Synthesis of Phosphine-Ligated Zinc Acetylide Dimers: Enhanced Reactivity in Carbonyl Additions

Transition Metal-Catalyzed Reactions of Alkynyl Halides

BACKGROUND

Transition metal-catalyzed reactions of alkynyl halides are a versatile means of synthesizing a wide array of products. Their use is of particular interest in cycloaddition reactions and in constructing new carbon-carbon and carbon-heteroatom bonds. Transition metal-catalyzed reactions of alkynyl halides have successfully been used in [4+2], [2+2], [2+2+2] and [3+2] cycloaddition reactions. Many carbon-carbon coupling reactions take advantage of metal-catalyzed reactions of alkynyl halides, including Cadiot-Chodkiewicz, Suzuki-Miyaura, Stille, Kumada-Corriu and Inverse Sonogashira reactions. All the methods of constructing carbon-nitrogen, carbon-oxygen, carbon-phosphorus, carbon-sulfur, carbon-silicon, carbon-selenium and carbon-tellurium bonds employed alkynyl halides.

OBJECTIVE

The purpose of this review is to highlight and summarize research conducted in transition metalcatalyzed reactions of alkynyl halides in recent years. The focus will be placed on cycloaddition and coupling reactions, and their scope and applicability to the synthesis of biologically important and industrially relevant compounds will be discussed.

CONCLUSION

It can be seen from the review that the work done on this topic has employed the use of many different transition metal catalysts to perform various cycloadditions, cyclizations, and couplings using alkynyl halides. The reactions involving alkynyl halides were efficient in generating both carbon-carbon and carbonheteroatom bonds. Proposed mechanisms were included to support the understanding of such reactions. Many of these reactions face retention of the halide moiety, allowing additional functionalization of the products, with some new products being inaccessible using their standard alkyne counterparts.

Transforming PPh3 into bidentate phosphine ligands at Ru-Zn heterobimetallic complexes

The reaction of [Ru(PPh₃)₃Cl₂] with excess ZnMe₂ led to P-C/C-H bond activation and P-C/C-C bond formation to generate a chelating diphenylphosphinobenzene ligand as well as a cyclometallated (diphenylphosphino)biphenyl group in the final product of the reaction, [Ru(dppbz)(PPh₂(biphenyl)')(ZnMe)] (1; dppbz = 1,2-bis(diphenylphosphino)benzene); PPh₂(biphenyl)' = cyclometallated PPh₂(biphenyl). The mechanism of reaction was studied and C-C coupling to give a bidentate 2,2'-bis(diphenylphosphino)biphenyl (BIPHEP) ligand was suggested to be one of the key steps of the process. This was confirmed by the reaction of [Ru(BIPHEP)(PPh₃)HCl] with ZnMe₂, which also gave 1. An analogous set of steps took place upon addition of ZnMe₂ to [Ru(rac-BINAP)(PPh₃)HCl] (rac-BINAP = racemic(2,2'-bis(diphenylphosphino)-1,1'-binaphthyl) to give [Ru(dppbz)(PPh₂(binaphthyl)')ZnMe] (3). H₂ and the C-H bond of PhC[triple bond, length as m-dash]CH added across the Ru-Zn bond of 1, and also reversed the phosphine cyclometallation, to give [Ru(dppbz)(Ph₂P(biphenyl))(H)₂(H)(ZnMe)] (4) and [Ru(dppbz)(Ph₂P(biphenyl))(C[triple bond, length as m-dash]CPh)₂(H)(ZnMe)] (5) respectively.

Monomeric 16-Electron π-Diborene Complexes of Zn(II) and Cd(II)

Despite the prevalence of stable π-complexes of most d¹⁰ metals, such as Cu(I) and Ni(0), with ethylene and other olefins, complexation of d¹⁰ Zn(II) to simple olefins is too weak to form isolable complexes due to the metal ion's limited capacity for π-backdonation. By employing more strongly donating π-ligands, namely neutral diborenes with a high-lying π(B═B) orbital, monomeric 16-electron M(II)-diborene (M = Zn, Cd) π-complexes were synthesized in good yields. Metal-B₂ π-interactions in both the solid and solution state were confirmed by single-crystal X-ray analyses and their solution NMR and UV-vis absorption spectroscopy, respectively. The M(II) centers adopt a trigonal planar geometry and interact almost symmetrically with both boron atoms. The MB₂ planes significantly twist out of the MX₂ planes about the M-centroid(B-B) vector, with angles ranging from 47.0° to 85.5°, depending on the steric interactions between the diborene ligand and the MX₂ fragment.

