Cp*M-Mediated P-H Activation Reactions: Activity and Mechanisms

Date

2022-09-22

Authors

Yang, Jin

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Abstract

This thesis presented the synthesis and reactivity of metal complexes for the hydrophosphination of alkenes and dehydrocoupling of phosphines. The mechanism of these metal-catalyzed P-H activation reactions was explored. Half-sandwich Cp*Ru complexes (Cp*= 1,2,3,4,5-pentamethylcyclopentadienyl) were developed as catalysts for hydrophosphination, based on the previous work in the Rosenberg group. Cp*Ru phosphido complexes, Ru(h 5 -Cp*)(PR2 )(PR2 H)2 (Ru-1) were found to be the vital intermediates for the hydrophosphination. Preliminary mechanistic studies also indicate that the catalyst resting state is Ru(h5 -Cp*)(PR2 )(PR2 H)(P) (P = hydrophosphination product) and intramolecular P-H bond cleavage is turnover-limiting. These investigations provide sufficient parallels to our established chemistry of the indenyl analogues to imply that conjugate addition of metal phosphido at alkene plays a significant role in these half-sandwich catalytic systems. The increased steric crowding at the Cp*Ru fragment and P-basicity/nucleophilicity of its phosphido complexes lead to a 30-fold increase in the hydrophosphination activity in the Cp* system compared to the indenyl catalysts. A half-sandwich Co catalyst, Co(h 5 -Cp*)I 2 (CO) (Co-1), was also developed for hydrophosphination along the lines of the conjugate addition mechanism. Similar to the Cp*Ru system, the substrate scope for alkene is limited to electron-deficient alkene. However, the Cp*Co catalyst significantly expands the substrate scope for phosphines (PR2 H and PRH 2 , R = alkyl and aryl). A detailed mechanistic study on the Cp*Co system was performed. The results show that the Co-catalyzed hydrophosphination occurs through iv an outer-sphere mechanism and the stoichiometric formation of diphosphine is a critical catalyst activation step. Since the side product diphosphine was formed during the Co-catalyzed hydrophosphination, using complex Co-1 as a catalyst for dehydrocoupling phosphines was investigated. The preliminary studies reveal the role of base and Cp* ligand in the catalysis. Additionally, the study highlights the importance of removing dihydrogen throughout the process. Thus, hydrogen acceptors (HA) were used to facilitate the dehydrocoupling reactions. Last, the novel P-H activation process between Cp*Co complexes [Co(h5 -Cp*)(NCCH 3 ) 3 ][SbF 6 ] 2 (Co-5) and excess PPh2 H was investigated through various analytical techniques.

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Keywords

hydrophosphination, catalysis, organometallics

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