Experimental and theoretical investigation of reversible interconversion, thermal reactions, and wavelength-dependent photochemistry of diazo Meldrum's acid and its diazirine isomer, 6,6-dimethyl-5,7-dioxa-1,2-diaza-spiro[2,5]oct-1-ene-4,8-dione

The photochemical or thermal decomposition of diazo Meldrum's acid (1) in methanolic solutions yields ketoester 3a, the product of the Wolff rearrangement, while products produced from the singlet carbene were not detected. This observation, combined with the analysis of activation parameters for the thermal decomposition of 1, as well as with the results of DFT B3PW91/6-311+G(3df,2p) and MP2/aug-cc-pVTZ//B3PW91/6-311+G(3df,2p) calculations, allows us to conclude that the Wolff rearrangement of 1 is a concerted process. The outcome of the photolysis of diazo Meldrum's acid depends on the wavelength of irradiation. Irradiation with 254 nm light results in an efficient (Phi(254) = 0.34) photo-Wolff reaction, while at 355 nm, the formation of diazirine 2 becomes the predominant process (Phi(350) = 0.024). This unusual wavelength selectivity indicates that Wolff rearrangement and isomerization originate from different electronically excited states of 1. The UV irradiation of diazirine 2 leads to the loss of nitrogen and the Wolff rearrangement, apparently via a carbene intermediate. This process is accompanied by a reverse isomerization to diazo Meldrum's acid. Triplet-sensitized photolysis of both isomers results in the formation of Meldrum's acid, the product of a formal reduction of 1 and 2. Mild heating of diazirine 2 produces quantitative yields of diazo Meldrum's acid. The activation parameters for thermal reactions of diazo 1 and diazirino 2 isomers were determined in aqueous and dioxane solutions.     

S2ClF的构型及其异构化反应的密度泛函理论研究

采用量子化学中的密度泛函理论B3LYP方法,在6-311+G(3df)水平上,全优化得到了S2ClF线型和分叉型2种异构体的平衡结构,同时对可能发生的分子内卤素原子迁移反应的过渡态进行了考察。计算结果表明,从能量角度看,线型的ClSSF为稳定构型,热力学和动力学计算表明,无论是F原子迁移还是Cl原子迁移,分子内的原子迁移需要较高的活化能,并且速度很慢。     

Gating the mechanistic pathway to the elusive 4-membered ring azeteoporphyrin

Photolysis of metalated (Cu and Ni) and free base 2-diazo-3-oxochlorins within a frozen matrix (λ = 457.9 nm, toluene, 80 K) generates a single photointermediate with a hypsochromically shifted electronic absorption spectrum relative to the starting diazochlorins. The appearance of ketene (~2131 cm(-1)) and azete (~1670 cm(-1)) vibrations in infrared absorption and Raman spectra, respectively, identifies this intermediate as resulting from the Wolff rearrangement of the diazochlorins upon N(2) loss. Computational modeling of the vibrational spectra and TDDFT simulation of the electronic transitions of potential photointermediates corroborate this assignment. Isolation and analysis of photoproducts of these diazochlorins formed within n-butanol-doped frozen toluene matrices indicate near exclusive formation of azeteoporphyrins. In sharp contrast, room temperature laser photolysis of these materials yields a mixture of photoproducts deriving from the presence of both carbene and ketene intermediates. Computational modeling of the intramolecular reactivity of the proposed sp(2) carbene intermediate shows exclusive bond insertion to the adjacent phenyl group, and no evidence of Wolff rearrangement. Computational reaction profile analyses reveal that the barrierless Wolff rearrangement proceeds via an out-of-plane carbene electronic configuration that is generated directly during the loss of N(2). The formation of out-of-plane carbene, resulting in the exclusive formation of the observed ketene photointermediate at low temperatures, is consistent with orbital symmetry considerations and by the geometric constraints imposed by the frozen matrix. Combined, this leads to a model showing that azeteoporphyrin formation via the Wolff rearrangement is dependent upon the structural disposition of the adjacent framework, and the specific reaction intermediate formed is very sensitive to this feature.     

