiabicyclo[4.2.1]nonane dioxide, undergo ring-closing metathesis to give a select few of the possible dimers and trimers. Only in the case of 3d were monomeric end products formed. The pronounced diastereoselectivities observed, particularly with the two lowest homologues, are suggested to be kinetically favored because of the operation of internal ruthenium/sulfonyl oxygen coordination during generation of the first intermolecular double bond. This ligation appears to be an important component of the overall reaction in that it serves to maximize unfavorable nonbonded steric interactions when the sulfone bridges adopt a syn relationship. MM3 calculations indicate the anti sulfone dimers also to be thermodynamically favored when n = 3. The preference for the anti sulfone arrangement appears to erode with an increase in the length of the tethers. Not unexpectedly, a ring size dependency is likely at play. The development of a ring-closing metathesis strategy for the incorporation of sulfone groups into stereochemically defined polybicyclic molecules has been realized.
Journal of the American Chemical Society