Dialkylcuprates

Organometallic compounds of copper were known for a long time before their versatil ity in synthetic organic chemistry was fully appreciated The most useful ones are the lithium dialkylcuprates which result when a copper(I) halide reacts with two equivalents of an alkyllithium in diethyl ether or tetrahydrofuran... 

The second molar equivalent of the alkyllithium adds to the alkylcopper to give a neg atively charged dialkyl substituted derivative of copper(I) called a dialkylcuprate It is formed as its lithium salt a lithium dialkylcuprate... 

Lithium dialkylcuprates react with alkyl halides to produce alkanes by carbon-carbon bond formation between the alkyl group of the alkyl halide and the alkyl group of the dialkylcuprate... 

Lithium diarylcuprates are prepared m the same way as lithium dialkylcuprates and undergo comparable reactions with primary alkyl halides... 

The most frequently used organocuprates are those m which the alkyl group is pri mary Steric hindrance makes secondary and tertiary dialkylcuprates less reactive and they tend to decompose before they react with the alkyl halide The reaction of cuprate reagents with alkyl halides follows the usual 8 2 order CH3 > primary > secondary > tertiary and I > Br > Cl > F p Toluenesulfonates are somewhat more reactive than halides Because the alkyl halide and dialkylcuprate reagent should both be primary m order to produce satisfactory yields of coupled products the reaction is limited to the formation of RCH2—CH2R and RCH2—CH3 bonds m alkanes... 

A key step in the reaction mechanism appears to be nucleophilic attack on the alkyl halide by the negatively charged copper atom but the details of the mechanism are not well understood Indeed there is probably more than one mechanism by which cuprates react with organic halogen compounds Vinyl halides and aryl halides are known to be very unreactive toward nucleophilic attack yet react with lithium dialkylcuprates... 

Preparation of alkanes using lithium di alkylcuprates (Section 14 11) Two alkyl groups may be coupled together to form an alkane by the reaction of an alkyl hal ide with a lithium dialkylcuprate Both alkyl groups must be primary (or meth yl) Aryl and vinyl halides may be used in place of alkyl halides... 

The preparation and some synthetic applications of lithium dialkylcuprates were described earlier (Section 14 11) The most prominent feature of these reagents is then-capacity to undergo conjugate addition to a p unsaturated aldehydes and ketones... 

Mentally disconnect one of the bonds to the p carbon so as to identify the group that comes from the lithium dialkylcuprate... 

The principal synthetic application of lithium dialkylcuprate reagents IS their reaction with a 3 unsatu rated carbonyl compounds Al kylation of the 3 carbon occurs... 

This procedure illustrates a new method for the preparation of 6-alkyl-a,g-unsaturated esters by coupling lithium dialkylcuprates with enol phosphates of g-keto esters. The procedure for the preparation of methyl 2-oxocyclohexanecarboxylate described in Part A Is based on one reported by Ruest, Blouin, and Deslongcharaps. Methyl 2-methyl-l-cyc1ohexene-l-carboxylate has been prepared by esterification of the corresponding acid with dlazomethane - and by reaction of methyl 2-chloro-l-cyclohexene-l-carboxyl ate with lithium dimethylcuprate. -... 

The formation of g-alkyl-a,g-unsaturated esters by reaction of lithium dialkylcuprates or Grignard reagents in the presence of copper(I) iodide, with g-phenylthio-, > g-acetoxy-g-chloro-, and g-phosphoryloxy-a,g-unsaturated esters has been reported. The principal advantage of the enol phosphate method is the ease and efficiency with which these compounds may be prepared from g-keto esters. A wide variety of cyclic and acyclic g-alkyl-a,g-unsaturated esters has been synthesized from the corresponding g-keto esters. However, the method is limited to primary dialkylcuprates. Acyclic g-keto esters afford (Zl-enol phosphates which undergo stereoselective substitution with lithium dialkylcuprates with predominant retention of stereochemistry (usually > 85-98i )). It is essential that the cuprate coupling reaction of the acyclic enol phosphates be carried out at lower temperatures (-47 to -9a°C) to achieve high stereoselectivity. When combined with they-... 

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