Pd-catalyzed Negishi coupling of pyrazole triflates with alkyl zinc halides

A mild, functional group tolerant palladium-catalyzed Negishi coupling of pyrazole triflates and nonaflates with alkyl, benzyl, and aryl zinc halides has been developed. It allows quick access to 3-substituted pyrazole analogs at late stage via common synthetic intermediates.     

Aryl triflates: useful coupling partners for the direct arylation of heteroaryl derivatives via Pd-catalyzed C-H activation-functionalization

Aryl triflates were found to be suitable partners for the palladium catalyzed direct arylation of heteroaromatics via C-H activation-functionalization. The reaction conditions and the catalyst have a determining influence on the yields. The system combining PPh3 and Pd(OAc)2 using KOAc as base and DMF as solvent promotes the selective 2- or 5-arylations in moderate to high yields. Several heteroaromatics such as furan, thiophene, thiazole or oxazole derivatives have been employed successfully. The electronic properties of the aryl triflates also have a decisive influence on the yields of the coupling products. Electron-rich aryl triflates gave satisfactory results, whereas the electron-poor ones led to the formation of phenols.     

Synthesis of diarylmethane derivatives from Pd-catalyzed cross-coupling reactions of benzylic halides with arylboronic acids

A simple catalyst precursor prepared in situ from palladium acetate and triphenylphosphine shows high activity for the Suzuki cross-coupling reaction of benzylic bromides and chlorides with aryl boronic acids. The reaction can be carried out at low catalyst loading (0.002-1 mol%) and under mild conditions (room temperature to 80 °C) furnishing diarylmethane derivatives in high yields (86-99%).     

A Ring Enlargement from Seven- to Ten-Membered-Ring sulfonamide derivatives

The reaction of 3-(dimethylamino)-2,2-dimethyl-2H-azirine ( 1a ) with 4,5-dihydro-7,8-dimethoxy-1,2-benzothiazepin-3-one 1,1-dioxide ( 4 ) in dioxane at room temperature gave the correspondingly substituted 4H-1,2,5-benzothiadiazecin-6-one 1,1-dioxide 5a in 64% yield (Scheme 2). The structure of this novel ten-membered ring-enlargement product was established by X-ray crystallography (Fig.). Under more vigorous conditions (refluxing dichloroethane), 5a was formed together with the isomeric 6a , both in low yield. The 3-(dimethylamino)-2H-azirines 1b and 1c reacted sluggishly to give the two isomeric ring-enlargement products of type 5 and 6 in yields of 24–29% and 2–4%, respectively (Table 1). Even less reactive is 2,2-dimethyl-3-(N-methyl-N-phenylamino)-2H-azirine ( 1d ), which reacted with 4 in MeCN only at 65°. Under these conditions, besides numerous decomposition products, only traces of 5d and 6d were formed. No ring enlargement was observed with the sterically crowded 1e , which bears an isopropyl group at C(2).     

Synthesis of 2-vinylic indoles and derivatives via a Pd-catalyzed tandem coupling reaction

A novel one-step synthesis of valuable 2-vinylic indoles and their tricycle derivatives is described. This reaction, which utilizes a gem-dibromovinyl unit as a readily available starting material, occurs via an efficient Pd-catalyzed tandem Buchwald-Hartwig/Heck reaction.     

Highly regioselective synthesis of trisubstituted allenes via lithiation of 1-aryl-3-alkylpropadiene, subsequent transmetalation, and Pd-catalyzed Negishi coupling reaction

A novel methodology for the synthesis of trisubstituted allenes is reported. Lithiation of 1-aryl-3-alkylpropadienes and subsequent transmetalation with zinc bromide followed by Pd-catalyzed Negishi coupling reactions with halides afforded the corresponding trisubstituted allenes in a highly regioselective fashion with moderate to excellent yields. A plausible regioselective lithiation mechanism was proposed on the basis of deuterium labeling experiments.     

A convenient one-pot Negishi coupling of amino-heteroaryl chlorides and alkyl bromides

A simple Ni-catalysed cross-coupling protocol for amino-heteroaryl chlorides with alkylzinc reagents has been developed. The alkylzinc reagents can be commercially available dialkylzincs or alkylzinc halides, or can be conveniently generated in situ from diethylzinc and primary alkyl bromides in the presence of the same inexpensive Ni catalyst used to effect the subsequent coupling reaction.     

A new approach to carbon-carbon bond formation: development of aerobic Pd-catalyzed reductive coupling reactions of organometallic reagents and styrenes

Alkenes are attractive starting materials for organic synthesis and the development of new selective functionalization reactions is desired. Previously, our laboratory discovered a unique Pd-catalyzed hydroalkoxylation reaction of styrenes containing a phenol. Based upon deuterium labeling experiments, a mechanism involving an aerobic alcohol oxidation coupled to alkene functionalization was proposed. These results inspired the development of a new Pd-catalyzed reductive coupling reaction of alkenes and organometallic reagents that generates a new carbon-carbon bond. Optimization of the conditions for the coupling of both organostannanes and organoboronic esters is described and the initial scope of the transformation is presented. Additionally, several mechanistic experiments are outlined and support the rationale for the development of the reaction based upon coupling alcohol oxidation to alkene functionalization.     

Pd-catalyzed benzylic C-h amidation with benzyl alcohols in water: a strategy to construct quinazolinones

A novel method for the synthesis of 4-phenylquinazolinones via a palladium-catalyzed domino reaction of o-aminobenzamides with benzyl alcohols is developed. This protocol involves N-benzylation, benzylic C-H amidation, and dehydrogenation in water, which may play an important role in the smooth generation of the (η(3)-benzyl)palladium species by activation of the hydroxyl group of the benzyl alcohol.     

