Diradical Intermediates Research Articles

Experimental and Theoretical Characterization of the Valence Isomerization of Bi‐2H‐azirin‐2‐yls to Diazabenzenes

3,4-diazidocyclobutenes 16 were prepared from the corresponding dihalides. Some of these diazides, such as parent compound 16 d and phenyl-substituted derivatives 16 c,f, underwent spontaneous stereoselective electrocyclic ring opening below room temperature, whereas the tetraalkyl derivatives of 16 had to be heated to force the same reaction. In most cases, the resulting 1,4-diazidobuta-1,3-dienes 8 were isolated to study their photochemical transformation into bi-2H-azirin-2-yls 9 via intermediate mono-azirines 17. Except for starting materials with a low number of substituents such as 9 d and 9 f, title compounds 9 underwent a thermal valence isomerization which led exclusively to pyridazines 18 at surprisingly low temperatures. Based on quantum-chemical calculations for the parent bi-2H-azirinyl-2-yl 9 d at the UB3LYP/6-31+G(d) and MR-MP2/TZV(2df,2p) levels, the valence isomerization process is best explained by simultaneous homolytic cleavage of both C--N single bonds of 9 to generate energetically favorable N,N' diradicals 26, which cyclize to 18. The theoretical studies indicate also that one stereoisomer of 9, namely, the rac compound, should undergo valence isomerization more easily than the other, which is in conformity with different rates of these rearrangement reactions found experimentally. For the tetramethyl-bi-2H-azirin-2-yls 9 g, which are better models for the experimentally studied compounds, simultaneous homolytic cleavage of both C--N single bonds is also predicted by the calculations, although the intermediate diradicals 26 g are significantly higher in energy than those of the parent system 9 d.

Why Does Perfluorination Render Bicyclo[2.2.0]hex-1(4)-ene Stable toward Dimerization? Calculations Provide the Answers

B3LYP calculations with two different basis sets have been performed to understand why bicyclo[2.2.0]hex-1(4)-ene (1a) undergoes dimerization with DeltaH(++) = 11.5 kcal/mol, but dimerization of perfluorobicyclo[2.2.0]hex-1(4)-ene (1b) has never been observed. The former reaction is computed to be exothermic by 37.2 kcal/mol, whereas the latter is calculated to be endothermic by 7.4 kcal/mol. The 44.6 kcal/mol difference between the enthalpies of these two reactions can be dissected into contributions of 24.5 kcal/mol for the difference between the enthalpies for forming diradical intermediates 2a and 2b and 20.1 kcal/mol for cyclization of 2a and 2b to, respectively, 3a and 3b. The latter enthalpy difference is largely attributable to repulsions between the endo-fluorines in the dimer, although the exo-fluorines also are found to contribute. The former enthalpy difference is attributable to the difference between the dissociation enthalpies of the pi bonds in 1a and 1b, which is shown to amount to 16 +/- 1 kcal/mol. About 25% of the stronger pi bond in fluoroalkene 1b is found to be due to hyperconjugation of the eight C-F bonds in 1b with the filled pi orbital. However, the major contributor to the stronger pi bond in 1b is shown to be the unfavorable interaction that results when a pyramidalized radical center is syn to a C-F bond. Both of these effects, which contribute to the greater strength of the pi bond in 1b, relative to that in 1a, are analyzed and discussed.

Theoretical Investigation of the Stereoselective Stepwise Cope Rearrangement of a 3-Vinylmethylenecyclobutane

The potential energy surface of the rearrangement of 3-vinylmethylenecyclobutane to 4-methylenecyclohexene has been studied computationally using density functional theory (B3LYP) and complete active space ab initio methods (CASSCF and CASPT2). The parent reaction is nonconcerted and occurs through several parallel diradical pathways. Transition structures and diradical intermediates are highly comparable in energy, with no deep potential energy well on the potential energy surface. In the substituted system, stereoelectronic effects of the trialkylsiloxy group regulate torquoselectivity in the bond-breaking processes and this, combined with low barriers to cyclization, leads to a stepwise Cope rearrangement that is, nevertheless, stereoselective.

