Solvolysis of 2-Bicyclo

Area of Science:

• Organic Chemistry
• Reaction Mechanisms
• Carbocation Chemistry

Background:

• Investigating the influence of structural features on solvolysis rates and reaction mechanisms.
• Understanding the nature of carbocations, particularly bicyclic systems, and their rearrangements.

Purpose of the Study:

• To determine if the ethylene bridge in 2-bicyclo[3.2.2]nonyl p-toluenesulfonate provides anchimeric assistance during solvolysis.
• To elucidate the structure of the 2-bicyclo[3.2.2]nonyl cation and the mechanisms of product formation using isotopic labeling.

Main Methods:

• Solvolysis of 2-bicyclo[3.2.2]nonyl p-toluenesulfonate and cycloheptyl p-toluenesulfonate in 2,2,2-trifluoroethanol (TFE).
• Synthesis and solvolysis of [1-(13)C]-2-bicyclo[3.2.2]nonyl p-toluenesulfonate in methanol and TFE.
• Quantitative (13)C Nuclear Magnetic Resonance (NMR) analysis with a relaxation reagent to determine label distribution in products.

Main Results:

• Solvolysis rates of 2-bicyclo[3.2.2]nonyl p-toluenesulfonate and cycloheptyl p-toluenesulfonate were comparable, indicating no significant rate enhancement from the ethylene bridge.
• (13)C labeling studies showed exclusive placement of the label at two positions in the products, with non-unity ratios.
• The bicyclo[3.2.2]nonene product (10) exhibited less label scrambling than other products, suggesting earlier formation. Exo-2-substituted bicyclo[3.3.1]nonane and bicyclo[3.3.1]nonene (11) showed similar (13)C redistribution patterns to the 2-substituted bicyclo[3.2.2]nonane.

Conclusions:

• The 2-bicyclo[3.2.2]nonyl cation is classical, lacking significant charge delocalization through anchimeric assistance.
• Product 10 is formed at an earlier stage of ionization than the 2-substituted bicyclo[3.2.2]nonane.
• The observed (13)C redistributions suggest that 1,3-hydride shifts, leading to exo-2-substituted bicyclo[3.3.1]nonane and 11, primarily occur at the solvent-separated ion pair stage.