Recent advances on the transition-metal-catalyzed synthesis of imidazopyridines: an updated coverage

• Gagandeep Kour Reen,
• Ashok Kumar and
• Pratibha Sharma

Beilstein J. Org. Chem. 2019, 15, 1612–1704, doi:10.3762/bjoc.15.165

Transient and intermediate carbocations in ruthenium tetroxide oxidation of saturated rings

• Manuel Pedrón,
• Laura Legnani,
• Maria-Assunta Chiacchio,
• Pierluigi Caramella,
• Tomás Tejero and
• Pedro Merino

Beilstein J. Org. Chem. 2019, 15, 1552–1562, doi:10.3762/bjoc.15.158

• carbocation. In the case of pyrrolidines, the carbocation is completely stabilized as an energy minimum in the form of an iminium ion and the reaction takes place in two steps. Keywords: alkanes; carbocations; DFT; oxidations; ruthenium tetroxide; Introduction Ruthenium-catalyzed oxidations [1][2] and, in

• , with P1, P2 and P3, respectively. A close inspection of the IRCs revealed a shoulder characteristic of a transient carbocation [23] which is more pronounced following the sequence R1 < R2 < R3. The preliminary analysis of the evolution of bonds along those IRCs further confirmed a high asynchronicity

• are broken and formed in two separate events, only a topological analysis of the ELF will provide the exact moment in which those events take place and provide evidences of the formation of a transient carbocation. The ELF analysis [39][40][72][73] allows calculation of the so-called basins of

Superelectrophilic carbocations: preparation and reactions of a substrate with six ionizable groups

• Sean H. Kennedy,
• Makafui Gasonoo and
• Douglas A. Klumpp

Beilstein J. Org. Chem. 2019, 15, 1515–1520, doi:10.3762/bjoc.15.153

• electrophiles based on the triarylmethyl cation scaffold (3–5, Scheme 1) [11][12]. These systems utilized pyridyl rings to produce increasing amounts of positive charge adjacent to the carbocation center. Both theoretical calculations and experimental results indicated that the carbocation center undergoes a

• studies of tri-, tetra-, and pentacationic systems [11][12]. This product, 10, is the result of charge migration involving the carbocation center and the phenyl group (vide infra). When compound 9 is treated with only superacid, the bis(pyrido[1,2-a]indole) 11 is formed as the major product. Likewise, the

• explained by mechanisms involving highly ionized intermediates. It is proposed that compound 9 initially reacts in excess superacid to give the tetracationic species 12 (Scheme 4). Protonation of the hydroxy groups leads to immediate ionization and formation of the carbocation centers. For related

Enantioselective Diels–Alder reaction of anthracene by chiral tritylium catalysis

• Qichao Zhang,
• Jian Lv and
• Sanzhong Luo

Beilstein J. Org. Chem. 2019, 15, 1304–1312, doi:10.3762/bjoc.15.129

• of a highly active carbocation Lewis acid catalyst. The stereocontrol potential of the chiral tritylium ion pair was demonstrated by its application in an enantioselective Diels–Alder reaction of anthracene. Keywords: anthracene; carbocation catalysis; Diels–Alder reaction; Fe(III)-based phosphate

• anion; tritylium salt; Introduction Carbocation Lewis acid catalysis has grown significantly over the last two decades [1][2][3][4][5][6][7][8][9][10][11][12][13]. The development of asymmetric carbocation catalysts has been long pursued but remains a challenging task. One strategy is to design and

• , the enantioselectivity was low in most cases. In addition, the synthetic efforts to access these chiral cations were generally non-trivial which limited their further development. Recently, we developed a chiral ion-pair strategy for asymmetric carbocation catalysis, with chiral trityl phosphate as

Multicomponent reactions (MCRs): a useful access to the synthesis of benzo-fused γ-lactams

• Edorta Martínez de Marigorta,
• Jesús M. de Los Santos,
• Ana M. Ochoa de Retana,
• Javier Vicario and
• Francisco Palacios

