RESUMO
Derived from structurally similar precursors, two different azidodiboranes went through distinct aryl migration reactions triggered by different boron-boron separation distances. Biphenylene based diborane with a shorter boron-boron distance underwent heterolateral aryl migration to form a seven-membered azadiborepin, while xanthrene based diborane with a longer boron-boron distance afforded a stable bis-azidoborane scaffold. The pyrolysis of such a bis-azidoborane led to eight-membered oxazadiborocine through homolateral aryl migration and subsequent [3+2] cycloaddition. Density functional theory (DFT) calculations unveiled that the boron-boron separation distances were the intrinsic factors for the distinct migrations.
RESUMO
The general strategies to stabilize a boryl radical involve single electron delocalization by π-system and the steric hinderance from bulky groups. Herein, a new class of boryl radicals is reported, with intramolecular mixed-valent B(III) Br-B(II) adducts ligated by a cyclic (alkyl)(amino)carbene (CAAC). The radicals feature a large spin density on the boron center, which is ascertained by EPR spectroscopy and DFT calculations. Structural and computational analyses revealed that the stability of radical species was assisted by the CAAC ligand and a weak but significant B(III)Br-B(II) interaction, suggesting a cooperative avenue for stabilization of boryl radicals. Two-electron reduction of these new boryl radicals provides C-H insertion products via a borylene intermediate.
RESUMO
Nitrogen fixation is essential for the maintenance of life and development of society, however, the large bond dissociation energy and nonpolarity of the triple bond constitute a considerable challenge. The transition metals, by virtue of their combination of empty and occupied d orbitals, are prevalent in the nitrogen fixation studies and are continuing to receive a significant focus. The main group metals have always been considered incapable in dinitrogen activation owing to the absence of energetically and symmetrically accessible orbitals. The past decades have witnessed significant breakthroughs in the dinitrogen activation with the main group elements and compounds via either matrix isolation, theoretical calculations or synthetic chemistry. The successful reactions of the low-valent species of the main group elements with inert dinitrogen have been reported via the π back-donation from either the d orbitals (Ca, Sr, Ba) or p orbitals (Be, B, C ). Herein, the significant achievements have been briefly summarized, along with predicting the future developments.
Assuntos
Fixação de Nitrogênio , Elementos de Transição , Ligantes , Metais/química , Elementos de Transição/químicaRESUMO
Hydrogen isotope exchange (HIE) represents one of the most attractive labeling methods to synthesize deuterium- and tritium-labeled compounds. Catalytic HIE methods that enable site-selective C-H bond activation and exchange labeling with gaseous isotopes D2 and T2 are of vital importance, in particular for high-specific-activity tritiation of pharmaceuticals. As part of our interest in exploring s-block metals for catalytic transformations, we found CsN(SiMe3 )2 to be an efficient catalyst for selective HIE of benzylic C-H bonds with D2 gas. The reaction proceeds through a kinetic deprotonative equilibrium that establishes an exchange pathway between C-H bonds and D2 gas. By virtue of multiple C-H bonds activation and high activity (isotope enrichment up to 99 %), the simple cesium amide catalyst provided a very powerful and practically convenient labeling protocol for synthesis of highly deuterated compounds and high-specific-activity tritiation of pharmaceuticals.
Assuntos
Hidrogênio , Deutério/química , Hidrogênio/química , Trítio/química , Catálise , Preparações FarmacêuticasRESUMO
Brønsted base catalyzed C-C bond formation reactions have been extensively utilized as reliable, efficient, and atom economical methods in organic synthesis. However, the electrophiles were mostly limited to polar ones such as imines, carbonyl compounds, α,ß-unsaturated compounds, styrenes and conjugated dienes. The use of α-alkenes as electrophiles in the C-C bond formation reactions always needs transition metal catalysts. Herein, we reported an alkyl lithium-catalyzed benzylic C-H bond addition of alkyl pyridines to α-alkenes. The alkyl lithium catalyst displayed quite different selectivity from those of transition metal catalysts.
RESUMO
Direct functionalization of the benzylic C-H bond of diarylmethanes is an important strategy for the synthesis of diarylmethine-containing compounds. However, the methods developed to date for this purpose require a stoichiometric amount (usually more) of either a strong base or an oxidant. Reported here is the first catalytic benzylic C-H bond addition of diarylmethanes to styrenes and conjugated dienes. A potassium zincate complex, generated from potassium benzyl and zinc amide, acts as a catalyst and displays good activity and chemoselectivity. Considering the atom economy of the reaction and the ready availability of the catalyst, this reaction constitutes a practical, efficient method for diarylalkane synthesis.
RESUMO
The benzylic functionalization of alkylpyridines is an important pathway for pyridine derivatives synthesis. The reaction partners, however, were mostly limited to highly reactive polar electrophiles. Herein, we report a potassium amide-catalyzed selective benzylic C-H bond addition of alkylpyridines to styrenes. Potassium bis(trimethylsilyl)amide (KHMDS), a readily available Brønsted base, showed excellent catalytic activity and chemoselectivity. A series of alkylpyridine derivatives, including benzylic quaternary carbon substituted pyridines, were obtained in good to high yield. Preliminary mechanistic studies revealed that the deprotonation equilibrium is probably responsible for the excellent selectivity.
