Anthracen-9-ylmethylene-(4-methoxyphenyl)amine: efficient promoter for silver-free palladium-catalyzed aerobic oxidative Sonogashira reactions

An efficient protocol for the synthesis of unsymmetrical substituted diarylacetylenes by the C(sp²)–C(sp) cross-coupling reactions of substituted phenylacetylenes and electronically different arylboronic acids has been developed. Anthracen-9-ylmethylene-(4-methoxyphenyl)amine was employed as an efficient promoter in this Pd(OAc)₂-catalyzed oxidative Sonogashira reaction in air in the absence of silver salt under optimized reaction conditions. The impact of reaction parameters such as solvent, base, reaction temperature and time in this silver-free aerobic oxidative Sonogashira cross-coupling reaction was also evaluated. Electron-deficient phenylacetylenes, which are sluggish coupling partners in the traditional Sonogashira reaction, underwent coupling in this protocol. The catalytic system is inexpensive, effortlessly attainable and highly flexible for functionalized phenylacetylenes and arylboronic acids.

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Stereoselective sp-sp bond formation via Negishi cross-coupling of vinylic tellurides and 2-heteroarylzinc chlorides

The Negishi cross-coupling reaction of vinylic- and aryltellurides with heteroarylzinc chlorides catalyzed by PdCl₂/CuI is described. This cross-coupling reaction is general and permits the formation of a new sp²-sp² carbon bond in good yields and high stereoselectivity.     

Synthesis of 2,2-diaryl-1,1-difluoroethenes via Pd-catalyzed dehydrosulfonylative cross-coupling of α-[difluoro(phenylsulfonyl)methyl]benzyl tosylates with arylboronic acids

By using PhSO₂CF₂H as the difluoromethylidene equivalent, a novel method for connecting aromatic aldehydes and arylboronic acids via consecutive reactions was developed to obtain structurally diverse 2,2-diaryl-1,1-difluoroethenes. The key step is the palladium-catalyzed dehydrosulfonylative cross-coupling of tosylates that are prepared from PhSO₂CF₂H, aromatic aldehydes and tosyl chloride. Mechanistic investigations showed that the reaction proceeds mainly through base-mediated dehydrosulfonylation followed by palladium-catalyzed C(sp²)-C(sp²) cross-coupling reaction.     

Sonogashira reaction catalyzed by palladium isocyanide complex modified <Emphasis Type="Italic">in situ</Emphasis>

Palladium isocyanide complex modified in situ by addition of benzohydrazide as nucleophile was shown to catalyze Sonogashira reactions of phenylacetylene with iodobenzenes in refluxing ethanol in the presence of potassium carbonate. The reactions were complete in 2 h, and neither preliminary degassing nor protection from atmospheric moisture and oxygen was necessary. The yields of the cross-coupling products, unsymmetrical diarylacetylenes were only slightly lower than those obtained with the preliminarily prepared catalyst. The proposed procedure was used to synthesize a series of diarylacetylenes containing both electrondonating and electron-withdrawing substituents in the benzene rings.     

The regio- and stereospecific synthesis of diarylpropenyllithium compounds from dimethylamino- and dimethylaminomethyl-substituted diarylacetylenes via transmetallation reactions involving diarylpropenylmagnesium and -tin compounds

Dimethylamino- and dimethylaminomethyl-substituted diarylacetylenes were prepared by the Pd(PPh₃)₄-catalyzed coupling reaction of dimethylamino- and dimethylaminomethyl-substituted aryl halides with arylacetylenes.Reaction of the asymmetric diarylacetylenes with methylmagnesium bromide in the presence of NiCl₂(PPh₃)₂ gave stereo- and regio-selectively cis addition products in which the aryl group bearing the built-in ligand (Me₂N or Me₂NCH₂) and the magnesium atom are bonded to the same unsaturated carbon atom. The E-diarylpropenylmagnesium bromides were converted in two successive transmetallation steps into the corresponding E-triphenyltin and Z-lithium compounds with retention of configuration. Depending on the type of solvent, complete inversion of configuration was observed for the Z-diarylpropenyllithium compounds. From the E-lithium compounds the corresponding Z-diarylpropenyltriphenyltin compounds were prepared (Scheme 1).Retention of configuration in the series of transmetallation reactions has unambiguously been established by comparison of the ¹¹⁹Sn¹H, ¹³C¹H and ¹¹⁹Sn¹³C coupling constants observed in the ¹H and ¹³C NMR spectra of the configurationally pure stereoisomeric pairs.     