Beilstein J. Org. Chem. 2019, 15, 1065–1085, doi:10.3762/bjoc.15.104

• , that rearranges to afford cationic isoindolinone moiety 95. Then, benzene derivative 86 would react by a Friedel–Crafts type reaction to form diaryl γ-lactam 88. When ketone 89 is used instead of arene 86, the enol form would act as nucleophile and upon reaction with carbocation 95, compounds 90 could

• be isolated. Finally, cyclopropyl ketone 91 would first rearrange by copper catalysis and the so-obtained furane derivative 96 would add to the carbocation in 95, followed by Friedel–Crafts cyclization, thus generating the polycyclic isoindolinones 92 in a formal hetero [4 + 2] cycloaddition process

Stereochemical investigations on the biosynthesis of achiral (Z)-γ-bisabolene in Cryptosporangium arvum

• Jan Rinkel and
• Jeroen S. Dickschat

Beilstein J. Org. Chem. 2019, 15, 789–794, doi:10.3762/bjoc.15.75

• ; carbocation chemistry; enzyme mechanisms; nerolidyl diphosphate; terpenes; Introduction Given the enormous impact of chirality within biomolecules for all forms of life, it is fascinating to see how nature is able to maintain and reproduce stereochemical information. This concept largely involves the

Synthesis of polydicyclopentadiene using the Cp₂TiCl₂/Et₂AlCl catalytic system and thin-layer oxidation of the polymer in air

• Zhargolma B. Bazarova,
• Ludmila S. Soroka,
• Alex A. Lyapkov,
• Мekhman S. Yusubov and
• Francis Verpoort

Beilstein J. Org. Chem. 2019, 15, 733–745, doi:10.3762/bjoc.15.69

• cyclopentene double bond, one of the options may be the formation of the D units (Scheme 2) as a result of the transannular rearrangement of the growing carbocation [1]. As it was found, A-type units (up to 70%) dominate in the structure of polymers formed as a result of cationic polymerization. The number of

Catalyst-free assembly of giant tris(heteroaryl)methanes: synthesis of novel pharmacophoric triads and model sterically crowded tris(heteroaryl/aryl)methyl cation salts

• Rodrigo Abonia,
• Luisa F. Gutiérrez,
• Braulio Insuasty,
• Jairo Quiroga,
• Kenneth K. Laali,
• Chunqing Zhao,
• Gabriela L. Borosky,
• Samantha M. Horwitz and
• Scott D. Bunge

Beilstein J. Org. Chem. 2019, 15, 642–654, doi:10.3762/bjoc.15.60

• Supporting Information File 1). Assignments of the quaternary carbons including the formal carbocation centers were made by HMBC correlations. NMR data suggest that the positive charge is more effectively delocalized into the indole rings. The GIAO-NMR data show the same general trend, as evidenced by the

• silica gel, using hexane/EtOAc (7:3) as eluent. The desired products 9 along with the side-products 8 were isolated and quantified. General procedure for the synthesis of carbocation salts 10{4,4,8} and 10{4,4,11}. DDQ (2 equiv) was added to a solution of 3,3'-(arylmethylene)bis(1-methyl-1H-indoles) 8

Syntheses and chemical properties of β-nicotinamide riboside and its analogues and derivatives

• Mikhail V. Makarov and
• Marie E. Migaud

Beilstein J. Org. Chem. 2019, 15, 401–430, doi:10.3762/bjoc.15.36

• intermediate F, in which the C2′-acyl group participates in delocalization of the carbocation on the C1′ position. Nam approaches and attacks this carbocation from the less sterically hindered face to give the β-nicotinamide riboside derivatives (Figure 3). A very comprehensive and thorough discussion of the

Tandem copper and photoredox catalysis in photocatalytic alkene difunctionalization reactions

• Nicholas L. Reed,
• Madeline I. Herman,
• Vladimir P. Miltchev and
• Tehshik P. Yoon

Beilstein J. Org. Chem. 2019, 15, 351–356, doi:10.3762/bjoc.15.30

• carbocation reactivity is an intriguing paradigm for the expansion of photoredox chemistry towards net oxidative transformations. Current studies in our laboratory are investigating the further application of copper cocatalysts to a wide range of alternate photoredox-mediated oxidative transformations. a

