Lithium Dimethylcuprate1

Me2CuLi

[15681-48-8]  · C2H6CuLi  · Lithium Dimethylcuprate  · (MW 100.57)

(methylating reagent; undergoes conjugate addition reactions,1a,b,e,f 1,2-addition reactions,2 substitution reactions with alkyl, vinyl, and allyl substrates,1a,c,e,f carbocupration of alkynes,1a and reduction of carbon-heteroatom bonds3)

Physical Data: colorless solution in THF or Et2O; 1H NMR (Et2O) d -1.46 to -1.55 ppm (at 25 to -70 °C);4,5 7Li NMR d 0.75 to -0.38 ppm (at 25 to -70 °C for Me2CuLi) and d 0.495 to -0.38 ppm (at 25 to -70 °C for Me2CuLi + LiI);5 X-ray scattering measurements;4 molecular association data.4,6

Solubility: sol THF, Et2O

Preparative Methods: prepared in situ from CuI salts (Copper(I) Iodide, Copper(I) Bromide, Copper(I) Trifluoromethanesulfonate, Copper(I) Chloride) and Methyllithium under N2 or argon.1a Impurities present in CuI or CuBr can promote decomposition of the reagent. Pure, light tan CuI is obtained by dissolving crude CuI in boiling NaI(aq) followed by cooling, precipitation of CuI by addition of H2O, filtration, sequentially washing the solid with H2O, EtOH, EtOAc, Et2O, and pentane, and then drying in vacuo for 24 h.7 CuBr can be purified in a similar manner8 with NaBr (aq) or by dissolving in 48% HBr, precipitating with H2O, sequentially washing the solid with H2O, EtOH, and Et2O, and drying in vacuo.9 CuI has also been purified by continuous extraction with THF in a Soxhlet extractor (12 h) followed by drying in vacuo.10 Me2CuLi free of LiI can be prepared from isolated Methylcopper,8 and the reagent can also be prepared in the presence of complexing agents from CuI.SBu2,8 Copper(I) Iodide-Tributylphosphine,8 or CuBr.SMe2.11 Polymer-supported Me2CuLi is also available.12

Handling, Storage, and Precautions: air- and moisture-sensitive; handle under inert atmosphere in a fume hood.

Organocuprate Reagents.

Organocuprate reactivity and thermal stability are functions of CuI precursor,1a,11b,13 solvent,1,14 temperature,1b cuprate composition,5,11b cation (e.g. Li,1a Mg,1a Zn,15a Mn15b), and alkyl1b ligand. Experimental evidence for ligand and metal lability has been interpreted in terms of a dynamic equilibrium between Me2CuLi, MeLi, and Me3Cu2Li.5 Me2CuLi is formulated as a dimer, and since cuprate reactivity is a function of composition (e.g. Me3Cu2Li, Me5Cu3Li216), stoichiometric considerations are important in preparing the reagent. For any given application the reactivity profile of Me2CuLi should be compared with a range of mixed homocuprates (e.g. MeCuC&tbond;CRLi, Lithium Cyano(methyl)cuprate, and Me2CuCNLi2 or Me2CuCN.LiCN),17 mixed heteroatom alkylcuprates (i.e. amido,10,18 phosphido ,18 thioxy,10 and alkoxy10), and methylcopper in the presence of additives (e.g. TMEDA/TMSCl,19 TMSI,20 and TMSCl + LiI20).

Conjugate Addition Reactions.

Me2CuLi transfers the methyl ligand in a 1,4-fashion to a wide range of a,b-unsaturated substrates which include enones,1,21,22b enals,22 enoates,1,14d,23 ynones,24a ynoates,24b and a,b-unsaturated lactones,25 imides,26 and phosphonates.27 a,b-Alkenyl sulfoxides,28 sulfones,29 and phosphine oxides,30 a,b-allenyl phosphine oxides,31a sulfoxides, sulfones,31b and ketones,31c,d ketoketenimines,31e and a,b-alkynyl sulfones32 also undergo 1,4-addition of methyl upon reaction with Me2CuLi. These reactions proceed most readily in nonpolar solvents (Et2O, PhMe, CH2Cl2, pentane), since good donor solvents (e.g. THF, DME, HMPA) diminish cuprate reactivity toward conjugate addition.1,14 Less reactive substrates can often be activated toward conjugate addition by use of Lewis acid additives1 and by solvent modifications (eq 114d).14 Utilization of low temperatures and nonpolar solvents22 gives, with the more reactive enals,22 better ratios of 1,4-/1,2-addition products, and Me5Cu3Li216 is generally more effective than Me2CuLi (see Dilithium Pentamethyltricuprate).