Kinetics of Thermal Rearrangements in the Δ2-Thujene System: A Full Quadrisection of a Perturbed Bicyclo[3.1.0]hex-2-ene

Further insight into the behavior of suppositional diradicals in a caldera is sought in the thermal rearrangements among the four "Delta2-thujenes", two 1-isopropyl-4-methylbicyclo[3.1.0]hex-2-enes [(-)-cis-1 and (+)-trans-2] and two isomers, exo- and endo-3-isopropyl-6-methylbicyclo[3.1.0]hex-2-ene [(+)-exo-3 and (-)-endo-4]. Optically pure trans-3-isopropyl-5-vinylcyclopentene (5) is the final, strongly thermochemically favored product, the result of an intramolecular homodienyl shift of a methyl hydrogen atom in (-)-endo-4. The set of twelve specific rate constants, four sets of three each, that define the interrelations among the four isomers has been extracted from data acquired starting from each isomer. An attractive mechanistic hypothesis involving an intermediate diradical of iso conformation, common, for example, to both (-)-cis-1 and (+)-exo-3 (as educts), that proceeds to an anticonformer common to both (+)-trans-2 and (-)-endo-4 does not lead to a satisfactory rationalization of the product distribution. Addition of a second mechanistic conceptual scheme, that of a diradical-in-transit behaving as if there were a measure of continuous bonding (for example, (+)-trans-2 proceeding directly to (-)-cis-1), improves agreement with experiment. Over a 30 degrees C range of temperature, there is no credible change in product distribution.

Thermal Isomerizations of 2-d-1-(E)-Propenylcyclobutanes to 4-d-3-Methylcyclohexenes

Racemic trans-2-d-1-(E)-propenylcyclobutane at 276 degrees C in the gas phase fragments to give ethylenes and pentadienes, equilibrates with its cis isomer, and rearranges to mixtures of 4-d- and 6-d-3-methylcyclohexenes through [1,3] carbon shifts. The time-dependent distributions of deuterium-labeled isomers of propenylcyclobutanes and 3-methylcyclohexenes reveal a significant secondary deuterium kinetic isotope effect favoring C1-C4 over C1-C2 bond breaking (kH/kD = 1.16 +/- 0.02) and a 72:28 preference for structural isomerizations giving (si + ar) rather than (sr + ai) products through conformationally flexible short-lived diradical intermediates.

Room Temperature Observation of p-Xylylenes by 1H NMR and Evidence for Diradical Intermediates in Their Oligomerization

p-Quinodimethanes (p-QDMs) are reactive molecules that have been invoked as transient intermediates in a number of reactions. Dilute solutions of benzene-based p-QDMs, p-xylylene (1), alpha-methyl-p-xylylene (10), and 2,5-dimethyl-p-xylylene (11) can be prepared by fluoride-induced elimination of trimethylsilyl acetate from the appropriate precursor. It has been found that these solutions are stable enough to allow these reactive p-QDMs to be observed by 1H NMR spectroscopy at room temperature. For the first time, the 13C NMR spectrum of p-QDM 1 was observed. After several hours at room temperature, these p-QDMs form dimers, trimers, and insoluble oligomers. Formation of trimers provides evidence that p-QDMs 1, 10, and 11 dimerize by a stepwise mechanism involving dimeric diradicals as intermediates.

Dynamics of the Degenerate Rearrangement of Bicyclo[3.1.0]hex-2-ene

Quasiclassical direct dynamics simulations are applied to a 4-fold degenerate rearrangement which yields a nonstatistical product distribution. The simulated product ratio agrees with experiment and is found to be entirely dynamically determined. Trajectory lifetimes are on the order of a low-frequency vibrational period. The interaction of reaction momentum with the geometric features of the potential surface produces selectivity despite a common energy barrier. A geometric model is described for qualitatively estimating much of the dynamically determined product ratio independently of trajectory calculations. The characteristics of this reaction are expected also to apply to others involving modestly stabilized diradical intermediates.

