We report the use of cationic gold
complexes [Au(NHC)(CH3CN)][BF4] and [{Au(NHC)}2(μ–OH)][BF4] (NHC = N-heterocyclic
carbene) as highly active catalysts
in the solvent-free hydroalkoxylation of internal alkynes using primary
and secondary alcohols. Using this simple protocol, a broad range
of (Z)-vinyl ethers were obtained in excellent yields
and high stereoselectivities. The methodology allows for the use of
catalyst loadings as low as 200 ppm for the addition of primary alcohols
to internal alkynes (TON = 35 000, TOF = 2188 h–1).
Thei mproved synthesis of g-, d-a nd elactones using ad inuclear N-heterocyclic carbene (NHC)-gold(I) catalyst is reported. This solventfree process provides access to g-a nd d-lactones in high regio-and stereoselectivity.R eactions were performed at low catalyst loadings andw ithout the need for any additives.T he use of adigold pre-catalyst providesanew synthetic route to functionalised e-lactones,p oorly accessible using previous methodologies.
ABSTRACT:The role of counterions in chemistry mediated by gold complexes stretches much further than merely providing charge balance to cationic gold species. Interplay between their basicities and coordination strengths influences interactions with both the gold center and substrates in catalysis. Actual monogold(I) active species are generally believed to be mono-coordinated species, formed from the abstraction or the decoordination of a second ligand from precursor complexes, but only little experimental evidence exists to underpin the existence of these transient species. The formation of a bench-stable neutral IPr Cl -gold(I) tetrafluoroborate complex is herein reported. Experimental studies by X-ray diffraction analysis, NMR spectroscopy and theoretical studies by DFT calculations were conducted to determine the composition, structure, and behavior of this complex. The absence of an auxiliary ligand resulted in inner-sphere coordination of the counterion in the solid state. In solution, an equilibrium between two conformations was found with the counterion occupying inner-sphere and outer-sphere positions, respectively. Stoichiometric and catalytic reactivity studies with the tetrafluoroborate complex have been conducted. These confirmed the lability of the inner-sphere coordinating counterion that gives the IPr Cl -gold(I) fragment similar behavior to related systems.
A well-defined NHC–gold(i) complex has been shown highly effective in the formation of ethers from secondary benzylic alcohols and phenols under mild reaction conditions.
It has been experimentally
observed that water–ice-embedded
polycyclic aromatic hydrocarbons (PAHs) form radical cations when
exposed to vacuum UV irradiation, whereas ammonia-embedded PAHs lead
to the formation of radical anions. In this study, we explain this
phenomenon by investigating the fundamental electronic differences
between water and ammonia, the implications of these differences on
the PAH–water and PAH–ammonia interaction, and the possible
ionization pathways in these complexes using density functional theory
(DFT) computations. In the framework of the Kohn–Sham molecular
orbital (MO) theory, we show that the ionic state of the PAH photoproducts
results from the degree of occupied–occupied MO mixing between
the PAHs and the matrix molecules. When interacting with the PAH,
the lone pair-type highest occupied molecular orbital (HOMO) of water
has poor orbital overlap and is too low in energy to mix with the
filled π-orbitals of the PAH. As the lone-pair HOMO of ammonia
is significantly higher in energy and has better overlap with filled
π-orbitals of the PAH, the subsequent Pauli repulsion leads
to mixed MOs with both PAH and ammonia character. By time-dependent
DFT calculations, we demonstrate that the formation of mixed PAH–ammonia
MOs opens alternative charge-transfer excitation pathways as now electronic
density from ammonia can be transferred to unoccupied PAH levels,
yielding anionic PAHs. As this pathway is much less available for
water-embedded PAHs, charge transfer mainly occurs from localized
PAH MOs to mixed PAH–water virtual levels, leading to cationic
PAHs.
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