Zhendong Zhu | CELGENE - Academia.edu (original) (raw)

Papers by Zhendong Zhu

Research paper thumbnail of Electronic structure of the naphthalene radical cation and some simple alkylated derivatives

Journal of the Chemical Society, Perkin Transactions 2, 1998

1 but more complete in the two derivatives. It is found that—in spite of the relatively small rel... more 1 but more complete in the two derivatives. It is found that—in spite of the relatively small relaxation energies of the vertically formed radical cations—the accompanying geometry changes on ionization may lead to quite substantial shifts in some of the excited state energies, notably that of the first (unobserved) one. Therefore, the common assumption that the good correlation between the

Research paper thumbnail of Rearrangement of 1-Noradamantyl and 1-Adamantylcarbene to Bridgehead Alkenes:  Lifetimes of Two Bridgehead Carbenes in Solution

Journal of Physical Chemistry A, 2001

B3LYP density functional calculations with the 6-31G* basis set predict that 3-noradamantylcarben... more B3LYP density functional calculations with the 6-31G* basis set predict that 3-noradamantylcarbene 7 rearranges to adamantene 9 and protoadmant-3-ene 10 over barriers of 0.35 and 7.84 kcal/mol, respectively, after zero-point energy correction. The ...

Research paper thumbnail of Electron Transfer Chemistry of Psoralen and Coumarin Derivatives by Means of Radiolytic and Electrochemical Experiments

The Journal of Physical Chemistry A, 1997

Bromopsoralens and bromocoumarins have been used to sensitize the inactivation of viruses. The br... more Bromopsoralens and bromocoumarins have been used to sensitize the inactivation of viruses. The bromine substituent increased the antiviral potency of the sensitizer, an effect attributed to cleavage of the C-Br bond of the aromatic radical anion produced by a light-induced electron transfer reaction with nucleic acids. This hypothesis was tested by a detailed mechanistic electrochemical investigation in acetonitrile and N,Ndimethylformamide of 5-bromo-8-methoxypsoralen, 8-methoxypsoralen, 3-bromocoumarin, and coumarin. A one-electron reduction process involves the formation of a radical anion, a step coupled with the cleavage of the carbon-bromine bond in the case of the brominated compounds, leading to the formation of an aryl radical moiety and a bromide anion. These experiments were further confirmed by radiolytic preparation coupled with UV-vis detection of the radical anions in glassy 2-methyltetrahydrofuran at 77 K and in ether, acetonitrile, and alcohol solutions at ambient temperature. This allows complete kinetic characterization of the ionic intermediates. The shorter lifetimes obtained with brominated radical anions relative to that of the parent structure are explained by scission of the carbon-halogen bond.

Research paper thumbnail of Chemistry and Kinetics of Dipropylcarbene in Solution

Journal of Physical Chemistry A, 1999

The photochemistry of 2-methoxy-2-methyl-5, 5-dipropyl-Δ3-1, 3, 4-oxadiazoline (1a) and 2, 2-dime... more The photochemistry of 2-methoxy-2-methyl-5, 5-dipropyl-Δ3-1, 3, 4-oxadiazoline (1a) and 2, 2-dimethoxy-5, 5-dipropyl-Δ3-1, 3, 4-oxadiazoline (1b) was investigated. Photolysis (300 nm) of these compounds in solution leads to fragmentation to 4-diazoheptane (major), ...

Research paper thumbnail of Observation of Perfluoromethylnitrene in Cryogenic Matrixes

The Journal of Physical Chemistry A, 2001

Photolysis (254 nm) of CF 3 N 3 in pentane at 6-10 K produced a persistent electron paramagnetic ... more Photolysis (254 nm) of CF 3 N 3 in pentane at 6-10 K produced a persistent electron paramagnetic resonance spectrum typical of a triplet nitrene centered at 8620 G. The spectrum (|D/hc| ) 1.736 cm -1 ) is attributed to triplet CF 3 -N, and is very similar to that of triplet NH (|D/hc| ) 1.863 cm -1 ) and CH 3-N (|D/hc| ) 1.720 cm -1 ) which were reported previously. CF 3 N 3 was also deposited in an argon matrix at 14 K. The azide (λ max ) 257 nm) was decomposed by exposure to 254 nm light with the concurrent formation of sharp, structured absorption bands at 342, 347.5, and 354 nm. The UV bands are assigned to the triplet nitrene CF 3 N on the basis of its similarity to the spectrum of CH 3 N and time-dependent density functional theory (B3LYP) calculations.

