Palladium-catalyzed amination reactions of aryl halides have undergone rapid development in the last 12 years, largely driven by the implementation of new classes of ligands. Biaryl phosphanes have proven to provide especially active catalysts in this context. This Review discusses the application of these catalysts in C-N cross-coupling reactions in the synthesis of heterocycles and pharmaceuticals, in materials science, and in natural product synthesis.

/pdf/biaryl-phosphane-ligands-in-palladium-catalyzed-amination-5180ia9hmu.pdf

Biaryl Phosphane Ligands in Palladium‐Catalyzed Amination

The widespread use of molecular machines in biology has long suggested that great rewards could come from bridging the gap between synthetic molecular systems and the machines of the macroscopic world. In the last two decades, it has proved possible to design synthetic molecular systems with architectures where triggered large amplitude positional changes of submolecular components occur. Perhaps the best way to appreciate the technological potential of controlled molecular-level motion is to recognize that nanomotors and molecular-level machines lie at the heart of every significant biological process. Over billions of years of evolution, nature has not repeatedly chosen this solution for performing complex tasks without good reason. When mankind learns how to build artificial structures that can control and exploit molecular level motion and interface their effects directly with other molecular-level substructures and the outside world, it will potentially impact on every aspect of functional molecule and materials design. An improved understanding of physics and biology will surely follow.

The first steps on the long path to the invention of artificial molecular machines were arguably taken in 1827 when the Scottish botanist Robert Brown observed the haphazard motion of tiny particles under his microscope.1,2 The explanation for Brownian motion, that it is caused by bombardment of the particles by molecules as a consequence of the kinetic theory of matter, was later provided by Einstein, followed by experimental verification by Perrin.3,4 The random thermal motion of molecules and its implications for the laws of thermodynamics in turn inspired Gedankenexperiments (“thought experiments”) that explored the interplay (and apparent paradoxes) of Brownian motion and the Second Law of Thermodynamics. Richard Feynman’s famous 1959 lecture “There’s plenty of room at the bottom” outlined some of the promise that manmade molecular machines might hold.5,6 However, Feynman’s talk came at a time before chemists had the necessary synthetic and analytical tools to make molecular machines. While interest among synthetic chemists began to grow in the 1970s and 1980s, progress accelerated in the 1990s, particularly with the invention of methods to make mechanically interlocked molecular systems (catenanes and rotaxanes) and control and switch the relative positions of their components.7−24

Here, we review triggered large-amplitude motions in molecular structures and the changes in properties these can produce. We concentrate on conformational and configurational changes in wholly covalently bonded molecules and on catenanes and rotaxanes in which switching is brought about by various stimuli (light, electrochemistry, pH, heat, solvent polarity, cation or anion binding, allosteric effects, temperature, reversible covalent bond formation, etc.). Finally, we discuss the latest generations of sophisticated synthetic molecular machine systems in which the controlled motion of subcomponents is used to perform complex tasks, paving the way to applications and the realization of a new era of “molecular nanotechnology”.

1.1. The Language Used To Describe Molecular Machines
Terminology needs to be properly and appropriately defined and these meanings used consistently to effectively convey scientific concepts. Nowhere is the need for accurate scientific language more apparent than in the field of molecular machines. Much of the terminology used to describe molecular-level machines has its origins in observations made by biologists and physicists, and their findings and descriptions have often been misinterpreted and misunderstood by chemists. In 2007 we formalized definitions of some common terms used in the field (e.g., “machine”, “switch”, “motor”, “ratchet”, etc.) so that chemists could use them in a manner consistent with the meanings understood by biologists and physicists who study molecular-level machines.14

The word “machine” implies a mechanical movement that accomplishes a useful task. This Review concentrates on systems where a stimulus triggers the controlled, relatively large amplitude (or directional) motion of one molecular or submolecular component relative to another that can potentially result in a net task being performed. Molecular machines can be further categorized into various classes such as “motors” and “switches” whose behavior differs significantly.14 For example, in a rotaxane-based “switch”, the change in position of a macrocycle on the thread of the rotaxane influences the system only as a function of state. Returning the components of a molecular switch to their original position undoes any work done, and so a switch cannot be used repetitively and progressively to do work. A “motor”, on the other hand, influences a system as a function of trajectory, meaning that when the components of a molecular motor return to their original positions, for example, after a 360° directional rotation, any work that has been done is not undone unless the motor is subsequently rotated by 360° in the reverse direction. This difference in behavior is significant; no “switch-based” molecular machine can be used to progressively perform work in the way that biological motors can, such as those from the kinesin, myosin, and dynein superfamilies, unless the switch is part of a larger ratchet mechanism.14

