RESUMEN
NMR spectra of 25 neat solvents have been recorded using No-D (no-deuterium proton) NMR, and water signals are visible in all spectra. Larger amounts of water can be measured by integration. Water can easily be detected at 0.01% (100 ppm), and amounts can be estimated by comparison with solvent 13C satellite peaks. Molecular sieves efficiently remove water from most solvents such that it cannot be detected by this NMR method. Additives in halogenated solvents and peroxides in ether and THF are also easily detected by No-D NMR.
RESUMEN
3-tert-Butyl- or phenyl-substituted-1-(trimethylsilylmethyl)cyclobutyl trifluoroacetates react in methanol via ß-trimethylsilyl carbocationic intermediates formed from loss of trifluoroacetate ion. These cationic intermediates react to give a significant amount of methyl ether substitution products along with the expected alkene elimination products. Different methyl ether substitution products are formed from isomeric trifluoroacetates, and complete retention of configuration of the ether group relative to the starting trifluoroacetates is observed. The proposed origin of this retention is the involvement of two different ß-trimethylsilyl carbocation intermediates that form while utilizing backside participation of the trimethylsilyl group. Computational studies have located three classical 1-(trimethylsilylmethyl)cyclobutyl carbocation energy minima. Two of these cations have a significant barrier (13-16 kcal/mol) to interconversion. It is proposed that methanol can capture each of these cations from different sides, leading to isomeric methyl ether products. A third energy minimum, formed by cyclobutane ring inversion, is thought to be unimportant in determining the stereochemistry of the methyl ether substitution products.
RESUMEN
Computational and experimental studies reveal two different modes of cation stabilization by the phenylazo group. The first mode involves a relatively weak conjugative interaction with the azo π-bond, while the second mode involves an interaction with the nitrogen nonbonding electrons. The 4-phenylazo group is slightly rate-retarding in the solvolysis of cumyl chloride and benzyl mesylate derivatives but rate-enhancing in the solvolysis of α-CF3 benzylic analogs. The phenylazo group can become a potent electron-donating group in cations such as [Me2CâNâNâPh]+. Nonbonding electron stabilization can be strong enough to offset the very powerful γ-silyl stabilization. In aromatic cyclopropenium and tropylium cations, the demand for stabilization is quite low, and the mode of phenylazo stabilization reverts back to the less-effective π-stabilization. The solvolysis of cis-4-phenylazo benzyl mesylate is faster than that of trans-4-phenylazo benzyl mesylate. Products formed suggest a stepwise ionization, cation isomerization, and nucleophile capture mechanism. Computational studies indicate a vanishingly small barrier for the isomerization of the cis-cation intermediate to the trans-cation.
RESUMEN
The stereochemistry of the substitution product formed when (1s,3s)-3-(tert-butyl)-1-methylcyclobutyl methanesulfonate, 7, reacts in trifluoroethanol is one of retention of configuration. This is consistent with the intermediacy of a nonclassical bicyclobutonium cation 8N. As the solvent becomes less highly ionizing, the product contains increasing amounts of the isomeric inverted product. This is indicative of the competitive involvement of a classical cation 8C that captures solvent from both sides. Solvolytic rate studies show no large rate enhancements due to the t-butyl group in 7. However, computational studies suggest that the combined stabilization of the bicyclobutonium cation 8N by C-C σ-interactions and the t-butyl group amounts to 10.8 kcal/mol in the gas phase. Computationally, the t-butyl group contributes to stabilization of 8N, but this stabilization is not revealed by kinetic studies. This discrepancy suggests caution when relating computational studies on carbocations to solvolytic studies in common solvents.
