Rapid homogeneous-phase Sonogashira coupling reactions using controlled microwave heating

Exploring SmBr2-, SmI2-, and YbI2-Mediated Reactions Assisted by Microwave Irradiation

The use of microwave heating in lanthanide(II) halide (LnX2 = SmBr2, SmI2, and YbI2) mediated reduction and coupling reactions has been investigated for a variety of functional groups including alpha,beta-unsaturated esters, aldehydes, ketones, imines, and alkyl halides. Good to quantitative transformations were obtained within a few minutes without the addition of any co-solvents, such as hexamethyl phosphoramide (HMPA). The redox potential of YbI2 in tetrahydrofuran (THF) has been determined as -1.02+/-0.05 V (versus Ag/AgNO3) by cyclic voltammetry. A large selectivity difference in various reactions was observed depending on the redox potential of the LnX2 reagent. The more powerful reductant, SmBr2, afforded mainly pinacol-coupling products of ketones whereas the weaker reductant YbI2 afforded mainly reduction products. The results indicate that the reducing power of LnX2 has a large impact on not only the pinacol coupling/reduction product ratio of ketones but also on other substrates in which there are competing coupling and reduction reactions. The use of in situ generated LnX2 has also been explored and proven useful in many of these reactions.

Copper- and Ligand-Free Sonogashira Reaction Catalyzed by Pd(0) Nanoparticles at Ambient Conditions under Ultrasound Irradiation

The Sonogashira reaction proceeds at ambient temperature (30 degrees C) in acetone or room-temperature ionic liquid, 1,3-di-n-butylimidazolium tetrafluoroborate ([bbim]BF4), as solvent under ultrasound irradiation to give enhanced reaction rates, excellent chemoselectivity, and high yields in the absence of a copper cocatalyst and a phosphine ligand. TEM analysis showed the formation of stable, crystalline, and polydispersed Pd(0) nanoparticles as catalyst for the reaction.

A ligand-free solid-supported system for Sonogashira couplings: applications in nucleoside chemistry

A mild heterogeneous, ligand-free protocol for Sonogashira and Heck couplings has been developed and used to access several biologically important deoxynucleoside derivatives in a facile manner.

Tandem Sonogashira Coupling: An Efficient Tool for the Synthesis of Diarylalkynes

[reaction: see text] The tandem Sonogashira coupling reaction of aryl halides provides an efficient method for the synthesis of diarylalkynes. Several aryl halides were coupled with 2-methyl-3-butyn-2-ol as acetylene source in the presence of PdCl2(PPh3)2 and CuI. Following the deprotection of the acetylene moiety in the same pot using a strong base, the Sonogashira coupling of a second aryl halide led to the formation of the appropriate diarylakyne. The established protocoll was successfully extended to the preparation of compound libraries.

Controlled Microwave Heating in Modern Organic Synthesis

Although fire is now rarely used in synthetic chemistry, it was not until Robert Bunsen invented the burner in 1855 that the energy from this heat source could be applied to a reaction vessel in a focused manner. The Bunsen burner was later superseded by the isomantle, oil bath, or hot plate as a source for applying heat to a chemical reaction. In the past few years, heating and driving chemical reactions by microwave energy has been an increasingly popular theme in the scientific community. This nonclassical heating technique is slowly moving from a laboratory curiosity to an established technique that is heavily used in both academia and industry. The efficiency of "microwave flash heating" in dramatically reducing reaction times (from days and hours to minutes and seconds) is just one of the many advantages. This Review highlights recent applications of controlled microwave heating in modern organic synthesis, and discusses some of the underlying phenomena and issues involved.

Palladium-Catalyzed Cross-Coupling of Aryl Triethylammonium Bis(catechol) Silicates with Aryl Bromides Using Microwave Irradiation

The scope of the palladium-catalyzed cross-coupling reaction of aryl bis(catechol) silicates has been extended to include the coupling of aryl bromides by employing microwave irradiation. This new set of coupling conditions is tolerant of electron-rich and -deficient aryl bromides. In addition, a variety of substituted aryl bis(catechol) silicates have been successfully cross-coupled.

