Examination of a Series of Ir and Rh PXL Pincer Complexes as (Pre)catalysts for Aromatic C–H Borylation

Abstract

This work follows a previously published study of high-turnover aromatic C–H borylation by Ir complexes supported by POCOP-type ligands incorporating −PiPr2 side donors (L1–L3) using HBpin in the presence of olefins. A variety of pincer-supported Ir and Rh complexes have been tested as precatalysts in the analogous aromatic C–H borylation. The ligands in this study included a number of aryl/bis­(phosphine) pincers of the PCP type, as well as PCS, PNP, PNSb, PSiP, and PBP ligands. The syntheses primarily targeted precursors of the (PXL)­M­(H)­(Cl) type (M = Rh, Ir); exceptions included complexes (L15)­Ir­(H)­(OAc), (L17)­Ir­(COE), (L18)­Ir­(H)4, and (L16)­Rh­(H2). The catalytic competence was tested using C6D6 as the substrate (and solvent) with HBpin and 1-hexene as reagents. C–H borylation and hydroboration of 1-hexene were the two competing catalytic reactions. None of the Rh complexes tested displayed any C–H borylation activity. Among the Ir complexes, only those possessing a central aryl site in the pincer showed significant C–H borylation activity with C6D6. The most promising precatalysts (L3)­Ir­(H)­(Cl), (L4)­Ir­(H)­(Cl), (L7)­Ir­(H)­(Cl), (L11)­Ir­(H)­(Cl), (L13)­Ir­(H)­(Cl), and (L15)­Ir­(H)­(OAc) were tested in the C–H borylation of C6H5F and C6H5CF3. All of these catalysts showed a roughly statistical preference for the borylation of only meta and para sites in C6H5CF3. For C6H5F borylation of all three sites (ortho, meta, para) was observed. Catalysts supported by the ligands L3 and L11 showed the highest rate of reaction and higher selectivity for C–H borylation vs hydroboration. Borylation of m-ClC6H4Me catalyzed by (L3)­Ir­(H)­(Cl) and (L11)­Ir­(H)­(Cl) resulted in only the borylation of only one aromatic C–H site (meta to both substituents), <10% of borylation of the benzylic site, and no borylation of the C–Cl moiety.

Rhodium‐Catalyzed Acrylate Synthesis from Carbon Dioxide and Ethylene by using a Guanidine‐Based Pincer Ligand: Perturbing Occupied d‐Orbitals by pπ‐dπ Repulsion Makes a Difference

“…This value is inferior to those of group-10-metal based d 10 -catalysts up to 514, [17] but is comparable to that of the isoelectronic d 8ruthenium(0) catalyst up to 15. [20] A control experiment using PCP-rhodium complex 9 [28] afforded only trace amount of acrylate under identical conditions (entry 4), which is in perfect agreement with our DFT calculations (Figure 2), For further screening, we focused on the use of 7 as a catalyst since mixing 5 and a base led to a formation of Rh(I) species reactive enough to etheric solvents (such as THF) as abovementioned. We next probed the effect of reaction temperature and found 160 °C as an optimal temperature (entries 5-7).…”

“…All other chemicals were purchased from commercial sources and used as received. The following compounds were prepared according to literature procedures: N , N ′‐(1,3‐phenylene)bis(2‐chloroacetamide), [30] [Rh(coe) 2 Cl] 2 , [31] complex 9 [28,32] . All NMR data of 1 – 8 are shown in Supporting Information.…”

Pd-pincer complexes containing six-membered rings that are more active allylation catalysts than five-membered ring analogues

“…This high activity of catalysts derived from L8 led us to investigate it further. The catalysis with L8 and Table2, entries[10][11][12][13][14][15]. These results demonstrated the greater activity of the Pd-TFA catalysts than the Pd-Cl catalysts: at 60 °C, after 2 h and at 1.25 mol% catalyst loading, full conversion was observed with Pd-TFA catalysts whilst only 48% conversion was observed with the Pd-Cl catalyst (Table2, entry 12).…”

Rhodium and Iridium Mediated C-H and O-H Bond Activation of Two Schiff Base Ligands: Synthesis, Characterization and Catalytic Properties of the Organometallic Complexes

“…In the last step, elimination of HCl takes place, which is believed to be facilitated by the closeness of Ir-Cl bond to the phenolic O-H fragment in the iridium(III) intermediate, and thus the organoiridium complexes are obtained as end products. It is relevant to mention here that like rhodium(I), iridium(I) is also known to bring about C-H bond activation (Boutry et al, 1997;Tang et al, 2009;Press et al, 2016;Hung et al, 2021;Liu et al, 2021;Slack and Colacot, 2021;Yoshino and Matsunaga, 2021).…”