Carbonylonium ions: the onium ions of the carbonyl group

• Daniel Blanco-Ania and
• Floris P. J. T. Rutjes

Beilstein J. Org. Chem. 2018, 14, 2568–2571, doi:10.3762/bjoc.14.233

• carbocation which contains at least one carbon atom having only six valence electrons [2]; Figure 1). The best way to represent these intermediates according to the literature is as a protonated (or organylated) carbonyl (1) [3][4], therefore the names given in the literature have mostly tried to represent

Glycosylation reactions mediated by hypervalent iodine: application to the synthesis of nucleosides and carbohydrates

• Yuichi Yoshimura,
• Hideaki Wakamatsu,
• Yoshihiro Natori,
• Yukako Saito and
• Noriaki Minakawa

Beilstein J. Org. Chem. 2018, 14, 1595–1618, doi:10.3762/bjoc.14.137

• its 1H NMR spectrum. The reaction of 96b also gave a similar result. In both reactions, the formation of cyclohexadiene 98 was observed. These results strongly supported that the reaction proceeded through the carbocation intermediate, as expected and depicted in Figure 4, since 98 was considered to

• developed. Since the Friedel–Crafts type reaction involved carbocation intermediate, the reaction always gave a mixture of products. Unfortunately, the reaction was not efficient. However, it is worthy that the oxidative coupling reaction contains a novel type of C–N bond formation and would help to

• of 2,4-bis(trimethylsilyl)uracil (29) occurs prior to an elimination. In path b, on the other hand, an allylic carbocation 110 formed from 108 reacts with 29. From the result that the reaction of 2,3-dihydrofuran gave side products generated from an intermediate resembling 107 (data not shown), it

Atom-economical group-transfer reactions with hypervalent iodine compounds

• Andreas Boelke,
• Peter Finkbeiner and
• Boris J. Nachtsheim

Beilstein J. Org. Chem. 2018, 14, 1263–1280, doi:10.3762/bjoc.14.108

• anion. Next, a SET from the enamide 45 to Fe(III) affords again Fe(II) and a further reaction with the radical A or A’ leads to the formation of carbocation B or iminium ion B’, respectively. Recombination of these positively charged intermediates with the azide anion finally affords the oxyazidated

Stereoselective nucleophilic addition reactions to cyclic N-acyliminium ions using the indirect cation pool method: Elucidation of stereoselectivity by spectroscopic conformational analysis and DFT calculations

• Koichi Mitsudo,
• Junya Yamamoto,
• Tomoya Akagi,
• Atsuhiro Yamashita,
• Masahiro Haisa,
• Kazuki Yoshioka,
• Hiroki Mandai,
• Koji Ueoka,
• Christian Hempel,
• Jun-ichi Yoshida and
• Seiji Suga

Beilstein J. Org. Chem. 2018, 14, 1192–1202, doi:10.3762/bjoc.14.100

• -generating reagent. For example, a pool of alkoxycarbenium ions can be rapidly generated by the reaction of thioacetals and ArS(ArSSAr)+ (Ar = p-FC6H4, Scheme 1) [11]. The cation pool method is advantageous over the conventional Lewis acid promoted generation of carbocation species because the reactive

• cation pool method as diaryldisulfides are the exclusive side products formed during carbocation generation. Thus, the reaction of acyliminium ion C1 with Me3Al afforded trans-1-(tert-butoxycarbonyl)-2-methyl-4-phenylpiperidine (2aa) in a good yield and virtually complete diastereoselectivity (cis/trans

One hundred years of benzotropone chemistry

• Arif Dastan,
• Haydar Kilic and
• Nurullah Saracoglu

Beilstein J. Org. Chem. 2018, 14, 1120–1180, doi:10.3762/bjoc.14.98

• Scheme 7. The acid-catalyzed cleavage of the oxo-bridge of 34 gives benzylic carbocation 35. Consequently, after deprotonation and dehydration, chloro benzotropilium cation 37 undergoes hydrolysis to give 4,5-benzotropone (11) in aqueous reaction media. Using o-xylylene dibromide (38) as starting