Research paper thumbnail of Singlet−Triplet Gap, and the Electronic and Vibrational Spectra of Chlorophenylcarbene:  A Combined Theoretical and Experimental Study

The Journal of Physical Chemistry A, 1999

... MG, 31270-901 Brazil. Sol Celebi, Zhendong Zhu, and Matthew S. Platz*. Newman and Wolfrom Lab... more ... MG, 31270-901 Brazil. Sol Celebi, Zhendong Zhu, and Matthew S. Platz*. Newman and Wolfrom Laboratory of Chemistry, The Ohio State University, 100 W. 18th Avenue, Columbus, Ohio 43210. J. Phys. Chem. A , 1999, 103 ...

Research paper thumbnail of A surprising rearrangement of a carbeneethylene sulfide ylide

Tetrahedron Letters, 1999

Efficient synthesis of benzofuroquinolizine ketone 1 was accomplished in four steps from ethyl 3b... more Efficient synthesis of benzofuroquinolizine ketone 1 was accomplished in four steps from ethyl 3benzofuranacetate. The O-analogue of the Pictet-Spengler cyclization was used to form the benzofuroquinolizine ring structure as a key step.

Research paper thumbnail of Ultrafast Spectroscopic and Matrix Isolation Studies of p- Biphenylyl, o -Biphenylyl, and 1-Naphthylnitrenium Cations

Journal of the American Chemical Society, 2007

p-Biphenylyl, o-biphenylyl, and 1-naphthyl azides were deposited in argon at low temperature in t... more p-Biphenylyl, o-biphenylyl, and 1-naphthyl azides were deposited in argon at low temperature in the presence and absence of HCl. In the absence of HCl, the known electronic and vibrational spectra of the corresponding triplet nitrenes, azirines, and didehydroazepines were observed, whereas in the presence of HCl, photolysis of these azides produces new electronic spectra assigned to the corresponding nitrenium cations. For p-biphenylyl azide the resulting spectrum of the nitrenium ion is very similar to the previously observed solution-phase spectrum of this species. The vibrational spectrum of this cation was recorded for the first time. Spectroscopic evidence for the previously unknown o-biphenylylnitrenium cation and 1-naphthylnitrenium cation are provided. The spectra of pand o-biphenylylnitrenium cations and 1-naphthylnitrenium cation are well reproduced by CASSCF and CASPT2 calculations. The same nitrenium cations were detected in solution by femtosecond time-resolved laser flash photolysis of the appropriate azides in 88% formic acid. The transient spectra of the nitrenium cations recorded in solution are in good agreement with the spectra obtained in HCl matrices. The rates of formation of these cations equal the rates of decay of the singlet nitrenes in 88% formic acid and are as follows: p-biphenylyl (τgrowth ) 11.5 ps), o-biphenylyl (τgrowth ) 7.7 ps), and 1-naphthylnitrenium cations (τgrowth ) 8.4 ps). The decay lifetimes of pand o-biphenylylnitrenium cations are 50 and 27 ns, respectively. The decay lifetimes of 1-naphthylnitrenium cation is 860 ps in 88% formic acid.

Research paper thumbnail of Reversible Interconversion between Singlet and Triplet 2-Naphthyl(carbomethoxy)carbene

