Continuous Flow Technology as an Enabler for Innovative Transformations Exploiting Carbenes, Nitrenes, and Benzynes

Influence of Curvature on the Physical Properties and Reactivity of Triplet Corannulene Nitrene

Although nitrene chemistry is promising for the light-induced modification of organic compounds, the reactivity of large polycyclic aromatic compounds and the effects of their curvature remain unexplored. Irradiation of azidocorannulene (1) in methanol/acetonitrile followed by HCl addition produced diastereomers 5 and 5'. Azirine 2 is apparently trapped by methanol to form diastereomeric acetal derivatives that are hydrolyzed with HCl to yield 5 and 5'. ESR spectroscopy in a glassy matrix at 77 K showed that irradiation of 1 yields corannulene nitrene 31N, which has significant 1,3-biradical character. Irradiation of 1 in a glassy matrix resulted in a new absorption band in the region of 360-440 nm, with λmax at 360 and 410 nm, attributed to 31N, as supported by time-dependent density function theory calculations, which placed the major electronic transitions of 31N at 367 nm (f = 0.0407) and 440 nm (f = 0.0353). Laser flash photolysis of 1 revealed a similar absorption spectrum. Nitrene 31N had a lifetime of only a few hundred nanoseconds and was efficiently quenched by oxygen, because of its 1,3-biradical character. CASPT2(12,11)/6-311G** calculations revealed small energy gap (7.2 kcal/mol) between singlet and triplet configurations, suggesting that 31N is formed by intersystem crossing of 11N to 31N. Spin-density, nucleus-independent chemical shift, and anisotropy of the induced current density calculations verified that 31N is a triplet vinylnitrene with unpaired electrons localized on the C═C-N moiety; decaying by intersystem crossing to 2, which is more stable owing to its aromaticity, as supported by calculations (SA-CASSCF/QD-NEVPT2/CBS).

Process Intensification of the Continuous Synthesis of Bio-Derived Monomers for Sustainable Coatings Using a Taylor Vortex Flow Reactor

We describe the optimization and scale-up of two consecutive reaction steps in the synthesis of bio-derived alkoxybutenolide monomers that have been reported as potential replacements for acrylate-based coatings (Sci. Adv.2020, 6, eabe0026). These monomers are synthesized by (i) oxidation of furfural with photogenerated singlet oxygen followed by (ii) thermal condensation of the desired 5-hydroxyfuranone intermediate product with an alcohol, a step which until now has involved a lengthy batch reaction. The two steps have been successfully telescoped into a single kilogram-scale process without any need to isolate the 5-hydroxyfuranone between the steps. Our process development involved FTIR reaction monitoring, FTIR data analysis via 2D visualization, and two different photoreactors: (i) a semicontinuous photoreactor based on a modified rotary evaporator, where FTIR and 2D correlation spectroscopy (2D-COS) revealed the loss of the methyl formate coproduct, and (ii) our fully continuous Taylor Vortex photoreactor, which enhanced the mass transfer and permitted the use of near-stoichiometric equivalents of O2. The use of in-line FTIR monitoring and modeling greatly accelerated process optimization in the Vortex reactor. This led to scale-up of the photo-oxidation in 85% yield with a projected productivity of 1.3 kg day-1 and a space-time yield of 0.06 mol day-1 mL-1. Higher productivities could be achieved while sacrificing yield (e.g., 4 kg day-1 at 40% yield). The use of superheated methanol at 200 °C in a pressurized thermal flow reactor accelerated the second step, the thermal condensation of 5-hydroxyfuranone, from a 20 h batch reflux reaction (0.5 L, 85 g) to a space time of <1 min in a reactor only 3 mL in volume operating with projected productivities of >700 g day-1. Proof of concept for telescoping the two steps was established with an overall two-step yield of 67%, producing a process with a projected productivity of 1.1 kg day-1 for the methoxybutenolide monomer without any purification of the 5-hydroxyfuranone intermediate.

Advances in the Continuous Flow Synthesis of 3- and 4-Membered Ring Systems

Small carbo- and heterocyclic ring systems have experienced a significant increase in importance in recent years due to their relevance in modern pharmaceuticals, as building blocks for designer materials or as synthetic intermediates. This necessitated the development of new synthetic methods for the preparation of these strained ring systems focusing on effectiveness and scalability. The high ring strain of these entities as well as the use of high-energy reagents and intermediates has often challenged their synthesis. Continuous flow approaches have thus emerged as highly effective means to safely and reliably access these strained scaffolds. In this short review, key developments in this field are summarised showcasing the power of continuous flow approaches for accessing 3- and 4-membered ring systems via thermal, photo- and electrochemical processes.

