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Red light has the advantages of low energy, less health risk and high penetration depth through various media. Herein, a helical carbenium ion (N,N’-di-n-propyl-1,13-dimethoxyquinacridinium (nPr-DM...

Most terpene synthase reactions follow Markovnikov rules for formation of high energy carbenium ion intermediates. However, there are notable exceptions. For example, pentalenene synthase (PS) undergoes an initial anti-Markovnikov cyclization reaction followed by a 1,2-hydride shift to form an intermediate humulyl cation with positive charge on the secondary carbon C9 of the farnesyl diphosphate substrate. The mechanism by which these enzymes stabilize and guide regioselectivity of secondary carbocations has not heretofore been elucidated. In an effort to better understand these reactions, we grew crystals of apo-PS, soaked them with the non-reactive substrate analog 12,13-difluorofarnesyl diphosphate, and solved the x-ray structure of the resulting complex at 2.2 Å resolution. The most striking feature of the active site structure is that C9 is positioned 3.5 Å above the center of the side chain benzene ring of residue F76, perfectly poised for stabilization of the charge through a cation-π interaction. In addition, the main chain carbonyl of I177 and neighboring intramolecular C6,C7-double bond are positioned to stabilize the carbocation by interaction with the face opposite that of F76. Mutagenesis experiments also support a role for residue 76 in cation-π interactions. Most interesting is the F76W mutant which gives a mixture of products that likely result from stabilizing a positive charge on the adjacent secondary carbon C10 in addition to C9 as in the wild-type enzyme. The crystal structure of the F76W mutant clearly shows carbons C9 and C10 centered above the fused benzene and pyrrole rings of the indole side chain, respectively, such that a carbocation at either position could be stabilized in this complex, and two anti-Markovnikov products, pentalenene and humulene, are formed. Finally, we show that there is a rough correlation (although not absolute) of an aromatic side chain (F or Y) at position 76 in related terpene synthases from Streptomyces that catalyze similar anti-Markovnikov addition reactions.

Alkylation of phenolics is of great importance in synthetic chemistry and the valorization of lignocellulosic-biomass-derived streams. Here, we unravel how alkylating reactants and solvents significantly alter the reaction pathways of zeolite-catalysed alkylation of phenol in the liquid phase. The carbenium ion formed from the dehydration of cyclohexanol or from the adsorption and protonation of cyclohexene acts as the electrophile, inducing carbon–carbon bond formation. Cyclohexanol at Bronsted acid sites (BAS) forms hydrogen-bonded monomers and protonated dimers in apolar solvents. The dimer appears to generate a much lower concentration of carbenium ions compared with the monomer. Higher alkylation rates in apolar solvents than in water are caused by the energetically more-favourable carbenium ion formation from either alcohol or olefin on non-hydrated zeolite BAS than on hydronium ions produced by BAS in pores filled with water. Zeolite-catalysed alkylations of phenolic compounds offer unique possibilities for the valorization of renewable aromatics into substituted arenes. Now, a mechanistic study reveals that the course of the reaction can be dramatically altered by changing the polarity of the solvent, which affects the nature of surface species and the pathway for the generation of the alkylating electrophile.

A series of novel 4-ferrocenyl-1,2,3,4-tetrahydroquinolines were synthesized in high-to-excellent yields (up to 99%) starting from the corresponding ferrocenoylethyl aryl amines. These Mannich bases were reduced (NaBH4) to the corresponding 3-(arylamino)-1-ferrocenylpropan-1-ols and submitted to an intramolecular cyclization prompted by acetic acid, proceeding via the corresponding alpha-ferrocenyl carbenium ion intermediates. Subsequently, the obtained tetrahydroquinolines were smoothly oxidized (aromatized) by means of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) to provide the corresponding 4-ferrocenylquinolines (up to 93%). (C) 2017 Elsevier Ltd. All rights reserved.

The solid acid, tin hydroxide-embedded montmorillonite, catalyzes the unprecedented alkylation of various silicon enolates with primary, secondary and tertiary benzylic alcohols as well as secondary allylic alcohols. The acid catalysis of Sn-Mont was not only higher than that of the other ion-exchanged montmorillonites (M-Mont; M = H, Ti, Fe and Al), but also higher than that of the typical homogeneous acid catalysts such as BF 3 ·OEt 2 , TMSOTf and TfOH.

Fully protected pyrazolidines can be readily obtained by acid-catalysed cyclisations of the corresponding allylic hydrazines by carbenium ion generation using concentrated sulfuric acid in dichloromethane.

A method for regioselective alkylation of the 3-thiono-1,2,4-triazinone 10 at the sulfur atom is reported. Subsequent Claisen rearrangements, triggered either thermally or using a palladium catalyst, deliver N-alkylated products 13, while acid-catalysed rearrangements of examples where a tertiary carbenium ion can be generated, result in the formation of N-thioalkyl derivatives.

