Organozinc mediated reactions

Reductive anti-Dizincation of Arylacetylenes

Arylacetylenes undergo anti-1,2-dizincation to afford trans-1,2-dizincioalkenes. The process employs sodium dispersion as a reducing agent and zinc chloride TMEDA complex as a reduction-resistant zinc electrophile. This reductive anti-dizincation contrasts with the conventional additive syn-dimetalation like silylzincation. The resulting dizincated alkenes undergo the cross-coupling to yield multi-substituted alkenes stereoselectively.

Ultrafast Halogen Dance Reactions of Bromoarenes Enabled by Catalytic Potassium Hexamethyldisilazide

Lochmann-Schlosser base, a stoichiometric combination of nBuLi and KOtBu, is commonly used as a superbase for deprotonating a wide range of organic compounds. In the present study, we report that catalytic potassium hexamethyldisilazide (KHMDS) exhibits higher catalytic activity than KOtBu for successive bromine-metal exchanges. Accordingly, 1-10 mol% of KHMDS dramatically enhances halogen dance reactions to introduce various electrophiles to bromopyridine, bromoimidazole, bromothiophene, bromofuran, and bromobenzene derivatives with the bromo group translocated from the original position. A dual catalytic cycle is proposed to explain the ultrafast bromine transfer.

Catalytic Asymmetric Ring-Opening Reactions of Unstrained Heterocycles Using Cobalt Vinylidenes

Cobalt catalysts promote highly enantioselective ring-opening reactions of 2,5-dihydrofurans using vinylidenes. The products are acyclic organozinc compounds that can be functionalized with an electrophile. The proposed mechanism involves the generation of a cobalt vinylidene species that adds to the alkene by a [2+2]-cycloaddition pathway. Ring-opening then occurs via outer-sphere β-O elimination assisted by coordination of a ZnX2 Lewis acid to the alkoxide leaving group. DFT models reveal that competing inner-sphere syn β-H and β-O elimination pathways are suppressed by the geometric constraints of the metallacycle intermediate. These models rationalize the observed stereochemical outcome of the reaction.

Formal Halogen Transfer of Bromoarenes via Stepwise Reactions

A two-step halogen transfer of bromoarenes is reported. Mono-, di-, and tribromoaryllithiums generated through deprotonative lithiation were converted into organozinc species by in situ zincation, which were then subjected to bromination to provide the corresponding di-, tri-, and tetrabromoarenes, respectively, in 41-95% yields. Regioselective bromine-magnesium exchange with ethylmagnesium chloride followed by electrophilic trapping afforded benzene, pyridine, quinoline, pyrimidine, and thiazole derivatives with the bromo group translocated from the original position in 28-86% yields.

Deprotonative Generation and Trapping of Haloaryllithium in a Batch Reactor

A method for the regioselective functionalization of haloarenes through deprotonative lithiation is disclosed. The generated haloaryllithiums were trapped in a batch reactor with a zinc chloride diamine complex to provide organozinc species without aryne formation, which reacted with electrophiles to afford the corresponding products in 38-98% yields. This method was applied to the five-step total synthesis of carbazomycin A on a gram scale in 33% overall yield.

C versus O Protonation in Zincate Anions: A Simple Gas-Phase Model for the Surprising Kinetic Stability of Organometallics

For better understanding the intrinsic reactivity of organozinc reagents, we have examined the protolysis of the isolated zincate ions Et₃ Zn^- , Et₂ Zn(OH)^- , and Et₂ Zn(OH)₂ Li^- by 2,2,2-trifluoroethanol in the gas phase. The protonation of the hydroxy groups and the release of water proceed much more efficiently than the protonation of the ethyl groups and the liberation of ethane. Quantum-chemical computations and statistical-rate theory calculations fully reproduce the experimental findings and attribute the lower reactivity of the more basic ethyl moiety to higher intrinsic barriers, which override the thermodynamic preference for its protonation. Thus, our minimalistic gas-phase model provides evidence for the intrinsically low reactivity of organozinc reagents toward proton donors and helps to explain their remarkable kinetic stability against moisture and even protic media.

