A Photochemical Macrocyclization Route to Asymmetric Strained [3.2] Paracyclophanes

The intricate frameworks of paracyclophanes are an important target for synthesis since they are found in various chiral auxiliaries, solar cells, high-performance plastics, pharmaceuticals, and molecular machines. Whereas numerous methods exist for the preparation of symmetric paracyclophanes, protocols for the efficient synthesis of strained asymmetric scaffolds are limited. Here we report a remarkably simple photochemical route to strained [3.2]paracyclophanes starting from readily available educts. By way of NMR and X-ray analyses, we discovered that UV-irradiation of an aromatic carboxylic ester tethered to a toluene moiety leads to the intramolecular formation of a new C-C bond, with loss of an alcohol. A systematic evaluation of the reaction conditions and substituents, as well as radical starter and triplet quenching experiments, point to a reaction mechanism involving an excited triplet state and hydrogen atom transfer. The new method proved to be robust and versatile enabling the synthesis of a range of cyclophanes with different substitutions, including an unusual diastereoisomer with two planar chiral centers, and thus proved to be a valuable addition to the synthetic toolbox.

Azulene-Naphthalene, Naphthalene-Naphthalene, and Azulene-Azulene Rearrangements

The mechanism(s) of thermal rearrangement of azulenes have been enigmatic for several decades. Herein, we have employed density functional theory (DFT) calculations at the M06-2X/6-311+G(d,p) level together with single-point calculations at the CCSD(T) level to assess possible mechanisms of the experimentally observed azulene and naphthalene automerizations. Of the two mechanisms proposed for naphthalene automerization, it is found that the benzofulvene (BF) route is favored over the naphthvalene mechanism by ∼6 kcal/mol and is energetically lower than the norcaradiene-vinylidene mechanism (NVM) for the azulene-naphthalene rearrangement (E_a ∼ 76.5 (74.6) kcal/mol). Moreover, contrary to older reports, we observe that a pathway involving indenylcarbene intermediates is a viable, alternate mechanism. Therefore, the naphthalene automerization is expected to take place during azulene pyrolysis, especially under conditions of low-pressure FVP, where it will be aided by chemical activation. Furthermore, thermal azulene-azulene isomerization is feasible through vinylidene-acetylene-vinylidene (VAV), dehydrotriquinacene (DTQ), and azulvalene (AV) pathways with activation energies lying below that required for the azulene-naphthalene conversion, i.e., the NVM. These results, together with the previously published NVM, provide reasonable explanations for most of the products of the thermal azulene-naphthalene rearrangement.

Glossary of terms used in physical organic chemistry (IUPAC Recommendations 2021)

This Glossary contains definitions, explanatory notes, and sources for terms used in physical organic chemistry. Its aim is to provide guidance on the terminology of physical organic chemistry, with a view to achieving a consensus on the meaning and applicability of useful terms and the abandonment of unsatisfactory ones. Owing to the substantial progress in the field, this 2021 revision of the Glossary is much expanded relative to the previous edition, and it includes terms from cognate fields.

Thermal Rearrangement of Azulenes to Naphthalenes: A Deeper Insight into the Mechanisms

The thermal rearrangement of azulene to naphthalene has been the subject of several experimental and computational studies. Here, we reexamine the proposed mechanisms at the DFT level. The use of different functionals showed that the HF-exchange contribution significantly affects reaction energies and barrier heights. Accordingly, all proposed pathways were investigated with the optimal method, M06-2X/6-311+G(d,p), which confirms the norcaradiene-vinylidene mechanism (A) as the dominant unimolecular route (E_a ≈ 76 kcal/mol) able to account for the major products of pyrolyses using ¹³C- or substituent-labeled azulenes. Moreover, a facile vinylidene-acetylene interconversion will scramble the terminal carbon atoms in the vinylidene. Several other potential intramolecular reaction mechanisms (B-E) are ruled out because of higher activation energies (>84 kcal/mol) and failure to reproduce the results obtained with substituted and ¹³C-labeled azulenes and benzazulenes. These experimental results also demonstrate that the proposed free radical or H atom-induced intermolecular methylene walk and spiran mechanisms cannot be major contributors, especially under flash vacuum pyrolysis conditions.

