Abstract: Highly useful carbon−carbon bond forming reactions using stable allenyl, propargyl, or allyl−propargyl hybrid cations have been developed. These carbocations could be generated from silyl 1-(π-donor)-substituted propargyl ethers by the action of trimethylsilyl trifluoromethanesulfonate in dichloromethane at −78 °C to room temperature and could be attacked nucleophilically by electron rich arenes, allylsilanes, or enol silyl ethers, giving rise to allenes, alkynes, and their derivatives. A novel method for regio- and stereoselective synthesis of conjugated enynes utilizing allyl−propargyl hybrid cations has also been established.

Abstract: Thermal elimination reactions on polycarbonates are investigated from both theoretical and experimental points of view, to obtain insight into the microscopic aspects that influence the reaction mechanism and rates. In particular, attention is focused on the influence of the type of substituents in the polymer chain on the reaction rates. Ab initio density functional theory calculations are performed on a series of model compound systems for the polycarbonates under study, in particular carbonates differing by the groups attached at the α and β carbon atoms. Reactants, products, and transition states are optimized at the B3LYP/6-311g** level of theory. The structures of the activated complex give insight into the mechanistic details of this type of Ei elimination reactions. The Cα−O bond dissociates before the Cβ−H bond, developing some carbocation character in the transition state on the Cα atom. The kinematics of the thermal decomposition reactions have been studied by means of transition state theory b...

Abstract: The substituted fluorenyl cation, 9-(diphenylmethyl)fluoren-9-yl cation (4), is formed under stable ion conditions (low temperature/strong acid) from its corresponding alcohol 3. This ion is transformed to a substituted diphenyl methyl cation 8 at ambient temperature via an apparent 1,2-hydrogen shift. Irradiation of 9-(diphenylmethyl)fluoren-9-ol in methanol gives products derived from the corresponding cation along with radical-derived products from C-C and C-O homolysis processes. The laser flash photolysis of this alcohol gave a transient corresponding to cation 4. All of the photoproducts are derived from cation 4 or radical pathways. High level MO calculations point to a high barrier (23.8 kcal x mol(-1)) for the 1,2-hydride shift. This barrier is the consequence of the minimum energy conformation of this fluorenyl cation which is less than ideal for the periplanar geometry necessary for this process.

Abstract: The probe bases used for measurement of strength of acid catalysts need to be structurally similar to the catalytic substrates. Traditional acidity measurements are inapplicable when the hydron transfer forms a tight ion pair, as on solid acids. Also, solid acids are much weaker than liquid acids of similar structure. Alkane activation by C–H and C–C cleavage, evidenced in strong superacids, does not occur in less strongly acidic media. The reactivity patterns and structure–reactivity relationships are the same for the cationoidic species (weakly coordinated carbocations) occurring in the less strong acids as for the full-fledged carbocations intervening in superacids. Even in trifluoromethanesulfonic acid, a weak superacid, the first step in alkane activation is an oxidation, forming unsaturated carbocations. On sulfated zirconia, the first step of alkane reactions is a one-electron oxidation. Mechanistic features incompatible with standard (or “traditional”) carbocationic processes, are observed in the reaction of 3-methylpentane on the zeolite HZSM-5.

Abstract: The first series of persistent carbocations derived from mono- and disubstituted chrysenes Ch (5- methyl- 3, 2-methoxy- 19, 2-methoxy-11-methyl- 20, 2-methoxy-5-methyl- 21, and 9-methyl-4H-cyclopenta[def]chrysene 22), monosubstituted benzo[c]phenanthrenes BcPh (3-methoxy- 23, 3-hydroxy- 24), and monosubstituted benzo[g]chrysenes BgCh (12-methoxy- 25; 12-hydroxy- 26) were generated in FSO3H/SO2ClF or FSO3H-SbF5 (4:1)/SO2ClF and studied by low-temperature NMR at 500 MHz. The methoxy and methyl substituents direct the protonation to their respective ortho positions. Whereas parent Ch 1 is protonated at C-6/C-12, 3 is protonated at C-6 (3aH+) and at C-12 (3bH+) with the latter being the thermodynamic cation. The 2-methoxy-Ch 19 is protonated at C-1 to give two conformationally distinct carboxonium ions (19aH+/19bH+). In the disubstituted Ch derivatives 20 and 21, the 2-methoxy overrides the 5-methyl and the predominant carbocations formed are via attack ortho to methoxy. For the methano derivative 22 (Me at C-9), a 3:1 mixture of 22aH+/22bH+ is formed. For parent BcPh 13, nitration and benzoylation are directed to C-5. With 3-methoxy-BcPh 23, the site of attack moves to C-4, thus producing two conformationally distinct carboxonium ions (23aH+/23bH+), whereas conventional nitration gave a 2:1 mixture of 23aNO2 and 23bNO2. In 3-hydroxy-BcPh 24, the carboxonium ion 24H+ is exclusively formed. For parent BgCh 16, protonation, nitration, and benzoylation are all directed to C-10 (16H+, 16NO2, 16COPh), but presence of OMe or OH substituent at C-12 changes the site of attack to C-11. Charge delocalization mode is probed based on magnitude of delta delta 13Cs and conformational aspects via NOED experiments. Complete NMR data are also reported for several benzoylation/nitration products. Using ab initio/GIAO (and NICS), the NMR chemical shifts (and aromaticity) in model carbocations A-D were evaluated. This work represents the first direct study of the carbocations derived from the methyl-, methoxy-/hydroxy-derivatives of three important classes of bay-region and fjord-region PAHs whose diol-epoxides extensively bind to DNA. It also extends the available data on electrophilic chemistry of BcPh and BgCh.