A Suzuki‐type cross‐coupling reaction of arylacetylene halides with arylboronic acids

A PdCl₂‐catalyzed direct alkynylation of arylboronic acids to give diarylacetylenes is described. The optimal conditions using PdCl₂ as catalyst, MeOH PhMe H₂O as solvent and K₂CO₃ as base effectively suppressed the formation of homo‐coupling product and afforded moderate to good yield of the desired unsymmetrical coupling product. This reaction represents a Suzuki‐type sp²(C B)–sp(C X) cross‐coupling. Copyright © 2011 John Wiley & Sons, Ltd.     

Preparation of 1,3-dienyl organotrifluoroborates and their Diels-Alder/cross-coupling reactions

2-BF₃-substituted 1,3-butadienes with potassium and tetrabutyl ammonium counterions have been prepared in gram quantities from chloroprene via a simple synthetic procedure. The potassium salt of this new main group element substituted diene has been characterized by ¹H, ¹³C, ¹¹B, and ¹⁹F NMR and the tetra n-butyl ammonium salt was also characterized by X-ray crystallography. Diels-Alder reactions of these dienes with dienophiles such as ethyl acrylate, methyl vinyl ketone, and N-phenylmaleimide are reported as well as subsequent Pd-catalyzed cross-coupling reactions of those Diels-Alder adducts. 4-Phenyl-2-BF₃-substituted 1,3-diene was prepared by magnesium-halogen exchange from the corresponding 2-bromo and iodo dienes. The 4-phenyl-2-bromo-1,3-butadiene was also characterized by X-ray crystallography. 4-Phenyl-2-BF₃-1,3-butadiene was used in Diels-Alder/cross-coupling reactions and the product of a Diels-Alder reaction with N-phenylmaleimide followed by cross-coupling with 4-bromo-benzonitrile was also characterized by X-ray crystallography.     

Carbon–Phosphorus Coupling from C^N Cyclometalated AuIII Complexes

With the aim of exploiting new organometallic species for cross-coupling reactions, we report here on the Au^III-mediated C_aryl−P bond formation occurring upon reaction of C^N cyclometalated Au^III complexes with phosphines. The [Au(C^N)Cl₂] complex 1 featuring the bidentate 2-benzoylpyridine (C^CON) scaffold was found to react with PTA (1,3,5-triaza-7-phosphaadamantane) under mild conditions, including in water, to afford the corresponding phosphonium 5 through C−P reductive elimination. A mechanism is proposed for the title reaction based on in situ ³¹P{¹H} NMR and HR-ESI-MS analyses combined with DFT calculations. The C−P coupling has been generalized to other C^N cyclometalated Au^III complexes and other tertiary phosphines. Overall, this work provides new insights into the reactivity of cyclometalated Au^III compounds and establishes initial structure–activity relationships to develop Au^III-mediated C−P cross-coupling reactions.     