All rights reserved.A range of enantiopure 1,2,3,4-tetrahydroisoquinolines have been prepared directly from α-hydroxy-β-amino esters. Activation of the α-hydroxy group upon treatment with Tf2O and 2,6-di-tert-butyl-4-methylpyridine promotes aziridinium formation, which is then followed by rupture of the C(3)-N bond and Friedel-Crafts alkylation-type cyclisation of an N-benzyl moiety onto the resultant benzylic carbenium ion. The nature of the N-protecting group was varied and it was found that superior yields were obtained for reactions employing two benzylic groups. In the cases where two different N-benzyl groups were used, the regioselectivity resulting from competitive cyclisation of either N-benzyl group was addressed by the introduction of a p-trifluoromethyl group on one of the N-benzyl moieties, which retarded the rate of cyclisation via this electron poor aryl ring. This methodology was employed in the asymmetric synthesis of a range of enantiopure 1,2,3,4-tetrahydroisoquinolines, which were isolated in good yields as single diastereoisomers.

Dehydration of ethyl 3-hydroxyquinuclidine-3-carboxylate is one of the key steps during production of the antihistamine drug Quifenadine (Phencarol). The side products of this process were isolated and their structures proved using NMR spectroscopy. Carbenium ion rearrangements of quinuclidine-3-carboxylate gave rise to 1-azabicyclo[3.2.1]octane and 5-azatricyclo[3.2.1.02,7]octane systems, which serve as impurity standards for the manufacturing process of Quifenadine and are also interesting building blocks for medicinal chemistry.

Fragmentation of electrochemically generated oxonium ions can be exploited to form carbenium ions at a low oxidation potential in the presence of a nucleophile. The application of this concept is demonstrated for the allylation of carbenium ions generated by the anodic oxidation of stannylmethylethers.

The development of a cation clock method based on the intramolecular Sakurai reaction for probing the concentration dependence of the nucleophile in glycosylation reactions is described. The method is developed for the sulfoxide and trichloroacetimidate glycosylation protocols. The method reveals that O-glycosylation reactions have stronger concentration dependencies than C-glycosylation reactions consistent with a more associative, S(N)2-like character. For the 4,6-O-benzylidene-directed mannosylation reaction a significant difference in concentration dependence is found for the formation of the β- and α-anomers, suggesting a difference in mechanism and a rationale for the optimization of selectivity regardless of the type of donor employed. In the mannose series the cyclization reaction employed as clock results in the formation of cis and trans-fused oxabicyclo[4,4,0]decanes as products with the latter being strongly indicative of the involvement of a conformationally mobile transient glycosyl oxocarbenium ion. With identical protecting group arrays cyclization in the glucopyranose series is more rapid than in the mannopyranose manifold. The potential application of related clock reactions in other carbenium ion-based branches of organic synthesis is considered.

An intermolecular dehydrogenative annulation (IDA) for carbazole synthesis via sequential C–C/C–N bond formation with a selective alkyl group migration is reported. Using the hypervalent iodine(III) reagent PhI(OAc)2 (PIDA), in a one-pot operation, up to five C(sp2)–H bonds, one N(sp3)–H bond functionalization, and one alkyl (Me, Et) group migration could all be achieved from non-prefunctionalized 1,3,5-trialkylbenzenes and anilides under ambient laboratory conditions. Mechanistically, it is shown that PIDA reacts with anilides to generate a nitrenium ion or an equivalent carbenium ion which influences the second aromatic ring to be activated for C–C/C–N bond formation. Strategically, regioselective fusion of arenes to anilides is described.

Hydrogen transfer is the major route in catalytic conversion of methanol to olefins (MTO) for the formation of nonolefinic byproducts, including alkanes and aromatics. Two separate, noninterlinked hydrogen transfer pathways have been identified. In the absence of methanol, hydrogen transfer occurs between olefins and naphthenes via protonation of the olefin and the transfer of the hydride to the carbenium ion. A hitherto unidentified hydride transfer pathway involving Lewis and Brønsted acid sites dominates as long as methanol is present in the reacting mixture, leading to aromatics and alkanes. Experiments with purely Lewis acidic ZSM-5 showed that methanol and propene react on Lewis acid sites to HCHO and propane. In turn, HCHO reacts with olefins stepwise to aromatic molecules on Brønsted acid sites. The aromatic molecules formed at Brønsted acid sites have a high tendency to convert to irreversibly adsorbed carbonaceous deposits and are responsible for the critical deactivation in the methanol to olefin process.

Electrophilic substitution of aromatics on zeolites is generally assumed to occur through the Wheland-type intermediate, although direct experimental evidence is lacking. Now, this carbenium ion has been identified as a stable intermediate in the alkylation of benzene with ethanol on an industrial zeolite catalyst.

Esters are chemical compounds with many practical uses. The common type of esterification is called the Fischer esterification. Another one is by the action of acid chlorides on alcohols but not with tertiary alcohols. The stable carbenium ions formed from tertiary alcohols favor elimination and the byproduct, hydrogen chloride prevents ester formation. In this new report, palladium inserted ArCOPdCl species reacts with tertiary alcohols and cross-coupling under microwave heating, minimizes the formation of probable carbenium ion, and promotes successful production of highly substituted esters in good to high yields.

