A Bis(silylene)‐Substituted ortho‐Carborane as a Superior Ligand in the Nickel‐Catalyzed Amination of Arenes

Abstract

The synthesis and structure of the first 1,2-bis-(NHSi)-substituted ortho-carborane [(LSi:)C] 2 B 10 H 10 (termed SiCCSi)i sr eported (NHSi = N-heterocyclic silylene;L = PhC(NtBu) 2 ). Its suitability to serve as ar eliable bis(silylene) chelating ligand for transition metals is demonstrated by the formation of [SiCCSi]NiBr 2 and [SiCCSi]Ni(CO) 2 complexes. The CO stretching vibration modes of the latter indicate that the Si II atoms in the SiCCSi ligand are even stronger s donors than the P III atoms in phosphines and C II atoms in Nheterocyclic carbene (NHC) ligands.M oreover,t he strong donor character of the [SiCCSi]ligand enables [SiCCSi]NiBr 2 to act as an outstanding precatalyst (0.5 mol %loading) in the catalytic aminations of arenes,s urpassing the activity of previously knownm olecular Ni-based precatalysts (1-10 mol %). Scheme 1. Multidentate NHSi ligands.

Synthesis and Catalytic Activity of Iron Hydride Ligated with Bidentate N-Heterocyclic Silylenes for Hydroboration of Carbonyl Compounds

“…1 Compared with N-heterocyclic carbenes (NHCs), the divalent Si center in a singlet state has unique σ-donating/π-accepting and steric hindrance properties. Therefore, transition metal complexes with NHSi ligands have been proved to be highly active precatalysts in the Suzuki 2 and Heck 3 coupling reactions, reduction of amides, 4 borylation 5 and amination of arenes, 6 the [2 + 2 + 2] cyclotrimeriztion reaction of phenylacetylene, 7 hydrosilyation of alkenes 8 or ketones, 9 and Sonogashira 10 and Kumada 11 crosscoupling reactions. However, there are few reports on multidentate NHSi ligands, and NHSi-ligated hydrido metal complexes remain virtually unknown.…”

Synthesis of N-Heterocycle Substituted Silyl Ligands within the Coordination Sphere of Iron

“…In a first set of experiments, the transformation of the trichlorosilyl ligand into tris(N-pyrrolyl)silyl was investigated. Complete substitution was achieved on the Fe(II) complex Cl 3 Si−FeCp(CO) 2 1: reaction with 3 equiv sodium pyrrolide produced the trisubstituted complex 4 (Scheme 2) resulting in a diagnostic shift of the 29 Additionally, the disappearance of the three distinct 13 C NMR resonances around 200 ppm for 3 62 and appearance of a single resonance at 217.9 ppm for Na-5 is consistent with the formation of a fluxional 5-coordinate structure with fast axial− equatorial exchange. 63−73 The independently synthesized ammonium salt NEt 4 -2 74 readily undergoes substitution under the same conditions, showing that nucleophilic substitution is feasible on Cl 3 Si bound to Fe(0) and may take place after deprotonation of 3.…”

“…The tmim moiety of crude product 9 in CD 3 CN gives rise to four resonances in 1 H NMR for the aromatic protons, indicating local 3-fold symmetry (see Supporting Information Figure S15). Furthermore, the spectrum of the reaction mixture of 9 displays four multiplet signals at δ = 3.66, 3.73, 4.00, and 4.10 ppm which couple in HMQC with two signals in 13 C NMR at 44.5 and 61.9 ppm, originating from the diastereotopic protons of N−CH 2 and O−CH 2 . An additional set of three multiplets with a 1:2:1 ratio at δ = 1.60, 1.77, and 2.34 ppm, corresponding to four protons, and coupling with two signals in 13 C NMR at 29.1 and 30.7 ppm, originates from the central CH 2 moieties.…”

“…The iPr residues give rise to 3 septets in a 1:1:2 ratio and 5 doublets in a 1:1:1:1:4 ratio, suggesting hindered rotation around the C aryl −N amine bond and free rotation around the C aryl −N imine bond. In 13 C NMR two resonances appear for the carbonyl carbons, which suggests that the substituent is bound to silicon in a bidentate fashion, rendering the silicon atom chiral and hence the carbonyls diastereotopic. The structure inferred from NMR was confirmed by the crystal structure (Figure 5).…”

“…Analytically pure material (44 mg, 0.11 mmol, 87%) was obtained by precipitation from toluene (2 mL) with hexane (12 mL), storage at −35 °C for 20 h and removal of the supernatant. 1 H NMR (400 MHz, C 6 D 6 , 25 °C) δ = 6.80 ('t', 2 J(H,H) = 2.0 Hz, 3 J(H,H) = 2.0 Hz, 6H, pyrrole-Hα), 6.44 ('t', 2 J(H,H) = 2.0 Hz, 3 J(H,H) = 2.0 Hz, 6H, pyrrole-Hβ), 3.89 ppm (s, 5H); 13 Synthesis of [(pyr 3 Si)Fe(CO) 4 ] − Na + (Na-5). A Et 2 O (6 mL) solution of 3 (96 mg, 0.32 mmol) was added dropwise to a precooled suspension of NaPyr (127 mg, 1.44 mmol) in Et 2 O (5 mL) at −79 °C, resulting in a pink solution.…”