Reactivity of Frustrated Lewis Pair: Carbocation versus Radical Intermediates

Area of Science:

• Computational Chemistry
• Organometallic Chemistry
• Frustrated Lewis Pairs (FLPs)

Background:

• Recent studies proposed single-electron transfer (SET) and radical formation as key mechanisms in frustrated Lewis pair (FLP) chemistry, particularly for C-C coupling reactions.
• The role of radical pathways in FLP-mediated C-C bond formation has been a subject of ongoing investigation and debate within the chemical community.

Purpose of the Study:

• To investigate the mechanistic pathways of reactions involving bulky phosphines, benzhydryl cations, and Lewis acids like tris(pentafluorophenyl)borane (B(C6F5)3).
• To computationally evaluate the kinetic favorability of radical versus non-radical pathways in FLP chemistry, specifically concerning C-C coupling and carbocation transfer.

Main Methods:

• Extensive dispersion-corrected Density Functional Theory (DFT) calculations were employed to model reaction pathways and determine kinetic barriers.
• Analysis focused on the generation of reactive intermediates, including benzhydryl radicals, phosphine radical cations, and phosphonium cations.
• Thermodynamic and kinetic assessments were performed for proposed radical C-C coupling and carbocation transfer mechanisms.

Main Results:

• DFT calculations indicate that while benzhydryl radicals and phosphine radical cations can be formed, direct P-C hetero-coupling to form phosphonium cations is kinetically favored.
• These phosphonium cations act as efficient carbocation transfer reagents to substrates like styrene, a pathway significantly more favorable than proposed radical C-C coupling.
• Meta-stable radical cation salts, such as Mes3P+•, are kinetically accessible via SET reactions involving Mes3P, B(C6F5)3, and specific oxidants like p-O2C6Cl4.

Conclusions:

• The study challenges the prevailing notion of radical-dominated C-C coupling in certain FLP systems, highlighting the importance of carbocation transfer mechanisms.
• Dispersion-corrected DFT calculations provide crucial insights into the kinetic landscape of FLP reactivity, favoring non-radical pathways under specific conditions.
• The findings suggest a re-evaluation of proposed radical mechanisms in FLP chemistry and emphasize the versatility of phosphonium cations as reactive intermediates.