Solution-Phase Microwave-Assisted Synthesis of Unsubstituted and Modified α-Quinque- and Sexithiophenes

The facile synthesis of poorly soluble unsubstituted and modified alpha-quinque- and sexithiophenes under microwave irradiation in the liquid phase is described. The use of microwave irradiation allowed these compounds to be prepared in a few minutes and at high yields by means of the Suzuki cross-coupling reaction. Unsubstituted sexithiophene was obtained in 10 min via the one-pot borylation/Suzuki reaction, purified according to a very simple procedure, and isolated in 84% yield. The efficient synthesis of two new methylated quinque- and sexithiophenes displaying liquid crystalline properties is reported. A new microwave-assisted methodology for the conversion of aldehyde-terminated quinque- and sexithiophenes into the corresponding cyano derivatives is also described. The use of microwaves was extended to the Sonogashira coupling reaction and found to be very effective in the preparation of a quinquethiophene containing acetylenic spacers. The electronic and optical characterization of this compound is reported and discussed in relation to that of unsubstituted quinquethiophene.

Cross-Coupling and Dehalogenation Reactions Catalyzed by (N-Heterocyclic carbene)Pd(allyl)Cl Complexes

A series of well-defined, air- and moisture-stable (NHC)Pd(allyl)Cl (NHC = N-heterocyclic carbene) complexes has been used in several catalytic reactions: Suzuki-Miyaura cross-coupling, catalytic dehalogenation of aryl halides, and aryl amination. The scope of the three processes using various substrates was examined. A general system involving the use of (IPr)Pd(allyl)Cl as catalyst and NaO(t)Bu as base has proven to be highly active for the Suzuki-Miyaura cross-coupling of activated and unactivated aryl chlorides and bromides, for the catalytic dehalogenation of aryl chlorides, and for the catalytic aryl amination of aryl triflates. All reactions proceed in short reaction times and at mild temperatures. The system has also proven to be compatible with the microwave-assisted Suzuki-Miyaura cross-coupling and catalytic dehalogenation processes, affording yields similar to those of the conventionally heated analogous reactions.

Synthesis and Target Identification of Hymenialdisine Analogs

Hymenialdisine (HMD) is a sponge-derived natural product kinase inhibitor with nanomolar activity against CDKs, Mek1, GSK3beta, and CK1 and micromolar activity against Chk1. In order to explore the broader application of the pyrrolo[2,3-c]azepine skeleton of HMD as a general kinase inhibitory scaffold, we searched for additional protein targets using affinity chromatography in conjunction with the synthesis of diverse HMD analogs and profiled HMD against a panel of 60 recombinant enzymes. This effort has led to nanomolar to micromolar inhibitors of 11 new targets including p90RSK, KDR, c-Kit, Fes, MAPK1, PAK2, PDK1, PKCtheta, PKD2, Rsk1, and SGK. The synthesis of HMD analogs has resulted in the identification of compounds with enhanced and/or dramatically altered selectivities relative to HMD (28n) and in molecules with antiproliferative activities 30-fold higher than HMD (28p).

Rapid microwave promoted Sonogashira coupling reactions on solid phase

A microwave-enhanced, rapid, and efficient solid-phase version of the Sonogashira reaction is presented. It has been applied to the coupling of aryl iodides and bromides with various acetylene derivatives giving excellent yields in 15-25 min. The scopes of homogeneous, solventless, and solid-phase conditions for Sonogashira coupling of aryl halides are compared.

Sonogashira coupling of aryl halides catalyzed by palladium on charcoal

With the proper choice of solvent, palladium on charcoal acts as an efficient catalyst in the Sonogashira cross-coupling reaction of aryl bromides. The catalytically active species in the process is probably palladium, which leaches into the solution but returns onto the surface of the charcoal at the end of the reaction.

Solid-phase synthesis of an A-B loop mimetic of the Cepsilon3 domain of human IgE: macrocyclization by Sonogashira coupling

The solid-phase synthesis of a cyclic peptide containing the 21-residue epitope found in the A-B loop of the Cepsilon3 domain of human immunoglobulin E has been carried out. The key macrocyclization step to form the 65-membered ring is achieved in approximately 15% yield via an "on-resin" Sonogashira coupling reaction which concomitantly installs a diphenylacetylene amino acid conformational constraint within the loop.