Regio- and Stereoselective Anti-Carbozincation of Alkynyl Ethers Using ZnBr2 toward (Z)-β-Zincated Enol Ether Synthesis

(Z)-β-Aryloxyalkenylzincs are synthesized stereoselectively via anti-carbozincation among alkynyl ethers, silyl ketene acetals, and ZnBr₂. X-ray analysis revealed the structure of the zinc species is a mononuclear two-coordinate dialkenylzinc that is transformed into functionalized enol ethers as a single isomer in the reaction of various electrophiles.

Simple ZnEt2as a catalyst in carbodiimide hydroalkynylation: structural and mechanistic studies

Simple ZnEt₂is an efficient catalyst for the addition of terminal alkynes to carbodiimides, through amidinate complexes, and consecutive isocyanate addition and intramolecular cyclohydroamination.

P,O-Phosphinophenolate zinc(II) species: synthesis, structure and use in the ring-opening polymerization (ROP) of lactide, ε-caprolactone and trimethylene carbonate

The P,O-type phosphinophenol proligands (1·H, 2-PPh2-4-Me-6-Me-C6H2OH; 2·H, 2-PPh2-4-Me-6-(t)Bu-C6H2OH) readily react with one equiv. of ZnEt2 to afford in high yields the corresponding Zn(II)-ethyl dimers of the type [(κ(2)-P,O)Zn-Et]2 (3 and 4) with two μ-OPh bridging oxygens connecting the two Zn(II) centers, as determined by X-ray diffraction (XRD) studies in the case of 3. Based on diffusion-ordered NMR spectroscopy (DOSY), both species 3 and 4 retain their dimeric structures in solution. The alcoholysis reaction of Zn(II) alkyls 3 and 4 with BnOH led to the high yield formation of the corresponding Zn(II) benzyloxide species [(κ(2)-P,O)Zn-OBn]2 (5 and 6), isolated in a pure form as colorless solids. The centrosymmetric and dimeric nature of Zn(II) alkoxides 5 and 6 in solution was deduced from DOSY NMR experiments and multinuclear NMR data. Though the heteroleptic species 5 is stable in solution, its analogue 6 is instable in CH2Cl2 solution at room temperature to slowly decompose to the corresponding homoleptic species 8via the transient formation of (κ(2)-P,O)2Zn2(μ-OBn)(μ–κ(1):κ(1)-P,O) (6′). Crystallization of compound 6 led to crystals of 6′, as established by XRD analysis. The reaction of ZnEt2 with two equiv. of 1·H and 2·H allowed access to the corresponding homoleptic species of the type [Zn(P,O)2] (7 and 8). All gathered data are consistent with compound 7 being a dinuclear species in the solid state and in solution. Data for species 8, which bears a sterically demanding P,O-ligand, are consistent with a mononuclear species in solution. The Zn(II) alkoxide species 5 and the [Zn(P,O)2]-type compounds 7 and 8 were evaluated as initiators of the ring-opening polymerization (ROP) of lactide (LA), ε-caprolactone (ε-CL) and trimethylene carbonate (TMC). Species 5 is a well-behaved ROP initiator for the homo-, co- and ter-polymerization of all three monomers with the production of narrow disperse materials under living and immortal conditions. Though species 7 and 8 are ROP inactive on their own, they readily polymerize LA in the presence of a nucleophile such as BnOH to produce narrow disperse PLA, presumably via an activated-monomer ROP mechanism.

Single electron transfer-induced coupling of alkynylzinc reagents with aryl and alkenyl iodides

An alkynyl–aryl coupling without the aid of transition metal catalysis has been achieved for the first time.