Speciation and kinetics of fluoride transfer from tetra-n-butylammonium difluorotriphenylsilicate ('TBAT')

Tetra-n-butylammonium difluorotriphenylsilicate (TBAT) is a conveniently handled anhydrous fluoride source, commonly used as a surrogate for tetra-n-butylammonium fluoride (TBAF). While prior studies indicate that TBAT reacts rapidly with fluoride acceptors, little is known about the mechanism(s) of fluoride transfer. We report on the interrogation of the kinetics of three processes in which fluoride is transferred from TBAT, in THF and in MeCN, using a variety of NMR methods, including chemical exchange saturation transfer, magnetisation transfer, diffusion analysis, and 1D NOESY. These studies reveal ion-pairing between the tetra-n-butylammonium and difluorotriphenylsilicate moieties, and a very low but detectable degree of fluoride dissociation, which then undergoes further equilibria and/or induces decomposition, depending on the conditions. Degenerate exchange between TBAT and fluorotriphenylsilane (FTPS) is very rapid in THF, inherently increases in rate over time, and is profoundly sensitive to the presence of water. Addition of 2,6-di-tert-butylpyridine and 3 Å molecular sieves stabilises the exchange rate, and both dissociative and direct fluoride transfer are shown to proceed in parallel under these conditions. Degenerate exchange between TBAT and 2-naphthalenyl fluorosulfate (ARSF) is not detected at the NMR timescale in THF, and is slow in MeCN. For the latter, the exchange is near-fully inhibited by exogenous FTPS, indicating a predominantly dissociative character to this exchange process. Fluorination of benzyl bromide (BzBr) with TBAT in MeCN-d3 exhibits moderate progressive autoinhibition, and the initial rate of the reaction is supressed by the presence of exogenous FTPS. Overall, TBAT can act as a genuine surrogate for TBAF, as well as a reservoir for rapidly-reversible release of traces of it, with the relative contribution of the pathways depending, inter alia, on the identity of the fluoride acceptor, the solvent, and the concentration of endogenous or exogenous FTPS.

Synthesis of substituted benzylboronates by light promoted homologation of boronic acids with N-sulfonylhydrazones

The synthesis of benzylboronates by photochemical homologation of boronic acids with N-tosylhydrazones under basic conditions is described. The reaction involves the photolysis of the N-tosylhydrazone salt to give a diazoalkane followed by the geminal carboborylation of the diazoalkane. Under the mild reaction conditions, the protodeboronation of the unstable benzylboronic acid is circumvented and the pinacolboronates can be isolated after reaction of the benzylboronic acid with pinacol. The metholodogy has been applied to the reactions of alkylboronic acids with N-tosylhydrazones of aromatic aldehydes and ketones, and to the reactions of arylboronic acids with N-tosylhydrazones of aliphatic ketones. Moreover, the employment of the DBU/DIPEA bases combination allows for homogeneous reactions which have been adapted to photochemical continuous flow conditions. Additionally, the synthetic versatility of boronates enables their further transformation via Csp3-C or Csp3-X bond forming reactions converting this methodology into a novel method for the geminal difunctionalization of carbonyls via N-tosylhydrazones.

Process Development of Heterogeneous Rh Catalyzed Carbene Transfer Reactions Under Continuous Flow Conditions

A very simple Rh-based catalyst operates under heterogeneous flow conditions for the carbene transfer of methyl diazoacetate (MDA) with several substrates. Two different methods for heterogenizing the catalyst in a column reactor have been applied. Different X-H (X=O, S, Si, CH2 ) were successfully functionalized by the carbene and cyclopropenation was performed under very mild continuous flow conditions. Following these promising results, catalyst recycling experiments using both methodologies were conducted in which up to 5 catalytic cycles have been achieved for the carbene O-H insertion reaction and interestingly, a sequential transformation of different substrates with up to 10 consecutive runs per reactor were achieved with no loss in the catalytic activity, thus allowing the production of families of compounds.

Diversification of Indoles and Pyrroles by Molecular Editing: New Frontiers in Heterocycle-to-Heterocycle Transmutation

Skeletal editing via single-atom insertion reactions involving nitrogen heterocycles have been reported by two innovative and complementary methods for the conversion of pyrroles and indoles to pyridines, quinolines and quinazolines. The use of electrophilic carbonyl cation equivalents and in situ generated nitrenes enables molecular editing to transform heterocycles forming the foundation of best-selling pharmaceuticals. Considering the importance of heterocycles in medicinal chemistry, biology and natural products, these methods offer innovative approach to complex molecular structures by heterocycle diversification and peripheral editing.