Formation of vinylidene and vinylene by selective reactivity of Si(1 0 0)2 × 1 towards iso, cis and trans isomers of dichloroethylene

The room temperature (RT) chemisorption of three ( iso, cis and trans) isomers of dichloroethylene (DCE) on Si(1 0 0)2 × 1 have been investigated by X-ray photoelectron spectroscopy (XPS) and temperature programmed desorption (TPD). Unlike ethylene, the lack of molecular desorption features in the TPD data effectively rules out the cycloaddition adsorption mechanism for all three isomers. XPS spectra show that cis- and trans-DCE adsorb dissociatively on the 2 × 1 surface in equal proportion as mono-σ bonded 2-chlorovinyl and di-σ bonded vinylene adspecies, which could be produced by dechlorination mechanisms involving the proposed tri-atom π-complex and diradical intermediates, respectively. Acetylene ( m/ z 26) evolution from 2-chlorovinyl adspecies at 590 K and vinylene at 750 K are also observed for both cis- and trans-DCE, further confirming the common adsorption mechanisms for these geometrical isomers and the relative stabilities of the adspecies. In contrast, only vinylidene adspecies is found for iso-DCE, which indicates that the high ionicity of the CCl 2 group favours the diradical dechlorination mechanism. The single m/ z 26 desorption peak for iso-DCE adspecies observed at a higher temperature (780 K) than cis and trans isomers is consistent with the higher adsorption energy of vinylidene than vinylene on Si(1 0 0) obtained in our ab initio calculations. The different relative locations of the Cl atoms in these isomers therefore play a crucial role in controlling the adsorption and thermal evolution on Si(1 0 0)2 × 1. The selective reactivity of the 2 × 1 surface towards these isomers can be used to generate vinylene or vinylidene templates from their corresponding adspecies.

A theoretical study of thermal [1,3]‐sigmatropic rearrangements of 3‐trimethylsilyl‐1‐pyrazoline: Concerted vs. stepwise mechanisms

Possible reaction mechanisms of 1,3-silyl and 1,3-hydrogen thermal rearrangements of trimethylsilyl-1-pyrazoline and its model systems were theoretically explored using B3LYP, MP2, CR-CCSD(T), CASSCF(6,5), and MRMP2(6,5) theories. Nitrogen substitution at the center position of allylic moiety turned out to have a special stabilizing effect on diradical intermediates, allowing a stepwise pathway. This substitutional effect was attributed to the nitrogen lone pair electrons, which form strong pi-conjugations with diradicals. The second nitrogen substitution at the terminal allylic position selectively reduces the reaction barrier of antarafacial retention pathway, creating a competition between concerted and stepwise channels. The introduction of a five-membered ring imposes ring strain on the allylic moiety and increases steric hindrance, allowing no antarafacial channels. The combined effect of the nitrogen substitution and the five-membered ring further removes the possibility of concerted reaction pathways. Therefore 1,3-silyl migrations of 3-trimethylsilyl-1-pyrazoline were found to occur only through stepwise mechanisms, implying that the Woodward-Hoffmann rule is not operative. The 1,3-hydrogen migration also occur via a stepwise mechanism; however, it would not occur easily because its reaction barrier is much higher than that of 1,3-silyl migrations. Current study shows that a stepwise mechanism can be the dominant reaction pathway of some particular [1,3]-sigmatropic rearrangements.

Thermal Reactions of 8-Methylbicyclo[4.2.0]oct-2-enes: Competitive Diradical-Mediated [1,3] Sigmatropic, Stereomutation, and Fragmentation Processes

[reaction: see text] At 275 degrees C, 8-exo-methylbicyclo[4.2.0]oct-2-ene (1a) undergoes a [1,3] sigmatropic rearrangement to 5-methylbicyclo[2.2.2]oct-2-enes, of which the orbital symmetry-allowed si product is only marginally favored over the forbidden sr product; that is, si/sr is 2.4. Accompanying the [1,3] shift are significant amounts of epimerization and fragmentation. The 8-endo epimer 1b, which yields no [1,3] product, experiences primarily direct fragmentation and secondarily epimerization. A diradical intermediate can account for all such observations.

