N-Heterocyclic carbene rhodium(<scp>i</scp>) complexes containing an axis of chirality: dynamics and catalysis

Cassani, Maria Cristina; Brucka, Marta Anna; Femoni, Cristina; Mancinelli, Michele; Mazzanti, Andrea; Mazzoni, Rita; Solinas, Gavino

doi:10.1039/c3nj01620j

“…Internal alkynes are generally considered to be fairly inert toward a variety of transformation reactions in comparison to their terminal alkyne counterparts, − and therefore a catalyst that can effectively and selectively hydrosilylate internal alkynes would be very useful. Previous studies on Rh-NHC catalysts with N -donor-functionalized ligands exhibited good activity toward the hydrosilylation of terminal alkynes, where the majority of these catalysts, however, produced low selectivities. − The hydrosilylation of internal alkynes provides four possible products, the α-( E ), α-( Z ), β-( E ), and β-( Z ) alkene isomers (Scheme ).…”

Section: Results and Discussionmentioning

confidence: 99%

Self-Isomerized–Cyclometalated Rhodium NHC Complexes as Active Catalysts in the Hydrosilylation of Internal Alkynes

Vuuren

¹

,

Cordier

²

,

Nell

³

et al. 2022

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A range of unique Rh-based bidentate NHC complexes that are formed in a base-free tandem isomerization/ cyclometalation process were synthesized (1−4) from a range of imidazolium salts with an N-alkenyl tether. Cyclometalation occurred with complex 1, leading to an unprecedented complex, which is the first and only example in the literature of a nonaromatic C(sp 2 )−H activation leading to a C(sp 3 )−Rh cyclometalated product with a concomitant intramolecular/ isomerization process. Dealkylation of the N-alkenyl substituent occurred to form byproducts that showed metal N-coordination (1b and 2b). These byproducts, 1b and 2b, were further reacted with the anion exchange reagent NH 4 PF 6 to form the dimeric complexes 1bd and 2bd. All of the complexes were applied as precatalysts in the hydrosilylation of internal alkynes with excellent performance (conversions of 66−100%) after only 1 h at 80 °C without the use of an additive. Anticancer studies showed that complexes presented half-maximum inhibitory concentrations ranging from 3.71 to 25.85 μM. Depending on the cell line, complex 4 was the most cytotoxic complex, especially in the BT-20 triple-negative breast carcinoma, MCF-12A nontumorigenic mammary gland cell, MDA-MB-231 triple-negative breast carcinoma, and MCF-7/TAMR-1 tamoxifen-resistant subtype of the MCF-7 estrogen-and progesterone-positive luminal breast carcinoma cell lines.

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“…3 Hz) and ( E )-triethyl(styryl)silane (β-( E ) isomer, J 3 HH = ca. 19 Hz)) ,, or the analysis and matching of chemical shifts for products against authentic samples. Quantifications were performed upon integration of the selected peaks of the product against peaks of 1,2-dimethoxyethane.…”

Section: Experimental Sectionmentioning

confidence: 99%

Platinum Complexes with Chelating Acyclic Aminocarbene Ligands Work as Catalysts for Hydrosilylation of Alkynes

Chay

¹

,

Rocha

²

,

Pombeiro

³

et al. 2018

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This work describes the preparation of a series of platinum–aminocarbene complexes [PtCl{ C (N=C a (C 6 R 2 R 3 R 4 R 5 CO N b ))=N(H)R 1 }(CNR 1 )] a–b ( 8 – 19 , 65–75% isolated yield) via the reaction of cis -[PtCl 2 (CNR 1 ) 2 ] (R 1 = Cy 1 , t -Bu 2 , Xyl 3 , 2-Cl-6-MeC 6 H 3 4 ) with 3-iminoisoindolin-1-ones HN=C a (C 6 R 2 R 3 R 4 R 5 CON b H) (R 2 –R 5 = H 5 ; R 3 = Me, R 2 , R 4 , R 5 = H 6 ; R 3 , R 4 = Cl, R 2 , R 5 = H 7 ). New complexes 17 – 19 were characterized by elemental analyses (C, H, N), ESI + -MS, Fourier transform infrared spectroscopy (FT-IR), one-dimensional ( 1 H, 13 C{ 1 H}), and two-dimensional ( 1 H, 1 H correlation spectroscopy (COSY), 1 H, 13 C heteronuclear multiple quantum correlation (HMQC)/ 1 H, 13 C heteronuclear single quantum coherence (HSQC), 1 H, 13 C heteronuclear multiple bond correlation (HMBC)) NMR spectroscopy, and authenticity of known species 8 – 16 was confirmed by FT-IR and 1 H and 13 C{ 1 H} NMR. Complexes 8 – 19 were assessed as catalysts for hydrosilylation of terminal alkynes with hydrosilanes to give vinyl silanes, and complex [PtCl{ C (N=C a (C 6 H 3 (5-Me)CO N b ))=N(H)(2-Cl-6-MeC 6 H 3 )}{CN(2-Cl-6-MeC 6 H 3 )}] a − b ( 18 ) showed the hi...