E-Stilbene derivatives synthesized by stereoselective reductive coupling of benzylic gem-dibromide promoted by Cu/polyamine

Stereoselective reductive coupling reaction of benzylic gem-dibromide promoted by Cu/polyamine produces E-stilbene derivatives with high yield under mild conditions. It provides a short pathway to synthesize symmetrical and asymmetrical E-stilbene derivatives using cheap reagents and alkenyl-free starting material together with easy workup.     

Highly selective synthesis of allylated arenes and diarylmethanes via palladium-catalyzed cross coupling involving benzylic derivatives

The Pd-catalyzed cross coupling of either benzylzincs with alkenyl halides or alkenylalanes with benzyl halides provides highly selective and expeditious routes to allylated arenes.     

A new strategy for the synthesis of benzylic sulfonamides: palladium-catalyzed arylation and sulfonamide metathesis

An efficient two-step strategy has been developed to access diversely functionalized benzylic sulfonamides. Execution of this strategy required the development of two reaction methods: the palladium-catalyzed cross-coupling of aryl halides with CH-acidic methanesulfonamides and a metathesis reaction between the resulting alpha-arylated sulfonamides and diverse amines. The broad scope of the cross-coupling process combined with a versatile sulfonamide metathesis constitutes an efficient strategy for the synthesis of various benzylic sulfonamides.     

Pd-catalyzed arylation/ring-closing metathesis approach to azabicycles

Palladium-catalyzed arylation followed by Grignard addition to imines and ring-closing metathesis, using Grubbs’ catalysts, provides a route to six-, seven-, and eight-membered azabicycles.     

Aryl ether as a Negishi coupling partner: an approach for constructing C-C bonds under mild conditions

An etheric Negishi coupling: The first cross-coupling reaction between aryl alkyl ethers and dianion-type zincate reagents to afford biaryl compounds through selective cleavage of the etheric C(sp(2))-O bond was developed. Dianion-type zincates showed excellent reactivity toward the aromatic ethers under mild conditions, with good functional group compatibility (see scheme).     

A three-component coupling approach to cyclopentanoids.

A new approach to 2,3-disubstituted cyclopentenones has been developed. This approach consists of a two-step protocol involving the cyclization of a Z-vinyl bromide under Barbier type conditions to form a cyclopentenol, which is then oxidatively rearranged to generate the cyclopentenone. The Z-vinyl bromide is in turn derived from a ruthenium catalyzed three-component coupling of an alkyne, an enone, and a HBr equivalent. A range of 2,3-disubstituted cyclopentenones has been generated, including short syntheses of jasmone and dihydrojasmone. Further applicability of this strategy is shown in the total syntheses of tetrahydrodicranenone B, rosaprostol, and a selective COX-2 inhibitor.     

Highly satisfactory procedures for the Pd-catalyzed cross coupling of aryl electrophiles with in situ generated alkynylzinc derivatives

[reaction: see text] Two new and very efficient procedures (Procedures A and B) are reported for the Pd-catalyzed cross coupling of aryl electrophiles with terminal alkynes via their in situ conversion into alkynylzinc derivatives. Procedure A is particularly valuable in cases where electron-deficient alkynes are used, while Procedure B is operationally simple and very satisfactory in less demanding cases.     

Efficient and stereoselective synthesis of yellow scale pheromone via alkyne haloboration, Zr-catalyzed asymmetric carboalumination of alkenes (ZACA reaction), and Pd-catalyzed tandem Negishi coupling

A Pd-catalyzed reaction of allylzincs with the 1-octyne bromoboration product gives the desired allyl-alkenyl coupling products in good yields except with H 2CCHCH 2ZnBr. This reaction is suitable for converting an alkyne bromoboration product 3 into 4 with no isomerization or beta-elimination. The Pd-catalyzed isoalkyl-alkenyl coupling of 4 with the isoalkylzinc reagent derived from 2 provides yellow scale pheromone ( 1) of >/=98% isomeric purity in 34% in six steps from TBDPS-protected homoallyl alcohol.     

Efficient synthesis of fluorinated biphenyl derivatives via Pd-catalyzed Suzuki coupling reactions in aqueous solvents at room temperature

A variety of fluorinated biphenyl derivatives were obtained in good yields in aqueous solvents at room temperature by Suzuki coupling reaction of aryl bromides and aryl boronic acid in the presence of high activity catalyst—some air-stable hemilabile PO coordinated cyclopalladated complexes. The structures of above catalysts were characterized by element analyses, IR, ¹H NMR, ¹³C NMR and ³¹P NMR.     

An efficient one-pot approach to phenanthrene derivatives using a catalyzed tandem Ullmann-pinacol coupling reaction

In the presence of catalyst (Ph₃P)₂NiCl₂ and reductant Zn, the Ullmann reactions of ortho-halo aryl aldehydes generate biaryl-dialdehydes and zinc halides. Subsequently, ZnX₂ can catalyze the intramolecular pinacol coupling reaction of biaryl-dialdehydes to form 9,10-dihydrophenanthrene-9,10-diols. One-pot synthesis of 9-phenanthrols can be achieved using this strategy.     

Efficient route to 1,3-Di-N-imidazolylbenzene. A comparison of monodentate vs bidentate carbenes in Pd-catalyzed cross coupling

A new bis(carbene) ligand architecture has been developed and was evaluated in the Suzuki-Miyaura cross-coupling reaction of various aryl halides with phenylboronic acid. Several new bis(carbene) ligands were tested in different carbene:Pd ratios. Pd(OAc)(2) and Pd(2)(dba)(3) were compared for efficiency as a Pd source. It was found that the Pd(OAc)(2)/bis(carbene) system formed a catalyst for the activation of chlorobenzene. [reaction: see text]