Directly Bridging Indoles to 3,3′‐Bisindolylmethanes by Using Carboxylic Acids and Hydrosilanes under Mild Conditions

A straightforward Lewis acid‐promoted protocol for 3,3′‐bisindolylmethanes (BIMs) synthesis by reductive alkylation of indoles at the C3 position with carboxylic acids in the presence of hydrosilane was developed for the first time. Instead of aldehydes, more readily available, stable, and easy‐to‐handle carboxylic acids have been employed as alternative alkylating agents. As an efficient organocatalyst, B(C₆F₅)₃ enables the reductive alkylation of various substituted indole derivatives with carboxylic acids with up to 98 % yield at room temperature and under neat conditions. This metal‐free strategy offers an alternative approach for the direct functionalization of indoles to BIMs with carboxylic acids and such protocol allows selective reduction of carboxylic acid to aldehyde in combination with C−C bond formation.     

Decarboxylation with Carbon Monoxide: The Direct Conversion of Carboxylic Acids into Potent Acid Triflate Electrophiles

We report a new strategy for the conversion of carboxylic acids into potent acid triflate electrophiles. The reaction involves oxidative carbonylation of carboxylic acids with I₂ in the presence of AgOTf, and is postulated to proceed via acyl hypoiodites that react with CO to form acid triflates. Coupling this chemistry with subsequent trapping with arenes offers a mild, room temperature approach to generate ketones directly from broadly available carboxylic acids without the use of corrosive and reactive Lewis or Bronsted acid additives, and instead from compounds that are readily available, stable, and functional group compatible.     

Steric effects in synthesis—steric limits to the alkylation of nitriles and carboxylic acids

The steric limits to the alkylation of aliphatic nitriles and carboxylic acids have been investigated in some detail. For the experimental conditions considered (ionization by i-Pr₂NLi in THF followed by alkylation with RI/THF/HMPA) the most hindered nitriles R-CN and carboxylic acids R-CO₂H have the same secondary alkyi group RtBuPⁱCH-, but different tertiary. i.e. Rt-BuPrⁱEtC- or i-Pr₃C- for RCN and REt₂MeC for RCO₂H. A comparison of the relative merits of alkylation of esters, carboxylic acids, and nitriles is considered.     

Trapping phosphodiester-quinone methide adducts through in situ lactonization

The goal of in situ modification of DNA via phosphodiester alkylation has led to our design of quinone methide derivatives capable of alkylating dialkyl phosphates. A series of catechol derivatives were investigated to trap the phosphodiester-quinone methide alkylation adduct through in situ lactonization. The catechol derivatives were uniquely capable of characterizable p-quinone methide formation for mechanistic clarity. These investigations revealed that with a highly reactive lactonization group (phenyl ester), lactonization competed with quinone methide formation. Lactone-forming groups of lower reactivity (methyl ester, n-propyl ester, and dimethyl amide) allowed quinone methide formation followed by phosphodiester alkylation; however, they were ineffective at in situ lactonization to drain the phosphodiester alkylation equilibrium to the desired phosphotriester product. The derivatives tethered with lactone-forming functionality of intermediate reactivity (chloro-, trichloro-, and trifluoroethyl esters), allowed quinone methide formation, phosphodiester alkylation, and in situ lactonization to efficiently afford the trapped phosphotriester adduct.     

Catalytic and Asymmetric Fluorolactonisations of Carboxylic Acids through Anion Phase Transfer

Catalytic fluorolactonisations of aromatic carboxylic acids have been developed. The reactions proceed under mild conditions using the commercially available reagent Selectfluor. A weak phase transfer of the reagent mediated by Na₂CO₃ allows the reaction to be conducted in non‐polar solvents. Furthermore, by the use of a catalytic amount of (DHQ)₂PHAL (hydroquinine 1,4‐phthalazinediyl diether), the first asymmetric fluorolactonisation has been achieved. The corresponding isobenzofuran core can be found in many biologically active molecules.     

