Structure and Bonding in Neutral and Cationic 14‐Electron Gold Alkyne π Complexes

Abstract

Cyclododecyne (5) as a prototype unstrained alkyne was coordinated to either the neutral [AuCl] fragment or to two different cationic [Au(NHC)](+) entities (NHC = N-heterocyclic carbene), and the resulting complexes 6, 8, and 10 were characterized by X-ray crystallography and NMR spectroscopy. Since the structure of cyclododecyne in the solid state could also be obtained after in situ crystallization, a comparison was possible that provides insights into structural changes imposed on the alkyne by the different gold fragments. These data are interpreted on the basis of a DFT analysis of the bonding situation in the individual compounds, which provides insights into the very first elementary step common to many gold-catalyzed transformations.

On the Dewar–Chatt–Duncanson Model for Catalytic Gold(I) Complexes

“…Closely related complexes have been experimentally characterized by Wu et al [34] and Flügge et al [35] The computed energy for the dissociation of com- , in line with those of previous DFT calculations using different exchange correlation functionals [34,35,49] and with those computed for other complexes of AuCl with simple alkynes (37.5 kcal mol À1 for cyclododecyne [35] and 3-hexyne [34] ). For the cationic complex 2 we obtained a dissociation energy of 36.9 kcal mol À1 .…”

“…Only when the presence of ancillary ligands (F À , bipyridines) was studied by natural bond order (NBO) analysis of orbital populations was a significantly larger or even dominating p back-donation found. [25,31] However, the most recent investigations carried out on realistic Au Ialkene [33] and Au I -alkyne [26,[34][35][36] catalytic intermediates using NBO orbital interaction energies all support the view of a dominating donation component.…”

“…Herein we report new theoretical results on the nature of the Au I -unsaturated C À C bond that clarify these issues as well as the significance and limits of the DCD model for Au I and other coinage-metal complexes. We have investigated a number of realistic catalytic intermediates, similar to some recently synthesized, [34,35] resulting from coordination of ethyne to AuCl and Au-nhc + (nhc = 2,3-dihydroimidazol-2-ylidene). The analogous Ag I and Cu I complexes have also been studied for comparison, as well as those of Au I with ethene.…”

“…We note in passing that our computed structural data are consistent with those of previous theoretical calculations on some related systems and with reported Xray crystallography data. [34,35] Returning now to the changes in density in Figure 1, we finally note that alkyne coordination also induces a significant perturbation of the system on the side of the ligand L, with an evident polarization of the AuÀL bond, of the chlorine atom and of the carbenic carbon atom directly bound to the gold. The far side of the nhc ligand seems by contrast only marginally affected.…”

Concise Total Synthesis of Enigmazole A

“…[12] We conceived that the most likely explanation [28] for this striking difference is the fact that the possible p-complexes [29] formed upon initial coordination of the catalyst to the alkyne in 33 are diastereomeric.I fo nly one …”