Structural and solvent effects on the C-S bond cleavage in aryl triphenylmethyl sulfide radical cations

Steady-state and laser flash photolysis (LFP) studies of a series of aryl triphenylmethyl sulfides [1, 3,4-(CH(3)O)(2)-C(6)H(3)SC(C(6)H(5))(3); 2, 4-CH(3)O-C(6)H(4)SC(C(6)H(5))(3); 3, 4-CH(3)-C(6)H(4)SC(C(6)H(5))(3); 4, C(6)H(5)SC(C(6)H(5))(3); and 5, 4-Br-C(6)H(4)SC(C(6)H(5))(3)] has been carried out in the presence of N-methoxyphenanthridinium hexafluorophosphate in CH(3)CN, CH(2)Cl(2), CH(2)Cl(2)/CH(3)CN, and CH(2)Cl(2)/CH(3)OH mixtures. Products deriving from the C-S bond cleavage in the radical cations 1(?+)-5(?+) have been observed in the steady-state photolysis experiments. Time-resolved LFP showed first-order decay of the radical cations accompanied by formation of the triphenylmethyl cation. A significant decrease of the C-S bond cleavage rate constants was observed by increasing the electron-donating power of the arylsulfenyl substituent, that is, by increasing the stability of the radical cations. DFT calculations showed that, in 2(?+) and 3(?+), charge and spin densities are mainly localized in the ArS group. In the TS of the C-S bond cleavage an increase of the positive charge in the trityl moiety and of the spin density on the ArS group is observed. The higher delocalization of the charge in the TS as compared to the initial state is probably at the origin of the observation that the C-S bond cleavage rates decrease by increasing the polarity of the solvent.     

An economic and efficient tetrahydrofuranylation of alcohols, imines and alkynes

The tetrahydrofuranyl radical, generated by heating tetrahydrofuran in the presence of air and allyl or benzyl chloride, becomes a useful tool in order to transform the hydroxyl functions into ethers, or the CN double bond into amine, or the C-C triple bond into vinyl derivatives. A radical mechanism is proposed followed by a nucleophilic substitution for the alcohol substrate and a radical addition for the iminic and the acetylenic reactants.     

DFT study on the cycloreversion of thietane radical cations

The molecular mechanism of the cycloreversion (CR) of thietane radical cations has been analyzed in detail at the UB3LYP/6-31G* level of theory. Results have shown that the process takes place via a stepwise mechanism leading to alkenes and thiobenzophenone; alternatively, formal [4+2] cycloadducts are obtained. Thus, the CR of radical cations 1a,b(?+) is initiated by C2-C3 bond breaking, giving common intermediates INa,b. At this stage, two reaction pathways are feasible involving ion molecule complexes IMCa,b (i) or radical cations 4a,b(?+) (ii). Calculations support that 1a(?+) follows reaction pathway ii (leading to the formal [4+2] cycloadducts 5a). By contrast, 1b(?+) follows pathway i, leading to trans-stilbene radical cation (2b(?+)) and thiobenzophenone.     

The Dewar benzene radical cation and its ring-opening reaction

The radical cation of Dewar benzene, 1*+, has been generated and observed by optical spectroscopy in cryogenic matrices. 1*+ distinguishes itself by a charge resonance band at 600 nm, very similar in shape and position to that observed for the related radical cation of norbornadiene. This coincidence indicates that in ground-state 1*+ the odd electron is also located in a pi-MO. The energy of the charge resonance transition, which is very sensitive to the dihedral angle between the four-membered rings in 1*+, is predicted consistently too low by TD-DFT and CASPT2. Probably this angle is too large in the B3LYP and CASSCF geometries. As 1*+ can be observed at 77 K, it must be separated by a barrier of at least 7-8 kcal/mol from its very exothermic decay to the radical cation of benzene, 2*+. An analysis shows that the ring-opening of 1*+ is a multistep process involving two avoided crossings between potential surfaces of different symmetry and electronic nature. Owing to the orbital symmetry-forbidden nature of the process, the energy of 1*+ starts by increasing steeply on stretching the central C-C bond, but then the system undergoes a crossing to a 2A1 surface which leads adiabatically to an excited state of 2*+. Therefore, another avoided crossing must be transited before the molecule can decay on the ground-state surface of 2*+. The rearrangement of 1*+ to 2*+ is an example of a "pseudodiabatic" thermal reaction that transits between potential surfaces representing very different electronic structures.     

