Aiming
to get knowledge on the reactivity of low-coordinate cobalt(0) species
toward primary phosphines, the reactions of [(IPr)Co(vtms)2] and [(ICy)2Co(vtms)] (IPr = 1,3-bis(2′,6′-diisopropylphenyl)imidazol-2-ylidene,
ICy = 1,3-dicyclohexylimidazol-2-ylidene, and vtms = vinyltrimethylsilane)
with several primary aryl phosphines have been examined. The reactions
of [(IPr)Co(vtms)2] and [(ICy)2Co(vtms)] with
H2PDmp (Dmp = 2,6-dimesitylphenyl) at 80 °C furnish
the diamagnetic cobalt(I) phosphido complexes [(NHC)Co(PHDmp)] (NHC
= IPr, 1; ICy, 2) that feature the Co–(η6-mesityl) interaction. Complex 1 can coordinate
CO to generate the terminal phosphido complex [(IPr)Co(CO)3(PHDmp)] (3) and can be oxidized by [Cp2Fe][BArF
4] to yield the cobalt(II) phosphido complex [(IPr)Co(PHDmp)][BArF
4] (4, BArF
4 = tetrakis(3,5-di(trifluoromethyl)phenyl)borate). For the reactions
with sterically less-hindered primary phosphines, [(IPr)Co(vtms)2] is inert toward H2PC6H2-2,4,6-Me3 (H2PMes) at room temperature, whereas
[(ICy)2Co(vtms)] can react with H2PMes at room
temperature to produce the cobalt(II) phosphido alkyl complex trans-[(ICy)2Co(CH2CH2SiMe3)(PHMes)] (5). At 80 °C, the cobalt(0) alkene
complexes [(IPr)Co(vtms)2] and [(ICy)2Co(vtms)]
and also the cobalt phosphido complexes, 1, 2, and 5 can serve as precatalysts for the dehydrocoupling
reaction of H2PMes to afford MesHPPHMes. NHC–Co(I)-phosphido
species are proposed as the in-cycle intermediates for these cobalt-catalyzed
dehydrocoupling reactions.
The reactions of nitrosoarenes with transition-metal species are fundamentally important for their relevance to metal-catalyzed transformations of organonitrogen compounds in organic synthesis and also the metabolization of nitroarenes and anilines in biology. In addition to the well-known reactivity of metal-mediated N−O bond activation and cleavage of nitrosoarenes, we present herein the first observation of a nitrosoarene C−N bond oxidative addition reaction upon the interaction of a three-coordinate cobalt(0) species [(IPr)Co(vtms) 2 ] with 2,4,6-tri(tert-butyl)-1-nitroso-benzene (Ar*NO). The reaction produces a cobalt nitrosyl aryl complex, [(IPr)Co-(Ar*)(NO)] (1), with a bis(nitrosoarene)cobalt complex, [(IPr)Co(η 2 -ONAr)(κ 1 -O-ONAr)] (2), as an intermediate. Spectroscopic characterizations, DFT calculations, and kinetic studies revealed that the redox non-innocence of nitrosoarene induces a stepwise pathway for the C−N bond oxidative addition reaction.
Selectivity control in the addition reactions of internal alkynes that bear two similar substituents presents a big challenge in modern organic synthesis. Herein we report that β-(E)-selective hydrosilylation of alk-2-ynes with tertiary silanes can be achieved by using Co 2 (CO) 8 as the catalyst. Under the catalytic reaction conditions, a wide range of alk-2-ynes can be converted into vinylsilanes with the β-(E) isomers as the major or sole hydrosilylation products. Mechanistic studies suggest that alkynebridged dicobalt species are the likely intermediates. Steric repulsion between substituents in alkenyl-bridged dicobalt silyl intermediates is proposed as the key factor inducing the observed regioselectivity.
Among the great efforts of developing cobalt-based catalysts for hydrosilylation reactions, cobalt (II) and cobalt(I) complexes are the extensively studied ones. In contrast, explorations on cobalt(0) complexes are relatively rare. Presented herein is the investigation on the catalytic performance of low-coordinate cobalt(0) N-heterocyclic carbene (NHC) complexes in the hydrosilylation reaction of alkynes, which disclosed the fine performance of [(CyIDep)Co(η 2 -CH 2 CHSiMe 3 ) 2 ] (CyIDep denotes for a 1,3-bis (2 0 ,6 0 -diethylphenyl)imidazole-2-ylidene that bears fused cyclohexyl group on the imidazole backbone) in catalyzing the syn-addition of a series of symmetric and unsymmetric internal alkynes with H 2 SiPh 2 , producing vinylsilanes with high regio-selectivity. Mechanistic study indicates that the catalytic reaction likely proceeds on a cobalt(0)/cobalt(II) cycle and that the high selectivity is governed by the steric nature of the NHC ligand.
Comprehensive SummaryHydrogermylation of alkenes and alkynes provides a straightforward synthetic route to organogermanium compounds that have been found applications in organic synthesis, medicinal chemistry, and material science. This paper reviews the advances in transition‐metal‐catalyzed hydrogermylation of unsaturated carbon‐carbon bonds, including alkenes, alkynes, dienes, and allenes, with the objective of providing readers with the status of the field. Progress, problems, and perspectives in this field are discussed.This article is protected by copyright. All rights reserved.
Summary of main observation and conclusionThe rich redox chemistry of nitrosoarenes has rendered these reactive molecules very useful in modern synthetic and material chemistry. Electrochemical studies have revealed the capability of nitrosoarenes to undergo one‐electron oxidation or reduction reaction for a long time. However, the isolation and structural characterization of nitrosoarene radical compounds deviating the stabilization of transition‐metal have not been achieved. Investigation on the reduction reaction of nitrosoarenes bearing steric demanding substituents has now revealed that the interaction of 2,6‐dimesityl‐1‐nitroso‐benzene (DmpNO) or 2,4,6‐tri(tert‐butyl)‐1‐nitroso‐benzene (TtpNO) with KC8 and crypt‐2,2,2 can produce the corresponding anion radical compound [K(crypt‐2,2,2)][DmpNO] (1) or [K(crypt‐2,2,2)][TtpNO] (2) in good isolated yield. Compounds 1 and 2 represent the first examples of isolable nitrosoarene radical compounds deviating the stabilization of transition‐metal, and have been characterized by single‐crystal X‐ray diffraction study, electron paramagnetic resonance (EPR) spectroscopy, and elemental analysis. Theoretical study in collaboration with the characterization data revealed that the unpaired spin in [DmpNO]•– and [TtpNO]•– delocalizes on the nitroso and the central phenyl groups.
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