Relative rates of alkylation for B-substituted triarylphosphines: an -Boron group enhances reactivity on phosphorus

Advancements in main-group catalysis are contingent on our ability to quantify effects that enhance reactivity in these systems. Herein we report the rates of alkylation for several substituted phosphines. We report that by incorporating a single pinacol boronic ester group in the ortho -position on triphenylphosphine, the rate of substitution with benzyl bromide is approximately 4.7 times faster than the parent compound as measured by initial rates. The corresponding meta - and para -isomers are only 1.3 and 1.5 times as fast, respectively. Using X-ray crystallographic data and quantum chemical calculations, we propose this rate acceleration occurs from an O to P electrostatic interaction that stabilizes the transition state. A slight rate enhancement for substitution reactions on P results from electrostatic interactions with an ortho -Bpin group.