Deuterium kinetic isotope effects on the thermal isomerizations of deuteriocyclopropane to deuterium-labeled propenes

The gas-phase thermal isomerizations of deuteriocyclopropane to the four possible monodeuterium-labeled propenes have been followed at 435 °C. The observed distribution of products provides estimates of two deuterium kinetic isotope effects, the secondary [Formula: see text] for the carbon–carbon bond cleavage leading to trimethylene diradical reactive intermediates and the primary [Formula: see text] ratio for a [1,2] shift of a hydrogen or deuterium leading from the diradical to a labeled propene. The values determined are [Formula: see text] = 1.09 ± 0.03 and [Formula: see text] = 1.55 ± 0.06. The experimental [Formula: see text] value found agrees well with some, but not all, earlier calculated values and conjectures. Key words: cyclopropane, thermal rearrangement, kinetic isotope effects.

"Concerted" transition state, stepwise mechanism. Dynamics effects in C2-C6 enyne allene cyclizations.

The C2-C6 (Schmittel)/ene cyclization of enyne-allenes is studied by a combination of kinetic isotope effects, theoretical calculations, and dynamics trajectories. For the cyclization of allenol acetate 9, the isotope effect (k(CH3)/k(CD3) is approximately 1.43. The isotope effect is interpreted in terms of a highly asynchronous transition state near the concerted/stepwise boundary. This is supported by density functional theory calculations that locate a highly asynchronous transition structure for the concerted ene reaction. However, calculations of both the experimental system and a model reaction were unable to locate a transition structure for formation of the diradical intermediate of a stepwise mechanism. The stepwise mechanism and the asynchronous concerted mechanism start out geometrically similar, and the two pathways appear to have merged as far as the initial transition structure. For the model reaction, quasiclassical direct dynamics trajectories emanating from the initial transition structure afforded the diradical intermediate in 29 out of 101 trajectories. A large portion of the remaining trajectories completes hydrogen transfer before carbon-carbon bond formation, despite the advanced carbon-carbon bond formation in the asynchronous transition structure. Overall, the single minimum-energy path from starting material to product is inadequate to describe the reaction, and a consideration of dynamic effects is necessary to understand the mechanism. The implications of these observations toward questions of concert in other reactions are discussed.

The Three Corrugated Surfaces of 1,4-Divinyltetramethylene Diradical Intermediates and Their Connections to 1,2-Divinylcyclobutane, 4-Vinylcyclohexene, 1,5-Cyclooctadiene, and Two Butadienes

The three potential energy surfaces of the trans-trans, cis-trans, and cis-cis divinyltetramethylene diradicals have been located with DFT calculations at the BPW91/6-311+G levels. The three surfaces account well for the experimental results reported for the thermolysis of optically active trans-1,2-divinylcyclobutane and optically active and deuterated 4-vinylcyclohexene. The surfaces account also for the outcome of the dimerization of butadiene and the thermolysis of cis,cis-1,5-cyclooctadiene. The three diradical intermediates are connected to the cyclization and dissociation products through conformations that are explored fully here.

1,4-Addition of Benzene to a Dihydrocyclopent[a]indene Diradical: Synthesis and DFT Study

Photochemical cyclization of compound 1, a homoenediyne (-CCC=CCH2CC-) bearing two ethynylanthracene chromophores, yields two isomeric dihydrocyclopent[a]indene ring systems, spiro-fused to the 9-position of a 9,10-dihydroanthracene moiety. Evidence of a photochemically initiated diradical cyclization pathway is proposed on the basis of (i) hydrogen abstraction from reaction with 1,4-cyclohexadiene (1,4-CHD) and (ii) the observation of 1,4-addition of benzene (solvent). The reaction was further analyzed by a complete density functional theory (DFT) study, using an unrestricted approach (UBLYP) with a 6-31G* basis set for the open-shell triplet states of the reactants, products, and diradical intermediates to model the photochemical nature of observed transformation. A mechanism detailing the observed cyclization/addition reaction is proposed.

