[39926-94-8] · C16H36Cl2CuN · Tetra-n-butylammonium Dichlorocuprate(I) · (MW 376.92)
Alternate Name: copper(I) chloride-tetra-n-butylammonium chloride.
Physical Data: mp 71-72 °C.
Solubility: sol haloalkanes, haloarenes, ketones, and other polar solvents; poorly sol alkanes.
Preparative Methods: in situ complexation of freshly prepared Copper(I) Chloride by tetrabutylammonium chloride in appropriate solvent (acetone, chloroform, etc.);2 pure material can best be prepared by reduction of Copper(II) Sulfate pentahydrate in aqueous solution with sodium sulfite or ascorbic acid in the presence of tetrabutylammonium hydrogen sulfate and sodium chloride, in the molar ratio 1:2:1:4. The resulting dichlorocuprate salt is extracted into CH2Cl2 and may be purified by recrystallization.3
Purification: the CH2Cl2 solution of the reagent may be dried, the solvent evaporated, and the residue recrystallized from ethyl acetate to yield colorless crystalline rhombic plates.
Handling, Storage, and Precautions: the solid cuprate salt is stable to short-term exposure to air, but should be used immediately or stored under an inert atmosphere.
The purified, solid copper(I) chloride-tetrabutylammonium chloride complex has been shown by X-ray crystallography to consist of monomeric, linear CuCl2- ions,4 and is thus best described as tetrabutylammonium dichlorocuprate(I). The reagent appears to function as a source of nucleophilic chloride soluble in common organic solvents. The exact role of the copper(I) is unclear, but the reactivity of the dichlorocuprate does not appear to be a simple extension of that of the chloride ion alone. The nucleophilic reactivity is strongly enhanced over that of tetrabutylammonium chloride alone in the same solvents,1 and it seems likely that the dichlorocuprate ion coordinates with the substrate during reaction.
The reagent provides a mild alternative to the mixtures of solid and dissolved copper(I) chloride in concentrated hydrochloric acid commonly used to effect rearrangement of propargyl chlorides to the isomeric allenyl chlorides and to effect halogen exchange in activated aryl halides. It is particularly useful where a strongly acidic reaction medium is undesirable, or in kinetic studies in which it is necessary to know the exact concentration of dichlorocuprate ion.
Use of this reagent promoted rearrangement of a propargyl chloride to the isomeric allene with a high degree of anti stereospecificity (eq 1).2
The purified dichlorocuprate reagent was used in kinetic studies of the halogen exchange reaction of 3-bromo-2-halonitrobenzenes (eq 2).1,5
Both chlorides of the dichlorocuprate are kinetically active, and the observed rate expression was first order in both substrate and chloride ion. Under the conditions of the kinetic study in chlorobenzene solution, the substitution was approximately 90% complete after 5 h, and >99% complete after 24 h reaction time. The 3-bromo-2-chloronitrobenzene product was not reactive with tetrabutylammonium dibromocuprate and tetrabutylammonium diiodocuprate; hence, the above reaction was not reversible. However, 2,3-dibromonitrobenzene and 2-iodo-3-bromonitrobenzene underwent reversible substitutions with the latter two dihalocuprates. Reaction rates for substitution of 2,3-dibromonitrobenzene by tetrabutylammonium dichlorocuprate are enhanced in aqueous DMSO.5
Oliver J. Muscio, Jr.
Murray State University, KY, USA