Zincation of Styrylsulfonium Salts

We report the formation of zinc reagents by the reaction of styrylsulfonium salts with zinc powder. Transition metals and other additives are not required for promoting zincation. Zincation tolerates a variety of sensitive functional groups, including esters, bromides, and boronic esters, and proceeds with complete retention of stereochemistry. This method presents a practical approach to the formation of zinc reagents that can be used in a variety of functionalizations, such as halogenation, carboxylation, and Negishi cross-couplings.

Gas Phase Protolysis of Trisarylzincate Anions

We have applied a combination of tandem-mass spectrometry, quantum-chemical calculations, and statistical rate theory computations to examine the gas phase reactions between the trisarylzincate anions Ar^XZnPh₂^- (Ar^X = p-X-C₆H₄, X = NMe₂, OMe, Me, H, F, and Cl) and 2,2,2-trifluoroethanol at T = 310 ± 20 K. The observed reactions bring about the protonation of one of the aryl anions, which is then released as the corresponding arene, while the formed alkoxide binds to the zinc center. The protonation is faster for the more electron-rich aryl groups and shows a linear Hammett plot if the rate constant for X = NMe₂ is discarded from the analysis. Although the reactions are highly exothermic, they proceed only with relatively low efficiencies (0.1% ≤ φ ≤ 1.3%). According to the quantum-chemical calculations, this behavior can be ascribed to the reactions proceeding through a double-well potential with a tight transition structure located at the central barrier. Based on these potential energy surfaces, the statistical rate theory computations can reproduce the measured rate constants within factors of 2 to 8. A comparison of the protolysis of the trisarylzincates with that of the corresponding free aryl anions demonstrates how the coordination to the metal center not only stabilizes the carbanions energetically but also moderates their reactivity. Thus, our gas phase study contributes to a better understanding of the fundamentals of organometallic reactivity.

Three-Component Difunctionalization of Cyclohexenyl Triflates: Direct Access to Versatile Cyclohexenes via Cyclohexynes

Difunctionalization of strained cyclic alkynes presents a powerful strategy to build richly functionalized cyclic alkenes in an expedient fashion. Herein we disclose an efficient and flexible approach to achieve carbohalogenation, dicarbofunctionalization, aminohalogenation and aminocarbonation of readily available cyclohexenyl triflates. We have demonstrated the novel use of zincate base/nucleophile system for effective formation of key cyclohexyne intermediates and selective addition of various carbon and nitrogen nucleophiles. Importantly, leveraging the resulting organozincates enables the incorporation of a broad range of electrophilic partners to deliver structurally diverse cyclohexene motifs. The importance and utility of this method is also exemplified by the modularity of this approach and the ease in which even highly complex polycyclic scaffolds can be accessed in one step.

A Ball-Milling-Enabled Reformatsky Reaction

An operationally simple one-jar one-step mechanochemical Reformatsky reaction using in situ generated organozinc intermediates under neat grinding conditions has been developed. Notable features of this reaction protocol are that it requires no solvent, no inert gases, and no pre-activation of the bulk zinc source. The developed process is demonstrated to have good substrate scope (39-82 % yield) and is effective irrespective of the initial morphology of the zinc source.

Nickel (II) Catalysed 1,4- Addition Reactions of Functionalised Organozinc Reagents with Enones

An efficient catalytic 1,4 addition reaction of functionalised organozinc reagents with enones has been developed using a nickel complex as catalyst.

Highly enantioselective addition of dimethylzinc to fluorinated alkyl ketones, and the mechanism behind it

Switching the solvent from toluene to dichloromethane results in a very important enhancement in the yield and enantiomeric excess of nucleophilic addition.

Synthesis of methyl 4-dihydrotrisporate B and methyl trisporate B, morphogenetic factors of Zygomycetes fungi

(9Z)-Methyl 4-dihydrotrisporate B and (9Z)-methyl trisporate B, pheromones of Zygomycetes fungi, have been synthesized using Stille cross-coupling from previously described cyclohexenone precursors. Conducting the coupling without protection groups allowed for a short and stereospecific synthesis route of the late trisporoids. Stability studies for both the compounds revealed (9Z)-methyl trisporate B to be very unstable against UV irradiation.

