Organocopper compound

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Lithium diphenylcuprate etherate dimer from crystal structure
Skeletal formula of lithium diphenylcuprate etherate dimer

Organocopper compounds in organometallic chemistry contain carbon to copper chemical bonds. Organocopper chemistry is the science of organocopper compounds describing their physical properties, synthesis and reactions.[1][2][3] They are reagents in organic chemistry.

The first organocopper compound, the explosive copper(I) acetylide Cu2C2 (Cu-C≡C-Cu), was synthesized by Rudolf Christian Böttger in 1859 by passing acetylene gas through copper(I) chloride solution:[4]

C2H2 + 2 CuCl → Cu2C2 + 2 HCl

Henry Gilman prepared methylcopper in 1936. In 1941, Kharash discovered that reaction of a Grignard reagent with cyclohexenone in presence of Cu(I) resulted in 1,4-addition instead of 1,2-addition.[5] In 1952 Gilman investigated for the first time dialkylcuprates. In the 1960s, complexes of alkenes and CO with copper(I) were established.

Chemical Properties[edit]

Organocopper compounds are very reactive towards oxygen and water, forming copper(I) oxide and tend to be thermally unstable. Because most cuprates are salts, many are generally insoluble in nonpolar solvents. Despite these difficulties, organocopper reagents are frequently generated and consumed in situ with no attempt to isolate them. They are used very frequently in organic chemistry as alkylating reagents because they exhibit more functional group tolerance than corresponding Grignard and organolithium reagents. The electronegativity of copper is much higher than its next-door neighbor in the group 12 elements, zinc, suggesting less nucleophilicity for its carbon ligands.

The oxidation state of copper can be +1 or +2 and intermediates can have oxidation state +3. Monovalent alkylcopper compounds (RCu) are polymeric but form cuprates (R2CuLi) upon treatment with organolithium compounds (RLi). These cuprates are sometimes referred to as Gilman reagents. Organocopper compounds can be stabilized by complexation to a variety of ligands such as alkylphosphines (R3P), thioethers (R2S), and cyanide (CN-).

The cuprate complexes form complicated aggregates both in crystalline form and in solution. Lithium dimethylcuprate is a dimer in diethyl ether forming an 8-membered ring with two lithium atoms coordinating between two methyl groups. Similarly, lithium diphenylcuprate forms a dimeric etherate, [{Li(OEt2)}(CuPh2)]2, in the solid state.[6]

Organocopper aggregates

The first ever crystal structure was determined in 1972 by Lappert for CuCH2SiMe3. This compound is relatively stable because the bulky trimethylsilyl groups provide steric protection. It is a tetramer, forming an 8-membered ring with alternating Cu-C bonds. In addition the four copper atoms form a planar Cu4 ring based on three-center two-electron bonds. The copper to copper bond length is 242 pm compared to 256 pm in bulk copper. In pentamesitylpentacopper a 5-membered copper ring is formed, similar to (2,4,6-Trimethylphenyl)gold, and pentafluorophenylcopper is a tetramer.[7]

With carbon monoxide copper forms a non-classical metal carbonyl.

Copper intermediates[edit]

Copper has four known intermediate with oxidative states ranging form 0 to +3.

Cu(III) intermediates[edit]

In many organometallic reactions involving copper, the reaction mechanism invokes a copper intermediate with oxidation state +3. For instance, in reductive elimination processes, Cu(III) is reduced to Cu(I). However Cu(III) compounds are rare in chemistry in general and until recently organocopper(III) species have been elusive. In 2007 the first spectroscopic evidence was obtained for the involvement of Cu(III) in the conjugate addition of the Gilman reagent to an enone:[8] In a so-called rapid-injection NMR experiment at -100 °C, the Gilman reagent Me2CuLi (stabilized by lithium iodide) was introduced to cyclohexenone (1) enabling the detection of the copper — alkene pi complex 2. On subsequent addition of trimethylsilyl cyanide the Cu(III) species 3 is formed (indefinitely stable at that temperature) and on increasing the temperature to -80 °C the conjugate addition product 4. According to an accompanying in silico experiments [9] the Cu(III) intermediate has a square planar molecular geometry with the cyano group in cis orientation with respect to the cyclohexenyl methine group and anti-parallel to the methine proton. With other ligands than the cyano group this study predicts room temperature stable Cu(III) compounds.

A Cu(III) intermediate characterized by NMR.[8]

Synthesis of organocopper compounds[edit]

Copper halides react with organolithium reagents to form the organocopper compound. Phenylcopper is prepared by reaction of phenyllithium with copper(I) bromide in diethyl ether. Reaction with a second equivalent of R-Li to R-Cu then gives the lithium diorganocopper compound. Copper halides also react with Grignard reagents. The compound pentamesitylpentacopper is prepared from mesityl magnesium bromide and copper(I) chloride. Copper salts add to terminal alkynes to form copper acetylides The copper metallocene (η-cyclopentadienyl triethylphosphine) copper can be prepared by reaction of copper(II) oxide with cyclopentadiene and triethylphosphine in pentane at reflux.