RESUMEN
The mesylate derivative of cis-1-hydroxymethyl-2-trimethylsilylcyclopropane has been prepared, along with a number of related mesylates and triflates with substituents on the 1-position. These substrates all solvolyze in CD3CO2D to give products derived from cyclopropylcarbinyl cations that undergo further rearrangement to give 3-trimethylsilylcyclobutyl cations. These 3-trimethylsilylcyclobutyl cations are stabilized by a long-range rear lobe interaction with the γ-trimethylsilyl group. When the substituent is electron-withdrawing (CF3, CN, or CO2CH3), significant amounts of bicyclobutane products are formed. The bicyclobutanes are a result of γ-trimethylsilyl elimination from the cationic intermediate that has an unusually long calculated Si-C bond. The solvolysis chemistry of mesylate and triflate derivatives of trans-1-hydroxymethyl-2-trimethylsilylcyclopropane and 1-substituted analogs can be quite different since these substrates do not generally lead to 3-trimethylsilylcyclobutyl cations.
RESUMEN
The rate of the methylenecyclopropane rearrangement is enhanced by an alkyne-Co2(CO)6 complex bonded to the para position of a benzene ring. This is explained by a stabilizing effect on the transition state leading to the biradical intermediate. Computational studies indicate that the benzylic-type biradical intermediate is stabilized by a delocalization mechanism, where spin is delocalized onto the two cobalt atoms. Silver cation also enhances the rate of the methylenecyclopropane rearrangement. Computational studies suggest that silver cation can also stabilize a benzylic radical by spin delocalization involving silver. In the case of the silver-promoted reactions, the rate enhancements in a series of aryl-substituted methylenecyclopropanes correlate with σ+ values. The question remains open as to whether the silver-catalyzed methylenecyclopropane rearrangement proceeds via an argento-stabilized biradical or whether the reaction involves an argento-substituted allylic cation.
RESUMEN
Three fluorobenzenes substituted with meta-triazole groups have been prepared, and 19F chemical shifts indicate that these triazole groups are all inductively electron-withdrawing in character, with the 1,5-triazole being the most electron-withdrawing. σ+ values for these three triazoles have also been determined from solvolysis rates of substituted cumyl trifluoroacetates. When substituted in the para-position, the 1,4 and the 2,4-triazoles are cation-stabilizing, whereas the 1,5-triazole is carbocation-destabilizing. γ+ values indicate that the 1,4 triazole group is cation-stabilizing relative to the phenyl group, albeit the 1,5 triazole is significantly destabilizing relative to phenyl. These studies all suggest that the 1,5-triazole group exerts a strong electron-withdrawing effect on carbocations that is not offset by a resonance effect. The three triazole groups all enhance the methylenecyclopropane rearrangement rate and are therefore radical stabilizers. The smallest stabilizing effect is seen for the 1,5-triazole, and this is attributed to the triazole group being twisted out of conjugation in the developing benzylic radical. Finally, the anionic triazole group is the most effective radical-stabilizing group. Computational studies indicate that these triazole groups all stabilize benzylic radicals by a spin delocalization mechanism.
RESUMEN
A series of γ-trimethylsilyl-substituted carbenes have been studied experimentally and by computational methods. In an acyclic system, 1,3-trimethylsilyl migration successfully competes with 1,3-hydrogen migration to the carbene center. The behavior of cyclic 3-trimethylsilyl-substituted carbenes contrasts with that of the acyclic system. Only 1,2-hydrogen migration processes are observed in the five-membered ring due to the high barrier to 1,3-hydrogen migration. In the cyclohexyl system, a small amount of a cyclopropane derived from 1,3-hydrogen migration occurs, as shown by a labeling study. In the cycloheptyl carbene system, a labeling study again showed that 1,3-hydrogen migration to the carbene center leads to the major product. Computational studies suggest that the cyclic carbenes all have lower energy conformations where the trimethylsilyl group is in a pseudo equatorial conformation where it cannot migrate to the carbene center. Computational studies also suggest that cyclohexyl and cycloheptyl carbene systems are slightly stabilized by a rear lobe interaction of the Si-C bond with the carbene center.