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“…Decreasing the steric demand of the bis(alkene) on the NHC can increase the rate of rotation around the Rh‐NHC axis, with the rotation barriers of norbornadiene (NBD) complexes measurable by 1 H VT NMR spectroscopy [58] unlike the COD complexes [59] . A series of [(BocNHCH 2 CH 2 ‐NHC−R)Rh(Cl)(NBD)] complexes are reported to have metal‐carbene bond rotational barriers between 55 and 59 kJ mol −1 , [58] and Δ G for Rh‐carbene rotation in [(R−NHC−Bn)Rh(Cl)(NBD)] complexes was measured as 57 to 63 kJ mol −1 [60] . An analogue of 2 was synthesised using [Rh(NBD)Cl] 2 in the place of [Rh(COD)Cl] 2 .…”

Section: Resultsmentioning

confidence: 99%

“… [59] A series of [(BocNHCH 2 CH 2 ‐NHC−R)Rh(Cl)(NBD)] complexes are reported to have metal‐carbene bond rotational barriers between 55 and 59 kJ mol −1 , [58] and Δ G for Rh‐carbene rotation in [(R−NHC−Bn)Rh(Cl)(NBD)] complexes was measured as 57 to 63 kJ mol −1 . [60] An analogue of 2 was synthesised using [Rh(NBD)Cl] 2 in the place of [Rh(COD)Cl] 2 . The resulting NBD complex, [(N 3 Aib 4 NH(CH 2 ) 2 ‐NHC−Ph)Rh(Cl)(NBD)] 8 (10 % overall yield) showed significant changes in the appearance of the 1 H NMR spectrum at room temperature, with broader resonances, for example from the Aib methyl groups in the foldamer body (Figure 4 ).…”

Section: Resultsmentioning

confidence: 99%

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α‐Amino‐iso‐Butyric Acid Foldamers Terminated with Rhodium(I) N‐Heterocyclic Carbene Catalysts

Tilly

¹

,

Cullen

²

,

Heng

³

et al. 2022

Chemistry A European J

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To investigate how remotely induced changes in ligand folding might affect catalysis by organometallic complexes, dynamic α‐amino‐iso‐butyric acid (Aib) peptide foldamers bearing rhodium(I) N‐heterocyclic carbene (NHC) complexes have been synthesized and studied. X‐ray crystallography of a foldamer with an N‐terminal azide and a C‐terminal Rh(NHC)(Cl)(diene) complex showed a racemate with a chiral axis in the Rh(NHC) complex and a distorted 310 helical body. Replacing the azide with either one or two chiral L‐α‐methylvaline (L‐αMeVal) residues gave diastereoisomeric foldamers that each possessed point, helical and axial chirality. NMR spectroscopy revealed an unequal ratio of diastereoisomers for some foldamers, indicating that the chiral conformational preference of the N‐terminal residue(s) was relayed down the 1 nm helical body to the axially chiral Rh(NHC) complex. Although the remote chiral residue(s) did not affect the stereoselectivity of hydrosilylation reactions catalysed by these foldamers, these studies suggest a potential pathway towards remote conformational control of organometallic catalysts.

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N-Heterocyclic carbene rhodium(i) complexes containing an axis of chirality: dynamics and catalysis

Abstract: Novel rhodium(i) complexes [RhCl(NBD)(NHC)] [NHC = 1-benzyl-3-R-imidazolin-2-ylidene; R = Me, Bz, Tr, tBu]: determination of the rotation barriers about the Rh-carbene and catalytic activity in the hydrosilylation of terminal alkynes.

Cited by 22 publications

References 61 publications

Self-Isomerized–Cyclometalated Rhodium NHC Complexes as Active Catalysts in the Hydrosilylation of Internal Alkynes

Self-Isomerized–Cyclometalated Rhodium NHC Complexes as Active Catalysts in the Hydrosilylation of Internal Alkynes

Platinum Complexes with Chelating Acyclic Aminocarbene Ligands Work as Catalysts for Hydrosilylation of Alkynes

α‐Amino‐iso‐Butyric Acid Foldamers Terminated with Rhodium(I) N‐Heterocyclic Carbene Catalysts

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