Ni‐catalyzed cross‐coupling between aryl alkyl ethers (ArOR) and Grignard reagents (RMgBr), known since 1979, proceeds under mild conditions in many cases. Although the reaction routes of various synthetic protocols involving transition‐metal‐catalyzed C?O bond activation have been elucidated, the mechanism of this etheric Kumada–Tamao–Curriu reaction remains enigmatic. This is because oxidative addition of inert etheric C?O to Ni0 is thermodynamically and kinetically unfavorable, making it hard to explain the observed high reactivity of ether toward Ni catalysts. In this work, we used DFT calculations to identify a plausible reaction pathway by the Ni0‐ate complex, which enables smooth C?O bond cleavage and R‐group transfer with reasonable activation barriers; this mechanism also accounts for the ineffectiveness of Pd catalysts. These results throw new light on both C?O activation and cross‐coupling, and should be valuable for further rational development of the methodologies. 相似文献
Mild substitution reactions of acetals with carbon nucleophiles via the pyridinium‐type salts generated by the treatment of acetals with TESOTf‐2,4,6‐collidine or 2,2′‐bipyridyl have been developed. Various carbon nucleophiles, such as organocuprates, silyl enol ethers, enamines, etc., reacted with the pyridinium‐type salts to give the corresponding substituted products in good yields. The reactions proceeded under very mild conditions (non‐acidic conditions) and thus acid‐sensitive functional groups can be tolerated during the reaction. In addition, only an acetal can form the pyridinium‐type salt and react with nucleophiles in the presence of a ketal. This unusual selectivity is in contrast to general methods conducted under acidic conditions. 相似文献
Catalytic insertion or addition of a metal‐imido/nitrene species, generated from reaction of a transition‐metal catalyst with iminoiodanes, to C? H and C?C bonds offers a convenient and atom economical method for the synthesis of nitrogen‐containing compounds. Following this groundbreaking discovery during the second half of the last century, the field has received an immense amount of attention with a myriad of impressive metal‐mediated methods for the synthesis of amines and aziridines having been developed. This review will cover the significant progress made in improving the efficiency, versatility and stereocontrol of this important reaction. This will include the various iminoiodanes, their in situ formation, and metal catalysts that could be employed and new ligands, both chiral and non‐chiral, which have been designed, as well as the application of this functional group transformation to natural product synthesis and the preparation of bioactive compounds of current therapeutic interest. DOI 10.1002/tcr.201100018 相似文献
A series of new titanium(IV) complexes with o‐metalated arylimine and/or cis‐9,10‐dihydrophenanthrenediamide ligands, [o‐C6H4(CH?NR)TiCl3] (R=2,6‐iPr2C6H3 ( 3 a ), 2,6‐Me2C6H3 ( 3 b ), tBu ( 3 c )), [cis‐9,10‐PhenH2(NR)2TiCl2] (PhenH2=9,10‐dihydrophenanthrene; R=2,6‐iPr2C6H3 ( 4 a ), 2,6‐Me2C6H3 ( 4 b ), tBu ( 4 c )), [{cis‐9,10‐PhenH2(NR)2}{o‐C6H4(HC?NR)}TiCl] (R=2,6‐iPr2C6H3 ( 5 a ), 2,6‐Me2C6H3 ( 5 b ), tBu ( 5 c )), have been synthesised from the reactions of TiCl4 with o‐C6H4(CH?NR)Li (R=2,6‐iPr2C6H3, 2,6‐Me2C6H3, tBu). Complexes 4 and 5 were formed unexpectedly from the reactions of TiCl4 with two or three equivalents of the corresponding o‐C6H4(CH?NR)Li followed by sequential intramolecular C? C bond‐forming reductive elimination and oxidative coupling reactions. Attempts to isolate the intermediates, [{o‐C6H4(CH?NR)}2TiCl2] ( 2 ), were unsuccessful. All complexes were characterised by 1H and 13C NMR spectroscopy, and the molecular structures of 3 a , 4 a – c , 5 a , and 5 c were determined by X‐ray crystallography. 相似文献
A detailed mechanistic study of the hydroxylation of alkane C? H bonds using H2O2 by a family of mononuclear non heme iron catalysts with the formula [FeII(CF3SO3)2(L)] is described, in which L is a tetradentate ligand containing a triazacyclononane tripod and a pyridine ring bearing different substituents at the α and γ positions, which tune the electronic or steric properties of the corresponding iron complexes. Two inequivalent cis‐labile exchangeable sites, occupied by triflate ions, complete the octahedral iron coordination sphere. The C? H hydroxylation mediated by this family of complexes takes place with retention of configuration. Oxygen atoms from water are incorporated into hydroxylated products and the extent of this incorporation depends in a systematic manner on the nature of the catalyst, and the substrate. Mechanistic probes and isotopic analyses, in combination with detailed density functional theory (DFT) calculations, provide strong evidence that C? H hydroxylation is performed by highly electrophilic [FeV(O)(OH)L] species through a concerted asynchronous mechanism, involving homolytic breakage of the C? H bond, followed by rebound of the hydroxyl ligand. The [FeV(O)(OH)L] species can exist in two tautomeric forms, differing in the position of oxo and hydroxide ligands. Isotopic‐labeling analysis shows that the relative reactivities of the two tautomeric forms are sensitively affected by the α substituent of the pyridine, and this reactivity behavior is rationalized by computational methods. 相似文献
Get a grip! NiII complexes of the new pincer amidobis(amine) ligand are described. The Ni chloride complex catalyzes Kumada–Corriu–Tamao coupling of unactivated alkyl halides with alkyl Grignard reagents, as well as double C–C coupling of CH2Cl2 with alkyl Grignard reagents (see schemes).
