An in-depth look at the effect of Lewis acid catalysts on Diels-Alder cycloadditions in ionic liquids

The present work explores in detail the Diels-Alder reaction between cyclopentadiene and a series of dienophiles, performed in an innovative medium such as an ionic liquid. The potential activation of different Lewis acid catalysts and their load effect when used in combination with this solvent have been explored, in order to settle the improvement on rates and selectivities.     

Metal bis(perfluorooctanesulfonyl)amides as highly efficient Lewis acid catalysts for fluorous biphase organic reactions

In fluorous biphase system, metal bis(perfluorooctanesulfonyl)amides are better Lewis acid catalysts than the analogous triflates toward either transesterifications, or direct esterifications, or Friedel-Crafts acylations or Baeyer-Villiger oxidations. These catalysts are selectively soluble in lower fluorous phase and can be recovered simply by phase separation. Furthermore, these catalysts can be reused without decrease of activity in most cases.     

Synthesis of 1-tetralones by intramolecular Friedel-Crafts reaction of 4-arylbutyric acids using Lewis acid catalysts

Intramolecular Friedel-Crafts reaction of 4-arylbutyric acids efficiently proceeded in the presence of catalytic amounts of Lewis acids such as Bi(NTf₂)₃ and M(OTf)₃ (M=Bi, Ga, In and rare-earth metals) to form 1-tetralones. Chroman-4-one and thiochroman-4-one were also obtained in good yields from 3-phenoxypropionic acid and 3-phenylthiopropionic acid, respectively.     

Efficient intramolecular Diels–Alder reactions of ester-tethered 1,7,9-decatrienoates catalyzed by bis-aluminated trifluoromethanesulfonamide

Bis-aluminated trifluoromethanesulfonamide generated in situ by mixing TfNH₂ (1 mol) and methylaluminum reagent (2 mol) is an effective catalyst for the IMDA reaction of ester-tethered 1,7,9-decatrienoates.     

Assessment of fluoroalkyltin compounds as fluorous Lewis acid catalysts

The synthesis of a variety of organotin compounds with 1H, 1H, 2H, 2H‐perfluorooctyl groups is reported, together with an improved method for the corresponding distannoxane. Unique properties of this compound are disclosed in terms of fluorophilicity and activity as a Lewis acid catalyst in comparison with other mono‐nuclear derivatives. A new criterion for obtaining high solubility in fluorocarbon solvents is presented. Copyright © 2003 John Wiley & Sons, Ltd.     

Lewis acid-mediated [3+2] cycloaddition between hydrazones and olefins

[3+2] Cycloaddition between hydrazones and olefins was accelerated in the presence of a stoichiometric amount of BF₃·OEt₂ or a catalytic amount of Zr(OTf)₄, Hf(OTf)₄, or Sc(OTf)₃ under mild conditions. The corresponding pyrazolidine derivatives were obtained in moderate to high yields using this novel [3+2] Lewis acid catalysis.     

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.     

Tricyclic-isoxazolidine analogues via intramolecular 1,3-dipolar cycloaddition reactions of nitrones

The tricyclic-isoxazolidine analogues tetrahydrothiochromenoisoxazoles, hexahydroisoxazolequinolines and tetrahydroisoxazolepyranopyridines were prepared by an intramolecular 1,3-dipolar cycloaddition reaction of a nitrone with an alkene. For N-alkylated hexahydroisoxazolequinolines, reduction of the reaction time from two days to 40 min was achieved using microwave heating. The cyclization to form tetrahydroisoxazolepyranopyridines only proceeded when the alkene was substituted with an electron withdrawing group.     

The search for tolerant Lewis acid catalysts. Part 2: Enantiopure cycloalkyldialkylsilyl triflimide catalysts

A series of 2-aryl- and arylmethyl-3-dialkylphenylsilyl cycloalkanones have been prepared and resolved. The pure enantiomers were reduced to the corresponding cycloalkane derivatives. These were used for the in situ generation of enantiopure cycloalkylsilyl triflimides by protodesilylation with bis(trifluoromethanesulfonyl)imide. The association of a bulky leaving group with a silicon atom carrying large substituents was again shown to favour the complexation of silicon with carbonyl groups: all these trialkylsilyl triflimides showed a high catalytic activity for Diels-Alder reactions. Enantiomeric excesses up to 59% were observed. This is the highest enantioselectivity ever observed for a Diels-Alder reaction catalysed by a silicon Lewis acid.     

ortho-Acidic aromatic thiols as efficient catalysts of intramolecular Morita-Baylis-Hillman and Rauhut-Currier reactions

ortho-Mercaptobenzoic acid and ortho-mercaptophenols were discovered as efficient thiol catalysts of both the intramolecular Morita-Baylis-Hillman (MBH) and Rauhut-Currier (RC) reaction. High reaction rates were achieved under mildly basic, aqueous conditions. The unprecedented catalytic activity of these protic nucleophiles could originate from a Brønsted acid induced destabilization of intermediate thioethers and thus represent a unique mechanism of multifunctional Lewis base catalysis.     