Conformational (eq 2)33 and steric factors often control diastereoselectivity, although electronic34 effects have been observed and stereodivergent pathways (eq 3)35a can sometimes be achieved by use of Lewis acid additives. Stereocontrol in conjugate addition to g-heteroatom substituted a,b-unsaturated substrates is a function of heteroatom identity and substitution patterns (eq 4).35b

The conjugate addition reaction is compatible with alkyl bromide36 and nitroalkane (eq 5)37 functionality, although substitution of mesylates is competitive36 with conjugate addition. Substrates with extended conjugation generally afford products of 1,6- or 1,8-addition.38 Reaction of Me2CuLi with ynenoates gives nonconjugated allenyl esters upon careful protonation of the enolate (eq 6).38c Bis-activation (e.g. alkylidenemalonates (eq 4), perhydro-1,4-oxazepine-5,7-diones,39a malononitriles,39b a-keto sulfoxides,39c and 2-acyl- or 2-alkoxycarbonyl-4-chromones39d) of the alkene facilitates conjugate addition, allowing participation of normally unreactive functionality (e.g. a,b-unsaturated nitriles and 4-chromones).

Transition metal-alkene complexes undergo conjugate addition (eq 7),40a and Me2CuLi and MeLi can effect different40b regioselectivities. Reaction at metal carbonyls can be a side reaction, and the efficiency of cuprate conjugate addition is dependent upon the ligands attached to the metal.40b

The enolate resulting from conjugate addition can be trapped with a variety of electrophiles (e.g. alkyl halides, halosilanes (eq 5), acid chlorides, formaldehyde, sulfenyl halides, selenenyl halides, aldehydes, Mannich salts, triflating agents, CO2, and chlorophosphates), although solvent modifications and use of additives (e.g. HMPA, Zinc Chloride) are sometimes required.41 The wide range and variability of reaction conditions required for successful tandem conjugate addition-enolate trapping reflect the complexity of these reaction mixtures and the generally diminished reactivity of the enolate, although enolates generated by conjugate addition of Me2CuLi to g-methylenebutenolides42a and simple butenolides42b participate in conjugate addition reactions with the starting lactone. Although acylation of these enolates is often problematic,43a O- vs. C-acylation can be controlled by use of Methyl Cyanoformate43b in either Et2O or THF (eq 8). A powerful application of cuprate 1,4-addition lies in the generation of regiospecific enolates which can undergo subsequent chemistry such as Dieckmann cyclization (eq 9)23a or Claisen rearrangement (eq 10).44

a,b-Unsaturated substrates containing a good leaving group (e.g. halide, alkoxycarbonyl, sulfonate esters, phosphonate esters, alkyl- or arylthioxy, alkoxy) on the b-carbon atom undergo substitution reactions,45 and tandem addition-elimination-addition pathways46 can be designed (eq 11)46b for substrates with an a-CH2L substituent; Chlorotrimethylsilane is necessary to prevent elimination of the nitro group in the reaction of Me2CuLi with an a-nitromethylenone37 (eq 5).

Several scalemic substrates26,37,39a,47 (eqs 11 and 1246b and 47c) afford good to excellent asymmetric induction in the conjugate addition reaction, while more modest ee's have been achieved with chiral coordinating47a ligands.

Although simple cyclopropyl ketones do not react with Me2CuLi, 1,1-bis-activated cyclopropanes (e.g. cyclopropylmalonates,48a b-keto esters,48a b-keto phosphonates48b) undergo a homoconjugate addition reaction (eq 13),48a and methylenecyclopropyl ketones48c participate in 1,5-additions.

Carbonyl Additions.

