Abstract: A number of trifluoroacetates, mesylates, and triflates have been studied in ionic liquids. Several lines of evidence indicate that all of these substrates react via ionization to give carbocationic intermediates. For example, cumyl trifluoroacetates give mainly the elimination products, but the Hammett ρ+ value of −3.74 is consistent with a carbocationic process. The analogous exo-2-phenyl-endo-3-deutero-endo-bicyclo[2.2.1]hept-2-yl trifluoroacetate gives an elimination where loss of the exo-hydrogen occurs from a cationic intermediate. 1-Adamantyl mesylate and 2-adamantyl triflate react to give simple substitution products derived from capture of 1- and 2-adamantyl carbocations by the residual water in the ionic liquid. The triflate derivative of pivaloin, trans-2-phenylcyclopropylcarbinyl mesylate, 2,2-dimethoxycyclobutyl triflate, the mesylate derivative of diethyl (phenylhydroxymethyl)-thiophosphonate, and Z-1-phenyl-5-trimethylsilyl-3-penten-1-yl trifluoroacetate all give products derived carbocatio...

Abstract: Two new photochemical reactions of 1-allyloxy-9,10-anthraquinones have been discovered. Both reactions involve initial photoinduced hydrogen abstraction and subsequent intramolecular electron transfers. The reactions were discovered as part of a project investigating the utility of intramolecular abstraction/SET processes for the release of biologically active aldehydes. The expected photochemistry for these compounds has been used to prepare trans-4-hydroxy-2-nonenal in 91% yield. However, under certain conditions, a number of unexpected reaction pathways became important. The first reaction included a remarkable C C bond formation; mechanistic possibilities are suggested. The second reaction involved a 6-endo cyclization of a photochemically generated phenolic radical on a tethered alkene, followed by an unprecedented electron transfer from a secondary carbon-centered radical to a semiquinone radical. The resulting carbocation was then trapped by solvent. In one molecule, both reactions were observed, along with a third photoinduced Claisen rearrangment.

Abstract: The photochemical reactions in methanol of the vinylic halides 1−4, halostyrenes with a methyl or a trifluoromethyl substituent at the α- or β-position, have been investigated quantitatively. Next to E/Z isomerization, the reactions are formation of vinyl radicals, leading to reductive dehalogenation products, and formation of vinyl cations, leading to elimination, nucleophilic substitution, and rearrangement products. The vinyl cations are parts of tight ion pairs with halide as the counterion. The elimination products are the result of β-proton loss from the primarily generated α-CH3 and α-CF3 vinyl cations, or from the α-CH3 vinyl cation formed from the β-CH3 vinyl cation via a 1,2-phenyl shift. The β-CF3 vinyl cation reacts with methanol yielding nucleophilic substitution products, no migration of the phenyl ring producing the α-CF3 vinyl cation occurs. The α-CF3 vinyl cation, which is the most destabilized vinyl cation generated thus far, gives a 1,2-fluorine shift in competition with proton loss. Th...

Abstract: A set of 40 finite temperature ab initio molecular dynamics trajectories is employed to investigate the distribution of addition products and underlying microscopic mechanism of the addition of 1,3-butadiene to the Si(100)-2 × 1 surface. The product yields are in good agreement with recent STM measurements and include a Diels−Alder [4 + 2] adduct with a surface dimer acting as the dienophile, a [4 + 2]-like adduct that bridges two dimers within a row, a [4 + 2]-like adduct that bridges two dimers in adjacent rows, and an interdimer [2 + 2]-like adduct. The trajectories indicate that a common mechanism underlies the distribution and is predominantly a nonconcerted stepwise mechanism that proceeds via an intermediate zwitterion composed of a carbocation bonded to a negatively charged surface dimer.

