Electron-rich triarylphosphines as nucleophilic catalysts for oxa-Michael reactions

• Susanne M. Fischer,
• Simon Renner,
• A. Daniel Boese and
• Christian Slugovc

Beilstein J. Org. Chem. 2021, 17, 1689–1697, doi:10.3762/bjoc.17.117

• formed by the conjugate addition of the phosphine to the Michael acceptor, is believed to be protonated by the alcohol forming the actual catalytically active species namely ion pair ii, consisting of a phosphonium cation and an alkoxide. The alkoxide in ii then reacts with another electrophile

• aromatic rings with methoxy groups increases the pKa value from 1.31 (TPP) to 4.20 (TMTPP) (Figure 1). Methyl cation affinities (MCA) which can be used as descriptors for the nucleophilicity of a compound were calculated by Lindner et al. who suggested TMTPP (651.0 kJ/mol) to be a stronger Lewis base than

Chemical approaches to discover the full potential of peptide nucleic acids in biomedical applications

• Nikita Brodyagin,
• Martins Katkevics,
• Venubabu Kotikam,
• Christopher A. Ryan and
• Eriks Rozners

Beilstein J. Org. Chem. 2021, 17, 1641–1688, doi:10.3762/bjoc.17.116

Methodologies for the synthesis of quaternary carbon centers via hydroalkylation of unactivated olefins: twenty years of advances

• Thiago S. Silva and
• Fernando Coelho

Beilstein J. Org. Chem. 2021, 17, 1565–1590, doi:10.3762/bjoc.17.112

• those with less activated phenyl unities. However, electron-withdrawing groups were not suitable substrates for this reaction, most probably because of the lower stability of the cyclohexadienyl cation intermediate formed (see Scheme 31, intermediate D). A replacement of the phenyl moiety with N

• reaction rate-determining step in a radical mechanism in which the alkane participates in both the initiation and propagation steps of the radical chain (Scheme 37C). Photoinduced electron transfer Under photoinduced electron transfer (PET) conditions, olefins generate cation radical species that are

• useful intermediates in polyene cyclization cascades [78][120]. Usually, the cation formed (see Scheme 38B) is trapped in an anti-Markovnikov fashion by a nucleophile, such as water or alcohol, present in the medium, while the radical engages in the cyclization cascade. Interestingly, in 2015, Luo

Breaking paracyclophane: the unexpected formation of non-symmetric disubstituted nitro[2.2]metaparacyclophanes

• Suraj Patel,
• Tyson N. Dais,
• Paul G. Plieger and
• Gareth J. Rowlands

Beilstein J. Org. Chem. 2021, 17, 1518–1526, doi:10.3762/bjoc.17.109

• , protonation might not be the key step, and the highly oxidizing nature of nitration conditions that can lead to the formation of a cationic intermediate via a radical cation might control this reaction [68]. A possible mechanism for the formation of 5 and 6 starts with protonation of 1 give the Wheland

Icilio Guareschi and his amazing “1897 reaction”

• Gian Cesare Tron,
• Alberto Minassi,
• Giovanni Sorba,
• Mara Fausone and
• Giovanni Appendino

Beilstein J. Org. Chem. 2021, 17, 1335–1351, doi:10.3762/bjoc.17.93

• of the first substrate gave a mixture of but-1-ene and but-3-en-1-ol, in accordance with the loss of the C-4 alkyl group as a radical, since the formation of a cylopropylmethyl cation would have generated a mixture of cyclobutanol and cyclopropylmethanol. The second substrate gave a mixture of

Photoinduced post-modification of graphitic carbon nitride-embedded hydrogels: synthesis of 'hydrophobic hydrogels' and pore substructuring

• Cansu Esen and
• Baris Kumru

Beilstein J. Org. Chem. 2021, 17, 1323–1334, doi:10.3762/bjoc.17.92

• two scenarios. The synthesis of ‘hydrophobic hydrogel’ is reported and its application in delayed cation delivery was investigated. Furthermore, pores of the initial hydrogel were modified by the formation of a secondary polymer network. Such a facile and straightforward synthetic protocol to