Types of Reactions[edit]

Substitution reactions[edit]

Substitution reactions of cuprates R2CuLi to alkyl halides R'-X give the alkylcopper compound R-Cu, the coupling product R-R', and the lithium halide Li-X. The reaction mechanism is based on nucleophilic attack, namely oxidative addition of the alkyl halide to Cu(I) elevating it to a planar Cu(III) intermediate followed by reductive elimination. The nucleophilic attack is the rate-determining step. In the case for substitution of iodide, single electron transfer mechanism is proposed (see figure).

Organocopper nucleophilic substitution

Many electrophiles will work. The approximate order of reactivity, beginning with the most reactive, is as follows: acid chlorides[10] > aldehydes > tosylates ~ epoxides > iodides > bromides > chlorides > ketones > esters > nitriles >> alkenes

Coupling reactions[edit]

Oxidative coupling is the coupling of copper acetylides to conjugated alkynes in the Glaser coupling (for example in the synthesis of cyclooctadecanonaene) or to aryl halides in the Castro-Stephens Coupling

Reductive coupling is a coupling reaction of aryl halides with a stoichiometric equivalent of copper metal that occurs in the Ullmann reaction. In an example of a present-day cross-coupling reaction called decarboxylative coupling, a catalytic amount of Cu(I) displaces a carboxyl group forming the arylcopper (ArCu) intermediate. Simultaneously, a palladium catalyst converts an aryl bromide to the organopalladium intermediate (Ar'PdBr), and on transmetallation the biaryl is formed from ArPdAr'.[11][12]

Decarboxylative aryl-aryl coupling

Redox neutral coupling is the coupling of terminal alkynes with halo-alkynes with a copper(I) salt in the Cadiot-Chodkiewicz coupling. Thermal coupling of two organocopper compounds is also possible.

Conjugate addition[edit]

Conjugate additions to enones are done with organocuprates. Note that if a Grignard reagent (such as RMgBr) is used, the reaction with an enone would instead proceed through a 1,2-addition.[13] The 1,4-addition mechanism of cuprates to enones goes through the nucleophilic addition of the Cu(I) species at the beta-carbon of the alkene to form a Cu(III) intermediate, followed by reductive elimination of Cu(I).[14] In the original paper describing this reaction, methylmagnesium bromide is reacted with isophorone with and without 1 mole percent of added copper(I) chloride (see figure).[5]

Addition of methylmagnesium bromide to isophorone.[5]

Without added salt the main products are alcohol B (42%) from nucleophilic addition to the carbonyl group and diene C (48%) as its dehydration reaction product. With added salt the main product is 1,4-adduct A (82%) with some C (7%).

A 1,6-addition is also possible, for example in one step of the commercial-scale production of fulvestrant:[15]

Fulvestrant organic synthesis brazier 2010

Carbocupration[edit]

Carbocupration is a nucleophilic addition of organocopper reagents (R-Cu) to acetylene or terminal alkynes resulting in an alkenylcopper compound (RC=C-Cu).[16] It is a special case of carbometalation and also called the Normant reaction.[17]

Figure: Catalytic cycle for carbocupration Muller,.[18]

Carbocupration mechanism

Copper was used as a catalyst for almost a century until Palladium cross coupling reaction was discovered. Palladium offered faster more selective reaction in comparison. Although in the recent years copper compounds have appeared again as a synthetically useful due too many of its advantages as an eco-friendly metal, its low cost, and its versatility of Carbon-Carbon and Carbon-heteroatom bonds formation.[19]

Synthetic applications[edit]

Ullman chemistry (1974)[edit]

Ullman used Goldeberg design synthesis to develop copper based reactions that enabled the formation of C-C, C-N and C-S bonds.[20] Ullmann chemistry is based on the formation of a carbon-carbon bond via condensation of aryl halides in the presence of a copper compound. This type of reaction has been useful for ring closures, aryl bond formation, synthesis of symmetrical and unsymmetrical biaryl compounds, synthesis of oligophenylenes and so on.

There are two types of Ullman reaction: Classic (Copper catalyzed synthesis of symmetric biaryl compounds) and Ullman type (copper catalyzed nucleophilic aromatic substitution). Electronegative groups in the ortho position of the aryl halogen are known to be strongly activated toward Ullman reaction.[21] On the other hand, this reaction is inhibited by the steric hindrance provided by bulky groups on ortho positions of the aryl groups.

Ullman condensation

Ullman condensation has been used to afford linear polyphenilene compounds as shown in the image below.