Artificial Molecular Machines

Background During the last few decades, progress has been made in manipulating the architecture of synthetic polymer materials. However, the primary structure—that is, the sequential arrangement of monomer units in a polymer chain—is generally poorly controlled in synthetic macromolecules. Common synthetic polymers are usually homopolymers, made of the same monomer unit, or copolymers with simple chain microstructures, such as random or block copolymers. These polymers are used in many areas but do not have the structural and functional complexity of sequence-defined biopolymers, such as nucleic acids or proteins. Indeed, monomer sequence regulation plays a key role in biology and is a prerequisite for crucial features of life, such as heredity, self-replication, complex self-assembly, and molecular recognition. In this context, developing synthetic polymers containing controlled monomer sequences is an important area for research. Precise molecular encoding of synthetic polymer chains. In most synthetic copolymers, monomer units (represented here as colored square boxes A, B, C, and D) are distributed randomly along the polymer chains (left). In sequence-controlled polymers, they are arranged in a speciﬁc order in all of the chains (right). Monomer sequence regularity strongly inﬂuences the molecular, supramolecular, andmacroscopic properties of polymer materials. Advances Various synthetic methods for controlling monomer sequences in polymers have been identified, and two major trends in the field of sequence-controlled polymers have emerged. Some approaches use biological concepts that have been optimized by nature for sequence regulation. For instance, DNA templates, enzymes, or even living organisms can be used to prepare sequence-defined polymers. These natural mechanisms can be adapted to tolerate nonnatural monomers. The other trend is the preparation of sequence-controlled polymers by synthetic chemistry. In the most popular approach, monomer units are attached one by one to a support, which is an efficient method but demanding in practice. Recently, some strategies have been proposed for controlling sequences in chain-growth and step-growth polymerizations. These mechanisms usually allow fast and large-scale synthesis of polymers. Specific kinetics and particular catalytic or template conditions allow sequence regulation in these processes. Outlook The possibility of controlling monomer sequences in synthetic macromolecules has many scientific and technological implications. Information can be controlled at the molecular level in synthetic polymer chains. This opens up interesting perspectives for the field of data storage. In addition, having power over monomer sequences could mean structural control of the resulting polymer, as it strongly influences macromolecular folding and self-assembly. For instance, functional synthetic assemblies that mimic the properties of globular proteins, such as enzymes and transporters, can be foreseen. Moreover, monomer sequence control influences some macroscopic properties. For example, bulk properties such as conductivity, rigidity, elasticity, or biodegradability can be finely tuned in sequence-controlled polymers. The behavior of polymers in solution, particularly in water, is also strongly dependent on monomer sequences. Thus, sequence regulation may enable a more effective control of structure-property relations in tomorrow’s polymer materials.

http://science.sciencemag.org/content/sci/341/6146/1238149.full.pdf

Sequence-controlled polymers.

Catalytic C-C bond-forming multi-component cascade or domino reactions: pushing the boundaries of complexity in asymmetric organocatalysis.

Heterocycles constitute the largest and the most diverse family of organic compounds Among them, aromatic heterocycles represent structural motifs found in a great number of biologically active natural and synthetic compounds, drugs, and agrochemicals Moreover, aromatic heterocycles are widely used for synthesis of dyes and polymeric materials of high value 1 There are numerous reports on employment of aromatic heterocycles as intermediates in organic synthesis 2

Although, a variety of highly efficient methodologies for synthesis of aromatic heterocycles and their derivatives have been reported in the past, the development of novel methodologies is in cuntinious demand Particlularly, development of new synthetic approaches toward heterocycles, aiming at achieving greater levels of molecular complexity and better functional group compatibilities in a convergent and atom economical fashions from readily accessible starting materials and under mild reaction conditions, is one of a major research endeavor in modern synthetic organic chemistry Transition metal-catalyzed transformations, which often help to meet the above criteria, are among the most attractive synthetic tools

Several excellent reviews dealing with transition metal-catalyzed synthesis of heterocyclic compounds have been published in literature during recent years Many of them highlighted the use of a particular transition metal, such as gold,3 silver,4 palladium,5 copper,6 cobalt,7 ruthenium,8 iron,9 mercury,10 rare-earth metals,11 and others Another array of reviews described the use of a specific kind of transformation, for instance, intramolecular nucleophilic attack of heteroatom at multiple C–C bonds,12 Sonogashira reaction,13 cycloaddition reactions,14 cycloisomerization reactions,15 C–H bond activation processes,16 metathesis reactions,17 etc Reviews devoted to an application of a particular type of starting materials have also been published Thus, for example, applications of isocyanides,18 diazocompounds,19 or azides20 have been discussed In addition, a significant attention was given to transition metal-catalyzed multicomponent syntheses of heterocycles21 Finally, syntheses of heterocycles featuring formation of intermediates, such as nitrenes,22 vinylidenes,23 carbenes, and carbenoids24 have also been reviewed