• -benzotropone (11) with dimethyl barbituric acid (62) and subsequent oxidative cyclization reaction using DDQ-Sc(OTf)3 or photoirradiation under aerobic conditions afforded 61+.BF4− (Scheme 12). The pKR+ value and reduction potential of the cation 61 were studied. The relative stability of a carbocation can be

• expressed by the pKR+ value, which is the affinity of the carbocation toward hydroxide ions. The pKR+ value for cation 61 was determined to be 4.7 spectrophotometrically. The reduction potential of the cation 61 was determined as −0.46 and −1.07 V by cyclic voltammetry in acetonitrile. The oxidizing ability

Anodic oxidation of bisamides from diaminoalkanes by constant current electrolysis

• Tatiana Golub and
• James Y. Becker

Beilstein J. Org. Chem. 2018, 14, 861–868, doi:10.3762/bjoc.14.72

• our case, the electro generated carbocation intermediate formed (RCONHCH2+) could meet with methanol (to form 'f') or water (to form 'g') preferentially at the electrode surface, dictating the ratio between products 'f' and 'g'. In order to increase the solubility of the substrates with a limited one

• formation of mono- and dimethoxylated amides is well-documented in the published literature [1][2][3][4][5][6][14][17][23]. It is generally accepted that the initial electron transfer forms an iminium cation radical followed by deprotonation and further oxidation to generate an iminium ion/carbocation that

Progress in copper-catalyzed trifluoromethylation

• Guan-bao Li,
• Chao Zhang,
• Chun Song and
• Yu-dao Ma

Beilstein J. Org. Chem. 2018, 14, 155–181, doi:10.3762/bjoc.14.11

• presence of copper reagent. Then, the trifluoromethyl radical reacts with substrates affording intermediate A, which may be oxidized to a carbocation B, followed by an oxidative dehydrogenation process delivering the target product. Copper-mediated/catalyzed direct trifluoromethylation of C(sp)–H

Stereochemical outcomes of C–F activation reactions of benzyl fluoride

• Neil S. Keddie,
• Pier Alexandre Champagne,
• Justine Desroches,
• Jean-François Paquin and
• David O'Hagan

Beilstein J. Org. Chem. 2018, 14, 106–113, doi:10.3762/bjoc.14.6

• benzylic carbocation and a formal equivalent of HF (which behaves in an autocatalytic manner as a stronger hydrogen bond donor than HFIP or TFA). Overall, there are three possible mechanistic pathways that these C–F activation reactions could follow: SN1, SN2, and a mixed SN1/SN2 pathway. Typically

• the actual attack of the nucleophile does not occur on the coordinated benzyl fluoride (A), or the fully solvated carbocation (D), as these scenarios would incur 100% or 0% enantiospecificity, respectively. Rather, the data suggest that attack occurs on a mixture of intimate (B) and solvent-separated

• of the TFA, and thus the more rapid generation of HF (vide infra), promotes a dissociative pathway via the solvent-separated ion pair. The greater ionic strength of the solution may also play a part in stabilising the partially dissociated carbocation. Pleasingly, decreasing the temperature (Table 3

Acid-catalyzed ring-opening reactions of a cyclopropanated 3-aza-2-oxabicyclo[2.2.1]hept-5-ene with alcohols

• Katrina Tait,
• Alysia Horvath,
• Nicolas Blanchard and
• William Tam

Beilstein J. Org. Chem. 2017, 13, 2888–2894, doi:10.3762/bjoc.13.281

• cleavage of the C–O bond as seen in path A, the free carbocation 28 would form in an SN1-like manner. The nucleophile could therefore attack from either the top or the bottom, forming products 26a and 29. Alternatively, in path B the oxygen atom could be protonated and undergo an SN2-like mechanism with

• the nucleophile attacking from the open top face seen in 30, forming sole product 26a. If a free carbocation was formed as shown in path A, both stereoisomers 26a and 29 should have been observed, which was not evident. Also, if a free carbocation was formed the product likely would have undergone

