Alkyl Zinc Research Articles

From Chain- to Graphene-like Hydroxyl-terminated (ZnO)n Clusters with n≤6 Obtained via Zinc Dimethoxide Hydrolysis and Condensation: Ab initio Structural, Electronic, Vibrational and Optical Properties Calculations.

Recent reports are focusing on the structural evolution from the atomic-scale and also at the expenses of alkyl zinc alkoxide precursors towards (ZnO)n clusters and nanostructures with different interesting motifs, but still not much is known about their electronic properties. In this manuscript, we present a theoretical study using DFT and TD-DFT methodologies on the hydrolysis and condensation of zinc dimethoxide precursor in its monomeric, dimeric and trimeric forms towards thermodynamically stable hydroxyl-terminated (ZnO)n clusters with novel chain- and graphene-like fashions. For all cases, distinct vibrational and optical spectra features were assigned evidencing a global monotonic decrease in the opto-electronic gap with increasing oligomerization and cyclization stages. In addition, the electron-affinity of all clusters was also observed to be enhanced with increasing oligomerization and cyclization stages and the electronic charge localization in -e charged clusters was observed to be strongly related to the presence of zinc-oxo subunits and other particular structural features. Our calculations also indicate that the stabilization through hydroxyl termination of both chain- and graphene-like ZnO clusters not only could be a promising driving force to obtain larger atomic-scale 1D and 2D nanostructures but also envisage interesting properties, particularly as electronic acceptor materials for energy applications.

Zinc Hydride-Catalyzed Hydrofuntionalization of Ketones.

Three new dimeric bis-guanidinate zinc(II) alkyl, halide, and hydride complexes [LZnEt]2 (1), [LZnI]2 (2) and [LZnH]2 (3) were prepared. Compound 3 was successfully employed for the hydrosilylation and hydroboration of a vast number of ketones. The catalytic performance of 3 in the hydroboration of acetophenone exhibits a turnover frequency, reaching up to 5800 h-1, outperforming that of reported zinc hydride catalysts. Notably, both intra- and intermolecular chemoselective hydrosilylation and hydroboration reactions have been investigated.

Regioselective Hydroboration and Hydrosilylation of N-Heteroarenes Catalyzed by a Zinc Alkyl Complex.

Readily available zinc alkyl complexes showed good activity and regioselectivity in catalyzing hydroboration and hydrosilylation of N-heteroarenes. Hydroboration of benzo-fused N-heterocycles gave exclusive 1,2-addition products in 80-97% yields. Reactions of pyridines afforded a mixture of 1,2-, 1,4-, and 1,6-products in yields of 55-95%, with 1,2-dihydropyridine as the main product. Bis-hydrosilylation was observed for quinoline derivatives, generating bis-1,2-hydrosilylation products in 76-96% yields. Kinetic studies and control experiments were conducted to gain some insights into the reaction mechanism.

Stereoselective Synthesis of Cis- and Trans-Tetrasubstituted Vinyl Silanes Using a Silyl-Heck Strategy and Hiyama Conditions for Their Cross-Coupling.

We report a palladium-catalyzed, three-component carbosilylation reaction of internal symmetrical alkynes, silicon electrophiles, and primary alkyl zinc iodides. Depending on the choice of ligand, stereoselective synthesis of either cis- or trans-tetrasubstituted vinyl silanes is possible. We also demonstrate conditions for the Hiyama cross-coupling of these products to prepare geometrically defined tetrasubstituted alkenes.

Dinucleating Amino-Phenolate Platform for Zinc Catalysts: Impact on Lactide Polymerization.

We report imine- and amine-based dinucleating ligands bearing a bisphenol backbone and explore their coordination chemistry with zinc to form zinc alkyl, alkoxide, acetate, and amide complexes. Full characterization of the complexes shows that this ligand framework can support dinuclear and trinuclear complexes. We explore the reactivity of the zinc alkyl and alkoxide complexes as catalysts for the ring opening polymerization of lactide and compared this reactivity to analogous mononuclear complexes. We show that 1) The amine-based complexes are more reactive than the imine-based analogues; 2) The trinuclear zinc alkyl species show unusual control and reproducibility for lactide polymerization; and 3) The extent of bimetallic cooperation is hampered by the ability of the ligand framework to form trinuclear clusters.