Site-Selective Copper-Catalyzed Amination and Azidation of Arenes and Heteroarenes via Deprotonative Zincation

Arene amination is achieved by site-selective C-H zincation followed by copper-catalyzed coupling with O-benzoylhydroxylamines under mild conditions. Key to this success is ortho-zincation mediated by lithium amidodiethylzincate base that is effective for a wide range of arenes, including nonactivated arenes bearing simple functionalities such as fluoride, chloride, ester, amide, ether, nitrile, and trifluoromethyl groups as well as heteroarenes including indole, thiophene, pyridine, and isoquinoline. An analogous C-H azidation is also accomplished using azidoiodinane for direct introduction of a useful azide group onto a broad scope of arenes and heteroarenes. These new transformations offer rapid access to valuable and diverse chemical space of aminoarenes. Their broad applications in organic synthesis and drug discovery are demonstrated in the synthesis of novel analogues of natural product (-)-nicotine and antidepressant sertraline by late-stage amination and azidation reactions.

Preparation of Organozinc Reagents via Catalyst Controlled Three-Component Coupling between Alkyne, Iodoarene, and Bis(iodozincio)methane

Three-component coupling between an alkyne, iodoarene, and bis(iodozincio)methane yields allylic zinc with a tetrasubstituted alkene moiety in the presence of a nickel catalyst. The reaction proceeds via aryl nickelation of the alkyne and subsequent cross-coupling with bis(iodozincio)methane. Meanwhile, the same combination in the presence of a palladium and cobalt catalyst gives tetrasubstituted alkenylzinc. The reaction proceeds via a palladium-catalyzed cross-coupling of iodoarene with bis(iodozincio)methane followed by a cobalt-catalyzed benzylzincation of alkyne.

Methods Utilizing First-Row Transition Metals in Natural Product Total Synthesis

First-row transition-metal-mediated reactions constitute an important and growing area of research due to the low cost, low toxicity, and exceptional synthetic versatility of these metals. Currently, there is considerable effort to replace existing precious-metal-catalyzed reactions with first-row analogs. More importantly, there are a plethora of unique transformations mediated by first-row metals, which have no classical second- or third-row counterpart. Herein, the application of first-row metal-mediated methods to the total synthesis of natural products is discussed. This Review is intended to highlight strategic uses of these metals to realize efficient syntheses and highlight the future potential of these reagents and catalysts in organic synthesis.

Palladium-Catalyzed Formylation of Arylzinc Reagents with S-Phenyl Thioformate

The first example of palladium-catalyzed direct formylation of arylzinc reagents using S-phenyl thioformate is reported. The reaction proceeded under mild conditions, allowing high functional group tolerance. In addition, the developed formylation method was used to prepare deuterated and ¹³C-labeled aryl aldehydes from isotope-labeled S-phenyl thioformates. Moreover, this procedure was applied to an alkenylzinc halide, affording the corresponding enal.

The synthesis of cardenolide and bufadienolide aglycones, and related steroids bearing a heterocyclic subunit

Covering: early studies through to March 2016Cardenolides and bufadienolides constitute an attractive class of biologically active steroid derivatives which have been used for the treatment of heart disease in traditional remedies as well as in modern medicinal therapy. Due to their application as therapeutic agents and their unique molecular structures, bearing unsaturated 5- or 6-membered lactones (or other heterocycles) attached to the steroid core, cardio-active steroids have received great attention, which has intensified during the last decade, in the synthetic organic community. Advances in the field of cross-coupling reactions have provided a powerful tool for the attachment of lactone subunits to the steroid core. This current review covers a methodological analysis of synthetic efforts to cardenolide and bufadienolide aglycones. Special emphasis is given to cross-coupling reactions applied for the attachment of lactone subunits at sterically very hindered positions of the steroid core. The carefully selected partial and total syntheses of representative cardio-active steroids will also be presented to exemplify recent achievements (improvements) in the field.