Direct Ruthenium‐Catalyzed Hydrogenation of Carboxylic Acids to Alcohols

The “green” reduction of carboxylic acids to alcohols is a challenging task in organic chemistry. Herein, we describe a general protocol for generation of alcohols by catalytic hydrogenation of carboxylic acids. Key to success is the use of a combination of Ru(acac)₃, triphos and Lewis acids. The novel method showed broad substrate tolerance and a variety of aliphatic carboxylic acids including biomass‐derived compounds can be smoothly reduced.     

Lewis acid-catalyzed Friedel-Crafts acylation reaction using carboxylic acids as acylating agents

Rare-earth metal Lewis acids, in particular Eu(NTf₂)₃, were found to be efficient catalysts for Friedel-Crafts acylation reaction using aliphatic as well as aromatic carboxylic acids as acylating agents at high temperature.     

N-Heterocyclic carbene–gold(I)-catalyzed carboheterofunctionalization of alkenes with arylboronic acids

A new approach to the synthesis of pyrrolidine, tetrahydrofuran, and imidazolidin-2-one via N-heterocyclic carbene–gold(I)-catalyzed intramolecular amino- or oxyarylation reactions from a wide variety of alkene substrates such as N-allyl amides, alcohols, carboxylic acids, and ureas in the presence of Selectfluor under mild conditions has been developed.     

A practical catalytic reductive amination of carboxylic acids

We report reductive alkylation reactions of amines using carboxylic acids as nominal electrophiles. The two-step reaction exploits the dual reactivity of phenylsilane and involves a silane-mediated amidation followed by a Zn(OAc)₂-catalyzed amide reduction. The reaction is applicable to a wide range of amines and carboxylic acids and has been demonstrated on a large scale (305 mmol of amine). The rate differential between the reduction of tertiary and secondary amide intermediates is exemplified in a convergent synthesis of the antiretroviral medicine maraviroc. Mechanistic studies demonstrate that a residual 0.5 equivalents of carboxylic acid from the amidation step is responsible for the generation of silane reductants with augmented reactivity, which allow secondary amides, previously unreactive in zinc/phenylsilane systems, to be reduced.

We report reductive alkylation reactions of amines using carboxylic acids as nominal electrophiles.     

Unstrained C–C bond activation and directed fluorination through photocatalytically-generated radical cations

Expanding the repertoire of controlled radical fluorination techniques, we present a photosensitized unstrained C–C bond activation/directed monofluorination method using Selectfluor and 9-fluorenone. The reaction is amenable to the opening of multiple 1-acetal-2-aryl substituted rings to yield ω-fluoro carboxylic acids, esters, alcohols, and ketones with relative ease. Initial mechanistic insight suggests radical ion intermediates.     

A new and efficient method for the facile synthesis of N-acyl sulfonamides under Lewis acid catalysis

The N-acylation of sulfonamides with carboxylic acid anhydrides in the presence of Lewis acids is described. Several Lewis acids such as BF₃·Et₂O, ZnCl₂, MoCl₅, TiCl₄, B(C₆F₅)₃, Sc(OTf)₃ and I₂ were found to catalyze the reaction efficiently to furnish the N-acylated products in good yields under solvent-free conditions. The reactions of various sulfonamides were studied with different carboxylic acid anhydrides including the less reactive benzoic and pivalic anhydrides, in the presence of 3 mol % ZnCl₂ as the catalyst. Carboxylic acids were also successfully used as acylating agents via the mixed anhydride method.     

The study of industrializable ionic liquid catalysts for long-chain alkenes Friedel–Crafts alkylation