Gas-phase synthesis of charged copper and silver Fischer carbenes from diazomalonates: mechanistic and conformational considerations in metal-mediated wolff rearrangements

Copper(I) and silver(I) Fischer carbenes are synthesized in the gas phase. Various diazomalonate-based compounds with an attached metal ion are introduced into the gas phase by electrospray ionization and subjected to collisional activation. Loss of N(2) generates a metastable Fischer carbene, which subsequently undergoes Wolff rearrangement and loss of CO. Further excitation leads to the loss of another CO molecule and the generation of a stable Fischer carbene. Isotopically labeled compounds are utilized to confirm the assignment of the products resulting from this process. DFT calculations are used to evaluate various mechanistic possibilities and to quantitatively assess the energetics of reactants and products. Silver(I) is shown to be more effective in facilitating Wolff rearrangement than copper(I), although both are more effective when compared to spectator charges such as sodium or a fixed quaternary nitrogen. Carbenes are not produced when copper(II), nickel(II), or a proton is used to form a quasi-molecular ion from the diazomalonate carbene precursor. Finally, trapping of the Fischer carbene by various functional groups attached through the open coordination site of the metal is investigated.     

Chemistry of diazocarbonyl compounds: XXVII. Thermolysis and photolysis of diazirines, derivatives of 1,3-dioxane-4,6-dione

Photolysis of diazirines, 1,3-dioxane-4,6-dione derivatives, occurs in the presence of methanol or dimethyl sulfide as carbene traps without a formation of carbene intermediates. It was established for the first time that the thermolysis and photolysis of diazirines from the 1,3-dioxane-4,6-dione series led mainly to Wolff rearrangement with a subsequent formation of 5-oxo-1,3-dioxolane-4-carboxylic acids or their derivatives. The data obtained show that the α-oxodiazirines are the key intermediates in the photolysis and Wolff rearrangement of α-diazocarbonyl compounds.     

Experimental and theoretical analysis of the photochemistry and thermal reactivity of ethyl diazomalonate and its diazirino isomer. The role of molecular geometry in the decomposition of diazocarbonyl compounds

The photochemical or thermal decomposition of ethyl diazomalonate (1) or ethyl 3,3-diazirinedicarboxylate in methanol solutions yields the O-H insertion product 6, while products of the Wolff rearrangement were not detected in both cases. The analysis of temperature-dependent (13)C NMR spectra and the results of DFT B3LYP/6-311+G(3df,2p) and MP2/aug-cc-pVTZ//B3LYP/6-311+G(3df,2p) calculations allow us to conclude that diazodiester 1 predominantly exists in the Z,Z-conformation. In contrast, photolysis of the cyclic isopropylidene diazomalonate (3), which also has a Z,Z-configuration of the diazodicarbonyl moiety, results in a clean Wolff rearrangement. These observations allow us to conclude that the direction of the photodecomposition of diazomalonates is not controlled by the ground-state conformation. The quantum-mechanical analysis of the potential energy surfaces for the dediazotization of 1 and 3 suggests that the formation of a carbene as a discrete intermediate is controlled by the ability of the latter to adopt a conformation in which carbonyl groups are almost orthogonal to the carbene plane. The outcome of the photolysis of ethyl diazomalonate depends on the wavelength of irradiation. Irradiation with 254 nm light results in the loss of nitrogen and the formation of dicarboethoxycarbene (5, Phi(254) = 0.31), while at longer wavelengths, diazirine 2 becomes an important byproduct (Phi(350) = 0.09). This observation suggests that the formation of carbene 5 and isomerization to diazirine proceed from different electronically excited states of ethyl diazomalonate.     

Potential energy surface of 1,6-Methano

The potential energy surface of 1,6-methano[10]annulen-11-ylidene and its isomers has been investigated by density functional (BLYP and B3LYP) molecular orbital methods. These calculations indicate the lowest energy annulene structure to be 56.9 kcal mol(-)(1) higher in energy than triplet 1-naphthylcarbene. These calculations, together with calculations on transition states connecting possible rearrangement products derived from this carbene, indicate that the trapping products reported by Carlton et al. J. Am. Chem. Soc. 1976, 98, 6068-6070 arise from rearrangement of the annulene-carbene to a tricyclic isomer.     

BrSSCl和SSBrCl相对稳定性的理论研究

采用量子化学中的密度泛函理论，在B3LYP／6-311 G(3df)水平上全优化得到了S2BrCl分子线型和分叉型2种异构体的平衡结构，同时对可能发生的分子内原子迁移过程的过渡态进行了考察。计算结果表明，从能量角度看，线型的BrSSCl为稳定构型。采用统计热力学及过渡态理论，研究了Z种平衡结构之间相互转化的热力学和动力学性质。根据计算结果，无论是Cl迁移还是Br迁移，分子内的原子迁移都需要较高的活化能，并且迁移速度较慢。     

Wolff Rearrangement of Oxidatively Generated α‐Oxo Gold Carbenes: An Effective Approach to Silylketenes