Stereoselective synthesis of 1'-C-branched arabinofuranosyl nucleosides via anomeric radicals generated by 1,2-acyloxy migration

Stereoselective C-C bond formation at the anomeric position of uracil and adenine nucleoside has been accomplished through reaction of the anomeric radical, generated by 1,2-acyloxy migration, with a radical acceptor. The present method consists of the following steps: (1) electrophilic addition (bromo-pivaloyloxylation) to 3',5'-O-(1,1,3,3-tetraisopropyldisiloxane-1,3-diyl)-protected 1',2'-unsaturated nucleoside, (2) tin radical-mediated reaction of the resulting adduct with a radical acceptor. The use of allyl(tributyl)tin gave the 1'-C-allylated uracil nucleoside 14 in 66% yield together with the unrearranged 2'-C-allylated product 15 (6%). Radical acceptors such as styryl(tributyl)tin and 3-bromo-2-methylacrylonitrile can also be used in the reaction of 5, giving 16 (70%) and 17 (76%) without the formation of unrearranged product. The radical-mediated C-C bond formation of the adenine counterpart 12 was also investigated.     

Solution-phase mechanistic study and solid-state structure of a tris(bipyridinium radical cation) inclusion complex

The ability of the diradical dicationic cyclobis(paraquat-p-phenylene) (CBPQT(2(?+))) ring to form inclusion complexes with 1,1'-dialkyl-4,4'-bipyridinium radical cationic (BIPY(?+)) guests has been investigated mechanistically and quantitatively. Two BIPY(?+) radical cations, methyl viologen (MV(?+)) and a dibutynyl derivative (V(?+)), were investigated as guests for the CBPQT(2(?+)) ring. Both guests form trisradical complexes, namely, CBPQT(2(?+))?MV(?+) and CBPQT(2(?+))?V(?+), respectively. The structural details of the CBPQT(2(?+))?MV(?+) complex, which were ascertained by single-crystal X-ray crystallography, reveal that MV(?+) is located inside the cavity of the ring in a centrosymmetric fashion: the 1:1 complexes pack in continuous radical cation stacks. A similar solid-state packing was observed in the case of CBPQT(2(?+)) by itself. Quantum mechanical calculations agree well with the superstructure revealed by X-ray crystallography for CBPQT(2(?+))?MV(?+) and further suggest an electronic asymmetry in the SOMO caused by radical-pairing interactions. The electronic asymmetry is maintained in solution. The thermodynamic stability of the CBPQT(2(?+))?MV(?+) complex was probed by both isothermal titration calorimetry (ITC) and UV/vis spectroscopy, leading to binding constants of (5.0 ± 0.6) × 10(4) M(-1) and (7.9 ± 5.5) × 10(4) M(-1), respectively. The kinetics of association and dissociation were determined by stopped-flow spectroscopy, yielding a k(f) and k(b) of (2.1 ± 0.3) × 10(6) M(-1) s(-1) and 250 ± 50 s(-1), respectively. The electrochemical mechanistic details were studied by variable scan rate cyclic voltammetry (CV), and the experimental data were compared digitally with simulated data, modeled on the proposed mechanism using the thermodynamic and kinetic parameters obtained from ITC, UV/vis, and stopped-flow spectroscopy. In particular, the electrochemical mechanism of association/dissociation involves a bisradical tetracationic intermediate CBPQT((2+)(?+))?V(?+) inclusion complex; in the case of the V(?+) guest, the rate of disassociation (k(b) = 10 ± 2 s(-1)) was slow enough that it could be detected and quantified by variable scan rate CV. All the experimental observations lead to the speculation that the CBPQT((2+)(?+)) ring of the bisradical tetracation complex might possess the unique property of being able to recognize both BIPY(?+) radical cation and π-electron-rich guests simultaneously. The findings reported herein lay the foundation for future studies where this radical-radical recognition motif is harnessed particularly in the context of mechanically interlocked molecules and increases our fundamental understanding of BIPY(?+) radical-radical interactions in solution as well as in the solid-state.     