• physical adsorption/releasing performance being under the control of intermolecular interactions. Hydrophobization might enhance the hydrophobic interactions between the dye core and the polymer network, thus releasing can be achieved in a longer term. As a last simulative experiment, cation (K+, Ca2+, Mg2

• +) releasing performances of HGCM and HGCM-vTA after an overnight immersion in separately prepared stock solutions were analysed via ICP-OES (Supporting Information File 1, Figure S4b). HGCM shows higher cation releasing which might be driven by osmosis, however, HGCM-vTA retains cations thus offering a slower

A comprehensive review of flow chemistry techniques tailored to the flavours and fragrances industries

• Guido Gambacorta,
• James S. Sharley and
• Ian R. Baxendale

Beilstein J. Org. Chem. 2021, 17, 1181–1312, doi:10.3762/bjoc.17.90

N-tert-Butanesulfinyl imines in the asymmetric synthesis of nitrogen-containing heterocycles

• Joseane A. Mendes,
• Paulo R. R. Costa,
• Miguel Yus,
• Francisco Foubelo and
• Camilla D. Buarque

Beilstein J. Org. Chem. 2021, 17, 1096–1140, doi:10.3762/bjoc.17.86

• state, the metal cation of the enolate is being coordinated with both nitrogen and oxygen atoms of the sulfinimine [25][26] (Scheme 3). In 1999, Ellman and co-workers described the reduction of sulfinyl imines using sodium borohydride (NaBH4) [34] or o ʟ-selectride [35]. Davis–Ellman transition state

• crystallographic analysis, and it was found that the configuration of the newly generated stereocenter was R. Huang and co-workers proposed a cyclic transition state determined by the coordination of the nitrogen of the imine, and the oxygen atoms of the sulfoxonium and sulfinyl units to the sodium cation, which

Synthesis of multiply fluorinated N-acetyl-D-glucosamine and D-galactosamine analogs via the corresponding deoxyfluorinated glucosazide and galactosazide phenyl thioglycosides

• Vojtěch Hamala,
• Lucie Červenková Šťastná,
• Martin Kurfiřt,
• Petra Cuřínová,
• Martin Dračínský and
• Jindřich Karban

Beilstein J. Org. Chem. 2021, 17, 1086–1095, doi:10.3762/bjoc.17.85

• coordination of ZnI2. When the α-anomer of thioglycoside 17 was separately subjected to the reaction conditions, the byproduct 20 started to form in trace amounts in accordance with the suggested mechanism. The ring contraction may involve formation of a transient oxiranium cation as suggested in Scheme 2 [42

Synthetic accesses to biguanide compounds

• Oleksandr Grytsai,
• Cyril Ronco and
• Rachid Benhida

Beilstein J. Org. Chem. 2021, 17, 1001–1040, doi:10.3762/bjoc.17.82

• heating for the synthesis of a series of alkyl- and arylbiguanides to study the drug uptake of biguanide derivatives by organic cation transporters OCT1 and OCT2. The reaction conditions involved refluxing acetonitrile overnight with 1.5 equivalents of TMSCl, resulting in the desired products in variable

Metal-free glycosylation with glycosyl fluorides in liquid SO₂

• Krista Gulbe,
• Jevgeņija Lugiņina,
• Edijs Jansons,
• Artis Kinens and
• Māris Turks

Beilstein J. Org. Chem. 2021, 17, 964–976, doi:10.3762/bjoc.17.78

• characterized by lithium cation basicity (LiCB) liquid SO2 (76.3) is similar to DCM (83) [67]. Thus, liquid SO2 could be classified as a non-coordinating solvent that unlikely coordinates to the oxocarbenium ion intermediate and affects the glycosylation stereoselectivity [1]. As a result, we can conclude that

Prins cyclization-mediated stereoselective synthesis of tetrahydropyrans and dihydropyrans: an inspection of twenty years

• Asha Budakoti,
• Pradip Kumar Mondal,
• Prachi Verma and
• Jagadish Khamrai

Beilstein J. Org. Chem. 2021, 17, 932–963, doi:10.3762/bjoc.17.77

• % yield and <5% ee. The poor enantiomeric excess of the product 23 indicates that the racemization takes place during the course of the reaction. It was explained that the reason for the loss of optical purity was due to the formation of a benzylic cation, which is stabilized by the electron-rich aromatic