Abstract: Reactive intermediates in the Friedel−Crafts acylation of aromatic donors are scrutinized upon their successful isolation and X-ray crystallography at very low temperatures. Detailed analyses of the X-ray parameters for the [1:1] complexes of different aliphatic and aromatic-acid chlorides with the Lewis acids antimony pentafluoride and pentachloride, gallium trichloride, titanium and zirconium tetrachlorides provide unexpected insight into the activation mechanism for the formation of the critical acylium carbocations. Likewise, the X-ray-structure examinations of aliphatic and aromatic acylium electrophiles also isolated as crystalline salts point to the origins of their electrophilic reactivity. Although the Wheland intermediates (as acylium adducts to arene donors) could not be isolated in crystalline form owing to their exceedingly short lifetimes, transient (UV−vis) spectra of benzenium adducts of acylium carbocations with hexamethylbenzene can be measured and directly related to Wheland intermediat...

Abstract: Vinyl type carbocations ArC+=CHX [X = CO2H, CO2Alk, C≡N, P(O)(OAlk)2] generated from alkyl 3-arylpropynoates and related compounds in fluorosulfonic acid at −75 to −20°C react with various benzene derivatives, following the mechanism of electrophilic substitution of hydrogen. A new procedure for the synthesis of alkyl 3,3-diarylpropenoates having various substituents in the aryl fragments has been developed on the basis of protonation of the triple bond in alkyl 3-arylpropynoates.

Abstract: In order to trap the carbocation intermediates formed during the squalene cyclization cascade, squalene analogs with threo- and erythro-diols at the 6,7- and 10,11-positions were incubated with the recombinant squalene cyclase from Alicyclobacillus acidocaldarius, leading to the construction of the triterpenes with tetrahydropyran, octahydrochromene, decahydronaphthalene with a carbonyl group, dodecahydrobenzo[f]chromene, tetradecahydronaphtho[2,1-b]oxepine and malabaricane skeletons, almost of which are novel compounds. These products indicate that 6-membered monocyclic, 6/6-fused bicyclic and 6/6/5-fused tricyclic cations were involved in the cyclization reaction in addition to acyclic cation. All the trapped cations were the stable tertiary cation, but not the secondary one, indicating that the polycyclization reaction proceeds with a Markovnikov closure. The product profiles revealed that the cyclization reactions proceeded with the product and substrate specificities in addition to enantioselectivity. Mechanistic insight into the observed stereochemical specificities indicated that the pre-organized chair-conformation of squalene-diols is tightly constricted by the cyclase and a free motion or a conformational change is not allowed in the reaction cavity, thus, the substrate and product specificities are dominantly directed by the least motion of the nucleophilic hydroxyl group toward the intermediary carbocation; a small rotation of the hydroxyl group afforded the cyclization products in a good yield, but a large rotation of the hydroxyl group gave a marginal or no detectable amount of products.

Abstract: The first formation of a Nicholas carbocation through cleavage of a carbon–carbon σ bond has allowed the preparation of highly substituted tetrahydrofurans in a formal dipolar cycloaddition reaction.

Abstract: Protonation of the epoxides, diol epoxides, and dihydrodiols of benzo[h]quinoline (BhQ), benzo[f]quinoline (BfQ), phenanthrene (Phe), benzo[c]phenanthridine (BcPhen), and chrysene (Chry) were studied by DFT at the B3LYP/6-31G* level, and selected cases were calculated with the 6-31+G* diffuse-function augmented basis set for comparison purposes Bay-region carbocations were formed from O-protonated epoxides via a barrierless processes Relative carbocation stabilities were determined in the gas phase and with water as solvent (PCM method) The presence of a heteroatom changes the regioselectivity of epoxide ring opening, in some cases favoring non-bay-region carbocations The epoxide ring opening mode is also greatly influenced by N-protonation The dications resulting from initial N-protonation followed by epoxide protonation were also studied by DFT Charge delocalization modes in the resulting mono- and dications were derived by GIAO-NMR (based on Δδ13C values) and via the NPA-derived changes in charges Relative aromaticity in different rings in the arenium ions was gauged by NICS In representative cases, the covalent adducts (syn and anti) formed by reaction of the benzylic carbocations derived from diol epoxides and dihydrodiols with methoxide and methanethiolate anions were studied Relative energies (in the gas phase and with water as solvent) and geometries of the adducts formed by quenching of the carbocations derived from BhQ and Phe-epoxides with guanine via the exocyclic amino group and via the N-7 were also investigated computationally Although aqueous phase calculations change the energy for the addition reactions because of greater stabilization of the reactants, relative reactivity trends remain the same The data are discussed, taking into account the available experimental results concerning the biological activity of these compounds