Journal of the American Chemical Society, 1999

Photolysis of methyl R-diazo-(2-naphthyl)acetate at 450 nm yields primarily the triplet ground st... more Photolysis of methyl R-diazo-(2-naphthyl)acetate at 450 nm yields primarily the triplet ground state of 2-naphthyl(carbomethoxy)carbene ( 3 NCC), which was characterized by its UV/visible, IR, and ESR spectra as well as by trapping with O 2 and CO. Bleaching of the weak visible bands of 3 NCC (λ > 515 nm) leads to the disappearance of the triplet ESR spectrum and to a new optical spectrum with broad bands in the near-UV region. The spectra of 3 NCC can be almost fully recovered in the dark at 12 K. The same process also occurs upon irradiation at 450 nm, with the exception that the recovery of 3 NCC is accompanied by the formation of small amounts of 2-naphthyl(methoxy)ketene and 2-(2-naphthyl)propiolactone. The observed optical and IR spectra of the intermediate that is formed reversibly from 3 NCC are in full accord with quantum chemical model calculations for the singlet state of the same carbene 1 NCC, but not with the tricyclic cyclopropene derivative observed previously for the parent 2-naphthylcarbene or the 1-(2-naphthyl)-2-methoxyoxirene. Exploration of the potential energy surfaces for singlet and triplet NCC show that a barrier between the two states is created by a pronounced change in conformation of the carbomethoxy group, which may furthermore be hindered in a solid matrix environment.

Research paper thumbnail of Laser Flash Photolysis and Computational Study of Singlet Phenylnitrene

Journal of the American Chemical Society, 1999

Laser flash photolysis (LFP, Nd:YAG laser, 35 ps, 266 nm, 10 mJ) of phenyl azide (PA) releases op... more Laser flash photolysis (LFP, Nd:YAG laser, 35 ps, 266 nm, 10 mJ) of phenyl azide (PA) releases open-shell singlet phenylnitrene ( 1 PN). 1 PN has a sharp absorption band at 350 nm and a very weak absorption band at 540 nm. These bands are assigned as n z -n y and a mixture of 52% π-n x and 28% n x -π* transitions, respectively, on the basis of CASSCF and CASPT2 calculations. A CASPT2 calculation of the spectrum of triplet phenylnitrene ( 3 PN) is in very good agreement with the experimental spectrum. A sharp absorption band of 3 PN at 308 nm and a broad, structured band at 370 nm are assigned as n z -n y and π-π transitions on the basis of CASSCF and CASPT2 calculations. The low intensity, long wavelength band of 3 PN tailing to 500 nm is assigned as a mixture of π-n x , n x -π* transitions. The decay of 1 PN in pentane was measured as a function of temperature to obtain observed rate constants of disappearance, k obs . The rate constant k obs is equal to k R + k ISC , where k R is the absolute rate constant of rearrangement of 1 PN to benzazirine and k ISC is the absolute rate constant of intersystem crossing to the lower energy triplet state ( 3 PN). When it is assumed that k ISC is independent of temperature, k obs can be dissected, and absolute values of k R and k ISC can be deduced. For 1 PN, k ISC ) 3.2 ( 0.3 × 10 6 s -1 and the Arrhenius parameters for k R are found to be E a ) 5.6 ( 0.3 kcal mol -1 and A ) 10 13.

Research paper thumbnail of Carbomethoxychlorocarbene:  Spectroscopy, Theory, Chemistry and Kinetics

Journal of the American Chemical Society, 2001

Photolysis (254 nm) of methyl 8-chloro-3a,7a-methanoindan-8-carboxylate (5) in argon at 14 K prod... more 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.

Research paper thumbnail of Pentalene:  Formation, Electronic, and Vibrational Structure

Journal of the American Chemical Society, 1997

Research paper thumbnail of Photochemical Generation and Matrix-Isolation Detection of Dimethylvinylidene

The Journal of Organic Chemistry, 2001

We report the spectroscopic characterization of dimethylvinylidene, (CH3)2C=C:, generated within ... more We report the spectroscopic characterization of dimethylvinylidene, (CH3)2C=C:, generated within an argon matrix at 14 K from a bisperoxyester precursor. The carbene was identified by comparison of the experimental IR spectrum with vibrational frequencies computed at the B3LYP/6-31G(d) level. Chemical trapping of the carbene within a 9% CO/Ar matrix to form dimethylpropadienone supports this analysis. Additional products produced during photolysis were identified by comparison to the appropriate computed vibrational frequencies. The potential energy surface of dimethylvinylidene and its intramolecular rearrangement products, 2-butyne and methylcyclopropene, were also investigated computationally at the B3LYP/6-31G(d) level. A spin-state analysis of this carbene using a variety of computational methods (CCSD(T), B3LYP, MP2) indicates the singlet state is more stable than the triplet by approximately 45 kcal mol-1. We anticipate the bisperoxyester precursor used here will be a convenient and general way for initiating future studies of alkylvinylidenes under matrix-isolation conditions.