Theoretical study of the [2+3] cycloaddition of nitrones to nitriles-influence of nitrile substituent,solvent and Lewis acid coordination

The [2+3] cycloaddition of nitrone PhCHdoublebondN(Me)O to nitriles RCtriplebondN (R=Me, Ph, CF(3)) was studied using quantum chemical calculations at the HF/6-31G* and B3LYP/6-31G* level of theory. With MeCN and PhCN, the reaction occurs through a concerted mechanism and leads selectively to Delta(4)-1,2,4-oxadiazolines rather than Delta(2)-1,2,5-oxadiazolines. Electron withdrawing substituents such as CF(3) at the nitrile provoke a non-synchronous bond formation, with the C-O bond being established on an earlier stage than the C-N bond. Additionally, the reaction becomes thermodynamically and kinetically more favourable. In the reaction of adducts of MeCN with BH(3) or BF(3) as model Lewis acids, the mechanism was found to change from fully concerted in the case of free MeCN towards a two-step reaction in the presence of BF(3), in which C-O bond formation occurs first. The BH(3)-mediated reaction occupies an intermediate stage where ring formation occurs in one-step but non-simultaneously, similar to the reaction of CF(3)CN. Interaction of the Lewis acid with the nitrile in the course of the reaction facilitates the cycloaddition by stabilizing transition states, intermediate and product rather than by activating the nitrile. The solvent influences the organic reaction mainly by lowering the energy of the reagents, thus leading to a higher activation barrier in a more polar solvent. In the Lewis acid mediated reaction, in contrast, the intermediate is strongly stabilised by a polar solvent and with that the synchronicity of the reaction changes in favour of a two-step mechanism.     

Chiral rare earth organophosphates as homogeneous Lewis acid catalysts for the highly enantioselective hetero-Diels–Alder reactions

Various trivalent rare earth-chiral phosphate complexes [(R)-1-RE, (R)-3-RE, and (R)-4-Ce] were prepared and evaluated as a Lewis acid catalyst for the asymmetric hetero-Diels–Alder reaction of aldehydes with the Danishefsky's diene. Some of them effectively promoted the reaction at room temperature in the presence or absence of achiral additives under homogeneous conditions to afford the corresponding cycloadducts with high ee's (up to 99% ee). During these reactions, remarkably high asymmetric amplifications (positive nonlinear effects) were observed as the first example in the metal ion–chiral ligand 1:3 catalytic system. A scandium catalyst bearing the H₈-BNP ligand, (R)-3-Sc, could be recovered after the reaction and successfully reused for the next round of reactions. In addition, the hetero-Diels–Alder reaction of -keto esters was effectively catalyzed by the ytterbium complex, (R)-1-Yb, without any additives thus producing the asymmetric quaternary carbon in excellent enantioselectivities (up to >99% ee).     

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.     

Rh(I)-catalyzed mild intramolecular [4+2] cycloaddition reactions of ester-tethered diene-yne compounds

The cationic rhodium(I) species derived from [Rh(COD)Cl]₂ and AgSbF₆ efficiently catalyze intramolecular [4+2] cycloadditions of ester-tethered 1,3-diene-8-yne derivatives such as 2-propynyl penta-2,4-dienoate and 2,4-pentadienyl propiolate derivatives in fluorinated alcohols.     

Cationic Rh(I) catalyst in fluorinated alcohol: mild intramolecular cycloaddition reactions of ester-tethered unsaturated compounds

In fluorinated alcohols, the cationic Rh(I) species, which is derived from [Rh(COD)Cl]2 and AgSbF6, efficiently catalyzed intramolecular [4+2] cycloaddition reactions of ester-tethered 1,3-diene-8-yne derivatives. The catalytic system was also effective in intramolecular [5+2] cycloaddition reactions of ester-tethered omega-alkynyl vinylcyclopropane compounds.     

Lewis acid promoted reactions of γ,γ-dialkoxyallylic zirconium species with various carbonyl compounds

The reactions of γ,γ-dialkoxyallylic zirconium species with carbonyl compounds in the presence of Lewis acid are reported. The reactivity of γ,γ-dialkoxyallylic zirconium species and reaction pathway were strongly dependent on the structure and electrostatic nature of the carbonyl compounds.