Nickel-catalyzed allylic carbonylative coupling of alkyl zinc reagents with tert-butyl isocyanide

Transition metal-catalyzed carbonylation with carbon nucleophiles is one of the most prominent methods to construct ketones, which are highly versatile motifs prevalent in a variety of organic compounds. In comparison to the well-established palladium catalytic system, the nickel-catalyzed carbonylative coupling is much underdeveloped due to the strong binding affinity of CO to nickel. By leveraging easily accessible tert-butyl isocyanide as the CO surrogate, we present a nickel-catalyzed allylic carbonylative coupling with alkyl zinc reagent, allowing for the practical and straightforward preparation of synthetically important β,γ-unsaturated ketones in a linear-selective fashion with excellent trans-selectivity under mild conditions. Moreover, the undesired polycarbonylation process which is often encountered in palladium chemistry could be completely suppressed. This nickel-based method features excellent functional group tolerance, even including the active aryl iodide functionality to allow the orthogonal derivatization of β,γ-unsaturated ketones. Preliminary mechanistic studies suggest that the reaction proceeds via a π-allylnickel intermediate.

Zn-mediated decarboxylative carbagermatranation of aliphatic N-hydroxyphthalimide esters: evidence for an alkylzinc intermediate.

Alkyl nucleophiles synthesized by decarboxylation of the corresponding N-hydroxyphthalimide esters (NHP esters) would inherit the complex structure of natural carboxylic acids and result in useful cross-coupling fragments. Herein, we report the synthesis of alkyl carbagermatranes via Zn-mediated decarboxylation of NHP esters without Ni catalysis or photocatalysis. Mechanistic studies indicate that an alkyl zinc intermediate was involved; however, the generation of alkyl zinc will be inhibited in the presence of Ni. Hence, this study provides valuable resolution to the perplexing problem about whether organozinc was involved in recently emerging catalytic systems of NHP ester-Zn. Meanwhile, alkyl zinc reagents from NHP esters are compatible with aryl/alkyl bromides and iodides; therefore the scope of carbagermatranation in this work precedes that of in situ-generated organozinc from alkyl halides.

Chemical modification of cotton by methyl methacrylate via emulsion polymerization

A highly efficient and environmentally friendly graft polymerization method of methyl methacrylate (MMA) onto cotton was performed using an alkyl zinc initiator in an emulsion system. Using this method, a high-molecular weight (Mw > 106) polymer was grafted onto a cotton fiber with a high graft yield at ambient temperature. The grafted cotton fiber was characterized by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and thermogravimetric analysis (TGA). The surface morphologies of the unmodified and grafted cotton were analyzed by scanning electron microscopy (SEM). Furthermore, the grafted cotton produced by this novel method that involved modifying the surface of the cotton improved its compatibility with a number of organic solvents. A highly efficient and environmentally friendly graft polymerization method of methyl methacrylate (MMA) onto cotton was performed using an alkyl zinc initiator in an emulsion system. Using this method, a high-molecular weight (Mw > 106) polymer was grafted onto a cotton fiber with a high graft yield at ambient temperature. The grafted cotton fiber was characterized by FTIR, XRD, TGA, and SEM. Furthermore, the grafted cotton produced by this novel method that involved modifying the surface of the cotton improved its compatibility with a number of organic solvents.

Direct Transformation of Aryl 2-Pyridyl Esters to Secondary Benzylic Alcohols by Nickel Relay Catalysis

A direct transformation of aryl esters to secondary benzylic alcohols via tandem Ni-catalyzed cross-coupling reactions of aromatic 2-pyridyl esters with alkyl zinc reagents and carbonyl group reduction by Ni-H species is achieved. Preliminary mechanistic studies reveal that the Ni-H species is generated in situ via β-hydride elimination of the Negishi reagents. The reaction is catalyzed by bench-stable nickel salts under mild conditions with wide functional group tolerance.

Cover Feature: Copper‐Catalyzed Electrophilic Thiolation of Organozinc Halides by Using N‐Thiophthalimides Leading to Polyfunctional Thioethers (Chem. Eur. J. 15/2019)

Electrophilic Thiolations are important reactions in organic synthesis. In this Communication, (hetero)aryl, benzylic, and alkyl zinc halides were thiolated with N-thiophthalimides at 25 °C within 1 h in the presence of 5–10% Cu(OAc)2⋅H2O to furnish the corresponding polyfunctionalized thioethers in good yields. This electrophilic thiolation was extended to the introduction of trifluoromethylthio (SCF3), thiocyanate (SCN), and selenophenyl (SePh) groups. The utility of this method was shown in a seven-step synthesis of a potent cathepsin D inhibitor in 34% overall yield. More information can be found in the Communication by P. Knochel et al. on page 3752.