• cyclization of substituted cyclopropylcarbinol 94 to 2,4,6-trisubstituted tetrahydropyran 97 was reported by Yadav and Kumar [55]. In this reaction, a homoallylic cation was generated from 94 by the opening of the cyclopropane ring in the presence of TFA, which upon reacting with an aldehyde delivered 2,4,6

• -trimethylsilylalkenyl cation 242 formed by the Grob-type fragmentation (Scheme 57), which was trapped by the subsequent attack of the halide anion, leading to the formation of Prins product 244. On the basis of theoretical calculations, the authors could conclude factors controlling the alkyne Prins cyclization over

Microwave-assisted multicomponent reactions in heterocyclic chemistry and mechanistic aspects

• Shivani Gulati,
• Stephy Elza John and
• Nagula Shankaraiah

Beilstein J. Org. Chem. 2021, 17, 819–865, doi:10.3762/bjoc.17.71

Synthetic reactions driven by electron-donor–acceptor (EDA) complexes

• Zhonglie Yang,
• Yutong Liu,
• Kun Cao,
• Xiaobin Zhang,
• Hezhong Jiang and
• Jiahong Li

Beilstein J. Org. Chem. 2021, 17, 771–799, doi:10.3762/bjoc.17.67

• additive to give corresponding thiophene radical 76 and aniline radical cation under irradiation with light. Then, 76 reacted with 73, giving rise to corresponding radical 77. Finally, product 74 was given via hydrogen atom transfer (Scheme 26). In contrast to (hetero)aryl halides with indispensable

• occurs, giving radical 146 and radical cation 147, respectively. Finally, radical 146 undergoes decarboxylation to afford an aryl radical and then combines with radical cation 147, yielding product 144 (Scheme 50). It should be noted that only when NHPI is firstly activated can it turn into an electron

Synthesis, structural characterization, and optical properties of benzo[f]naphtho[2,3-b]phosphoindoles

• Mio Matsumura,
• Takahiro Teramoto,
• Masato Kawakubo,
• Masatoshi Kawahata,
• Yuki Murata,
• Kentaro Yamaguchi,
• Masanobu Uchiyama and
• Shuji Yasuike

Beilstein J. Org. Chem. 2021, 17, 671–677, doi:10.3762/bjoc.17.56

• approximately 50 nm longer than that of our oxide 3 [16]. The fluorescence wavelength, including the maximum emission (λem), and the quantum yield depend on the nature of the P-modification. Phosphine oxide 3, cation 5, and boron complex 6 emitted blue fluorescence in the visible-light region, with λem at 395

• –426 nm (Table 2). P-methylated cation 5 exhibited the longest wavelength and the highest quantum yield. The electrochemical properties of benzonaphthophosphoindoles were investigated by using cyclic voltammetry, and the electrochemical data are summarized in Table 3 and Figure S4, Supporting

Valorisation of plastic waste via metal-catalysed depolymerisation

• Francesca Liguori,
• Carmen Moreno-Marrodán and
• Pierluigi Barbaro

Beilstein J. Org. Chem. 2021, 17, 589–621, doi:10.3762/bjoc.17.53

• and the possible product contamination by homogeneous catalysts [199]. The conventional catalysts for this reaction are EG-soluble metal acetates, the activity of which showed a decrease in the order Zn2+ > Mn2+ > Co2+, which was attributed to the diverse interaction between the metal cation promoter

• , based on the activation of the C=O bond by the IL Lewis acid cation and of the hydroxy group in EG by the IL anion (Scheme 6). The IL catalysts could be easily separated by distillation and reused up to six times with no significant efficiency drop. Catalytic glycolysis by heterogenised, metallated

Amino- and polyaminophthalazin-1(2H)-ones: synthesis, coordination properties, and biological activity

• Zbigniew Malinowski,
• Emilia Fornal,
• Agata Sumara,
• Renata Kontek,
• Karol Bukowski,
• Beata Pasternak,
• Dariusz Sroczyński,
• Joachim Kusz,
• Magdalena Małecka and
• Monika Nowak