Microwave-enhanced enzyme reaction for protein mapping by mass spectrometry: a new approach to protein digestion in minutes

Accelerated proteolytic cleavage of proteins under controlled microwave irradiation has been achieved. Selective peptide fragmentation by endoproteases trypsin or lysine C led to smaller peptides that were analyzed by matrix-assisted laser desorption ionization (MALDI) or liquid chromatography-electrospray ionization (LC-ESI) techniques. The efficacy of this technique for protein mapping was demonstrated by the mass spectral analyses of the peptide fragmentation of several biologically active proteins, including cytochrome c, ubiquitin, lysozyme, myoglobin, and interferon alpha-2b. Most important, using this novel approach digestion of proteins occurs in minutes, in contrast to the hours required by conventional methods.

Palladium-catalyzed cross-alkynylation of aryl bromides by sodium tetraalkynylaluminates

Sodium tetraalkynylaluminates (1-4), prepared from NaAlH4 and terminal alkynes, cross-couple with aryl bromides in the presence of Pd(0) and Pd(II) catalysts. The reactions take place in boiling THF or DME. The process is applicable to both homo- and heterocyclic aryl bromides and can be used for conversion of polybromo compounds into polyalkynes. The reactions are high yielding and selective, free of undesired homocoupling and hydrogenolysis processes. The reagents selectively react with the ring-bound bromine atoms but do not affect chloro, cyano, triflate, or ester functions.

Microwave-accelerated homogeneous catalysis in organic chemistry

The efficiency of microwave flash heating in accelerating organic transformations (reaction times reduced from days and hours to minutes and seconds) has recently been proven in several different fields of organic chemistry. This specific account mainly summarizes our own experiences in developing rapid, robust, and selective microwave-assisted transition metal-catalyzed homogeneous reactions. Applications include selective Heck couplings, cross-couplings, and asymmetric substitutions. The science of green chemistry was developed to meet the increasing demand for environmentally benign chemical processes. We believe the combination of metal catalysis and microwave heating will be of importance in the search for green laboratory-scale synthesis.

Microwave enhanced Akabori reaction for peptide analysis

The Akabori reaction, devised in 1952 for the identification of C-terminus amino acids, involves the heating of a linear peptide in the presence of anhydrous hydrazine in a sealed tube for several hours. We report here a modified Akabori reaction that rapidly identifies the C-terminus amino acid in a polypeptide including its amino acid sequence information at both the C-terminus and the N-terminus. This modified methodology demonstrates the fundamentals of microwave chemistry applied to bioanalytical problems. In this modified process, hydrazinolysis has been accelerated by the application of microwave irradiation. In our reaction, the linear peptide and hydrazine solution, contained in a loosely covered conical flask, was exposed to a few minutes of irradiation using an unmodified domestic microwave oven. While the classical Akabori reaction required several hours, the microwave assisted reaction takes just minutes. If dimethyl sulfoxide is added to dilute the reaction mixture, the process is retarded enough to allow aliquots of the reaction mixture to be drawn every few minutes over a period of about an hour in order to study the progress of hydrazinolysis. Reaction products were monitored by mass spectrometry-primarily FAB-MS. In addition to providing sequence information, the microwave enhanced Akabori reaction quickly detects the presence of arginine (Arg) by converting each Arg to ornithine (Orn). Furthermore, certain amino acids, containing beta-SH, CO2H, and CONH2 groups in their side chain, are susceptible to modification by hydrazine, thereby, providing rapid confirmation of the presence of these amino acid residues. In these preliminary studies, the following oligopeptides were analyzed to demonstrate the effectiveness of our approach; the dipeptide (Trp-Phe), the tripeptide (Tyr-Gly-Gly), the tetrapeptide (Pro-Phe-Gly-Lys), the heptapeptide (Ala-Pro-Arg-Leu-Arg-Phe-Tyr), and a N-terminal blocked tripeptide (N-acetyl-Met-Leu-Phe).

A copper-free Sonogashira coupling reaction in ionic liquids and its application to a microflow system for efficient catalyst recycling

[reaction: see text] The PdCl2(PPh3)2-catalyzed Sonogashira coupling reaction, in good to high yields, was performed in an ionic liquid ([BMIm][PF6]) in the absence of a copper salt. The use of an ionic liquid allows for the facile separation and recycling of the catalyst. The application of the above reaction in a microflow system in conjunction with an IMM micromixer was also successful.