Copper-Catalyzed Electrophilic Thiolation of Organozinc Halides by Using N-Thiophthalimides Leading to Polyfunctional Thioethers.

(Hetero)aryl, benzylic, and alkyl zinc halides were thiolated with N-thiophthalimides at 25 °C within 1 h in the presence of 5-10 % Cu(OAc)2 ⋅H2 O to furnish the corresponding polyfunctionalized thioethers in good yields. This electrophilic thiolation was extended to the introduction of trifluoromethylthio (SCF3 ), thiocyanate (SCN), and selenophenyl (SePh) groups. The utility of this method was shown in a seven-step synthesis of a potent cathepsin D inhibitor in 34 % overall yield.

An imidazolin-2-iminato ligand organozinc complex as a catalyst for hydroboration of organic nitriles

Catalytic hydroboration of nitriles with pinacolborane (HBpin) using the imidazolin-2-iminato zinc alkyl complex [{(ImtBuN)Zn(CH2CH3)}2] (1c) under mild and solvent-free conditions to afford corresponding diboronate esters in high yield is reported.

Transition-Metal-Free Arylation and Alkylation of Diarylmethyl p-Tolyl Sulfones with Zinc Reagents.

The transition-metal-free synthesis of unsymmetrical and highly functionalized triarylmethanes through arylation of the situ generated o-QMs from diarylmethyl p-tolyl sulfones with aryl zinc reagents is described. Alkyl zinc reagents are also well tolerated in this reaction. Additionally, the straightforward synthesis of the analogue of the antituberculosis agent A and the key precursor of the anti-breast-cancer agent B are achieved by this strategy.

Ethylene–co–norbornene copolymerization in the presence of a chain transfer agent

Ethylene (E)-co-norbornene (N) copolymerizations were conducted by using two ansa-metallocenes, isopropyliden(η5-cyclopentadienyl)(η5-indenyl)zirconium dichloride (1) and isopropyliden(η5-3-methylcyclopentadienyl)(η5-fluorenyl)zirconium dichloride (2), activated with dimethylanilinium tetrakis(pentafluorophenyl)borate, in presence of TIBA and variable amounts of Zn(Et)2 as a potential chain transfer agent (CTA). The study evaluated the effect of CTA on microstructure, molecular weight, chain end group and structural, thermal and mechanical properties of poly(E-co-N)s. The analysis of the data revealed that in all copolymerization reactions with metallocenes 1 and 2 in presence of diethyl zinc chain transfer to zinc alkyl occurs along with the typical chain transfers of E-co-N polymerization. The chain transfer to zinc alkyl is more efficient at low N content in copolymers. The two catalysts show different behavior in copolymerization at high N content: molar masses of copolymers produced by 2 are more affected by CTA concentration, that is, the chain shortening with increasing CTA content is greater in copolymers obtained with catalyst 2. The analysis of mechanical properties revealed that CTA affects the polymer chain structure. An increase of CTA caused a change of ductility, rigidity, and mechanical strength; chains produced in presence of Zn(Et)2 are, at least in part, characterized by segments produced as a result of exchange between different metal centers, possibly jointed through ethylene units.

Chain transfer reaction to diethylzinc in ethylene polymerization by metallocene catalysts

The addition of diethyl zinc into the metallocene catalytic system for ethylene polymerization efficiently reduces the molecular weight of the resulted polyethylene due to the transfer reaction of the propagation chain to the dialkyl zinc. The alkyl exchanging reaction between diethyl zinc and the methylaluminoxane or trimethyl aluminum may result in the new zinc compounds. Therefore, the result of regression analysis shows that the chain transfer is not an ideal first order reaction. The regression result also shows that the transfer reaction to zinc is much stronger than other transfer reaction. The reaction results in the long alkyl zinc, and the latter is easily be oxidized, and then forms the polyethylene contains an abundance of hydroxyl capped chain end, which has been identified by means of 13C-NMR.

Carbonylative Coupling of Alkyl Zinc Reagents with Benzyl Bromides Catalyzed by a Nickel/NN2 Pincer Ligand Complex.

An efficient catalytic protocol for the three-component assembly of benzyl bromides, carbon monoxide, and alkyl zinc reagents to give benzyl alkyl ketones is described, and represents the first nickel-catalyzed carbonylative coupling of two sp3 -carbon fragments. The method, which relies on the application of nickel complexed with an NN2 -type pincer ligand and a controlled release of CO gas from a solid precursor, works well with a range of benzylic bromides. Mechanistic studies suggest the intermediacy of carbon-centered radicals.