Beilstein J. Org. Chem. 2021, 17, 558–568, doi:10.3762/bjoc.17.50

• HCl/CO or 2) with the loss of HCl/N2 (≈64 Da). In both cases, the pyridazinone moiety undergoes degradation to different ions 11, 12 with the same m/z = 287.3 and 289.3 Da. In the next stage, the copper cation is detached to form ions 13 and 14 (224.3 Da) indicating that the copper is well fitted into

Unexpected rearrangements and a novel synthesis of 1,1-dichloro-1-alkenones from 1,1,1-trifluoroalkanones with aluminium trichloride

• Beatrice Lansbergen,
• Catherine S. Meister and
• Michael C. McLeod

Beilstein J. Org. Chem. 2021, 17, 404–409, doi:10.3762/bjoc.17.36

• ), a single product was isolated from the reaction that, after careful elucidation (see Supporting Information File 1), was identified as the bicycle 17. This compound could potentially arise via an initial conversion to the dichloroalkyl cation 18, followed by a ring closure and elimination of HCl to

Helicene synthesis by Brønsted acid-catalyzed cycloaromatization in HFIP [(CF₃)₂CHOH]

• Takeshi Fujita,
• Noriaki Shoji,
• Nao Yoshikawa and
• Junji Ichikawa

Beilstein J. Org. Chem. 2021, 17, 396–403, doi:10.3762/bjoc.17.35

• bisacetal precursors, which were readily prepared through C–C bond formation by Suzuki–Miyaura coupling. This cyclization was efficiently realized by a catalytic amount of trifluoromethanesulfonic acid (TfOH) in a cation-stabilizing solvent, 1,1,1,3,3,3-hexafluoropropan-2-ol (HFIP), which readily allowed

• types of helicenes using a sequence of (i) Suzuki–Miyaura coupling of acetal-containing components and (ii) catalytic cycloaromatization with the aid of a cation-stabilizing fluoroalcohol, HFIP, as solvent. Particularly, helicene synthesis using tandem cyclization of bisacetals with teraryl structures

Mesoionic tetrazolium-5-aminides: Synthesis, molecular and crystal structures, UV–vis spectra, and DFT calculations

• Vladislav A. Budevich,
• Sergei V. Voitekhovich,
• Alexander V. Zuraev,
• Vadim E. Matulis,
• Vitaly E. Matulis,
• Alexander S. Lyakhov,
• Ludmila S. Ivashkevich and
• Oleg A. Ivashkevich

Beilstein J. Org. Chem. 2021, 17, 385–395, doi:10.3762/bjoc.17.34

• -regioisomers (152–156 and 143.4 ppm for 1-methyl-5H-tetrazole) [31]. The de-tert-butylation selectivity observed for compound 11b can be explained by the higher stability of the t-Bu cation versus the Me cation. Crystal structures The mesoionic compounds 8a, 10, 11a, and salt 9 were characterized by single

• cation, shown in Figure 2, and two bromide anions. The structure of the cation is close to C2 symmetry, with an rms deviation of its non-hydrogen atoms from ideal positions of 0.1320 Å. In the salt 9, the lengths of the tetrazole ring bonds and the exocyclic C–N bond are given in Table 3 together with

• derivatives. Molecules of compounds 8a, 10, 11a, and the bistetrazolium cation 9, with displacement ellipsoids drawn at the 50% probability level. The hydrogen atoms are shown as spheres of arbitrary radii. The atom numbering is done for the asymmetric unit. Experimental (a) and TD-tHCTHhyb/6-311+G(2d,p

CF₃-substituted carbocations: underexploited intermediates with great potential in modern synthetic chemistry

• Anthony J. Fernandes,
• Armen Panossian,
• Bastien Michelet,
• Agnès Martin-Mingot,
• Frédéric R. Leroux and
• Sébastien Thibaudeau

Beilstein J. Org. Chem. 2021, 17, 343–378, doi:10.3762/bjoc.17.32

• material sciences [14][15][16][17]. In this context, deciphering the impact that can be exerted by the trifluoromethyl group on a cation and the associated consequences when facing the challenge of developing innovative synthetic methods are the subjects of this review. Review Quantitative parameters