The direct catalytic azidation of 2-hydroxy-1,2,2-triarylethanones occurs at room temperature using 2 mol % of InBr3 as Lewis acid and TMSN3 as soluble azide source. 2-Azido-1,2,2-triarylethanones have been isolated in excellent yields. The role of aryl group and stereoelectronic factors indicate that the mechanism may involve the formation of a stable carbenium ion towards azidation.

Two new dodecaferrocenyl dendrimers have been prepared using a sixfold Sonogashira coupling reaction and sixfold Huisgen cycloaddition, respectively. In addition, a dendron containing four ferrocenyl groups has been synthesised by conventional synthetic methods. Electrochemical studies have shown that all the ferrocene units are electrochemically equivalent. Moreover, when treated with acid these compounds form multicharged methylium near-infrared-absorbing dendritic dyes with a high extinction coefficient.

A one-step synthesis of the stable [4]helicenium dye, 1,13-dimethoxy-chromeno[2,3,4- kl ]xanthenium hexafluorophosphate (DMCX + ) from the readily available tris(2,6-dimethoxyphenyl)carbenium ion is reported. The crystal structure, the chemical stability, and dye properties of the DMCX + helicenium system are described.

What do quantum cellular automata (QCA), "on water" reactions, and SN 1-type organocatalytic transformations have in common? The link between these distant arguments is the practical access to useful intermediates and key products through the use of stabilized carbenium ions. Over 10 years, starting with a carbenium ion bearing a ferrocenyl group, to the 1,3-benzodithiolylium carbenium ion, our group has exploited the use of these intermediates in useful and practical synthetic transformations. In particular, we have applied the use of carbenium ions to stereoselective organocatalytic alkylation reactions, showing a possible solution for the "holy grail of organocatalysis". Examples of the use of these quite stabilized intermediates are now also considered in organometallic chemistry. On the other hand, the stable carbenium ions are also applied to tailored molecules adapted to quantum cellular automata, a new possible paradigm for computation. Carbenium ions are not a problem, they can be a/the solution!

A new route toward 4-acylamino- and 4-amino-substituted tetrahydroindazoles is disclosed. The title compounds are obtained in good to excellent yields in the Ritter reaction between 4-hydroxy-tetrahydroindazoles and various nitriles. The reactivity of the tetrahydroindazole-derived carbenium ion is both sufficiently high to react with trichloroacetonitrile and sufficiently selective to resist the azide functionality within its structure. The present approach adds a method to the toolbox of tetrahydroindazole chemistry and facilitates the structural modifications of the scaffold, which has found important applications in medicinal chemistry.

The indole alkaloid α-cyclopiazonic acid 1 has been synthesised by a route, which features at its core an acid-catalysed cationic cascade cyclisation terminated by a sulfonamide group.

An efficient method for the synthesis of 2-aryl-2,3-dihydro-3-sulfanyl-1H-isoindol-1-ones 1via Pummerer-type cyclization of N-aryl-2-(sulfinylmethyl)benzamides 2 is described. Thus, treatment of these sulfinyl-benzamides 2, easily prepared from 2-(bromomethyl)benzoates 3 in three steps, with Ac2O at ca. 100° resulted in the formation of the desired isoindolones 1 in generally good yields.

Cyclisations of homoallylic sulfonamides proceed smoothly via carbeniumion generation using trifluoromethanesulfonic (triflic) acid, the ease of cyclisation being directly related to the ion stability to give good to excellent yields of the corresponding pyrrolidines. Both toluene- and nitrophenyl-sulfonyl groups are suitable for all substrates tested whereas the corresponding carbamates are only useful in cases of tertiary and highly stabilised carbeniumions. Polyene-derived sulfonamides can also be cyclised to the corresponding polycyclic systems in remarkably high yields, in reactions reminiscent of related cascades encountered in terpene biosynthesis.

We developed a kinetic model for the reversible direct addition of propionic acid to linear tetradecences and linear hexadecenes on Amberlyst 15 at 93°C. The model addresses concurrent double bond isomerization of the olefins and assumes carbenium ion formations to be rate limiting for all conversions; it is detailed enough to include terms for multiple isomers of esters and olefins. The reaction was first order in olefin and in ester but was one-half order in propionic acid. Gas chromatography was the analytical method employed to follow the reactions, and this study represents a novel use of this analytical method to elucidate carbenium ion behavior. The model includes five rate-determining steps and five fast steps. © 2010 Wiley Periodicals, Inc. Int J Chem Kinet 42: 354–371, 2010