Abstract: The curing behavior of bisphenol-A-type epoxide oligomers (Ep) was evaluated by differential scanning calorimetry in the presence of S-alkylsulfonium salts of dibenzothiophene, phenoxathiin, thianthrene, thioanisole, and tetrahydrothiophene as thermal latent initiators. These initiators dissolved homogeneously in Ep, except for 2,8-dimethoxy-5-methyldibenzothiophenium tetrafluoroborate, and the curing reaction of the resulting mixtures occurred on heating, except for S-methyltetrahydrothiophenium tetrafluoroborate. The initiation activity of these salts was controlled by the character of the substituents on the benzene ring, the leaving sulfide group, and the S-alkyl group. Presumably, the electron density on the sulfide moieties and the stability of the carbocation released from the sulfonium salts affected the initiating temperature. A good correlation was obtained between the initiating temperature and the electron density of the sulfur atom of the corresponding sulfides, estimated from ab initio molecular orbital calculations in which the initiating temperature became higher as the electron density of the sulfur atom increased. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 868–871, 2001

Abstract: The probe bases used for measurement of strength of acid catalysts need to be structurally similar to the catalytic substrates. Traditional acidity measurements are inapplicable when the hydron transfer forms a tight ion pair, as on solid acids. Also, solid acids are much weaker than liquid acids of similar structure. Alkane activation by C–H and C–C cleavage, evidenced in strong superacids, does not occur in less strongly acidic media. The reactivity patterns and structure–reactivity relationships are the same for the cationoidic species (weakly coordinated carbocations) occurring in the less strong acids as for the full-fledged carbocations intervening in superacids. Even in trifluoromethanesulfonic acid, a weak superacid, the first step in alkane activation is an oxidation, forming unsaturated carbocations. On sulfated zirconia, the first step of alkane reactions is a one-electron oxidation. Mechanistic features incompatible with standard (or “traditional”) carbocationic processes, are observed in the reaction of 3-methylpentane on the zeolite HZSM-5.

Abstract: Cations are thermodynamically stable on certain transition metal surfaces, but so far no one has determined whether cations are catalytic intermediates or merely spectators. In this paper TPD and HREELS are used to determine whether methoxonium cations ([CH3OH2]+ (ad)) or closely related species are intermediates during the reaction H2+CH3OH→CH4+H2O on Pt(110). HREELS shows that the methoxonium cations are produced at the same temperature where reaction starts, and that the methoxonium cations decay in the same temperature range where the reaction stops, suggesting that methoxonium is a likely intermediate during the reaction. Replacement of CH3OH with CD3OH produces a 1.8-fold change in the yield of water and methane at 180 K due to a secondary kinetic isotope effect. The strong secondary kinetic isotope effect would be expected only if the reaction occurred via an SN1 pathway with a fully ionized transition state with the positive charge on the carbon. Together the HREELS and TPD results provide strong evidence that reaction goes via a carbocation pathway, with methoxonium cations or some closely related species as likely intermediates. This paper demonstrates for the first time that carbocations can be reactive intermediates during reactions on metal surfaces.

Abstract: The rates and products of the reactions of (±)-7β,8α-dihydroxy-9β,10β-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (1) and (±)-7β,8α-dihydroxy-9α,10α-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (2) in water and dioxane−water mixtures have been determined over a pH range wider than that of earlier studies. This study provides additional insight on the mechanisms of the pH-independent reactions of 1 and 2. The rate profile for reaction of 1 shows acid-catalyzed hydrolysis at pH 11.5. The rate decrease between pH 10 and pH 11.5 is accompanied by a decrease in the yield of tetrols from 60% (pH 8) to 29% (pH 11.2) and is interpreted to be the result of a partial change in mechanism brought about by attack of hydroxide ion acting as a base to deprotonate a carbocation intermediate and regenerate 1 at pH >10, thus reducing the contribution of the pathway for tetrol formation in which water attacks the carbocation. The rate pro...

Abstract: The ratio of rate constants for the hydration of the bis(p-methoxyphenyl)methyl cation and the (p-hydroxyphenyl)(p-methoxyphenyl)methyl cation is 1.75 ∶ 1. Thus a methoxybenzyl carbocation is a good model for a protonated p-quinone methide, and the stabilizing effect of hydration on the OH compound is minimal.