Research paper thumbnail of The Radical Cation ofanti-Tricyclooctadiene and Its Rearrangement Products

Chemistry - A European Journal, 2000

The anti dimer of cyclobutadiene (anti-tricyclo[4.2.0.0 2,5 ]octa-3,7-diene, TOD) is subjected to... more The anti dimer of cyclobutadiene (anti-tricyclo[4.2.0.0 2,5 ]octa-3,7-diene, TOD) is subjected to ionization by g-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 C 8 H 8

Research paper thumbnail of Efficient Use of the Iron Ortho-Nitrophenylporphyrin Chloride to Mimic Biological Oxidations of Dimethylaminoantipyrine

Chemical Biology & Drug Design, 2007

Major metabolites of dimethylaminoantipyrine have been synthesized using iron ortho-nitrophenylpo... more Major metabolites of dimethylaminoantipyrine have been synthesized using iron ortho-nitrophenylporphyrin chloride as biomimetic catalyst. Reactivity of iron tetrakis-ortho-nitrophenylporphyrin chloride [Fe(TNO2PP)Cl] has been compared with iron tetrakis-pentafluorophenylporphyrin chloride and iron tetrakis-2,6-dichlorophenylporphyrin chloride using various oxidants such as hydrogen peroxide, iodosobenzene, and cumene hydroperoxide in either protic or aprotic solvent. Effect of imidazole has been showed on the reactivity of Fe(TNO2PP)Cl/cumene hydroperoxide system.

Research paper thumbnail of A Laser Flash Photolysis and Quantum Chemical Study of the Fluorinated Derivatives of Singlet Phenylnitrene

Journal of the American Chemical Society, 2001

Laser flash photolysis (LFP, Nd:YAG laser, 35 ps, 266 nm, 10 mJ or KrF excimer laser, 10 ns, 249 ... more Laser flash photolysis (LFP, Nd:YAG laser, 35 ps, 266 nm, 10 mJ or KrF excimer laser, 10 ns, 249 nm, 50 mJ) of 2-fluoro, 4-fluoro, 3,5-difluoro, 2,6-difluoro, and 2,3,4,5,6-pentafluorophenyl azides produces the corresponding singlet nitrenes. The singlet nitrenes were detected by transient absorption spectroscopy, and their spectra are characterized by sharp absorption bands with maxima in the range of 300-365 nm. The kinetics of their decay were analyzed as a function of temperature to yield observed decay rate constants, k OBS . The observed rate constant in inert solvents is the sum of k R + k ISC where k R is the absolute rate constant of rearrangement of singlet nitrene to an azirine and k ISC is the absolute rate constant of nitrene intersystem crossing (ISC). Values of k R and k ISC were deduced after assuming that k ISC is independent of temperature. Barriers to cyclization of 4-fluoro-, 3,5-difluoro-, 2-fluoro-, 2,6-difluoro-, and 2,3,4,5,6-pentafluorophenylnitrene in inert solvents are 5.3 ( 0.3, 5.5 ( 0.3, 6.7 ( 0.3, 8.0 ( 1.5, and 8.8 ( 0.4 kcal/mol, respectively. The barrier to cyclization of parent singlet phenylnitrene is 5.6 ( 0.3 kcal/mol. All of these values are in good quantitative agreement with CASPT2 calculations of the relative barrier heights for the conversion of fluorosubstituted singlet aryl nitrenes to benzazirines (Karney, W. L. and Borden, W. T. J. Am. Chem. Soc. 1997, 119, 3347). A single ortho-fluorine substituent exerts a small but significant bystander effect on remote cyclization that is not steric in origin. The influence of two ortho-fluorine substituents on the cyclization is pronounced. In the case of the singlet 2-fluorophenylnitrene system, evidence is presented that the benzazirine is an intermediate and that the corresponding singlet nitrene and benzazirine interconvert. Ab initio calculations at different levels of theory on a series of benzazirines, their isomeric ketenimines, and the transition states converting the benzazirines to ketenimines were performed. The computational results are in good qualitative and quantitative agreement with the experimental findings.

