Bismuth catalysis has traditionally relied on the Lewis acidic properties of the element in a fixed oxidation state. In this paper, we report a series of bismuth complexes that can undergo oxidative addition, reductive elimination, and transmetallation in a manner akin to transition metals. Rational ligand optimization featuring a sulfoximine moiety produced an active catalyst for the fluorination of aryl boronic esters through a bismuth (III)/bismuth (V) redox cycle. Crystallographic characterization of the different bismuth species involved, together with a mechanistic investigation of the carbon-fluorine bond-forming event, identified the crucial features that were combined to implement the full catalytic cycle.

A catalytic transfer-hydrogenation utilizing a well-defined Bi(I) complex as catalyst and ammonia-borane as transfer agent has been developed. This transformation represents a unique example of low-valent pnictogen catalysis cycling between oxidation states I and III, and proved useful for the hydrogenation of azoarenes and the partial reduction of nitroarenes. Interestingly, the bismuthinidene catalyst performs well in the presence of low-valent transition-metal sensitive functional groups and presents orthogonal reactivity compared to analogous phosphorus-based catalysis. Mechanistic investigations suggest the intermediacy of an elusive bismuthine species, which is proposed to be responsible for the hydrogenation and the formation of hydrogen.

A structural synthetic model of the iron(IV)-oxo-halide active species of non-heme iron dependent halogenases is reported. Compounds with general formula [Fe(IV)(O)(X)(Pytacn)](+) (1-X, X = Cl, Br) have been prepared and characterized spectroscopically and chemically with regard to their oxidizing ability. 1-X performs hydrogen-atom abstraction of C-H bonds at reaction rates 2-3 times faster than the corresponding solvato dicationic species, thus modelling the first step in C-H functionalization taking place in natural halogenation.

Herein we present a Bi-catalyzed cross-coupling of arylboronic acids with perfluoroalkyl sulfonate salts based on a Bi­(III)/Bi­(V) redox cycle. An electron-deficient sulfone ligand proved to be key for the successful implementation of this protocol, which allows the unusual construction of C­(sp2)–O bonds using commercially available NaOTf and KONf as coupling partners. Preliminary mechanistic studies as well as theoretical investigations reveal the intermediacy of a highly electrophilic Bi­(V) species, which rapidly eliminates phenyl triflate.

Thed evelopment of new C À Hf unctionalization protocols based on inexpensive cobalt catalysts is currently attracting significant interest. Functionalized 8-aminoquinoline compounds are high-potential building blocks in organic chemistry andpharmaceutical compoundsa nd new facile routes for their preparation would be highly valuable.R ecently, copper has been applied as catalyst for the functionalization of 8-aminoquinoline compoundsa nd found to operate through as ingle electron transfer (SET) mechanism, although requiring elevated reaction temperatures.H erein, we described the first example of ac obalt-catalyzed remote C À Hf unctionalization of 8-aminoquinoline compoundso perating through aS ET mechanism, exemplified using ap ractical and mild nitration protocol. Ther eactionu ses inexpensive cobaltn itrate hexahydrate [Co(NO 3 ) 2 ·6 H 2 O] as catalyst and tert-butyl nitrite (TBN) as nitro source. This methodology offers the basis for the facile preparationo fm any newf unctionalized 8-aminoquinoline derivatives. Scheme 4. Proposed mechanism for the nitration reaction at C-5. Scheme 5. (a) Competition reactions performed to investigate relative reactivity of substrates 17 and 19.Y ields calculated from 1 HNMR spectra of the crude reaction mixture using 1,3,5-trimethoxybenzene as internal standarda nd are based on conversion of corresponding substrate. Reaction conditions: substrate( 0.5 mmol;0 .25 mmol of each substrate), Co(NO 3 ) 2 ·6 H 2 O( 29.1 mg, 0.1 mmol, 20 mol%),T BN (267 mL, 90%, 4.0 equiv., 2.0 mmol), acetic acid (3.5 mL), room temperature, 18 h. (b) Summary of kinetic isotopeexperiment(KIE);for further details see the Supporting Information. 1686 asc.wiley-vch.de

The selective annulation reaction of alkynes with substrates containing inert C-H bonds using cobalt as catalyst is currently a topic attracting significant interest. Unfortunately, the mechanism of this transformation is still relatively poorly understood, with little experimental evidence for intermediates, although an organometallic Co(III) species is generally implicated. Herein, we describe a rare example of the preparation and characterization of benchtop-stable organometallic aryl-Co(III) compounds (NMR, HRMS, XAS, and XRD) prepared through a C(sp)-H activation, using a model macrocyclic arene substrate. Furthermore, we provide crystallographic evidence of an organometallic aryl-Co(III) intermediate proposed in 8-aminoquinoline-directed Co-catalyzed C-H activation processes. Subsequent insights obtained from the application of our new organometallic aryl-Co(III) compounds in alkyne annulation reactions are also disclosed. Evidence obtained from the resulting regioselectivity of the annulation reactions and DFT studies indicates that a mechanism involving an organometallic aryl-Co(III)-alkynyl intermediate species is preferred for terminal alkynes, in contrast to the generally accepted migratory insertion pathway.

Reported herein are organometallic Ni(ii) complexes that react with mild oxidants such as O2 and H2O2 and exhibit oxidatively-induced reactivity to yield C–C and C–O bond formation products via observable high-valent Ni intermediates.

Herein we describe the synthesis of a family of aryl-Co(III)-carboxylate complexes and their reactivity with ethyl diazoacetate. Crystallographic, full spectroscopic characterization, and theoretical evidence of unique C-metalated aryl-Co(III) enolate intermediates is provided, unraveling a carboxylate-assisted formation of aryl-Co(III) masked-carbenes. Moreover, additional evidence for an unprecedented Co(III)-mediated intramolecular S2-type C-C bond formation in which the carboxylate moiety acts as a relay is disclosed. This novel strategy is key to tame the hot reactivity of a metastable Co(III)-carbene and elicit C-C coupling products in a productive manner.