One-pot synthesis of ultrastable pentanuclear alkylzinc complexes

The first ever reported organometallic compounds featuring a Kuratowski-type bond topology were found to be unexpectedly chemically and thermally stable.

Stability and Unimolecular Reactivity of Palladate(II) Complexes [Ln PdR3 ]- (L=Phosphine, R=Organyl, n=0 and 1)

The reduction of Pd^II precatalysts to catalytically active Pd⁰ species is a key step in many palladium-mediated cross-coupling reactions. Besides phosphines, the stoichiometrically used organometallic reagents can afford this reduction, but do so in a poorly understood way. To elucidate the mechanism of this reaction, we have treated solutions of Pd(OAc)₂ and a phosphine ligand L in tetrahydrofuran with RMgCl (R=Ph, Bn, Bu) as well as other organometallic reagents. Analysis of these model systems by electrospray- ionization mass spectrometry found palladate(II) complexes [L_n PdR₃ ]^- (n=0 and 1), thus pointing to the occurrence of transmetallation reactions. Upon gas-phase fragmentation, the [L_n PdR₃ ]^- anions preferentially underwent a reductive elimination to yield Pd⁰ species. The sequence of the transmetallation and reductive elimination, thus, constitutes a feasible mechanism for the reduction of the Pd(OAc)₂ precatalyst. Other species of interest observed include the Pd^IV complex [PdBn₅ ]^- , which did not fragment via a reductive elimination but lost BnH instead.

Iron-Catalyzed Acylation of Polyfunctionalized Aryl- and Benzylzinc Halides with Acid Chlorides

FeCl2 (5 mol %) catalyzes a smooth and convenient acylation of functionalized arylzinc halides at 50 °C (2-4 h) and benzylic zinc chlorides at 25 °C (0.5-4 h) with a variety of acid chlorides leading to polyfunctionalized diaryl and aryl heteroaryl ketones.

Nickel N-heterocyclic carbene-catalyzed cross-coupling reaction of aryl aldehydes with organozinc reagents to produce aryl ketones

The transformation of aromatic aldehydes into aryl ketones by nickel-catalyzed cross-coupling has been developed. This transformation represents an efficient and attractive synthetic utilization of organozinc reagents.

Structure-kinetic relationship study of organozinc reagents

Phenylzinc reagents prepared from various zinc halides show distinct kinetic features in the palladium-catalyzed Negishi-type oxidative coupling reaction, in which the phenylzinc reagent prepared from ZnI2 gives the highest rate. In situ infrared and X-ray absorption spectroscopy studies show that the higher reaction rate was observed for longer Zn-C bond distances.

Copper-catalyzed electrophilic amination of heteroarenes and arenes by C-H zincation

Direct amination of heteroarenes and arenes has been achieved in a one-pot CH zincation/copper-catalyzed electrophilic amination procedure. This amination method provides an efficient and rapid approach to access a diverse range of heteroaromatic and aromatic amines including those previously inaccessible using CH amination methods. The mild reaction conditions and good functional-group compatibility demonstrate its great potential for the synthesis of important and complex amines.

Zinc-mediated addition of diethyl bromomalonate to alkynes for the cascade reaction towards polysubstituted pyranones and tetracarbonyl derivatives

The zinc-mediated regioselective addition reactions of diethyl bromomalonate and aromatic and aliphatic alkynes were investigated for the synthesis of vinyl malonates. When the vinyl organo-zinc intermediates were reacted with acid chlorides 2H-pyran-2-ones were obtained while the application of oxalyl chloride and an amine led to tetracarbonyl derivatives in a one-pot multi-step reaction sequence.

Selective monofluorination of active methylene compounds: the important role of ZnCl2 in inhibiting overfluorination

ZnCl₂ effectively facilitates selective monofluorination of methylene compounds by inhibiting the undesired overfluorination.

Selective α-amination and α-acylation of esters and amides via dual reactivity of O-acylhydroxylamines toward zinc enolates

α-Amination and α-acylation of esters and amides have been developed, employing O-acylhydroxylamines as a dually reactive aminating and acylating reagent.