Theoretical studies of quantum amplified isomerizations for imaging systems involving hexamethyl Dewar benzene and related systems

The ring-opening reactions of the radical cations of hexamethyl Dewar benzene (1) and Dewar benzene have been studied using density functional theory (DFT) and complete active-space self-consistent field (CASSCF) calculations. Compound 1 is known to undergo photoinitiated ring opening by a radical cation chain mechanism, termed "quantum amplified isomerization" (QAI), which is due to the high quantum yield. Why QAI is efficient for 1 but not other reactions is explained computationally. Two radical cation minima of 1 and transition states located near avoided crossings are identified. The state crossings are characterized by conical intersections corresponding to degeneracy between doublet surfaces. Ring opening occurs by formation of the radical cation followed by a decrease in the flap dihedral angle. A rate-limiting Cs transition state leads to a second stable radical cation with an elongated transannular C-C bond and an increased flap dihedral. This structure proceeds through a conrotatory-like pathway of Cs symmetry to give the benzene radical cation. The role of electron transfer was investigated by evaluating oxidation of various systems using adiabatic ionization energies and electron affinities calculated from neutral and cation geometries. Electron-transfer theory was applied to 1 to investigate the limiting effects of back-electron transfer as it is related to the unusual stability of the two radical cations. Expected changes in optical properties between reactants and products of Dewar benzene compounds and other systems known to undergo QAI were characterized by computing frequency-dependent indices of refraction from isotropic polarizabilities. In particular, the reaction of 1 shows greater contrast in index of refraction than that of the Dewar benzene parent system.     

The rearrangement of the cubane radical cation in solution

The rearrangement of the cubane radical cation (1*+) was examined both experimentally (anodic as well as (photo)chemical oxidation of cubane 1 in acetonitrile) and computationally at coupled cluster, DFT, and MP2 [BCCD(T)/cc-pVDZ//B3LYP/6-31G* ZPVE as well as BCCD(T)/cc-pVDZ//MP2/6-31G* + ZPVE] levels of theory. The interconversion of the twelve C2v degenerate structures of 1*+ is associated with a sizable activation energy of 1.6 kcalmol(-1). The barriers for the isomerization of 1*- to the cuneane radical cation (2*+) and for the C-C bond fragmentation to the secocubane-4,7-diyl radical cation (10*+) are virtually identical (deltaH0++ = 7.8 and 7.9 kcalmol(-1), respectively). The low-barrier rearrangement of 10*+ to the more stable syn-tricyclooctadiene radical cation 3*+ favors the fragmentation pathway that terminates with the cyclooctatetraene radical cation 6*+. Experimental single-electron transfer (SET) oxidation of cubane in acetonitrile with photoexcited 1,2,4,5-tetracyanobenzene, in combination with back electron transfer to the transient radical cation, also shows that 1*+ preferentially follows a multistep rearrangement to 6*+ through 10*+ and 3*+ rather than through 2*+. This was confirmed by the oxidation of syn-tricyclooctadiene (3), which, like 1, also forms 6 in the SET oxidation/rearrangement/electron-recapture process. In contrast, cuneane (2) is oxidized exclusively to semibullvalene (9) under analogous conditions. The rearrangement of 1*+ to 6*+ via 3*+, which was recently observed spectroscopically upon ionization in a hydrocarbon glass matrix, is also favored in solution.     

Trimethyl phosphite as a trap for alkoxy radicals formed from the ring opening of oxiranylcarbinyl radicals. Conversion to alkenes. Mechanistic applications to the study of C-C versus C-O ring cleavage

