The radical cation of syn-tricyclooctadiene and its rearrangement products

The syn dimer of cyclobutadiene (tricyclo[4.2.0.0(2.5)]octa-3,7-diene, TOD) is subjected to ionization under different conditions and the resulting species are probed by optical and ESR spectroscopy. By means of quantum chemical modelling of the potential energy surfaces and the optical spectra, it is possible to assign the different products that arise spontaneously after ionization or after subsequent warming or illumination of the samples. Based on these findings, we propose a mechanistic scheme which involves a partitioning of the incipient radical cation of TOD between two electronic states. These two states engage in (near) activation-less decay to the more stable valence isomers, cyclooctatetraene (COT*+) and a bis-cyclobutenylium radical cation BCB*+. The latter product undergoes further rearrangement, first to tetracyclo[4.2.0.0(2,4).0(3,5]oct-7-ene (TCO*+) and eventually to bicyclo[4.2.0]octa-2,4,7-triene (BOT*+) which can also be generated photochemically from BCB*+ or TCO*+. The surprising departure of syn-TOD*+ from the least-motion reaction path leading to BOT*+ can be traced to strong vibronic interactions (second-order Jahn-Teller effects) which prevail in both possible ground states of syn-TOD*+. Such effects seem to be more important in determining the intramolecular reactivity of radical cations than orbital or state symmetry rules.     

The radical cation of anti-tricyclooctadiene and its rearrangement products

The anti dimer of cyclobutadiene (anti-tricyclo[4.2.0.0(2.5)]octa-3,7-diene, TOD) is subjected to ionization by gamma-irradiation in Freon matrices, pulse radiolysis in hydrocarbon matrices, and photoinduced electron transfer in solution. The resulting species are probed by optical and ESR spectroscopy (solid phase) as well as by CIDNP spectroscopy (solution). Thereby it is found that ionization of anti-TOD invariably leads to spontaneous decay to two products, that is bicyclo[4.2.0]octa-2,4,7-triene (BOT) and 1,4-dihydropentalene (1,4-DHP), whose relative yield strongly depends on the conditions of the experiment. Exploration of the C8H8*+ potential energy surface by the B3LYP/6-31G* density functional method leads to a mechanistic hypothesis for the observed rearrangements which involves a bifurcation between a pathway leading to the simple valence isomer, BOT*+, and another one leading to an unprecedented other valence isomer, the anti form of the bicyclo[3.3.0]octa-2,6-diene-4,8-diyl radical cation (anti-BOD*+). The latter product undergoes a very facile H-shift to yield the radical cation of 1,3a-dihydropentalene (1,3a-DHP*+) which ultimately rearrranges by a further H-shift to the observed product, 1,4-DHP*+.     

Car-Parrinello molecular dynamics study of the rearrangement of the valeramide radical cation

Car-Parrinello molecular dynamics (CPMD) studies of neutral (1) and ionized (1 (+.)) valeramide are performed with the aim of providing a rationalization for the unusual temperature effect on the dissociation pattern of 1(+.) observed in mass spectrometric experiments. According to CPMD simulations of neutral valeramide 1 performed at approximately 500 K, the conformation with the fully relaxed carbon backbone predominates (96 %). Conformational changes involving folding of the carbon backbone into conformers that would allow intramolecular H transfers are predicted not to take place spontaneously at this temperature because of the barrier heights associated with these transitions (3.5 and 6.9 kcal mol(-1)), which cannot be overcome by thermal motion alone. For 1(+.), CPMD simulations performed at approximately 300 K reveal a substantial stability of a conformation in which the carbon backbone is fully relaxed; no reaction is observed even after 7 ps. However, when conformers with already folded carbon-backbones are used as initial geometries in the CPMD simulations, the gamma-hydrogen migration (McLafferty rearrangement resulting in C(3)H(6)) is already completed within 2 ps. For this important process, the free activation energy associated with both a required conformational change and the subsequent H transfer equals 4.5 kcal mol(-1), while for the formally related delta-H shift (which eventually gives rise to the elimination of C(2)H(4)/C(2)H(5.)) it amounts to 7.0 kcal mol(-1). Since the barriers associated with conformational changes are energetically more demanding than those of the corresponding hydrogen transfers, 1(+.) is essentially trapped by conformational barriers and long-lived at approximately 300 K. At elevated temperatures (500 K), the preferred reaction (within 7.3 ps) in the CPMD simulation corresponds to the McLafferty rearrangement. The estimated free activation energy associated with this process amounts to 2.5 kcal mol(-1), while the free activation energy for the delta-H transfer equals 4.4 kcal mol(-1). This relatively small free activation energy for the McLafferty rearrangement might cause dissociation of a substantial fraction of 1(+.) prior to the time-delayed mass selection, which would reduce the C3/C2 ratio in the experiments conducted with metastable ions that have a lifetime in the order of some micros at a source temperature of 500 K.     

