Palladium-catalyzed sequential alkylation-alkenylation reactions. Application to the synthesis of 2-substituted-4-benzoxepines and 2,5-disubstituted-4-benzoxepines

The synthesis of 2-substituted-4-benzoxepines and 2,5-disubstituted-4-benzoxepines from aryl iodides and bromoenoates is described. This methodology is based on a palladium-catalyzed aromatic substitution followed by an intramolecular Heck sequence. Under the reaction conditions (Pd(OAc)(2) (10 mol %), tri-2-furylphosphine (20 mol %), norbornene (2 equiv), Cs(2)CO(3) (2 equiv), CH(3)CN, 85 degrees C), moderate to excellent yields of benzoxepines bearing numerous substituents (Me, F, Cl, etc.) are obtained.     

Highly efficient Heck reactions of aryl bromides with n-butyl acrylate mediated by a palladium/phosphine-imidazolium salt system

[reaction: see text] A new phosphine-imidazolium salt, L.HBr (1, L = (1-ethylenediphenylphosphino-3-(mesityl))imidazol-2-ylidene), has been prepared. A combination of 0.5 mol % of Pd(dba)(2) and 0.5 mol % of L.HBr in the presence of 2 equiv of Cs(2)CO(3) as base has proven to be highly efficient in the Heck coupling reactions of aryl bromides (from electron-deficient to electron-rich aryl bromides) with n-butyl acrylate.     

Suzuki cross-coupling on enantiomerically pure epoxides: efficient synthesis of diverse, modular amino alcohols from single enantiopure precursors

Suzuki arylation of enantiopure (4-bromophenyl)- or (3,5-dibromophenyl)glycidyl ethers has been achieved in toluene under Buchwald conditions [ArB(OH)2 (1.1 equiv), Cs2CO3 (2 equiv), Pd2(dba)3.C6H6 (1 mol %), S-Phos (4 mol %), toluene, 100 degrees C] allowing for the formation of modular arylglycidyl ethers not directly available in enantiopure form by epoxidation routes. These bulky ethers, when submitted to regioselective and stereospecific ring opening with ammonia [aq NH3, LiClO4 (1 equiv), THF, microwave irradiation (80 W), 125 degrees C] in a sealed tube, provide access to novel enantiopure beta-amino alcohols which, in turn, provide an easy access to structurally complex C2 symmetrical bisoxazolines.     

Fast and regioselective Heck couplings with N-acyl-N-vinylamine derivatives

Highly regioselective Heck couplings of aryl triflates with N-acyl-N-vinylamines lacking an N-alkyl substituent were achieved with reaction times of approximately 1 h in yields ranging from 62 to 98% using 1.5 mol % of Pd(2)(dba)(3), 3 mol % of DPPF, and diethylisopropylamine in dioxane. The efficiency of these cross-couplings were studied with several N-vinylamides and an example each of an N-vinylcarbamate and an N-vinylurea. The Heck coupling products easily underwent acidic hydrolysis to the corresponding aryl methyl ketone or in situ hydrogenation in the presence of (Ph(3)P)(3)RhCl under a hydrogen atmosphere to provide the N-acyl derivatives of pharmaceutically relevant benzylic amines. The coupling of a vinyl triflate and more interestingly a vinyl tosylate to N-vinyl acetamide was also studied affording a 2-acylamino-1,3-butadiene with the same high regioselectivity in preference for the alpha-isomer. This result suggests that Heck couplings of electron-rich alkenes with vinyl tosylates also follow a cationic pathway.     

Palladium-catalyzed inter- and intramolecular coupling reactions of aryl and vinyl halides mediated by indium

Treatment of aryl and vinyl halides with 50 mol % of 100 mesh indium, 2.5 mol % of Pd-C, and 1.5 equiv of LiCl under mild conditions (DMF, 100 degrees C, 1-3 h) could produce coupling products efficiently in good to excellent yields in which the C(sp2)-C(sp2) bond was formed. This reagent worked equally well with both intermolecular and intramolecular coupling reactions, producing a variety of biaryls, 1,3-dienes, and cyclic compounds.     

Palladium-catalyzed intramolecular hydroalkylation of unactivated olefins with dialkyl ketones

[reaction: see text] Treatment of 3-butenyl heptyl ketone with substoichiometric amounts of PdCl(2)(CH(3)CN)(2) (10 mol %), HCl (0.1 equiv), and CuCl(2) (0.3 equiv) in dioxane at 70 degrees C for 12 h in a sealed tube formed 2-hexylcyclohexanone in 77% isolated yield. A number of alkyl 3-butenyl ketones underwent hydroalkylation under these conditions to form 2-substituted cyclohexanones in moderate to good yield.     

Convenient synthesis of alkylhydrazides by the cobalt-catalyzed hydrohydrazination reaction of olefins and azodicarboxylates

Treatment of olefins 2 with 1.5 equiv of di-tert-butyl azodicarboxylate (3), 1 equiv of PhSiH3, and 1.5-5 mol % of the simple Co(III) catalyst 1 in ethanol at 23 degrees C affords the Markovnikov hydrazide product for a broad range of olefins in 62-94% yield.     