Trimethyl phosphite, (MeO)(3)P, is introduced as an efficient and selective trap in oxiranylcarbinyl radical (2) systems, formed from haloepoxides 8-13 under thermal AIBN/n-Bu(3)SnH conditions at about 80 degrees C. Initially, the transformations of 8-13, in the absence of phosphite, to allyl alcohol 7 and/or vinyl ether 5 were measured quantitatively (Table 1). Structural variations in the intermediate oxiranylcarbinyl (2), allyloxy (3), and vinyloxycarbinyl (4) radicals involve influences of the thermodynamics and kinetics of the C-O (2 --> 3, k(1)) and C-C (2 --> 4, k(2)) radical scission processes and readily account for the changes in the amounts of product vinyl ether (5) and allyl alcohol (7) formed. Added (MeO)(3)P is inert to vinyloxycarbinyl radical 4 and selectively and rapidly traps allyloxy radical 3, diverting it to trimethyl phosphate and allyl radical 6. Allyl radicals (6) dimerize or are trapped by n-Bu(3)SnH to give alkenes, formed from haloepoxides 8, 9, and 13 in 69-95% yields. Intermediate vinyloxycarbinyl radicals (4), in the presence or absence of (MeO)(3)P, are trapped by n-Bu(3)SnH to give vinyl ethers (5). The concentrations of (MeO)(3)P and n-Bu(3)SnH were varied independently, and the amounts of phosphate, vinyl ether (5), and/or alkene from haloepoxides 10, 11, and 13 were carefully monitored. The results reflect readily understood influences of changes in the structures of radicals 2-4, particularly as they influence the C-O (k(1)) and C-C (k(2)) cleavages of intermediate oxiranylcarbinyl radical 2 and their reverse (k(-1), k(-2)). Diversion by (MeO)(3)P of allyloxy radicals (3) from haloepoxides 11 and 12 fulfills a prior prediction that under conditions closer to kinetic control, products of C-O scission, not just those of C-C scission, may result. Thus, for oxiranylcarbinyl radicals from haloepoxides 11, 12, and 13, C-O scission (k(1), 2 --> 3) competes readily with C-C cleavage (k(2), 2 --> 4), even though C-C scission is favored thermodynamically.     

Reaction of vinyl radical with oxygen: a matrix isolation infrared spectroscopic and theoretical study

The reaction of vinyl radical with molecular oxygen in solid argon has been studied using matrix isolation infrared absorption spectroscopy. The vinyl radical was produced through high frequency discharge of ethylene. The vinyl radical reacted with oxygen spontaneously on annealing to form the vinylperoxy radical C(2)H(3)OO with the O-O bond in a trans position relative to the C-C bond, which is characterized by O-O stretching and out-of-plane CH(2) bending vibrations at 1140.7 and 875.5 cm(-1). The vinylperoxy radical underwent visible photon-induced dissociation to the CH(2)OH(CO) complex or CH(2)OH+CO, which has never been considered in previous studies. The CH(2)OH(CO) product was predicted to be more thermodynamically accessible than the previously reported major HCO+H(2)CO channel, and is most likely produced by hydrogen atom transfer from the first-formed H(2)CO-HCO pair in solid argon.     

Dinitro and quinodimethane derivatives of terthiophene that can be both oxidized and reduced. Crystal structures,spectra, and a method for analyzing quinoid contributions to structure

Two new oligothiophenes, the dinitro compound 3',4'-dibutyl-5,5' '-dinitro-2,2':5',2' '-terthiophene (1) and the quinodimethane 3',4'-dibutyl-5,5' '-bis(dicyanomethylene)-5,5' '-dihydro-2,2':5',2' '-terthiophene (2), have been synthesized and studied with electrochemistry, UV-vis-NIR-IR spectroscopy, ESR, and X-ray crystallography. These compounds, designed to be both electron and hole carriers, show redox properties that are unusual for oligothiophenes. Cyclic voltammetry and spectroelectrochemistry demonstrated that each compound could be oxidized to a cation radical and reduced to an anion radical and dianion. The spectra of 2 and its three redox partners were analyzed in terms of a limiting structure in which the neutral 2 has orbitals corresponding to those of a substituted-terthiophene dication. Compound 1 crystallizes with the thiophene rings held in an unusual nonplanar, cisoid configuration in face-to-face pi-stacks, with a spacing between molecules of 3.65 A. The C-C bond lengths of the outer nitro-substituted rings have quinoid character. Compound 2 crystallizes with the thiophene rings in a planar, transoid configuration. The molecules are held in pi-stacks formed from pi-dimers with a spacing between molecules of 3.47 and 3.63 A. The C-C bond distances of the thiophene rings of 1 and 2 and other oligomers were analyzed by a principal component analysis. The analysis found that 93% of the structural variance resided in one principal component related to the quinoid structure of the oligothiophene moiety. The analysis reliably demonstrated a quinoid contribution to the structure of 1. This method should be applicable to understanding the structure of other conjugated molecules in which quinoid structures contribute.     