Ullman cond

Asymmetric Ullman reaction [22]

Ullman synthesis of symmetrical biaryl compounds has been varied to obtain asymmetric reaction conditions.[22] Nelson and collaborators worked on the synthesis of asymmetric biaryl compounds and obtained the thermodinamically controlled product.

Ullman Assymetric reaction

The diasterometric rate of the product is enhanced with bulkier R groups in the Auxiliary oxazoline group. Tert-butyl group has higher level of selectivity in the Ullman Coupling. Using an oxazoline auxiliary group provides steric effect which influences the high diasteroselectivity of asymmetric Ullman reaction.

Selected applications of Ullman chemistry [21]

Synthesis of biphenyls

Biphenyls had been obtained before with reasonable yields using 2, 2 diiodobiphenyl or 2, 2 diiodobiphenylonium ion as starting material; the reaction proceed when heating with Cu0 or Cu (I) 2O.

Ullman Byphenyl Synthesis

Ring closing reactions

5- Memebered ring closures are reported to be favorable more facile, but larger rings have also been made.

Ring close sonogashira

Copper catalyzed Cross coupling reactions

Evidence suggests that copper has a similar mechanism to that of palladium in cross coupling reactions (see figure bellow). Copper in contrast to palladium has accessible oxidations states from 0 to +3, while palladium only has two stable oxidation states. Another difference between copper and palladium is that since copper can take part in single electron transfer processes, an alternative free radical process should be taken into consideration(1) when using this metal as a catalyst.


General scheme of copper catalyzed cross coupling reaction


Copper cross coupling proposed mechanism

Although it is known that Copper based cross reaction mechanisms do not proceed via a general mechanism like palladium cross coupling reactions. The mechanisms by which cooper based cross coupling reactions proceed depend on the type of reaction, substrate, and solvent.[23]

Types of reactions.[23]


1. Thermal dimerization:

It Proceeds via a copper hydride mechanism with complete retention of organocopper configuration.

Copper hydride based mechanism [23]

Copper hydride proposed mechanism

2. Oxidative dimerization:

Proceeds via oxidation of dialkylcuprate I to neutral transient diakyl copper (II) which decomposes to give the desired akyl alkyl dimer. This reaction is first order kinetics for both the organocopper and the substrate with inversion as the stereochemical consequence.

SN2 like mechanism for oxidative dimerization and direct displacement

Oxid dimer

3. Direct displacement

Reactions with alkyl halides and organocopper compound are an example of direct displacement mechanism, similar to an SN2 reaction, where inversion of the substrates configuration occurs, as in oxidative dimerization. On the other hand, reactions of organocopper compound with alkenyl halides proceed with retention of subtrate’s configuration; two possibe mechanisms have been considered[23].In the case of organocopper coupling with aryl halides the reaction proceed via aromatic nucleophilic substitution reaction. These reactions are reported to occur with lithium diaryl cuprates with a transmetalation step that forms mixed homocuprate compounds. The composition of this mixture can be generally determined statistically from the amount of substrate that is present before oxidation.

Organocopper and aryl halides coupling general mechanism

Ullman aryl halide

Cross coupling and Transmetallation reactions[edit]

Sonogashira Cross coupling reaction

The Sonogashira cross-coupling reaction utilizes copper as a [co-catalyst], and palladium as the main catalyst. Its main synthetically application is toward the coupling of aryl and/or vinyl halides with terminal alkynes. The Sonogashira reaction has provided efficient routes to synthesize cyclic alkynes, which are used for a variety of applications including the well know click chemistry.[24]

'Synthesis of 2-amino-1-(2-propynyl)pyridinium bromide'.[24] One of many applications of the Sonogashira cross coupling reactions is in the synthesis of imidazopyridine derivatives which have a diverse range of biological activities.

Synthesis of imidazopyridine derivatives

Imidazopyridine


A general mechanism for the Pd-Cu transmetallation on the Sonogashira cross coupling reaction is shown below. Sonogashira reactions have been modify to use other catalysts and co-catalyst metals, e.g. Palladium free Sonogashira reactions have been published using copper as the main catalyst,[25] although it is well know that palladium impurities can accelerates Sonogashira reaction rates.[26]

Proposed Sonogashira Copper- Palladium synergistic coupling of acetylene and aryl halides

Pc-cu sonogash

It is unknown which reaction cycle proceeds first in this cross coupling mechanism, therefore we are uncertain on which of the cycle produce the coupled product. Sonogashira Pd-free reactions had already been developed; these reactions are economically favorable since the expensive metal is removed. Although it was demonstrated that this cross coupling reactions is very sensitive to Pd and even ppb can make a huge difference in the reaction rate. Pd act as an engine for the Sonogashira reaction while copper acts as fuel by coordination to acetylide.[26]

Palladium free, copper catalyzed Sonogashira reaction: Proposed mechanism [25]

Pd-free Sonogashira

The use of CuI and a carboxamide ligand readily improves the efficiency of copper catalyze (Pd-Free) reaction, and it inhibited the cyclization by- products and thus provides regioselectivity.

