Methylene Capping Facilitates Cross-Metathesis Reactions of Enals: A Short Synthesis of 7-Methoxywutaifuranal from the Xylochemical Isoeugenol

Groß J., Grundke C., Rocker J., Arduengo A.J., Opatz T.

More than 100 useful ‘xylochemicals’ and their natural origins are offered combined with an overview of comparative petro- and xylochemical approaches to synthetic chemistry.

Avendaño Villarreal J.A., Granato A.V., Delolo F.G., dos Santos E.N.

• Styrene derivatives are important flavor and locust pheromone. • The synthesis is made in one step from renewable 1-propenylbenzenes. • Commercial ruthenium metathesis catalysts were employed. • Unprecedented turnover numbers for this reaction were achieved. Functionalized styrenes were obtained by the ethenolysis of renewable 1-propenylbenzenes in a very efficient synthetic pathway. Some of the products are valuable food & flavor ingredients (4-vinylguaiacol) or locust pheromone (4-vinylanisole). The catalysts employed were ruthenium-alkylidene complexes bearing a N-heterocyclic carbene as a ligand, which bulkiness proved to be important for the catalysis output. The judicious choice the reaction conditions was critical to enable near quantitative yields under mild conditions in short reaction times. More strikingly, the catalyst load could be reduced to 0.01 mol%, keeping good conversion and selectivity.

Mo F., Qiu D., Zhang L., Wang J.

Aryl diazonium salts are versatile building blocks in organic synthesis. In light of the ever-increasing importance of aryl diazonium salts spanning most disciplines of the chemical sciences, we review the recent development of aryl diazonium chemistry over the past seven years (2013-2020). Special emphasis is put on various new transformations involving the generation of radical intermediates via thermal, photochemical, and electrochemical means. Recent advances in the development of transition metal-catalyzed reactions using aryl diazonium salts are also reviewed. Together, these newly developed transformations significantly expand the synthetic chemist's repertoire of aromatic carbon-carbon and carbon-heteroatom bond forming methods using aryl diazonium precursors, providing powerful tools for the synthesis and modification of complex molecular scaffolds.

Ulanowska M., Olas B.

Eugenol is a phenolic aromatic compound obtained mainly from clove oil. Due to its known antibacterial, antiviral, antifungal, anticancer, anti-inflammatory and antioxidant properties, it has long been used in various areas, such as cosmetology, medicine, and pharmacology. However, high concentrations can be toxic. A dose of 2.5 mg/kg body weight is regarded as safe. This paper reviews the current state of knowledge regarding the activities and application of eugenol and its derivatives and recent research of these compounds. This review is based on information concerning eugenol characteristics and recent research from articles in PubMed. Eugenol remains of great interest to researchers, since its multidirectional action allows it to be a potential component of drugs and other products with therapeutic potential against a range of diseases.

Paul V., Rai D.C., T.S R.L., Srivastava S.K., Tripathi A.D.

Hoveyda A.H., Liu Z., Qin C., Koengeter T., Mu Y.

Ethylene is the byproduct of olefin metathesis reactions that involve one or more terminal alkenes. Its volatility is one reason why many cross-metathesis or ring-closing metathesis processes, which are reversible transformations, are efficient. However, because ethylene can be converted to a methylidene complex, which is a highly reactive but relatively unstable species, its concentration can impact olefin metathesis in other ways. In some cases, introducing excess ethylene can increase reaction rate owing to faster catalyst initiation. Ethylene and a derived methylidene complex can also advantageously inhibit substrate or product homocoupling, and/or divert a less selective pathway. In other instances, a methylidene's low stability and high activity may lead to erosion of efficiency and/or kinetic selectivity, making it preferable that ethylene is removed while being generated. If methylidene decomposition is so fast that there is little or no product formation, it is best that ethylene and methylidene complex formation is avoided altogether. This is accomplished by the use of di- or trisubstituted alkenes in stereoretentive processes, which includes adopting methylene capping strategy. Here, we analyze the different scenarios through which ethylene and the involvement of methylidene complexes can be manipulated and managed so that an olefin metathesis reaction may occur more efficiently and/or more stereoselectively.

Bilel H., Hamdi N., Fischmeister C., Bruneau C.