Synthesis and characterization of ferrocene-chelating heteroscorpionate complexes of nickel(II) and zinc(II)

The first example of a ferrocene-chelating heteroscorpionate, [Li(THF)2][fc(PPh2)(BH[(3,5-Me)2pz]2)] ((fc(P,B))Li(THF)2, fc = 1,1'-ferrocenediyl) is described. Starting from a previously reported compound, fcBr(PPh2), a series of ferrocene derivatives, fc(PPh2)(B[OMe]2), [Li(OEt2)][fc(PPh2)(BH3)], [Li(THF)2][fc(PPh2)(BH[(3,5-Me)2pz]2)] (pz = pyrazole), was isolated and characterized. Compound (fc(P,B))Li(THF)2 allowed the synthesis of the corresponding nickel and zinc complexes, (fc(P,B))NiCl, (fc(P,B))NiMe, (fc(P,B))ZnCl, and (fc(P,B))ZnMe. All compounds were characterized by NMR spectroscopy, while the zinc and nickel complexes were also characterized by X-ray crystallography. The redox behavior of (fc(P,B))NiCl, (fc(P,B))NiMe, (fc(P,B))ZnCl, and (fc(P,B))ZnMe was studied by cyclic voltammetry and supported by density functional theory calculations.

Design, synthesis, and evaluation of curcumin-derived arylheptanoids for glioblastoma and neuroblastoma cytotoxicity

Using an innovative approach toward multiple carbon-carbon bond-formations that relies on the multifaceted catalytic properties of titanocene complexes we constructed a series of C1-C7 analogs of curcumin for evaluation as brain and peripheral nervous system anti-cancer agents. C2-Arylated analogs proved efficacious against neuroblastoma (SK-N-SH & SK-N-FI) and glioblastoma multiforme (U87MG) cell lines. Similar inhibitory activity was also evident in p53 knockdown U87MG GBM cells. Furthermore, lead compounds showed limited growth inhibition in vitro against normal primary human CD34+hematopoietic progenitor cells. Taken together, the present findings indicate that these curcumin analogs are viable lead compounds for the development of new central and peripheral nervous system cancer chemotherapeutics with the potential for little effects on normal hematopoietic progenitor cells.

Bifunctional titanocene catalysis in multicomponent couplings: a convergent assembly of β-alkynyl ketones

Herein is described a titanium-catalyzed three-component coupling to assemble β-alkynyl ketones in a single operation. Treatment of an aryl aldehyde with an acetylide and silyl enol ether in the presence of a bifunctional titanocene catalyst enables the highly convergent assembly of β-alkynyl ketones in good to excellent yields.

Cooperative titanocene and phosphine catalysis: accelerated C-X activation for the generation of reactive organometallics

The study presented herein describes a reductive transmetalation approach toward the generation of Grignard and organozinc reagents mediated by a titanocene catalyst. This method enables the metalation of functionalized substrates without loss of functional group compatibility. Allyl zinc reagents and allyl, vinyl, and alkyl Grignard reagents were generated in situ and used in the addition to carbonyl substrates to provide the corresponding carbinols in yields up to 99%. It was discovered that phosphine ligands effectively accelerate the reductive transmetalation event to enable the metalation of C-X bonds at temperatures as low as -40 °C. Performing the reactions in the presence of chiral diamines and amino alcohols led to the enantioselective allylation of aldehydes.

Titanocene-catalyzed multicomponent coupling approach to diarylethynyl methanes

A titanocene-catalyzed multicomponent coupling to provide diarylethynyl methanes is described. By combining the multifunctionality of Cp(2)TiCl(2) with the traceless dielectrophilicity of aryl aldehydes, all-carbon tertiary centers are obtained in 55-99% yield.

Bis(allyl)zinc revisited: sigma versus pi bonding of allyl coordination

The reinvestigation of two allyl zinc compounds, parent bis(allyl)zinc [Zn(C(3)H(5))(2)] (1) and 2-methallyl chloro zinc [Zn(C(4)H(7))Cl] (2), revealed two new coordination modes in the solid state for the allyl ligand, viz cis- and trans-μ(2)-η(1):η(1). These results call for modification of the conventional interpretation of zinc-allyl interactions. Computational results indicate that the classical η(3)-bonding mode of the allyl ligand is not favored in zinc compounds. A rare case of a zinc-olefin interaction in the dimer of [Zn(η(1)-C(3)H(5))(OC(C(3)H(5))Ph(2))] was found in the monoinsertion product of 1 with benzophenone.