The alkyl-bridged iron(II) complexes [{Cp(CO)2Fe}2{μ-(CnH2n)}] (n = 6–10, Cp = η5-C5H5) undergo both single and double hydride abstraction when reacted with one equivalent of Ph3CPF6 to give both the monocationic complexes, [{Cp(CO)2Fe}2{μ-(CnH2n−1)}]PF6, and the dicationic complexes, [{Cp(CO)2Fe}2{μ-(CnH2n−2)}](PF6)2. The ratios of monocationic to dicationic complexes decrease with the increase in the value of n. The complexes where n = 4 and 5 undergo only single hydride abstraction under similar conditions. When reacted with two equivalents of Ph3CPF6, the complexes where n = 6–10 undergo double hydride abstraction to give dicationic complexes only. In contrast, the complex where n = 5 gives equal amounts of the monocationic and the dicationic complexes, while the complex where n = 4 only gives the monocationic complex. 1H and 13C NMR data show that in the monocationic complexes one metal is σ-bonded to the carbenium ion moiety while the other is bonded in a η2-fashion forming a chiral metallacylopropane type structure. In the dicationic complexes both metals are bonded in the η2-fashion. The monocationic complexes where n = 4–6, react with methanol to give η1-alkenyl complexes[Cp(CO)2Fe(CH2)nCH CH2] (n = 2–4) as the major products and σ-bonded ether products [{Cp(CO)2Fe}2{μ-(CH2)nCH(OCH3)CH2}] as the minor products. The complex where n = 8 reacted with iso-propanol to give the η1-alkenyl complex [Cp(CO)2Fe(CH2)6CH CH2]. The dicationic complexes where n = 5, 8 and 9 were reacted with NaI to give the respective α, ω-dienes and [Cp(CO)2FeI].

Acidolytic release of an amide from a solid support by C–N bond cleavage is an ubiquitous and crucial step in many solid-phase syntheses. We have used molecular modeling of a pseudo-equilibrium to explore substituent and steric effects in the release of peptides. The high acid-lability of the backbone amide linkage (BAL), which releases sec. amides, compared to C-terminal amide anchoring, which releases primary amides, was rationalized by steric relief upon cleavage. Thus, the relative stability of the carbenium ion formed from the linker in the acidolytic release is an insufficient measure of the lability of a linkage. In addition, predictions indicated that steric effects from the Cα-substituent in a BAL anchored amino acid residue should accelerate the acidolytic release. The finding that steric crowding leads to increased acid-lability will be important for further development and use of handles.

A series of triarylmethanol derivatives containing combinations of phenyl, 1-naphthyl, 4-dimethylamino-4′-stilbenyl, ferrocenyl or 4-[2-ferrocenylethenyl]phenyl groups have been prepared by reaction of 4-[4-Me 2 NC 6 H 4 C C]C 6 H 4 Li or 4-[FcC C]C 6 H 4 Li (Fc = ferrocenyl) with ferrocenecarboxaldehyde, ferrocenyl phenyl ketone, methyl ferrocenecarboxylate, 1-naphthyl phenyl ketone, ethyl benzoate or diethyl carbonate. The carbinols readily form the corresponding intensely coloured carbenium ions in acid solution which have significant electronic absorption in the near infrared. Initial studies indicate that they also possess substantial first hyperpolarisabilities.

Enthalpies of addition of hydride ion to eleven carbonyl acceptors (X-CHO), two conjugate addition sites (X-CH=CH2; X = CHO, NO2), eight carbenium ion acceptors, fulvene, borane, and SiH3(+) were calculated at the MP2/6-311+G level. Correlation between calculated and experimental enthalpies of addition of hydride ion is excellent. Transition states (ts) for the identity hydride transfers between the acceptors and their corresponding hydride adducts (hydride donors) were also calculated. The carbonyl and fulvene reactions have transition states with one imaginary frequency: the hydrogen transfer coordinate. The carbenium ions, borane, and SiH3(+) gave not transition states but stable compounds upon addition of the hydride donor. Computational differences between these hydride transfers and previously reported proton transfers include shorter partial C...H bonds and a tendency toward bent C...H...C angles for the hydride transfer ts and addition compound structures, particularly when a bent geometry improves interactions elsewhere in the structure. These and other differences are explained by modeling the hydride transfer ts and addition compounds as two-electron, three-center systems involving the transfer termini and the shared hydrogen but the proton transfer ts structures as four-electron, three-center systems. Charge and geometry measures suggest transition states in which these features change synchronously, again in contrast to many proton transfer reactions. For the X-CHO set, polar effects dominate enthalpies of hydride addition, with resonance effects also important for resonance donors; these preferentially stabilize the acceptor, reducing its hydride ion affinity. Activation enthalpies are dominated by resonance stabilization of the acceptors, greatly attenuated in the transition states.

A novel, acid catalyzed rearrangement of the 11-trimethylsilyl-tetracyclo[8.1.0.0 3,7 .0 4,11 ]undeca-5,8-diene ring system was observed. The trimethylsilyl substitution on the cyclopropane moiety was found to be crucial for rapid, high yielding formation of the unusual homotriquinacene ring system. The transformation is presumed to proceed via a series of carbenium ion intermediates that can be trapped with suitable nucleophiles.

The structures and reactivities of the complexes between carbenium ions (R(+)) and acetylene or propyne have been investigated with the aid of electron-correlated quantum mechanical calculations (hybrid density functional, perturbation theory, and coupled cluster methods). Depending on the R group, the acetylene/carbenium ion interaction can produce either an "open" 3c-2e structure or the conventional vinyl cation structure. The "open" 3c-2e C-C-C bonding geometry exists as a minimum for R = methyl and primary/secondary/tertiary alkyl, and hence is the most notable. The alignment of three carbon centers is neither bridged nor linear, but L-shaped, and represents a new type of "open" 3c-2e bonding that has so far escaped proper attention.

