Zwitterionic Relatives to the Classic [(P–P)Rh(solv)₂]⁺ Ions: Neutral Catalysts Active for HE Bond Additions to Olefins (E=C, Si, B)

Abstract

Since their introduction by Osborne and Schrock three decades ago, cationic rhodium [(P-P)Rh(solv) 2 ] + complexes have been used as precatalysts for a wide range of EÀH (E = C, Si, B) bond-forming processes.[1-4] Herein, we introduce a new and complementary class of neutral, formally zwitterionic rhodium complexes supported by bis(phosphanyl)-and bis(amino)borate ligands that provide an alternative

Zwitterionic Relatives of Cationic Platinum Group Metal Complexes: Applications in Stoichiometric and Catalytic σ‐Bond Activation

“…[93,94] In keeping with previously characterized PGM zwitterions of this type, crystallographic data obtained for 46-48 and 51 revealed no inter-or intramolecular ion-pairing interactions, Scheme 14. Cationic and zwitterionic platinum complexes, including benzene CÀH bond activation by the platinum zwitterion 32, leading to 34. and the observed Rh···B distances (> 3.6 ) confirmed that the borate fragment is spatially separated from the rhodium center.…”

“…However, whereas the catalytic activity of these rhodium cations was completely inhibited upon addition of appreciable quantities of CH 3 CN, the zwitterions 47 and 50 proved tolerant to these conditions, even when using up to 50 % by volume CH 3 CN as the reaction medium. [93] Vedernikov and co-workers have observed that exposure of the anionic borate complexes 61 to an O 2 /R'OH mixture (R' = H, Me, or Et) in benzene or THF affords the corresponding platinum(IV) species 62 (Scheme 19). [96] This methyl migration process was proposed to occur as an electrophilic substitution at one of the BÀCH 3 sites by a five-coordinate platinum(IV) zwitterion, accompanied by nucleophilic attack of solvent at boron.…”

“…[3,4] In a survey of rhodiumcatalyzed styrene hydrogenation, the zwitterions 48 and 51 out-performed their cationic counterparts (54 and 55) in terms of activity; perhaps surprisingly, the [Ph 2 BN 2 ]-ligated zwitterion 51 was the most active of the catalysts surveyed. [93] However, despite the well-established utility of P,N-ligated rhodium and iridium cations including Crabtrees catalyst 56 [5] in alkene hydrogenation, [2] PGM complexes featuring heterobidentate borato P,N-ligands have yet to be reported, and zwitterionic iridium derivatives of [R 2 2 ] (tpy = 2-p-tolylpyridyl), which was prepared in the context of an investigation of ligand effects on the excited-state properties of iridium(III) complexes. [95] In surveying the rhodium-mediated hydroacylation of 4-methyl-4-pentenal (57) to 3-methylcyclopentanone (58) in acetone (Scheme 17), zwitterionic pre-catalysts provided an advantage over cationic pre-catalysts in maintaining excellent activity in the presence of both high-and low-polarity solvents.…”

“…Such divergent reactivity behavior may be attributable to the poor solubility of cationic species in low-polarity media, such as benzene, as well as solvent inhibition by more polar solvents, such as acetonitrile. [93] A comparison of the performance of cationic and zwitterionic rhodium pre-catalysts, including 47, 50, 53, and 55, in the hydroboration and hydrosilylation of styrene (using HBcat in THF, and Ph 2 SiH 2 in acetone, respectively) was also examined (Scheme 18). In all cases, the [Ph 2 SiN 2 ]-ligated cation 55 performed poorly, providing inconsistent results owing to a propensity to form rhodium(0); otherwise, in all the reactions, only 59 or 60 were obtained as products.…”

Synthesis and Molecular Structure of [<i>cis</i>-Rh(PMePh₂)₂(NCCH₃)₂][BC₂₈H₄₀O₄]

“…8 Bond distances and angles are similar to those reported for analogous cis-and trans-complexes, 8 including those containing chelating bisphosphines. 9,10 The B-O bond distances are typical for four coordinate borates containing catechol groups. There does not appear to be any significant interaction of the arylspiroborate anion and the rhodium center.…”