Very fast, reversible, polyethylene (PE) chain transfer or complex-catalysed "Aufbaureaktion" describes a "living" chain-growing process on a main-group metal or zinc atom; this process is catalysed by an organo-transition-metal or lanthanide complex. PE chains are transferred very fast between the two metal sites and chain growth takes place through ethylene insertion into the transition-metal- or lanthanide-carbon bond-coordinative chain-transfer polymerisation (CCTP). The transferred chains "rest" at the main-group or zinc centre, at which chain-termination processes like beta-H transfer/elimination are of low significance. Such protocols can be used to synthesise very narrowly distributed PE materials (M(w)/M(n)<1.1 up to a molecular weight of about 4000 g mol(-1)) with differently functionalised end groups. Higher molecular-weight polymers can be obtained with a slightly increased M(w)/M(n), since diffusion control and precipitation of the polymers influences the chain-transfer process. Recently, a few transition-metal- or lanthanide-based catalyst systems that catalyse such a highly reversible chain-growing process have been described. They are summarised and compared within this contribution.
The reductive amination,
the reaction of an aldehyde or a ketone
with ammonia or an amine in the presence of a reducing agent and often
a catalyst, is an important amine synthesis and has been intensively
investigated in academia and industry for a century. Besides aldehydes,
ketones, or amines, starting materials have been used that can be
converted into an aldehyde or ketone (for instance, carboxylic acids
or organic carbonate or nitriles) or into an amine (for instance,
a nitro compound) in the presence of the same reducing agent and catalyst.
Mechanistically, the reaction starts with a condensation step during
which the carbonyl compound reacts with ammonia or an amine, forming
the corresponding imine followed by the reduction of the imine to
the alkyl amine product. Many of these reduction steps require the
presence of a catalyst to activate the reducing agent. The reductive
amination is impressive with regard to the product scope since primary,
secondary, and tertiary alkyl amines are accessible and hydrogen is
the most attractive reducing agent, especially if large-scale product
formation is an issue, since hydrogen is inexpensive and abundantly
available. Alkyl amines are intensively produced and use fine and
bulk chemicals. They are key functional groups in many pharmaceuticals,
agro chemicals, or materials. In this review, we summarize the work
published on reductive amination employing hydrogen as the reducing
agent. No comprehensive review focusing on this subject has been published
since 1948, albeit many interesting summaries dealing with one or
the other aspect of reductive amination have appeared. Impressive
progress in using catalysts based on earth-abundant metals, especially
nanostructured heterogeneous catalysts, has been made during the early
development of the field and in recent years.
Die Umsetzung von Monohydridkomplexen später Übergangsmetalle mit Lanthanoidalkylen führt zu heterodimetallischen Verbindungen mit direkten Metall‐Metall‐Bindungen (siehe Schema). Diese kovalenten Bindungen sind stark polar und können als Donor‐Akzeptor‐Bindungen verstanden werden.
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