Abstract: The fundamentals: the IR spectrum of the adamantane cation, C(10)H(16)(+), has been derived by resonant IR photodissociation of weakly bound C(10)H(16)(+)⋅L(n) clusters. The analysis of the IR spectrum provides the first spectroscopic characterization of this fundamental cycloalkane carbocation in the gas phase and direct evidence for the Jahn-Teller distortion in the (2)A(1) ground electronic state.

Abstract: UV irradiation (266 or 280 nm) of benzhydryl triarylphosphonium salts Ar2CH-PAr3+X– yields benzhydryl cations Ar2CH+ and/or benzhydryl radicals Ar2CH•. The efficiency and mechanism of the photo-cleavage were studied by nanosecond laser flash photolysis and by ultrafast spectroscopy with a state-of-the-art femtosecond transient spectrometer. The influences of the photo-electrofuge (Ar2CH+), the photo-nucleofuge (PPh3 or P(p-Cl-C6H4)3), the counterion (X– = BF4–, SbF6–, Cl–, or Br–), and the solvent (CH2Cl2 or CH3CN) were investigated. Photogeneration of carbocations from Ar2CH-PAr3+BF4– or -SbF6– is considerably more efficient than from typical neutral precursors (e.g., benzhydryl chlorides or bromides). The photochemistry of phosphonium salts is controlled by the degree of ion pairing, which depends on the solvent and the concentration of the phosphonium salts. High yields of carbocations are obtained by photolyses of phosphonium salts with complex counterions (X– = BF4– or SbF6–), while photolyses of pho...

Abstract: A survey of computational studies on the mechanisms of dyotropic, Schmidt, and related reactions is presented. Connections between synthetically applied versions of these processes and those predicted to occur during biosynthetic terpene-forming carbocation cascades are highlighted.

Abstract: It takes al-kynes: The formation of furyl-substituted heterocycles from furanynes with donor groups on the furan-alkyne tether and mechanistic control experiments indicate the involvement of open-chained carbenium ions in the overall insertion of an alkyne into a C-C bond, rather than the usual spirocyclic intermediates.

Abstract: Oxatriquinanes are tricyclic oxonium ions which are known to possess remarkable solvolytic stability compared to simple alkyl oxonium salts. Their rigid, hemispherical structure presents an oxygen at the apex of three fused five-membered rings. While trivalent oxygen species like these have been well described in the literature, the ability of oxygen to enter into a fourth covalent bonding relationship has been visited in theory and suggested by the outcome of certain reactions conducted in superacidic media, but has never been established by the characterization of a stable, persistent R3OH2+ or R4O2+ ion. In this study, the nucleophilicity of the oxatriquinane oxygen was evaluated first by a series of protonation studies using the Bronsted superacid H(CHB11Cl11) both in the solid state and in liquid HCl solution. The interaction of the oxatriquinane oxygen with a bridging carbocation was also examined. A strong case could be made for the occurrence of hydrogen bonding between H(CHB11Cl11) and oxatriquin...

Abstract: Car-Parrinello molecular dynamics simulations (CPMD) coupled with metadynamics (MTD) simulations were conducted to investigate glucose isomerization to fructose in acidic aqueous solution. Glucose to fructose isomerization is initiated by protonation of the C2-OH and the formation of a furanose aldehyde intermediate. Fructose is produced via a hydride transfer from C2 to C1 on the furanose aldehyde followed by the rehydration of the C2 carbocation. Hydride 1,2 shift to form a C2 carbocation is an energetically favorable process but the barrier is relatively high at around 35 kcal/mol. The final step during glucose to fructose isomerization involves the rehydration of the C2 carbocation with an estimated barrier of 25 kcal/mol from our CPMD-MTD simulations.

Abstract: Definitely different: the path towards sorting out a long-standing dichotomy in carbocation chemistry is disclosed by infrared multiple photon dissociation spectroscopy of tropylium and benzylium isomers of C(8)H(9)(+) ions.

