Frustrated Lewis Pair Reactivity of Phosphino-Phosphenium Cations

The field of “frustrated Lewis pairs” (FLPs) broke the century-old paradigm that activation and cleavage of small molecules such as H2 belonged to the realm of transition metal chemistry. Since Stephan and co-workers’ seminal report on reversible H2 activation using a main-group phosphinoborane, the field of FLPs has grown immensely to show that main-group compounds can activate a wide range of organic molecules. While FLP reactivity is often thought to occur from the unquenched reactivity of sterically bulky Lewis acids and bases that cannot form a traditional Lewis adduct, recent reports have shown that FLP reactivity can also be accessed by compounds which are in equilibrium between their free and bound adduct forms. Herein, contributions on expanding the field of FLPs by utilization of the seemingly stable adducts of phosphino-phosphenium cations (PPCs) will be reported. PPCs have been known for over four decades, and PPCs [Ph2ClPPPh2]+ and [Ph3PPPh2]+ have been known for two decades. Yet no one had applied them in FLP chemistry. Initially, a dibenzo-7-phosphanorbornadiene and B(C6F5)3 were found to undergo FLP addition reactions with a terminal aryl alkyne. This addition product was further functionalized to make several anionic phosphines and a phosphino-phosphenium species. The latter compound reacted with another terminal alkyne, thus giving a valuable clue into the FLP reactivity of PPCs. This was further expanded upon by the use of PPCs for FLP addition reactions to terminal alkynes. This reactivity was used for the synthesis of cis-olefin-linked bidentate phosphines, thus showing an FLP route to these valuable ligands and bypassing traditional routes using pyrophoric lithium reagents, radicals, and transition metals. PPCs were shown to react with a diazodicarboxylate by addition across the N=N bond, and to react with azobenzenes to affect N=N double bond cleavage to result in four-membered phosphorus-nitrogen heterocycles. These heterocycles could undergo subsequent substitution reactions and ring-opening. Finally, PPCs were shown to react with disulfides and diselenides to affect E–E bond cleavage (E = S, Se). Attempts were made to apply PPCs as FLPs in reactions with H2, benzyne and other small molecules. Future work on the FLP chemistry of PPCs will be proposed.

Department dc:contributor.department

Chemistry

Year dc:date.issued

2024

Author dc:creator

• Kim, Hyehwang

Advisor dc:contributor.advisor

• Stephan, Douglas W

• Frustrated Lewis Pairs
• Phosphenium
• Phosphorus

Statement dc:rights

• Attribution-NoDerivatives 4.0 International

Licence dc:rights.uri

• http://creativecommons.org/licenses/by-nd/4.0/

Handle dc:identifier.uri

http://hdl.handle.net/1807/140071

OAI identifier oai:identifier

oai:utoronto.scholaris.ca:1807/140071