Regioselective oxidative coupling reactions of 3-substituted thiophenes with arylboronic acids

Under optimized conditions, 3-substituted thiophenes (EWG = COOEt, PO(OEt)(2)) undergo a facile and regioselective oxidative coupling reaction at carbon atom C4. The reactions were performed with various aryl boronic acids as nucleophiles in the presence of silver oxide (2.0 equiv), cesium trifluoroacetate (tfa) (1.0 equiv), benzoquinone (BQ) (0.5 equiv), and catalytic amounts of Pd(tfa)(2) (10 mol %) employing trifluoroacetic acid (TFA) as the solvent.     

Synthesis of isoquinolines and pyridines by the palladium-catalyzed iminoannulation of internal alkynes.

A wide variety of substituted isoquinoline, tetrahydroisoquinoline, 5,6-dihydrobenz[f]isoquinoline, pyrindine, and pyridine heterocycles have been prepared in good to excellent yields via annulation of internal acetylenes with the tert-butylimines of o-iodobenzaldehydes and 3-halo-2-alkenals in the presence of a palladium catalyst. The best results are obtained by employing 5 mol % of Pd(OAc)(2), an excess of the alkyne, 1 equiv of Na(2)CO(3) as a base, and 10 mol % of PPh(3) in DMF as the solvent. This annulation methodology is particularly effective for aryl- or alkenyl-substituted alkynes. When electron-rich imines are employed, this chemistry can be extended to alkyl-substituted alkynes. Trimethylsilyl-substituted alkynes also undergo this annulation process to afford monosubstituted heterocyclic products absent the silyl group.     

New highly asymmetric Henry reaction catalyzed by Cu(II) and a C(1)-symmetric aminopyridine ligand, and its application to the synthesis of miconazole

A new catalytic asymmetric Henry reaction has been developed that uses a C(1)-symmetric chiral aminopyridine ligand derived from camphor and picolylamine. A variety of aromatic, heteroaromatic, aliphatic, and unsaturated aldehydes react with nitromethane and other nitroalkanes in the presence of DIPEA (1.0 equiv), Cu(OAc)(2)*H(2)O (5 mol %), and an aminopyridine ligand (5 mol %) to give the expected products in high yields (up to 99 %), moderate-to-good diastereoselectivites (up to 82:18), and excellent enantioselectivities (up to 98 %). The reaction is air-tolerant and has been used in the synthesis of the antifungal agent miconazole.     

Development of a decarboxylative palladation reaction and its use in a Heck-type olefination of arene carboxylates

The development of a palladium-catalyzed decarboxylative coupling reaction of arene carboxylates with olefinic substrates is described. The optimized procedure for decarboxylative palladation employs Pd(O2CCF3)2 as catalyst (0.2 equiv) in the presence of Ag2CO3 (3 equiv) in the solvent 5% DMSO-DMF and proceeds at temperatures of 80-120 degrees C with a wide range of arene carboxylates and alkenes as substrates. The process is proposed to proceed by an initial Ar-SE reaction involving ipso attack of an electrophilic Pd(II) intermediate on an arene carboxylate to form an arylpalladium(II) species with loss of carbon dioxide. This intermediate is then proposed to react with an olefinic substrate by steps common to the Heck coupling process. Reoxidation of the liberated Pd(0) in situ is proposed to establish the catalytic cycle.     

Synthesis of 2-alkylidenecyclopentanones via palladium-catalyzed carbopalladation/ring expansion of 1-(1-alkynyl)cyclobutanols

The palladium-catalyzed cross-coupling of aryl halides or vinylic halides or triflates and 1-(1-alkynyl)cyclobutanols affords good yields of stereoisomerically pure 2-arylidene- or 2-(2-alkenylidene)cyclopentanones, respectively. The process involves (1) oxidative addition of the organic halide or triflate to Pd(0), (2) regioselective, intermolecular carbopalladation of the carbon-carbon triple bond of the 1-(1-alkynyl)cyclobutanol to produce a vinylic palladium intermediate, (3) regioselective ring expansion to a palladacycle, and (4) reductive elimination of the 2-alkylidenecyclopentanone with simultaneous regeneration of the Pd(0) catalyst. Generally, the best results are obtained by employing 10 mol % of Pd(OAc)(2), 20 mol % of PPh(3), 2 equiv of the aryl or vinylic iodide or vinylic triflate, 2 equiv of diisopropylethylamine, and n-Bu(4)NCl in DMF as the solvent.     

Copper-catalyzed, one-pot, three-component synthesis of benzimidazoles by condensation and C-N bond formation

Benzimidazoles were synthesized by the copper-catalyzed, one-pot, three-component reaction of 2-haloanilines, aldehydes, and NaN(3). The reaction was optimized when 2-iodo- or 2-bromoanilines (1.0 equiv), aldehydes (1.2 equiv), NaN(3) (2.0 equiv), 5 mol% of CuCl, and 5 mol % of TMEDA were reacted in DMSO at 120 °C for 12 h. Good yields resulted, and the reaction showed tolerance toward functional groups such as ester, nitro, and chloro. Aliphatic and heteroaromatic aldehydes also afforded the desired products in moderate to good yields.     