Theoretical study of the photochemical reaction mechanism of bicyclo[4.1.0]hept‐2‐ene upon direct photolysis

AbstractAb initio multiconfigurational CASSCF/MP2 method with the 6‐31G* basis set has been employed in studying the photochemistry of bicyclo[4.1.0]hept‐2‐ene upon direct photolysis. Our calculations involve the ground state (S0) and excited states (S1, T1, and T2). The ground‐state reaction pathways corresponding to the formation of the six products derived from bicyclo[4.1.0]hept‐2‐ene via two important diradical intermediates (D1 and D2) were mapped. It was found that there are various crossing points (conical intersections and singlet–triplet crossings) in the regions near D1 and D2. These crossing points imply that direct photolysis can lead to two possible radiationless relaxation routes: (1) S1 → S0, (2) S1 → T2 → T1 → S0. Computation indicates that the second route is not a competitive path with the first route during direct photolysis. The first route is initiated by barrierless cyclopropane bond cleavage to form two singlet excited diradical intermediates, followed by efficient decay to the ground‐state surface via three S1/S0 conical intersections in the regions near the diradical intermediates. All six ground‐state products can be formed via the three conical intersections almost without barrier after the decays. The barriers separating the diradical minima on S1 from the S1/S0 conical intersections were found to be very small with respect to the vertical excitation energy, which can explain why the product distribution is independent of excitation wavelength. Triplet surfaces are not involved in the first route, which agrees with the fact that the product contribution was unchanged by the addition of naphthalene. © 2005 Wiley Periodicals, Inc. Int J Quantum Chem, 2005

Mechanism of photoisomerization of optically pure trans-2,3-diphenylcyclopropane-1-carboxylic acid derivatives

The photochemistry of optically pure isomers of alpha-methylbenzylamide of trans-2,3-diphenylcyclopropane-1-carboxylic acid has been examined in isotropic solution and within zeolites. The results suggest that these isomerize through cleavage of C2-C3 bond. The direct excitation in solution leads to non-equilibrating 1,3-singlet diradical intermediates whereas triplet sensitization results in equilibrating 1,3-triplet diradical intermediates. The direct excitation within NaY zeolite seems to result in equilibrating zwitterionic intermediates. Studies on the optically pure trans isomers allow one to understand the mechanism of chiral induction during the photoisomerization of mesocis-2,3-diphenylcyclopropane-1-carboxylic acid. The current study has clarified the nature of the excited states involved during the classic (R)-N-acetyl-1-naphthylethylamine sensitized isomerization of 1,2-diphenylcyclopropane.

Regioselectivity of the Intramolecular Photocycloaddition of α,β-Butenolides to a Terminal Alkene

The intramolecular [2+2] photocycloaddition of α,β-butenolides to a terminal double bond tethered to the lactone through the γ-position and located at a suitable distance has been studied. The regioselectivity of the photoisomerization depends on the substitution pattern of the substrate and can be rationalized by simple theoretical calculations performed on the diradical intermediates.

An Unusual Domino Retro‐Ene–Conia Reaction: Regio‐ and Stereoselective One‐Carbon Ring Expansion of Fenchol Derivatives