Photochemistry of cation radicals in solution : photoinduced oxidation by the phenothiazine cation radical

Irradiation of phenothiazine cation radical, Ph^.+, with 1,1-diphenylethylene, DPE, causes its reduction to Ph and oxidation of DPE. Cyclodimeric products are formed from DPE^.+.     

The theoretical reaction path for the cation radical vinylcyclobutane rearrangement: A concerted SR path

Ab initio reaction path calculations for the cation radical vinylcyclobutane rearrangement at the MP2/ 6-31G*//3-21G level reveal a concerted, sr reaction path with an activation energy of 9.4 kcal/mol. The vinylcyclobutane cation radical itself, at both the MP2 and MP3 levels of theory has predominant olefin cation radical character but with modest stretching of one of the adjacent ring carbon—carbon bonds.     

A 4 k matrix esr study of the radical cations of methyl formate: The primary radical cation and its rearrangement

It is found that an oxygen-centred n_π radical of HCOOCH₃ is produced radiolytically in CFCl₃ at 4.2 K without forming a σ* complex with a matrix molecule. This cation converts into the carbon-centred radical cation HC⁺(OH)OCH₂ by an intramolecular hydrogen-atom transfer upon warming to 77 K. This is clear experimental evidence for a McLafferty rearrangement of ester radical cations.     

Aminium cation radical of glycylglycine and its deprotonation to aminyl radical in aqueous solution

The photochemical reaction between glycylglycine and triplet 4-carboxybenzophenone has been investigated using time-resolved chemically induced dynamic nuclear polarization (CIDNP). It is shown that the mechanism of the peptide reaction with triplet excited carboxybenzophenone is electron transfer from the amino group of the peptide, leading to the formation of an aminium cation radical that deprotonates to a neutral aminyl radical. Simulation of the CIDNP kinetics leads to an estimation of the paramagnetic relaxation time for the alpha-protons at the N-terminus at 20 to 40 mus with the best-fit value of 25 mus.     

Carbocationic rearrangement of pivaloyl cation and protonated pivalaldehyde in superacid medium: A novel solution equivalent of the McLafferty rearrangement

Both pivaloyl cation in the presence of hydride donors and protonated pivalaldehyde in superacid media (both aprotic and protic) rearrange to protonated methyl isopropyl ketone involving gitionic dicationic intermediates. In our earlier studies we have found that the rearrangement of pivaladehyde to methyl isopropyl ketone occurs quantitatively in the presence of various superacidic media such as anhydrous HF, triflic acid, boron trifluoride-2,2,2-trifluoroethanol complex (BF(3).2CF(3)CH(2)OH) etc. Our present study with environmentally more benign and stable amine:HF complexes, namely pyridinium poly(hydrogen fluoride) (PPHF) (5), poly(4-vinylpyridinium) poly(hydrogen fluoride) (6), and poly(ethyleniminium) poly(hydrogen fluoride) (PEIHF) (7) shows that these modified HF equivalents can carry sufficient amount of immobilized HF and provide ample acidity for complete isomerization of pivalaldehyde to methyl isopropyl ketone. Calculations on protioformyl, acetyl and pivaloyl dications at the B3LYP/6-311 ++ G(d,p) and CCSD(T)/6-311 ++ G(d,p)//B3LYP/6-311 ++ G(d,p) levels have been performed to compare the nature of protosolvation of formyl, acetyl, pivaloyl cations and protonated pivaladehyde in superacid media. These studies further suggest protosolvation of protonated pivalaldehyde leading to gitionic dications at high acidities resulting in the carbocatioinic rearrangement. The reported carbocationic rearrangement under superacidic activation represents a novel solution chemistry equivalent of the well known gas-phase McLafferty rearrangement.     

Ab initio evidence for the stepwise mechanism of the McLafferty rearrangement of the butanal radical cation

The mechanism of the McLafferty rearrangement of the butanal radical cation to ethylene and vinyl alcohol cation is found, by ab initio calculations, to be stepwise. The results of a previous ab initio study are inconclusive because of symmetry restriction in their geometry optimization.     

The diene component in the cation radical diels-alder

The scope of and structural constraints upon the diene component in the cation radical Diels-Alder are investigated, with special attention to electronic, steric, and conformational effects. The major factors which control the competition between Diels-Alder and cyclobutane adduct formation are also illustrated.     