Steady-state and laser flash photolysis study of the carbon-carbon bond fragmentation reactions of 2-arylsulfanyl alcohol radical cations

The N-methylquinolinium tetrafluoroborate (NMQ(+))-sensitized photolysis of the erythro-1,2-diphenyl-2-arylsulfanylethanols 1-3 (1, aryl = phenyl; 2, aryl = 4-methylphenyl; 3, aryl = 3-chlorophenyl) has been investigated in MeCN, under laser flash and steady-state photolysis. Under laser irradiation, the formation of sulfide radical cations of 1-3, in the monomeric (lambda(max) = 520-540 nm) and dimeric form (lambda(max) = 720-->800 nm), was observed within the laser pulse. The radical cations decayed by first-order kinetics, and under nitrogen, the formation of ArSCH(*)Ph (lambda(max) = 350-360 nm) was clearly observed. This indicates that the decay of the radical cation is due to a fragmentation process involving the heterolytic C-C bond cleavage, a conclusion fully confirmed by steady-state photolysis experiments (formation of benzaldehyde and the dimer of the alpha-arylsulfanyl carbon radical). Whereas the fragmentation rate decreases as the C-C bond dissociation energy (BDE) increases, no rate change was observed by the replacement of OH by OD in the sulfide radical cation (k(OH)/k(OD) = 1). This suggests a transition state structure with partial C-C bond cleavage where the main effect of the OH group is the stabilization of the transition state by hydrogen bonding with the solvent. The fragmentation rate of 2-hydroxy sulfanyl radical cations turned out to be significantly slower than that of nitrogen analogues of comparable reduction potential, probably due to a more efficient overlap between the SOMO in the heteroatom and the C-C bond sigma-orbital in the second case. The fragmentation rates of 1(+*)-3(+*) were found to increase by addition of a pyridine, and plots of k(base) against base strength were linear, allowing calculation of the beta Bronsted values, which were found to increase as the reduction potential of the radical cation decreases, beta = 0.21 (3(+*)), 0.34 (1(+*)), and 0.48 (2(+*)). The reactions of 1(+*) exhibit a deuterium kinetic isotope effect with values that increase as the base strength increases: k(OH)/k(OD) = 1.3 (pyridine), 1.9 (4-ethylpyridine), and 2.3 (4-methoxypyridine). This finding and the observation that with the above three bases the rate decreases in the order 3(+*) > 1(+*) > 2(+*), i.e., as the C-C BDE increases, suggest that C-C and O-H bond cleavages are concerted but not synchronous, with the role of OH bond breaking increasing as the base becomes stronger (variable transition state). It is probable that, with the much stronger base, 4-(dimethylamino)pyridine, a change to a stepwise mechanism may occur where the slow step is the formation of a radical zwitterion that then rapidly fragmentates to products.     

Theoretical calculations on the cycloreversion of oxetane radical cations

The molecular mechanism for the cycloreversion of oxetane radical cations has been studied at the UB3LYP/6-31G* level. Calculations support that the cycloreversion takes place via a concerted but asynchronous process, where C-C bond breaking at the transition state is more advanced than O-C breaking. This allows a favorable rearrangement of the spin electron density from the oxetane radical cation (with the spin density located mainly on the oxygen atom) to the alkene radical cation which is one of the final products. Inclusion of solvent effects does not modify the gas-phase results.     

Vibronic coupling in benzene cation and anion: vibronic coupling and frontier electron density in Jahn-Teller molecules

Vibronic coupling constants of Jahn-Teller molecules, benzene radical cation and anion, are computed as matrix elements of the electronic part of the vibronic coupling operator using the electronic wave functions calculated by generalized restricted Hartree-Fock and state-averaged complete active space self-consistent-field methods. The calculated vibronic coupling constants for benzene cation agree well with the experimental and theoretical values. Vibronic coupling density analysis, which illustrates the local properties of the coupling, is performed in order to explain the order of magnitude of the coupling constant from view of the electronic and vibrational structures. This analysis reveals that the couplings of the e2g2 and e2g3 modes in which the large displacements locate on C-C bonds are strong in the cation. On the other hand, they are greatly weakened in the anion because of the decrease of electron density in the region of the C-C bonds, which originates from the antibonding nature of the singly occupied molecular orbital of the anion. However, the difference of the electronic structure has a little influence on the vibronic coupling of the e2g4 mode. These results indicate that the vibronic coupling depends not only on the direction of the nuclear displacement but also on the frontier electron density.     