Gold‐catalyzed oxidations of alkynes by N‐oxides offer direct access to reactive α‐oxo gold carbene intermediates from benign and readily available alkynes instead of hazardous diazo carbonyl compounds. Despite various versatile synthetic methods developed based on this strategy, one of the hallmarks of α‐oxo carbene/carbenoid chemistry, that is, the Wolff rearrangement, has not been realized in this context. This study discloses the first examples that show the Wolff rearrangement can be readily realized by α‐oxo gold carbenes oxidatively generated from TBS‐terminated alkynes (TBS=tert‐butyldimethylsilyl). The thus‐generated silylketenes can be either isolated pure or subsequently trapped by various internal or external nucleophiles in one pot to afford α‐silylated carboxylic acids, their derivatives, or TBS‐substituted allenes.     

Are Copper(I) Carbenes Capable Intermediates for Cyclopropanations? The Case for Ylide Intermediates

A novel approach is used to synthesize a stable, ligated copper(I) carbene in the gas phase that is capable of typical metal carbenoid chemistry. However, it is shown that copper(I) carbenes generally undergo rapid unimolecular rearrangements including insertions into copper‐ligand bonds and Wolff rearrangements. The results indicate that most copper(I) carbenes are inherently unstable and would not be viable intermediates in condensed‐phase applications; an alternative intermediate that is less prone to rearrangements is required. Computational data suggest that ylides formed by the complexation of the carbene with solvent or other weak nucleophiles are viable intermediates in the reactions of copper(I) carbenes.     

Microwave specific Wolff rearrangement of alpha-diazoketones and its relevance to the nonthermal and thermal effect

alpha-Diazoketones possess high electric dipole moments, as a consequence of the dipolar nature of the diazocarbonyl functional group. The vectorial analysis, theoretical calculations (PM3 and ab initio), and literature reports based on experimental and theoretical calculations reveal a higher dipole moment for the Z-configuration of the diazo functional group. Microwave irradiation of alpha-diazoketone (1a-m) (Figure 1) promotes Wolff rearrangement specifically via the Z-configuration in excellent yields. The dielectric properties of the solvent govern the course of the microwave rearrangement. 3-Diazocamphor (1m) on microwave irradiation in benzylamine exhibits nonthermal effects to furnish exclusively the Wolff rearrangement product (4m), equivalent to its photochemical behavior. In the presence of an aqueous medium, through solvent heating predominates, leading to the formation of a tricyclic ketone (5) as the principal product, arising from an intramolecular C-H insertion. This behavior is similar to its known thermal and transition metal catalyzed reactivity pattern.     

Carbene formation in its lower singlet state from photoexcited 3H-diazirine or diazomethane. A combined CASPT2 and ab initio direct dynamics trajectory study

The potential energy surfaces of the ground and valence excited states of both 3H-diazirine and diazomethane have been studied computationally by mean of the CASSCF method in conjunction with the cc-pVTZ basis set. The energies of the critical points found on such surfaces have been recomputed at the CASPT2/cc-pVTZ level. Additionally, ab initio direct dynamic trajectory calculations have been carried out on the S(1) and S(2) surfaces, starting each trajectory run at the region dominated by the conformational molecular rearrangement of diazomethane. It is found that both isomers are interconnected along a C(s)() reaction coordinate on each potential surface. Radiationless deactivation of the corresponding S(1) state of each isomer occurs through the same point on the surface, an S(1)/S(0) conical intersection. Thereafter, the system has enough energy to surmount the barrier which leads to dissociation products (CH(2) + N(2)) on S(0) state. Therefore, photoexcitation to S(1) state of either diazirine of diazomethane produces methylene in its lower singlet state on a very short time scale (ca. 100 fs). Furthermore, both isomers can generate excited singlet carbene when they are excited onto the S(2) surface; in this case, they lose the activation energy passing through another common S(2)/S(1) conical intersection and then proceed to dissociation into carbene and N(2) on the S(1) surface. For the special case of methylene, it rapidly experiences deexcitation to S(0) state.     

Chemistry of diazocarbonyl compounds: XXV. Comparative photochemistry of diazo compounds and sulfur ylides of the 1,3-dioxane-4,6-dione series

Photochemical decomposition of 2,2-dialkyl-5-diazo-1,3-dioxane-4,6-diones in the presence of pyridine, methanol, or dimethyl sulfide as carbene traps involves mainly the Wolff rearrangement which is likely to follow a concerted pattern, while the yield of the “carbene” products does not exceed 27–28%. No carbene intermediates are formed in the photolysis of the corresponding dioxo sulfonium ylides under analogous conditions, and the main photochemical process is 1,2-methyl shift (Stevens rearrangement), followed by photochemical transformations of the primary products according to the Norrish type II pattern.     