Research paper thumbnail of Electronic structure of the naphthalene radical cation and some simple alkylated derivatives

Journal of the Chemical Society, Perkin Transactions 2, 1998

1 but more complete in the two derivatives. It is found that—in spite of the relatively small rel... more 1 but more complete in the two derivatives. It is found that—in spite of the relatively small relaxation energies of the vertically formed radical cations—the accompanying geometry changes on ionization may lead to quite substantial shifts in some of the excited state energies, notably that of the first (unobserved) one. Therefore, the common assumption that the good correlation between the

Research paper thumbnail of Rearrangement of 1-Noradamantyl and 1-Adamantylcarbene to Bridgehead Alkenes:  Lifetimes of Two Bridgehead Carbenes in Solution

Journal of Physical Chemistry A, 2001

B3LYP density functional calculations with the 6-31G* basis set predict that 3-noradamantylcarben... more B3LYP density functional calculations with the 6-31G* basis set predict that 3-noradamantylcarbene 7 rearranges to adamantene 9 and protoadmant-3-ene 10 over barriers of 0.35 and 7.84 kcal/mol, respectively, after zero-point energy correction. The ...

Research paper thumbnail of Electron Transfer Chemistry of Psoralen and Coumarin Derivatives by Means of Radiolytic and Electrochemical Experiments

The Journal of Physical Chemistry A, 1997

Bromopsoralens and bromocoumarins have been used to sensitize the inactivation of viruses. The br... more Bromopsoralens and bromocoumarins have been used to sensitize the inactivation of viruses. The bromine substituent increased the antiviral potency of the sensitizer, an effect attributed to cleavage of the C-Br bond of the aromatic radical anion produced by a light-induced electron transfer reaction with nucleic acids. This hypothesis was tested by a detailed mechanistic electrochemical investigation in acetonitrile and N,Ndimethylformamide of 5-bromo-8-methoxypsoralen, 8-methoxypsoralen, 3-bromocoumarin, and coumarin. A one-electron reduction process involves the formation of a radical anion, a step coupled with the cleavage of the carbon-bromine bond in the case of the brominated compounds, leading to the formation of an aryl radical moiety and a bromide anion. These experiments were further confirmed by radiolytic preparation coupled with UV-vis detection of the radical anions in glassy 2-methyltetrahydrofuran at 77 K and in ether, acetonitrile, and alcohol solutions at ambient temperature. This allows complete kinetic characterization of the ionic intermediates. The shorter lifetimes obtained with brominated radical anions relative to that of the parent structure are explained by scission of the carbon-halogen bond.

Research paper thumbnail of Chemistry and Kinetics of Dipropylcarbene in Solution

Journal of Physical Chemistry A, 1999

The photochemistry of 2-methoxy-2-methyl-5, 5-dipropyl-Δ3-1, 3, 4-oxadiazoline (1a) and 2, 2-dime... more The photochemistry of 2-methoxy-2-methyl-5, 5-dipropyl-Δ3-1, 3, 4-oxadiazoline (1a) and 2, 2-dimethoxy-5, 5-dipropyl-Δ3-1, 3, 4-oxadiazoline (1b) was investigated. Photolysis (300 nm) of these compounds in solution leads to fragmentation to 4-diazoheptane (major), ...

Research paper thumbnail of Observation of Perfluoromethylnitrene in Cryogenic Matrixes

The Journal of Physical Chemistry A, 2001

Photolysis (254 nm) of CF 3 N 3 in pentane at 6-10 K produced a persistent electron paramagnetic ... more Photolysis (254 nm) of CF 3 N 3 in pentane at 6-10 K produced a persistent electron paramagnetic resonance spectrum typical of a triplet nitrene centered at 8620 G. The spectrum (|D/hc| ) 1.736 cm -1 ) is attributed to triplet CF 3 -N, and is very similar to that of triplet NH (|D/hc| ) 1.863 cm -1 ) and CH 3-N (|D/hc| ) 1.720 cm -1 ) which were reported previously. CF 3 N 3 was also deposited in an argon matrix at 14 K. The azide (λ max ) 257 nm) was decomposed by exposure to 254 nm light with the concurrent formation of sharp, structured absorption bands at 342, 347.5, and 354 nm. The UV bands are assigned to the triplet nitrene CF 3 N on the basis of its similarity to the spectrum of CH 3 N and time-dependent density functional theory (B3LYP) calculations.

