The cyclopropylcarbinyl route to γ-silyl carbocations

• Xavier Creary

Beilstein J. Org. Chem. 2019, 15, 1769–1780, doi:10.3762/bjoc.15.170

• ; rearrangement; silicon; Introduction Carbocations, positively charged trivalent carbon compounds and reactive intermediates, have continued to fascinate chemists since the early discoveries of tropylium [1][2] and trityl [3][4][5][6][7] salts. Many of the giants of organic chemistry during the last century

• contributed heavily to the development of carbocation chemistry. This article will deal with three types of carbocations that have been of intense and fundamental interest over the years, i.e., cyclopropylcarbinyl cations, electron-deficient cations, and silyl substituted carbocations. A brief overview of

• these types of carbocations is warranted. Cyclopropylcarbinyl cations are an extensively studied system [8][9]. Initial interest was derived from the fact that both cyclopropylcarbinyl and cyclobutyl substrates 1 and 2, where X represents diazonium ion [10][11], chloride [10], or naphthalenesulfonate

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

• -mediated oxidation of tertiary amines [26] by trapping them with cyanide anion [27][28]. These results point out the formation of transient carbocations III that can be stabilized by the presence of heteroatoms in the alpha position. The formation of transient carbocations do not contradict, necessarily

• can lead to different products in a ratio that depends on reaction dynamics [31][32][33]. The study of molecular dynamics trajectories has allowed characterization of ambimodal transition states in reactions involving carbocations [34][35]. We have demonstrated computationally the presence 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

• conversions, computational and experimental data indicated that the protonated hydroxy groups (oxonium ions) are not persistent intermediates, but rather cleavage of the carbon–oxygen bond is almost instantaneous [12]. It is assumed that ionization to the carbocations occurs in a stepwise process, first

• from diol 9. Proposed mechanisms leading to products 10 and 11. Products and relative yields from the reaction of alcohol 18 with CF3SO3H and C6H6 [12]. Comparison of superelectrophilic carbocations (3–5 and 14) and their chemistry. DFT calculated relative energies of pentacations 16 and 21 [14

Cyclobutane dication, (CH₂)₄²⁺: a model for a two-electron four-center (2e-4c) Woodward–Hoffmann frozen transition state

• G. K. Surya Prakash and
• Golam Rasul

Beilstein J. Org. Chem. 2019, 15, 1475–1479, doi:10.3762/bjoc.15.148

• carbocations, could not be interpreted by the classical tetravalency bonding concept [2][3] also requiring the engagement of a 2e-3c bond as suggested by Olah in 1969 [4][5]. A compelling number and huge array of carbocations involving higher coordinate carbon is by now realized by experimental studies

• . Hypercarbon chemistry covers in addition to carbocations also carboranes, carbon-bridged organometallics, carbonyl clusters, along with others. The rapidly evolving field has been extensively surveyed [6]. In comparison, structures of the carbocations involving a two-electron four-center (2e-4c) bond are rare

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

• synthesize stabilized chiral carbocations with chirality installed onto their backbones. Pioneering efforts along this line by Kagan, Sammakia, and Chen have shown that chiral catalysis with such chiral carbocations was indeed plausible to achieve stereocontrol (Scheme 1a). [14][15][16][17][18][19]. However

• with weakly coordinating metal-based phosphate anion. a) The reaction with 9,10-dimethylanthracene (3b). b) Gram-scale reaction of 3a and 4k, and transformation of cycloadduct 5k. Screening and optimization for the asymmetric catalyzed Diels–Alder reaction of anthracene by carbocations. Scope for the

• asymmetric catalyzed Diels–Alder reaction of anthracene (3a) with ketoesters 4 by carbocations. Supporting Information Supporting Information File 282: Experimental procedures and characterization data of all products, copies of 1H and 13C NMR, IR, HRMS, and HPLC spectra of all compounds. Acknowledgements

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

• enzymatic step [1][2][3]. With this approach, nature makes perfect use of the versatile chemistry of carbocations with its hydride or proton shifts and Wagner–Meerwein rearrangements leading to a large variety of possible structures. Among terpenoid natural products, achiral compounds are rarely found, but

Sigmatropic rearrangements of cyclopropenylcarbinol derivatives. Access to diversely substituted alkylidenecyclopropanes

• Guillaume Ernouf,
• Jean-Louis Brayer,
• Christophe Meyer and
• Janine Cossy

Beilstein J. Org. Chem. 2019, 15, 333–350, doi:10.3762/bjoc.15.29

• resulting α-allenic carbocations 24a–c (Scheme 14) [48]. As a complementary strategy, our group examined the [3,3]-sigmatropic rearrangement of cyanates derived from cyclopropenylcarbinols [53]. The allyl cyanate to isocyanate rearrangement displays many interesting features such as the possibility to

Synthesis of new tricyclic 5,6-dihydro-4H-benzo[b][1,2,4]triazolo[1,5-d][1,4]diazepine derivatives by [3⁺ + 2]-cycloaddition/rearrangement reactions