1,2-Addition of Me2CuLi to ketones is generally limited to alkyl aryl49a and diaryl49b ketones, although an a,a-dialkoxy ketone (eq 14)49c gave good yields of the 1,2-addition product. Although enals preferentially react in a 1,4-fashion, 1,2-addition is often a serious side reaction and is favored by addition of TMSCl (which also facilitates 1,2-addition to ketones2); this additive also increases Cram stereoselectivity.2,50

Carbocupration.

Carbocupration of alkynes with Me2CuLi appears limited to propynal diethyl acetal;51a the reaction fails with a higher homolog. Carbocuprations of 1-methoxyallene51a and of cyclopropene derivatives (eq 15)51b have been reported.

Substitution Reactions.

Me2CuLi participates in SN2-type substitution reactions1a,c,e with alkyl halides8,52 (e.g. a-halo ketones52a and b-alkoxy-b-OLi intermediates52b), sulfonate esters,53 oxiranes (epoxides),54 tosylated aziridines,55a oxetanes,55b cyclic sulfinates,56a trifluoromethyl sulfonimides,56b b-lactones,57 and with vinyl halides,58a triflates,58b,c and selenones.58d 1,1-Dibromocyclopropanes59 can undergo a double substitution reaction with Me2CuLi. Proximate heteroatoms facilitate the reaction of secondary tosylates.53b Alkyl substrates generally react with inversion of configuration, while substitution on alkenyl substrates (eq 16)58a proceeds with retention of configuration. Although Me2CuLi reacts with iodobenzene, it is unreactive towards aryl triflates and bromides and thus the reaction8 is unsatisfactory for effecting aryl substitution.

Regioselective cleavage of oxiranes is a powerful synthetic method.1a,c,e Me2CuLi cleaves epoxides at the least substituted center; 1,2-disubstituted epoxides afford mixtures of regioisomers. Regiocontrol is possible in 2,3-epoxy acids,54a esters,54b,c and 4-alkyl-substituted 2,3-epoxy alcohols,54d and regiocontrol is enhanced by solvent effects with 2,3-epoxy alcohols54e lacking a C-4 substituent.

Allylic substrates react with (SN2) or without (SN2) rearrangement via a p-allyl copper complex, with the anti SN2 pathway predominating. Regio- and stereochemistry is generally governed by steric factors, although stereoelectronic effects may also operate.1a Allylic carbamates display high syn SN2 stereo- and regioselectivity (eq 1760a).60 Propargyl substrates uniformly give SN2 substitution.1a Vinyloxetanes,61 vinyloxiranes (eq 18),62a and allylic sulfoxides and sulfones62b undergo SN2 substitutions, and the vinyloxiranes react with good diastereoselectivity.

Reaction of Me2CuLi with acyl halides1c or thiol esters63 results in nucleophilic acyl substitution, providing a useful route to ketones. Substitution reactions also occur between Me2CuLi and sulfonium salts,64 and via benzyne65 intermediates.

Miscellaneous Reactions.

The reagent adds to 1-acylpyridinium salts66 and to transition metal carbene complexes.67 Me2CuLi can reduce a-halo3a and a-acetoxy ketones3a and a,b-epoxy3b ketones. Reduction of a,b-epoxy sulfoxides affords enolates.68 Me2CuLi reacts with a,a-dibromo ketones to afford a-methyl ketones via a cyclopropanone intermediate (eq 19).36a,68 Reaction of Me2CuLi with a-nucleofuge a,b-enones leads to mixtures of reduction and substitution products.70 Chiral allenes have been prepared by reaction of Me2CuLi with allylic sulfinyl mesylates.71

Related Reagents.

Lithium Diallylcuprate; Lithium Di-n-butylcuprate; Lithium Diethylcuprate; Lithium Diethylcuprate-Tributylphosphine; Lithium Dimethylcuprate-Boron Trifluoride; Lithium Diphenylcuprate; Lithium Di-n-propylcuprate; Lithium Di-p-tolylcuprate; Lithium Divinylcuprate; Lithium Divinylcuprate-Tributylphosphine.


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R. Karl Dieter

Clemson University, SC, USA



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