Research paper thumbnail of Singlet−Triplet Gap, and the Electronic and Vibrational Spectra of Chlorophenylcarbene:  A Combined Theoretical and Experimental Study

The Journal of Physical Chemistry A, 1999

... MG, 31270-901 Brazil. Sol Celebi, Zhendong Zhu, and Matthew S. Platz*. Newman and Wolfrom Lab... more ... MG, 31270-901 Brazil. Sol Celebi, Zhendong Zhu, and Matthew S. Platz*. Newman and Wolfrom Laboratory of Chemistry, The Ohio State University, 100 W. 18th Avenue, Columbus, Ohio 43210. J. Phys. Chem. A , 1999, 103 ...

Research paper thumbnail of A surprising rearrangement of a carbeneethylene sulfide ylide

Tetrahedron Letters, 1999

Efficient synthesis of benzofuroquinolizine ketone 1 was accomplished in four steps from ethyl 3b... more Efficient synthesis of benzofuroquinolizine ketone 1 was accomplished in four steps from ethyl 3benzofuranacetate. The O-analogue of the Pictet-Spengler cyclization was used to form the benzofuroquinolizine ring structure as a key step.

Research paper thumbnail of Ultrafast Spectroscopic and Matrix Isolation Studies of p- Biphenylyl, o -Biphenylyl, and 1-Naphthylnitrenium Cations

Journal of the American Chemical Society, 2007

p-Biphenylyl, o-biphenylyl, and 1-naphthyl azides were deposited in argon at low temperature in t... more p-Biphenylyl, o-biphenylyl, and 1-naphthyl azides were deposited in argon at low temperature in the presence and absence of HCl. In the absence of HCl, the known electronic and vibrational spectra of the corresponding triplet nitrenes, azirines, and didehydroazepines were observed, whereas in the presence of HCl, photolysis of these azides produces new electronic spectra assigned to the corresponding nitrenium cations. For p-biphenylyl azide the resulting spectrum of the nitrenium ion is very similar to the previously observed solution-phase spectrum of this species. The vibrational spectrum of this cation was recorded for the first time. Spectroscopic evidence for the previously unknown o-biphenylylnitrenium cation and 1-naphthylnitrenium cation are provided. The spectra of pand o-biphenylylnitrenium cations and 1-naphthylnitrenium cation are well reproduced by CASSCF and CASPT2 calculations. The same nitrenium cations were detected in solution by femtosecond time-resolved laser flash photolysis of the appropriate azides in 88% formic acid. The transient spectra of the nitrenium cations recorded in solution are in good agreement with the spectra obtained in HCl matrices. The rates of formation of these cations equal the rates of decay of the singlet nitrenes in 88% formic acid and are as follows: p-biphenylyl (τgrowth ) 11.5 ps), o-biphenylyl (τgrowth ) 7.7 ps), and 1-naphthylnitrenium cations (τgrowth ) 8.4 ps). The decay lifetimes of pand o-biphenylylnitrenium cations are 50 and 27 ns, respectively. The decay lifetimes of 1-naphthylnitrenium cation is 860 ps in 88% formic acid.

Research paper thumbnail of Reversible Interconversion between Singlet and Triplet 2-Naphthyl(carbomethoxy)carbene