Carbon subsulfide C3S2 – synthesis by flash vacuum pyrolysis and crystal structure determination

Carbon subsulfide C3S2 can be produced on a preparative scale by flash vacuum pyrolysis (FVP). The precursor 5-(methylthio)-3H-1,2-dithiole-3-thione (C4H4S4) proved to be particularly suitable and yields up to 8% could be achieved on evaporation at T = 180 °C and pyrolysis of the vapour at 950 °C. The other precursors tested, C4S6 and C6S8, were far less productive. Insight into the thermal conversion of C4S6 was gained by isolation and structure determination of a new isomer of the sulur-carbon compound C8S8, which is formed on thermal treatment of C4S6 at T = 330 °C. The formation of C8S8 can be interpreted by sulfur cleavage from C4S6. Crystal growth by sublimation below 0 °C allowed for the determination of the crystal structure of C3S2. The five-atomic molecules are linear and arranged in a typical pattern analogous to the crystal structures of I2, CS2 and CSe2. The reaction of C3S2 with bromine is known to give C3Br6S2 of yet unknown structure. By sublimation of C3Br6S2 in air, 4,5-dibromo-1,2-dithiol-3-one (C3Br2OS2) was obtained, representing the product of bromine abstraction and oxidation. This substantiates the former suggestion for C3Br6S2 to have the structure of a hexabromodithiolane.

Ørsted and Bunsen: Voltaic Batteries, Electric Arcs, Electromagnetism, and Electrolysis

200 years ago Ørsted laid the foundation of electromagnetism in his famous experiment in which a magnetic needle is deflected in the electrical field of a platinum wire. For this he used his own Cu-Zn trough battery, which was among the best then available, but 21 years later it was surpassed by the coal-zinc battery invented by Bunsen, which became highly successful and acclaimed. That year, 1841, Bunsen made his first direct contact with Scandinavia when he visited Berzelius in Stockholm, Palmstedt in Gothenburg, and Ørsted, Scharling, and Zeise in Copenhagen. Like almost everybody in continental Europe, they adopted Bunsen's battery, and Ørsted used it for his experiments with a very large electromagnet. The paths of Ørsted's and Bunsen's research crossed again much later through the synthesis of elemental aluminum, which was first achieved by Ørsted in 1825 (although it was probably not obtained as the pure metal) and performed quite differently by Bunsen, by electrolysis using his coal-zinc battery, in 1854.

Synthesis and Crystal Structure of Dimorphic Dibenzo[cde,opq]rubicene

Dibenzo[cde,opq]rubicene has been synthesized by an eight-step reaction sequence including an iron-mediated [2+2+1] cycloaddition and a flash vacuum pyrolysis as key steps. Two crystal modifications of the S-shaped, planar polycyclic aromatic hydrocarbon have been obtained and characterized by X-ray diffractometry.

Benefits of Unconventional Methods in the Total Synthesis of Natural Products

This article provides a survey of four "unconventional" methods employed in the synthesis of natural products in the Hudlicky group. The utility of flash vacuum pyrolysis is highlighted by examples of many natural products attained via vinylcyclopropane-cyclopentene rearrangement and its heterocyclic variants. Preparative organic electrochemistry was used in oxidations and reductions with levels of selectivity unattainable by conventional methods. Yeast reduction of ketoesters was featured in the total synthesis of pyrrolizidine alkaloids. Finally, the use of toluene dioxygenase-mediated dihydroxylations in enantioselective synthesis of natural products concludes this presentation. Recently, synthesized targets in the period 2010-2019 are listed in the accompanying table. The results of research from the Hudlicky group are placed in appropriate context with the work of others, and a detailed guide to the current literature is provided.

Flash Vacuum Pyrolysis: Techniques and Reactions

Flash vacuum pyrolysis (FVP) had its beginnings in the 1940s and 1950s, mainly through mass spectrometric detection of pyrolytically formed free radicals. In the 1960s many organic chemists started performing FVP experiments with the purpose of isolating new and interesting compounds and understanding pyrolysis processes. Meanwhile, many different types of apparatus and techniques have been developed, and it is the purpose of this review to present the most important methods as well as a survey of typical reactions and observations that can be achieved with the various techniques. This includes preparative FVP, chemical trapping reactions, matrix isolation, and low temperature spectroscopy of reactive intermediates and unstable molecules, the use of online mass, photoelectron, microwave, and millimeterwave spectroscopies, gas-phase laser pyrolysis, pulsed pyrolysis with supersonic jet expansion, very low pressure pyrolysis for kinetic investigations, solution-spray and falling-solid FVP for involatile compounds, and pyrolysis over solid supports and reagents. Moreover, the combination of FVP with matrix isolation and photochemistry is a powerful tool for investigations of reaction mechanism.