• Lin-bo Luan,
• Zi-jie Song,
• Zhi-ming Li and
• Quan-rui Wang

Beilstein J. Org. Chem. 2018, 14, 1826–1833, doi:10.3762/bjoc.14.155

• competes with the aliphatic side. It has been reported that the migratory tendency of substituents prefer those with a higher ability to accommodate the respective carbocations [46]. As anticipated, it was the phenyl moiety not the aliphatic moiety that moves from C(3) to N(2) to furnish the isolated

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

Cationic Pd(II)-catalyzed C–H activation/cross-coupling reactions at room temperature: synthetic and mechanistic studies

• Takashi Nishikata,
• Alexander R. Abela,
• Shenlin Huang and
• Bruce H. Lipshutz

Beilstein J. Org. Chem. 2016, 12, 1040–1064, doi:10.3762/bjoc.12.99

• with arylboronic [132][133][134], arylbismuth [135], and arylsilicon [136] reagents. Although carbocations react with arenes through electrophilic aromatic hydrogen substitution in a Friedel–Crafts reaction, the potential for metal cations to participate in similar chemistry has been far less widely

Dynamic behavior of rearranging carbocations – implications for terpene biosynthesis

• Stephanie R. Hare and
• Dean J. Tantillo

Beilstein J. Org. Chem. 2016, 12, 377–390, doi:10.3762/bjoc.12.41

• Stephanie R. Hare Dean J. Tantillo Department of Chemistry, University of California–Davis, 1 Shields Avenue, Davis, CA 95616, USA 10.3762/bjoc.12.41 Abstract This review describes unexpected dynamical behaviors of rearranging carbocations and the modern computational methods used to elucidate

• rings that are transformed in only one or two enzyme-promoted reactions. These reactions involve generation of a carbocation by protonation or loss of a diphosphate group followed by cyclization, alkyl shift, hydride shift and/or proton transfer reactions to generate new, more complex, carbocations

• . Ultimately these carbocations are either trapped by a nucleophile (e.g., water, diphosphate) or deprotonated to form alkenes. The details of terpene-forming carbocation cyclization/rearrangement processes have been of interest for decades [1][2][3][4][5][6]. Although much has been learned, new observations

Smart molecules for imaging, sensing and health (SMITH)

• Bradley D. Smith

Beilstein J. Org. Chem. 2015, 11, 2540–2548, doi:10.3762/bjoc.11.274

• . David Kelly and worked on an NMR project on the structure of carbocations. I was lucky enough to stumble upon an unusual naphthalenium rearrangement process and I worked hard enough to earn co-authorship of a full paper in the Journal of the American Chemical Society [1]. As the year progressed I began

Investigation of the role of stereoelectronic effects in the conformation of piperidones by NMR spectroscopy and X-ray diffraction

• Cesar Garcias-Morales,
• David Ortegón-Reyna and
• Armando Ariza-Castolo

Beilstein J. Org. Chem. 2015, 11, 1973–1984, doi:10.3762/bjoc.11.213

• carbanions [13][14], carbocations [15][16][17][18], and free radicals [19][20][21] which has been explained by negative (nX→σ*C–Y) or positive hyperconjugation (σC–Y→π* or p). Hyperconjugation is commonly described as the interaction between electronic orbitals where one filled orbital (donor) interacts with

Attempts to prepare an all-carbon indigoid system

• Şeref Yildizhan,
• Henning Hopf and
• Peter G. Jones

Beilstein J. Org. Chem. 2015, 11, 363–372, doi:10.3762/bjoc.11.42

• converted into cross-conjugated organic salts during the color-generating process. This is illustrated by indigo (1, Scheme 1) and its derivatives (e.g., Tyrian purple) and the triphenylmethane-derived carbocations. Starting from the generalized indigoid structure 2, the role of the heteroatoms X and Y can

Switching the reaction pathways of electrochemically generated β-haloalkoxysulfonium ions – synthesis of halohydrins and epoxides

• Akihiro Shimizu,
• Ryutaro Hayashi,
• Yosuke Ashikari,
• Toshiki Nokami and
• Jun-ichi Yoshida

Beilstein J. Org. Chem. 2015, 11, 242–248, doi:10.3762/bjoc.11.27

• + or I+ and DMSO to unsymmetrically substituted olefins 2c and 2d regioselectively gave bromohydrins as single regioisomers (Table 2, entries 5–10). The regioselectivity of the products can be explained by the stability of carbocations (benzyl > secondary > primary). In the case of terminal alkene 2c

Cross-dehydrogenative coupling for the intermolecular C–O bond formation

• Igor B. Krylov,
• Vera A. Vil’ and
• Alexander O. Terent’ev

Beilstein J. Org. Chem. 2015, 11, 92–146, doi:10.3762/bjoc.11.13

The Shono-type electroorganic oxidation of unfunctionalised amides. Carbon–carbon bond formation via electrogenerated N-acyliminium ions

• Alan M. Jones and
• Craig E. Banks

Beilstein J. Org. Chem. 2014, 10, 3056–3072, doi:10.3762/bjoc.10.323

• between anodically generated carbocations occurs by mass transfer diffusion across the liquid–liquid contact area. Therefore, one liquid can be oxidized and the other liquid containing the nucleophile can intercept the N-acyliminium ion formed in the microflow reactor (when the two sides of the channel