Good quality ab initio calculations (MP2) show that the water adducts of BX(3) and CX(3)(+) have totally different structures (X = F-I). While all H(2)O-BX(3) complexes have classical C(s) symmetric structures with strong B-O bonds and additional H-bonding, the heavier CX(3)(+) cations (X = Cl-I) form weakly bonded "non-classical" water adducts that maximize C-X pi-bonding rather than C-O sigma-bonding. The delocalization of the positive charge as the driving force for pi-bond formation is absent in BX(3), and therefore, pi-bonding is only weak and not structure determining in H(2)O-BX(3). Since the PES of all H(2)O --EX(3)(0/+1) particles (E = B, C) is very flat, flexible basis sets (like TZVPP) are required to rigorously characterize the adducts. In earlier calculations (J. Am. Chem. Soc. 1997, 119, 6648), classical structures were reported for all H(2)O --EX(3)(0/+1) (E = B, C) complexes, likely resulting from the insufficient quality of the basis sets employed. By introducing a positive charge to three coordinate boron-halogen cations Do --BX(2)(+) (Do = NH(3), OH(2), X-H), also the B-X bonds shrink due to the stronger pi-bonding induced by the positive charge delocalization and if compared to the respective neutral compounds like H(2)N-BX(2) or BX(3). The "non-classical" water adducts also suggest that the mechanism of organic reactions involving carbenium ion intermediates with alpha-bromine or -iodine substituents and a nucleophile may proceed through halogen- rather than carbon coordination.

Steric effect in the complexes of tri-coordinate silylenium and carbenium ions with model nucleophiles is discussed based on calculated energies of complex formation and on natural steric analysis (a part of the NBO theory). While the energies of complexation of CH 3 + are greater than those of SiH 3 + , for trimethyl-substituted cations the order is reversed. This observation is interpreted in terms of smaller steric hindrance of trimethylsilyl cation compared to t -butyl cation. Natural steric analysis points to the potentials and difficulties in separate treatment of steric and electron delocalization effects on stabilization of these species.

Treatment of (+)-sclareolide (1) with polyphosphoric acid or Eaton's reagent furnished, besides the anticipated cyclopentenone (−)-12 and its isomer (−)-15, two diastereoisomeric [4.3.3]propellanes (−)-13 and (−)-14, which possess interesting woody-ambery odors. The hydrogenated derivative (−)-17 possessed an even more-powerful odor reminiscent of natural ambergris tincture. Mechanistic insight into this rearrangement was provided by a by-product 24 of the reaction of sclareolide (1) with Eaton's reagent. The carbenium ion rearrangement was then employed in the synthesis of four related [4.3.3]propellanes 40–43, illustrating the utility and scope of this reaction. The olfactory properties of the synthesized [4.3.3]propellanes as well as of the original target structures 10, 33, and 34, prepared from (−)-12 and (−)-15, are discussed. Especially the pronounced ambra odor of (−)-17 vividly contradicts the ‘triaxial rule of amber sensation' and provides new insight into the structural requirements for ambra odorants.

On Rearrangements by Cyclialkylations of Arylpentanols to 2,3-Dihydro-1H-indene Derivatives. Part 3. The Acid-Catalyzed Cyclialkylation of 3,4-Dimethyl- and 3-([2H3]Methyl)-4-methyl-3-phenylpentan-2-ol The cyclialkylation of 2-([2H3]methyl)-4-methyl-4-phenyl[1,1,1-2H3]pentan-3-ol (4) yielded a 1 : 1 mixture of 1,1-di([2H3]methyl)-2,3-dimethyl-1H-indene (5) and of 2,3-dihydro-2,3-di([2H3]methyl)-1,1-dimethyl-1H-indene (6) (Scheme 1) [1]. However, it was not clear whether the transposition takes place through the successive migration of a Ph, a Me and again the Ph group (Scheme 2, Path A: shift IVVIIVIIa) or through Ph-, Me-, and then i-Pr-group (Scheme 2, Path B:IVVIIVIIb). The cyclialkylation of 3-([2H3]methyl)-4-methyl-3-phenylpentan-2-ol (7) yielded only one product, the 2,3-dihydro-2-([2H3]methyl)-1,1,3-trimethyl-1H-indene (8), in accordance with the migrations according to Path A. This result is also a support for the total mechanism proposed for the cyclialkylation of 4 (Scheme 2). The transition of a tertiary to a secondary carbenium ion is not definitely ensured (see [1]).