A highly efficient memantine-modified palladium catalyst for Suzuki–Miyaura cross-coupling reaction

A new simple Pd(memantine)₂Cl₂ complex was synthesized and characterized by ¹H NMR, ¹³C NMR and X-ray single crystal structure determination. The Suzuki–Miyaura reaction of aryl bromides catalyzed by Pd(memantine)₂Cl₂ complex was investigated in air with different temperature. The high turnover numbers of 650,000 have been obtained in the reaction of 4-bromonitrobenzene with phenylboronic acid at 80 °C. At room temperature, the complex also showed high activity for Suzuki–Miyaura cross-coupling reaction of aryl bromides with a wide range of functional groups under air, and the turnover number of up to 99,000 was achieved. The catalytic system also gives good yields toward the reaction of several heteroaryl bromides with thiophenylboronic acid.     

Palladium-catalyzed cross-coupling reaction of resin-bound chlorotriazines

To introduce the biaryl structure as a triazine functionality, we have developed a new synthetic route via the Suzuki cross-coupling reaction of resin-bound chlorotriazines. The Suzuki cross-coupling reaction was achieved using various arylboronic acids, Pd(PPh₃)₄, Cs₂CO₃, and dioxane. With the integration of this chemistry and our previous orthogonal methodology, the triazine library is greatly expanded to a biaryl scaffold.     

Symmetric diarylacetylenes: one-pot syntheses and solution photoluminescence

Bis(tri-n-butylstannyl)acetylene was synthesized and used to create a series of symmetric diarylacetylenes via a one-step Stille coupling protocol with Pd(PPh₃)₄ as the catalyst. In many cases, the product simply crystallized in good yields from the reaction mixture upon cooling after reflux at 100 °C or upon removal of solvent. The diarylacetylenes were studied using UV-vis and fluorescence spectroscopies, which showed that naphthyl- and biphenyl-substituted acetylenes had very high solution-state fluorescence quantum yields.     

A new organic transformation by introducing crotyl/allyltrifluoroborates in cross-coupling reaction with aroyl chlorides

Microwave irradiated PdCl₂(d^tbpf) catalyzed direct cross-coupling reaction of aroyl chlorides with potassium crotyl/allyltrifluoroborates has been developed. Regioselectivity of the cross-coupling product is varied from crotyltrifluoroborate to allyltrifluoroborate.     

Alkaloids of the flora of Siberia and Altai: XX. Synthesis of 5-aryl(hetaryl)-substituted anthranilic acid esters

Cross-coupling of methyl 2-acetylamino-5-bromobenzoate and 5′-bromolappaconitine with aryl-, furyl-, pyridyl-, and 5-acetylthiophen-2-ylboronic acids or 1-(2-fluoroquinolin-3-yl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane gave the corresponding 5-aryl(hetaryl)-substituted anthranilic acid derivatives. The use of the two-phase toluene-water system as reaction medium and addition of tetrabutylammonium bromide allows the cross-coupling to be accomplished under mild conditions. The catalytic system Pd(dba)₂-AsPh₃ was found to be efficient in the cross-coupling of methyl 2-acetylamino-5-bromobenzoate with furyl- and pyridylboronic acids, whereas the system Pd(OAc)₂-(o-Tol)₃P ensured good results in the reactions of 5′-bromolappaconitine with hetarylboronic acids. Facile esterification at the C⁸-OH and C⁹-OH groups of the aconitane skeleton was observed in the reactions of 5′-bromolappaconitine and 5′-phenyllappaconitine with phenylboronic acid. 5′-Bromo-8,9-O-(phenylboranediyl)lappaconitine under the Suzuki reaction conditions underwent hydrolysis of the boronic ester moiety with formation of the cross-coupling product of 5′-bromolappaconitine with phenylboronic acid.     

Visible-Light-Initiated Palladium-Catalyzed Cross-coupling by PPh3 Uncaging from an Azobenzene Ruthenium–Arene Complex

Photo-release of triphenylphosphine from a sulfonamide azobenzene ruthenium–arene complex was exploited to activate Pd^IICl₂ into Pd⁰ catalyst, for the photo-initiation of Sonogashira cross-coupling. The transformation was initiated on demand – by using simple white LED strip lights – with a high temporal response and the ability to control reaction rate by changing the irradiation time. Various substrates were successfully applied to this photo-initiated cross-coupling, thus illustrating the wide functional-group tolerance of our photo-caged catalyst activator, without any need for sophisticated photochemistry apparatus.     