Palladium-catalyzed cycloalkylations of 2-bromo-1,n-dienes with organoboronic acids

Cascade palladium-catalyzed cycloalkylations of 2-bromo-1,n-dienes were accomplished in good to excellent yields, where the alkylpalladium intermediates, formed via an intramolecular Heck reaction of 2-bromo-1,n-dienes (n = 6 or 7), were successfully cross-coupled with various organoboronic acids. The optimal yields were achieved by the use of cesium carbonate in ethanol with Pd(PPh(3))(4) as catalyst, with 2-bromo-1,n-dienes and organoboronic acids at concentrations of 0.2 and 0.3 M (1.5 equiv), respectively.     

Palladium-catalyzed carbonylative annulation of internal alkynes: synthesis of 3,4-disubstituted coumarins

The palladium-catalyzed annulation of internal alkynes by o-iodophenols in the presence of CO results in exclusive formation of coumarins. No isomeric chromones have been observed. The best reaction conditions utilize the 2-iodophenol, 5 equiv of alkyne, 1 atm of CO, 5 mol % Pd(OAc)2, 2 equiv of pyridine, and 1 equiv of n-Bu4NCl in DMF at 120 degrees C. The use of a sterically unhindered pyridine base is essential to achieve high yields. A wide variety of 3,4-disubstituted coumarins containing alkyl, aryl, silyl, alkoxy, acyl, and ester groups have been prepared in moderate to good yields. Mixtures of regioisomers have been obtained when unsymmetrical alkynes are employed. 2-iodophenols with electron-withdrawing and electron-donating substituents and 3-iodo-2-pyridone are effective in this annulation process. The reaction is believed to proceed via (1) oxidative addition of the 2-iodophenol to Pd(0), (2) insertion of the alkyne triple bond into the aryl-palladium bond, (3) CO insertion into the resulting vinylic carbon-palladium bond, and (4) nucleophilic attack of the phenolic oxygen on the carbonyl carbon of the acylpalladium complex with simultaneous regeneration of the Pd(0) catalyst. This annulation process is the first example of intermolecular insertion of an alkyne occurring in preference to CO insertion.     

Palladium(0)-catalyzed suzuki-miyaura cross-coupling reactions of potassium aryl- and heteroaryltrifluoroborates with alkenyl bromides

Efficient palladium(0)-catalyzed Suzuki-Miyaura cross-couplings are described. The reactions involving potassium aryl- and heteroaryltrifluoroborates with alkenyl bromides can generally be carried out using < or =2 mol % of palladium catalyst and 3 equiv of base in toluene/H2O. When stereodefined alkenyl bromides are employed, the resulting styrene derivatives are accessed stereospecifically. A variety of functional groups are tolerated in both coupling partners.     

Stereoselective Suzuki-Miyaura cross-coupling reactions of potassium alkenyltrifluoroborates with alkenyl bromides

[reaction: see text] The stereoselective synthesis of conjugated dienes using air-stable potassium alkenyltrifluoroborates as coupling partners is described. The palladium-catalyzed cross-coupling reaction of potassium (E)- and (Z)-alkenyltrifluoroborates with either (E)- or (Z)-alkenyl bromides proceeds readily with moderate to excellent yields to give the corresponding (E,E)-, (E,Z)-, (Z,E)-, or (Z,Z)-conjugated dienes stereospecifically. The cross-coupling can generally be effected using 5 mol % of Pd(OAc)2, 10 mol % of PPh3, and 3 equiv of Cs2CO3 in THF-H2O (10:1). A variety of functional groups are tolerated in both coupling partners.     

Practical Oppenauer (OPP) oxidation of alcohols with a modified aluminum catalyst

[reaction: see text] Modified aluminum catalyst 2 was found to be highly effective for Oppenauer (OPP) oxidation of alcohols under mild conditions. For example, oxidation of carveol in toluene with t-BuCHO (1.2 equiv) as a hydride acceptor in the presence of 2 (1 mol %) proceeded smoothly at 21 degrees C to give carvone in 94% yield after 1 h. A practical aspect of this system was highlighted by the facile OPP oxidation of terpenoids and steroids with 1.2-3.0 equiv of acetone and 2 (3-5 mol %).     

Pd(II)-mediated coupling of 1,1-dimethyl-2-propynyl acetate with itself and 1-decene

Heating 1,1-dimethyl-2-propynyl acetate (1) with PdCl₂(MeCN)₂ (0.05–0.1 mol equiv) and MeCN alone afforded (E)-2,7-dimethylocta-2,4,6-trien-3,6-diyl diacetate (2) (yield ca. 20%) and with 1-decene (7 mol equiv) present (E)-2-methyltetradeca-2,4-dien-3-yl acetate (7) (30%) together with 2 (12%).