Chan Lam coupling Reaction

This reaction enables the formation of aryl carbon- hetoroatom bond. Chan Lam reaction is an oxidative coupling of boronic acids, stannanes or siloxanes with NH or OH containing compounds, the reaction is catalyzed by stoichiometric amounts of copper (II). It has been use as a milder approach, compared to Baeyer-Villiger oxidation, to afford direct esterification of carboxylic acids.[27] The reaction's stereochemistry is controlled by electronic substituents at the aryl group; electron donating groups on the aryl enhance the yield of the reaction.

Cu(OTf)2-mediated Chan-Lam reaction of carboxylic acids

Cham Lam

Chan Lam reaction has also been useful for selective transformations e.g. s- arylation [28]

Copper based reduction reactions[edit]

Phosphine Copper hydrides Copper hydrides are generally considered mild reducing agent which meaning that they can afford better selectivity in reaction.[18] Copper hydrides are used generally in organic synthesis as mild reducing agents. Their use became a hot area of study after Stryker reagent report on 1988 where he described the use of his reagents to selectively reduce the β position on α,β - unsaturated carbonyl derivatives.[18]

Stryker reagent The copper hydride compound [(PPH3)CuH]6 is known as the Stryker reagent. The Stryker reagent is used in organic reactions as a source of hydride ions and is generally used in conjugate addition reactions. Stryker’s reagent provides remarkable regioselectivity favoring the formation of 1, 4- addition products when reacted with α,β- unsaturated carbonyl compounds. .[29]

Stryker

Buchwald Cooper Reaction The Buckwald reaction is a Copper catalyzed assymetric reduction of activated alkenes using bidentate ligands such as (S)-T- BINAP,.[18][30]

Catalytic conjugate reduction of α,β unsaturated esters using Buckwald reagent

Buckwald selective- Cooper based chemistry

Other copper mediated reductions

Synthesis of Z- flourolefins

More Specific applications of copper based catalyst leads to the stereoselective synthesis of Z-flourolefin derivatives. Synthesis of Z-Flouro alkene dipeptide isosteres,.[31][32] Other effort to make this a more selective reactions inclodes the use of oxidation reduction condition for the reaction.[33]Fluoride acts as a leaving group and it enhances regioselectivity in the tranformation the Z- Fluoroalkene.

Z flouroalkene

Cu alkylation reaction[edit]

γ- Alkylation of allylic alcohols.[34]

Generally, the alkylation reaction of organocopper reagents proceed via gamma- alkylation. Cis- gamma attack occurs better in cyclohexyl carbamate due to sterics.

Alkylation of allylic alcohols

Alkylation of amines using the Gilman reagent Yamamoto and coworkers described an efficient synthetic method for the alkylation of amines. The reaction is based on the oxidative coupling of lithium alkyl copper amide which is reported to form in situ during the reaction between lithium dialkylcuprates and primary or secondary amides.[35]

Amine alkylation reaction

Cooper based Amine alkylation

The reaction is reported to be favorable in ethereal solvents. This method was proved to be very effective for the oxidative coupling of amines and alkyl, including tertbutyl, and aryl halides.[35]

Vicinal functionalization reactions[edit]

Vicinal functionalization using a Carbocupration- Mukaiyama aldol reaction sequence [36]

Vicinal funct

Muller and collaborators reported a vicinal funtionalization of α,β- acetylenic esters using a Carbocupration/ Mukaiyama aldol reaction sequence (as shown in fig above) carbocupration favors the formation of the Z- aldol.

See also[edit]

  • Chemistries of carbon with other elements of the periodic table:
CH He
CLi CBe CB CC CN CO CF Ne
CNa CMg CAl CSi CP CS CCl CAr
CK CCa CSc CTi CV CCr CMn CFe CCo CNi CCu CZn CGa CGe CAs CSe CBr CKr
CRb CSr CY CZr CNb CMo CTc CRu CRh CPd CAg CCd CIn CSn CSb CTe CI CXe
CCs CBa CHf CTa CW CRe COs CIr CPt CAu CHg CTl CPb CBi CPo CAt Rn
Fr CRa Rf Db Sg Bh Hs Mt Ds Rg Cn Uut Fl Uup Lv Uus Uuo
CLa CCe CPr CNd CPm CSm CEu CGd CTb CDy CHo CEr CTm CYb CLu
Ac CTh CPa CU CNp CPu CAm CCm CBk CCf CEs Fm Md No Lr
Chemical bonds to carbon
Core organic chemistry Many uses in chemistry
Academic research, but no widespread use Bond unknown

References[edit]

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