Abstract: NphB is an aromatic prenyltransferase that catalyzes the attachment of a 10-carbon geranyl group to aromatic substrates. Importantly, NphB exhibits a rich substrate selectivity and product regioselectivity. A systematic computational study has been conducted in order to address several question associated with NphB-catalyzed geranylation. The reaction mechanism of the prenylation step has been characterized as a SN1 type dissociative mechanism with a weakly stable carbocation intermediate. A novel π-chamber composed of Tyr121, Tyr216, and 1,6-DHN is found to be important in stabilizing the carbocation. The observed difference in the rates of product formation from 5- and 2-prenylation arises from the differing orientations of the aromatic substrate in the resting state. 4-Prenylation shares the same resting state with 5-prenylation, but the lower free energy barrier for carbocation formation makes the latter reaction more facile. The high free energy barrier associated with 7-prenylation is caused by the ...

Abstract: Density functional theory calculations have been employed to investigate the mechanism of gold(I)-catalysed rearrangements of cyclopropenes. Product formation is controlled by the initial ring-opening step which results in the formation of a gold-stabilised carbocation/gold carbene intermediate. With 3-phenylcyclopropene-3-methylcarboxylate, the preferred intermediate allows cyclisation via nucleophilic attack of the carbonyl group and hence butenolide formation. Further calculations on simple model systems show that substituent effects can be rationalised by the charge distribution in the ring-opening transition state and, in particular, a loss of negative charge at what becomes the β-position of the intermediate. With 1-C3H3R cyclopropenes (R = Me, vinyl, Ph), ring-opening therefore places the substituent at the β-position.

Abstract: As model substrates for the desired tandemreaction, 1-hexene (1a) as active terminal olefin and alcohol2 as carbocation precursor were chosen. For the hydrofor-mylation, a rhodium precatalyst was selected and synthesisgas (1:1) with pressure of 30 bars was applied.Proline was selected as initial catalyst for the a-alkylationwith additional acid for carbocation generation. In a preced-ing solvent screening, DMF, THF, acetonitrile, and toluenedid not lead to significant conversion (see the Supporting In-formation), whereas the combination of chloroform with tri-fluoroacetic acid gave more encouraging results. Previousexperiments in the a-alkylation had shown Bronsted acidsto be beneficial in the organocatalytic reaction step(Table 1, entry 1).To accelerate the hydroformylation of 1-hexene (1a), sev-eral rhodium precursors were tested in this reaction. Al-though all rhodium precursors were active in this reaction,surprisingly, the frequently used [Rh(CO)

Abstract: The prenyltransferase CymD catalyzes the reverse N-prenylation of tryptophan using dimethylallyl diphosphate (DMAPP) in the biosynthesis of the cyclic peptides cyclomarin and cyclomarazine. The mechanism is of interest because a non-nucleophilic indole nitrogen must be alkylated in this process. Three mechanisms were initially considered, including (A) a direct addition of a carbocation to the nitrogen, (B) an addition of a carbocation to C-3 followed by an aza-Cope rearrangement, and (C) deprotonation of the indole followed by an SN2′ addition to DMAPP. The use of 4-fluorotryptophan and 6-fluorotryptophan revealed that the reaction kinetics are only modestly affected by these substitutions, consistent with the notion that positive charge does not accumulate on the indole ring during catalysis. When (E)-3-(fluoromethyl)-2-buten-1-yl diphosphate was used in place of DMAPP, the maximal rate was reduced by a factor of 100, consistent with the development of positive charge on the dimethylallyl moiety. Positi...

Abstract: The nature of the recently described “triple shift” rearrangement of a biologically relevant carbocation (computed in the absence of a surrounding enzyme) is characterized by examining the evolution of charge distribution, molecular volume, accessible surface area, and multicenter bonding indices along its reaction coordinate. Implications for interaction of the rearranging carbocation with a terpene synthase active site are discussed.

