Decarbonylative Alkynylation of Aryl Anhydrides via Palladium Catalysis.

A robust palladium-catalyzed decarbonylative alkynylation of aryl anhydrides is reported. The catalytic system of Pd(OAc)2/XantPhos and DMAP as a nucleophilic additive has been identified as effective promoters for decarbonylative Sonogashira alkynylation. Recently, activated esters, amides, and carboxylic acids were applied as electrophiles in transition-metal-catalyzed decarbonylative alkynylation. The present process expands this reactivity to readily available aryl anhydrides as electrophilic reagents for decarbonylative alkynylation. It is worth noting that the reactivity of aryl anhydrides is higher than that of esters, amides, and carboxylic acids in decarbonylative alkynylation. Broad substrate scope and excellent functional group tolerance are presented, demonstrating that aryl anhydrides may serve as a general and practical class of electrophiles to achieve the synthesis of internal alkynes.

Palladium-Catalyzed Decarbonylative Borylation of Aryl Anhydrides.

A palladium-catalyzed base-free decarbonylative borylation of aryl anhydrides has been developed. Catalyst system consisting of Pd(OAc)2/dppb enables readily available aryl anhydrides to be employed as electrophiles for the synthesis of versatile arylboronate esters via O-C(O) bond activation and decarbonylation. This method is characterized by an excellent functional group tolerance and broad substrate scope, using bench stable aryl anhydrides as aryl electrophiles in C-B bond formation. Mechanistic studies and functionalization of late-stage pharmaceutical molecules are disclosed.

Pd-Catalyzed Double-Decarbonylative Aryl Sulfide Synthesis through Aryl Exchange between Amides and Thioesters.

We report the palladium-catalyzed double-decarbonylative synthesis of aryl thioethers by an aryl exchange reaction between amides and thioesters. In this method, amides serve as aryl donors and thioesters are sulfide donors, enabling the synthesis of valuable aryl sulfides. The use of Pd/Xantphos without any additives has been identified as the catalytic system promoting the aryl exchange by C(O)-N/C(O)-S cleavages. The method is amenable to a wide variety of amides and sulfides.

Rh(I)-Catalyzed Intramolecular Decarbonylation of Thioesters.

Decarbonylative synthesis of thioethers from thioesters proceeds in the presence of a catalytic amount of [Rh(cod)Cl]2 (2 mol %). The protocol represents the first Rh-catalyzed decarbonylative thioetherification of thioesters to yield valuable thioethers. Notable features include the absence of phosphine ligands, inorganic bases, and other additives and excellent group tolerance to aryl chlorides and bromides that are problematic using other metals to promote decarbonylation. Gram scale synthesis, late-stage pharmaceutical derivatization, and orthogonal site-selective cross-couplings by C-S/C-Br cleavage are reported.

Conversion of esters to thioesters under mild conditions.

We report conversion of esters to thioesters via selective C-O bond cleavage/weak C-S bond formation under transition-metal-free conditions. The method is notable for a general and practical transition-metal-free system, broad substrate scope and excellent functional group tolerance. The strategy was successfully deployed in late-stage thioesterification, site-selective cross-coupling/thioesterification/decarbonylation and easy-to-handle gram scale thioesterification. Selectivity and computational studies were performed to gain insight into the formation of weak C-S bonds by C-O bond cleavage, which contrasts with the traditional trend of nucleophilic additions to carboxylic acid derivatives.

Rh-Catalyzed Base-Free Decarbonylative Borylation of Twisted Amides.

We report the rhodium-catalyzed base-free decarbonylative borylation of twisted amides. The synthesis of versatile arylboronate esters from aryl twisted amides is achieved via decarbonylative rhodium(I) catalysis and highly selective N-C(O) insertion. The method is notable for a very practical, additive-free Rh(I) catalyst system. The method shows broad functional group tolerance and excellent substrate scope, including site-selective decarbonylative borylation/Heck cross-coupling via divergent N-C/C-Br cleavage and late-stage pharmaceutical borylation.

Diels-Alder reactions between cyclopentadiene analogs and benzoquinone in water and their application in the synthesis of polycyclic cage compounds.

Diels-Alder reactions between cyclopentadiene analogs and p-benzoquinone were explored in water and yielded 83-97% product, higher than the results reported in water with a catalyst or cetrimonium bromide (CTAB) micelles. The novel adduct 10 was synthesized and further used to synthesize the bi-cage hydrocarbon 4,4'-spirobi[pentacyclo[5.4.0.02,6.03,10.05,9]undecane], which has a high density (1.2663 g cm-3) and a high volumetric heat of combustion (53.353 MJ L-1). Four novel bi-cage hydrocarbon compounds were synthesized in water using this method starting from 2,2'-bi(p-benzoquinone) and cyclopentadiene analogs.

