The
facile oxidation of alcohols to carboxylate salts and H2 is achieved using a simple and readily accessible cobalt pincer
catalyst (NNNHtBuCoBr2). The reaction follows
an acceptorless dehydrogenation pathway and displays good functional
group tolerance. The amine–amide metal–ligand cooperation
in cobalt catalyst is suggested to facilitate this transformation.
The mechanistic studies indicate that in-situ-formed aldehydes react
with a base through a Cannizzaro-type pathway, resulting in potassium
hemiacetolate, which further undergoes catalytic dehydrogenation to
provide the carboxylate salts and H2.
Selective syntheses
of symmetrical siloxanes and cyclotetrasiloxanes
are attained from reactions of silanes and dihydrosilanes, respectively,
with water, and the reactions are catalyzed by a NNNHtBu cobalt(II) pincer complex. Interestingly, when
phenylsilane was subjected to catalysis with water, a siloxane cage
consisting 12 silicon and 18 oxygen centers was obtained and remarkably
the reaction proceeded with liberation of 3 equiv of molecular hydrogen
(36 H2) under mild experimental conditions. Upon reaction
of silane with different silanols, highly selective and controlled
syntheses of higher order monohydrosiloxanes and disiloxymonohydrosilanes
were achieved by cobalt catalysis. The liberated molecular hydrogen
is the only byproduct observed in all of these transformations. Mechanistic
studies indicated that the reactions occur via a homogeneous pathway.
Kinetic and independent experiments confirmed the catalytic oxidation
of silane to silanol, and further dehydrocoupling processes are involved
in syntheses of symmetrical siloxanes, cyclotetrasiloxanes, and siloxane
cage compounds, whereas the unsymmetrical monohydrosiloxane syntheses
from silanes and silanols proceeded via dehydrogenative coupling reactions.
Overall these cobalt-catalyzed oxidative coupling reactions are based
on the Si–H, Si–OH, and O–H bond activation of
silane, silanol, and water, respectively. Catalytic cycles consisting
of Co(II) intermediates are suggested to be operative.
A ruthenium-catalyzed
reaction of HBpin with substituted organic
ethers leads to the activation of C–O bonds, resulting in the
formation of alkanes and boronate esters via hydroboronolysis. A ruthenium precatalyst, [Ru (p-cymene)Cl]2Cl2 (1), is employed, and the reactions
proceed under neat conditions at 135 °C and atmospheric pressure
(ca. 1.5 bar at 135 °C). Unsymmetrical dibenzyl ethers undergo
selective hydroboronolysis on relatively electron-poor
C–O bonds. In arylbenzyl or alkylbenzyl ethers, C–O
bond cleavage occurs selectively on CBn–OR bonds
(Bn = benzyl); in alkylmethyl ethers, selective deconstruction of
CMe–OR bonds leads to the formation of alkylboronate
esters and methane. Cyclic ethers are also amenable to catalytic hydroboronolysis. Mechanistic studies indicated the immediate
in situ formation of a mono-hydridobridged dinuclear ruthenium complex
[{(η6-p-cymene)RuCl}2(μ–H−μ–Cl)] (2), which
is highly active for hydroboronolysis of ethers.
Over time, the dinuclear species decompose to produce ruthenium nanoparticles
that are also active for this transformation. Using this catalytic
system, hydroboronolysis could be applied effectively
to a very large scope of ethers, demonstrating its great potential
to cleave C–O bonds in ethers as an alternative to traditional
hydrogenolysis.
Herein, a catalytic cross-coupling of methyldiphenylphosphine oxide with arylmethyl alcohols leading to the alkenylphosphine oxides is reported. A manganese pincer catalyst catalyzes the reactions, which provides exclusive formation of trans-alkenylphosphine oxides. Mechanistic studies indicate that reactions proceed via aldehyde intermediacy and the catalyst promotes the CC bond formation. Reactions are facilitated by dearomatization, and aromatization metal-ligand cooperation operates in catalyst. Use of abundant base metal catalyst and formation of water and H 2 as the only byproducts make this catalytic protocol sustainable and environmentally benign.
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