The widespread use of controlled molecular-level motion in key natural processes suggests that great rewards could come from bridging the gap between the present generation of synthetic molecular systems, which by and large rely upon electronic and chemical effects to carry out their functions, and the machines of the macroscopic world, which utilize the synchronized movements of smaller parts to perform specific tasks. This is a scientific area of great contemporary interest and extraordinary recent growth, yet the notion of molecular-level machines dates back to a time when the ideas surrounding the statistical nature of matter and the laws of thermodynamics were first being formulated. Here we outline the exciting successes in taming molecular-level movement thus far, the underlying principles that all experimental designs must follow, and the early progress made towards utilizing synthetic molecular structures to perform tasks using mechanical motion. We also highlight some of the issues and challenges that still need to be overcome.

Synthetic molecular motors and mechanical machines.

Dynamic covalent chemistry relates to chemical reactions carried out reversibly under conditions of equilibrium control. The reversible nature of the reactions introduces the prospects of "error checking" and "proof-reading" into synthetic processes where dynamic covalent chemistry operates. Since the formation of products occurs under thermodynamic control, product distributions depend only on the relative stabilities of the final products. In kinetically controlled reactions, however, it is the free energy differences between the transition states leading to the products that determines their relative proportions. Supramolecular chemistry has had a huge impact on synthesis at two levels: one is noncovalent synthesis, or strict self-assembly, and the other is supramolecular assistance to molecular synthesis, also referred to as self-assembly followed by covalent modification. Noncovalent synthesis has given us access to finite supermolecules and infinite supramolecular arrays. Supramolecular assistance to covalent synthesis has been exploited in the construction of more-complex systems, such as interlocked molecular compounds (for example, catenanes and rotaxanes) as well as container molecules (molecular capsules). The appealing prospect of also synthesizing these types of compounds with complex molecular architectures using reversible covalent bond forming chemistry has led to the development of dynamic covalent chemistry. Historically, dynamic covalent chemistry has played a central role in the development of conformational analysis by opening up the possibility to be able to equilibrate configurational isomers, sometimes with base (for example, esters) and sometimes with acid (for example, acetals). These stereochemical "balancing acts" revealed another major advantage that dynamic covalent chemistry offers the chemist, which is not so easily accessible in the kinetically controlled regime: the ability to re-adjust the product distribution of a reaction, even once the initial products have been formed, by changing the reaction's environment (for example, concentration, temperature, presence or absence of a template). This highly transparent, yet tremendously subtle, characteristic of dynamic covalent chemistry has led to key discoveries in polymer chemistry. In this review, some recent examples where dynamic covalent chemistry has been demonstrated are shown to emphasise the basic concepts of this area of science.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/1521-3773%2820020503%2941%3A9%3C1460%3A%3AAID-ANIE11111460%3E3.0.CO%3B2-N

Dynamic covalent chemistry.

For more than 70 years, researchers in several areas of science have been intrigued by the physical and chemical properties of the lowest excited states of molecular oxygen. With two singlet states lying close above its triplet ground state, the O2 molecule possesses a very unique configuration, which gives rise to a very rich and easily accessible chemistry, and also to a number of important photophysical interactions. In particular, photosensitized reactions of the first excited state, O2(∆g), play a key role in many natural photochemical and photobiological processes, such as photodegradation and aging processes including even photocarcinogenesis. Reactions of O2(∆g) are associated with significant applications in several fields, including organic synthesis, bleaching processes, and, most importantly, the photodynamic therapy of cancer, which has now obtained regulatory approval in most countries for the treatment of several types of tumors. The development of both applications and novel observation techniques has strongly accelerated during the past few years. Significant recent advances include, for example, the development of novel luminescent singlet oxygen probes,1-4 the time-resolved detection of O2(∆g) in a transmission microscope,5 the first time-resolved measurements of singlet oxygen luminescence in vivo,6 and the observation of oxygen quenching of triplet-excited single molecules.7 Experimental and theoretical studies on the mechanisms of photosensitized formation of excited O2 states and of their deactivation have been performed for almost 40 years. While most early liquid-phase studies were exclusively concerned with O2(∆g), recent technological advances also made possible time-resolved investigations of the second excited state, O2(Σg), which can be formed in competition with O2(∆g) in many cases. A significant number of * Corresponding author. Tel.: ++49 69 79829448. Fax: ++49 69 79829445. E-mail: R.Schmidt@chemie.uni-frankfurt.de. 1685 Chem. Rev. 2003, 103, 1685−1757

Physical mechanisms of generation and deactivation of singlet oxygen.