Abstract: A set of design rules for the prediction of relative stabilities of methoxy substituted naphthyl methyl carbocations are presented based on a series of DFT calculations. The peri-effect, over-crowding, substitutions on the ring carrying the CH2+ group and substitution on the opposite ring are the principal factors that influence the stability of the carbocations. All of these factors have to be taken simultaneously into account. The most pronounced destabilization occurs when the methyl part of the methoxy substituent lies out of the plane of the aromatic core because this causes the resonance stabilization of the carbocation to become hindered. The performance of the DFT-calculations was assessed on the results of a G3(MP2)//B3LYP calculation—a method that is known to predict energies to within chemical accuracy. These values were found to compare well with those obtained at the B3LYP/6-31G(d) level. Thus, a computationally inexpensive method such as the B3LYP/6-31G(d) might prove to be a powerful tool in the design of future complex extended aromatic systems.

Abstract: The detection during solvolysis of competing signature reactions such as the racemization of a chiral substrate, or the scrambling of oxygen isotopes at the leaving group serves as evidence for a stepwise mechanism that proceeds through a reversibly formed ion-pair intermediate. We have examined these signature reactions and determined the relative rates of competing solvolysis, isomerization, and racemization reactions of neutral ring-substituted 1-phenylethyl derivatives. These data were used to define the relative rate constants for partitioning of carbocation–anion pairs between dynamic processes such as reorganization of the ion pair within a solvent cage, and migration of the leaving group anion from the “top” to the “bottom” face of the planar carbocation. We present here a well-defined picture of the dynamics of these reactions in the mostly aqueous solvent of 50/50 (v/v) trifluoroethanol/water. A more complicated problem is the description of the changes in the reaction energy profiles for solvolysis, isomerization, and racemization reactions that occur as one destabilizes the carbocation intermediate of the stepwise reaction. We have examined the signature isomerization and racemization reactions of a ring-substituted 1-phenylethyl derivative that reacts by a “borderline” mechanism. We suggest that the signature isomerization reaction may in fact follow a concerted mechanism that bypasses the putative carbocation–anion pair intermediate. © 2005 The Japan Chemical Journal Forum and Wiley Periodicals, Inc. Chem Rec 5: 94–106; 2005: Published online in Wiley InterScience (www.interscience.wiley.com) DOI 10.1002/tcr.20038

Abstract: Publisher Summary This chapter discusses the dynamics for the reactions of ion pair intermediates of solvolysis. It focuses on determining partition rate constant ratios for a variety of reactions of ion pairs, and of absolute rate constants from these ratios. This has been accomplished by use of one of the rate constants from this product ratio as a “clock” for the second reaction. The addition of water to a free carbocation intermediate of solvolysis has been distinguished from addition to an ion-pair intermediate by an examination of common ion inhibition of solvolysis. The observation of “hidden” reactions during solvolysis, through the use of chiral or isotopically labeled substrates has created considerable excitement in communities interested in the mechanisms of non-enzymatic and enzyme catalyzed reactions. The racemization of chiral substrate or the exchange of bridging and nonbridging oxygen during solvolysis may occur through an ion-pair reaction intermediate that is sufficiently long-lived to undergo reorganization in a solvent cage, or it may proceed by an effectively “concerted” mechanism over an energy maximum that closely resembles the carbocation–anion pair intermediate. Another historically important reaction is the reorganization of “chiral” ion pair intermediates of solvolysis of a chiral substrate that leads to racemization of substrate during solvolysis.