Abstract: We showed in [1] that a-hydroxy thiocyanates can be alkylated by tertiary carbocations, following the Ritter reaction pattern; intermediate thionitrilium cation is stabilized through intramolecular cyclization involving the neighboring hydroxy group to form 2-alkylimino-1,3-oxathiolanes. With the goal of searching for other functional groups capable of stabilizing thionitrilium cations via intramolecular interaction, we examined the reactions of 1-hydroxyadamantane (I) with thiocyanatoacetamide (II) and thiocyanatoacetylurea (III) in concentrated sulfuric acid. However, these reactions gave unexpected products formally resulting from replacement of the cyano group by adamantyl (compounds IV, V, and VI).

Abstract: The reaction of ethylene radical cations with a broad selection of uncharged nucleophiles (hydrogen fluoride, dichloromethane, trifluoroethanol, water, formaldehyde, methanol, ethanol, ethylene glycol, dimethyl formamide, acetonitrile, ammonia) has been studied at the BHandHLYP/6-31 + G(d,p) level of theory. With the exception of hydrogen fluoride and dichloromethane, all of these nucleophiles form covalent addition products with ethylene radical cations. Transition states for these addition reactions could, however, not be identified. The reaction energies calculated for the addition reactions correlate well with the proton affinities of the respective nucleophiles. Based on this correlation the reactivity of ethylene radical cations can be characterized as intrinsically cationic. The reaction of ammonia, the water dimer, and the water trimer with the radical cations of some substituted alkenes (propene, trans-but-2-ene, butadiene, styrene) have been studied at the same level of theory. In all cases the most exothermic reaction occurs for the water trimer. Comparison of the reaction energies calculated for these radical cations with those calculated for the analogously substituted carbocations shows that alkene radical cations can best be understood as strongly stabilized carbocations. The stabilizing effect of the radical center on cationic intermediates is not constant and varies between 40 kcal mol−1 for the smaller systems and 20 kcal mol−1 for the larger systems investigated in this study.

Abstract: The nucleophilic substitution reaction by gas−solid contact has been investigated. When 9-thienothienylfluoren-9-ol derivatives were coground with dichlorodicyanoquinone (DDQ) and then exposed to methanol vapor, the corresponding 9-methoxyfluorenes were obtained in 15−70% yields. Throughout the whole procedure the solid state was retained. The generation of a radical cation in the coground solids via charge-transfer interaction between the substrate alcohol and DDQ was suggested by the ESR spectrum. The mechanism involving the collapse of the radical cation to generate a proton, which acts as a catalyst to afford the carbocation, was deduced based on the electrochemical oxidation of the substrate in solution. The propagation of the substitution reaction in the solid state has been shown for the carbocation upon contact with methanol vapor. The crystalline inclusion compounds of 9-thienothienylfluoren-9-ol derivatives incorporating methanol as a guest were exposed to HCl gas. This gas−solid reaction also l...

Abstract: A novel migration of the trimethylsilyl group during reaction of methoxy[(trimethylsilyl)ethoxy]carbene with N-phenylmaleimide (NPM) and with C(60), reported earlier, was examined by means of deuterium labeling of the carbene. For the NPM case it was found that the CD(2)CH(2)SiMe(3) group, initially bound to oxygen, became the CH(2)CD(2)SiMe(3) group bound to carbon in the end product. Not only had the trimethylsilylethyl group moved from oxygen to carbon, but the TMS group had also migrated 1,2 along the ethyl chain. For the C(60) case, complete scrambling of the CD(2) group was observed, strongly implying the involvement of a silacyclopropane carbocation responsible for product formation. The labeling study supports the mechanism that was tentatively advanced earlier for addition to NPM and one of the possibilities suggested for addition to C(60).

Abstract: The 5-trifluoroacetoxy-5-heptafluoropropylcyclopentadiene 15 rearranges to the isomeric trifluoroacetate 16 with a rate constant 5 × 105 less than that for solvolysis of the corresponding 5-CH3 derivative 5 Labeling of 15 with 18O shows the rearrangement occurs by a [1,5]-sigmatropic rearrangement Solvolysis of 16 occurs at a rate 4 times slower than its formation from 15 and leads to the extensively rearranged fulvene 18, implicating formation of the doubly destabilized cation 20 Carbocation formation from 15 is retarded by a factor of 1020 relative to the model 11, showing cumulative destabilizing effects due to formation of the antiaromatic cyclopentadienyl carbocation and electron withdrawal by the fluoroalkyl group

Abstract: This overview summerizes the most important aspects of our present day knowledge about the title species. The material is presented in two sections; the first one reminds salient physico-chemical results related to the stabilization of carbenium ions by selenium moieties and the second one gives examples of the synthetic usefullness of these electrophiles. Gas phase proton affinity measurements complemented by ab initio MO calculations revealed that the intrinsic carbocation stabilizing ability of the Group 16 elements decreases from oxygen to tellurium. However, this ability is not connected in a straightforward manner to the dispersion of the cation's positive charge. The latter is the largest for the most stable species, i.e. oxocarbenium ions, and it decreases downwards in Group 16. Therefore, coulombic (i.e. electrostatic) factors must play a significant role in the overall stabilization. It follows that oxocarbenium should be considered as hard electrophiles undergoing rapid charge contolle...