Abstract: The rearrangement of epoxidized vegetable oils to produce fatty ketones, catalyzed by acidic resins, is disclosed. The non-microbial production of fatty ketones from epoxidized vegetable oils has not been previously reported. Some of the variables affecting the ketone formation such as acidic strength of the catalyst, temperature and solvent are studied in order to determine the conditions that favor the rearrangement. Epoxidized vegetable oils can be easily transformed to the respective ketones via a rearrangement reaction catalyzed by acidic resins. Other kind of acidic catalysts are active for this reaction if their Bronsted acid sites are accessible to protonate the epoxide. Formation of ketones, from the rearrangement of epoxidized methyl oleate, is favored in the presence of strongly acidic catalysts and enhanced by increasing temperature. Polar-protic solvents increase the ketone yield but decrease the ketone selectivity because they are added to the epoxide ring. The mechanism for the epoxide rearrangement is very likely to take place through a hydride migration to the carbocation generated in the acid-catalyzed epoxide ring opening.

Abstract: A first computational insight into the intrinsic strength of exocyclic bonds to phosphorus in oxaphosphirane κP-pentacarbonylmetal(0) complexes 1a-f (M=Cr, Mo) is provided using a set of P-R derivatives (R=Me, tBu, CPh(3)). Whereas homolytic cleavage of the exocyclic P-R bond was found to be always unfavored (for neutral complexes), heterolytic cleavage leading to a carbocation R(+) moiety and the oxaphosphiranide complex 2(-) constitutes the lowest-energy process, especially if R is bulky and can stabilize the positive charge, that is, triphenylmethyl (trityl), efficiently. The energies required for P-M bond cleavage are about 30 kcal mol(-1), and decrease with the increasing bulk of the R substituent (from Me to trityl) and ongoing from Cr to Mo. The reactivities of complexes 1a-f towards oxidative and reductive single electron transfer (SET) reactions were analyzed using the facile variation of bond-strength-related descriptors (VBSD) methodology, thus enabling the design of synthetically useful strategies addressing decomplexation and P-functionalization. Reductive SET reactions with sodium naphthalenides enable selective P-M bond cleavage (i.e., decomplexation) for the case of P-Me and P-tBu substitution, whereas reductive P-R bond cleavage is favored in the case of the P-trityl complexes 1c,f, and results in the formation of the (anionic) oxaphosphiranide complex 2(-), which may be regarded as a potential key intermediate for further P-functionalization.

Abstract: The 1,2-bis[(diarylphosphino)ethynyl]benzene derivatives 1a (R=Ph) and 1b (R=o-tolyl) undergo 1,1-carboboration at one of their acetylene units upon treatment with (C6F5)3B at elevated temperature to give the products 5a and 5b, respectively. At room temperature, we observed the formation of the corresponding phosphireniumborate zwitterions, 7a and 7b, respectively, which may be intermediates of the 1,1-carboboration reactions. The reaction of the more bulky 1,2-bis[(dimesitylphosphino)ethynyl]benzene 1c with (C6F5)3B takes a different course. At 110°, we observed the complete conversion to the benzopentafulvene derivative 8 which is probably formed in a typical carbocation rearrangement sequence after the initial (C6F5)3B Lewis acid-addition step. The compounds 5a, 5b, 7b, and 8 were characterized by X-ray crystal-structure analyses.

Abstract: The ring-shift isomerization of sym-octahydrophenanthrene (sym-OHP) to sym-octahydroanthracene (sym-OHA) catalyzed by acidic zeolites (Mordenite (MOR) and Faujasite (FAU)) was investigated by the ONIOM(DFT:UFF) and DFT approaches. A “five-membered ring” mechanism through carbocation rearrangement via 1–2 migration was proved to be kinetically favored over a “six-membered ring” mechanism through Friedel–Crafts reactions. Computational studies based on the “five-membered ring” mechanism demonstrate that a decreasing Bronsted acid site strength from Al-H-MOR to Ga-H-MOR to B-H-MOR reduces the catalytic activity. The catalyst acid site strength would thereby impact the yield of sym-OHA. The isomerization barrier increases when using an Al-H-FAU catalyst that has a similar Bronsted acid site strength as Al-H-MOR but considerably bigger cages, indicating that apart from the desired density and strength of acid sites, optimal zeolite catalysts should have a pore size that better fits the intermediates and transition states. DFT calculations on Gibbs free energy were performed to evaluate the equilibrium ratios of sym-OHA to sym-OHP at specific reaction temperatures from 175 to 325 °C. The results indicate that reaction temperature has a moderate impact on the equilibrium yield of sym-OHA, whose formation is relatively favorable at a lower temperature under experimental conditions.