Abstract: Direct deoxygenation of ketoneswas carried out by the reaction of carbocation intermediatewith hydride. Elements which belong to the same group in the Periodictable revealed the same chemical reactivity. We paidattention to the reactivity of gallium which belongs to thesame group as boron, aluminum, and indium. However,there had been only few examples of synthetic reactionusing gallium compounds.

Abstract: In a model computational study aimed at understanding structure-reactivity relationships and substituent effects on carbocation stability in aza-polycyclic aromatic hydrocarbons, the epoxides, diol epoxides, and the dihydrodiols of dibenzo[a,h]acridine (DB[a,h]ACR) were studied by density functional theory at the B3LYP/6-31G level. Bay region carbocations were formed via the O-protonated epoxides in barrierless processes. Relative carbocation stabilities were determined in the gas phase and in water as solvent (polarized continuum model method). Charge delocalization modes in the resulting carbocations were deduced by gauge-independent atomic orbitals (GIAO) NMR (based on Delta delta13C values) and via the natural population analysis (NPA)-derived changes in charges. Although the solvent decreases the exothermicity of the epoxide ring-opening reactions due to greater stabilization of the reactants, relative reactivity trends remain the same. Whereas fluorine substitution at ring positions bearing significant positive charge leads to carbocation stabilization by fluorine p-pi back-bonding, fluorine substitution at a ring position that presented negative charge density in the unsubstituted compound leads to inductive destabilization. Methylated derivatives exhibit less sensitivity to substituent effects as compared to the fluorinated analogues. A bay region methyl group produces structural distortion, and this deviation from planarity destabilizes the epoxide, favoring ring opening. Relative energies, changes in NPA charges, and GIAO NMR data in the resulting "benzylic" carbocations are examined collectively and discussed, taking into account the available biological activity data on these compounds.

Abstract: The diastereoselective synthesis of unsaturated 1.4-amino alcohols can be achieved using chial allylic ethers with a hydroxyl group attached to the π-system and chlorosulfonyl isocyanate. The enantioselectivity of the CSI reaction with the chiral allylic and benzylic ethers was examined in various solvents and temperatures. Based on these results, it was proposed that the CSI reaction is a competitive reaction of a SNi (retention) and a SN1 mechanism (racemization) according to the stability of the carbocation intermediate. This means that there is a greater proportion of retention with the less stable the carbocation intermediate and vise versa.