Practical synthesis of biaryl colchicinoids containing 3,4-catechol ether-based A-rings via Suzuki cross-coupling with ligandless palladium in water

Eight new biaryl colchicinoids containing 3^′,4^′-methylene or benzodioxy ether bridges were synthesized. The key synthetic step employed a ligandless, aqueous Suzuki cross-coupling reaction catalyzed by Pd(OAc)₂ with tetrabutylammonium bromide (TBAB) and potassium carbonate (K₂CO₃). The biaryl Suzuki products were typically formed in 5-30 min and always in less than 1 h.     

A carrier-free and recyclable protocol for the cross-coupling of terminal alkynes with arylboronic acids in H2O/TBAB

A new and recyclable protocol was developed for Pd(OAc)₂-catalyzed the cross-coupling reaction of terminal alkynes with arylboronic acids using environmentally friendly H₂O/TBAB as reaction medium. A series of cross-coupling products containing internal acetylenic bond can be obtained with good selectivity and yield. The Pd(OAc)₂/H₂O/TBAB system was stable in the Sonogashira-type cross-coupling reaction and could be used at least three cycles without considerable decrease in catalytic performance. The results suggest that the unsupported and recyclable systems can be extended to the other realm of CC bond formation in synthetic organic chemistry.     

Synthesis of [carbonyl-C]acetophenone via the Stille cross-coupling reaction of [1-C]acetyl chloride with tributylphenylstannane mediated by Pd2(dba)3/P(MeNCH2CH2)3N·HCl

The Stille cross-coupling reaction of [1-¹¹C]acetyl chloride with tributylphenylstannane leading to [carbonyl-¹¹C]acetophenone was studied with the goal of developing a new ¹¹C-labeling method for positron emission tomography tracer synthesis. The coupled product [carbonyl-¹¹C]acetophenone was synthesized using the Pd₂(dba)₃/P(MeNCH₂CH₂)₃N·HCl system with a 60-61% radiochemical conversion from [1-¹¹C]acetyl chloride (decay-corrected, n = 3).     

Preparation of new (η6-arene)Mn(CO)3+ complexes involving Pd-catalysed/exo-hydride abstraction sequence

A novel methodology consisting in a selective palladium cross-coupling reaction starting from (η⁵-chlorocyclohexadienyl)Mn(CO)₃ followed by rearomatisation allows the preparation of new cationic (η⁶-arene)Mn(CO)₃ complexes, which cannot be obtained directly from the coordination of the corresponding free arenes.     

Remarkable regioselectivity in microwave-enhanced palladium-catalyzed allylation reaction involving allyltrifluoroborates and aryl halides

An unprecedented cross-coupling reaction of potassium allyltrifluoroborates and aryl halides to the corresponding trans-β-methylstyrenes in the presence of PdCl₂(d^tbpf) catalyst under microwave heating was developed.     

Perfluoroalkylphosphines and arsines obtained by Pd-catalyzed cross-coupling reaction with organoheteroatom stannanes

Pd-catalyzed cross-coupling reaction of organoheteroatom stannanes containing elements of the groups 15 (P, As) and 16 (Se) with perfluoroalkyl iodides (R_fI) was studied. Herein, a one-pot two-step reaction to form P–R_f, As–R_f and Se–R_f bonds is reported. The stannanes n-Bu₃SnZPh_n (Z = P, As, Se; n = 1–2) were generated in situ by the reaction of the Ph_nZ⁻ anion with n-Bu₃SnCl. The cross-coupling reactions of these stannanes with R_fI afforded C-heteroatom products, new perfluoroalkylarsines and perfluoroalkylselenides in good yields (47–90%) and perfluoroalkylphosphines in moderate yields (15–48%).