Laser flash photolysis of diphenylsulfonyldiazomethane: detection of the sulfene and a sulfene-pyridine ylide

The photochemistry of diphenylsulfonyldiazomethane (DSD) was studied by means of nanosecond laser flash photolysis. The photochemical behavior of this molecule upon UV irradiation is characterized by sulfene formation, presumed to arise via Wolff rearrangement of a carbene. We were able to detect the sulfene and the sulfene ylide formed upon sulfene trapping by pyridine. Sulfene quenching by nucleophiles was also examined.     

H2S2的构型及其异构化反应的密度泛函理论研究

B3LYP calculations of density functional theory with the 6-311 + G（3df,2p）basis set level are used to investigate the equilibrium structures and intramolecular rearrangement reaction between the linear HSSH and branched H2SS isomers. The predicted geometrical parameters and scaled harmonic vibrational frequencies for HSSH are in good agreement with the available values experimentally. The predicted S-S and S-H bond lengths in the thiosulfoxide structure H2SS are 0.1986 and 0.1366 nm respectively and the values of 5SSH and 5HSH bond angles are 108. 3o and 89. 5o respectively. The transition-state for the unimolecular isomerization is suitably characterized by diagonalizing the matrix of energy second derivatives to determine the unique imaginary vibrational frequency and confirmed by the IRC calculation. The calculated results show that the linear structure is stable with respect to the branched form（lower 109.8 kJ/mol corrected with zero point vibrational energy）energetically. The calculated energy barrier for the direct intramolecular hydrogen atom transfer isomerization process is 190.3 kJ/mol. The kinetic results demonstrate that the isomerization is a unimolecular process,but the reaction rate is pretty slow. This agrees with the thermodynamical results. So the isomerization process should proceed via the other likely processes.     

Theoretical study of the thermal unimolecular rearrangement of fluoroethylidenes

The thermal unimolecular isomerization of fluoroethylidenes to the corresponding fluoroethylenes has been studied by the MNDO method. It has been shown that fluorine substitution on the carbene carbon increases the activation energy in comparison with the ethylidene rearrangement. To understand the reason for this increase in the activation energy, the charge-transfer effects have been analyzed. Fluorine substitution at other positions does not significantly affect the activation energies. The thermodynamic parameters for the reaction have been evaluated, using vibrational and rotational spectral data calculated in this work. RRKM calculations have been performed and high-pressure Arrhenius parameters calculated. Hydrogen–deuterium kinetic isotope effects indicate that the reaction rates are altered considerably on isotopic substitution, and the change in reaction rates depends upon the position of deuterium substitution, as well as on the number of hydrogens replaced by deuterium atoms. © 1992 John Wiley & Sons, Inc.     

Is the bisphenol A biradical formed in the pyrolysis of polycarbonate?

An unknown species has been detected in the analysis of the products in a pyrolysis of polycarbonate using Li(+) ion-attachment mass spectrometry (IAMS). The mass spectra exhibited a Li(+) adduct peak at m/z 233 that was tentatively assigned to bisphenol A (BPA) biradical. Experimentally, this assignment was supported by the observation that the production rate increased under an inert nitrogen atmosphere. To further confirm the assignment, the stability of the BPA biradical to intramolecular rearrangement reactions as well as unimolecular decomposition has been analyzed via density functional theory calculations [B3LYP/6-311+G(3df,2p)]. The results show that the bisphenol A biradical is an open-shell biradical singlet that is stable to unimolecular decomposition. Although some of the proposed intramolecular rearrangement products have lower energies than those of the BPA diradical, these pathways have large reaction barriers and the kinetic lifetime of the radical is expected to be of the order of hours under the conditions of the experiment. The calculations also reveal that the bisphenol A diradical has large Li(+) affinities supporting the fact that these Li(+) complexes could be detected in the Li(+) ion attachment mass spectrometry. On the basis of these results the Li(+) adduct peak at m/z 233 detected in the pyrolysis of polycarbonate is assigned to the bisphenol A biradical.     