RESUMEN
A series of isomeric 3-trimethylsilyl-1-arylcyclobutyl carbocations, 10 and 11, where the cross-ring 3-trimethylsilyl group has the potential to interact with the cationic center, have been generated under solvolytic conditions. When the cationic center can interact with the rear lobe of the carbon-silicon bond, rate enhancements become progressively larger as the substituent on the aryl group becomes more electron-withdrawing. When the potential interaction with the trimethylsilyl group is via a front lobe interaction, there is minimal rate enhancement over the range of substituents. Computational studies have also been carried out on these cations 10 and 11. Calculated trimethylsilyl stabilization energies progressively increase with electron-withdrawing character of the aryl groups when the trimethylsilyl interaction is via the rear lobe. By way of contrast, there are minimal changes in stabilization energies when the potential trimethylsilyl interaction is via the front lobe of the carbon-silicon bond. These computational studies, along with the solvolytic studies, point to a significant rear lobe 3-trimethylsilyl stabilization of arylcyclobutyl cations. They also argue against any front lobe stabilization of the isomeric arylcyclobutyl cations.
RESUMEN
endo-2-Trimethylsilyl-anti-7-norbornyl triflate undergoes solvolysis reactions 1.8 × 10(4) faster than 7-norbornyl triflate in CD3CO2D and 1.3 × 10(5) times faster in CF3CH2OH. The exclusive substitution products with retained stereochemistry point to a significantly stabilized γ-trimethylsilyl carbocation intermediate. The endo-2-trimethylsilyl-7-norbornyl carbene gives a major rearrangement product where the trimethylsilyl-activated hydrogen migrates to the carbenic center. This rearrangement product implies stabilization of the carbene by the γ-trimethylsilyl group. Isodesmic computational studies (M062X/6-311+G**) indicate that the endo-2-trimethylsilyl-7-norbornyl cation is stabilized by 16.2 kcal/mol and that the endo-2-trimethylsilyl-7-norbornyl carbene is stabilized by a smaller factor of 1.8 kcal/mol. By way of contrast, anti-7-trimethylsilyl-endo-2-norbornyl mesylate undergoes solvolysis in CD3CO2D only 2.6 times faster than endo-2-norbornyl mesylate and 9.4 times faster in CF3CH2OH. The substitution products have only partially retained stereochemistry, and significant rearrangements are observed. The anti-7-trimethylsilyl-2-norbornyl carbene gives a rearrangement product via 1,3-hydrogen migration of the C6 hydrogen, which is completely analogous to the behavior of the unsubstituted 2-norbornyl carbene. Isodesmic calculations show that the anti-7-trimethylsilyl-2-norbornyl cation is stabilized by only 3.2 kcal/mol relative to the 2-norbornyl cation, and the corresponding anti-7-trimethylsilyl-2-norbornyl carbene is stabilized by a negligible 0.9 kcal/mol.
RESUMEN
3-Trimethylsilylcyclobutylidene was generated by pyrolysis of the sodium salt of the tosylhydrazone derivative of 3-trimethylsilylcyclobutanone. This carbene converts to 1-trimethylsilylbicyclobutane as the major product. A labeling study shows that this intramolecular rearrangement product comes from 1,3-hydrogen migration to the carbenic center and not 1,3-silyl migration. Computational studies show two carbene minimum energy conformations, with the lower energy conformation displaying a large stabilizing interaction of the carbene center with the rear lobe of the C3-Si bond. In this conformation, the trimethylsilyl group cannot migrate to the carbene center, and the most favorable process is 1,3-hydrogen migration. When the carbene is generated photochemically in methanol, it reacts by a protonation mechanism giving the highly stabilized 3-trimethylsilylcyclobutyl carbocation as an intermediate. When generated in dimethylamine as solvent, the carbene undergoes preferred attack of this nucleophilic solvent from the back of this C-Si rear lobe stabilized carbene.
RESUMEN
1,4-Disubstituted-1H-1,2,3-triazoles 1 can easily be distinguished from the isomeric 1,5-disubstituted-1H-1,2,3-triazoles 2 by simple one-dimensional (13)C NMR spectroscopy using gated decoupling. The C(5) signal of 1 appears at δ â¼120 ppm, while the C(4) signal of 2 appears at δ â¼133 ppm. Computational studies also predict the upfield shift of C(5) of 1 relative to C(4) in 2.