The reaction mechanism of solid-acid-catalyzed phenol alkylation with cyclohexanol and cyclohexene in the apolar solvent decalin has been studied using in situ 13C MAS NMR spectroscopy. Phenol alkylation with cyclohexanol sets in only after a majority of cyclohexanol is dehydrated to cyclohexene. As phenol and cyclohexanol show similar adsorption strength, this strict reaction sequence is not caused by the limited access of phenol to cyclohexanol, but is due to the absence of a reactive electrophile as long as a significant fraction of cyclohexanol is present. 13C isotope labeling demonstrates that the reactive electrophile, the cyclohexyl carbenium ion, is directly formed in a protonation step when cyclohexene is the coreactant. In the presence of cyclohexanol, its protonated dimers at Brønsted acid sites hinder the adsorption of cyclohexene and the formation of a carbenium ion. Thus, it is demonstrated that protonated cyclohexanol dimers dehydrate without the formation of a carbenium ion, which would otherwise have contributed to the alkylation in the kinetically relevant step. Isotope scrambling shows that intramolecular rearrangement of cyclohexyl phenyl ether does not significantly contribute to alkylation at the aromatic ring.

The following kinetic isotope effects, KIEs (k(light)/k(heavy)), have been measured for the hydrolyses of methyl alpha- and beta-xylopyranosides, respectively, in aqueous HClO(4) (mu = 1.0 M, NaClO(4)) at 80 degrees C: alpha-D, 1.128 +/- 0.004, 1.098 +/- 0.005; beta-D, 1.088 +/- 0.008, 1.042 +/- 0.004; gamma-D(2), (C5) 0.986 +/- 0.001, 0.967 +/- 0.003; leaving-group (18)O, 1.023 +/- 0.002, 1.023 +/- 0.003; ring (18)O, 0.983 +/- 0.001, 0.978 +/- 0.001; anomeric (13)C, 1.006 +/- 0.001, 1.006 +/- 0.003; and solvent, 0.434 +/- 0.017, 0.446 +/- 0.012. In conjunction with the reported (J. Am. Chem. Soc. 1986, 108, 7287-7294) KIEs for the acid-catalyzed hydrolysis of methyl alpha- and beta-glucopyranosides, it is possible to conclude that at the transition state for xylopyranoside hydrolysis resonance stabilization of the developing carbenium ion by the ring oxygen atom is coupled to exocyclic C-O bond cleavage, and the corresponding methyl glucopyranosides hydrolyze via transition states in which charge delocalization lags behind aglycon departure. In the analogous hydrolysis reactions of methyl 5-thioxylopyranosides, the measured KIEs in aqueous HClO(4) (mu = 1.0 M, NaClO(4)) at 80 degrees C for the alpha- and beta-anomers were, respectively, alpha-D, 1.142 +/- 0.010, 1.094 +/- 0.002; beta-D 1.061 +/- 0.003, 1.018(5) +/- 0.001; gamma-D(2), (C5) 0.999 +/- 0.001, 0.986 +/- 0.002; leaving-group (18)O, 1.027 +/- 0.001, 1.035 +/- 0.001; anomeric (13)C, 1.031 +/- 0.002, 1.028 +/- 0.002; solvent, 0.423 +/- 0.015, 0.380 +/- 0.014. The acid-catalyzed hydrolyses of methyl 5-thio-alpha- and beta-xylopyranosides, which occur faster than methyl alpha- and beta-xylopyranosides by factors of 13.6 and 18.5, respectively, proceed via reversibly formed O-protonated conjugate acids that undergo slow, rate-determining exocyclic C-O bond cleavage. These hydrolysis reactions do not have a nucleophilic solvent component as a feature of the thiacarbenium ion-like transition states.

2-(1,3-Dimethylimidazolidinio)diselenocarboxylate (16) was obtained in 48% yield as thermally stable, dark green crystals by reaction of 2-methylene-1,3-dimethylimidazolidine (7) with Se2Cl2 in the presence of Et3N. The reaction of 16 with elemental sulfur gave 2-(1,3-dimethylimidazolidinio)dithiocarboxylate (20) in 95% yield by exhaustive selenium−sulfur exchange, whereas the use of a limited amount of sulfur allowed the preparation of 2-(1,3-dimethylimidazolidinio)selenothiocarboxylate (19) as dark red crystals. X-ray crystallographic analyses revealed that the planes of the carbenium ion and CX2- (X = S, Se) parts of 16 and 20 are nearly perpendicular to each other. The observed geometries (including bond angles and lengths) of 16 and 20 were reproduced satisfactorily by ab initio calculations (B3LYP/6-31G* level). Discussion is also made on the structures of 16, 19, and 20 on the basis of spectroscopic data (NMR, IR, and UV/vis). Methylation of 16 with MeI furnished the carbenium iodide (26a), which p...

In situ 13C NMR measurements on samples prepared using a pulse-quench catalytic reactor show that the 1,3-dimethylcyclopentenyl carbenium ion (1) is an intermediate in the synthesis of toluene from ethylene on zeolite catalyst HZSM-5. Cation 1 forms in less than 0.5 s when ethylene is pulsed onto the catalyst bed at 623 K, and its presence obviates the kinetic induction period for conversion of a subsequent pulse of dimethyl ether, or methanol, into olefins (MTO chemistry). The kinetic induction period returns when the interval between pulses is many times the half-life of 1 in the catalyst bed. Density functional theory calculations (B3LYP/ 6-311G**) on a cluster model of the zeolite confirm that 1 is stable in the zeolite as a free cation and suggest why the alternative framework alkoxy is not observed. A π complex of the neutral cyclic diene is only 2.2 kcal/mol higher in energy than that of the ion pair. Theoretical (GIAO-MP2/tzp) 13C isotropic shifts of isolated 1 are in good agreement with the exper...