Groß J., Kühlborn J., Opatz T.

Xylochemicals, their utilisation in total synthesis and industrial scale production as well as their advantages over petroleum-based chemicals are highlighted.

Kühlborn J., Groß J., Opatz T.

This review highlights the utilization of biomass-derived building blocks in the total synthesis of natural products.

Tachibana K., Abe H.

Novel bio-based polyurethane (PU) was synthesized using vanillyl alcohol (VA) and lysine diisocyanate (LDI) by one-step solution polyaddition process with 1,4-butanediol (BD) as a co-monomer. The ratios of VA/BD were varied in the PU synthesis, and the properties of the obtained PUs with different vanillin contents were characterized. The molecular weights of PUs ranged from 32 to 44 kDa, and slight decreases of the molar masses were found as VA contents increased due to lower reactivity of phenolic-OH in VA structure with respect to aliphatic BD. The low VA contents in the PU provided flexible material with high elongation up to 600%, while the PU with high VA contents presented stiff characteristics with high glass transition temperature (Tg) of 70 °C and Young's Modulus of 1.8 GPa. Furthermore, thermo gravimetrical analysis (TGA) revealed lower thermal degradation temperature for the PU with higher VA contents, suggesting weaker thermal stability of the urethane bond connected with VA unit. Pyrolysis-gas chromatography/mass spectrometry and nuclear magnetic resonance were employed for the investigation of thermal degradation manner of the PU, which detected the formation of urea compound during the thermal degradation via scission of the urethane bond at benzyl-side accompanied by the elimination of carbon dioxide. Based on these findings, the effect of vanillin-derived diol structure on thermal properties for the resulted PUs was studied by using modified diols of diphenol (methoxyhydroquinone, MHQ) and stilbene-type bisphenol (vanillin-derived stilbene, VS). TGA, differential scanning calorimetry and wide angle X-ray diffraction results indicate that the modified diphenol and stilbene structure improve thermo-mechanical properties of the obtained PU. The introduction of the modified diols into PU increased thermal degradation temperature up to 220 °C. The VS-based PU showed highest Tg of approximately 120 °C due to the stiff structure and strong interaction of the stilbene.

Cai Y., Jiang T., Zhang Q., Li G., Cheng X.

Phenylpropanoid natural products containing a cinnamaldehyde motif were easily synthesized from allylarenes mediated by 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) oxidation. Representative examples described herein are five types of 4-hydroxycinnamaldehyde derivatives from monolignols biosynthesis, Boropinal C, and 7-methoxywutaifuranal from plant extracts. Especially, simple synthesis of 7-methoxywutaifuranal was exploited through selective mono-oxidation and subsequent isomerization–ring-closing-metathesis strategy.

Lancefield C.S., Wienk H.J., Boelens R., Weckhuysen B.M., Bruijnincx P.C.

Detailed structural analysis of industrial and model kraft lignins reveals an important new reaction intermediate and condensation pathway operating during kraft pulping.

Xu T., Jiang T., Han X., Xu Y., Qiao J.

A practical route to prepare (E)-cinnamaldehydes and natural products randainal and geranyloxy sinapyl aldehyde has been achieved via a metal-free DDQ-mediated oxidative transformation of allylarenes.

Gao R., Li Y., Kim H., Mobley J.K., Ralph J.

Quick and easy: An effective and efficient method was developed for the oxidation and cleavage of lignin model compounds using easily prepared and environmentally friendly MoO2Cl2(DMSO)2 as catalyst under microwave irradiation. Owing to the broad scope of this oxidation system and the insight gleaned with regard to its mechanism, this strategy could be adapted and applied in a general sense to the production of useful aromatic chemicals from phenolics and lignin.

Bailey G.A., Foscato M., Higman C.S., Day C.S., Jensen V.R., Fogg D.E.