CF₃SO₂X (X = Na, Cl) as reagents for trifluoromethylation, trifluoromethylsulfenyl-, -sulfinyl- and -sulfonylation. Part 1: Use of CF₃SO₂Na

• Hélène Guyon,
• Hélène Chachignon and
• Dominique Cahard

Beilstein J. Org. Chem. 2017, 13, 2764–2799, doi:10.3762/bjoc.13.272

• give intermediate radical 65, which was oxidised by Mn(OAc)3 to form the carbocation 66 and, after deprotonation, the trifluoromethyl compounds (Scheme 39) [62]. The same group also reported a straightforward method for the trifluoromethylation of pyrimidinones and pyridinones under the same reaction

Herpetopanone, a diterpene from Herpetosiphon aurantiacus discovered by isotope labeling

• Xinli Pan,
• Nicole Domin,
• Sebastian Schieferdecker,
• Hirokazu Kage,
• Martin Roth and
• Markus Nett

Beilstein J. Org. Chem. 2017, 13, 2458–2465, doi:10.3762/bjoc.13.242

• configuration, thereby facilitating an intramolecular cyclization to a cyclodeca-1,5-diene by electrophilic attack of the allylic carbocation onto the corresponding double bond. A 1,3-hydride shift by Wagner–Meerwein rearrangement followed by another cyclization would then give rise to an octahydronaphthalene

Phosphonic acid: preparation and applications

• Charlotte M. Sevrain,
• Mathieu Berchel,
• Hélène Couthon and
• Paul-Alain Jaffrès

Beilstein J. Org. Chem. 2017, 13, 2186–2213, doi:10.3762/bjoc.13.219

• occur on the oxygen atom which is doubly bonded to the phosphorus atom. This protonation likely yields the intermediate I (Figure 8) which exists as two mesomeric forms. Then, two competitive mechanisms can occur. Intermediate I can lose a carbocation according to a SN1 mechanism whereas the second way

• could consist in a nucleophilic substitution (SN2) involving chloride ions as nucleophilic species to produce the intermediate II. Then, a repetition of this mechanism yields phosphonic acid. The preponderant route is likely governed by the stability of the carbocation and the steric hindrance around

Mechanochemical synthesis of small organic molecules

• Tapas Kumar Achar,
• Anima Bose and
• Prasenjit Mal

Beilstein J. Org. Chem. 2017, 13, 1907–1931, doi:10.3762/bjoc.13.186

• process to stabilize the carbocation species. This method was proved to be efficient by performing the reaction at 9.7 mmol scale to obtain 84% yield of the product. C–O bond formation reaction Carbon–oxygen (C–O) bonds are widely present in molecules containing ester, carbamate and amino acid, etc. [88

Oxidative dehydrogenation of C–C and C–N bonds: A convenient approach to access diverse (dihydro)heteroaromatic compounds

• Santanu Hati,
• Ulrike Holzgrabe and
• Subhabrata Sen

Beilstein J. Org. Chem. 2017, 13, 1670–1692, doi:10.3762/bjoc.13.162

• hydrogen is then transferred to TEMPO resulting in a radical–TEMPO–Cu intermediate Z. The benzyl radical in Z is then further oxidized to the corresponding carbocation which gets deprotonated to afford dihydroquinazoline Z´ along with Y´ and TEMPOH. Finally, Z´ is further oxidized to 73. TEMPOH oxidizes

Correction: Dynamic behavior of rearranging carbocations – implications for terpene biosynthesis

• Stephanie R. Hare and
• Dean J. Tantillo

Beilstein J. Org. Chem. 2017, 13, 1669–1669, doi:10.3762/bjoc.13.161

• Stephanie R. Hare Dean J. Tantillo Department of Chemistry, University of California–Davis, 1 Shields Avenue, Davis, CA 95616, USA 10.3762/bjoc.13.161 Keywords: carbocation; density functional theory; dynamics; mechanism; terpene; The originally published Figure 6 had several mis-drawn