A new, short total synthesis of dihydroactinidiolide 1 is described using selenium carbenium ion-promoted carbon–carbon bond formation as the key step. Our synthetic strategy starts with a lactonization reaction between 1,3,3-trimethylcyclohexene 13 and α-chloro-α-phenylseleno ethyl acetate 14, affording the key intermediate, α-phenylseleno-γ-butyro lactone 15, which reacted via a selenoxide elimination to the target compound 1.

5,10,15,20-Tetralkyl-2-formylporphyrins were treated with various organolithium reagents to form porphyrins with exocyclic double bonds. The reaction involved conversion with LiR to the respective alcohol. Subsequence dehydratization of the alcohols yielded olefinic systems in which the double bond formed was located in the meso substituent neighboring the β position, i.e., the result of a 1,5-hydride shift. Depending on the organolithium reagent used various olefinic porphyrins are accessible, provided the stability of the intermediary carbenium ion is high enough. Furthermore, use of a Peterson olefination allowed the facile synthesis of 2-vinyl-5,10,15,20-tetralkylporphyrins and use of an organodilithium reagent gave convenient access to functionalized bisporphyrins.

Product analysis of dealkylation in P(S)C(S)-soman-inhibited electric eel acetylcholinesterase (AChE) by GC−MS using the selected ion monitoring mode has been carried out. The instrument was calibrated with pure standards of 2,3-dimethyl-1-butene and 2,3-dimethyl-2-butene in the gas phase and methylene chloride extracts of 2,3-dimethyl-2-butanol and 3,3-dimethyl-2-butanol from the aqueous phase. The dealkylation in soman-inhibited AChE at pH 5.0 ± 0.2 and 25 °C produces close to 40% alkenes and 50−60% 2,3-dimethyl-2-butanol. No 3,3-dimethyl-2-butanol could be detected to provide direct evidence of the intervention of a secondary carbenium ion in the reaction path. All the products of the reaction originate from a tertiary carbenium ion. These findings are in good agreement with the results of Michel et al. [(1967) Arch. Biochem. Biophys. 121, 29], which were obtained by countercurrent distribution of tritium-labeled products and their identification by scintillation counting. The early experiments were pe...

The free energies for hydride donation (ΔGM+) have been determined in acetonitrile solution for a series of seven molybdenum and tungsten compounds (1−7) of general formula (C5R5)M(CO)2(L)H, which yield the salts [(C5R5)M(CO)2(L)(NCMe)][BF4] in these reactions. These data constitute the first thermodynamic data for hydride transfer by transition metal hydrides, and were gathered from equilibrium studies with carbenium ion salts of known hydride ion affinities in acetonitrile. The metal hydride ΔGM+ values range from ca. 79 to 89 kcal/mol, and these values may be compared with pKas for related compounds to demonstrate that proton-transfer processes are somewhat more sensitive to changes in co-ligands than are hydride transfer processes. Additionally, kinetic studies of hydride transfer reactions with hydride acceptor [(p-MeOPh)2CPh][BF4] exhibit second-order rate constants ranging from ca. 200 to 7500 M-1 s-1. These rates show a correlation with thermodynamic driving force, and a Bronsted plot yields a slo...

Addition of carbon disulfide to 2-isopropylidene- and 2-methylene-1,3-dimethylimidazolidines gave the thermally stable, crystalline inner salts, 1,3-dimethylimidazolidiniodithioacetates (10 and 12), where carbenium ion and dithiocarboxylate parts are insulated by an sp3 carbon atom. X-Ray diffraction analyses of these compounds and the reaction of 10 with DMAD are described.

The aqueous ethanolysis reactions of the adamantylideneadamantyl halides (2-Cl, 2-Br, and 2-I) exhibit Grunwald−Winstein sensitivity parameters (m) of 0.74 ± 0.06, 0.90 ± 0.01, and 0.88 ± 0.03 for the chloride, bromide, and iodide compounds, respectively. All reaction products are formed with retention of both the ring structure and the stereochemistry of the reaction center. Common-ion rate depressions are observed during the solvolyses of all three of the homoallylic halides, a result that is consistent with these reactions proceeding via a free-solvated homoallylic carbenium ion. The rate of nucleophilic capture of the homoallylic carbenium ion exhibits a Swain−Scott parameter (s) of 0.20 ± 0.01, indicating that these reactions show a low sensitivity to the nature of the nucleophile. From the data it is estimated that the lifetime of the homoallylic carbenium ion (1) in 60:40 v/v EtOH−H2O at 25 °C is at least 6.4 × 10-9 s.