RESUMO
An efficient and general protocol for the ortho-alkylation of pyridines via C-H addition to olefins has been developed, using cationic half-sandwich rare-earth catalysts, which provides an atom-economical method for the synthesis of alkylated pyridine derivatives. A wide range of pyridine and olefin substrates including α-olefins, styrenes, and conjugated dienes are compatible with the catalysts.
Assuntos
Alcenos/química , Metais Terras Raras/química , Compostos Organometálicos/química , Piridinas/química , Piridinas/síntese química , Catálise , Estrutura Molecular , EstereoisomerismoRESUMO
Amides are a fundamental and widespread functional group, and are usually considered as poor electrophiles owing to resonance stabilization of the amide bond. Various approaches have been developed to address challenges in amide transformations. Nonetheless, most methods use activated amides, organometallic reagents or transition metal catalysts. Here, we report the direct alkylation of N,N-dialkyl benzamides with methyl sulfides promoted by the readily available base LDA (lithium diisopropylamide). This approach successfully achieves an efficient and selective synthesis of α-sulfenylated ketones without using transition-metal catalysts or organometallic reagents. Preliminary mechanism studies reveal that the deprotonative aroylation of methyl sulfides is promoted by the directed ortho-lithiation of the tertiary benzamide with LDA.
RESUMO
Iron-catalyzed cross-coupling of alkenyl/aryl carboxylates with primary alkyl Grignard reagent was described. This reaction brought a new family of electrophiles to iron catalysis. The combination of an inexpensive carboxylate electrophile and an iron catalyst would generate ample advantages.
Assuntos
Alcenos/química , Carbono/química , Ácidos Carboxílicos/química , Ferro/química , Oxigênio/química , Catálise , Indicadores e Reagentes/químicaRESUMO
The first catalytic α-alkylation reaction of benzyl sulfides and 1,3-dithianes with styrenes and conjugated dienes was developed under mild conditions by using a readily available Brønsted base potassium bis(trimethylsilyl)amide (KHMDS) as catalyst. The reaction displayed good functional group tolerance, high efficiency, and excellent chemoselectivity. A series of desired alkylation products were obtained in good to high yield. Preliminary mechanism studies suggested that two of the potassium amide catalyst molecules worked together in the catalytic cycle.
RESUMO
Biaryl scaffolds were constructed via Ni-catalyzed aryl C-O activation by avoiding cleavage of the more reactive acyl C-O bond of aryl carboxylates. Now aryl esters, in general, can be successfully employed in cross-coupling reactions for the first time. The substrate scope and synthetic utility of the chemistry were demonstrated by the syntheses of more than 40 biaryls and by constructing complex organic molecules. Water was observed to play an important role in facilitating this transformation.
Assuntos
Compostos de Bifenilo/síntese química , Ácidos Carboxílicos/química , Fenóis/química , Catálise , Naftóis/química , Níquel/químicaRESUMO
Aryl C-O and C-F can be transformed into C-Me via Ni-catalyzed coupling with MeMgBr under mild conditions.
Assuntos
Éteres/química , Hidrocarbonetos Aromáticos/síntese química , Níquel/química , Compostos Organometálicos/química , Catálise , Hidrocarbonetos Aromáticos/química , Metilação , Estrutura Molecular , EstereoisomerismoRESUMO
Allylic substitution reactions, a well-established approach for new bond construction, often need transition-metal catalysts and stoichiometric amounts of organometallic reagents, strong bases, or oxidants. Lithium diisopropylamide (LDA), a widely used and commercially available Brønsted base, is herein reported to catalyze the allylic C-H bond addition of 1,3-diarylpropenes to styrenes. Preliminary mechanism studies have provided a solid structure of the π-allyllithium intermediate and revealed the unique catalytic roles of LDA and its conjugate acid diisopropylamine.
RESUMO
A nickel-catalyzed, efficient C-N bond reduction of aromatic and benzylic ammonium triflates has been developed using sodium isopropoxide as a reducing agent. The high efficiency, mild conditions, and good compatibility with various substituents made this method an effective supplement to the current catalytic hydrogenation reactions. Combining this reductive deamination reaction with directed aromatic functionalization reactions, a powerful strategy for regioselective functionalization of arenes was demonstrated using dialkylamine groups as traceless directing groups.
RESUMO
A general and efficient method for the synthesis of tertiary amides has been developed via the copper-catalyzed aerobic oxidative amidation of tertiary amines. Due to the use of the O2 oxidant, various functional groups were well tolerated under the present conditions. Extensive substrates studies demonstrated its potential as a practical approach for the synthesis of tertiary amides.
RESUMO
A mild and efficient protocol for the high para-selective arylation of monosubstituted arenes with tertiary benzamides has been developed via palladium-catalyzed oxidative coupling reactions. Due to the mild conditions and the easy availability of substrates and oxidant, this method could potentially provide a practical approach for the synthesis of para-substituted biaryl compounds.