Journal of the American Chemical Society, 1999

Photolysis of methyl R-diazo-(2-naphthyl)acetate at 450 nm yields primarily the triplet ground st... more Photolysis of methyl R-diazo-(2-naphthyl)acetate at 450 nm yields primarily the triplet ground state of 2-naphthyl(carbomethoxy)carbene ( 3 NCC), which was characterized by its UV/visible, IR, and ESR spectra as well as by trapping with O 2 and CO. Bleaching of the weak visible bands of 3 NCC (λ > 515 nm) leads to the disappearance of the triplet ESR spectrum and to a new optical spectrum with broad bands in the near-UV region. The spectra of 3 NCC can be almost fully recovered in the dark at 12 K. The same process also occurs upon irradiation at 450 nm, with the exception that the recovery of 3 NCC is accompanied by the formation of small amounts of 2-naphthyl(methoxy)ketene and 2-(2-naphthyl)propiolactone. The observed optical and IR spectra of the intermediate that is formed reversibly from 3 NCC are in full accord with quantum chemical model calculations for the singlet state of the same carbene 1 NCC, but not with the tricyclic cyclopropene derivative observed previously for the parent 2-naphthylcarbene or the 1-(2-naphthyl)-2-methoxyoxirene. Exploration of the potential energy surfaces for singlet and triplet NCC show that a barrier between the two states is created by a pronounced change in conformation of the carbomethoxy group, which may furthermore be hindered in a solid matrix environment.

Research paper thumbnail of Laser Flash Photolysis and Computational Study of Singlet Phenylnitrene

Journal of the American Chemical Society, 1999

Laser flash photolysis (LFP, Nd:YAG laser, 35 ps, 266 nm, 10 mJ) of phenyl azide (PA) releases op... more Laser flash photolysis (LFP, Nd:YAG laser, 35 ps, 266 nm, 10 mJ) of phenyl azide (PA) releases open-shell singlet phenylnitrene ( 1 PN). 1 PN has a sharp absorption band at 350 nm and a very weak absorption band at 540 nm. These bands are assigned as n z -n y and a mixture of 52% π-n x and 28% n x -π* transitions, respectively, on the basis of CASSCF and CASPT2 calculations. A CASPT2 calculation of the spectrum of triplet phenylnitrene ( 3 PN) is in very good agreement with the experimental spectrum. A sharp absorption band of 3 PN at 308 nm and a broad, structured band at 370 nm are assigned as n z -n y and π-π transitions on the basis of CASSCF and CASPT2 calculations. The low intensity, long wavelength band of 3 PN tailing to 500 nm is assigned as a mixture of π-n x , n x -π* transitions. The decay of 1 PN in pentane was measured as a function of temperature to obtain observed rate constants of disappearance, k obs . The rate constant k obs is equal to k R + k ISC , where k R is the absolute rate constant of rearrangement of 1 PN to benzazirine and k ISC is the absolute rate constant of intersystem crossing to the lower energy triplet state ( 3 PN). When it is assumed that k ISC is independent of temperature, k obs can be dissected, and absolute values of k R and k ISC can be deduced. For 1 PN, k ISC ) 3.2 ( 0.3 × 10 6 s -1 and the Arrhenius parameters for k R are found to be E a ) 5.6 ( 0.3 kcal mol -1 and A ) 10 13.

Research paper thumbnail of Carbomethoxychlorocarbene:  Spectroscopy, Theory, Chemistry and Kinetics

Journal of the American Chemical Society, 2001

Photolysis (254 nm) of methyl 8-chloro-3a,7a-methanoindan-8-carboxylate (5) in argon at 14 K prod... more 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.

Research paper thumbnail of Pentalene:  Formation, Electronic, and Vibrational Structure

Journal of the American Chemical Society, 1997

Research paper thumbnail of Photochemical Generation and Matrix-Isolation Detection of Dimethylvinylidene

The Journal of Organic Chemistry, 2001

We report the spectroscopic characterization of dimethylvinylidene, (CH3)2C=C:, generated within ... more We report the spectroscopic characterization of dimethylvinylidene, (CH3)2C=C:, generated within an argon matrix at 14 K from a bisperoxyester precursor. The carbene was identified by comparison of the experimental IR spectrum with vibrational frequencies computed at the B3LYP/6-31G(d) level. Chemical trapping of the carbene within a 9% CO/Ar matrix to form dimethylpropadienone supports this analysis. Additional products produced during photolysis were identified by comparison to the appropriate computed vibrational frequencies. The potential energy surface of dimethylvinylidene and its intramolecular rearrangement products, 2-butyne and methylcyclopropene, were also investigated computationally at the B3LYP/6-31G(d) level. A spin-state analysis of this carbene using a variety of computational methods (CCSD(T), B3LYP, MP2) indicates the singlet state is more stable than the triplet by approximately 45 kcal mol-1. We anticipate the bisperoxyester precursor used here will be a convenient and general way for initiating future studies of alkylvinylidenes under matrix-isolation conditions.