Morpholine-phthalocyanine (donor-acceptor) construct: photoinduced intramolecular electron transfer and triplet formation from its charge separation state

Silicon phthalocyanine (SiPc) with two axially attached morpholine (MP) units was prepared, and its photophysics was studied by laser flash photolysis, steady state and time-resolved fluorescence methods. Both the fluorescence efficiency and lifetime of SiPc moiety were remarkably quenched, because of the efficient intramolecular photoinduced electron transfer (PET) from morpholine donors to SiPc moiety. The generated charge separation state (CSS), SiPc(?-)-MP(?+), which was observed by transient absorption spectra, showed a lifetime of 4.8 ns. The triplet quantum yield of SiPc unit in the supra-molecule is unexpectedly high, and the predominant spectral signal in microsecond-scale is triplet-triplet (T(1)-T(n)) absorption. This high triplet yield is due to the charge recombination of CSS that generates T(1) in 32% efficiency: SiPc(?-)-MP(?+) → (3)SiPc-MP. The T(1) formation process occurred efficiently because the CSS SiPc(?-)-MP(?+) has a higher energy (1.65 eV) than that of the triplet state (3)SiPc-MP (1.0 eV). Emission from the CSS was also observed: SiPc(?-)-MP(?+) → SiPc-MP + hν'.     

The possibility of the decomposition of 2'-deoxyribose moiety of thymidine induced by the low energy electron attachment

To evaluate the possibility of the decomposition of 2-deoxyribose moiety of thymidine induced by low energy electrons (LEE) attachment, the transition states and the energy barriers of the bond breaking processes of the ribose of the nucleoside have been studied theoretically by applying the density functional theory with the double zeta basis sets (DZP++). The energy barriers for the breakage of the C-C bonds (C(1')-C(2'), C(2')-C(3'), C(3')-C(4'), and C(4')-C(5')) of the ribose group of the radical anion of thymidine are found to be high (ca. 42-57 kcal/mol). The total energies of the C-C bond-broken products are significantly higher than that of the radical anion dT(*-). The decomposition of dT(*-) through the C-C bond rupture is unlikely to take place. The rupture of the C(1')-O(4') bond of dT(*-) needs an activation energy as low as 10.4 kcal/mol. However, the reversed reaction (C(1')-O(4') bond formation) needs the activation energy low as 0.3 kcal/mol. Therefore, the intermediate product LM1(C1')-(O4') is unlikely to be stable and the C(1')-O(4') bond-broken is not favored. The activation energy of the C(4')-O(4') bond rupture process amounts to 20.5 kcal/mol. The total energy of the C(4')-O(4') bond broken product is about 6.5 kcal/mol lower than that of the reactant dT(*-). The subsequent N1-glycosidic bond breaking process is found to have a very low energy barrier. Therefore, the LEE-induced base release through the C(4')-O(4') bond rupture might be a possible pathway.     

Palladium(II)-catalyzed claisen rearrangement of allyl vinyl ethers

The Claisen rearrangement of allyl vinyl ethers is catalyzed by PdCl₂(CH₃CN)₂, provided that alkyl substituents protect the vinyl ether double bond from coordination by the metal catalyst.     

Rearrangement of allyl homoallyl ethers to gamma,delta-unsaturated carbonyl compounds catalyzed by iridium complexes

Iridium complexes were found to promote the conversion of allyl homoallyl ethers to gamma,delta-unsaturated carbonyl compounds. For example, treatment of 1-allyl-1-allyloxycyclohexane in the presence of catalytic amounts of [Ir(cod)Cl](2), PCy(3), and Cs(2)CO(3) in toluene at 100 degrees C afforded 4-cyclohexyliden-2, 3-dimethylbutanal in 74% yield. The reaction presumably proceeds through double bond migration to allyl vinyl ethers, which then undergo the Claisen rearrangement.     