Abstract: OH(+) is an extraordinarily strong oxidant. Complexed forms (L--OH(+)), such as H(2)OOH(+), H(3)NOH(+), or iron-porphyrin-OH(+) are the anticipated oxidants in many chemical reactions. While these molecules are typically not stable in solution, their isolation can be achieved in the gas phase. We report a systematic survey of the influence on L on the reactivity of L--OH(+) towards alkanes and halogenated alkanes, showing the tremendous influence of L on the reactivity of L--OH(+). With the help of with quantum chemical calculations, detailed mechanistic insights on these very general reactions are gained. The gas-phase pseudo-first-order reaction rates of H(2)OOH(+), H(3)NOH(+), and protonated 4-picoline-N-oxide towards isobutane and different halogenated alkanes C(n)H(2n+1)Cl (n=1-4), HCF(3), CF(4), and CF(2)Cl(2) have been determined by means of Fourier transform ion cyclotron resonance measurements. Reaction rates for H(2)OOH(+) are generally fast (7.2x10(-10)-3.0x10(-9) cm(3) mol(-1) s(-1)) and only in the cases HCF(3) and CF(4) no reactivity is observed. In contrast to this H(3)NOH(+) only reacts with tC(4)H(9)Cl (k(obs)=9.2x10(-10)), while 4-CH(3)-C(5)H(4)N-OH(+) is completely unreactive. While H(2)OOH(+) oxidizes alkanes by an initial hydride abstraction upon formation of a carbocation, it reacts with halogenated alkanes at the chlorine atom. Two mechanistic scenarios, namely oxidation at the halogen atom or proton transfer are found. Accurate proton affinities for HOOH, NH(2)OH, a series of alkanes C(n)H(2n+2) (n=1-4), and halogenated alkanes C(n)H(2n+1)Cl (n=1-4), HCF(3), CF(4), and CF(2)Cl(2), were calculated by using the G3 method and are in excellent agreement with experimental values, where available. The G3 enthalpies of reaction are also consistent with the observed products. The tendency for oxidation of alkanes by hydride abstraction is expressed in terms of G3 hydride affinities of the corresponding cationic products C(n)H(2n+1) (+) (n=1-4) and C(n)H(2n)Cl(+) (n=1-4). The hypersurface for the reaction of H(2)OOH(+) with CH(3)Cl and C(2)H(5)Cl was calculated at the B3 LYP, MP2, and G3(m*) level, underlining the three mechanistic scenarios in which the reaction is either induced by oxidation at the hydrogen or the halogen atom, or by proton transfer.

Abstract: NEW SPECTROSCOPIC STUDY of protonated methane, CH 5 +, is allowing long-standing questions about the mysterious ion's structure and dynamics to be answered with greater clarity than ever before. CH 5 +, also called methonium ion, is a carbocation formed in the gas phase by a chemical reaction that adds a proton to methane, and it has also been reported to be present in very acidic solution. It has resisted complete spectroscopic characterization since it was discovered in the early 1950s, but researchers have now obtained the first broad-frequency infrared (IR) spectrum of the ion. The spectrum reveals new details about the structure and dynamics of CH 5 +. More work still needs to be done for the ion's structure and dynamics to be fully characterized, but the new study represents a key step toward that goal. The work was carried out by a collaborative team including Britta Redlich, manager of the Free Electron Laser for Infrared ...

Abstract: Sec-butyl and tert-butyl cations were first stabilized upon cocondensation of aluminum chloride and haloalkanes (2-chlorobutane and 2-chloro-2-methylpropane, respectively) and their IR spectra and thermal stability investigated. Aluminum chloride anions participating in stabilization of carbenium ions were revealed. Ionic complexes and both 1:1 and 2:1 molecular complexes of reagents were detected and their IR spectra studied. Quantum-chemical calculations at the PBE level of density functional theory with inclusion of electron correlation were employed to optimize the geometric parameters and to determine the vibrational frequencies of aluminum chloride, butyl chlorides, and their complexes of various compositions. The structures of the associates observed were determined on the basis of comparison of the experimental and calculated vibrational spectra.

Abstract: The potential energy surface relating to the C7H11+ bicyclobutonium cation and its isomers was investigated in the gas phase using the ab initio and density functional calculations. The unsymmetrical bicyclobutonium ion 5 was identified with the MP2/6-31G* method as a global minimum, which is slightly more stable by 0.8 kcal/mol than the boat conformer of the bicyclo[3.1.0]heptyl-1-carbinyl cation 6. Similar results were obtained when larger basis sets, such as 6-311+G**, were used to optimize the geometries. However, the energy gap between the bicyclobutonium ion 5 and the chair conformer of cyclopropylcarbinyl cation 6 became even smaller when the MP4(SDQ)/6-31G* method was used. The small differences in terms of relative energies between these minima located at the MP2/6-31G* and MP4(SDQ)/6-31G* levels reflect the flat profile of the corresponding part of C7H11+ potential energy surfaces. The geometries of other types of C7H11+ isomeric carbocations were also optimized such as the allyl cation 7, the chair conformer of bicyclo[3.1.0]heptyl-1-carbinyl ion 8, and π-complex 9, with the latter species being clearly less stable by 15.2 kcal/mol than 5. Becke's hybrid density functional, B3LYP, combined with the 6-31G* and 6-311+G** basis sets, was also utilized to investigate the C7H11+ potential energy surface. The majority of B3LYP results were comparable to those obtained with the post Hartree–Fock methods; however, the unsymmetrical bicyclobutonium ion 5 could not be found with this density functional method. Low energy barriers separate the two cyclopropylcarbinyl cations 6 and 8, preventing them from rearranging into the bicyclobutonium type of ion 5. A much higher barrier is found for the rearrangement of the two cyclopropylcarbinyl cations 6 and 8 into allyl and π-complex species. The carbocation with the positive charge localized at the bridgehead position does not represent a minimum at any level of theory used throughout this investigation.