Research paper thumbnail of The Radical Cation ofanti-Tricyclooctadiene and Its Rearrangement Products

Chemistry - A European Journal, 2000

The anti dimer of cyclobutadiene (anti-tricyclo[4.2.0.0 2,5 ]octa-3,7-diene, TOD) is subjected to... more The anti dimer of cyclobutadiene (anti-tricyclo[4.2.0.0 2,5 ]octa-3,7-diene, TOD) is subjected to ionization by g-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 C 8 H 8

Research paper thumbnail of Efficient Use of the Iron Ortho-Nitrophenylporphyrin Chloride to Mimic Biological Oxidations of Dimethylaminoantipyrine

Chemical Biology & Drug Design, 2007

Major metabolites of dimethylaminoantipyrine have been synthesized using iron ortho-nitrophenylpo... more Major metabolites of dimethylaminoantipyrine have been synthesized using iron ortho-nitrophenylporphyrin chloride as biomimetic catalyst. Reactivity of iron tetrakis-ortho-nitrophenylporphyrin chloride [Fe(TNO2PP)Cl] has been compared with iron tetrakis-pentafluorophenylporphyrin chloride and iron tetrakis-2,6-dichlorophenylporphyrin chloride using various oxidants such as hydrogen peroxide, iodosobenzene, and cumene hydroperoxide in either protic or aprotic solvent. Effect of imidazole has been showed on the reactivity of Fe(TNO2PP)Cl/cumene hydroperoxide system.

Research paper thumbnail of A Laser Flash Photolysis and Quantum Chemical Study of the Fluorinated Derivatives of Singlet Phenylnitrene

Journal of the American Chemical Society, 2001

Laser flash photolysis (LFP, Nd:YAG laser, 35 ps, 266 nm, 10 mJ or KrF excimer laser, 10 ns, 249 ... more Laser flash photolysis (LFP, Nd:YAG laser, 35 ps, 266 nm, 10 mJ or KrF excimer laser, 10 ns, 249 nm, 50 mJ) of 2-fluoro, 4-fluoro, 3,5-difluoro, 2,6-difluoro, and 2,3,4,5,6-pentafluorophenyl azides produces the corresponding singlet nitrenes. The singlet nitrenes were detected by transient absorption spectroscopy, and their spectra are characterized by sharp absorption bands with maxima in the range of 300-365 nm. The kinetics of their decay were analyzed as a function of temperature to yield observed decay rate constants, k OBS . The observed rate constant in inert solvents is the sum of k R + k ISC where k R is the absolute rate constant of rearrangement of singlet nitrene to an azirine and k ISC is the absolute rate constant of nitrene intersystem crossing (ISC). Values of k R and k ISC were deduced after assuming that k ISC is independent of temperature. Barriers to cyclization of 4-fluoro-, 3,5-difluoro-, 2-fluoro-, 2,6-difluoro-, and 2,3,4,5,6-pentafluorophenylnitrene in inert solvents are 5.3 ( 0.3, 5.5 ( 0.3, 6.7 ( 0.3, 8.0 ( 1.5, and 8.8 ( 0.4 kcal/mol, respectively. The barrier to cyclization of parent singlet phenylnitrene is 5.6 ( 0.3 kcal/mol. All of these values are in good quantitative agreement with CASPT2 calculations of the relative barrier heights for the conversion of fluorosubstituted singlet aryl nitrenes to benzazirines (Karney, W. L. and Borden, W. T. J. Am. Chem. Soc. 1997, 119, 3347). A single ortho-fluorine substituent exerts a small but significant bystander effect on remote cyclization that is not steric in origin. The influence of two ortho-fluorine substituents on the cyclization is pronounced. In the case of the singlet 2-fluorophenylnitrene system, evidence is presented that the benzazirine is an intermediate and that the corresponding singlet nitrene and benzazirine interconvert. Ab initio calculations at different levels of theory on a series of benzazirines, their isomeric ketenimines, and the transition states converting the benzazirines to ketenimines were performed. The computational results are in good qualitative and quantitative agreement with the experimental findings.