Regioselective C–H
bond alkylation of indolines and benzo[h]quinoline
with a wide range of unactivated and highly
demanded primary and secondary alkyl chlorides is accomplished using
a low-cost iron catalyst. This reaction tolerates diverse functionalities,
such as C(sp2)–Cl, fluoro, alkenyl, silyl, ether,
thioether, pyrrolyl, and carbazolyl groups including cyclic and acyclic
alkyls as well as alkyl-bearing fatty-alcohol and polycyclic-steroid
moieties. The demonstrated iron-catalyzed protocol proceeded via either
a five-membered or a six-membered metallacycle. Intriguingly, the
C-7-alkylated indolines can be readily functionalized into free-NH indolines/indoles and tryptamine derivatives. A detailed
mechanistic investigation highlights the participation of an active
Fe(I) catalyst and the involvement of a halogen-atom transfer process
via a single-electron-based mechanism. Deuterium labeling and kinetics
analysis indicate that the C–H metalation of indoline is the
probable turnover-limiting step. Overall, the experimental and theoretical
studies supported an Fe(I)/Fe(III) pathway for the alkylation reaction
comprising the two-step, one-electron oxidative addition of alkyl
chloride.
The indium-catalyzed regiospecific coupling of substituted
phenol
derivatives and quaternary peroxyoxindoles for the synthesis of C2
or C4 benzoxazin-3-one-substituted phenols via skeletal rearrangement
is described. This reaction is demonstrated with 17 examples with
good yields and diverse aryl substituents. In contrast to the indium-catalyzed
reaction, the Cu(OTf)2-catalyzed reaction of the phenol
with quaternary peroxyoxindoles afforded C2 or C4 2-oxindole-substituted
phenol derivatives. This diverse catalytic reaction generated various
biologically important phenol-substituted 2-oxindole derivatives directly
without any skeleton rearrangement and was demonstrated with 19 examples
in high yield. The regiospecificity and the reaction pathways were
explained with the support of density functional theory (DFT).
The hydrodeoxygenation (HDO) reaction
of oxygenated compounds such
as lignin-derived phenolics is well studied using molecular H2 as a hydrogen source, yet the use of high pressures discourages
its use on an industrial scale. As an alternative, the catalytic transfer
hydrogenation (CTH) pathway provides in situ hydrogenation species,
which reduces the need for the high-pressure infrastructure required
when molecular hydrogen is used. Nevertheless, this strategy is scantly
studied, and in lieu with this, herein we report the kinetic and mechanistic
investigations of the CTH strategy for the HDO of guaiacol, phenol,
anisole veratrole, and eugenol to their respective products. For potential
commercialization purposes, low loading of metal, milder reaction
conditions, and high selectivity toward desired products with a high
H/C ratio were considered while designing catalysts (0.5 wt % Ru on
SiO2–Al2O3, SiO2, Al2O3-acidic, Al2O3-basic, and Al2O3-neutral) for these reactions.
As high as 74% cyclohexanol yield from guaiacol was achieved at 225
°C in the presence of isopropyl alcohol (IPA) as the hydrogen
source and over the Ru/Al2O3-acidic catalyst
reduced at 150 °C. A detailed kinetic study is carried out to
understand the interaction of the substrate and intermediates with
the catalyst and the influence of reaction parameters on the product
formation. It was observed that the cis-isomer of 2-methoxycyclohexanol
rapidly undergoes further conversion than the trans-isomer. The experimental
observations are substantiated through density functional theory (DFT)
studies on Ru(0001) and guaiacol molecule complexes. DFT studies indicate
that the adsorption of the cis-isomer is more exothermic as compared
to that of the trans counterpart, and the underlying electronic factors
are elucidated using charge density difference and density of states
plots.
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