Catalysts based on different halo-alkanes structures with durable catalytic performance were synthesized and applied to the Friedel–Crafts alkylation of long-chain alkenes (mixed C_16–24 olefins) with toluene. Surprisingly, compared with the usual industrial catalysts (~10 runs), the cyclic times of the ionic liquid (IL) catalysts reached up to 24 runs, which greatly promotes the industrialization process. Then, Lewis acids of catalysts with different precursor/AlCl₃ molar ratios were investigated and a close relation was discovered between the Lewis acid and catalytic activity. In addition, a comparison of the different halo-alkanes structures about those catalysts was made. The results showed that the [C₆Et₃N]Cl–AlCl₃ had the strongest Lewis acid, corresponding to the highest catalytic performance. Also, the structures of precursors and the specific gravity and active site species of catalysts were investigated by Fourier transform infrared and Magic Angle Spinning Nuclear Magnetic Resonance (MAS NMR). Meanwhile, the various parameters (catalyst dosage, toluene/olefin molar ratio, reaction temperature and reaction time) of long-chain alkenes alkylation with toluene were studied. Finally, under the optimized reaction conditions, the conversion and selectivity of long-chain alkenes alkylation reached 99.92 and 32.99%, respectively.     

Cationic polymerization of cyclocarbonates

Trimethylenecarbonate (TMC) and neopentane diol carbonate (NPC) were polymerized with two groups of initiators, proton and carbenium ion donors or Lewis acids. Initiation with methyltriflate, triflic acid or triethyloxonium tetrafluoroborate in solution gave satisfactory yields (up to 90%) but only low molecular weights (M_n < 5000), due to rapid back-biting degradation. IR- and NMR-spectroscopy demonstrate that the propagation steps involve alkylation of the carbonyl oxygen and cleavage of the alkyl-0 bond by analogy with lactones. Whereas borontribromide and trichloride form solid complexes with NPC or TMC, but do not initiate a polymerization, boron trifluoride is a good initiator. High yields (up to 99,5%) and high molecular weights (M_w > 10⁵) were obtained. However, in analogy to triflic acid initiated polymerizations all polycarbonates contain ether groups. The molar fraction of the ether groups increases with the reaction temperature. High molecular-weight polycarbonates containing ether groups were also obtained with other strong Lewis acids such as SnCl₄, SnBr₄ and TiCl₄. In contrast, weak Lewis acids such as Bu₂SnBr₂ Bu₃SnOMe and Sn(II)2-ethylhexanoate yield polycarbonates free of ether groups. This finding and the NMR-spectroscopically identified endgroups suggest that these weak Lewis acids initiate an insertion mechanism.     

Spectroscopic and Crystallographic Characterization of the R3N+−C−H⋅⋅⋅X Interaction

As appreciation for nonclassical hydrogen bonds has progressively increased, so have efforts to characterize these interesting interactions. Whereas several kinds of C−H hydrogen bonds have been well-studied, much less is known about the R₃N⁺−C−H⋅⋅⋅X variety. Herein, we present crystallographic and spectroscopic evidence for the existence of these interactions, with special relevance to Selectfluor chemistry. Of particular note is the propensity for Lewis bases to engage in nonclassical hydrogen bonding over halogen bonding with the electrophilic F atom of Selectfluor. Further, the first examples of ¹H NMR experiments detailing R₃N⁺−C−H⋅⋅⋅X (X=O, N) hydrogen bonds are described.     

New C(4)‐Functionalized Colchicine Derivatives by a Versatile Multicomponent Electrophilic Aromatic Substitution

Electrophilic alkylation of colchicine at C(4) was accomplished by a multicomponent aromatic electrophilic substitution reaction with electrophilic aldehydes and carboxylic acids or amides in H₂SO₄. A series of new derivatives were obtained and evaluated for their antiproliferative effect towards various tumor cell lines, and their stimulatory effect on the development of polarity in human neutrophils.     

An efficient FeCl3‐promoted O‐alkyl cleavage of esters to carboxylic acids

A reliable and practical procedure for FeCl₃‐promoted ester cleavage has been developed. Lewis acids including TiCl₄, ZnO and FeCl₃ etc. were investigated as promoters for O‐alkyl cleavage of carboxylic acid ester. Under optimal reaction conditions, FeCl₃ (1.5 equiv.) was found to possess the highest activity and efficiently enhanced dealkylation of aryl esters, alkyl esters and aromatic heterocyclic esters to give their corresponding carboxylic acids in 54–98% yield, the method provides a complementary access to dealkylation of ester under neutral condition. Copyright © 2011 John Wiley & Sons, Ltd.     