An iron(III)–catecholate complex [L1FeIII(DBC)] ( 2 ) and an iron(II)–o‐aminophenolate complex [L1FeII(HAP)] ( 3 ; where L1=tris(2‐pyridylthio)methanido anion, DBC=dianionic 3,5‐di‐tert‐butylcatecholate, and HAP=monoanionic 4,6‐di‐tert‐butyl‐2‐aminophenolate) have been synthesised from an iron(II)–acetonitrile complex [L1FeII(CH3CN)2](ClO4) ( 1 ). Complex 2 reacts with dioxygen to oxidatively cleave the aromatic C? C bond of DBC giving rise to selective extradiol cleavage products. Controlled chemical or electrochemical oxidation of 2 , on the other hand, forms an iron(III)–semiquinone radical complex [L1FeIII(SQ)](PF6) ( 2ox‐PF6 ; SQ=3,5‐di‐tert‐butylsemiquinonate). The iron(II)–o‐aminophenolate complex ( 3 ) reacts with dioxygen to afford an iron(III)–o‐iminosemiquinonato radical complex [L1FeIII(ISQ)](ClO4) ( 3ox‐ClO4 ; ISQ=4,6‐di‐tert‐butyl‐o‐iminobenzosemiquinonato radical) via an iron(III)–o‐amidophenolate intermediate species. Structural characterisations of 1 , 2 , 2ox and 3ox reveal the presence of a strong iron? carbon bonding interaction in all the complexes. The bond parameters of 2ox and 3ox clearly establish the radical nature of catecholate‐ and o‐aminophenolate‐derived ligand, respectively. The effect of iron? carbon bonding interaction on the dioxygen reactivity of biomimetic iron–catecholate and iron–o‐aminophenolate complexes is discussed. 相似文献
Luminescent pincer‐type PtII complexes supported by C‐deprotonated π‐extended tridentate R C^N^N R′ ligands and pentafluorophenylacetylide ligands show emission quantum yields up to almost unity. Femtosecond time‐resolved fluorescence measurements and time‐dependent DFT calculations together reveal the dependence of excited‐state structural distortions of [Pt(R C^N^N R′)(CC‐C6F5)] on the positional isomers of the tridentate ligand. Pt complexes [Pt(R‐C^N^N R′)(CC‐Ar)] are efficient photocatalysts for visible‐light‐induced reductive C C bond formation. The [Pt(R‐C^N^N R′)(CC‐C6F5)] complexes perform strongly as phosphorescent dopants for green‐ and red‐emitting organic light‐emitting diodes (OLEDs) with external quantum efficiency values over 22.1 %. These complexes are also applied in two‐photon cellular imaging when incorporated into mesoporous silica nanoparticles (MSNs). 相似文献
Bridging the gap : Snapshots of 1,6‐H‐shift precursors indicate that a narrower C? H???C+ separation (D in the ORTEP diagram) in the title complexes induces faster degenerate rearrangement of 1 +. A contact distance of less than 2.7 Å is necessary to realize the organic three‐center two‐electron bond of [C? H? C]+, as indicated by extrapolation of the X‐ray data.
Check M(etal)ate: The chessboard and the figures represent a special reaction in which different low‐polarity metals can metalate arenes directly when they are brought into the right position. In a combination of queen (sodium) and knight (chromium or iron), it is possible for the knight (usually the weaker piece) to make a direct deadly hit on the king (benzene) in this game of elemental chess.
A series of hydrogen‐abstraction barriers of a nonheme iron(IV)–oxo oxidant mimicking the active species of taurine/α‐ketoglutarate dioxygenase (TauD) are rationalized by using a valence‐bond curve‐crossing diagram (see figure). It is shown that the barriers correlate with the strength of the C? H bond. Furthermore, electronic differences explain the differences between nonheme and heme iron(IV)–oxo hydrogen‐abstraction barriers.