Photo‐Wolff Rearrangement of 2‐Diazo‐1,2‐naphthoquinone: Stern–Volmer Analysis of the Stepwise Reaction Pathway

2‐Diazo‐1,2‐naphthoquinone (1) and its derivatives are the photoactive components in Novolak photoresists. A femtosecond infrared study has established that the photoreaction of 1 proceeds largely by a concerted Wolff rearrangement yielding the ketene 1H‐inden‐1‐ylidene‐methanone (3) within 300 fs after excitation, but earlier trapping studies gave evidence for a minor reaction path via a carbene intermediate 1‐oxo‐2(1H)‐naphthalenylidene (2) with a lifetime of about 10 ps. Here, we provide a quantitative assessment of the stepwise pathway by Stern–Volmer analysis of the trapping of 2 by methanol to yield 2‐methoxy‐1‐naphthol (4). We conclude that the lifetime of the carbene 2 is at least 200 ps. Moreover, [3 + 2]cycloaddition of 2 and acetonitrile yielding 2‐methylnaphth[2,1‐d]oxazole (5) was observed. A comparison of the yields of 5 formed upon photolysis and upon thermolysis of 1 in acetonitrile provides evidence that a substantial part of the hot nascent carbene 2 formed photolytically rearranges to the ketene 3 during its vibrational relaxation (hot ground‐state reaction).     

Carbomethoxychlorocarbene: spectroscopy, theory, chemistry and kinetics.

Photolysis (254 nm) of methyl 8-chloro-3a,7a-methanoindan-8-carboxylate (5) in argon at 14 K produces carbomethoxychlorocarbene (6) as a persistent species. The IR and UV-vis spectra of the carbene were recorded and analyzed with the aid of density functional calculations (B3-LYP/6-31G). The IR spectrum of 6 is consistent with the carbene having a nonplanar singlet ground state, in agreement with the G3(MP2)//B3-LYP calculations of Scott and Radom (accompanying paper). Irradiation (300 nm) of 5 in solution produces indane in 97% yield. In cyclohexane, carbene 6 is trapped by insertion into a CH bond, whereas in 2,3-dimethylbutene it adds to the double bond to form a cyclopropane. Laser flash photolysis of 5 (308 nm, 17 ns, XeCl excimer) produces carbene 6 which reacts with pyridine to form an ylide (lambda(max) = 440 nm). It was possible to resolve the growth of the ylide in Freon-113 (CF(2)ClCFCl(2)) to measure the lifetime (tau = 114 ns, ambient temperature) of the carbene and the absolute rate constant of its reaction with pyridine (k(pyr) = 2 x 10(9) M(-)(1) s(-)(1)). A plot of log(1/tau) versus 1/T in CF(2)ClCFCl(2) is linear with Arrhenius parameters E(a) = 10.9 +/- 0.8 kJ/mol and A = 10(9.1)(+/-)(0.2) s(-)(1). In perfluorohexane, a less reactive solvent than Freon-113, E(a) = 23.4 +/- 1.7 kJ/mol, A = 10(10.6)(+/-)(0.) s(-)(1), and tau = 354 ns at 293 K. It is argued that the activation barrier to carbene disappearance in perfluorohexane represents the lower limit to the barrier to Wolff rearrangement of the carbene.     

Singlet-triplet splittings and barriers to Wolff rearrangement for carbonyl carbenes.

High-level ab initio calculations at the G3(MP2)//B3-LYP level have been used to study carbomethoxychlorocarbene and related halogenocarbenes and carbonyl carbenes. Initial calculations at the more accurate W1' level on the subset CH(2), HCCl, HCF, CCl(2), and CF(2) provide support for the reliability of G3(MP2)//B3-LYP for this type of problem. The W1' calculations also suggest that the experimental S-T splitting is slightly underestimated for HCCl and CF(2) and substantially underestimated for CCl(2), in keeping with other recent high-level studies. Whereas the parent carbonyl carbenes, namely formylcarbene, carbohydroxycarbene, and carbomethoxycarbene, are all predicted to have triplet ground states, their chloro and fluoro derivatives are predicted to have singlet ground states. In particular, carbomethoxychlorocarbene is predicted to have a singlet ground state, with the singlet-triplet splitting estimated as -16.0 kJ mol(-)(1). The barriers to Wolff rearrangement of the singlet carbonyl carbenes generally (but not always) correlate with the exothermicity accompanying the production of ketenes. In the case of the parent carbonyl carbenes, for which the rearrangement reaction is most exothermic, the barriers lie between about 10 and 30 kJ mol(-)(1), whereas for the less exothermic rearrangements of the chloro- and fluoro-substituted carbonyl carbenes, the Wolff rearrangement barriers increase significantly to between 58 and 75 kJ mol(-)(1). The calculated barrier for carbomethoxychlorocarbene is 58.2 kJ mol(-)(1).