Abstract: Trityl chloride (triphenylmethyl chloride, TrCl, Ph3CCl) is utilized as an efficient and homogeneous organocatalyst for the synthesis of 14-aryl-14H-dibenzo[a,j]xanthenes from β-naphthol and arylaldehydes under solvent-free conditions. Moreover, a plausible mechanism is suggested based on the literature and on in situ formation of trityl carbocation with inherent instability during the reaction.

Abstract: The invention relates to the use of a compound according to formula (I) as a contact biocide, wherein A is a fragment that coordinates as a cation on the free electron pair of a nitrogen atom, B is a fragment that reacts as an anion with a carbocation to form an uncharged product, R is an organic radical from a saturated aliphatic chain C 6 to C 30 , a saturated cycloaliphatic radical, a mono- or polyunsaturated aliphatic chain having at least 6 carbon atoms, an alkyl spacer to crosslinking oligooxazolines, an aryl spacer to crosslinking oligooxazolines, a phenyl, an alkylphenyl, a heteroaryl or copolymers thereof, and X is a biocidally ineffective anion, wherein (n+m) > 10.

Abstract: The effect of β-trimethylsilyl (TMS) substituent on the structure, stability, natural charges, electrostatic potential map, natural bond orders, rotational energy barrier, and hyperconjugative interactions of five acyclic β-silyl carbocation derivatives of RR′C+–CH2Si(Me)3 including α-dimethyl 1 (R,R′ = Me), α-methyl phenyl 2 (R = Me, R′ = Ph), α-methyl para-aminophenyl 3 (R = Me, R′ = p-NH2Ph), α-methyl para-nitrophenyl 4 (R = Me, R′ = p-NO2Ph) and diphenyl 5 (R,R′ = Ph) was investigated in the gas phase and in solution using polarized continuum model (PCM) at B3LYP/6-311 ++G** level of theory. The resonance structures weighting of cations 1–5 were determined using natural resonance theory (NRT). The contribution of carbenium ion (RR′C+–CH2Si(Me)3) and silylium ion (RR′C=CH2 Si(Me) 3 + ) to the stability depend upon substituents. The former form dominants when R,R′ = Ph, but the latter is major the contributor when R,R′ = Me. The weighting of carbocation forms of β-silyl benzyl cation overwhelms silylium cation due to the delocalization of positive charge on the phenyl ring. The calculated molecular orbital (MO) diagrams, energy decomposition analysis (EDA) and 29Si and 13C nuclear magnetic resonance (NMR) chemical shifts complement these predictions.

Abstract: Rate constants for acid-catalyzed dehydration of cis-2-substituted 1,2-dihydro-naphthols are well correlated by the Taft relationship log k = −0.49 – 8.8σI, with minor negative deviations for OH and OMe. By contrast the trans substituents show a poor correlation with σI and in most cases react more slowly than their cis isomers. The behavior is consistent with rate-determining formation of a 2-substituted carbocation (naphthalenium ion) intermediate that for cis reactants possesses a 2-C–H bond suitably oriented for hyperconjugation with the charge center. For the trans isomers the 2-substituent itself is oriented for hyperconjugation in the initially formed conformation of the cation. It is argued that kcis/ktrans rate ratios for substituents (Me, 8.4; But, 12.7; Ph, 3.8; NH3+, 160; OH, 440) reflect their hyperconjugating ability relative to hydrogen. Faster reactions of trans isomers are observed for substitutents known (RS, N3) or suspected (EtSO, EtSO2) of stabilizing the cation by a π or σ neighborin...

Abstract: Homoallylic amines were prepared by asymmetric deprotonation of benzylamines using n-BuLi and (–)-sparteine to give chiral organolithiums to which were added prenyl bromide. Removal of the Boc protecting group gave anilines that were found to undergo Bronsted acid catalyzed or iodine-mediated cyclization to give aryl-substituted pyrrolidine heterocyclic products with high enantioselectivity.