Lewis acid-catalyzed tandem acylation–Nazarov cyclization for the syntheses of fused cyclopentenones

Treatment of N-tosylpyrroles or N-tosylindoles with α-unsubstituted α,β-unsaturated carboxylic acids or unsaturated carboxylic acids having an α electron-withdrawing substituent in the presence of TFAA and a Lewis acid catalyst resulted in the formation of fused cyclopentenones via a tandem acylation–Nazarov cyclization sequence, while either the acylation product obtained or no reaction occurred in the absence of the Lewis acid catalyst.     

Visible Light‐Induced Decarboxylative Alkylation of Heterocyclic Aromatics with Carboxylic Acids via Anthocyanin as a Photocatalyst

A visible light‐induced decarboxylative alkylation of heterocyclic aromatics with aliphatic carboxylic acids was developed by using anthocyanins as a photocatalyst under mild conditions. A series of alkylated heterocyclic compounds were obtained in moderate to good yields by using the metal‐free decarboxylative coupling reaction under blue light. This strategy uses cheap and readily available carboxylic acids as alkylation reagents with good functional group tolerance and environmental friendliness. It is worth noting that this is the first time that anthocyanin has been used to catalyze the Minisci‐type C?H alkylation. The mechanism of decarboxylation alkylation was studied by capturing the adduct of alkyl radical and hydroquinone, thus confirming a radical mechanism. This protocol provides an alternative visible light‐induced decarboxylative alkylation for the functionalization of heterocyclic aromatics.     

An effective method for the synthesis of carboxylic esters and lactones using substituted benzoic anhydrides with Lewis acid catalysts

An efficient mixed-anhydride method for the synthesis of carboxylic esters and lactones using benzoic anhydride having electron withdrawing substituent(s) is developed by the promotion of Lewis acid catalysts. In the presence of a catalytic amount of TiCl₂(ClO₄)₂, various carboxylic esters are prepared in high yields through the formation of the corresponding mixed-anhydrides from 3,5-bis(trifluoromethyl)benzoic anhydride and carboxylic acids. The combined catalyst consisting of TiCl₂(ClO₄)₂ together with chlorotrimethylsilane functions as an effective catalyst for the synthesis of carboxylic esters from free carboxylic acids and alcohols with 4-(trifluoromethyl)benzoic anhydride. Various macrolactones are prepared from the free ω-hydroxycarboxylic acids by the combined use of 4-(trifluoromethyl)benzoic anhydride and titanium(IV) catalysts together with chlorotrimethylsilane under mild reaction conditions. The lactonization of trimethylsilyl ω-(trimethylsiloxy)carboxylates using 4-(trifluoromethyl)benzoic anhydride is also promoted at room temperature in the presence of a catalytic amount of TiCl₂(ClO₄)₂. An 8-membered ring lactone, a synthetic intermediate of cephalosporolide D, is successfully synthesized according to this mixed-anhydride method using 4-(trifluoromethyl)benzoic anhydride by the promotion of a catalytic amount of Hf(OTf)₄.     

A New Mode of Chemical Reactivity for Metal‐Free Hydrogen Activation by Lewis Acidic Boranes

We herein explore whether tris(aryl)borane Lewis acids are capable of cleaving H₂ outside of the usual Lewis acid/base chemistry described by the concept of frustrated Lewis pairs (FLPs). Instead of a Lewis base we use a chemical reductant to generate stable radical anions of two highly hindered boranes: tris(3,5‐dinitromesityl)borane and tris(mesityl)borane. NMR spectroscopic characterization reveals that the corresponding borane radical anions activate (cleave) dihydrogen, whilst EPR spectroscopic characterization, supported by computational analysis, reveals the intermediates along the hydrogen activation pathway. This radical‐based, redox pathway involves the homolytic cleavage of H₂, in contrast to conventional models of FLP chemistry, which invoke a heterolytic cleavage pathway. This represents a new mode of chemical reactivity for hydrogen activation by borane Lewis acids.