• alkylcarbenium ions, in which a higher electronic contribution from neighboring substituents is required. Detailed ab initio studies have been focused on the stability of the CF3CH2+ cation and provide pieces of thoughts on the origins of the stabilizing interactions in α-(trifluoromethyl)carbenium ions. The

• optimization of the geometry for CF3CH2+ at the STO-3G level led to an energy minimum, in which one of the fluorine atoms is significantly closer to the positive carbon center (Figure 1, top, θ = 101°) [24]. However, exactly the same structural distortion was calculated for the ethyl cation. Furthermore, the

Hydrazides in the reaction with hydroxypyrrolines: less nucleophilicity – more diversity

• Dmitrii A. Shabalin,
• Evgeniya E. Ivanova,
• Igor A. Ushakov,
• Elena Yu. Schmidt and
• Boris A. Trofimov

Beilstein J. Org. Chem. 2021, 17, 319–324, doi:10.3762/bjoc.17.29

• (Scheme 4). A proposed mechanism for the recyclization of hydroxypyrrolines 1 with hydrazides 2 is shown in Scheme 5. The protonation of the starting hydroxypyrroline 1 with TFA leads to the formation of cation A, which reacts with hydrazide 2 to give the pyrrolidine derivative B. The latter undergoes a

The preparation and properties of 1,1-difluorocyclopropane derivatives

• Kymbat S. Adekenova,
• Peter B. Wyatt and
• Sergazy M. Adekenov

Beilstein J. Org. Chem. 2021, 17, 245–272, doi:10.3762/bjoc.17.25

• - or dibromomethyl carbanions. The so-obtained carbanions form lipophilic ion pairs with the catalyst cation and move into the organic phase, where they react with dibromodifluoromethane. Consequently, carbon tetrabromide (or bromoform) and the ion pair CBrF2−N+Bu4 are formed. The ion pair decomposes

• by a dipolar resonance structure containing the 2-fluoroallyl cation and fluoride anion (Scheme 39). A subsequent comparison of the rates of racemization with those of epimerization confirmed experimentally the preference for coupled disrotatory motions in the opening and closing of 2,3-dialkyl-1,1

• opening was attributed to the stabilization of the developing cationic center by the +M effect of the fluorine atoms. The formation of the 2,2-difluorohomoallyl cation or 3,3-difluorocyclobutyl cation did not occur as a result of the strong destabilization by the −I effect of the fluorine atoms [96]. On

Multiswitchable photoacid–hydroxyflavylium–polyelectrolyte nano-assemblies

• Alexander Zika and
• Franziska Gröhn

Beilstein J. Org. Chem. 2021, 17, 166–185, doi:10.3762/bjoc.17.17

• MicroCal ITC data analysis software for Origin 7.0. Top: pelargonidin cation (Flavy) and network of chemical reactions; bottom: corresponding UV–vis spectra of the different states of Flavy. Characterization of Flavy: a) 1H NMR spectrum at pH 7.0 (form A) before and after irradiation; b) 13C NMR spectrum

Novel library synthesis of 3,4-disubstituted pyridin-2(1H)-ones via cleavage of pyridine-2-oxy-7-azabenzotriazole ethers under ionic hydrogenation conditions at room temperature

• Romain Pierre,
• Anne Brethon,
• Sylvain A. Jacques,
• Aurélie Blond,
• Sandrine Chambon,
• Sandrine Talano,
• Catherine Raffin,
• Branislav Musicki,
• Claire Bouix-Peter,
• Loic Tomas,
• Gilles Ouvry,
• Rémy Morgentin,
• Laurent F. Hennequin and
• Craig S. Harris

Beilstein J. Org. Chem. 2021, 17, 156–165, doi:10.3762/bjoc.17.16

• a sacrificial metal cation, preventing Pd ligation and permitting the reaction to turnover resulting in albeit poor isolated yields of 9d,e (Table 2). All attempts to carry out the reaction using only CuI failed, confirming that Pd was necessary to catalyze this transformation. It was preferential