The main focus of the present review is a transition metal-catalyzed synthesis of aromatic monocyclic heterocycles The organization of the review is rather classical and is based on a heterocycle, categorized in the following order: (a) ring size of heterocycle, (b) number of heteroatoms, (c) type of heterocycle, and (d) a class of transformation involved A brief mechanistic discussion is given to provide information about a possible reaction pathway when necessary The review mostly discusses recent literature, starting from 200425 until the end of 2011, however, some earlier parent transformations are discussed when needed

Transition Metal-Mediated Synthesis of Monocyclic Aromatic Heterocycles

For a long time multi-component syntheses of heterocycles have undeniably been a domain of classical carbonyl condensation chemistry. However, the advent of transition-metal catalysis not only has fertilized strategies in heterocyclic synthesis by uni- and bimolecular transformations but the past decade has also witnessed a rapid development of transition-metal catalysis in new multi-component reactions (MCR). Expectedly, palladium catalyzed processes have received a dominant position, yet, other transition-metal complexes are catching up implying organometallic elementary steps that reach even further than cross-coupling and carbometallation. Besides domino MCRs that are purely based upon organometallic catalysis the sequential and consecutive combination with condensation, addition and cycloaddition steps opens a vast playground for the invention of new sequences in heterocyclic synthesis. This tutorial review outlines the underlying reaction based principles of transition-metal catalysis in multi-component syntheses of heterocycles, summarizes recent developments of palladium catalyzed MCR, and highlights the more recent contributions to MCR based heterocyclic synthesis by virtue of rhodium, ruthenium, and copper catalysis.

Multi-component syntheses of heterocycles by transition-metal catalysis

The ribosome builds proteins by joining together amino acids in an order determined by messenger RNA. Here, we report on the design, synthesis, and operation of an artificial small-molecule machine that travels along a molecular strand, picking up amino acids that block its path, to synthesize a peptide in a sequence-specific manner. The chemical structure is based on a rotaxane, a molecular ring threaded onto a molecular axle. The ring carries a thiolate group that iteratively removes amino acids in order from the strand and transfers them to a peptide-elongation site through native chemical ligation. The synthesis is demonstrated with ~1018 molecular machines acting in parallel; this process generates milligram quantities of a peptide with a single sequence confirmed by tandem mass spectrometry.

/pdf/sequence-specific-peptide-synthesis-by-an-artificial-small-2zo3kqu98d.pdf

Sequence-Specific Peptide Synthesis by an Artificial Small-Molecule Machine

A domino sequence consisting of insertion, coupling, isomerization, and Diels-Alder steps yields highly fluorescent spirocycles.

An alkynoyl ortho-iodo phenolester or alkynoyl ortho-iodo anilides and propargyl allyl ethers react under Sonogashira coupling conditions in the sense of an insertion-coupling-isomerization-Diels-Alder hetero domino reaction to furnish (tetrahydroisobenzofuran)-spirobenzofuranones and -spirodihydroindolones in good yields. Many representatives can be crystallized and single crystal structure analyses display steric and electronic substituent effects on the torsional angles of the terminal (hetero)aryl groups and the central cis,-trans-butadiene fragment. DFT computations reveal that in the final pericyclic step the Diels-Alder termination is by far thermodynamically and kinetically favored over a possible Claisen rearrangement. Compounds of this new class of spirocyclic compounds possess large Stokes shifts and fluoresce intensively with blue over green to orange colors. As a consequence of the spirocyclic rigidity fluorescence lifetimes and quantum yields are rather high in some cases.

Synthesis, structure and emission properties of spirocyclic benzofuranones and dihydroindolones: a domino insertion-coupling-isomerization- Diels-Alder approach to rigid fluorophores.

Functional π-electron systems can be synthesized in a diversity-oriented fashion by applying multicomponent and domino reactions. Based upon alkyne activation by Sonogashira coupling, reactive triple or double bonds become the key functionality for se- quential and consecutive synthesis of heterocycles, such as β-amino vinyl nitrothiophenes, δ-amino propenylidene indolones, indolizines, furans, pyrroles, annelated 2-amino pyridines, and spirocyclic benzofuranones and indolones. All these chromophores and fluorophores possess peculiar properties such as nonlinear optical (NLO) activity, pH-sensitivity, distinct solid-state fluorescence, or large Stokes shifts.

/pdf/diversity-oriented-syntheses-of-functional-p-systems-by-3j89fbwl52.pdf

Daniel M. D'Souza

Papers

Multi-component syntheses of heterocycles by transition-metal catalysis

Sequence-Specific Peptide Synthesis by an Artificial Small-Molecule Machine

A domino sequence consisting of insertion, coupling, isomerization, and Diels-Alder steps yields highly fluorescent spirocycles.

Synthesis, structure and emission properties of spirocyclic benzofuranones and dihydroindolones: a domino insertion-coupling-isomerization- Diels-Alder approach to rigid fluorophores.

Diversity-oriented syntheses of functional π-systems by multicomponent and domino reactions