π-Complexes from acyl cyanides and lithium dimethylcuprate(i)

Abstract

Rapid injection of pyruvonitrile or benzoyl cyanide into solutions of Me2CuLi in THF-d8 at -100 °C gave complexes that were stable at this temperature. 1D NMR with multiply labelled substrates ((13)C/(15)N) and 2D NMR ((1)H/(13)C) identified them as the first cuprate-carbonyl π-complexes.

The X‐ray Crystal Structure of a Cuprate–Carbonyl π‐Complex

“…Therefore, we applied our rapid‐injection techniques, which were previously used to prepare several important intermediates,5–8 to screen a number of typical aldehydes and ketones for complex formation with Me 2 CuLi,9 and we discovered such a species with unusual stability, which has allowed us to prepare it on a larger scale and grow high‐quality crystals (see the Supporting Information). We now report the first X‐ray crystal structure10 of a cuprate–carbonyl π‐complex, 1 (Scheme and Figure 1).…”

The X‐ray Crystal Structure of a Cuprate–Carbonyl π‐Complex

“…The most significant feature of 1 is the doubly complexed carbonyl group involving the relatively rare side-on pbonding to the copper atom, as well as the more common end-on s-bonding to the lithium atom via a lone pair. [11] The Me a -Cu-Me b angle in 1 (C1-Cu-C2 1048) is considerably distorted from the linear arrangement in [Li( [12]crown-4) 2 ] + [MeCuMe] À (C-Cu-C 1808, Cu À C 1.935 (8) ). [12] The Me a À Cu bond is significantly longer than the Me b À Cu bond (see Figure 1 caption).…”

“…Therefore, dialkyl cuprate complexes of double-bonds can be described as pseudo square-planar, which was originally proposed on the basis of NMR spectroscopy. [7,8] Also noteworthy is the approximately tetrahedral lithium atom, bonded to the cuprated carbonyl group as well as three tetrahydrofuran (THF) ligands. The former lithium-oxygen bond length is significantly less than the latter bond lengths (see Figure 1 caption).…”

“…Trans coupling across copper is typically larger than cis. [5][6][7][8][9] A heteronuclear multiple quantum coherence plot (HMQC, Figure 3, right) has a cross-peak (lower right quadrant) between the upfield methyl hydrogen atoms (d = À1.54 ppm) and the downfield methyl carbon (d = 6.04 ppm), which are assigned to Me b . It is highly unusual for the 1 H NMR shift of Me b to be the upfield signal.…”

“…It is highly unusual for the 1 H NMR shift of Me b to be the upfield signal. [8,9] A possible explanation for the anomalous upfield shift of Me b is shielding by an aromatic ring current, owing to the position of this methyl group vis-a-vis the central 5-membered ring (Figure 1). This aromaticity requires a cyclopentadienyl anion, which in turn requires the transfer of electron density from the dimethylcuprate moiety to the cyclopentadiene substructure, and it implies a significant degree of Cu III character, as proposed by Nakamura et al [1] Complex 1 was stable indefinitely at À100 8C, but decomposed at a significant rate upon warming to À10 8C (t 1/2 = 30 min) to afford a 9:1 mixture of 1,2-adduct and ethane, the two possible reductive elimination products.…”