Computational studies of carbenium ions relevant to sterol biosynthesis via lanosterol synthase were undertaken to determine fundamental energetics underlying cyclization steps. Ab initio B3LYP/6-31G*//B3LYP/6-31G* calculations were performed for the addition of 2-methyl-2-propyl cation and 2-methylpropene to represent a tertiary cation → tertiary cation cyclization. Solvent effects were included by Monte Carlo (MC) simulations in methylene chloride, methanol, and THF. The picture that emerges for a cation−olefin cyclization is one of barrierless collapse at short distance, while desolvation and conformational barriers are expected for initial separations beyond ca. 5 A. Thus, the cyclization that forms the sterol B ring likely proceeds in barrierless concert with A-ring formation in the preorganized environment of a cyclase enzyme. However, C-ring formation appears to involve a tertiary → secondary cation rearrangement. This was modeled by ab initio and MC calculations for the interconversion of the C11 ...

The structures and energies of lithiated cyclopropenyl cations and their acyclic isomers (C3H3-nLin+, n = 0−3) have been calculated employing ab initio MO (HF/6-31G*) and density functional theory (DFT, Becke3LYP/6-311+G*) methods. The cyclic isomers (4, 6, 10, and 14) are always favored, but when lithium is substituted sequentially along the C3H3+, C3H2Li+, C3HLi2+, and C3Li3+ series, the acyclic forms (5, 7, 11, 16) become progressively less competitive energetically. A triply bridged c-C3(μ-Li)3+ geometry, 14, was preferred over the classical form 3 by 8.7 kcal/mol. A single lithium substituent results in a very large (67 kcal/mol) stabilization of the cyclopropenyl cation. The favorable effects of further lithium substitution are attenuated but are still large: 48.2 and 40.5 kcal/mol for the second and third replacements, respectively. Comparison with polyamino-substituted cyclopropenyl cations suggest c-C3Li3+ (3 and 14) to be a good candidate for the thermodynamically most stable carbenium ion. The...

A series of the novel M 2 L 6 -complexed alkynyl-substituted C10-anthrone derivatives were prepared and fully characterized. Furthermore, the X-ray molecular structures of three complexes belonging to this family were determined with the general formula M 2 L 6 ( μ , η 2 , η 2 -R 3 CCC(OR 1 )(C 13 H 8 O))], M 2 L 6  Co 2 ( CO ) 6 . R 1  CH 3 . R 2  H 5; M 2 L 6  MoCOCp ( CO ) 5 . R 1  CH 3 , R 2  H 7 and M 2 L 6  CO 2 ( CO ) 6 . R 1  CH 3 , R 2  C 6 H 5 8. Addition of HBF 4 to the compounds [MoCoCp(CO) 5 ( μ , η 2 , η 2 -HCCC(OR 1 )(C 13 H 8 O))], R 1 H 6: R 1 CH 1 7, produced the cationic counterpart [MoCOCp(CO) 5 ( μ , η 2 , η 2 -HCCC(C 13 H 8 O))] + BF 4 9 The charge stabilization in this novel carbenium ion possessing an anthrone fragment was investigated and showed that the dinuclear cluster alleviates the positive charge at the C10 carbon center. Further, the reactivity of these carbenium ion towards nucleophiles was also examined.

Bis(diethylamino)[(methylthio)thiocarbonyl]carbenium salts (1), a carbenium ion carrying an α-thiocarbonyl group, shown ambident reactivities toward a series of nucleophiles. Thus, they reacted with a typically hard nucleophile, hydroxide, at the carbenium carbon atom, whereas they smoothly reacted with soft nucleophiles, such as carbon, nitrogen, sulfur, and phosphorus nucleophiles, at the thiocarbonyl sulfur atom.

N-Arylnitrilium salts 1 react with nucleophilic alkenes 2 to afford 3,4-dihydroquinolinium salts 3, which can be transformed into the free bases with aqueous sodium hydroxide. Dehydrogenation of the 3,4-dihydroquinolinium salts 3 with 2,3-dichloro-5,6-dicyano-p-benzoquinone furnishes quinolinium salts 7. If the intermediate carbenium ion A formed by electrophilic attack of 1 on the alkene 2 is conjugatively or hyperconjugatively stabilized, instead of 3,4-dihydroquinolinium salts 3 iminium salts 4 resulting from a Houben-Hoesch reaction, or iminium salts 5 arising from a formal ene reaction are formed. For the 3,4-dihydorquinolinium salt 3ac X-ray structural analysis has been carried out.

Heterolytic splitting of the Si-H bond mediated by a Ru-S bond forms a sulfur-stabilized silicon cation that is sufficiently electrophilic to abstract fluoride from CF(3) groups attached to selected anilines. The ability of the Ru-H complex, generated in the cooperative activation step, to intramolecularly transfer its hydride to the intermediate carbenium ion (stabilized in the form of a cationic thioether complex) is markedly dependent on the electronic nature of its phosphine ligand. An electron-deficient phosphine thwarts the reduction step but, based on the Ru-S catalyst, half of an equivalent of an added alkoxide not only facilitates but also accelerates the catalysis. The intriguing effect is rationalized by the formation of a hydride-bridged Ru-S dimer that was detected by (1)H NMR spectroscopy. A refined catalytic cycle is proposed.