The correlation between rapid initiation and rapid decomposition in olefin metathesis is probed for a series of fast-initiating, phosphine-free Ru catalysts: the Hoveyda catalyst HII, RuCl2(L)(═CHC6H4- o-O iPr); the Grela catalyst nG (a derivative of HII with a nitro group para to O iPr); the Piers catalyst PII, [RuCl2(L)(═CHPCy3)]OTf; the third-generation Grubbs catalyst GIII, RuCl2(L)(py)2(═CHPh); and dianiline catalyst DA, RuCl2(L)( o-dianiline)(═CHPh), in all of which L = H2IMes = N,N'-bis(mesityl)imidazolin-2-ylidene. Prior studies of ethylene metathesis have established that various Ru metathesis catalysts can decompose by β-elimination of propene from the metallacyclobutane intermediate RuCl2(H2IMes)(κ2-C3H6), Ru-2. The present work demonstrates that in metathesis of terminal olefins, β-elimination yields only ca. 25-40% propenes for HII, nG, PII, or DA, and none for GIII. The discrepancy is attributed to competing decomposition via bimolecular coupling of methylidene intermediate RuCl2(H2IMes)(═CH2), Ru-1. Direct evidence for methylidene coupling is presented, via the controlled decomposition of transiently stabilized adducts of Ru-1, RuCl2(H2IMes)Ln(═CH2) (Ln = py n'; n' = 1, 2, or o-dianiline). These adducts were synthesized by treating in situ-generated metallacyclobutane Ru-2 with pyridine or o-dianiline, and were isolated by precipitating at low temperature (-116 or -78 °C, respectively). On warming, both undergo methylidene coupling, liberating ethylene and forming RuCl2(H2IMes)Ln. A mechanism is proposed based on kinetic studies and molecular-level computational analysis. Bimolecular coupling emerges as an important contributor to the instability of Ru-1, and a potentially major pathway for decomposition of fast-initiating, phosphine-free metathesis catalysts.

Antonova Alexandra S., Zubkov Fedor I.

Catalytic olefin metathesis using Hoveyda-Grubbs type ruthenium complexes is a powerful tool for creating complex molecules possessing a variety of practically useful properties. This method is also applied for obtaining modern polymer materials from low-demand petroleum products. Among all ruthenium complexes containing five- or six-membered chelate rings, the commercially available HG-II catalyst is the most common. In addition, other Hoveyda-Grubbs type complexes, which include a Het→Ru donor–acceptor bond in the chelate ring, often exhibit metathesis activity equal to or superior to that of HG-II. This review considers second-generation N-heterocyclic ruthenium carbene Hoveyda-Grubbs type complexes with donor–acceptor bonds such as O→Ru, S→Ru, Se→Ru, N→Ru, P→Ru and Hal→Ru in the chelate ring. Methods of preparation, analysis of stability and catalytic activity of such complexes are compared, and examples of the application of these organometallic ruthenium derivatives in the synthesis of practically relevant products are provided. The literature from 2010 to 2023 is summarized, making this review useful for a broad audience of chemists working in heterocyclic and organometallic chemistry, as well as practitioners involved in the production of catalysts and polymers.The bibliography includes 174 references.

Rogalski S., Pietraszuk C.

The olefin metathesis reaction has found numerous applications in organic synthesis. This is due to a number of advantages, such as the tolerance of most functional groups and sterically demanding olefins. This article reviews recent advances in the application of the metathesis reaction, particularly the metathetic cyclization of dienes and enynes, in synthesis protocols leading to (hetero)aromatic compounds.

Kwesiga G., Greese J., Kelling A., Sperlich E., Schmidt B.

Isoflavones were synthesized via Suzuki–Miyaura coupling of 3-iodochromones and para-methoxybenzene- and para-phenolboronic acid. In our hands, conditions commonly used for similar cross couplings turned out to be unsuccessful or difficult to reproduce, for example, due to the unplanned partial cleavage of MOM-protecting groups. Using Pd(dba)2 as a precatalyst and tricyclohexylphosphine as an activating ligand, reliable cross-coupling conditions were identified. In all cases, notably higher yields of isoflavones were obtained with para-phenolboronic acid than with para-methoxybenzene boronic acid. This observation and the commercial availability of para-phenolboronic acid suggest that for the synthesis of the important 3′-prenyl- or 3′,5′-diprenylisoflavone substitution pattern a synthetic route that introduces the prenyl substituents after the Pd-catalyzed cross-coupling step, thereby avoiding laborious and protecting-group-intensive multistep syntheses of C-prenylated arene boronic acids, is advantageous.