AbstractThe 2‐exo‐substituted fenchol derivatives 1–7, easily prepared from (−)‐fenchone in good‐to‐excellent yields, were pyrolyzed by dynamic gas‐phase thermo‐isomerization (DGPTI). At temperatures of ca. 620°, the substrates with a hydroxyallyl (1–4) or a hydroxypropargyl moiety (6) underwent an initial retro‐ene reaction under cleavage of the C(2)C(3) bond to form enol‐ene intermediates with no loss of optical activity. These intermediates then experience either tautomerization to the corresponding α,β‐unsaturated ketones or subsequent Conia rearrangement under one‐carbon ring expansion of the fenchone system to a bicyclo[3.2.1]octane framework. In the case of the isopropenyl substrate 3, the sterically crowded Conia product underwent a new type of ‘deethanation’ reaction by stepwise loss of two Me radicals, giving rise to the thermodynamically favored enone 21. A similar relaxation behavior was observed in the case of the ethynyl substrate 6, which showed a remarkable 1,3‐Me shift after the Conia reaction, leading to the α,β‐unsaturated cyclic ketone 25. The homolytic cleavage of the weakest single bond in 1–3 turned out to be a competing reaction pathway. Intramolecular H‐abstraction within the generated diradical intermediates produced the monocyclic ketones 8, 16, and 19, besides the products obtained by tautomerization and Conia reaction. In contrast, a Ph substituent at C(2) in 7 allowed only the passage through a diradical species to provide phenone 26, which was converted by regioselective Baeyer–Villiger oxidation to the optically active cyclopentanol 29. Both reaction channels, the domino retro‐ene–Conia rearrangement and the diradical‐promoted H‐transfer, have been shown to proceed highly stereoselectively. The absolute configuration of the newly formed stereogenic centers in all compounds was assigned by 1H‐NOE experiments. The reaction mechanism of the novel domino retro‐ene–Conia reaction was established by both a series of 2H‐ and 13C‐labeling experiments, as well as by a detailed computational analysis.

Producing Reactive Species on Si(100), Ge(100), and Si(111) Surfaces by Attachments of Diacetylenes

Formation of a bilayer or a multilayer organic thin film on semiconductor surfaces is promising for the manufacture of new-generation microelectronic/nanoelectronic materials. A prerequisite for realization of such a goal is the formation of monolayer organic thin films with reactive functional groups exposed as a template for further chemical manipulations. We report herein a theoretical prediction that attachments of diacetylenes on Si(100)-2 × 1, Ge(100)-2 × 1, and Si(111)-7 × 7 surfaces could produce monolayers of reactive [3]cumulenic or/and enynic surface species. Our density-functional cluster-model calculations revealed the following: (i) Diacetylene can undergo either stepwise [4 + 2]-like or stepwise [2 + 2]-like cycloadditions onto a XX dimer of X(100)-2 × 1 surfaces (X = Si and Ge) via a common singlet−diradical intermediate. (ii) On Si(100), the [2 + 2]-like pathway is kinetically favored over the [4 + 2]-like one, preferentially giving rise to enynic adspecies. (iii) On Ge(100), both pathways are competitive, resulting concomitantly in [3]cumulenic and enynic adspecies. (iv) On Si(111)-7 × 7, diacetylene can undergo either 1,4 addition or 1,2 addition onto a rest atom−adatom pair with the former process being favored over the latter both thermodynamically and kinetically and leading preferentially to the formation of [3]cumulenic adspecies on the surface. It is noteworthy that the [4 + 2]-like cycloaddition of diacetylene on Ge(100) 2 × 1 is among the few examples of [4 + 2] cycloaddition with a 1,3-diyne acting as a four-electron component affording six-membered cyclic [3]cumulenes.

Kinetics and mechanism of the thermal decomposition reaction of acetone cyclic diperoxide in methyl tert‐butyl ether solution

AbstractThe thermal decomposition reaction of acetone cyclic diperoxide (3,3,6,6‐tetramethyl‐1,2,4,5‐tetroxane, ACDP), in the temperature range of 130.0–166.0°C and initial concentrations range of 0.4–3.1 × 10−2 mol kg−1 has been studied in methyl t‐butyl ether solution. The thermolysis follows first‐order kinetic laws up to at least ca 60% ACDP conversion. Under the experimental conditions, the activation parameters of the initial step of the reaction (ΔH# = 33.6 ± 1.1 kcal mol−1; ΔS# = −4.1 ± 0.7 cal mol−1 K−1; ΔG# = 35.0 ± 1.1 kcal mol−1) and acetone, as the only organic product, support a stepwise reaction mechanism with the homolytic rupture of one of its peroxidic bond. Also, participation of solvent molecules in the reaction is postulated given an intermediate diradical, which further decomposes by CO bond ruptures, yielding a stoichiometric amount of acetone (2 mol per mole of ACDP decomposed). The results are compared with those obtained for the above diperoxide thermolysis in other solvents. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 302–307, 2004