Structural characterization of organocopper reagents.
Huang, Hui
1997
Abstract
Organocopper reagents are widely used in organic synthesis. They are derived from transmetallation of cuprous salts with organolithiums, Grignard reagents or other metal organyls. The solution structures of organocopper reagents, although extremely relevant to an understanding of their reactivity, are mainly unknown. This dissertation is focused on characterization of the structure around copper in the tetrahydrofuran (THF) solutions of organocopper reagents, using infrared (IR) spectroscopy and x-ray absorption spectroscopy (including EXAFS, extended x-ray absorption fine structure, and XANES, x-ray absorption near edge structure). The structures of the organocopper reagents are dependent on the identity of the precursors. Cuprous cyanide (CuCN) can be converted to (MeCuCN) $\rm\sp- Li\sp+$ or (CuMe$\rm\sb2\rbrack\sp-$ CNLi$\sb2\sp+$ by adding one or two equivalents of MeLi. Low temperature (${-}78\sp\circ$C) IR spectra show an intense cyanide stretching absorption at $\rm\nu\sb{CN}$ = 2133 cm$\sp{-1}$ for the former and a weak cyanide peak at $\rm\nu\sb{CN}$ = 2115 cm$\sp{-1}$ for the latter. An IR detected titration shows that the formation constant for (CuMe$\sb2\rm\rbrack\sp- CNLi\sb2\sp+$ is $7.8\times 10\sp3\ {\rm M}\sp{-1}$. In contrast, no matter how many equivalents of MeMgCl are added to CuCN, only one species is formed, with $\rm\nu\sb{CN}$ = 2144 cm$\sp{-1}$ and an intensity similar to that for (MeCuCN) $\sp- {\rm Li}\sp+$. EXAFS data show that this species has a cyanide bridged Cu/Mg structure best represented by (Me-Cu-CN-Mg) $\sp+$. A mechanism is proposed for the $\rm S\sb{N}2\sp\prime$ regiospecificity of CuCN catalyzed reaction between Grignard reagent and allylic derivatives based on this structure. In samples prepared from CuI, the linear, two-coordinate dimethylcuprate( (CuMe$\rm\sb2\rbrack\sp- Li\sp+)$ is always formed, regardless of whether MeLi or MeMgCl is used. Organocopper reagents derived from phenyllithium have always been considered complicated due to the fact that a number of different crystal structures are reported with various phenyl:Cu ratios. A XANES detected titration shows that only two phenyl-copper species, with phenyl:Cu ratios of 1.2 and 2.0, exist in significant concentration in solutions with phenyl:Cu ratios ranging from 0 to 3. EXAFS data show that the species with a phenyl:Cu ratio of 1.2 has a structure consistent with previously reported ($\rm Cu\sb5Ph\sb6\rbrack\sp-$ anion, while the species with a phenyl:Cu ratio of 2.0 is a diphenylcuprate, with copper bound to two phenyls, CuPh$\sb2\sp-$. The structures of organocopper compounds are also dependent on solvent. The aggregation state of dimethylcuprate derived from cuprous halide and two equivalents of MeLi is slightly different in THF and diethyl ether, although the first coordination shell of the coppers is identical. Little solvent dependence is observed for the samples prepared from CuCN. The role of halides in organocopper chemistry is two-fold. Firstly, addition of lithium halide to cuprous salts can provide an extra ligand that is necessary for solubilization of the copper. Secondly, halide can improve the reactivity of monoalkylcopper reagent by acting as an auxiliary ligand to form a heteroleptic alkylcuprate, RCuX$\sp-$. The coppers are three-coordinate in reagents derived from alkyl aluminum and CuCN. This copper coordination is significantly different from that found in the traditional organocopper reagents derived from organolithium and Grignard reagents, where the copper is two-coordinate.Subjects
Characterization Cyanocuprate Exafs Organocopper Reagents Structural Tetrahydrofuran
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