Azulene-Naphthalene-Type Rearrangements in Benz[a]azulene and Cyclohepta[b]indole

Flash vacuum pyrolysis (FVP) of benz[a]azulene yields phenanthrene and 2-ethynylbiphenyl. FVP of cyclohepta[b]indole similarly yields phenanthridine and 2-cyanobiphenyl. The reversibility of the reactions is demonstrated by FVP of 2-ethynylbiphenyl and 2-isocyanobiphenyl. All the observed reactions are in accord with the norcaradiene-vinylidene mechanism of the azulene-naphthalene rearrangement, whereas other proposed mechanisms are ruled out.

Continuous-flow technology—a tool for the safe manufacturing of active pharmaceutical ingredients

In the past few years, continuous-flow reactors with channel dimensions in the micro- or millimeter region have found widespread application in organic synthesis. The characteristic properties of these reactors are their exceptionally fast heat and mass transfer. In microstructured devices of this type, virtually instantaneous mixing can be achieved for all but the fastest reactions. Similarly, the accumulation of heat, formation of hot spots, and dangers of thermal runaways can be prevented. As a result of the small reactor volumes, the overall safety of the process is significantly improved, even when harsh reaction conditions are used. Thus, microreactor technology offers a unique way to perform ultrafast, exothermic reactions, and allows the execution of reactions which proceed via highly unstable or even explosive intermediates. This Review discusses recent literature examples of continuous-flow organic synthesis where hazardous reactions or extreme process windows have been employed, with a focus on applications of relevance to the preparation of pharmaceuticals.

Ketenes from N-(2-Pyridyl)amides

Methoxycarbonylketene 4a, methoxycarbonyl(methyl)ketene 4b, chloroketene 4c, cyanoketene 4d, diphenylketene 4e, and 2-pyridylketene 4f have been generated by flash vacuum thermolysis of the corresponding 2-pyridylacetamide derivatives 3a–f and isolated in Ar matrices for FT-IR spectroscopic characterisation. The N-(2-pyridyl)-2-pyridylacetamide 3f yielded 2-pyridyl isocyanate in addition to 2-pyridylketene.

Use of External Radical Sources in Flash Vacuum Pyrolysis to Facilitate Cyclodehydrogenation Reactions in Polycyclic Aromatic Hydrocarbons

A new process to facilitate the cyclodehydrogenation of polycyclic aromatic hydrocarbons (PAHs) in flash vacuum pyrolysis (FVP) using an external radical source is described. Using hexanes as an external radical source the conversion of various PAHs to their cyclodehydrogenated products is vastly increased. Various other volatile organic compounds were also examined to determine their ability to act as external radical sources in FVP.

Microwave Flash Pyrolysis: C9H8 Interconversions and Dimerisations

The pyrolysis of 2-ethynyltoluene, indene, fluorene, and related compounds has been studied by sealed tube microwave flash pyrolysis (MFP), in concert with modelling of putative mechanistic pathways by density functional theory (DFT) computations. In the MFP technique, samples are admixed with graphite and subjected to intense microwave power (150–300 W) in a quartz reaction tube under a nitrogen atmosphere. The MFP reaction of 2-ethynyltoluene gave mostly indene, the product of a Roger Brown rearrangement (1,2-H shift to a vinylidene) followed by insertion. An additional product was chrysene, the likely result of hydrogen atom loss from indene followed by dimerisation. The intermediacy of dimeric bi-indene structures was supported by pyrolysis of bi-indene and by computational models. Benzo[a]anthracene and benzo[c]phenanthrene are minor products in these reactions. These are shown to arise from pyrolysis of chrysene under the same MFP conditions. MFP reaction of fluorene gave primarily bi-fluorene, bifluorenylidene, and dibenzochrysene, the latter derived from a known Stone–Wales rearrangement.