RESUMEN
p-Cyclopropylbenzophenone, 20, gives no photoreduction when irradiated in i-PrOH solvent. This is a general phenomenon and a number of cyclopropyl-substituted benzophenones, including 4-(endo-6-bicyclo[3.1.0]hexyl)benzophenone, 19, 4-(cis-2,3-dimethylcyclopropyl)benzophenone, 21, 4-(cis-2-vinylcyclopropyl)benzophenone, 22, and 4-(endo-7-bicyclo[4.1.0]hept-2-enyl)benzophenone, 23, also fail to undergo photoreduction. Instead these latter compounds undergo cis-trans isomerization when irradiated. A mechanism involving formation of an (n, π*) triplet, which subsequently fragments the strained cyclopropane bond to give a lower energy and unreactive open triplet, has been suggested. p-Cyclopropylvalerophenone, 25, and p-(endo-6-bicyclo[3.1.0]hexyl)valerophenone, 24, also undergo photoisomerization and fail to undergo the Norrish Type II photoreactions. Triplet energy dissipation by fragmentation of the cyclopropane bond is also proposed. In addition to the Norrish Type II reaction, p-cyclobutylvalerophenone, 27, undergoes a photofragmentation to give ethylene and p-vinylvalerophenone, 60, by an energy dissipation mechanism involving a 1,4-biradical derived from cyclobutane bond fragmentation.
RESUMEN
A series of 3-trimethylsilyl-1-substituted cyclobutyl trifluoroacetates have been prepared and reacted in CD(3)CO(2)D. Rate data indicate that the substrates with the trimethylsilyl group cis to the leaving group react with assistance due to gamma-silyl participation. Rate enhancements range from a factor of 209 for alpha-phenyl-substituted cations to 4.6 x 10(4) for alpha-methyl-substituted cations to >10(10) for the unsubstituted gamma-trimethylsilylcyclobutyl cation. Acetate substitution products are formed with net retention of stereochemistry. These experimental studies, as well as B3LYP/6-31G* computational studies, are consistent with the involvement of carbocations where the rear lobe of the gamma-Si-C bond interacts strongly with the developing cationic center. Solvolytic rate studies, as well as computational studies, suggest that the secondary gamma-trimethylsilylcyclobutyl cation is even more stable than the beta-trimethylsilylcyclobutyl cation, i.e., the gamma-silyl effect actually outweighs the potent beta-silyl effect. Although computational studies suggest the existence of certain isomeric cations, where the front lobe of the Si-C bond interacts with the cationic center, solvolytic evidence for the involvement of these front lobe stabilized cations is less compelling.
RESUMEN
Solvolysis of 1-(trimethylsilylmethyl)cyclopropyl mesylate in CD(3)CO(2)D gives ring-opened products as well as methylenecyclopropane. The rate enhancement due to the beta-trimethylsilyl group is a factor of about 10(6). The large stabilizing effect of a beta-silyl group (which can cause rate enhancements of up to 10(12)) on the intermediate cation has been repressed. B3LYP/6-31G* computational studies indicate a carbocation stabilization energy of 16.6 kcal/mol. Rates of solvolyses of 1-phenyl-2-trimethylsilylcyclopropyl chlorides are enhanced by a factor of 10(3)-10(4). The intermediate cyclopropyl cation undergoes substantial ring opening since beta-silyl stabilization is not large (calculated stabilization energy of 12 kcal/mol). Solvolysis rates of 2-trimethylsilylbenzocyclobutyl derivatives are not significantly enhanced by the beta-trimethylsilyl group. Beta-silyl stabilization of benzocyclobutenyl carbocations generated in solution has been effectively eliminated due to antiaromatic considerations (calculated stabilization energy of 3.7 kcal/mol when R = Ph). While computational studies parallel solvolytic rate studies, they overestimate the extent of beta-trimethylsilyl stabilization of solvolytically generated carbocations.