Automated solid-phase peptide synthesis to obtain therapeutic peptides

• Veronika Mäde,
• Sylvia Els-Heindl and
• Annette G. Beck-Sickinger

Beilstein J. Org. Chem. 2014, 10, 1197–1212, doi:10.3762/bjoc.10.118

• deprotected and conditions to remove both, side-chain protecting groups and the peptide from the resin, have to be used (Scheme 2) [20]. The last step of SPPS should be performed in the presence of scavengers to trap highly reactive carbocations that are formed during the cleavage procedure and that might

A new intermediate in the Prins reaction

• Shinichi Yamabe,
• Takeshi Fukuda and
• Shoko Yamazaki

Beilstein J. Org. Chem. 2013, 9, 476–485, doi:10.3762/bjoc.9.51

• , which leads to formation of the 1,3-diol and the subsequent allylic alcohol. In (ii), the second H2C=O is bound to the cation center, leading to formation of the 1,3-dioxane. While tertiary carbocations might intervene, the presence or absence of secondary ones would be critical in the aqueous media

• . This is because the water cluster has high nucleophilic strength and tends to make C–O bonds to form alcohols , overcoming the intervention of carbocations. In this respect, the mechanism depicted in Scheme 5 needs to be examined by the use of some alkenes theoretically. A variety of protic acids and

Caryolene-forming carbocation rearrangements

• Quynh Nhu N. Nguyen and
• Dean J. Tantillo

Beilstein J. Org. Chem. 2013, 9, 323–331, doi:10.3762/bjoc.9.37

• encountered in the very first step involving carbocations. We were unable to locate a minimum for B in a productive conformation, despite the fact that alternative conformers of this secondary carbocation had been found to be involved in pathways to pentalenene and presilphiperfolanol [18][19][20][21][22

• represents the inherent reactivity of the carbocations involved in the formation of E, but how might the mechanism change if we allowed for an enzymatic base to be involved? Would the same unusual sequence of events associated with the intramolecular proton transfer persist? Would a lower energetic pathway

The chemistry of bisallenes

• Henning Hopf and
• Georgios Markopoulos

Beilstein J. Org. Chem. 2012, 8, 1936–1998, doi:10.3762/bjoc.8.225

• are summarized under this title, whether they involve carbocations or carbanions, or “only” polarized intermediates, ionic reactions involving 2 and its derivatives have only been studied to a limited extent so far. And this situation does not change very much if we include the chemical behavior of

A quantitative approach to nucleophilic organocatalysis

• Herbert Mayr,
• Sami Lakhdar,
• Biplab Maji and
• Armin R. Ofial

Beilstein J. Org. Chem. 2012, 8, 1458–1478, doi:10.3762/bjoc.8.166

• -one (E = –6.75) [87] and, in particular, stabilized carbocations, which are generated in situ from the corresponding alcohols under weakly acidic conditions [14][88][89]. Suggestions for further promising electrophilic reaction partners in enamine activated reactions [90] can be derived from the

Cation affinity numbers of Lewis bases

• Christoph Lindner,
• Raman Tandon,
• Boris Maryasin,
• Evgeny Larionov and
• Hendrik Zipse

Beilstein J. Org. Chem. 2012, 8, 1406–1442, doi:10.3762/bjoc.8.163

• through charge delocalization [22]. Affinity numbers obtained for larger carbocations such as the benzhydryl cation may thus more closely mimic the steric and electronic properties of synthetically used carbon electrophiles. The corresponding benzhydryl cation affinity (BHCA) of a neutral Lewis base (LB

• towards different carbocations span an extraordinarily large energy range. In order to find out, whether different nucleophiles respond to changes in the electrophile in a systematically comparable manner, we have selected a small group of nucleophiles of different type for a direct comparison of affinity

• parameters N measured recently for these compounds and indicate that carbon basicities parallel the kinetics of base addition to carbocations for this class of compounds [47]. Guanidinyl pyridines such as 433 have, in contrast, surprisingly low ACA values around 230 kJ/mol. Structural changes in the

A practical microreactor for electrochemistry in flow

• Kevin Watts,
• William Gattrell and
• Thomas Wirth

Beilstein J. Org. Chem. 2011, 7, 1108–1114, doi:10.3762/bjoc.7.127

• the product at a flow rate of 10–15 µL·min−1 with a current of 0.6 mA. Yoshida reported a method of producing carbocations such as 1 in the absence of a suitable nucleophile (the "cation pool" method). This is an unconventional method because the ions generated are unstable and usually need to be

Synthetic applications of gold-catalyzed ring expansions

• David Garayalde and
• Cristina Nevado

Beilstein J. Org. Chem. 2011, 7, 767–780, doi:10.3762/bjoc.7.87

• was gathered which proved the reversible nature of the [3,3]-rearrangement in these cyclopropane probes. However, these transformations proved to be stereospecific in nature through gold-stabilized non-classical carbocations 100 and 100', even if the stereochemical information transfer to the product