Abstract: This chapter describes the formation of carbanions and some of their reactions. Directed deprotonation offers a regioselective route to carbanions. Diastereo- and enantioselective deprotonation can be achieved by using either sparteine or atropoisomerism. Enantioselectivity can also be induced by the introduction of chiral auxiliaries, like in chiral cnolates. Oxidative addition of some metals to halogen–carbon bond provides an entry to carbanions. Single-electron transfer from a metal to halides also gives carbanions; this process can be facilitated by arenes. Halogen, chalcogen, or tin-metal exchanges provide a convenient and stereoselective access to carbanions. Permanent or transient nucleophilic addition to enones gave enolates, which can be trapped by various electrophiles in a 1,4 or 1,3 addition (Michael or Baylis-Willman reactions) Carbo- or hydrometallation also offers a stereo- and regioselective access to carbanions. Such processes can be applied to the formation of 1,1-dicarbanions. Transmetallation allows to modulate carbanion reactivity and selectivity. Transmetallation sometimes produces dicarbanions, The Kulinkovich and Dzelmilev reactions are examples of such behaviors. Since the discovery of the ionization in sulfuric acid of triphenylmethanol into triphenylmethyl (trityl) cation in 1901, and the Meerwein's proposal in 1922 that the Wagner rearrangement of camphene hydrochloride into isonorbornyl chloride involves alkyl cation intermediates, carbocations have been found everywhere, from interstellar space to plants and other living systems, either as reactive intermediates or as stable salts. Gasoline production (alkane alkylation) and important materials (living cationic polymerization of alkenes) in our civilization rely upon procedures involving carbocations. The study of the carbocations has played a fundamental role in the understanding of organic reactivity and biochemistry: it has helped us to develop better models for chemical bond of organic and organometallic compounds. Thanks to mass spectrometric methods accurate thermodynamic and kinetic data have been collected for carbocations and their reactions in the gas phase. Vertical and non-vertical substituent effects on the relative stability of ionized species are well understood. Accurate characterization of electrophilicity and nucleophilicity has now been realized. Carboranes have been used to generate powerful super-acids capable to protonate systems such as the fullerenes. Important catalysts such as zeolites generate carbocationic intermediates. Unstable “antiaromatic” cyclopentadienyl cation has been generated in a matrix, the IR spectrum of phenyl cation has been recorded. The technique called “heavy-electron” photoelectron spectroscopy has permitted to measure vibrational frequencies and rotational constant of methyl cation. Instable carbocations can be generated by fragmentation of alkoxy(chloro)carbenes.

Abstract: A new example is provided of completely diastereoselective polycyclization, affording the octahydrophenanthrene framework. Generation of an iron tricarbonyl stabilized pentadienyl carbocation is the triggering event of the cascade reaction. The carbocation is generated by anchimerically assisted regiospecific protonation of a double bond adjacent to the iron tricarbonyl diene moiety. Tetrafluoroboric acid ether complex appears to be the optimum reagent, affording good yields, even under catalytic conditions.