Esr studies of the ring opening of cyclopropane radical cations in freon matrices

The radical cations of cyclopropane and several of its methyl derivatives have been characterized by ESR spectroscopy following their generation by γ irradiation of dilute solutions of the parent compounds in Freon matrices at 77 K. In the CFCl₃, CF₃CCl₃, and CF₂ClCCl₃, matrices, only the ring-closed species is usually observed in the accessible temperature range up to ca 160 K. In the CFCl₂CF₂Cl matrix, however, the ring-closed radical cations initially formed at 77 K undergo ring opening between 83 and 110 K, the more highly substituted radical cations requiring a higher temperature for this transformation. The ring-closed radical cations are ²A₁ species for C_2v symmetry, the most substituted cyclopropane C-C bond being elongated with the spin density largely confined to the basal carbons in a face-to-face (90°, 90°) structure. In the ring-opened radical cations, the radical center is localized on the most substituted carbon atom following the breaking of the weakened C-C bond of the ring-closed species. The radical conformations of the ring-opened species have been determined, the RCH₂CH₂· center produced from cyclopropane having a bisected conformation while the RCH₂CMe₂· center obtained from 1,1,2,2-tetramethylcyclopropane is eclipsed, as expected for the presence of α-methyl substituents at the radical site. The nature of the putative carbocation center in the ring-opened radical cations is discussed with reference to recent proposals that this center is strongly coordinated to an electrophile (Cl^- or RCl) thereby negating the requirement for an orthogonal structure. Consideration of the strong matrix dependence of the ring-opening reaction suggests a possible solvation effect, however, in which the CFCl₂CF₂Cl matrix assists the twisting of one of the CR₁R₂ groups at the most substituted bond, leading to the rupture of this one-electron σ bond. A strong solvation effect also explains why ring-opening can occur in a suitable polar solvent despite theoretical calculations of unfavourable energetics for a similar gas-phase reaction. Experiments are also described on spiro[2.5]octane, the cyclopropane ring undergoing scission at the CH₂-CH₂ bond of this radical cation to give an RCH₂· radical center. this radical then undergoes a H-atom abstraction with a neutral spiro[2.5]octane molecule in the CFCl₂CF₂Cl matrix at higher temperature to give the spiro[2.5]oct-6-yl radical.     

A theoretical study of cyclohexyne addition to carbonyl-Cα bonds: allowed and forbidden electrocyclic and nonpericyclic ring-openings of strained cyclobutenes

The mechanism of cyclohexyne insertion into a C(O)-C(α) bond of cyclic ketones, explored experimentally by the Carreira group, has been investigated using density functional theory. B3LYP and M06-2X calculations were performed in both gas phase and THF (CPCM, UAKS radii). The reaction proceeds through a stepwise [2 + 2] cycloaddition of cyclohexyne to the enolate, followed by three disparate ring-opening possibilities of the cyclobutene alkoxide to give the product: (1) thermally allowed conrotatory electrocyclic ring-opening, (2) thermally forbidden disrotatory electrocyclic ring-opening, or (3) nonpericyclic C-C bond cleavage. Our computational results for the model alkoxide and potassium alkoxide systems show that the thermally allowed electrocyclic ring-opening pathway is favored by less than 1 kcal/mol. In more complex systems containing a potassium alkoxide (e-f), the barrier of the allowed conrotatory ring-opening is disfavored by 4-8 kcal/mol. This suggests that the thermodynamically more stable disrotatory product can be formed directly through a "forbidden" pathway. Analysis of geometrical parameters and atomic charges throughout the ring-opening pathways provides evidence for a nonpericyclic C-C bond cleavage, rather than a thermally forbidden disrotatory ring-opening. A true forbidden disrotatory ring-opening transition structure was computed for the cyclobutene alcohol; however, it was 19 kcal/mol higher in energy than the allowed conrotatory transition structure. An alternate mechanism in which the disrotatory product forms via isomerization of the conrotatory product was also explored for the alkoxide and potassium alkoxide systems.