Carbonylative Coupling of Alkyl Zinc Reagents with Benzyl Bromides Catalyzed by a Nickel/NN2 Pincer Ligand Complex

AbstractAn efficient catalytic protocol for the three‐component assembly of benzyl bromides, carbon monoxide, and alkyl zinc reagents to give benzyl alkyl ketones is described, and represents the first nickel‐catalyzed carbonylative coupling of two sp3‐carbon fragments. The method, which relies on the application of nickel complexed with an NN2‐type pincer ligand and a controlled release of CO gas from a solid precursor, works well with a range of benzylic bromides. Mechanistic studies suggest the intermediacy of carbon‐centered radicals.

Rhodium-Catalyzed Desymmetrization of meso-Glutaric Anhydrides to Access Enantioenriched anti,anti-Polypropionates

An expedient desymmetrization of 3,5-dimethyl-4-alkoxyglutaric anhydrides to access anti,anti-polypropionates is described. The previously unknown anhydrides are rapidly assembled from readily available precursors. A Rh(I)·t-BuPHOX catalyst system was found to provide good yield and high selectivities. With these conditions, the trisubstituted anhydrides were desymmetrized with various alkyl zinc reagents to provide synthetically useful enantioenriched anti,anti-2,4-dimethyl-3-hydroxy-δ-ketoacids. An identical catalyst system also affords access to syn,syn-stereotriads as well as a partial kinetic resolution of a chiral anhydride.

Toward Factors Affecting the Degree of Zinc Alkyls Oxygenation: A Case of Organozinc Guanidinate Complexes

While extensive research has been carried out on the oxygenation of alkylzinc complexes for decades, this issue still remains unresolved and significant uncertainties concerning the mechanism of these reactions and the composition of the resulting products persist. These reactions are believed to proceed via the initial formation of ROOZn(L) species, but studies on the oxygenation reactions have been substantially impeded by the low stability of the alkylperoxides. This report describes the oxygenation of a tert-butylzinc guanidinate, i.e. a tBuZn(L)-type complex (L = deprotonated 1,4,6-triazabicyclo[3.3.0]oct-4-ene), and the isolation and characterization of an unprecedented aggregate based on a combination of a ROOZn(L) moiety and two parent tBuZn(L) molecules, {[tBuZn(L)]2[tBuOOZn(L)]}. Further study revealed that examined tert-butylzinc guanidinate molecules exhibit an ability to entrap other oxygenated species, which was demonstrated by an aggregate containing a tert-butylzinc tert-butylperoxide and ...

Unprecedented Variety of Outcomes in the Oxygenation of Dinuclear Alkylzinc Derivatives of an N,N-Coupled Bis(β-diketimine).

Reactions between O2 and organometallics with non-redox-active metal centers have received continuous interest for over 150 years, although significant uncertainties concerning the character and details of the actual mechanism of these reactions persist. Harnessing dinuclear three-coordinate alkylzinc derivatives of an N,N-coupled bis(β-diketimine) proligand (LH2 ) as a model system, we demonstrate for the first time that a slight modification of the reaction conditions might have a dramatic influence on the oxygenation reaction outcomes, leading to an unprecedented variety of products originating from a single reaction system, that is, partially and fully oxygenated zinc alkoxides, zinc alkylperoxides, and zinc hydroxide compounds. Our studies indicate that accessibility of the three-coordinate zinc center by the O2 molecule, coupled with the lower reactivity of Zn-Me vs. Zn-Et units towards dioxygen, are key factors in the oxygenation process, providing a novel tetranuclear methyl(methoxy)zinc {[L][ZnMe][Zn(μ-OMe]}2 and zinc ethoxide {[L][Zn(μ-OEt)]2 }2 . Remarkably, oxygenation of three-coordinate alkylzinc [L][ZnR]2 complexes at ambient temperature afforded a unique hydroxide {[L][Zn(μ-OH)]2 }2 . Oxygenation of the [L][ZnEt]2 complex in the presence of 4-methylpyridine (py-Me) at low temperature led to the isolation of a dinuclear zinc ethylperoxide [L][Zn(OOEt)(py-Me)]2 , which nicely substantiates the intermediacy of an unstable zinc alkylperoxide in the formation of the subsequent zinc alkoxide and hydroxide compounds. Finally, our investigations provide compelling evidence that a non-redox-active metal center plays a crucial role in the oxygenation process through assisting in single-electron transfer from an M-C bond to an O2 molecule. Although the oxygenation of zinc alkyls occurs by radical pathways, the reported results stand in clear contradiction to the widely accepted free-radical chain mechanism.