Donald H. Aue

Experimental and Computational Evidence for Gold Vinylidenes: Generation from Terminal Alkynes via a Bifurcation Pathway and Facile C-H Insertions

Facile cycloisomerization of (2-ethynylphenyl)alkynes is proposed to be promoted synergistically by two molecules of BrettPhosAuNTf(2), affording tricyclic indenes in mostly good yields. A gold vinylidene is most likely generated as one of the reaction intermediates on the basis of both mechanistic studies and theoretical calculations. Different from the well-known Rh, Ru, and W counterparts, this novel gold species is highly reactive and undergoes facile intramolecular C(sp(3))-H insertions as well as O-H and N-H insertions. The formation step for the gold vinylidene is predicted theoretically to be complex with a bifurcated reaction pathway. A pyridine N-oxide acts as a weak base to facilitate the formation of an alkynylgold intermediate, and the bulky BrettPhos ligand in the gold catalyst likely plays a role in sterically steering the reaction toward formation of the gold vinylidene.

[3,3]-Sigmatropic Rearrangement versus Carbene Formation in Gold-Catalyzed Transformations of Alkynyl Aryl Sulfoxides: Mechanistic Studies and Expanded Reaction Scope

Gold-catalyzed intramolecular oxidation of terminal alkynes with an arenesulfinyl group as the tethered oxidant is a reaction of high impact in gold chemistry, as it introduced to the field the highly valued concept of gold carbene generation via alkyne oxidation. The proposed intermediacy of α-oxo gold carbenes in these reactions, however, has never been substantiated. Detailed experimental studies suggest that the involvement of such reactive intermediates in the formation of dihydrobenzothiepinones is highly unlikely. Instead, a [3,3]-sigmatropic rearrangement of the initial cyclization intermediate offers a reaction path that can readily explain the high reaction efficiency and the lack of sulfonium formation. With internal alkyne substrates, however, the generation of a gold carbene species becomes competitive with the [3,3]-sigmatropic rearrangement. This reactive intermediate, nevertheless, does not proceed to afford the Friedel-Crafts-type cyclization product. Extensive density functional theory studies support the mechanistic conclusion that the cyclized product is formed via an intramolecular [3,3]-sigmatropic rearrangement instead of the previously proposed Friedel-Crafts-type cyclization. With the new mechanistic insight, the product scope of this versatile formation of mid-sized sulfur-containing cycloalkenones has been expanded readily to various dihydrobenzothiocinones, a tetrahydrobenzocyclononenone, and even those without the entanglement of a fused benzene ring. Besides gold, Hg(OTf)2 can be an effective catalyst, thereby offering a cheap alternative for this intramolecular redox reaction.

Gold‐Catalyzed Cyclizations of cis‐Enediynes: Insights into the Nature of Gold–Aryne Interactions

Golden aryne? Gold aryne complexes are inferred as transition states in dual gold-catalyzed cyclizations of cis-enediynes (see scheme; DCE = 1,2-dichloroethane). They are better described as ortho-aurophenyl cations, which react with weak nucleophiles and undergo facile intramolecular insertions into C(sp(3))-H bonds. Indanes, fused heteroarenes, and phenol derivatives are readily prepared using this method.

Asymmetric Gold‐Catalyzed Lactonizations in Water at Room Temperature

Asymmetric gold-catalyzed hydrocarboxylations are reported that show broad substrate scope. The hydrophobic effect associated with in situ-formed aqueous nanomicelles gives good to excellent ees of product lactones. In-flask product isolation, along with the recycling of the catalyst and the reaction medium, are combined to arrive at an especially environmentally friendly process.

Ab initio calculated gas-phase basicities of polynuclear aromatic hydrocarbons

Abstract The gas-phase basicities (GBs) of polynuclear aromatic hydrocarbons (PAHs) have been calculated using the semiempirical Austin model 1 method, ab initio quantum mechanical methods at the HF/3-21G, HF/6-31G(d), MP2/6-31G(d), MP2/6-31+G(d,p) levels, and the B3LYP/6-311G(d,p) density functional method. GBs calculated at these levels of theory are compared to experimentally known GBs measured by equilibrium mass spectrometric methods. Theoretically calculated entropies for PAHs and their protonated carbocations are used to reevaluate original experimental equilibrium data producing a revised set of experimental GBs and proton affinities for PAHs and related hydrocarbons. Experimental GBs for 40 hydrocarbons are found to correlate well with AM1, Hartree–Fock, DFT and MP2 calculated GBs, with regression analyses showing standard errors of 2.12, 1.53, 1.36, and 1.55 kcal mol −1 for each of the four methods, respectively. The results permit an evaluation of the reliability of experimental data and suggest a need for new experimental work for some molecules. Predictions are made for GBs for 12 new PAHs whose GBs have not yet been measured.

On the measurement of gas-phase ion-molecule equilibrium constants in an ion cyclotron resonance spectrometer

Abstract The technique used in measuring ion-molecule equilibrium constants, in particular proton-transfer equilibrium constants, with a high pressure drift cell ion cyclotron resonance spectrometer is described. Experimental and kinetic problems which occur in such measurements and their effect on the determination of accurate equilibrium constants are discussed. Non-attainment of equilibrium may result from either slow proton-transfer rates or fast competitive side reactions. With proper experimental technique, errors in free energies caused by such kinetic problems may usually be kept below 0.2 kcal mol−1. A number of examples are given. While the analysis is applied to high pressure drift cell ICR, most of the constraints are applicable to equilibrium measurements with any type of apparatus.

An ab initio molecular orbital study on the Lewis acidity of TMS-Cl and TMS-CN toward an α,β-unsaturated aldehyde: Are these acid-base interactions important in organocuprate 1,4-additions to enones?

Abstract High-level ab initio calculations were used to determine the effectiveness of commonly used silanes as Lewis acids toward acrolein as a model for Gilman cuprate conjugate addition reactions. The data suggest that complexation with an enone is by a weak dipole-dipole interaction and is not directly responsible for the rate accelerating effect of TMS-X.

Reaction of 1-methylcyclopropene with ketens. A ready ene reaction and evidence for unstable enol and cyclopropanone intermediates

The addition of t-butylcyanoketen and bis(trifluoromethyl)keten to 1-methylcyclopropene gives mixtures of ene products and rearranged products (5) and (12) from competing stepwise pathways via dipolar ions (9) and (14) and a cyclopropanone (15).

Reaction of 3,3-dimethyl- and 1,3,3-trimethylcyclopropene with t-butylcyanoketen. Formation of bicyclo[2,1,0]pentan-2-ones

Addition of t-butylcyanoketen to 3,3-dimethylcyclopropene and 1,3,3-trimethylcyclopropene gives mixtures of the bicyclo[2,1,0]pentan-2-ones (3a,b) and rearranged products (4a,b) and (5b); the ketone (3b) rearranges to (6b) but not to (4b).

EvanPhos: a ligand for ppm level Pd-catalyzed Suzuki–Miyaura couplings in either organic solvent or water

A new biaryl phosphine-containing platform can be constructed in only two steps. It complexes Pd(OAc)2 forming a precursor of a very active catalyst useful for Suzuki–Miyaura cross-couplings of functionalized substrates. By a combination of pre-activation, used together with the uncommon solvent EtOAc, the resulting catalyst system is effective at loadings in the ppm (0.1–0.5 mol%) range with highly functionalized reaction partners. Similar reactions run in water containing nanomicelles are as fast or faster. The derived Pd-complexed pre-catalyst possesses extended bench stability. The resulting technology represents an attractive green synthetic advance in highly valued Suzuki–Miyaura couplings.