We report a switchable synthesis of acylindoles and quinoline derivatives via gold‐catalyzed annulations of anthranils and ynamides. α‐Imino gold carbenes, generated in situ from anthranils and an N,O‐coordinated gold(III) catalyst, undergo electrophilic attack to the aryl π‐bond, followed by unexpected and highly selective 1,4‐ or 1,3‐acyl migrations to form 6‐acylindoles or 5‐acylindoles. With the (2‐biphenyl)di‐tert‐butylphosphine (JohnPhos) ligand, gold(I) carbenes experienced carbene/carbonyl additions to deliver quinoline oxides. Some of these epoxides are valuable substrates for the preparation of 3‐hydroxylquinolines, quinolin‐3(4H)‐ones, and polycyclic compounds via facile in situ rearrangements. The reaction can be efficiently conducted on a gram scale and the obtained products are valuable substrates for preparing other potentially useful compounds. A computational study explained the unexpected selectivities and the dependency of the reaction pathway on the oxidation state and ligands of gold. With gold(III) the barrier for the formation of the strained oxirane ring is too high; whereas with gold(I) this transition state becomes accessible. Furthermore, energetic barriers to migration of the substituents on the intermediate sigma‐complexes support the observed substitution pattern in the final product.
2,4-bis(4-methoxyphenyl)-1,3,2,4-dithiadiphosphetane-2,4-disulfide (Lawesson's reagent) was made to react with benzylamine to produce [(PhCH2NH)(p-C6H4OMe)PS2] -[PhCH2NH3] + in a very facile manner. From the abovementioned product, two new complexes {Pd[(PhCH2NH)(p-C6H4OMe)PS2]2} (C1) and {Cu2(PPh3)2[(PhCH2NH)(p-C6H4OMe)PS2]2} (C2) were obtained in high yields whose molecular structures were ascertained by X-ray diffraction analysis, IR and NMR spectroscopy, and elemental analysis. The catalytic properties of both complexes were evaluated in the Heck reaction. High turnover numbers (TONs) and yields were observed for palladium catalyst and It was revealed that dicopper(I) complex by a distance of 2.84 Å between metal ions, bearing triphenylphosphine and dithiophosphorus ligands, can catalyze the Heck reaction. This is the first report of Cu(I) complex as catalyst in the Heck reaction. Natural bonding orbital (NBO) analysis for C1 indicated that natural charge on Pd atom is -0.07e and Pd atom has formed four sigma bonds with S atoms. Similarly, NBO analysis revealed no significant Cu…Cu interaction in dicopper complex C2.
Density
functional theory (DFT) at the SMD/M06-2X/def2-TZVP//SMD/M06-2X/LANL2DZ,6-31G(d)
level was employed to explore mechanistic aspects of BF3-catalyzed alcohol oxidation using a hypervalent iodine(III) compound,
[ArI(OAc)2], to yield aldehydes/ketones as the final products.
The reaction is composed of two main processes: (i) ligand exchange
and (ii) the redox reaction. Our study for 1-propanol discovered that
ligand exchange is preferentially accelerated if BF3 first
coordinates to the alcohol. This coordination increases the acidity
of the alcohol hydroxyl proton, resulting in ligand exchange between
the iodane and the alcohol proceeding via a concerted interchange
associative mechanism with an activation free energy of ∼10
kcal/mol. For the redox process, the calculations rule out the feasibility
of the conventional mechanism (alkoxy Cα deprotonation)
and introduce a replacement for it. This alternative route commences
with α-hydride elimination of the alkoxy group promoted by BF3 coordination, which yields a BF3-stabilized aldehyde/ketone
product and the iodane [ArI(OAc)(H)]. The ensuing iodane is extremely
reactive toward reductive elimination to give ArI + HOAc in a highly
exergonic fashion (ΔG = −62.1 kcal/mol).
The reductive elimination reaction is the thermodynamic driving force
for the alcohol oxidation to be irreversible. Consistent with the
kinetic isotope effect reported experimentally, the α-hydride
elimination is calculated to be the rate-determining step with an
overall activation free energy of ∼24 kcal/mol.
Although Pd(OAc)2-catalysed alkoxylation of the C(sp3)-H bonds mediated by hypervalent iodine(III) reagents (ArIX2) has been developed by several prominent researchers, there is no clear mechanism yet for such crucial transformations....
Density functional theory was utilized to investigate
plausible
mechanisms for amine and alcohol oxidation by an iodine(V) hypervalent
reagent (IBX). In this contribution, we found that amine and alcohol
oxidation both proceed by similar mechanisms. The reactions initiate
from ligand exchange to give four coordinate intermediates followed
by a redox process giving an iodine(III) species and oxidized substrates.
Interestingly, for both the ligand-exchange and the redox steps a
hypervalent twist is required for the reaction to proceed via an energetically
more accessible route. The ligand-exchange process was found to be
mediated by a proton-shuttling agent such as water, a second IBX,
or a second substrate. While the ligand-exchange step for both amine
and alcohol occurs with almost identical activation energy (particularly
when water is considered as the shuttling agent), the redox step for
the amine takes place with much lower activation energy than that
for the alcohol. Finally, we ascertained that five coordinate amide
iodine(V) complexes are unreactive toward redox reactions due to the
fact that in such cases two electrons from the coordinated amide are
required to occupy a 3c–4e σ* orbital which is too high
in energy to be reachable.
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