RESUMEN
3-Aryl-3-hydroxy-1-methylazetidine-2-thiones react with HCl in DMSO to give 3-methyl-5-aryloxazole-2-thiones. Substituent effects correlate with rate effects on hydrolyses of acetals of benzaldehyde. An (17)O labeling experiment indicates that the oxygen atom of the product is derived from the hydroxyl group. Trifluoroacetic anhydride/DMSO in CH2Cl2 can also promote the reaction. Mechanisms involving a Grob-type fragmentation of an activated substrate, followed by recyclization, or a cyclopropylcarbinyl type of rearrangement can account for this oxidative rearrangement.
RESUMEN
Certain 1,1-dimethyl-2-aryl-3-methylenecyclopropanes containing carbonyl substituents undergo rearrangement when irradiated with 350 nm light. These rearrangements occur via the (n,pi*) triplet state, which fragments the strained cyclopropane bond. Intersystem crossing followed by ring closure gives the observed products. No photoreduction is seen in i-PrOH. Potential Norrish type II processes are also bypassed. It is suggested that the cyclopropane bond fragmentation dissipates the triplet energy and that the new intermediates are not energetic enough to abstract hydrogen atoms in an intramolecular fashion or from solvent. Nitro substituted systems undergo analogous photoinitiated rearrangements. Benzophenone sensitization of naphthyl, biphenyl, styrene, and phenylacetylene analogues also leads to rearrangement, presumably via the sensitized generation of triplet states. When triplet states cannot be accessed by direct irradiation or by sensitized processes, methylenecyclopropane rearrangements do not occur. An exception is the ferrocenyl analogue, which does not photorearrange, presumably due to the very short lifetime of the triplet intermediate.
RESUMEN
A series of E- and Z-1-aryl-5-trimethylsilyl-3-buten-1-yl trifluoroacetates were solvolyzed in CD3CO2D, and rates of reaction as well as products derived from these reactions were determined. Hammett plots showed a break, which was indicative of a mechanistic change from a kC process when the most electron-donating substituents were attached to the aryl group to a kDelta process involving formation of cyclized beta-silyl carbocation intermediates for electron-withdrawing groups. In the case of p-CH3O substitution (a kC extreme), the cationic intermediate captures solvent (95%) or loses a proton (5%). In the case of m-CF3 substitution (a kDelta extreme), the beta-silyl cation intermediate desilylates to give vinylcyclopropane products. Substituents with intermediate electronic properties give more complex product mixtures. Solvolysis of pure Z-trifluoroacetate (p-CH3) gives small amounts of E-trifluoroacetate (p-CH3) along with the E-substitution product. This isomerization suggests that the cyclized beta-silyl cation can isomerize and then reopen to a classical aryl-stabilized cation. By way of contrast, B3LYP/6-31G* computational studies show only cyclized beta-silyl cations as energy minima. Open kC cations are higher-energy nonminimum energy structures.
RESUMEN
A comprehensive series of substituted 1,1-dimethyl-2-methylenecyclopropanes have been thermally rearranged. These rearrangements proceed via singlet biradical intermediates that can be stabilized by substituents. Rates are greatly enhanced by certain groups that are termed super radical stabilizers. Substituents included 4-pyridyl N-oxide, 2-(1,6-methano[10]annulenyl), and a number of anion-substituted phenyl groups. Simple valence bond theory, as well as more sophisticated computational studies, gives insights into modes of radical stabilization.
RESUMEN
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 rho+ 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 carbocation rearrangements (kDelta processes). anti-7-Norbornenyl mesylate gives products with complete retention of configuration, indicative of involvement of the delocalized 7-norbornenyl cation. 1,6-Methano[10]annulen-11-yl triflate reacts in [BMIM][NTf2] to give 1,6-methano[10]annulen-11-ol, along with naphthalene, an oxidized product derived from loss of trifluoromethanesulfinate ion. Analogous loss of CF3SO2- can be seen in reaction of PhCH(CF3)OTf. Ionic liquids are therefore viable solvents for formation of carbocationic intermediates via kC and kDelta processes.