Ruthenium(II) Complexes of Aryl Triazole Foldamers as Receptors for Anions.

Ruthenium(II) complexes of oligo(bipyridine-phenyl triazole)s were synthesized as receptors for anions. The receptors, which are partially preorganized through metal-ligand interactions, can fold into a helical conformation to bind chloride, bromide, iodide, or nitrate anions in their inner cavities in acetone or competitive H-bonding media such as 5% water in acetone and DMSO. The short oligomer 1 can sandwich the studied anions with high affinity in acetone. Addition of 5% water to acetone results in overall decreases in the binding strength due to the binding competition from water, but does not change the basic binding mode. The highly competitive H-bonding solvent DMSO causes the receptor-anion interactions to become even weaker. However, in DMSO, the short receptor 1 is still able to bind the halide anions with moderate affinities in a 1:1 stoichiometry, whereas the longer oligomer 2 forms double-strand helices with an anion wrapped inside for all anions investigated so far.

Aromatic triazole foldamers induced by C-H···X (X = F, Cl) intramolecular hydrogen bonding.

Aryl-triazole oligomers based on isobutyl 4-fluorobenzoate and isobutyl 4-chlorobenzoate were designed and synthesized. Crystal structure and (1)H-(1)H NOESY experiments demonstrate that the oligomers adopt stable helical conformation, which are induced by C(5)-H···X-C (X = F, Cl) intramolecular hydrogen bonding between triazole protons and halogen atoms. The stabilities of the folded conformations are confirmed by DFT calculations, which show that each C(5)-H···F-C planar interaction lowers the energy by ~3 kcal mol(-1) on average, and by ~1 kcal mol(-1) when C(5)-H···Cl-C bridges are formed. The hydrogen-bonding networks are disrupted in competitive hydrogen-bonding media such as DMSO, generating the unfolded oligomers.

Controlling binding affinities for anions by a photoswitchable foldamer.

A photoswitchable foldamer containing an azobenzene and phenyl-1,2,3-triazole motif was found to show photochemical/dark isomerization in a controllable and reversible manner in both the absence and presence of anions. The transformation in conformation of the foldamer leads to changes in binding affinities for anions to a certain extent that depends on the size and the geometrical shape of the anions.

Chiral eta(6)-arene/N-tosylethylenediamine-ruthenium(II) complexes: solution behavior and catalytic activity for asymmetric hydrogenation.

Aromatic ketones are enantioseletively hydrogenated in alcohols containing [RuX{(S,S)-Tsdpen}(eta(6)-p-cymene)] (Tsdpen=TsNCH(C(6)H(5))CH(C(6)H(5))NH(2); X=TfO, Cl) as precatalysts. The corresponding Ru hydride (X=H) acts as a reducing species. The solution structures and complete spectral assignment of these complexes have been determined using 2D NMR ((1)H-(1)H DQF-COSY, (1)H-(13)C HMQC, (1)H-(15)N HSQC, and (1)H-(19)F HOESY). Depending on the nature of the solvents and conditions, the precatalysts exist as a covalently bound complex, tight ion pair of [Ru(+)(Tsdpen)(cymene)] and X(-), solvent-separated ion pair, or discrete free ions. Solvent effects on the NH(2) chemical shifts of the Ru complexes and the hydrodynamic radius and volume of the Ru(+) and TfO(-) ions elucidate the process of precatalyst activation for hydrogenation. Most notably, the Ru triflate possessing a high ionizability, substantiated by cyclic voltammetry, exists in alcoholic solvents largely as a solvent-separated ion pair and/or free ions. Accordingly, its diffusion-derived data in CD(3)OD reflect the independent motion of [Ru(+)(Tsdpen)(cymene)] and TfO(-). In CDCl(3), the complex largely retains the covalent structure showing similar diffusion data for the cation and anion. The Ru triflate and chloride show similar but distinct solution behavior in various solvents. Conductivity measurements and catalytic behavior demonstrate that both complexes ionize in CH(3)OH to generate a common [Ru(+)(Tsdpen)(cymene)] and X(-), although the extent is significantly greater for X=TfO(-). The activation of [RuX(Tsdpen)(cymene)] during catalytic hydrogenation in alcoholic solvent occurs by simple ionization to generate [Ru(+)(Tsdpen)(cymene)]. The catalytic activity is thus significantly influenced by the reaction conditions.