Description of the Scoring Method
The "Score" in the Lewis Acid table is a semi-quantitative measure of Lewis acidity designed to provide a general ranking of the Lewis acids for C-F bond activation. The scoring system ranges from 0.01 (weakest) to 1.99 (strongest), with the calcium ion (Ca2+) assigned a score of 1.00 as a reference point. This table's data, including scores, are designed as a quick reference and are updated frequently.
The scoring system considers a combination of factors, including:
Charge Density: Higher charge density generally correlates with stronger Lewis acidity. This is estimated based on the charge and ionic radius of the Lewis acid.
Fluoride Affinity: The predicted affinity of the Lewis acid for fluoride ions is an important consideration, as stronger fluoride binding can enhance C-F bond activation.
Literature Precedent: The prevalence of a Lewis acid in reported C-F activation reactions is taken into account. Lewis acids commonly used for this purpose are generally assigned higher scores.
Electronic and Steric Effects: The electronic and steric properties of the Lewis acid, including the presence of electron-withdrawing or electron-donating groups and steric hindrance around the Lewis acidic center, are considered.
Qualitative Comparisons: In cases where quantitative data is limited, qualitative comparisons to other Lewis acids are used to estimate the score.
Important Notes:
The scores are approximate and intended for general comparison purposes. The actual Lewis acidity can vary depending on the specific reaction conditions, solvent, and substrate.
The scoring system is designed to be flexible and will be adjusted as new information becomes available or the understanding of Lewis acidity evolves.
This scoring system, in conjunction with the other information in and off the table (ionic radius, charge density, fluoride affinity, etc.), offers a comprehensive guide for selecting and evaluating Lewis acids for re-functionalization reactions up to C-F bond activation. By considering these factors, researchers can make informed decisions about the most suitable catalysts for their specific needs and optimize reaction conditions to achieve efficient and selective C-F bond transformations.