In contrast to transition metal-catalysed C-H functionalisation, highly efficient construction of C-C and C-X (X = N, O, S, B, Si,
etc.
) bonds through metal-free catalytic C-H functionalisation ...remains one of the most challenging tasks for synthetic chemists. In recent years, electron-deficient boron-based catalyst systems have exhibited great potential for C-H bond transformations. Such emerging systems may greatly enrich the chemistry of C-H functionalisation and main-group element catalysis, and will also provide enormous opportunities in synthetic chemistry, materials chemistry, and chemical biology. This article aims to give a timely comprehensive overview to recognise the current status of electron-deficient boron-based catalysis in C-H functionalisation and stimulate the development of more efficient catalytic systems.
This review article provides a comprehensive overview to recognise the current status of electron-deficient boron-based catalysis in C-H functionalisations.
The catalytic asymmetric construction of silicon‐stereogenic silanes is of great interest and significance, but has met with only limited success to date. We herein report the enantioselective ...hydrosilylation of alkenes with dihydrosilanes by a chiral half‐sandwich scandium catalyst, which constitutes an efficient and general route for the synthesis of a wide range of enantioenriched silicon‐stereogenic silanes from easily accessible starting materials. This reaction features a broad substrate scope, high yields, and high enantioselectivity. Some of the chiral tertiary silane products were also converted into valuable derivatives, such as chiral silanol, quaternary silane, and benzosilole compounds.
A chiral half‐sandwich scandium catalyst enables the highly enantioselective hydrosilylation of alkenes with dihydrosilanes. This process constitutes an efficient and general route for the synthesis of a wide range of enantioenriched silicon‐stereogenic silanes from easily accessible starting materials.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Transition metal catalyzed C−H functionalization of organic compounds has proved to be a useful atom‐efficient strategy in organic synthesis. In contrast, main‐group‐element‐based catalytic processes ...for C−H functionalization have remained underexplored to date. Reported herein is the catalytic C(sp)−H silylation of a wide range of terminal alkynes with hydrosilanes by using a combination of B(C6F5)3 and an organic base such as triethylenediamine (DABCO). This protocol constitutes the first example of boron‐catalyzed C(sp)−H functionalization, offering a convenient route for the synthesis of a variety of alkynylsilanes. Experimental and computational studies have revealed that DABCO plays two crucial roles (Lewis base and Brønsted base) in this catalytic transformation.
Double duty: The combination of B(C6F5)3 and an organic base, such as triethylenediamine (DABCO), serve as an excellent catalytic system for the C(sp)−H silylation of terminal alkynes with hydrosilanes. DABCO plays two crucial roles (Lewis base and Brønsted base) as revealed by experimental and DFT studies.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
The 2+2 cycloaddition of allenes with alkenes is of much interest and importance as a straightforward route for the construction of four‐membered carbocycles but has remained much underexplored to ...date. Herein we report for the first time the intermolecular regio‐ and diastereoselective formal 2+2 cycloaddition of a wide range of allenes with amino‐functionalized alkenes by half‐sandwich rare‐earth catalysts. The reaction proceeded through an allene C(sp2)−H activation mechanism initiated by the site‐selective deprotonation of the allene unit by a rare‐earth metal alkyl species followed by alkene insertion into the resulting metal‐allenyl bond and the subsequent intramolecular cycloaddition to an allene C=C bond. This protocol offers a unique route for the synthesis of a new family of cyclobutane and cyclobutene derivatives which were difficult to access previously.
Half‐sandwich rare‐earth catalysts serve as a unique platform for the regio‐ and diastereoselective formal 2+2 cycloaddition of a wide range of allenes with amino‐functionalized alkenes via allene C(sp2)−H activation, affording a new family of cyclobutane and cyclobutene derivatives which were difficult to access previously.
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
A novel and facile CH bond fluorination proceeds under remarkably mild conditions (close to room temperature in most cases). Both aromatic and olefinic C(sp2)H bonds with a wide range of electronic ...properties are selectively fluorinated in the presence of a catalytic amount of simple, cheap, and nontoxic nitrate as the promoter. A PdII/PdIV catalytic cycle that is initiated by an in situ generated cationic Pd(NO3)+ species was proposed based on preliminary mechanistic studies.
Nitrate makes it possible: A novel and facile method for CH bond fluorination entails remarkably mild reaction conditions (close to room temperature in most cases). Both aromatic and olefinic C(sp2)H bonds were selectively fluorinated in the presence of a catalytic amount of inexpensive and nontoxic nitrate as the promoter.
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Selective introduction of fluorine into molecules by the cleavage of inert C−H bonds is of central academic and synthetic interest, yet remains challenging. Given the central role of alcohols in ...organic chemistry as the most ubiquitous building blocks, a versatile and selective C(sp3)−H and C(sp2)−H fluorination of simple alcohols, enabled by novel designed exo‐directing groups, is described. C(sp2)−H bond fluorination was achieved by using a simple acetone oxime as auxiliary, whereas a new, modular and easily accessible bidentate auxiliary was developed for the efficient and site‐selective fluorination of various primary methyl, methylene, and benzylic C(sp3)−H bonds. Fluorinated alcohols can readily be accessed by the removal of auxiliaries, and significantly expands the synthetic prospect of the present procedure.
A versatile and selective C(sp3)−H and C(sp2)−H fluorination of simple alcohols, enabled by an exo‐directing group (DG) is described. C(sp2)−H bond fluorination was achieved by using simple acetone oxime as an auxiliary, whereas a new and modularly accessible bidentate auxiliary was developed for the efficient and site‐selective fluorination of C(sp3)−H bonds. The auxiliary is removable and demonstrates broad substrate scope and excellent selectivity.
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A novel palladium catalyzed highly para-selective C-H difluoromethylation of electron-deficient aromatic carbonyls was developed. Diverse substituted aromatic ketones and benzoates were selectively ...difluoromethylated at the remote para-site of carbonyl groups in moderate to good yields. Moreover, the difuoromethylation was also compatible with several complex bioactive molecules.
We report herein the palladium-catalysed mono-selective C-H arylation of 2.2paracyclophane (PCP) with diverse aryl iodides in the absence of any pendant directing groups, providing straightforward ...and modular access to C4-arylated 2.2paracyclophanes. Moreover, a new PCP-containing biaryl monophosphine complex could be readily obtained through further derivation of the arylated product.
A non-directed palladium-catalyzed mono-selective C-H arylation of 2.2paracyclophane (PCP) with diverse aryl iodides was reported.
Nitration of N‐heteroaromatics: The first example of palladium‐catalyzed direct ortho‐nitration of aryl CH bonds is described. A range of azaarenes, such as 2‐arylquinoxalines, pyridines, pyrazoles, ...and O‐methyl oximes, were nitrated with excellent chemo‐ and regioselectivity (see scheme; DCE=1,2‐dichloroethane). Preliminary mechanistic investigations support a silver‐mediated radical mechanism involving palladium(II/III) and/or palladium(II/IV) catalytic cycles under oxidizing conditions.
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A versatile and site-selective nitrate-promoted C–H bond fluorination using various weak coordinating amides as intrinsic directing groups was developed. Diverse tertiary and secondary amides ...underwent selective aromatic C–H bond fluorination, which features broad substrate scope, good regioselectivity, and mild conditions. Moreover, the late-stage C–H bond fluorination of the challenging benzeneacetamides via distal directing was reported for the first time.
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IJS, KILJ, NUK, PNG, UL, UM