Generation and absolute reactivity of an aryl enol radical cation in solution

Laser flash photolysis of 1-bromo-1-(4-methoxyphenyl)acetone in acetonitrile leads to the formation of the alpha-acyl 4-methoxybenzyl radical that under acidic conditions rapidly protonates to give detectable amounts of the radical cation of the enol of 4-methoxyphenylacetone. This enol radical cation is relatively long-lived in acidic acetonitrile (tau approximately equal to 200 micros), which is on the same order of magnitude as the radical cations of other 4-methoxystyrene derivatives. Rate constants for deprotonation of the radical cation and the acid dissociation constant for the enol radical cation were also determined using time-resolved absorption spectroscopy. Deprotonation is rapid, taking place with a rate constant of 3.9 x 10(6) s(-1), but the enol radical cation is found to be only moderately acidic in acetonitrile having a pK(a) = 3.2. The lifetime of the enol radical cation was also found to be sensitive to the presence of oxygen and chloride. The sensitivity toward oxygen is explained by oxygen trapping the vinyloxy radical component of the enol radical cation/vinyloxy equilibrium, while chloride acts as a nucleophile to trap the enol radical cation.     

A highly selective rearrangement of a housane-derived cation radical: an electrochemically mediated transformation

To utilize housane-derived cation radicals as intermediates for the synthesis of the bicyclo (n.3.0) framework of natural products, a highly regioselective [1,2] shift of carbon to either a radical or an electron-deficient site is required. Herein we describe how this has been accomplished, provide a set of guidelines to assess housane oxidizability prior to its synthesis, and describe a synthesis of housane 18 that capitalizes upon the facility of [1,5] hydrogen shifts in substituted cyclopentadienes. The catalytic electrochemically mediated oxidation of 18 leads to a cation radical that engages in a rearrangement leading to the (4.3.0) adduct 23. The appearance of a catalytic current in the cyclic voltammogram of a solution containing the tris(aryl)amine and housane 18 is an excellent indicator that the amminium cation radical 14*+ is able to oxidize the housane and return the mediator to the original redox couple. DFT calculations show electron density on both the aryl and strained sigma framework in the HOMO of housane 18. From the spin density and electrostatic potential map for the cation radical, a picture where the spin resides on the side that is distal to the substituent emerges, while the hole is proximal to it. Both experiment and theory show that the rearrangement is best characterized as a [1,2] carbon shift toward an electron-deficient site and that migration toward the substituent-bearing carbon is much preferred over the alternative pathway.     

Tetramethylcyclobutadiene radical cation

The tetramethylcyclobutadiene radical cation has been generated photochemically in solutions of aluminum halide σ complexes of tetramethylcyclobutadiene. It decays thermally to a “dimeric” radical cation.     

The ∏-states of tetraacetylene radical cation

The energies of the four ∏-states of tetraacetylene radical cation have been determined by He(I) photoelectron spectroscopy.     

A nonelectrocyclic path from the cyclobutene cation radical to the 1,3-butadiene cation radical

The conversion of the cyclobutene cation radical to the 1,3-butadiene cation radical has been studied using MINDO /3 and ab initio SCF MO methods. Not only smooth electrocyclic but also stepwise, non-electrocyclic routes were considered. Both calculational methods agree that the preferred reaction path is a novel nonelectrocyclic one proceeding through an intermediate “cyclopropylcarbinyl cation radical.” The quantitative agreement in the activation parameters calculated by the two methods is excellent. The proposed intermediate also provides an attractive explanation for the mass spectrometric fragmentation patterns of the cyclobutene and butadiene cation radicals.     

The resonance Raman spectrum of the aniline radical cation

The time-resolved resonance Raman spectrum of the aniline radical cation has been observed using pulse radiolysis methods. This radical exhibits a large inverse secondary isotope effect on deuteration of the NH₂ group on the CN stretching frequency. Various spectral features indicate that this radical is structurally similar to the phenoxyl radical.     

A new rearrangement process in tert-amyl cation

13C NMR spectroscopy of the 2-methyl-2-butyl-1-13C cation (13C-labeled tert-amyl cation) indicates that interchange of the inside and outside carbons occurs via a barrier of 19.5 +/- 2.0 kcal/mol. A plausible mechanism involves hydride migration in the proposed 2-pentyl cation 4 to form 3-pentyl cation 5. Via the protonated cyclopropane intermediate 6, which undergoes degenerate corner-to-corner hydride shift, the secondary 3-pentyl cation 5' with the label shifted to the central carbon atom is formed. The tert-amyl cation obtained from 5' in the reverse process has the 13C label on an inside carbon atom. All intermediates and transition structures were located on the PES theoretically at the MP2/6-31G(d,p) level of theory. The rearrangement rate of the doubly labeled tert-amyl cation (methyl-13C-butyl-1-13C cation), followed by means of 13C NMR, revealed that the process that interchanges inside and outside carbons has the highest barrier. Comparison of the initial rates revealed that isotopomer 1e arises considerably more slowly than other isotopomers, indicating that in the overall rearrangement process transition structure 5-TS has the highest energy.