Hydrogen bonds are like human beings in the sense that they exhibit typical grouplike behavior. As an individual they are feeble, easy to break, and sometimes hard to detect. However, when acting together they become much stronger and lean on each other. This phenomenon, which in scientific terms is called cooperativity, is based on the fact that 1+1 is more than 2. By using this principle, chemists have developed a wide variety of chemically stable structures that are based on the reversible formation of multiple hydrogen bonds. More than 20 years of fundamental studies on these phenomena have gradually developed into a new discipline within the field of organic synthesis, and is nowadays called noncovalent synthesis. This review describes noncovalent synthesis based on the reversible formation of multiple hydrogen bonds. Starting with a thorough description of what the hydrogen bond really is, it guides the reader through a variety of bimolecular and higher order assemblies and exemplifies the general principles that determine their stability. Special focus is given to reversible capsules based on hydrogen-bonding interactions that exhibit interesting encapsulation phenomena. Furthermore, the role of hydrogen-bond formation in self-replicating processes is actively discussed, and finally the review briefly summarizes the development of novel materials (nanotubes, liquid crystals, polymers, etc.) and principles (dynamic libraries) that recently have emanated from this intriguing field of research.

Noncovalent synthesis using hydrogen bonding

Self-Assembling Capsules

Before 1965, only a few examples of reactions of this type were known, and it was generally believed that steric effects determined the stereochemical course of such reactions. As anomalies were discovered, theories were developed to explain them. These theories led to predictions, which were tested by experiment and verified. Eventually, a comprehensive quantitative theory of stereoselectivity was developed, such that now the products of reactions of this type can be predicted with confidence. Cyclobutene was first prepared by Willstatter1 in 1905. Although its ready thermal conversion to 1,3butadiene was noted often in the early literature, it was not until the 1950s that mechanistic studies were undertaken. In 1958 Walters reported that the conversion of cyclobutene to butadiene was a wellbehaved unimolecular process occurring at 150 °C,2 a relatively low temperature compared to the 350 °C required for thermal cleavage of the almost equally strained cyclobutane. Walters reported an activation energy of 32.5 kcal/mol for the reaction along with a log A of 13.1. By that time initial reports of the stereochemistry of the ring-opening of 3-substituted and 3,4-disubstituted cyclobutenes had begun to appear, and by 1959 the substantial investigations of Vogel3,4 and particularly of Criegee and his co-workers5-7 had led to considerable mechanistic understanding of the reaction.

/pdf/electronic-control-of-stereoselectivities-of-electrocyclic-13bwc4yhi8.pdf

Electronic Control of Stereoselectivities of Electrocyclic Reactions of Cyclobutenes: A Triumph of Theory in the Prediction of Organic Reactions

phoric. 2 and 3: THF (8.5 mL) was added to a mixture of 1 (0.58 g, 0.94 mmol) and dry InCI, (0.87 g, 4.10 mmol). The color changed immediately from green to blue. The solution was stirred for 5 min, methanol (1.5 mL) was added, and the mixture was separated on a chromatography column (MPLC column 46 x 2.6 cm. silica gel 40-63 F, eluent THFjmethanol l0jl). Ayellow and a blue fraction that have not yet been able to be characterized were eluted first in low yields. The third violet fraction was treated with AsPh,CI monohydrate (100 mg. 0.229 mmol). thesolvent removed at 10-' mbar, and the residue taken upina mixtureofn-hexane.ethylacetate.andacetone(5:20:15 mL).Theviolet mother liquor was filtered off through a G3 Schlenk filter; at f4 'C 2-(AsPh,), . (CH,),CO precipitates as red-brown crystals. They are separated from the mother liquor by decantation. washed three times with diethyl ether, and dried in vacuum. Yield: 90 mg (X%O, calculated on initial amount of I). correct elemental analyses. The fourth. brown fraction was treated with AsPh,Cl monohydrate (50 mg, 0.115 mmol). the solvent removed at lo-' mbar. and the residue taken up in ethyl acetate. Excess AsPh,CI and KCI remained undissolved. The brown inother liquor was filtered through a G-3 Schlenk filter. At +4T, 3-(AsPh,), precipitated in the form of dark brown crystals. which were separated by decantation from the mother liquor, washed three times with diethyl ether, and dried at lo-' mbar. Yield: 15mg (2%, calculated on initial ainonnt of 1). correct elemental analyses. A light brown residue which cannot be eluted remains on the column.

Reaction of C60 with Dimethyldioxirane—Formation of an Epoxide and a 1,3‐Dioxolane Derivative

Theoretical modeling of the dynamics of complexation and decomplexation of guest molecules by container molecules reveals that gating has a critical influence on the ease of formation and stability of host-guest complexes. Hosts equipped with gates can form very stable complexes with a variety of guests under readily achievable conditions. Gating involves conformational processes of the host molecule that alter the size of the portals through which guest molecules pass. “French door” and “sliding door” mechanisms of gate opening are identified.

Chimin Sheu

Papers

Electronic Control of Stereoselectivities of Electrocyclic Reactions of Cyclobutenes: A Triumph of Theory in the Prediction of Organic Reactions

Reaction of C60 with Dimethyldioxirane—Formation of an Epoxide and a 1,3‐Dioxolane Derivative

Gating as a Control Element in Constrictive Binding and Guest Release by Hemicarcerands

Reactivity toward singlet oxygen of a 7,8-dihydro-8-oxoguanosine ( 8-hydroxyguanosine ) formed by photooxidation of a guanosine derivative

Endoperoxide formation in a guanosine derivative